November 2012

Understanding 

Particulate 

Matter: 

Protecting Public 

Health in the 

San Francisco Bay Area
The Air District has prepared this report for informational purposes only. This report builds upon the Bay Area 2010 Clean 

Air Plan, which defines the Air District’s current particulate matter (PM) control strategy. This report will inform future PM 

planning in the Bay Area, but does not commit the Air District to any specific course of regulatory action.

Understanding Particulate Matter: 

Protecting Public Health in the 

San Francisco Bay Area
 Table of 

Contents

Executive Summary vii

Introduction 1

Reducing Population Exposure to PM 1

What is PM? 2

Public Health Impacts 2

PM Planning in the Bay Area 3

Progress in Reducing PM Levels in the Bay Area 5

Why We Need to Reduce PM Further 5

Challenges 5

Section 1 : PM IMpacts 7

Section 1-A: The Public Health Effects of PM 7

Challenges in Analyzing the Health Effects of PM 8

Early Evidence of PM Health Effects 8

Analyzing the Health Effects of PM 9

Recent Research on Health Effects of PM 10

PM Impact on Premature Mortality and Life Expectancy 11

Reaction to Findings 13

Summary of PM Health Effects 13

No Safe Threshold Has Been Identified 17

Recent Findings 17

Biological Pathways: How PM Harms the Body 18

Which sizes and types of particles are most dangerous? 20

PM Damage to Cells 21

Relationship Between Particle Composition and Health Effects 21

Estimating PM Health Impacts in the Bay Area 25

Areas that Require Additional Research 28

Section 1-B: Population Exposure to PM 30

Linking PM to Health Effects 30

Sensitive Populations 33

Which Environments Pose the Greatest Risk? 35

Understanding particulate matter | 2012 | Bay Area Air Quality Management District i
 Table of 

Contents

Exposure to PM from Busy Roadways 36

Indoor Exposure to PM 41

Summary 46

Section 1-C: PM and Climate Change 47

How PM Affects Climate Change 47

Climate Forcing 48

Short-Term vs. Long-Term Impacts 52

Geographic Scale of Impacts 52

Net impact of PM on the Climate 53

Impacts of Climate Change on PM Levels 55

Trade-offs 58

Next Steps 58

Section 1-D: PM Impacts on Ecosystems & Visibility 59

Effects of PM on Vegetation and Soils 60

PM and Acid Rain 61

Effects of PM on Water, Aquatic Systems, and Wildlife 63

PM and Visibility (Haze) 64

Effects of PM on Materials and Property Values 66

Section 2: PM Technical information 68

PM Characteristics 68

PM Formation & Dynamics in the Bay Area 75

Source Contributions to Ambient PM Concentrations 79

PM Emissions Inventory 81

Sources of Precursors Pollutants That Form Secondary PM 91

Predicting Future Emissions Trends 94

Projected Trends for Primary PM and Precursors To Secondary PM 96

Section 3: PM Air quality standards and trends 100

Section 3-A: PM Monitoring Program 100

Regional PM Monitoring Network 100

Measuring PM Concentrations at the Local Scale 102

Measuring Ultrafine PM 104

ii understanding Particulate matter | 2012 | Bay Area Air Quality Management District
 Table of 

Contents

Section 3-B: PM Standards and PM Planning Requirements 106

PM Standards 106

Section 3-C: Trends in Ambient PM Concentrations 112

PM2.5 Trends 112

PM10 Trend 117

Section 4: Summary of PM Control Program 118

CARB PM Reduction Program 118

EPA Actions to Reduce PM from “Federal Sources” 122

BAAQMD PM Reduction Program 122

Reducing Population Exposure to PM 137

Section 5: Looking Forward 144

Why It’s Important to Continue Reducing PM 144

Challenges We Face 145

Policy Guidelines to Inform Future PM Planning 146

Areas for Further Evaluation 147

Technical Enhancements Needed to Support 

the Air District’s Efforts 151

How Bay Area Residences Can Reduce 

Their Personal Exposure to PM 155

Conclusion 156

Appendix 157

Table 1: Bay Area Winter Emissions Inventory for 

Primary PM & PM Precursors 2010-2030 157

List of Sources 161

Understanding particulate matter | 2012 | Bay Area Air Quality Management District iii
 List of

Tables

Table 1-1 US EPA Findings on Health Effects for Particulate Matter 14

Table 1-2 Estimated increase in hospital admissions 

 rate for a 10 μg/m3 increase for short-term and long-term 

 exposure to PM2.5 by cause of admission 16

Table 1-3 Comparison of Ultrafine Particle Concentrations 39

Table 1-4 Climate-Forcing Properties of PM Components 49

Table 1-5 Summary of Negative Impacts of PM on 

 Ecosystems, Visibility, and Materials 60

Table 2-1 Characteristics of Particulate Matter by Size Range 70

Table 2-2 Bay Area Annual Average Primary PM2.5 Emissions – 

 Year 2010 On-Road and Off-Road Motor Vehicles 84

Table 3-1 Bay Area PM Monitoring Sites 102 

Table 3-2 Basic Design Value Calculation Method for National PM2.5

 Standards 107

Table 3-3 PM Standards, Bay Area Attainment Status, and Design Values 109

Table 3-4 Exceedances of 24-Hour PM2.5 (35 μg/m3) 114

Table 4-1 CARB Diesel Air Toxic Control Measures for 

 Heavy-Duty Vehicles, Equipment and Ships 119

Table 4-2 Estimated Reductions in Primary PM & PM 

 Precursors from BAAQMD Regulations (tons per day) 125

Table 4-3 Stationary Source Measures to Reduce PM & PM 

 Precursors in Bay Area 2010 Clean Air Plan 126

Table 4-4 Mobile Source Enforcement Summary 

 for Port of Oakland: 1/1/2010-12/31/2011 129

Table 4-5 Air District Grant Programs: Projects Funded from 

 2007 through 2011 130

iv understanding Particulate matter | 2012 | Bay Area Air Quality Management District
 List of

Figures

Figure 1-1 How Particulate Matter Enters Our Body 19

Figure 1-2 Cells Exposed to Fine PM (PM2.5) for 16 Hours 21

Figure 1-3 Cells Exposed to Ultrafine PM for 16 Hours 21

Figure 1-4 Bay Area Air Pollution Health Burden: Past & Present 27

Figure 1-5 Number of Particles Versus Distance from Roadway: Two Freeways in Southern 

California (Zhu, 2002) 37

Figure 1-6 Formation and Deposition of Acid Rain 62

Figure 2-1 Comparison of PM10, PM2.5, and Ultrafine PM 69

Figure 2-2 Ratio of surface area to volume 71

Figure 2-3 Spatial distribution of simulated 24-hr primary and secondary PM2.5 

levels averaged across the 52 simulated days when measured Bay Area 

24-hr PM2.5 level exceeded 35 μg/m3 78

Figure 2-4 Estimated Source Contributions to Annual PM2.5 Concentrations 80

Figure 2-5 Estimated Source Contributions to Peak PM2.5 Concentrations 81

Figure 2-6 2010 Annual Primary Emissions of PM2.5, 47 tons/day

2010 Winter PM2.5 Emissions 88

Figure 2-7 2010 Winter Primary Emissions of PM2.5, 49 tons/day 89

Figure 2-8 2010 Annual Average Primary Emissions of PM10, 106 tons/day 90

Figure 2-9 2010 Winter Primary Emissions of PM10, 104 tons/day 90

Figure 2-10 2010 Winter Emissions Reactive Organic Gases (ROG) 294 tons/day 91

Figure 2-11 2010 Winter Emissions Oxides of Nitrogen (NOx), 347 tons/day 92

Figure 2-12 2010 Winter Emissions Sulfur Dioxides (SO2), 29 tons/day 93

Figure 2-13 Bay Area Winter Primary PM2.5 Emissions Trends 96

Figure 2-14 Bay Area Winter PrimaryPM10 Emissions Trends 97

Figure 2-15 Bay Area Winter ROG Emissions Trends 98

Figure 2-16 Bay Area Winter NOx Emission Trends 99

Understanding particulate matter | 2012 | Bay Area Air Quality Management District v
 List of

Figures

Figure 2-17 Bay Area Winter SO2 Emissions Trends 90

Figure 3-1 Bay Area PM2.5 Annual Design Value 1999-2001 through 2009-2011 112

Figure 3-2 Bay Area Design Values for the 24-Hour Standard

1999-2001 through 2009-2011 113

Figure 3-3 Bay Area PM2.5 Exceedances by Winter Number of days exceeding the 24‐hour 

NAAQS, November 15 – February 15 113

Figure 3-4 Bay Area PM2.5 Exceedances and Conducive Days by Winter 115

Figure 3-5 Design Values for Annual PM2.5 Standard for 3 Central California Air Basins 116

Figure 3-6 Design Values* for 24-Hour PM2.5 Standard for 3 Central California Air Basins 116

Figure 3-7 Bay Area Annual Mean PM10 117

Figure 4-1 Air District Grants Awarded by Project Type from 2007 through 2011

 (Total value = $308 million) 131

Figure 4-2 Grant Funds Awarded to Projects in Impacted Communities in 

2011 (Total value = $60 million) 131

Figure 4-3 BAAQMD Clean Air Communities Initiative 139

vi understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Executive Summary

The air we breathe is saturated with millions of microscopic particles. This particulate matter (PM), 

especially the finest particles, can evade the body’s natural defenses and penetrate deep into the 

lungs, blood stream, brain and other vital organs, and individual cells. Health studies in recent 

decades have shown that exposure to PM, on both a short-term and a long-term basis, can have a 

wide range of negative health effects, including respiratory effects such as asthma, bronchitis, and 

impaired lung development in children, as well as serious cardiovascular effects such as hardening of 

the arteries, heart attacks and strokes. Health studies in cities throughout the world have found that 

mortality (death) rates increase with higher PM levels.

PM is a complex pollutant composed of a diverse assortment of extremely small airborne particles, 

including a mixture of solid particles and liquid droplets that vary greatly in terms of their size and 

mass, chemical composition, and toxicity. PM originates from a variety of man-made processes and 

sources such as fossil fuel combustion, residential wood burning, and cooking, as well as from natural 

sources such as wildfires, volcanoes, sea salt, and geological dust. PM is generated indoors as well 

as outdoors. Indoor sources include stoves, heaters, fireplaces, consumer products, and cigarettes. 

Although researchers have been making progress in analyzing PM, there are still major gaps in our 

understanding of PM and its effects on public health, climate change, and ecosystems. 

The Bay Area Air Quality Management District (Air District) and the California Air Resources Board 

(ARB) have been working to reduce PM levels and human exposure. Thanks to these efforts, PM 

levels in the Bay Area have been significantly reduced in recent decades. The reduction in PM levels 

has provided significant public health benefits and has helped to increase average life expectancy in 

the region. But despite this progress, PM is still by far the air pollutant most harmful to public health 

in the Bay Area. Although monitoring data shows that the Bay Area currently meets all national (and 

most State) standards for PM, health studies show that there are negative health effects associated 

with exposure to PM even below the current air quality standards.

The purpose of this report is to synthesize the latest scientific information about PM and its impacts, 

describe the Air District’s current efforts to analyze and reduce PM in the Bay Area, and lay the 

groundwork for the Air District’s continuing efforts to further reduce PM and better protect public 

health. The report provides technical information about how PM is emitted and formed in the 

Bay Area; describes progress in reducing PM levels in the San Francisco Bay Area in recent years; 

summarizes current regulations and programs to reduce PM emissions and concentrations; identifies 

future technical work needed to improve the Air District’s understanding of PM; and explains the 

importance of continuing and enhancing the Air District’s efforts to reduce PM in order to protect 

public health, the climate, and the environment.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District vii
Introduction

The mission of the Bay Area Air Quality Management District (Air District or BAAQMD) is to protect and 

improve air quality, public health, and the global climate. Although we have made great progress in 

improving Bay Area air quality over the past 50 years, new challenges continue to emerge. Research 

in recent years indicates that particulate matter (PM) suspended in the air we breathe is the air 

pollutant that imposes the greatest health burden on Bay Area residents. The available evidence 

indicates that the most effective way that the Air District can fulfill its mission of protecting public 

health is to reduce the population exposure of Bay Area residents to PM. 

Although PM has been regulated by U.S. EPA as one of the original “criteria air pollutants” since 

the early 1970’s, early efforts to improve air quality primarily focused on other pollutants such as 

ground-level ozone (smog), carbon monoxide, and toxic air contaminants. PM moved to the forefront 

of the air quality agenda only in recent years, beginning in the mid-1990’s, in response to a series of 

compelling health studies that linked population exposure to PM with a wide range of respiratory and 

cardiovascular health effects, including premature death. Indeed, the recognition that PM must be 

treated as an air pollutant of the highest priority represents perhaps the most important development 

in the air quality arena in recent years.

This report describes particulate matter and its impacts on public health, climate change, and 

ecosystems; summarizes technical information about PM, and how it is emitted and formed in the 

Bay Area; describes progress in recent years in reducing PM levels in the San Francisco Bay Area 

in relation to State and national PM standards; describes current regulations and programs to 

reduce PM emissions and concentrations; identifies future technical work needed to improve our 

understanding of PM; and provides a roadmap to focus Air District resources in the effort to reduce 

PM and protect public health in the Bay Area in the years to come.

Reducing Population Exposure to PM

Air quality planning to date, in the Bay Area and elsewhere, has generally focused on reducing 

emissions and ambient concentrations of air pollutants in order to attain State and national ambient 

air quality standards. This approach has enabled the Bay Area and other regions to make substantial 

progress in improving air quality, especially for pollutants that are regional in nature, such as ozone. 

However, in recent years there is a growing recognition that if we want to achieve the ultimate goal of 

protecting public health, then reducing emissions and concentrations of air pollutants at the regional 

scale may not suffice. We need to directly consider where, when, and how people are being exposed 

to air pollution. This is especially true in the case of PM, a complex pollutant whose concentrations in 

the air can vary substantially depending upon location and time. Therefore, the major objective of this 

report is to advance our understanding of how the Bay Area public is exposed to PM, which sources 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 1
and types of PM are most harmful, and where we should focus efforts to reduce PM in order to better 

protect public health.

What is PM?

The term particulate matter (PM) describes a complex pollutant composed of a diverse assortment 

of extremely small airborne particles, including a mixture of solid particles and liquid droplets known 

as aerosols. Most air pollutants (such as ozone, carbon monoxide, and 

sulfur dioxide) consist of a single molecule or compound. This means 

that the pollutant will have the same physical properties and the same 

impacts on public health and the environment, regardless of the source, 

or combination of sources, from which it is emitted. PM, by contrast, 

includes a wide range of disparate particles that vary greatly in terms 

of their size and mass (ultrafine, fine, and coarse), physical state (solid 

or liquid), chemical composition, toxicity, and how they behave and 

transform in the atmosphere. 

Particles originate from a variety of man-made processes and sources 

such as fossil fuel combustion, residential wood-burning, and cooking, 

as well as from natural sources such as wildfires, volcanoes, sea salt, and geological dust. PM is 

emitted directly from tailpipes, smokestacks, and fireplaces, and also formed indirectly by chemical 

reactions among precursor pollutants. Particulate matter is generated indoors as well as outdoors. 

Indoor sources include stoves, heaters, fireplaces, and consumer products, and cigarettes (if smoked 

inside). As described in Section 1-B, most people experience a significant percentage of their 

personal exposure to PM in the indoor environment where they are 

exposed to both ambient (outdoor) PM that penetrates inside as well 

as PM emissions produced by indoor sources.

Public Health Impacts

PM is a stealthy pollutant - it is generally tasteless and odorless, and 

most particles are too small to be seen by the naked eye (though 

PM makes the air look hazy in the distance). But even on clear days 

when ambient PM concentrations are low and well within air quality 

standards, we inhale contains millions of tiny particles with each 

breath. Health studies show that these airborne particles cause and/

or contribute to a wide range of respiratory and cardiovascular problems. In addition to effects such 

as asthma and bronchitis, exposure to PM can trigger major health impacts such as heart attacks or 

strokes. In fact, the evidence indicates that exposure to PM contributes to the death of more Bay Area 

residents each year than more visible causes such as auto accidents. Analysis shows that, even at 

the relatively moderate concentrations that prevail in the Bay Area, PM imposes economic and social 

costs to Bay Area residents and employers (in terms of sickness, lost productivity, and premature 

mortality) that run to billions of dollars per year, as discussed in Section 1-A. 

Every breath we 

take contains 

millions of tiny 

airborne particles.

PM includes a 

wide range of 

particles that vary 

in size, chemical 

composition, 

and toxicity.

2 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
PM Planning in the Bay Area

In response to concern about the health impacts of PM, in 2003 the State legislature enacted SB 

656 (codified as Health & Safety Code Section 39614). This legislation required the Air Resources 

Board and local air districts to evaluate potential PM control measures and to develop a PM 

implementation schedule for appropriate PM-reduction measures. The Air District complied with this 

legislation; staff developed a Particulate Matter Implementation Schedule that was adopted by the Air 

District’s Board of DIrectors in November 2005, and the Air District adopted the measures identified 

in the Implementation Schedule.

How many particles do we breathe?

The number of airborne particles that we are exposed 

to on a daily basis is truly staggering. The air we breathe 

contains a very minute amount of PM in terms of its overall 

mass. However, even such a miniscule amount of mass 

contains enormous numbers of particles. For example, 

air with an ambient concentration of 10 micrograms of 

fine particulate matter (PM2.5, or particles less than 2.5 

microns in diameter) per cubic meter - roughly the average 

concentration of Bay Area PM2.5 on an annual basis - may 

contain on the order of 100 million particles per cubic 

meter. 

For this reason, we inhale huge numbers of particles. For 

purposes of illustration, consider the fact that urban air 

typically contains in the range of 5,000 to 30,000 particles 

per cubic centimeter, primarily in the ultrafine size range 

(particles less than 0.1 microns in diameter). The average adult inhales 450 cubic centimeters 

(roughly one pint) of air per breath. This means that in a typical urban environment we inhale from 

1 million to 10 million minute particles with every breath we take. But that figure can spike to much 

higher levels in close proximity to high-volume roadways or other major outdoor emission sources, or 

indoor sources such as stoves and ovens. The bottom line is that, during the course of a single day, 

we inhale many trillions of fine and ultrafine particles, even when the air we are breathing meets air 

quality standards. Fortunately, our bodies have defenses in the nasal passages, throat, and lungs 

to filter out particles, so not all the pollutants that we inhale actually reach the air sacs (alveoli) 

where they can damage the lungs. But some of these tiny particles - which may be coated with 

acids, metals, and other toxic substances – are able to evade the body’s defense mechanisms and 

penetrate deep into the lungs, bloodstream, cells, and vital organs where they can trigger various 

biological responses that harm the body. 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 3
 In fall 2010, the Air District adopted the Bay Area 2010 Clean Air Plan (2010 CAP) to update the 

region’s plan to control ground-level ozone. In developing the 2010 CAP, the Air District was inspired 

by the recommendations issued by the National Research Council in 2004 which called for a new 

approach to air quality planning based on integrated multi-pollutant planning focused on achieving 

key outcomes such as protecting public health, the global climate, and ecosystems. 

The Bay Area 2010 CAP identified two key goals: (1) protecting public health, and (2) protecting the 

climate. The 2010 CAP also pursued a multi-pollutant approach in developing an integrated control 

strategy to reduce four types of air pollutants: ground-level ozone; PM; toxic air contaminants (TACs); 

and greenhouse gases, such as carbon dioxide and methane, that contribute to climate change.

To inform the development of the 2010 CAP, Air District staff performed a “health burden” analysis, 

based upon the results of peer-reviewed health studies, in order to estimate:

• the public health effects of air pollution in the Bay Area, based upon key 

health endpoints such as chronic bronchitis, asthma emergency room visits, 

hospital admissions for respiratory or cardiovascular diseases, heart attacks, 

and premature mortality;

• the role of each air pollutant in causing or contributing to these health 

effects;

• the health benefits due to progress in improving Bay Area air quality in recent 

years; and

• the economic benefit of the improvement in public health.

Air quality issues are often presented in highly technical terms, using a specialized vocabulary and 

arcane units of measurement (such as parts per million or micro-grams per cubic meter) that mean 

little to the average person. The health burden analysis, by contrast, served to put a “human face” 

on the benefits of better air quality, by expressing its results in terms of tangible outcomes that make 

sense to policy-makers and the public. 

From the policy perspective, the most important finding from the health burden analysis is that 

PM is the air pollutant that poses by far the greatest health risk to Bay Area residents, as shown 

in Figure 1-4. Although evidence of the public health risk related to PM has been mounting in 

recent years, it was only by performing this side-by-side health burden analysis for the 2010 CAP 

4 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
that the predominant impact of PM on public health in comparison to other air pollutants was so 

starkly revealed.

Progress in Reducing PM Levels in the Bay Area 

Although revelations about the health impacts of PM are cause for concern, the good news is that we 

have already achieved major progress in reducing PM levels in the Bay Area, as described in Section 

3-C. Thanks to this progress, the Bay Area currently meets the national ambient air quality standards 

that apply to PM, and is making steady progress toward attaining more stringent, health-protective 

California PM standards. The reduction in PM levels in recent years translates into improved public 

health and vitality, and longer average life span. These benefits are worth billions of dollars in cost 

savings to Bay Area residents and employers.

Why We Need to Reduce PM Further

The fact that the Bay Area has made substantial progress in reducing PM levels does not mean that 

we can rest easy, however. There are several reasons why it is important to continue to enhance our 

efforts to reduce PM emissions, concentrations, and population exposure. 

• Researchers have not been able to establish a safe threshold for 

population exposure to PM. Epidemiological studies have shown that there 

are health effects from PM2.5 exposure even at concentrations below 

current standards.

• The U.S. Environmental Protection Agency (EPA) reviews the national PM2.5 

standards on a regular basis and may issue more stringent standards in 

the future.

• Even at the current, relatively low concentrations, PM is the most hazardous 

air pollutant in the Bay Area in terms of health impacts, including premature 

mortality, heart attacks, chronic bronchitis and other key health endpoints.

• PM concentrations – and population exposure to PM – can vary significantly 

at the local scale, as discussed in Section 1-B). Even though the Bay Area 

currently meets national PM standards based on the measurements from 

the regional PM monitoring network, some communities and individuals are 

exposed to higher concentrations of PM. People who live or work near major 

roadways, ports, distribution centers, or other major emission sources, or in 

proximity to wood-burning activities, may be disproportionately exposed to 

certain types of PM (e.g. ultrafine particles), so it is important to implement 

effective measures to reduce their exposure and health risks.

Challenges

Because PM is a complex pollutant and has become the focus of intense research only in recent 

years, there are still major gaps in our understanding of PM and its effects on public health, climate 

change, and ecosystems. These gaps are especially profound in regard to ultrafine PM, the smallest 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 5
particles. As discussed in Section 1-A, health researchers are working to better understand which 

sizes and types of PM are most damaging to public health, and to explain the precise biological 

mechanisms by which PM damages our health. Similarly, climate scientists are striving to better 

define the various mechanisms by which different types of particles act upon the climate, and the 

overall effect of ambient PM on the climate. 

Although many health studies to date suggest that breathing PM of all types may have negative 

health effects, researchers suspect that certain types of particles may be especially harmful. And 

intuitively, it makes sense that the size and chemical composition of particles should make a 

difference in terms of their health effects. Because health studies have not yet clearly defined the 

specific particle types that are most harful, regulators currently treat all fine PM as equal in terms of 

its health impacts.1 However, determining the types of particles which are the most harmful is vitally 

important in the quest to understand PM. The current across-the-board approach to reducing PM has 

clearly provided major benefits in terms of reducing PM concentrations and protecting public health. 

But if certain particle types can be identified as the key culprits, and if specific sources account for 

the bulk of their emissions, then we may be able to identify appropriate control measures with a 

higher degree of precision, rather than pursuing reductions in all types of PM across the board. The 

ability to target the particles with the most severe health impacts would enable us to better protect 

public health, and also to identify measures that would achieve the greatest benefit at the lowest 

cost.

Since it is clear that we need to enhance our efforts to reduce PM in order to protect public health, 

PM will continue to be a major focus of air quality planning, regulation, and public education in the 

Bay Area over the next decade. Despite the gaps in our understanding of PM, we must develop and 

implement policies to control PM in the near-term, while refining our policies and priorities to reflect 

new information as it becomes available. Because we still have a great deal to learn, this report 

cannot serve as the final word on how to address PM in the Bay Area. But it is meant to lay the 

groundwork to guide the Air District’s efforts to reduce PM in the coming years.

1 Certain types of particles, such as metals, polyaromatic hydrocarbons (PAHs), and diesel PM, are classified as toxic air contaminants 

(TACs), and are thus subject to regulation as TACs.

6 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Section 1 : PM IMpacts

Section 1-A: The Public Health Effects of PM

This section summarizes methods used to study PM health effects, the current evidence 

regarding public health impacts related to exposure to PM; the biological pathways by which PM 

affects the body; which sizes and types of particles pose the greatest health risk; the estimated 

health burden from PM in the Bay Area; and why it is important to further reduce PM levels to 

protect the health of Bay Area residents.

This section will consider several key questions regarding the health effects of particulate matter:

• What types of negative health impacts are associated with exposure to PM?

• Does evidence show that exposure to PM is bad for public health?

• How does PM damage the body?

• Which types and sizes of particles are most harmful to health?

• Are there safe levels of PM?

• How does PM affect public health in the San Francisco Bay Area?

The discussion presented below attempts to synthesize information from the vast literature of studies 

that have analyzed the health effects of various particle sizes, including PM10 (particles less than 10 

microns in diameter), PM2.5 (particles less than 2.5 microns in diameter), and ultrafine PM (particles 

less than 0.1 microns in diameter). An explanation of the various PM size categories is presented in 

Section 2 of this report.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 7
A great deal of research has been performed in the past 25 years to identify and quantify the health 

risks of particulate matter. Health studies have linked exposure to PM with a wide range of negative 

health effects. The research provides evidence that exposure to PM, even at low and moderate levels, 

can cause or contribute to a wide range of respiratory and cardiovascular disease, including: 

• irritation of the airways, coughing, or difficulty breathing 

• decreased lung function

• aggravated asthma

• chronic bronchitis

• irregular heartbeat

• strokes

• heart attacks

• premature death in people with

 heart or lung disease

Challenges in Analyzing the 

Health Effects of PM

Determining the health effects of air pollutants is inherently 

difficult. But because PM is a heterogeneous pollutant 

comprised of particles that vary in size, mass, and chemical 

composition, this presents special challenges in determining 

its health effects. In the case of air pollutants composed 

of a single molecule, such as ozone (O3) or carbon 

monoxide (CO), the pollutant has exactly the same chemical 

composition – and thus the same potential health effects – 

regardless of the emissions source. However, in the case of 

PM, the composition of particles in a given air sample – and the corresponding health effects – will 

vary depending on the mix of emission sources.

Early Evidence of PM Health Effects

Several dramatic episodes in the first half of the 20th century demonstrated that very high levels 

of PM and other air pollutants can cause sickness and death. Early scientific research into the 

health effects of PM and air pollution was triggered by the December 1930 episode in the heavily 

populated and industrialized Meuse Valley of Belgium; this extreme air pollution episode killed 

more than 60 people over a three-day period. A similar tragedy occurred in Donora, Pennsylvania in 

October 1948 when an inversion layer trapped a lethal mix of PM, sulfuric acid, nitrogen dioxide, and 

other pollutants from local industrial plants for five days. Nearly half of the town’s 14,000 residents 

became sick, twenty people perished, and 800 animals died. 

Perhaps the most infamous air pollution episode occurred in London in December 1952, when a 

combination of coal combustion, cold weather and windless conditions trapped a thick layer of PM and 

8 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
other pollutants over the city, killing thousands. By comparing death rates during this event with normal 

conditions, London’s Ministry of Health estimated at the time that nearly 4,000 deaths occurred as a 

result of the extreme air pollution. However, more recent research (Bell et al. 2004) suggests that the 

number of fatalities was considerably greater, on the order of 12,000. Death certificates and autopsies 

show that the main causes of death were respiratory and cardiovascular disease. These extreme 

episodes showed the need to study the effects of air pollution on public health. 

Analyzing the Health Effects of PM

Researchers have developed various methods to analyze the health effects of PM and other air 

pollutants in terms of both morbidity (disease or illness) and mortality (premature death). Two 

of the most important techniques used to analyze the health effects of PM are epidemiological 

studies and clinical studies. Epidemiological studies analyze health data for a defined population 

group; the objective is to tease out the health effects of air pollution by looking for correlations 

between the amount of exposure to a pollutant and the observed incidence rate for various health 

endpoints (e.g., cases of respiratory or cardiovascular disease, hospital admissions, or premature 

mortality). If a correlation is observed, researchers must then try to determine whether the 

relationship may be inferred to be causal, meaning that exposure to the pollutant actually causes 

the observed health effect. 

Establishing a causal relationship between PM and a given health effect is difficult because exposure 

to PM is only one factor among many that may cause, contribute to, or exacerbate a specific health 

effect. Other factors that affect our health include genetic and biological factors, environmental 

conditions (air quality, water quality, climate), and lifestyle (diet, exercise, drinking, and smoking), 

to name but a few. Therefore, in designing studies to analyze the health effects of air pollutants, 

epidemiologists attempt to isolate the effect of the air pollutants by controlling for (masking) the 

effect of these other socioeconomic (e.g. income and education), demographic (age, gender, etc.), 

environmental, and lifestyle factors that impact public health. One of the difficulties in air pollution 

epidemiology is that the health risks associated with current ambient levels of air pollution, while 

significant, are nonetheless extremely small when compared to other known risk factors, such as 

cigarette smoking, lack of physical activity, obesity, etc. Because these other risk factors have a 

powerful impact on health, it is difficult to distinguish the more subtle effects of air pollution from the 

health effects attributable to these other factors.

One of the key challenges in epidemiology is estimating how much the people in a study group 

have been exposed to the pollutant in question. Exposure estimates are generally based upon PM 

monitoring data and/or results of computer modeling to simulate ambient PM concentrations. Many 

studies rely on ambient air quality data from monitoring networks, but these data may not capture 

exposures in micro-environments. And even if accurate estimates of population exposure to ambient 

(outdoor) PM are available, epidemiological studies generally do not include indoor exposure to PM, 

which accounts for a significant portion of total exposure for many people, as discussed in Section 

1-B. Improved methods to estimate population exposure to PM across the full range of indoor and 

outdoor environments would be valuable to enable epidemiologists to better analyze the health 

effects related to exposure to PM. 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 9
Analyzing health effects related to PM is also complicated by the fact that the various species of 

PM, as well as a variety of other pollutants and toxic air contaminants, are all mixed together in the 

air that we breathe. Therefore, isolating the health effects of PM from other types of pollutants, or 

distinguishing the effects of a certain size fraction of PM, or a certain chemical species of PM, from 

the overall mass of PM is a difficult task. 

Despite these caveats, epidemiological studies are of great value in helping to illuminate the 

relationship between air pollutants and health effects. For example, exposure to PM is rarely, 

if ever, cited as the cause of death in a coroner’s report when someone dies of a heart attack 

or stroke or lung disease. However, epidemiological studies indicate that exposure to PM is an 

important contributing factor in hundreds, perhaps thousands, of deaths in the Bay Area each year. 

Epidemiological studies are used to analyze:

• Correlations between exposure to air pollutants and the incidence rate of 

both acute (short-term) and chronic (long-term) health effects among the 

general population or defined sub-groups;

• Concentration-response functions; i.e., how a change in the ambient 

concentration of a pollutant may affect the incidence rate of a specific 

health effect. Concentration-response functions can be used to estimate the 

reduction in health effects that can be expected from a given improvement in 

air quality, such as attaining an ambient air quality standard in the Bay Area 

or other metropolitan area.

• Safe thresholds, i.e., a “no-effects” threshold such as an ambient 

concentration below which no health effects can be observed. This is 

especially important for purposes of ensuring that ambient air quality 

standards are sufficiently health protective.

Epidemiological studies are an essential tool to discern correlations between exposures to air 

pollutants and health outcomes. However, these studies are based on statistical analysis of large 

population groups; they cannot definitively “prove” that air pollution causes a specific health effect 

either in an individual case or among a larger population group. Nor can they explain the precise 

biological mechanisms by which PM causes or contributes to the negative health effects observed. 

To investigate these issues, researchers perform clinical studies of small groups of people or 

animals in which they can carefully control the exposure and dosage and observe the impacts over 

a specific timeframe. These clinical studies are valuable in terms of confirming results of largescale epidemiological studies at the individual level. Clinical studies also help to define biological 

mechanisms; that is, exactly how PM or other air pollutants act on and harm the body.

Recent Research on Health Effects of PM

By the 1970s, a link between exposure to PM and respiratory disease, such as triggering asthma 

episodes or other pulmonary disorders, had been well established, although there was uncertainty 

as to the level of PM exposure required to trigger significant public health impacts. Based on the 

10 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
evidence regarding the respiratory effects of particulate matter, such as asthma, bronchitis, and 

diminished lung function, PM was included among the six original “criteria pollutants” identified in 

the ground-breaking federal Clean Air Act of 1970. However, in the ensuing two or three decades 

after the Clean Air Act was enacted, other air pollutants such as ozone, carbon monoxide, and lead, 

were thought to pose a greater health risk than PM. As a result, air pollution control efforts focused 

primarily on reducing these other pollutants. 

PM began to move to the forefront of concern about the health impacts of air pollution beginning 

in the 1990s in response to a new series of studies on PM health effects. Research in recent years 

provides evidence that, even at moderate or low levels, PM has a wide range of negative health 

impacts and can contribute to premature mortality. Whereas earlier research focused primarily 

on respiratory effects of PM, recent 

years have seen a great deal of 

research into the effects of PM on the 

cardiovascular system, the heart and 

blood system which takes oxygen from 

the lungs and distributes it throughout 

the body. Negative impacts of PM on 

the cardiovascular system include 

atherosclerosis (hardening of the 

arteries), ischemic strokes (caused by 

obstruction of the blood supply), and 

heart attacks. These new findings about 

the cardiovascular effects of exposure 

to PM, especially the increase in premature mortality in adults, have given greater urgency to the 

need to reduce PM.

PM Impact on Premature Mortality and Life Expectancy

Concern about PM health impacts was crystalized in the early to mid-1990s by a series of 

epidemiological studies that analyzed the correlation between PM and premature mortality (death).

• Studies in various cities with different climates, pollution mixes, and 

demographics consistently found a correlation between daily changes in PM 

levels and daily mortality. 

• The two most important studies were the Harvard “Six Cities Study” (Dockery 

et al. 1993) which followed the health of over 8,000 people for a period of 

14 to 16 years, and the March 1995 American Cancer Society study (Pope 

et al. 1995) which analyzed a study group of over half a million people in 

151 cities. Both studies found that long-term exposure to PM is associated 

with cardiopulmonary mortality in adults. The Six Cities Study found that an 

increase of 10 μg/m3 in ambient PM2.5 concentrations increases the risk of 

death from all cardiovascular causes by 19%. The American Cancer Society 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 11
study found that an increase of 10 μg/m3 in ambient PM2.5 levels increases 

the risk of death from all cardiovascular causes by 13%. 

It was already known, based on the severe pollution episodes in years past described above, that 

exposure to extremely high concentrations of PM and other air pollutants can kill people. But these 

new studies found that people may experience serious health effects, including premature mortality, 

from exposure to ambient PM at concentrations that most people would not even notice, including 

clear days when PM levels are below the current national ambient PM standards. For example, a 

recent study (Wellenius et al. 2012) on the link between PM and ischemic strokes in the Boston area 

(a region which attains the national 24-hour PM2.5 standard) found 

that the risk of stroke was 34% higher on days with “moderate” PM2.5 

levels compared with days with “good” PM2.5 levels, as defined by the 

EPA Air Quality Index. The study also found that exposure to PM2.5 

levels considered safe by the EPA increases the risk of ischemic stroke 

onset within hours of exposure and that the increase in risk was 

greatest within 12 to 14 hours after exposure to PM2.5.

In assessing the linkage between PM and premature mortality, it 

is instructive to consider the leading causes of death in America. 

According to Center for Disease Control data for 2009, heart disease 

(25%), chronic lower respiratory disease (5%), and strokes (5%) are 

three of the four leading causes of death in the U.S., and collectively they account for 35% of all 

mortality. So if exposure to PM exacerbates cardiovascular and respiratory conditions even to a 

modest extent, this can be expected to exert a tangible impact in terms of increasing the overall 

mortality rate.

Since exposure to PM has been found to increase the incidence of premature mortality, it stands to 

reason that reducing PM levels should prevent premature death and thus help to extend average 

life expectancy. One recent study (Pope et al., 2009) analyzed the change in life expectancy as PM 

levels declined over the 20-year period from 1980 through 2000, based on data from 211 U.S. 

counties in 51 metropolitan areas. This study found that a 10 μg/m3 decrease in PM2.5 levels 

was associated with a 7.3 (± 2.4) month increase in life expectancy. Analysis by Air District staff 

estimated that the improvement in air quality from 1990 to 2008 increased average Bay Area life 

expectancy by approximately six months per person during this period.2 Since PM is estimated to be 

responsible for roughly 90% of the premature mortality related to air pollution in the Bay Area, most 

of this improvement in life expectancy due to improved air quality can be attributed to reduced PM 

concentrations.

2 See Appendix A in Bay Area 2010 Clean Air Plan: www.baaqmd.gov/Divisions/Planning-and-Research/Plans/Clean-Air-Plans.aspx 

People may experience 

negative health effects 

from exposure to PM

even on clear days 

when PM levels are 

below the current 

national standards.

12 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Reaction to Findings

The findings of these studies in the 1990s linking PM to premature mortality provoked controversy 

and skepticism. Researchers were surprised by results showing a broad range of health impacts from 

exposure to low ambient concentrations of PM. Health experts were perplexed by these findings, 

because at that time there were no known biological mechanisms to explain how exposure to 

relatively low concentrations of PM would produce the health effects observed in the epidemiological 

studies, especially in terms of cardiovascular disease and death. 

In response to the controversy generated by these studies, researchers reexamined the results 

of the studies, and also embarked on a search for 

biological mechanisms to explain the health effects 

observed in these studies. To address concerns about 

methodological issues with previous studies, the 

Health Effects Institute funded the National Morbidity, 

Mortality, and Air Pollution Study (NMMAPS) (Samet 

et al. 2000). Over a five-year period through 2005, 

NMMAPS performed a time-series study using a 

consistent method to analyze health impacts of PM10 

in the 90 largest American cities, cities that cover a 

wide geographic area and have varying levels of air 

pollutants. The NMMAPS largely confirmed the the 

findings in the original studies that, on average, for 

every 10 μg/m3 increase in PM10, there was a 0.5% 

increase in overall mortality on the following day, as well as a 2% increase in hospital admissions for 

pneumonia and chronic obstructive pulmonary disease. 

Summary of PM Health Effects

To inform its period review (once every five years) of PM air quality standards, U.S. EPA prepared a 

detailed synthesis of the vast body of literature on PM health effects and issued its December 2009 

Integrated Science Assessment for Particulate Matter. Based on the cumulative weight of the studies 

in the literature, EPA’s conclusions regarding the strength of the evidence to support a finding of 

causality between exposure to PM2.5 and key health effects are summarized in Table 1-1. EPA also 

reviewed the evidence as to whether exposure to coarse PM and to ultra-fine PM has been proven to 

cause negative health effects; they found that the evidence currently available was either suggestive

of causality or inadequate to establish causality in the case of coarse PM and ultra-fine PM. 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 13
Table 1-1 US EPA Findings on Health Effects for Particulate Matter

Health Outcome Causality 

Determination Examples of Health Effects

Size Fraction: PM10-2.5 Coarse PM

Short -Term Exposure

Cardiovascular Effects Suggestive

Increase in hospital admissions and emergency room visits 

for cardiovascular causes

Reduction in heart rate variability

Respiratory Effects Suggestive

Increase in hospital admissions and emergency room visits 

for respiratory causes, particularly in children

Pulmonary inflammation

Mortality Suggestive

Central Nervous System Inadequate

Long -Term Exposure

Cardiovascular Effects Inadequate

Increase in hospital admissions for ischemic heart disease

Arrhythmia

Reduction in heart rate variability

Respiratory Effects Inadequate

Mortality Inadequate

Reproductive & 

Developmental Inadequate Low birth weight

Cancer, Mutagenicity, 

Genotoxicity Inadequate

Size Fraction: PM2.5 Fine PM

Short -Term Exposure

Cardiovascular Effects Causal

Myocardial ischemia (reduced blood flow to the heart)

Congestive heart failure

Altered vasomotor function (stiffening and reduced 

flexibility of blood vessels)

Respiratory Effects Likely to be causal

Alterations in lung function & respiratory symptoms in 

asthmatic children

Chronic obstructive pulmonary disease

Respiratory infections

14 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Health Outcome Causality 

Determination Examples of Health Effects

Mortality Causal Cardiovascular- and respiratory-related mortality

Central Nervous System Inadequate Pro-inflammatory responses in the brain that may lead to 

neurodegenerative diseases

Long -Term Exposure

Cardiovascular Effects Causal

Higher blood pressure

Increased blood coagulation

Enhanced development of atherosclerosis (hardening of 

the arteries)

Reduction in heart rate variability

Increased risk of heart disease and stroke

Respiratory Effects Likely to be causal

Impaired lung development 

Increased respiratory symptoms

Asthma

Altered pulmonary function

Chronic bronchitis

Cancer, Mutagenicity, 

Genotoxicity Suggestive Lung cancer

Reproductive & 

Developmental Suggestive Low birth weight

Mortality Causal Cardiovascular mortality, lung cancer mortality, and Infant 

mortality due to respiratory causes

Size Fraction: Ultrafine PM

Short -Term Exposure

Cardiovascular Effects Suggestive

Increased markers of oxidative stress

Changes in vasomotor function

Alterations in heart rate variability parameters

Respiratory Effects Suggestive

Oxidative, inflammatory and allergic responses 

Decreases in pulmonary function

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 15
Health Outcome Causality 

Determination Examples of Health Effects

Mortality Inadequate

Central Nervous System Inadequate

Long -Term Exposure

Cardiovascular Effects Inadequate

Respiratory Effects Inadequate

Pulmonary inflammation 

Oxidative and allergic responses

Mortality Inadequate

Reproductive & 

Developmental Inadequate

Cancer, Mutagenicity, 

Genotoxicity Inadequate

Source: EPA Integrated Science Assessment, December 2009, Table 2-6

Table 1-2 presents the findings of a recent Harvard School of Public Health study (Kloog et al. 2012) 

that analyzed hospital admission rates throughout New England in terms of the correlation between 

PM2.5 exposure and hospital admission rates for respiratory disease, cardiovascular disease, strokes 

and diabetes. The effects from long-term exposure to PM2.5 are significantly higher than for shortterm exposure for all four causes of admission.

Table 1-2: Estimated increase in hospital admissions rate for a 10 μg/m3 increase for 

short-term and long-term exposure to PM2.5 by cause of admission 

PM2.5 Exposure 

Type All Respiratory Cardiovascular Disease Stroke Diabetes

Short-term 0.70% 1.03% 0.24% 0.96%

Long-term 4.22% 3.12 % 3.49% 6.33%

Source: Kloog et al. Acute and Chronic Effects of Particles on Hospital Admissions in New England. Harvard School of Public Health, 2012. 

www.hsph.harvard.edu/clarc/sac2012/kloog-ne.pdf 

16 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
No Safe Threshold Has Been Identified

The federal Clean Air Act requires US EPA to adopt ambient air quality standards for PM and other 

criteria pollutants at a level that provides an “adequate margin of safety ... requisite to protect the 

public health.” EPA is charged with reviewing the standards every five years based on the latest 

scientific evidence on health effects. A key issue in setting standards is whether researchers 

can identify a safe threshold below which level no negative health effects are observed. To date, 

researchers have not been able to identify a “no-effects” threshold for PM. The evidence suggests 

that in terms of the effect of PM on premature mortality, the concentration-response function (i.e., 

how the incidence of a given health effect varies in response to a change in ambient concentration of 

the pollutant) is essentially linear (EPA Integrated Science Assessment for PM, 2009). These findings 

suggest that people exposed to PM at levels below the current EPA standards may still experience 

negative health effects. (PM air quality standards are discussed in 

Section 3.) 

Recent Findings 

Research on the health effects of PM is on-going. Our 

understanding of PM health impacts is gradually enhanced as 

new studies and journal articles appear at a steady rate. The new 

research reinforces earlier findings regarding negative impacts of 

PM on both respiratory and cardiovascular health, and increased 

rates of health impacts such as heart attacks, strokes, and 

premature death in response to PM exposure. However, in addition 

to confirming the results of earlier research, new research is also 

uncovering evidence of a wider range of potential health effects 

from exposure to PM, including, linkages to diabetes, reduced 

cognitive function in older adults, and oxidative damage to DNA.

Diabetes: The incidence of Type 2 diabetes (sometimes referred to as “adult onset” diabetes) has 

increased rapidly in recent years in response to sedentary lifestyles, changes in diet, and higher rates 

of obesity. People are also contracting Type 2 diabetes at a younger age as well. Experts predict major 

impacts on public health and enormous costs to the health care system as a result of increasing 

diabetes rates. Although diet and lifestyle are key factors in diabetes incidence, a recent nationwide 

study (Pearson et al. 2010) found that air pollution may also be a risk factor for diabetes. The study 

concluded that diabetes prevalence increases with increasing PM2.5 concentrations, with a 1% 

increase in diabetes prevalence seen with a 10 μg/m3 increase in PM2.5 exposure. These results 

suggest that PM2.5 may contribute to increased diabetes prevalence in the adult U.S. population and 

that air pollution is a risk factor for diabetes. There is also some evidence that people with diabetes 

may be more vulnerable to the negative health effects of PM. A recent study (O’Donnell et al. 2011) 

by the Harvard School of Public Health found that diabetics exposed to PM may be at higher risk for 

ischemic stroke compared to the background population. 

New research is uncovering 

evidence of a wider 

range of potential health 

effects from exposure to 

PM, including linkages to 

diabetes, reduced cognitive 

function in older adults, and 

oxidative damage to DNA.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 17
Impacts on the Brain and Cognition: Although the lungs and the circulatory system are the primary 

path by which particles are transported throughout the body, studies (e.g., Oberdörster et al. 2010) 

have found that particles can also enter the central nervous system and then the brain via olfactory 

neurons in the nasal passages. When ultrafine particles travel via olfactory nerves to the brain, they 

are able to bypass the blood-brain barrier, the defensive shield that blocks unwanted chemicals from 

reaching sensitive brain cells. Studies which exposed mice to both fine and ultrafine particles showed 

inflammatory responses in the brain. Ultrafine particles can also damage brain cells in the basal 

ganglia, the region of the brain impacted by degenerative nerve diseases such as Parkinson’s (Peters 

et al. 2006).

A recent study (Weuve et al. 2012) found an association between long-term exposure to both fine PM 

and coarse PM and cognitive capability in older women. This study, based on the longitudinal Nurses’ 

Health Study Cognitive Cohort, examined the effects of PM exposure over 7-14 years for nearly 

20,000 American women aged 70 to 81. The study found that women exposed to higher levels of PM 

experienced more rapid cognitive decline, and concluded that the effect of a 10 μg/m3 increment 

increase in long-term PM exposure is equivalent to approximately two years of cognitive aging. 

The study noted that higher levels of exposure to ambient PM are associated with worse cognitive 

decline, and that the effects observed occurred at levels of exposure typical of many areas of the 

United States. The authors suggest that reducing particulate levels may help to reduce the future 

incidence of age-related cognitive decline and dementia. In a clinical postmortem study (CalderonGarciduenas et al. 2004) that also points to a potential connection between PM and cognitive 

impairment, researchers found higher levels of amyloid-B42, a hallmark of Alzheimer’s disease, in the 

brains of people who lived in cities with higher pollution levels. Clinical studies in animals have shown 

increased brain inflammation in response to PM exposures. 

Biological Pathways: How PM Harms the Body

External exposure to PM can cause short-term impacts to external organs, such as irritation of the 

eyes. But the most damaging effects are caused when pollution enters the body via the respiratory 

system. The mechanisms by which PM and other air pollutants damage the lungs and the respiratory 

system are well understood. Our lungs serve as the entry point to the body for PM and other 

pollutants, so they are the organ most directly impacted by air pollution. In addition, the lungs are 

especially sensitive to air pollutants because they contain a large surface of exposed membrane to 

facilitate the delivery of oxygen to the blood system. Our respiratory system has defenses in the nasal 

passages, throat, and lungs that filter out particles, but the smallest particles are most likely to elude 

the body’s filtration mechanisms. For example, as much as 50% of ultrafine particles with a diameter 

of 0.02 microns or smaller are estimated to be deposited in the alveolar region of the lung. Particles 

inhaled deep into the lungs can then be transported to cells and organs throughout the body. And 

once particles become deeply embedded in our body, they can remain there for weeks, months, or 

even years. 

Figure 1-1 depicts how PM enters the body. The larger particles are typically filtered out; in contrast, 

particles less than 2.5 microns in diameter can penetrate deep into the lungs which are where most 

health problems begin. 

18 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
On the individual basis, the health impact of longterm exposure to PM is likely to be determined by the 

number of particles that are transported from the 

lungs into the body, the chemical composition of the 

particles, and how quickly the particles are cleared 

from the body.

Researchers have been making progress in 

recent years in explaining how PM damages the 

cardiovascular system and other organs and systems, 

but this is still an area of on-going research. Research 

to date indicates that inflammation and oxidative stress 

are two of the key ways that PM damages the body. 

Inflammation: When foreign substances are 

deposited in the body, this irritates the impacted 

area and causes an inflammatory response. Studies 

(e.g., Araujo et al. 2010) have found that exposure 

to PM can lead to chronic, low-level inflammation. An 

inflammatory response to PM or other air pollutants 

can damage the body in many ways. In the vascular 

system, an inflammatory response to PM can stiffen 

blood vessels and reduce their flexibility, leading to 

higher blood pressure, increased blood coagulation, 

hardening of the arteries (atherosclerosis), altered 

cardiac autonomic function (the system that controls the heart), and reduction in heart rate variability 

(a risk factor for future cardiovascular problems). All these effects can increase long-term risk of 

heart disease or stroke. Based on high particle numbers, high lung deposition efficiency and surface 

chemistry, ultrafine PM may be especially dangerous in terms of its potential to induce inflammation. 

Oxidative Damage to DNA: Studies (e.g., Risom et al. 2005) indicate that exposure to PM 

increases oxidative stress. This term describes the effect of oxidation in which an elevated level of 

reactive oxygen species, such as free radicals (e.g. hydroxyl, nitric acid, superoxides) or non-radicals 

(e.g. hydrogen peroxide, lipid peroxide) causes oxidative damage to specific molecules, thereby 

injuring cells or tissue. There is evidence that ultrafine PM may cause oxidative damage to DNA. For 

example, a Danish study (Vinzents et al. 2005) found that participants who rode bicycles in traffic in 

Copenhagen, and were thus subjected to increased exposure to ultra-fine PM, sustained oxidative 

damage to their DNA, thus demonstrating an association between DNA damage and ultrafine PM 

exposure in live subjects. 

Cardiovascular effects: In terms of explaining how PM damages the cardiovascular system, 

more study is needed to determine how exposure to PM affects intermediate health outcomes such 

as heart rate variability and inflammation markers. In a paper (Pope & Dockery 2006) reviewing 

research on the health effects of PM, two of the leading researchers summarize their discussion 

Source: British Columbia Air Quality (www.bcairquality.ca/health/

air-quality-and-health.html)

Figure 1-1 How Particulate Matter 

Enters Our Body

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 19
of biological mechanisms as follows: “Various plausible pathways have been identified. However, 

none has been definitively demonstrated to be the pathway that directly links exposure to PM with 

cardiopulmonary morbidity and mortality. In fact, it is unlikely that any single pathway is responsible. 

There are almost certainly multiple pathways with complex interactions and interdependencies.” 

Which sizes and types of particles are most dangerous?

Evidence suggests that PM health effects depend upon both particle size and particle composition.

Particle Size: Research indicates that the health effects of PM depend upon particle size. Smaller 

particles (in the fine and ultrafine size ranges) are generally more harmful than coarse particles. 

Smaller particles typically remain suspended in the air for longer periods; penetrate more readily and 

deeply into the lungs, bloodstream and organs; and present a large amount of reactive surface area 

relative to their mass. 

Ultrafine PM: A growing body of evidence documents public health effects from ultrafine particles. 

Motor vehicles are a major source of ultrafine particle emissions, and these particles are highly 

reactive when emitted from internal combustion engines. Because ultrafine particles are so 

miniscule, they can travel deep into the lungs and organs and pass through cell membranes. These 

particles can also carry toxic compounds into the body. Based on high particle numbers, high lung 

deposition efficiency, and surface chemistry, ultrafine particles may have a greater potential than 

PM2.5 for inducing inflammation and oxidative stress, key mechanisms by which PM harms the body. 

In clinical studies, greater inflammatory and oxidative stress (cell, tissue or organ damage), resulting 

in damage to DNA, has been associated with exposure to ultrafine particles compared to the larger 

particles at comparable mass doses. In some cases, the substances absorbed on to the ultrafine 

particles may be responsible for some of the effects observed, including oxidative stress, rather than 

the particles themselves. A study (Oberdoster et al. 2010) that examined the effects of combustions 

fumes on laboratory rats found that, compared to larger particles, ultrafine particles cause a greater 

inflammatory response in the lungs of rats and increased antioxidant levels in their lung tissues.

Numerous studies (e.g., Wichmann 2000) have found a correlation between exposure to ultrafine 

PM and increased incidence of health effects such as premature mortality, hospital admissions, lung 

cancer and other cancers, cardiovascular disease, adverse birth outcomes, effects on the immune 

system, and neurotoxicity. In cell cultures exposed to ambient particles, ultrafine particles were found 

in the mitochondria where they induced structural damage. 

Research on health effects related to exposure to ultrafine PM is still very limited compared to 

the amount of research that has been performed into the health effects of PM2.5. For example, 

specific mechanisms of health effects from exposure to ultra-fine nitrate and sulfate particles 

are not well defined, nor are the health effects of semi-volatile organic compounds and trace 

metals found in ultrafine PM. However, existing studies suggest that ultrafine PM may have 

significant health effects, and that some of the health effects related to ultrafine particles may be 

independent of the effects from exposure to PM2.5 and/or PM10.

20 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
PM Damage to Cells

Figure 1-2 depicts an image of what happens when PM passes through the lungs and penetrates 

into cells. Photos E and F (magnified x 6,000 and 21,000, respectively) show a cell exposed 

for 16 hours to fine PM (PM2.5). The “P” indicates damage to cristae, the inner membrane of 

the mitochondria, which are studded with proteins and increase the surface area for chemical 

reactions, such as cellular respiration, to occur. The “M” points to the presence of particles inside 

the mitochondria as well as ultra-structural 

damage to the mitochondria. Mitochondria, 

sometimes described as “cellular power 

generators” supply cellular energy, and are 

involved in a number of processes such as 

signaling, cellular differentiation, cell death 

and the control of the cell cycle and cell 

growth. Mitochondria have a central place 

in cell metabolism and their damage plays 

an important role in a wide range of health 

effects. 

Figure 1-3 shows images of a cell exposed 

to ultrafine PM for 16 hours. As in the figure 

above, the “P” points to damage to the 

cristae, and the “M “shows the presence 

of particles inside the mitochondria as well 

as structural damage. The degree of ultrastructural damage in this study was found to 

be greater in the cells exposed to ultrafine PM 

than the cells exposed to fine PM (and even 

more so than those exposed to coarse PM, 

which experienced little if any mitochondrial 

damage).

Relationship 

Between Particle Composition and Health Effects

The available evidence from epidemiological studies to date suggests that fine particles themselves 

are harmful, regardless of their emissions source or chemical composition. Isolating and pinpointing 

the health effects of a specific particle type through epidemiological studies is difficult, because 

many particle types and sizes are co-emitted by the same sources and processes, and the air we 

breathe always includes a diverse mix of particle sizes and types. Because risk estimates for any 

specific particle type are subject to confounding by co-pollutants, the evidence for differential health 

risk among PM2.5 components is not as robust as for PM2.5 as a whole. Therefore, when estimating 

the health impacts of PM, most researchers currently assume that all mixtures of PM2.5 are equally 

potent. And in the absence of information to clearly distinguish the relative risk of different particle 

Figure 1-2 Cells Exposed to Fine PM

(PM2.5) for 16 Hours

Source: Cho et al. Ultrafine Particulate Pollutants Induce Oxidative Street and 

Mitochondrial Damage. Environmental Health Perspectives. 2003 April; 111(4): 455–460.

Figure 1-3 Cells Exposed to Ultrafine PM for 16 Hours

Source: Cho et al. 2003.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 21
types, US EPA and ARB currently treat all particles alike, regardless of their chemical make-up, for 

purposes of PM air quality standards. (It should be noted, however, that ARB does call out diesel PM 

for special attention as a toxic air contaminant.)

But even though it is difficult to determine the relative harmfulness of different particle types, 

variation in particle composition and behavior suggests that their health effects are likely to differ 

as well. There is some evidence that specific particle types, such as black carbon and diesel PM2.5, 

may be especially harmful. For example, fine and ultrafine particles produced by fuel combustion 

may be more toxic, because they are highly reactive and because they include sulfates, nitrates, 

acids, trace metals and other toxic contaminants. Researchers 

have also hypothesized that insoluble ultrafine particles with a solid 

core may be more harmful than soluble particles. Whereas soluble 

particles will dissolve as they interact with blood and body liquids, 

the insoluble particles persist in solid form and can thus penetrate 

through protective barriers to irritate and inflame deep within the 

body (Ostiguy et al., IRSST, 2006).

As the science advances, at some future date it may be possible to 

link the various health effects associated with PM to specific particle 

types, or combinations thereof. From the standpoint of protecting 

public health, determining which sizes or types of particles are most 

harmful is vitally important in the quest to understand PM. If new 

information becomes available to identify and help target the most 

harmful particle types, then we should be able to develop more 

effective control measures to maximize public health benefits in the 

most cost-effective manner.

Motor Vehicle Emissions

From the standpoint of population exposure, PM from both gasoline 

and diesel powered vehicles is of special concern, because of the large number of people exposed to 

motor vehicles emissions in urban areas, including pedestrians, bicyclists, motorists, and people who 

live, work or go to school in proximity to busy roadways. Vehicle tailpipe emissions include primary 

ultrafine and fine PM from fuel combustion and lubricating oil, as well as gaseous PM precursors 

such as ROG, NOx, and ammonia. Vehicles also produce particles from brake and tire wear that 

include toxic chemicals such as copper and cadmium; these toxic substances pollute soil and water 

as well as the air. In addition, motor vehicles re-suspend dust (primarily coarse particles) that has 

been deposited on roadways from a variety of emission sources, so that these particles once again 

become available to be inhaled.

ARB’s Air Quality and Land Use Handbook (April 2005) cites several key findings regarding health 

impacts from near-roadway exposures.

From the 

standpoint 

of protecting 

public health, 

determining 

which sizes or 

types of particles 

are most harmful 

is vitally important 

in the quest to 

understand PM.

22 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
• Reduced lung function in children was associated with traffic density, 

especially trucks, within 1,000 feet; this association was strongest within 

300 feet. (Brunekreef, 1997)

• Increased asthma hospitalizations were associated with living within 650 

feet of heavy traffic and heavy truck volume. (Lin, 2000)

• Asthma symptoms increased with proximity to roadways; the risk was 

greatest within 300 feet. (Venn, 2001)

• Asthma and bronchitis symptoms in children were associated with proximity 

to high traffic in a San Francisco Bay Area community with good overall 

regional air quality. (Kim, 2004)

• A San Diego study found increased medical visits in children living within 550 

feet of heavy traffic. (English, 1999)

The ARB Handbook notes that truck traffic densities and distance from the roadway are key factors 

affecting the strength of the association with adverse health effects. In the health studies cited in the 

ARB Handbook, adverse health effects diminished with distance; adverse health effects were seen 

within 1,000 feet of high-volume roadways, with the strongest effects within 300 feet.

Exposure to vehicle emissions has been linked to increased blood pressure and thickening of the 

arteries. A recent clinical study (Brook et al. 2009) that exposed subjects to PM levels typically found 

near highways showed an immediate increase in blood pressure. This study also found that exposure 

to PM induced inflammation; this effect typically manifested within roughly 24 hours after exposure. 

Another recent study (Kunzli et al. 2010) found that thickening of artery walls progressed more than 

twice as quickly among people living within 100 meters of a Los Angeles freeway (where ultrafine PM 

levels are typically elevated) compared to those who lived farther away. 

The Brugge (2007) review of health studies concludes that there is strong evidence that exposure to 

high volume roadways is linked to higher asthma rates and to reduction of lung function in children. 

In addition, recent studies (Schwartz et al. 2005, and Adar et al. 2007) have found that heart rate 

variability, a risk factor for future cardiovascular problems, is altered by traffic-related pollutants, 

particularly in older people and people with heart disease. 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 23
Lubricating Oil 

Recent research indicates that engine lubricating oil may contribute to PM emissions from motor 

vehicles. To investigate how lubricating oil (and fuels) contributes to the formation of PM and semivolatile organic compounds (SVOC) in vehicle exhaust, the National Renewal Energy Lab, ARB, South 

Coast AQMD and other partners worked together on the Collaborative Lubricating Oil Study on 

Emissions (CLOSE). The objective was to evaluate how much PM emissions come from lubricating oil 

and explore ways to reformulate oil so as to reduce PM emissions.

The CLOSE project found that in the vehicles tested unburned lubricating oil makes up more than 70% 

of organic carbon (OC), which constitutes a large portion of UFPM. The results of the CLOSE project 

indicate that lubricating oil and fuels may lead to the formation of PM. However, further study is needed 

to test a wider range of vehicles, and to investigate the effects of oil type on PM and SVOC formation.

Diesel PM

Diesel PM is a subset of PM2.5 that is emitted by diesel engines. Although diesel PM accounts for 

a small portion (less than 10%) of the overall PM2.5 emission inventory, it has been called out for 

special attention by ARB because of its toxicity. In 1998, in response to a comprehensive health 

assessment of diesel exhaust, ARB formally identified diesel PM as a toxic air contaminant (TAC), a 

special class of air pollutants that can impair public health even at very low exposures or dosages. 

TACs can cause both acute and chronic effects, including cancer. Diesel exhaust also contains more 

than 40 other TACs, including carcinogens such as benzene, arsenic, nickel, and formaldehyde. The 

Air District performed an analysis of TACs for its Community Risk Evaluation (CARE) program and 

found that diesel PM accounts for approximately 85% of the total cancer risk from TACs in the Bay 

Area. As discussed in Section 4, diesel PM 

has been the focus of control efforts by 

both ARB and the Air District.

Wood Smoke

Although wood fires may have an aesthetic 

appeal and some people may perceive 

wood smoke as natural or even healthy, 

findings from health studies to date 

indicates that wood smoke particles cause 

the same types of negative health effects 

as other types of fine PM. Wood smoke 

accounts for a major portion (38%) of 

the Bay Area PM2.5 inventory during the 

winter season.

Wood smoke is produced by incomplete 

combustion from residential fireplaces 

and wood stoves. Whereas combustion in 

diesel and gasoline engines is carefully controlled to regulate oxygen supply and the fuel-to-air ratio 

24 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
in order to maximize the efficiency of the combustion process, the wood-burning combustion process 

in fireplaces is largely uncontrolled. This leads to inefficient combustion, resulting in a high rate of 

smoke and PM emissions compared to other types of combustion. In addition to PM, wood smoke 

contains thousands of chemicals, including criteria pollutants such 

as sulfur oxides (SOx), nitrogen oxides (NOx), and carbon monoxide 

(CO); as well as several dozen toxic air contaminants such as acrolein 

and acetaldehyde, and carcinogenic compounds such as polycyclic 

aromatic hydrocarbons (PAHs), benzene, formaldehyde and dioxins. 

More research is needed to evaluate how the various pollutants in wood 

smoke may interact in terms of health impacts. 

A review of research on the health effects of residential wood smoke 

(Zelikoff et al. 2002) found that prolonged inhalation of wood smoke 

contributes to chronic bronchitis, chronic interstitial lung disease, 

pulmonary arterial hypertension, and altered pulmonary immune 

defense mechanisms in adults. Studies (e.g., Larsen & Koenig, 1994) 

found that young children living in homes heated by a wood-burning 

stove had a greater occurrence of moderate and severe chronic respiratory symptoms than children 

who did not live in homes heated with a wood-burning stove. Effects on preschool children living in 

homes heated with wood burning stoves or in houses with open fireplaces include decreased lung 

function in young asthmatics, increased incidence of acute bronchitis, and increased incidence 

and duration of acute respiratory infections. One study (Danielsen et al. 2011) found that wood 

smoke PM has small particle size and a high level of polycyclic aromatic hydrocarbons (PAH). This 

study found that, in terms of health effects, exposure to wood smoke produces high levels of free 

radicals, may trigger inflammatory and oxidative stress in cultured human cells, and may damage 

DNA.

Studies of wood smoke health effects to date have focused primarily on respiratory impacts. Additional 

research is needed to evaluate potential cardiovascular effects from exposure to wood smoke. 

Estimating PM Health Impacts in the Bay Area

The Air District has performed two recent analyses to estimate the public health impacts (morbidity 

and premature mortality) of PM and other air pollutants in the Bay Area. 

• Bay Area Air Pollution Burden: Past and Present (see Appendix A in the Bay 

Area 2010 Clean Air Plan, issued September 2010).

• Health Impact Analysis of Fine Particulate Matter in the San Francisco Bay 

Area, September 2011.

These analyses were based on methodologies employed by US EPA and ARB to estimate the 

health impacts and the monetary costs of air pollution. They rely on the results of peer-reviewed 

In addition 

to PM, wood 

smoke contains 

various toxic and 

carcinogenic 

compounds.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 25
epidemiological studies and US EPA’s BenMAP program, in combination with air quality modeling 

results performed in-house by Air District staff. Key findings from these studies include the following:

• Improvements in Bay Area air quality in recent decades have greatly reduced 

the health burden from air pollution. Most of the public health benefit is due 

to the substantial reduction in PM levels in recent years, as 

discussed in Section 3-C. 

• The reduction in health impacts has provided cost 

savings (in terms of reduced treatment costs, increased 

productivity, and longer life span) to the region valued at 

multiple billions of dollars per year. Most of this economic 

benefit is due to progress in reducing PM levels. 

• Average life expectancy in the Bay Area increased from 

75.7 years in 1990 to 80.5 years in 2006. Improved air 

quality – primarily the reduction in PM levels – in the Bay 

Area during this period accounts for roughly 6 months of 

the overall increase in average Bay Area life expectancy.

• Despite progress in reducing PM levels and related health 

impacts, exposure to fine PM (PM2.5) remains the leading 

public health risk, and contributor to premature death, from air pollution 

in the Bay Area. The vast majority of premature deaths associated with 

air pollution - more than 90% - are related to exposure to fine particulate 

matter (PM2.5). Most of the deaths associated with PM2.5 are related to 

cardiovascular problems. Based on current PM levels, exposure to PM is 

estimated to contribute to approximately 1,700 premature deaths per year 

in the Bay Area; this represents roughly 3-5% of total annual deaths in the 

region. The economic cost of these negative health impacts is estimated at 

multiple billions of dollars per year.

• PM emitted by diesel engines is believed to be the leading toxic air 

contaminant (TAC) in the Bay Area. However, the current evidence suggests 

that only 10-20% of PM-related deaths in the Bay Area are linked to diesel 

exhaust. PM from other (non-diesel) sources (such as wood smoke, cooking, 

combustion of other (non-diesel) fossil fuels, and secondary PM formed by 

precursors such as NOx, SO2, and ammonia) appears to be responsible for 

most of the PM and the PM-related deaths in the Bay Area.

• The Air Resources Board has identified diesel PM as a carcinogenic pollutant 

that may cause lung cancer. Although lung cancer is clearly a major public 

health issue, it should be noted that exposure to diesel PM may cause a wide 

range of respiratory and cardiovascular effects in addition to lung cancer. In 

fact, to the extent that diesel PM contributes to premature mortality, analysis 

suggests that this is primarily due to its role as a component of PM2.5, 

in which it contributes to mortality related to heart attacks, emphysema, 

strokes, etc. 

PM is the air 

pollutant that 

causes by far the 

greatest harm to 

public health in 

the Bay Area.

26 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Figure 1-4 depicts findings from the health burden analysis performed for the Bay Area 2010 Clean 

Air Plan. The graph shows the number of cases of seven key health efects that are related to 

population exposure to current Bay Area air pollution levels (2008, labeled “now) compared to the 

estimated number of cases that would have occurred if the quantifiable air quality improvements 

had not been made (labeled “then”). The “then” data is based on the earliest data available - 1970 

for ozone, and the late 1980’s for toxics and PM. Figure 1-4 shows that the annual cases of health 

effects associated with exposure to air pollutants in the Bay Area has dropped dramatically, by more 

than half. Of particular interest, premature mortality related to air pollution has decreased from an 

estimated 6,400 per year to an estimated 2,800 per year. Despite this substantial progress, the 

health impacts from air pollution are still significant. And as the graph shows, PM2.5 accounts for the 

vast majority of the health effects in comparison to ozone or other (non-diesel) toxic air contaminants.

Figure 1-4 Bay Area Air Pollution Health Burden: Past & Present

Estimated contribution of PM, ozone, and key air toxics to health endpoints among Bay Area residents, based on 

Bay Area air pollution data for year 2008.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 27
Summary of Key Points on the Health Effects of PM

• A robust body of epidemiological studies has established that there are 

a wide range of negative health impacts from both short-term and longterm exposure to PM2.5. These impacts include both respiratory and 

cardiovascular effects, including key health endpoints such as heart attacks 

and strokes.

• Although researchers are still working to define which specific particle 

types are more harmful to human health, the available evidence indicates 

that smaller particles are most dangerous because they can most easily 

penetrate into the lungs, bloodstream, organs, and cells of the body.

• There are documented health effects from exposure to PM2.5 even below 

current PM air quality standards, and researchers have not been able to 

establish a safe threshold below which there are no health risks. 

• Even although the Bay Area either attains or is close to attaining State and 

national air quality standards for PM, analysis by Air District staff indicates 

that PM is the air pollutant that imposes by far the greatest harm to public 

health in the Bay Area.

• Even in regions with relatively low PM concentrations, such as the Bay Area, 

efforts to further reduce PM levels will results in public health improvements 

and longer average life expectancy, thus providing significant social and 

economic benefits.

Areas that Require Additional Research

This chapter has attempted to summarize current information regarding the health effects related to 

PM. 

Although there is robust evidence that exposure to PM can cause a wide range of respiratory and 

cardiovascular effects, many fundamental questions have not yet been fully answered. Researchers 

are actively working to provide better answers to key questions, including the following:

• Which particle types are most dangerous?

• What biological mechanisms cause the observed health effects?

28 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
• Is there a safe level (no-effects threshold) for short-term or long-term 

exposure to PM, below which no health effects occur? 

• Do ultrafine particles cause health effects independent of the health effects 

caused by exposure to PM2.5?

• If ultrafine particles do cause health effects independent of the effects 

caused by exposure to larger particles, are these effects related to short-term 

peak exposure, chronic exposure to lower levels of UFPM, or a combination of 

both acute and chronic exposure? 

• How does exposure to PM affect intermediate health outcomes such as heart 

rate variability and inflammation markers?

• What is the relationship between exposure to PM emissions from roadways 

and health effects such as cardiovascular disease, lung cancer, and 

premature mortality?

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 29
Section 1-B: Population Exposure to PM

This section summarizes key information regarding population exposure to PM. 

In order to protect Bay Area residents from the negative health effects related to PM described above, 

we need to better understand population exposure to PM. The key questions are: Who is exposed to 

PM? And when, where, and how much PM are people exposed to?

Reducing personal (individual) exposure and population exposure (the aggregate exposure 

experienced by all Bay Area residents combined) to PM can provide important public health benefits. 

Key questions considered in this section include:

• Who is at greatest risk from exposure to PM?

• How do PM concentrations vary at the local scale?

• Which types of environments pose the greatest risk?

• How much exposure to PM occurs in the indoor environment compared to 

outdoors?

• How do PM concentrations and population exposure to PM change based 

upon distance from emission sources?

Linking PM to Health Effects

The connection between PM and negative health effects can be explained by a pathway that includes 

the following links:

1. Emissions: A wide range of sources release primary PM and PM precursors 

into the air.

2. Ambient PM concentrations: The combination of emissions and 

meteorological conditions determines the ambient concentration of PM, that 

is, the level of PM in the air.

3. Exposure: Exposure occurs when people actually inhale PM into their 

lungs. The level of exposure to PM is closely linked to the ambient PM 

concentration.

4. Health effects: Health effects may occur as a result of exposure to PM, 

depending upon the intensity, duration and frequency of population 

exposure, as well as the size and physical condition of the receptor 

population.

The Air District has detailed information about PM emissions and ambient concentrations of PM3

at the regional scale, and there is a wealth of epidemiological studies analyzing the relationship 

3 The emissions inventory described in Section 2 provides information on emissions by source. Information regarding ambient 

concentrations of PM is available from monitoring data and photochemical modeling.

30 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
between population exposure and health effects. However, as discussed below, estimating 

population exposure to PM at the regional scale is difficult. In analyzing how PM effects public health 

via the steps described above, the greatest uncertainty relates to estimating population exposure.

Measuring population exposure to PM, and the related health risks, is challenging for several 

reasons:

• PM concentrations vary both temporally and spatially

• The composition of PM varies depending upon the mix of emission sources 

and meteorology

• Personal activity patterns are complex

• Indoor exposure may account for a significant share of total exposure

Emissions: Many different sources, both 

stationary (factories, refineries, etc.) and mobile 

(cars, trucks, locomotives, marine vessels, and 

farm and construction equipment) emit direct 

emissions of PM and/or PM precursors such 

as NOx and SOx. Identifying the key emission 

sources and developing strategies to reduce 

emissions from these sources is the first and 

most fundamental step to improve air quality.

Ambient Concentrations: This term refers 

to the level of pollutants that are measured in 

the air. PM air quality standards are expressed in terms of ambient concentrations, as discussed in 

Section 3. The relationship between emissions and ambient concentrations is complex and depends 

upon many factors, including meteorological conditions (temperature, humidity, wind speed and 

direction, vertical mixing, etc.) the ratio of precursor pollutants, and regional topography. Ambient 

concentrations of PM can vary greatly at the local scale.

Population Exposure: 

From the standpoint of protecting public health, the key objective is to reduce population exposure to 

PM and other air pollutants. The issue is not 

simply how much pollution may be degrading 

the quality of the air that we breathe, but 

rather how much people are exposed to the 

pollution and how much each individual 

actually inhales and absorbs (dosage). Key 

factors in determining population exposure 

include how much pollution is in the air at 

the time and location that exposure occurs, 

the number of people exposed, the duration 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 31
of exposure, and the frequency of exposure. People are exposed to air pollutants in both the outdoor 

and indoor environments. In the case of PM, the evidence suggests that most people experience a 

major portion of their total PM exposure when they are indoors, as discussed later in this Section.

Population exposure to PM can also be analyzed in terms of intake fraction, i.e., the fraction of 

an emitted pollutant that is actually inhaled by the population. High PM emissions in a sparselypopulated area may cause little damage to public health, because the intake fraction and population 

exposure are low. Conversely, moderate levels of emissions in proximity to a densely populated 

environment may result in a higher intake fraction, more population exposure, and greater public 

health impacts. 

Factors that determine individual exposure to PM and other air pollutants include:

• The pollution profile: the level of pollution, mix of co-pollutants, the specific 

mix of particle sizes and their chemical composition, all of which vary 

depending on geographical location and emissions sources, season of the 

year, and weather conditions.

• Where people live, work, and play. 

• Activity patterns and lifestyle choices such as how much time people spend 

outside, or how much time they spend driving on busy roadways.

Personal exposure to PM for a given individual on a specific day can be calculated by multiplying the 

time spent in each activity by the PM concentration in each location, and then summing the exposure 

for each activity. The degree of exposure may vary greatly depending on the type and location of the 

activity. Therefore, an activity that involves high exposure for a relatively short period of time, such as 

driving on a freeway for 30 minutes, may account for a major portion of total daily exposure.

Estimating total population exposure to PM is challenging because people are mobile and PM 

levels may vary substantially from place to place. Estimating population exposure to PM requires 

three steps:

1. Documenting activity patterns for a specific individual: i.e., where, when, and 

for how long he or she performs various activities.

2. Estimating the ambient concentration of PM that the individual is exposed 

to at each time and location. These estimates are derived from air quality 

monitoring data and/or computer modeling results.

3. Aggregating each individual’s personal exposure across an entire defined 

population. 

Dosage: The amount of an air pollutant that an individual actually inhales is called dosage. And 

the amount that is absorbed by the body and becomes available for interaction with biologically 

significant organs and tissues is called the internal dose. Once absorbed, the chemical can undergo 

32 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
metabolism, storage, excretion, or transport within the body. The amount transported to a specific 

organ or system is termed the delivered dose.

The air we breathe serves as the carrier medium by which harmful pollutants can enter the body. The 

average adult takes about 25,000 breaths and inhales a total volume of approximately 14,000 liters 

of air every day. So even though we may only be exposed to a tiny amount of pollution per breath, on a 

cumulative basis this can result in a substantial amount of pollution inhaled over time.

The pollutant dosage depends upon the amount of pollution in the air and an individual’s inhalation 

rate relative to their body weight. Inhalation rate varies depending upon age, metabolic rate, body 

weight, and type of activity. When people are exercising, they inhale more frequently and more deeply, 

resulting in a higher dosage. So walkers, runners, and bicyclists experience greater dosage from a 

given exposure; this may be a concern if people engage in these activities in close proximity to busy 

roadways or other major emission sources. 

Children may experience higher dosage than adults, 

because they are more physically active, have a higher 

metabolic rate and inhale more air on a per-pound basis 

than adults. Children also tend to breathe more through 

their mouths more than adults; as a result, more PM may 

reach their lungs because the mouth is less effective 

than the nose at filtering out PM. For these reasons, 

children typically experience a greater dosage into their 

bodies from a given exposure to pollution.

Sensitive Populations

Just as individual exposure differs, so does the ability of our bodies to tolerate exposure to pollutants. 

People vary in their susceptibility to health effects from air pollution depending upon factors such as 

genetic features, gender, age, lifestyle (e.g., smoking status and nutrition), and their health status. To 

protect public health, we need to focus on reducing exposure among the most sensitive populations 

and the most heavily impacted communities. 

The key aspect to protecting public health is to reduce personal exposure for the people who are most 

susceptible to air pollution; these “sensitive populations” include children, pregnant women, seniors, 

and people burdened with existing cardiovascular or respiratory conditions. Children are especially 

vulnerable to air pollution due to their higher inhalation rates, narrower airways, less mature immune 

systems, and the fact that their lungs and other key organs are still developing. In addition, children 

with allergies may have an enhanced allergic response when exposed to pollutants such as diesel 

exhaust. Children also tend to spend more time outside than adults, so they may be more exposed 

to pollutants in the ambient air. Seniors and people with existing cardiovascular or respiratory 

conditions are more vulnerable to the effects of air pollution than healthy adults because their lungs, 

hearts, and immune systems may already be weakened or compromised.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 33
Some epidemiological studies suggest that PM health effects may be influenced by gender. For 

example, a 2007 study by the Women’s Health Initiative (Miller et al. 2007), which examined the 

impacts of PM exposure on post-menopausal women, found that a relatively modest increase in 

PM2.5 levels (an increase of 10 μg/m3 or micrograms per cubic meter) was associated with a 76% 

increase in the risk of death from cardiovascular disease. This is much higher than the increase in 

risk associated with an increase of 10 μg/m3 that was observed for the general population in the 

Harvard Six Cities study (19%) and the American Cancer Society study (13%). 

Although members of sensitive populations live in communities throughout the Bay Area, we know 

that certain communities experience higher-than-average levels of air pollution. And in many cases, 

people who live in communities that are disproportionately impacted by air pollution may be especially 

vulnerable to the negative effects of air pollution because of their demographic and socioeconomic 

status. Studies have shown that socioeconomic factors such as income, race, access to health care, 

and level of educational attainment, can profoundly affect health status and life expectancy. 

But although a correlation between socioeconomic factors and health outcomes has been well 

documented, determining the precise correlation between a particular socioeconomic factor and 

a specific health outcome is complicated because the various factors are closely intertwined in 

most communities. Identifying the specific mechanism(s) by which socioeconomic factors affect 

health outcomes is also difficult. But researchers have found that factors such as poverty and race 

contribute to stress, thus increasing susceptibility to the health effects of air pollution, since stress 

weakens the immune system and may be a factor in initiating some types of disease.

Recognizing that certain neighborhoods and communities in the Bay Area are disproportionately 

impacted by local air pollutants, the Air District launched the Community Air Risk Evaluation (CARE) 

program in 2004. Air District staff performed technical analysis which identified six impacted 

communities most affected by toxic air contaminants. The Air District has been making a concerted 

effort to reduce emissions and 

population exposure to PM and 

toxic air contaminants in these 

communities, as described in 

Section 4. 

To sum up, many factors go into 

determining population exposure 

to air pollutants and the degree 

to which an individual’s health 

may be affected by that exposure. 

The amount of pollution in the air 

and the frequency and duration 

of the exposure are key factors. But individual attributes such as age and gender; personal activity 

patterns; lifestyle (active or sedentary); socioeconomic status; and health status all play a role as 

well in determining how much pollution people are exposed to and how their bodies respond to that 

exposure.

34 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Which Environments Pose the Greatest Risk?

In the course of their daily activities and routines, Bay Area residents are exposed to PM and other 

air pollutants in a variety of locations and environments. Several of these environments may expose 

people to elevated levels of PM and account for a significant portion of total exposure to PM.

Population Exposure to Motor Vehicle Emissions

Most Bay Area residents are exposed to motor vehicle emissions on a daily basis. Motor vehicles emit 

the entire spectrum of ultrafine, fine, and coarse PM by means of several mechanisms. Gasoline and 

diesel vehicles produce tailpipe emissions of both primary PM in the fine and ultrafine size ranges, 

as well precursor pollutants that form secondary PM, such as ROG, NOx, and ammonia. Lubricating 

oil has also been identified as a potential important source of ultrafine PM. In addition to these 

combustion-related emissions, motor vehicles generate PM from brake wear and tire wear, and 

they cause dust that has settled on roads to become re-suspended in the air. Studies have found a 

wide range of negative health effects among people who are exposed to PM emissions from motor 

vehicles, as discussed in Section 1-A.

The amount of primary PM and PM precursors emitted by vehicles on a given roadway, and the 

chemical composition of those emissions, depend upon many factors, including:

• the volume of traffic and the level of traffic congestion;

• the composition of the vehicle fleet, and whether there are any restrictions 

for certain types of vehicles, such as trucks; 

• the age mix of the vehicles, the effectiveness of their emissions control 

devices, and the fraction of vehicles that are high-emitters;

• the mix of fuel types: gasoline, diesel, or alternative fuel vehicles;

• season of the year and weather conditions; and

• vehicle speed and operating mode; vehicles emit more particles when 

accelerating and when traveling at high speed (Hall & Dickens, 1999). 

Near-Roadway Concentrations and Population Exposure

Although the emissions inventory (see Section 2) indicates that on-road vehicles account for a 

relatively modest share of overall primary PM2.5 emissions in the Bay Area, anyone who drives in 

traffic, walks or cycles on urban streets, or lives in close proximity to a busy roadway incurs significant 

exposure to PM. Exposure to roadway emissions has emerged as an important social equity issue 

because major roadways, especially those that carry a high volume of heavy-duty diesel-powered 

trucks, often run through or in close proximity to low-income and minority communities.

Studies show that ambient concentrations of ultrafine and fine PM are generally much higher than 

average near major roadways, especially in the downwind direction. Numerous studies have found 

increased incidence of respiratory and cardiovascular disease among people who live in close 

proximity to heavily-traveled roadways. The good news is that PM concentrations, exposure, and 

health effects all tend to decrease rapidly with greater distance from the roadway.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 35
Insert Caption Here

Population exposure to roadway emissions depends upon the amount and the chemical composition 

of emissions generated by the vehicles on a given roadway; meteorological conditions, such as wind 

speed and prevailing wind direction; the number of people in proximity to the roadway, their distance 

from the roadway, and their demographic characteristics; the type of land uses and buildings near the 

roadway; and the presence of buildings, trees, sound walls or other barriers that affect air pollution 

dispersion patterns. 

Exposure to PM from Busy Roadways

Key findings from studies on population exposure to PM emissions from roadways are summarized 

here:

• Numerous studies have found increased rates of respiratory and 

cardiovascular disease among people who live in close proximity to busy 

roadways.

• In close proximity to high-volume roadways, ambient concentrations of PM 

may be greatly elevated compared to background levels, especially in the 

downwind direction.

• When vehicle emissions are trapped in enclosed areas, such as urban street 

canyons and tunnels, this can lead to much higher pollution concentrations 

and population exposure on a local basis.

• In-vehicle exposure may be a leading source of exposure to PM and other air 

pollutants for people who drive on freeways or major arterials on a regular 

basis. Exposure rates may be 5 to 10 times higher than average when driving 

on busy roadways. Driving on a freeway or busy arterial road for even a 

modest time or distance can account for a significant portion of total daily 

exposure to ultrafine particles.

• Both PM2.5 mass and ultrafine particle number decrease as distance from 

the roadway increases. The concentration of ultrafine particles drops off 

rapidly within the first 50 to 100 meters from the source, and generally 

reverts to background levels 100-300 meters of the roadway. 

• How quickly the particle concentration declines as a function of distance 

from the roadway depends upon the specific mix of particles emitted by the 

vehicles, as well as the atmospheric conditions and wind speed and direction 

in the area of the roadway.

A number of studies have analyzed how PM concentrations change in relation to distance from 

roadways. For example, a study in Southern California (Zhu et al., 2002) found that the concentration 

of ultrafine particles decreased rapidly within approximately 300 feet (~ 100 meters) of the I-710 

and I-405 freeways, as shown in Figure 1-5. Although the 710 freeway had much more diesel traffic 

(more than 25% of the vehicles) than the 405 freeway (less than 5% of the vehicles), the results were 

similar for both freeways.

36 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Figure 1-5 Number of Particles Versus Distance from Roadway: Two 

Freeways in Southern California (Zhu, 2002)

The chemical composition and size distribution of freshly-emitted ultrafine and fine PM are 

transformed during transport to downwind locations by the processes of condensation, evaporation, 

and dilution. Although PM2.5 mass and ultrafine particle number both decrease with distance from a 

roadway, the number of ultrafine particles generally drops off much more rapidly than PM2.5 mass. 

This is due to the fact that ultrafine particles coagulate very rapidly to form larger particles upon 

exposure to ambient air; coagulation reduces particle number, but does not significantly affect overall 

PM2.5 particle mass. 

Another way to analyze how roadway emissions affect population exposure to PM is to consider 

intake fraction; i.e., the percentage of emitted PM that is actually inhaled by a human receptor. A 

study by the EPA-funded Harvard Center for Ambient Particle Health Effects performed dispersion 

modeling of primary PM2.5 emissions for 23,000 road segments in the Boston area. For each 

segment modeled, the study estimated how much of the intake fraction occurs among people in 

close proximity to the roadway. The study found that although the intake fraction varied considerably 

among the different segments, on average 46% of the total population exposure occurs within 200 

meters of the road segment. These findings support the notion that in a dense urban environment, 

such as Boston or many parts of the San Francisco Bay Area, a considerable portion of the overall 

population exposure to roadway emissions occurs in close proximity to the roadway. However, the 

findings also suggest that even though emissions from a roadway become diluted and disperse 

rapidly within a few hundred meters of the road, a significant share of the total population exposure 

to primary PM2.5 emissions from a roadway still occurs at a distance greater than 200 meters from 

the roadway.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 37
The spatial distribution and extent of roadway emissions may vary based upon temporal factors, such 

as time of day and season of the year. A study (Zhu et al. 2006) which compared ultrafine particle 

numbers for daytime and nighttime conditions near a major freeway (I-405) in Los Angeles found 

that the rate of decrease in ultrafine particles downwind of the freeway was much less at night than 

during the day. Although traffic volume on I-405 at night was only 25% of the daytime volume, the 

particle count 30 meters downwind of the freeway was about 80% of the daytime value. The authors 

attribute the higher ratio of particles to traffic volume at night to a combination of lower wind speed 

and weaker atmospheric dilution, as well as cooler temperatures which cause increased particle 

formation in the vehicle exhaust. The study also found that particle counts near the freeway were 

higher in winter than in summer, for similar reasons to the factors that lead to higher particle counts 

at night. 

Dispersion is key to reducing ambient concentrations and exposure to PM. However, it is important to 

note that some urban environments, such as tunnels and “urban street canyons”, are not conducive 

to dispersion of air pollutants. When emissions are trapped in enclosed areas, this can lead to much 

higher local concentrations, and thus much higher population exposure. One study (Morwaska et 

al. 2008) found that ultrafine particle numbers in the near-roadway environment were roughly 18 

times higher than in a non-urban background environment, while measured concentrations in street 

canyons and tunnels were 27 and 64 times higher, respectively, than background. Another study 

(Zhou et al. 2008) found that, due to high population density, combined with the lack of dispersion, 

the intake fraction of emissions in urban street canyons is very high, similar in magnitude to the 

intake fraction associated with indoor tobacco smoke.

In-Vehicle Exposure 

Concerns about elevated exposure to PM near major roadways also apply to drivers and 

passengers traveling in vehicles on high-volume roads. In fact, the evidence suggests that invehicle exposure may be a leading source of exposure to PM and other air pollutants for people 

who drive on freeways or major arterials on a regular basis. In-vehicle exposure depends on the 

volume and mix of vehicles on a given road, as well as the type of ventilation system used in the 

vehicle. Moving vehicles typically have high air exchange rates, allowing emissions from the stream 

of traffic to penetrate into vehicles. One study (Fruin et al. 2008) found that 36% of total daily 

exposure to ultrafine particles occurred during a daily commute of 1.5 hours round trip (6% of the 

day) in Los Angeles, and that 22% of total exposure occurred during 0.5 hours (just 2% of the day) 

that was spend on freeways. This indicates that exposure rates may be 5 to 10 times higher than 

average when driving on busy roadways. Thus, even limited time on a freeway can account for a 

significant portion of total daily exposure to ultrafine particles. 

Freeways are also where people are most likely to experience higher exposure to diesel PM, which 

has been classified by the Air Resources Board as a toxic air contaminant. The 2008 Fruin study 

found that on freeways in Los Angeles, concentrations of ultrafine PM, black carbon, nitric oxide, and 

polycyclic aromatic hydrocarbons (PAH) bound to small particles are generated primarily by dieselpowered vehicles, even though diesel vehicles account for only a small fraction (6%) of the traffic 

on LA freeways. This study also found, however, that on arterial roads concentrations of ultrafine 

38 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
particles appear to be emitted primarily by gasoline-powered vehicle undergoing hard accelerations. 

Concentrations of ultrafine particles on arterials were roughly one-third those of freeways. 

“Self-pollution”, which occurs when the exhaust from a vehicle infiltrates its own passenger cabin, 

may also contribute to in-vehicle exposure. This has raised concern about risks to children who ride 

to school on diesel-powered buses. One study (Adar et al. 2008) found that PM2.5 on school buses 

was double the on-road levels, and that 35% of PM2.5 measured in school buses came from selfpollution. (See description of the Lower-Emission School Bus Program in Section 4 regarding actions 

to address this issue.) 

Aircraft and Airports

Studies conducted by the South Coast 

AQMD suggest that jet aircraft may be 

major emitters of ultrafine particles. 

Typical ultrafine particle concentrations 

are on the order of 50,000-200,000 

particles per cm3 near freeways; by 

contrast, ultrafine particle concentrations 

near jet exhaust can reach 6,000,000 

particles per cm3. As shown in Table 1-3.

Table 1-3 Comparison of Ultrafine Particle Concentrations4

Environment Ultrafine Particle Concentration

Clean background 500 – 2,000 particles per cubic centimeter

Typical urban air 5,000 – 30,000 particles per cc

Freeway 50,000 – 200,000 particles per cc

Jet exhaust Up to 6,000,000 particles per cc

A study (Hu et al. 2009) that measured ultrafine particles near the Santa Monica Airport, at the 

residence closest to the airport, and at a nearby school showed correlations of ultrafine particle 

concentrations from jet exhaust at all three locations. Aircraft operations resulted in average ultrafine 

particle concentrations elevated by a factor of 10 at 100 meters downwind and by a factor of 2.5 at 

660 meters downwind. In fact, the area impacted by elevated UFPM concentrations was found to 

extend beyond 660m downwind and 250m perpendicular to the wind on the downwind side of the 

4 Presentation by Dr. Philip Fine of South Coast AQMD to BAAQMD Advisory Council Meeting: Ultrafine Particles 2012 Atmospheric 

Monitoring of Ultrafine Particles, February 2012.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 39
Santa Monica Airport. This study demonstrated that there may be health implications for residences 

living in proximity to jetports, especially in the downwind direction.

A study by Carnegie Mellon University researchers (Miracolo et al. 2011) evaluated the effects of 

photo-oxidation on ultrafine PM emissions from a gas turbine engine designed to mimic a jet aircraft 

engine. The study found that photo-oxidation created substantial secondary PM, suggesting that it 

is also important to consider potential secondary PM formation when assessing the influence of jet 

aircraft emissions. 

Back-up Generators

Back-up generators (BUGs), also known as stationary engines and emergency generators, are 

used frequently by hospitals, office buildings, schools, grocery stories, and government facilities to 

supply power to a building during a power failure. While power failures are generally rare, BUGs are 

operated several times a year for testing. Diesel BUGs emit diesel particulate matter and other toxic 

air contaminants and may contribute significantly to people’s exposure to toxics and health risks. In 

addition, BUGs tend to be concentrated in populated areas, where high numbers of people may be 

already exposed to high levels of pollution.

A new BUG installed today in the Bay Area poses little health risk during its operating testing hours 

due to the Air District’s and ARB’s regulations. However, old BUGs that were installed prior to 

regulations and continue to be in use today generate high levels of toxics and pose a serious health 

risk challenge. Even though these BUGs may be used as little as 100-50 hours a year, they can emit 

enormous amounts of diesel PM since their engines do not comply with any emission standards or 

contain retrofit technologies. In the Air District’s general screening of health risks for BUGs in the 

Bay Area, the cancer risk for grandfathered BUGs ranges from 20 to 200 in a million in some cases. 

There are close to 3,000 BUGs in the Bay Area, approximately 1,500 of which may have cancer risks 

over 10 in a million. The majority of these BUGs are located in Bay Area urban centers. These BUGs 

contribute heavily to health risks already experienced by people living near roadways and other 

mobile emissions of diesel PM. The Air District’s general health risk screening for stationary sources 

indicates that addressing emissions from grandfathered back-up generators could significantly 

reduce exposure to diesel PM, especially in urban areas with already high exposure rates.

Indoor Exposure to PM

Studies have found that most people experience a major portion of 

their total PM exposure when they are indoors. This is not surprising, 

since people spend the majority of their time indoors, in the home, 

office, school, stores, restaurants, etc. According to one study (Qing 

Yu Meng et al. 2005), adults typically spent 87% of their time indoors, 

7% in vehicles, and just 6% outside. The PM that we breathe indoors 

is a combination of ambient (outdoor) PM that penetrates to the 

indoor environment, as well as PM emissions produced by indoor 

sources. 

Most people 

experience a 

major portion of 

their exposure 

to PM when they 

are indoors.

40 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Studies to date to measure indoor PM levels and population exposure have generally been limited 

to small numbers of sites, because gaining access to suitable sites (private homes, schools, etc.), 

installing monitors, and analyzing data requires substantial time and resources. Analyzing indoor 

concentrations and exposures in multi-unit buildings, such as apartment buildings, is complicated 

by the fact that PM created indoors can move between units, as well as the fact that heating and 

ventilation systems, if not properly designed and maintained, can transfer pollutants between units. 

Nonetheless, the findings of existing studies suggest that indoor exposure to PM is a serious issue 

that merits more attention. 

Factors that determine indoor exposure to PM include (1) the ambient (outdoor) PM concentration 

in the vicinity of the building, 2) the infiltration rate: i.e., how much of the ambient outdoor PM 

penetrates indoors, 3) the air exchange rate: how quickly indoor air is replaced by outdoor air, and 

(4) the amount of primary PM emissions and PM precursors produced in the indoor environment from 

sources such as cooking, wood-burning, and cigarette smoking. These factors can vary considerably 

depending upon building type and location, the type of heating and ventilation system, and 

meteorological conditions.

The infiltration rate of ambient (outdoor) PM to the indoor environment depends upon building 

materials, characteristics, and design, such as the type of ventilation system, the location of air 

intake units, whether windows are open or closed, and whether a building has air conditioning or an 

air filtration system. The PM infiltration rate also varies upon the size and composition of the particles 

present in the ambient PM. Because different sizes and types of particles have different infiltration 

rates, the composition of PM in the indoor environment generally differs from the ambient outdoor 

PM. Ammonium nitrate levels, for example, are generally higher outside than indoors. Ammonium 

nitrate can exist in either particle or gaseous form in the atmosphere, depending upon temperature. 

In colder weather, ammonium nitrate particles account for a sizable portion of total ambient PM2.5 

in the Bay Area. However, when they encounter warmer air in the indoor environment, ammonium 

nitrate particles generally volatize (convert to the gaseous form), such that they no longer exist in 

particle form. 

Ultrafine particles are less likely to penetrate through a building envelope because they deposit 

more rapidly on building surfaces due to Brownian motion at the molecular level. Whereas typical 

infiltration factors for PM10 and PM2.5 are in the range of 50%, (Ott et al. 2000), infiltration factors 

for ultrafine particles are on the order of 30% (Wallace & Howard-Reed, 2002). Since ultrafine 

particles do not easily penetrate to the indoors, this suggests that indoor sources of ultrafine particles 

play an important role in determining total personal exposure to UFPM.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 41
Sources of PM in the 

Home Environment

Although PM in outdoor air does 

penetrate to the indoor environment, 

particles generated within the home 

often account for a substantial share 

of indoor PM levels and exposure. 

Indoor sources of PM include 

fireplaces and wood stoves, cooking, 

gas stoves, cleaning products, 

cigarette smoking, candles, and 

incense, laser printers, as well as 

human activities that may re-suspend PM2.5. Indoor PM may also include a mixture of dander from 

pets, other types of allergens, chemical substances, mineral particulate, mold spores, viruses and 

bacteria. The RIOPA study (Polidari, A. et al. 2006) found that fine organic particles dominates indoorgenerated PM2.5 in the homes that were studied. Indoor sources of PM can cause PM levels to 

spike, especially because the emissions are often retained within a confined area. Several of the key 

sources of PM generated in the home environment are briefly described below.

Cooking: Studies have found that cooking is a leading source 

of ultrafine particles in many homes. Indoor monitors show 

that ultrafine particle counts spike whenever cooking occurs. 

Studies suggest that emissions of UFPM are higher from natural 

gas stoves than from electric stoves, but the particle emission 

rates are high in both cases. Ultrafine particle levels tend to be 

significantly higher in homes with gas stoves that use a pilot 

light (compared to pilot-less 

stoves). Emission rates when 

the oven is in use may be 

greater than for stove-top 

cooking. One study found that the indoor concentration of ultrafine 

particles jumped from 5,000 particles per cubic centimeter to 1 

million particles – a 200-fold increase – within a few minutes after 

the oven in a residential kitchen was turned on. 5

Wood-burning devices: People are exposed to wood smoke in 

both indoor and outdoor environments. In addition to its negative 

impact on outdoor air quality, residential wood-burning can be a 

major source of indoor PM, especially if the chimney or stovepipe 

does not vent smoke to the outdoors effectively. This problem 

occurs most commonly when a fire is first ignited and the fireplace 

flue is not warmed up, thus failing to draw smoke efficiently. One 

5 Presentation by Susanne Hering, Ph. D., of Aerosol Dynamics to BAAQMD Advisory Council on March 9, 2011.

42 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
study (Pierson et al. 1989) found that 70% of smoke from chimneys can reenter the home where it 

originated and/or neighboring dwellings. 

Appliances: Common household appliances, such as clothes dryers, toaster ovens, irons, and laser 

printers can also produce ultrafine particles, especially appliances that operate by heating metal 

surfaces. 

Cleaning products: Household cleaning products can also produce ultrafine and fine particles in 

the indoor environment. Scented cleaning products contain terpenes such as pinene (pine scent) and 

limonene (citrus scent); these terpenes can react with ozone to form ultrafine particles.

Contribution of Indoor Exposure to Total PM Exposure

Lance Wallace and Wayne Ott have done pioneering work using portable particle counters to measure 

personal exposure to ultrafine particles. In one of their recent studies (Wallace & Ott, 2010) using 

personal monitors to measure exposures in environments such as homes, cars, and restaurants, they 

estimated that, on average, 47% of daily personal exposure to ultrafine particles for the participants 

in the study can be attributed to indoor sources, 36% to outdoor sources, and 17% to in-vehicle 

exposure. Consistent with the SHEDS-PM estimates for PM2.5 described below, cooking and cigarette 

smoking were the dominant sources of indoor emission of UFPM. In households with one or more 

smokers, the cigarette smoke more than doubled the exposure from all other sources. By measuring 

the particle count per cubic centimeter (cm3) and multiplying this by the size of the impacted indoor 

area, this study estimates that smoking a single cigarette emits approximately 2 trillion (2 x 1012) 

ultra-fine particles.

Lynn Hildebrand at Stanford University and William Nazaroff at UC Berkeley have also done important 

research to advance our understanding of exposure to PM in various micro-environments. A recent 

study directed by Professor Nazaroff (Bhangar at al. 2011) monitored ultrafine particle concentrations 

and exposures in seven residences (with non-smoking inhabitants) in urban and suburban Alameda 

County. This study provides several findings of interest: 

• Ultrafine particle concentrations in the home environment are heavily 

impacted by episodic indoor source events that cause sharp spikes in 

particle counts. These events are triggered by activities such as cooking on 

the stove; uses of appliances such as toaster ovens, steam irons, or clothes 

dryers; burning candles; and use of the furnace.

• Frequency of use of the cooking range (either gas or electric) is the single 

most important determinant of exposure from episodic indoor sources.

• Gas stoves with pilot lights are a key source of indoor emissions and 

exposures to ultrafine particles.

• Indoor particle counts are much higher when occupants are at home and 

active (thus generating particles via indoor source events), compared to 

when they are away from home, or at home but asleep.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 43
• Emissions from indoor sources of ultrafine particles accounted for roughly 

60% of the indoor particles; the remaining 40% represent particles that 

infiltrated from outdoor air. 

• Active particle removal systems can reduce indoor particle levels (of both 

particles generated indoors, as well as particles that infiltrate from outdoors) 

by a factor of 2 to 4.

The papers cited above analyzed personal exposure to PM at the individual level. Efforts have also 

been made to estimate the major sources of aggregate population exposure to PM in various urban 

areas. Many of these studies have employed the Stochastic Human Exposure & Dosage Simulation 

for PM (SHEDS-PM) model developed by the US EPA National Exposure Research Laboratory. 

Synthesizing data from many sources, including personal activity logs, ambient PM2.5 concentrations 

for outdoor air, and results from studies of indoor PM, the SHEDS-PM model has been used to 

estimate the contribution of outdoor exposure and indoor exposure to total population exposure, and 

to examine the role of key indoor sources of PM2.5 such as cigarette smoking and cooking. 

An analysis (Burke et al. 2001) using SHEDS-PM for Philadelphia found that, on average, ambient 

(outdoor) PM2.5 accounted for only 37.5% of total exposure; however, this percentage varied greatly 

within the population. The study found relatively low variation in personal exposure to ambient 

(outdoor) PM2.5; however, exposure to PM in the indoor environment varied greatly, with high 

levels of indoor exposure caused primarily by emissions from cigarette smoking and/or cooking. 

Another study (Cao & Frey, 2011) had similar findings, using SHEDS-PM to analyze and compare 

PM exposures in three different areas and climate zones (New York City; Harris County, Texas; and 

six counties along the I-40 corridor in North Carolina). This study found that ambient exposure 

accounted for approximately 40% of the estimated total daily average PM2.5 exposure in each of the 

three areas. As in the case of the Burke study of Philadelphia, the Cao study also found that some 

individuals have extremely high PM exposures, primarily due to indoor emissions from cigarette 

smoking and/or cooking. 

The Relationship of Indoor, Outdoor and Personal Air (RIOPA) study (Polidari et al. 2006) investigated 

residential indoor, outdoor and personal exposures to PM2.5 in three cities with different climates: 

Houston, TX; Los Angeles, CA; and Elizabeth, NJ. The study found that the median contribution of 

ambient (outdoor) sources to indoor PM2.5 concentrations was 56% for all study homes (63%, 52% 

and 33% for California, New Jersey and Texas study homes, respectively).

44 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Exposure to PM in Schools

Another recent study directed by Professor Nazaroff (Mullen et al. 2011) measured PM concentrations 

in six elementary school classrooms in Alameda County; measurements were performed for a total 

of 18 days (from 2-4 days in each classroom). None of the schools was in close proximity to a major 

freeway; distance from the nearest freeway was 0.5 km or greater in all cases. Two of the classrooms 

were equipped with mechanical ventilation systems; the other four used natural ventilation (windows 

and doors that open). The study found that (1) indoor particle counts were typically about half of the 

outdoor concentrations, and (2) roughly 90% of the ultrafine particles measured in the classrooms 

originated outdoors. The authors compared exposure in the classrooms to exposure in the homes 

(per Bhangar 2011), noting that the results suggest that elementary school students are subject 

to much greater overall exposure to ultrafine particles in the home environment, because in-home 

particle counts are higher and because the students spend more time at home than at school. The 

authors attribute the difference in concentrations to the fact that fewer ultrafine particles are emitted 

in classrooms than in homes. In particular, indoor source events, such as cooking, that lead to sharp 

spikes in UF particle levels, are common in the home, but much less prevalent in the school setting. 

Summary of Indoor Population Exposure to PM

Key findings regarding indoor exposure to PM can be summarized as follows:

• Ambient contribution to indoor PM exposure depends on outdoor 

concentrations in combination with the infiltration rate.

• When indoor sources are present, indoor PM concentrations can be 

substantially higher than outdoor PM concentrations.

• Indoor PM emissions are generated primarily by specific activities and 

sources: cooking, cleaning, ironing clothes, burning candles, use of forced-air 

furnaces, fireplaces, etc.

• PM levels in the home are characterized by sharp spikes triggered by the 

types of activities mentioned above.

• Ventilation to control PM spikes can greatly reduce indoor concentrations 

and population exposure.

• PM concentrations in the home are generally much lower at night (when 

people are sleeping, and PM-generating activities are not occurring) than 

when people are at home and active.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 45
Occupational Exposure

Exposure to PM and other pollutants on the 

job is regulated by the Occupational Safety & 

Health Administration (OSHA). Occupational 

exposure to PM may differ from ambient 

exposure in terms of particle type and 

composition, as well as the intensity, frequency, 

and duration of exposure. Certain job types 

may expose workers to significant occupational 

exposures. For example, truck drivers and 

other people who drive a lot on the job may be 

exposed to higher levels of PM from both diesel and gasoline vehicles. Restaurant workers may 

be exposed to PM from cooking and wood smoke from charbroilers. Construction workers and 

quarry workers may be exposed to diesel PM, as well as to geologic dust particles from mechanical 

processes. Firefighters, especially those who combat wildfires, may be subject to extremely high 

acute exposures to PM. Janitorial workers may be exposed to high levels of PM in the indoor 

environment when they use cleaning products that contain chemicals which react with ambient 

ozone to form PM. Researchers (Morawska et al. 2007) have founds that people who work in office 

buildings may be exposed to PM (as well as VOCs) from printers. 

Brigham and Woman’s Health Hospital conducted a study (Laden et al. 2007) of mortality patterns 

associated with job-specific exposure to fine particulate and especially particulate matter from 

vehicle exhaust. They examined rates of cause-specific mortality and compared this to the general 

population. This study concluded that in the U.S. trucking industry there was an excess of mortality 

due to lung cancer and heart disease particularly among drivers. 

Summary

Population exposure to PM is heavily dependent on individual activity patterns and the types of PM 

emissions sources that people are exposed to in the course of their day-to-day activities. PM levels, 

and population exposure to PM, may be greatly elevated in certain micro-environments, such as invehicle, near-roadway, and in the home. 

The key to avoiding negative health impacts from PM is to reduce population exposure to PM among 

Bay Area residents. Recognizing the importance of reducing population exposure to air pollutants, the 

Air District has been working to identify areas that are disproportionately impacted and implementing 

policies and programs to protect these communities, as described in Section 4.

But to better protect public health, we need to improve our understanding of population exposure to 

PM in the Bay Area. Future steps to enhance our understanding of population exposure to PM are 

discussed in Section 5.

Simple steps that Bay Area residents can take to reduce their exposure to PM in the course of their 

day-to-day activities are also described in Section 5.

46 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Section 1-C: PM and Climate Change

This section describes the complex interplay between particulate matter and climate change, 

including how PM affects climate, as well as how higher temperatures due to climate change 

may impact local PM levels. 

Although more work is needed to fully discern the connections, research reveals a two-way 

relationship in which air pollutants impact the climate at both the local and global scale, while 

changes in climate impact air quality. Most discussion has focused on the need to reduce emissions 

of carbon dioxide and other greenhouse gases, but researchers have found that particulate matter 

also affects the climate, especially the type of PM known as black carbon.

How PM Affects Climate Change

The thin atmosphere that surrounds the Earth enables our planet to support life and the complex 

ecosystems that sustain us. There is irrefutable scientific evidence 

that the Earth’s atmosphere is getting hotter, and that a wide range 

of human activities, such as combustion of fossil fuels, emit carbon 

dioxide (CO2) and other greenhouse gases (GHG) that are building up 

in the atmosphere and changing the climate at the global scale. The 

effects of this man-made global heating are already being experienced 

in California and on a global basis in terms of temperature trends, 

extreme weather events (e.g., drought, frequency and intensity of 

hurricanes and cyclones), sea-level rise, changes in precipitation 

patterns, the frequency and intensity of wildfires, changes in habitat for 

flora and fauna, etc.

Efforts to date to protect the climate have focused primarily on reducing 

man-made emissions of GHGs that trap solar radiation (heat) that would 

normally escape back into space. Reducing emissions of CO2 has been the main focus of climate 

protection efforts to date, because on a mass basis emissions of CO2 dwarf the other GHGs, and 

because CO2 remains in the atmosphere for a very long time. 

However, in recent years researchers have discovered that other short-lived air pollutants, including 

particulate matter and tropospheric ozone, also affect the climate. Although the effects are complex, 

there is evidence that certain types of particulate matter, especially black carbon, can have a 

potent effect in heating the climate at both the local scale (in the area where PM is emitted) and the 

global scale. In response to this research, there is a growing recognition that we need to incorporate 

strategies to reduce emissions of black carbon into climate protections efforts. Reducing black 

carbon can help to slow the rate of atmospheric heating in the near-term, while also protecting 

air quality and public health. Emission control opportunities that provide co-benefits in terms of 

protecting both air quality and the climate are highly desirable from the policy perspective. 

Certain types of 

PM, especially 

black carbon, can 

have a potent 

effect in heating 

the climate.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 47
Climate Forcing

Climate change is primarily caused by man-made activities that impact the Earth’s energy balance 

(Denman et al. IPCC, 2007). Energy constantly flows to the Earth in the form of sunlight and other 

forms of solar radiation. Some of this solar energy is reflected back into space, and the rest is 

absorbed by the planet and stored in the atmosphere, as well as in oceans, forests, etc. Factors 

external to the natural energy system – so-called external forcings - can disturb the Earth’s energy 

balance. These external forcings can be positive or negative. Positive forcers, such as carbon dioxide, 

methane, and other greenhouse gases, cause more of the sun’s energy to be retained by the planet. 

In contrast, negative forcers, such as volcanic dust that reflects sunlight back into space, cause less 

of the sun’s energy to be retained by the planet. The overall impact of human activities on the climate 

depends upon the net sum of positive and negative forcings caused by a wide spectrum of man-made 

activities, including emissions of GHGs and other air pollutants, agriculture and forestry practices, 

land development and road-paving that affect the reflectivity (albedo) of the Earth’s surface. 

Climate Forcing Effects of Particulate Matter (PM)

Particulate matter is composed of solid or liquid particles that are suspended in the air; these 

particles are sometimes referred to as atmospheric aerosols. Fine particles affect the climate 

by means of several direct and indirect processes, some of which heat, and others of which cool, 

the climate. All PM in the atmosphere can affect the Earth’s climate either by absorbing light or by 

scattering light. Particles that absorb sunlight add energy to the earth’s system; they act as positive 

forcers that lead to climate heating. Particles that scatter light increase the reflection of incoming 

sunlight back to space; they serve as negative forcers that cool the climate. In addition to the direct 

effect caused by absorbing or scattering incoming sunlight, fine particles may also have indirect 

effects on the climate by altering the properties of clouds in various ways. 6 More analysis is needed 

to fully define the impacts of particles on clouds, but researchers have noted various different 

processes by which aerosols can affect the reflectivity and lifespan of clouds, in ways that can have 

both heating and cooling effects, as further describe below. (The 2007 IPCC report discusses five 

processes; Jacobson 2002 lists 12 processes.)

For purposes of analyzing the impacts of PM on climate, scientists have identified several types of 

carbon: black carbon, brown carbon, and organic carbon. The effect of primary (directly-emitted) 

PM on sunlight spans a continuum from light-absorbing to light-scattering, with black carbon at the 

light-absorbing end of the spectrum, most organic carbon at the opposite, light-scattering end of 

the spectrum, and brown carbon (a subset of organic carbon) somewhere in the middle. The ratio 

of black carbon, brown carbon and organic carbon produced by fuel combustion depends upon the 

specific fuel being burned and the type of combustion conditions. PM emitted by diesel engines is 

primarily black carbon, whereas the PM emitted by gasoline engines is mostly organic carbon.

Table 1-4 lists the most significant types of anthropogenic (man-made) aerosol particles in terms of 

impact on the climate, and their most common sources. At the global scale, the dominant negative 

6 “Atmospheric Aerosol Properties and Climate Impacts” U.S. Climate Change Science Program Synthesis and Assessment Product 2.3; 

January 2009.

48 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
forcing agent is sulfate,7 whereas the dominant particle as a positive forcing agent is black carbon. 

But organic carbon, brown carbon, and ammonium nitrate also affect the climate in various ways 

that can have both heating and cooling effects. In the Bay Area, ammonium nitrate levels are 

greater than sulfates. 

Table 1-4 Climate-Forcing Properties of PM Components

Negative 

Forcer 

(Cooling 

Agent)

Positive 

Forcer 

(Heating 

Agent)

Direct Effect Indirect Effect Source

Sulfates

X Reflects 

sunlight Increases reflectivity of clouds

Secondary PM formed 

by SO2 emissions from 

fossil fuel-burning

Ammonium nitrate

X Reflects 

sunlight Increase reflectivity of clouds

Secondary PM formed 

by combination of 

NOx and ammonia 

emissions.

Black carbon

X Absorbs 

sunlight

1) Reduces reflectivity of clouds; 

impacts cloud formation. 

2) Heats snow & ice by reducing 

their reflectivity in polar regions.

Incomplete combustion 

of fossil fuels, biofuels, 

and biomass (woodburning)

Brown carbon

X

Absorbs 

some 

wavelengths 

of sunlight 

Incomplete combustion 

of fossil fuels, biofuels, 

and biomass (woodburning)

Organic carbon

?

Mildly 

absorbs 

sunlight

Incomplete combustion 

of fossil fuels, biofuels, 

and biomass (woodburning) 

The various particle types are never emitted into the atmosphere in isolation. The emissions 

produced by a given combustion process or event contain a mixture of black carbon, brown carbon, 

7 Text from NASA Fact Sheet: “While a large fraction of human-made aerosols come in the form of smoke from burning tropical forests, the 

major component comes in the form of sulfate aerosols created by the burning of coal and oil. The concentration of human-made sulfate 

aerosols in the atmosphere has grown rapidly since the start of the industrial revolution. At current production levels, human-made sulfate 

aerosols are thought to outweigh the naturally produced sulfate aerosols.” 

http://www.nasa.gov/centers/langley/news/factsheets/Aerosols.html

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 49
and organic carbon, as well as other co-pollutants including nitrates and sulfates, and various air 

toxics. The climate effects from a given emissions source will depend on the mix and ratio of the 

PM components and other co-pollutants, and the way these pollutants interact in the atmosphere. 

Therefore, unlike greenhouse gases, the climate effects from aerosol particles can vary regionally.

PM as Negative Forcer (Cooling Agent): Sulfates & Ammonium Nitrate

As discussed in Section 2, sulfates and ammonium nitrate are the two leading forms of secondary 

PM, formed by interaction of precursor chemicals such as SOx, NOx, and ammonia (NH3). Sulfate 

particles reflect sunlight, thereby acting as a negative forcer (cooling agent) on the climate. Sulfate 

aerosols also have an indirect cooling effect on the climate by increasing the reflectivity of clouds. 

Sulfate particles serve as nuclei for the condensation of 

water vapor; higher rates of condensation increase the 

brightness of clouds and thus their reflectivity. Sulfates 

and other aerosols may also lengthen cloud lifetimes. The 

overall impact of sulfate and ammonium nitrate particles is 

to increase the Earth’s atmospheric albedo, or reflectivity. 

This causes less sunlight to reach the Earth and thus 

has a cooling effect on the climate. The cooling effects of 

sulfates are somewhat regional in nature, that is, the cooling effects seem to be concentrated near 

areas where the emissions occur, such as areas of industrial activity.

Due to the atmospheric presence of sulfate and nitrate particles which function as cooling agents, 

the evidence suggests that the climate has experienced less heating in recent decades than would 

have otherwise occurred. However, since these particles are harmful to human health, we need to 

further reduce sulfate and nitrate levels. As we reduce sulfate and nitrate levels to protect public 

health, this will lend increasing urgency to the need to reduce emissions of the greenhouse gases, as 

well as particles such as black carbon that contribute to climate heating.

PM as Positive Forcer (Heating Agent): Black Carbon & Brown Carbon 

Several types of particles act as heating agents. Black carbon, often referred to as “soot”, is a solid 

particle formed of mostly pure carbon that absorbs solar radiation (light) at all wavelengths. Black 

carbon has been identified as a potent climate heating agent. In fact, black carbon can absorb 

a million times more energy than carbon dioxide per unit of mass. Black carbon is black in color 

because it is highly efficient in absorbing all the wavelengths of light in the visible spectrum. The vast 

majority of black carbon is man-made. Black carbon is produced by incomplete combustion of fossil 

fuels, biofuels and biomass (wood-burning). 

Combustion of fossil fuels (gasoline, diesel, coal, and natural gas) and biomass (wood and 

vegetation) are the major sources of black carbon on a global basis. In general, fossil fuel 

combustion from diesel engines, energy production and industrial processes accounts for most black 

carbon in developed countries, with the major contribution coming from diesel engines. Approximately 

75% of the PM in diesel exhaust is black carbon. By contrast, biomass burning produces most of the 

50 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
black carbon in developing countries. In developing countries, burning of biomass occurs both to 

clear land and for domestic uses such as cooking and home-heating.

A recent analysis by Air District staff, based on the Bay Area emissions inventory and chemical mass 

balance (CMB) analysis of PM captured on filters, indicates that fossil fuel combustion accounts for 

approximately 65% of black carbon emissions (diesel engines accounts for 50%; other fossil fuels 

15%) in the Bay Area. Wood smoke from all sources accounts for 35% of black carbon emissions in 

the Bay Area, most of which (25%) is produced by residential wood-burning.

CO2 and other GHGs heat the atmosphere primarily by retaining heat from the outgoing infrared 

radiation (produced when sunlight is reflected off the Earth’s surface) that would otherwise escape 

the atmosphere. By contrast, the impact of black carbon on the climate is more complex. It acts on 

the climate through multiple mechanisms while suspended as a particle in the atmosphere, and also 

when deposited on snow and ice. The impact of the black carbon heating mechanisms described 

below can be greater if other pollutants in the air adhere to the black carbon, thus making the 

particles bigger and their heat absorption greater.

Direct effect: Black carbon absorbs both incoming and outgoing radiation of all wavelengths 

(whereas GHGs only absorb outgoing radiation in the infrared range). When sunlight hits black carbon 

in the atmosphere, the carbon particle absorbs that solar radiation and heats the atmosphere. Black 

carbon also has a heating impact when it absorbs solar radiation reflected by the Earth and clouds, 

thus reducing the amount of heat that would otherwise radiate back into space.

Snow/ice albedo effect: When black carbon falls on to snow or ice, it darkens their surface. 

This decreases the reflectivity (albedo) of the snow or ice, so that more sunlight is absorbed, thus 

accelerating the melting of ice caps and glaciers. On a global basis, the albedo effect of black 

carbon on ice and snow accounts for about 25% of the total heating effect of black carbon (Hansen 

& Nazarenko, 2004). Nonetheless, the melting of ice caps and glaciers, which reduces and alters 

habitat for key arctic species such as polar bears, is one of the most dramatic manifestations 

of climate change. In addition, the impact of black carbon in polar regions is of special concern, 

because it may lead to abrupt transitions or “tipping points”. 

Two examples of potential tipping point phenomena include the melting of ice in the Arctic Sea, 

as well as the release from thawing permafrost of vast quantities of CO2 and methane which 

could further accelerate the process of global heating. While the most dramatic manifestations of 

the snow/ice albedo effects may occur in polar regions, it has implications for California and the 

western U.S. as well. For example, one study (Hadley et al. 2010) has found deposition of black 

carbon on snowpack in the Sierra Nevada and Rocky mountains contributes to faster melting of 

the snowpack earlier in the spring, thus reducing the amount of snowmelt that would normally 

occur later in the spring and summer. This may have impacts on water supply in the western U.S. 

by reducing the supply that can be captured for human use. Given the importance of the Sierra 

snowpack to water supplies in California, this could be one of the most important effects of black 

carbon emissions within the state.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 51
Indirect and semi-direct effects: Black carbon also alters the 

properties of clouds, affecting cloud reflectivity, precipitation, and 

the surface dimming caused by cloud cover. Clouds permeated with 

black carbon reflect less sunlight back into space, thus causing more 

heat to be absorbed in the atmosphere. Because these effects are 

so complex, estimating the indirect heating effects of black carbon 

and other particles in terms of cloud formation and albedo and 

precipitation patterns is one of the greatest challenges for modelers 

who study climate change. 

Net effect of black carbon: Although black carbon has a complex 

mix of both cooling and heating effects, the evidence suggests that its 

net climate impact is positive forcing with significant climate heating 

potential. In fact, recent studies indicate that black carbon may be 

the second biggest contributor to global heating, after carbon dioxide 

(Jacobson 2010). However, because of the complexities related to 

analyzing the climate impacts of black carbon, most of the global-scale 

climate change models currently in use only consider black carbon in a simplified way by addressing 

a subset of its various forcing mechanisms.

Short-Term vs. Long-Term Impacts

Black carbon typically remains suspended in the atmosphere for a relatively short time, on the order 

of 10-12 days. This is a very brief timespan compared with greenhouse gases such as methane, 

which typically remains in the atmosphere for approximately 10-15 years, or carbon dioxide (CO2), 

which stays in the atmosphere for decades or even up to hundreds of years. Because its atmospheric 

timespan is brief, this means that reducing emissions of black carbon in the near-term will provide 

immediate climate cooling benefits. This could mitigate, in the short-term, the heating that continues 

to occur in the absence of effective national and international policies to reduce the on-going 

increase in emissions of longer-lived GHG emissions such as CO2 and methane. In other words, 

efforts to reduce black carbon can yield immediate cooling benefits, thus buying us time to address 

the longer-term solutions needed to reduce longer-lived GHG emissions. 

Geographic Scale of Impacts

Whereas the climate impacts of the traditional greenhouse gases are global in scale, the evidence 

suggests that the climate impact of black carbon is more localized in nature. Certain regions of the 

world are more likely to be impacted by black carbon heating effects, either due to transport and 

deposition, such as polar regions, or to high levels of PM emissions in the region, such as Asia. This 

has several implications: 

• Efforts to reduce black carbon can be targeted on the sources and locations 

where the heating effect of black carbon is most damaging. One of the great 

unknowns in terms of global heating is where and when climate change may 

Grinnell Glacier 2005.jpg courtesy 

of the U.S. Geological Survey

52 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
go beyond irreversible tipping points that could trigger disastrous impacts, 

such as greatly accelerated melting of the Greenland ice sheet or thawing 

permafrost. Well-targeted efforts to reduce black carbon emissions in the 

most sensitive regions may provide a means to avert or delay such tipping 

point scenarios.

Reductions in black carbon at the local level will provide direct local benefits, both in terms of 

reducing heating in the specific region where the black carbon reductions occur, as well as reducing 

the serious health impacts related to exposure to PM and black carbon.

Brown Carbon

The term “brown carbon” refers to organic carbon compounds that absorb visible and/or ultraviolet 

light, and thus heat the atmosphere. Like black carbon, it is a product of incomplete combustion. Brown 

carbon compounds are chemically diverse, so that the wavelengths of light they absorb, and thus their 

color, vary. The mixture of colors of brown carbon compounds appears brown to the human eye. The total 

quantity of solar energy absorbed by a brown carbon mixture depends upon the molecular structure of 

the compounds and the total mass of material. The net contribution of brown carbon to climate change is 

presently uncertain; this represents a key gap in our understanding of the net impact of PM on climate.

Net impact of PM on the Climate

Assessing the net impact of PM on the climate system is challenging. It requires analyzing the mix 

of the various particle types in the atmosphere in a given region, and then evaluating the different 

heating and/or cooling properties (both direct effects and the diverse indirect effects) for each 

particle type. And because fine particles are generally short-lived in the atmosphere, it is difficult to 

measure them on a global scale.

Despite the uncertainties, the available evidence suggests that, even though black carbon and 

brown carbon have a heating effect, fine particles as a whole currently have a net cooling effect 

on the climate.8 In fact, a recent study (Sriver 2011) led by the National Oceanic and Atmospheric 

Administration found that a rapid build-up of aerosols (fine particles) in the stratosphere over the 

past decade has offset about one-third of the climate heating influence of CO2 during this period. 

The NOAA study concludes that the amount of aerosols in the stratosphere will play an important part 

in determining the overall change in climate in coming decades. As noted above, it is important to 

reduce emissions of fine particles to protect public health, but doing so may exacerbate the challenge 

we face in attempting to control the climate heating impacts of greenhouse gases and black carbon. 

8 Stratospheric Pollution Helps Slow Global Warming,” David Biello, Scientific American, July 22, 2011; www.scientificamerican.com/article.

cfm?id=stratospheric-pollution-helps-slow-global-warming.

Quote from this article: “By analyzing satellite data and other measures, Daniel and his colleagues found that such aerosols have been 

on the rise in Earth’s atmosphere in the past decade, nearly doubling in concentration. That concentration has reflected roughly 0.1 watts 

per meter squared of sunlight away from the planet, enough to offset roughly one-third of the 0.28 watts per meter squared of extra heat 

trapped by rising atmospheric concentrations of greenhouse gases such as carbon dioxide. The researchers calculate that the aerosols 

prevented 0.07 degrees Celsius of warming in average temperatures since 2000.”

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 53
Air District staff has performed a preliminary analysis to examine how decreasing PM levels in the 

Bay Area may be affecting the local climate. Staff looked at pyranometer9 readings for three sites 

with data stretching back to 1990: Bethel Island, 

Santa Rosa and San Martin. The data suggest that 

insolation (the amount of solar radiation hitting the 

earth’s surface) for these sites has increased on the 

order of 2 Watts/m2 per decade, or about 1% per 

decade. This finding, though preliminary, is consistent 

with the results from one paper (Wild et al. 2008) 

which found that surface net radiation over land 

rapidly increased by about 2 W/m2 per decade on a 

global basis for the 15-year period 1986–2000.

An April 2012 study (Leibensperger et al.) by the Harvard School of Engineering & Applied Sciences 

also sheds light on the cooling effect of sulfate particles and other aerosols. The Harvard study found 

that in the later part of the 20th century particulate pollution created a “cold patch” over the eastern 

United States where the effects of global warming were temporarily obscured. In addition to directly 

scattering incoming sunlight, the particles also helped form clouds that further reflected sunlight, 

thus indirectly leading to greater cooling at the earth’s surface. The study found that as a result 

of efforts to reduce sulfates and other particles in recent years to protect public health, this “cold 

patch” effect has now been largely removed. In the words of the authors, “What we’ve shown is that 

particulate pollution over the eastern United States has delayed the warming that we would expect to 

see from increasing greenhouse gases. For the sake of protecting human health and reducing acid 

rain, we’ve now cut the emissions that lead to particulate pollution, but these cuts have caused the 

greenhouse warming in this region to ramp up to match the global trend. No one is suggesting that 

we should stop improving air quality, but it’s important to understand the consequences. Clearing the 

air could lead to regional warming.” 

In analyzing the effects of PM on climate, one of the key technical issues is how to compare the 

climate forcing effects of fine particles with the effects of carbon dioxide and other greenhouse 

gases. A metric called Global Warming Potential (GWP) is generally used to compare the heating 

potential of various greenhouse gases in comparison to carbon dioxide. Although a range of values 

has been published in the literature, there is as yet no consensus as to the appropriate GWP value for 

black carbon or for fine PM as a whole; this is due to the complex combination of heating and cooling 

effects described above, as well as the fact that the impacts of PM on climate may vary from region to 

region depending upon the specific sources and composition of PM in a given area. 

In comparing the impact of different climate forcing agents, it is important to consider the different 

atmospheric lifetimes of the various pollutants that impact the climate. For example, CO2 and 

methane remain in the atmosphere for many years; therefore, the benefit of reducing these gases 

in cooling the atmosphere will be spread over many years. By contrast, black carbon and other 

9 A pyranometer measures solar irradiance, that is, the amount of solar energy hitting a flat horizontal surface. It works by using a blackcoated flat disk called a thermopile: The black surface absorbs the solar energy and converts it to heat. The thermopile converts this 

thermal energy to electrical energy.

54 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
fine particles remain in the atmosphere for just a few days or weeks; therefore, the cooling effect 

from reducing black carbon is experienced immediately. So for purposes of policy decisions with a 

short-term time horizon, it may be appropriate to give greater weight to black carbon, since reducing 

black carbon (1) provides immediate climate protection benefits by helping to offset the rise in 

temperatures caused by the continued build-up of greenhouse gases in the atmosphere, and (2) 

provides localized health benefits by reducing PM concentrations and population exposure to PM. 

Impacts of PM on Precipitation Patterns and Storms

In addition to the impacts of fine particles on climate summarized 

above, recent studies also describe potential impacts of PM on 

cloud formation and precipitation patterns. A study (Zhanqing 

Li et al. 2011) that analyzed rainfall patterns and aerosol level 

over a 10-year period in the southern Great Plains of the U.S. 

found that aerosol pollution will suppress cloud formation and 

reduce precipitation in relatively dry environments. Conversely, 

aerosols are likely to increase cloud formation and rainfall in the 

summertime in areas with an existing moist environment, thus 

worsen flooding. The authors conclude that “These findings have 

important implications for the redistribution, availability, and usability of water resources in different 

regions of the world.” 

Impacts of Climate Change on PM Levels

Meteorology plays a critical role in determining air pollution levels. Climate change may impact future 

PM levels by affecting key meteorological variables such as surface temperature, relative humidity, 

precipitation rate and patterns, wind speed, and mixing height (vertical mixing). Atmospheric mixing is 

important to disperse PM and prevent it from building up in the air, so any climate effects that would 

reduce horizontal or vertical mixing could lead to higher ambient concentrations of PM. In considering 

potential impacts of climate change on future PM levels, one of the main concerns is to determine 

whether there will be a “climate penalty” – i.e., whether climate change will increase ambient PM 

levels. Any such climate penalty would either lead to higher PM levels, or require additional controls, 

beyond those already enacted or anticipated, in order to achieve PM standards. 

Researchers are still attempting to evaluate the potential impacts of climate change on future PM 

levels. There are many factors that introduce uncertainty into this exercise, including uncertainty 

about the degree of future change in the climate and future emissions of PM and its precursors. 

Key findings from a June 2010 report to ARB prepared by University of California scientists entitled 

Climate Change Impact on Air Quality in California include the following:

• The impact of climate heating on PM levels is difficult to pin down because 

some of the likely effects act in opposite directions. For example, higher 

temperatures discourage the formation of ammonium nitrate, a component 

of secondary PM that constitutes a sizable fraction of Bay Area PM2.5. 

However, as an offsetting effect, higher temperatures will also lead to 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 55
increased background concentrations of ozone, which encourages the 

formation of ammonium nitrate. 

• Impacts of climate change on PM levels may vary by region, but, overall, 

climate change is expected to have only a small effect on PM levels and 

population exposure to PM in California’s major air basins on an annual 

average basis.

• Climate change is likely to increase average wind speeds in coastal regions 

of California; this may lead to lower concentrations of primary PM, especially 

in coastal regions, such as the Bay Area.

• However, climate change may cause PM levels to be higher during extreme 

pollution events in the future, because future stagnation events which trap 

pollutants close to the emissions source will increase in strength.

Increased Wildfires

Climate change may cause an increase in the frequency and severity of wildfires by altering snowmelt 

and precipitation patterns. At least one study (Westerling, et al. 2006) has found an association 

between climate change and increased wildfires in forests in the western US. Wildfires can emit 

huge quantities of fine particles such as black carbon, as well as other air pollutants, such as carbon 

monoxide, NOx, and air toxics. Most of the particles from wildfires are in the very fine size range, 

the types of particles that can most effectively penetrate deep into the lungs. Wildfires can cause 

dramatic short-term spikes in pollution levels, and greatly increase population exposure to PM and 

other harmful pollutants. The outbreak of wildfires that swept across California in late June 2008 

caused ambient concentrations of ozone and PM to soar to unprecedented levels.10 A recent study 

(Wegesser et al. 2009) found that the PM concentrations not only reached high levels, but that the 

PM released by these June 2008 fires was much more toxic than the PM more typically present in the 

California atmosphere. Smoke from wildfires can cause a variety of acute health effects, including 

irritation of the eyes and the respiratory tract, reduced lung function, bronchitis, exacerbation of 

asthma, and premature death. 

10 During the final week of June 2008, PM2.5 levels increased five or ten-fold compared to normal readings at several Bay Area monitoring 

stations.

56 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
In addition to these health effects, wildfires also release immense quantities of carbon dioxide stored 

in trees and vegetation into the atmosphere. Therefore, to the extent that climate change increases 

wildfires, this will increase emissions and atmospheric concentrations of black carbon and GHGs that 

contribute to climate change, in an unwelcome feedback loop.

Policy Considerations

The evidence suggests that there is a compelling rationale to reduce black carbon emissions for 

several reasons.

• Reducing black carbon will provide immediate climate protection benefits 

by (1) helping to offset temperature increases related to greenhouse gases, 

and (2) potentially averting or delaying the onset of “tipping point” scenarios, 

such as collapse of arctic ice cap or the sudden release of carbon stored 

in permafrost, which could have profound and irreversible impacts at the 

global level. Since emissions of greenhouse gases, especially CO2, continue 

to increase at the national and global scale, and efforts to reverse this trend 

have made little headway to date, reducing black carbon may buy society 

time to implement the more fundamental changes required to stem the rise 

of greenhouse gas emissions.

• In addition to protecting the climate, reducing black carbon will provide 

important public health benefits by reducing population exposure to PM2.5 

and diesel PM, both of which have been shown to cause a range of negative 

health effects.

• Reducing black carbon will provide these health and climate benefits at the 

local scale; i.e. in the region where the emissions reductions occur.

The good news is that, thanks to efforts over the past 10-15 years to reduce emissions of fine PM 

and diesel PM in California and the Bay Area (described in Section 4) in order to protect public 

health, we have already made major progress in reducing ambient levels of fine PM and black 

carbon. Analysis of elemental carbon, which is closely associated with black carbon, indicate 

that Bay Area levels decreased 73% from 1989-91 through 2008-10. 11 Reductions in ambient 

concentrations of black carbon in California and the Bay Area have presumably helped to reduce 

the amount of climate heating caused by man-made emissions of GHGs and other climate forcers 

at both the local and global scale. Looking forward, black carbon emissions will be further reduced 

in response to adopted regulations, such as ARB’s heavy-duty diesel engine regulations, that will 

be implemented over the next 10-15 years. The other side of the coin, however, is that because 

we have a “head start” on reducing diesel PM and black carbon in California, much of the “lowhanging fruit” has already been picked. Therefore, measures to reduce the remaining increment of 

fine PM and black carbon may entail higher cost or effort per unit of emissions reduced. However, 

because reducing black carbon provides benefits in protecting both public health and the climate, 

11 See pages 36-37 in Trends in Bay Area Ambient Particulates, BAAQMD, November 2011.

www.baaqmd.gov/Divisions/Planning-and-Research/Research-and-Modeling/Publications/Reports.aspx 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 57
it should still be possible to craft additional black carbon control measures or incentive programs 

that reduce black carbon at a favorable benefit/cost ratio. 

Trade-offs

Because PM includes a variety of particle types with differing effects on climate, it is important to 

identify and mitigate potential trade-offs in developing control measures to reduce PM. As noted 

above, reducing emissions of particles that act as cooling agents (e.g., sulfates and ammonium 

nitrate) should protect public health, but may entail a trade-off in terms of protecting the climate. In 

another example of trade-offs, a recent study (Anenberg et al. 2011) that analyzed the global-scale 

health benefits of reducing black carbon noted that reducing black carbon could lead to a modest 

increase in sulfate (SO4) concentrations. Because sulfate particles scatter incoming sunlight, an 

increase in sulfates could provide additional benefit in terms of cooling the atmosphere. However, 

an increase in sulfates could have a negative impact on public health, depending on how this would 

affect PM2.5 levels and local population exposure to PM. As this example suggests, identifying and 

evaluating potential trade-offs, and determining the net benefit of potential control strategies, can be 

a complex exercise. The Air District will closely track ongoing research on these issues to inform policy 

decisions.

Next Steps

Although the impact of PM and aerosols on the global climate is complex, some type of particles 

such as black carbon contribute to climate heating. This suggests that efforts to reduce PM should 

place high priority on reducing emissions of those particles, such as black carbon, that both damage 

public health and contribute to climate heating. Potential policies and actions to address the climate 

impacts of PM are discussed in Section 5.

58 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Section 1-D: PM Impacts on 

Ecosystems & Visibility

This section describes impacts of PM on ecosystems, visibility, and the built environment.

In addition to directly impacting public health and the climate, particulate matter also can have 

negative effects on water quality and on the ecosystems and environment that sustain us. These 

effects, as summarized in Table 1-5, include acid rain which leads to acidification of lakes and 

streams, changes in the nutrient balance of coastal waters and river basins, leaching of nutrients 

from soil and reduced nutrient uptake in plants, damage to forests and crops, reduced diversity 

and productivity of ecosystems, damage to stone and man-made materials, and reduced visibility 

and aesthetic values. According to the US EPA, the scientific evidence is sufficient to conclude 

that a causal relationship is likely to exist between deposition of PM and a variety of effects on 

individual ecosystems. 

It is important to note that the varying chemical compositions found in PM concentrations affect 

different aspects of the ecosystem. As discussed in Section 2, PM is a complex pollutant made 

up of a number of compounds and originates from a variety of sources and processes. As such, 

specific chemicals and components of PM are linked to specific ecosystem affects. For example, 

elemental carbon and some crustal minerals are the most commonly occurring airborne particle 

components that absorb light. Reduced visibility is caused by light absorption (as well as light 

scattering). In another example, toxic responses in plants and foliage have been documented 

when exposed to PM concentrations containing acids, trace metal content, or saline compounds. 

Also, different chemical compounds can negatively affect segments of the environment to varying 

degrees. One significant trace metal component of PM is mercury, which is toxic and can move 

readily through ecosystems and food chains. Mercury emitted from a smoke stack of a factory, for 

instance, may settle into soil, then be transported from soils into a nearby water body, accumulate 

in the bodies of plankton and other smaller fish who are then consumed by larger predators such 

as animals and humans, thereby transferring the mercury content all the way up the food chain.

In summary, the overall impact of PM on the environment depends upon many factors, 

including the chemical composition of the particles, meteorology, season of the year, and the 

characteristics and viability of the impacted ecosystem.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 59
Table 1-5: Summary of Negative Impacts of PM on Ecosystems, Visibility, and Materials

Effects of PM on: Effects: Economic/Social Ramifications:

Vegetation

Damaged leaf tissue; stunted 

growth; reduced starch storage 

capacity. 

Reduced crop yields; less robust growth of 

vegetation that humans, animals, and insects 

depend upon.

Soils Altered soil chemistry.

Plants absorb metals and other toxic elements, 

which are then transferred to animals & people 

that eat the plants. Can lead to an increase in 

invasive plant species. Polluted soil can run off 

into stream/water bodies.

Water & Aquatic 

Systems

Altered chemistry; increased 

acidity; increased mortality in fish, 

decreased stamina in fish.

Decline in fish populations; bio-accumulation 

effect which impacts plants, fish, and 

mammals from the bottom of the food chain to 

the top

Wildlife Lung damage; neurological 

damage; chromosome damage.

Stress on wildlife populations. Bioaccumulation of pollutants up the entire food 

chain.

Visibility Reduced visibility; aesthetic 

degradation.

Reduction in tourism & economic benefits from 

tourism. Reduced ability to enjoy scenic and/or 

historical vistas.

Materials Damage and degradation of 

building materials and finishes.

Increased maintenance costs; degradation of 

historic structures.

Property Values Reduction in property values. 

Economic disbenefit to homeowners & 

landlords. Reduction in local government tax 

revenue.

Effects of PM on Vegetation and Soils 

Airborne particles are deposited to a variety of surfaces, including soil and open ground; forests, crops, 

and other vegetation; water bodies such as lakes, streams, and oceans; and man-made surfaces such 

as buildings, roads, and parking lots. Particles are deposited via two processes: wet deposition (rain 

or snow) and dry deposition. Coarse particles typically are deposited by means of dry deposition, and 

generally settle close to the location where they were emitted. Conversely, fine particles are typically 

deposited via wet deposition and may travel farther from the point of origin. 

PM can affect plant life through direct deposition on surfaces, or indirectly through altered soil 

chemistry. When directly deposited onto vegetation, PM can affect the metabolism and photosynthesis 

of plants by blocking light, obstructing stomata apertures, increasing their temperature, and altering 

pigment and mineral content. In general, the toxic responses documented in plants after exposure to 

high levels of PM are typically associated with acidity, trace metal content, or salinity of the deposited 

particles. Fine PM has been shown to enter the leaf through the stomata, penetrate the structure of the 

60 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
leaf, and alter its chemistry. Coarse PM can form a “crust” on the leaf, which reduces photosynthesis, 

damages the leaf tissues, inhibits new growth of the tissue, and reduces starch storage. 

The soil environment is one of the most dynamic sites of biological interaction in nature. Deposition 

of PM on soil can have a negative effect not just on the chemical composition of the soil itself, but 

also upon the plants which grow in it, the animals and people who eat those plants, and even nearby 

groundwater systems which run through the impacted soils. This phenomenon, whereby a substance 

(such as trace metal) moves up the food chain and becomes more concentrated during ascension, is 

also known as biomagnification. In general, plant growth is negatively impacted by the presence of trace 

elements and heavy metals in soils which can then enter the plant tissue. As the plants absorb heavy 

metals and other pollutants via PM deposition into the soil, this can have a biomagnification effect and 

negatively impact the health of the people or animals that eat them. 

PM may have another potential impact on soil, flora, and fauna, to the extent that the ammonium nitrate 

component of PM acts as a source of reactive nitrogen. Deposition of reactive nitrogen on land acts as an 

unintended fertilizer which can have impacts on terrestrial flora and fauna. A growing body of literature 

documents the impacts of nitrogen deposition on ecosystems in western states. Studies demonstrate that 

increased nitrogen deposition is negatively affecting native plant communities which are adapted to live in 

low-nitrogen environments. These changes have enhanced invasion of exotic plant species such as annual 

grasses. Of the 225 plant species in California listed as threatened or endangered by the state or federal 

government, 101 are exposed to levels of nitrogen suspected of causing ecological disruption (Weiss, CEC 

2006). In areas where reactive nitrogen is deposited on nutrient-poor soil, this can facilitate the expansion 

of invasive, non-native species that choke out native plants. As the flora changes, animal species that 

depend on the native vegetation may be adversely impacted. 

The case of the Bay Checkerspot Butterfly, which has been on the federal endangered species list 

since 1987, provides an example of the impact of reactive nitrogen on diversity of native flora and 

fauna. The Checkerspot depends on native grasses that grow on nutrient-poor serpentine soils. The 

serpentine ecosystem provides food for both the larval and adult stages of the butterfly. Edgewood 

Natural Preserve in San Mateo County historically supported a healthy population of Checkerspots. 

However, nitrogen deposition from vehicles on Interstate 280, which is adjacent to the Preserve, has 

allowed aggressive, non-native grasses, such as Italian rye grass, to crowd out native grass species in 

recent years (Weiss 2002). As a result of habitat reduction, the Checkerspot population at Edgewood 

is in jeopardy.

PM and Acid Rain

Acid deposition, more commonly known as acid rain, is a widespread problem which effects water 

quality and ecosystems, and its effects have been well studied. Acid rain is a broad term which refers 

to a mix of wet and dry deposited particles from the atmosphere which contain high proportions of 

nitric and sulfuric acids. The precursors of such acids result from both natural and manmade sources. 

Natural sources include volcanoes and decaying vegetation, and manmade sources include emissions 

of SO2 and NOx from fossil fuel combustion. Figure 1-6 shows the process whereby natural and manmade emissions combine in the atmosphere to produce acid rain.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 61
Figure 1-6 Formation and Deposition of Acid Rain

Acid Rain. Source: U.S. Environmental Protection Agency

Acid rain has negative effects on soil, water (freshwater and saltwater), aquatic ecosystems, and 

building materials. Regions where a high percent of ambient PM2.5 is composed of secondary particles 

such ammonium sulfate and ammonium nitrate (e.g., the eastern US) are more likely to experience 

greater negative impacts of acid rain. While a majority of the PM2.5 in the Bay Area is attributable 

to primary PM from wood smoke and fossil fuel combustion, during winter months a large portion of 

PM2.5, on the order of 35%, is composed of ammonium nitrate. Accordingly, while acid rain is not a 

serious problem in the western United States, due to the levels of ammonium nitrate in overall PM 

concentrations in the Bay Area, it is worthwhile to take precautions. 

On land, acid rain can damage trees, especially at higher elevations, where exposure to acid-heavy 

clouds and mist is greater. The ability of a forest to cope with acid rain depends on the buffering 

capacity of its soil. Acid dissolves and removes the nutrients in forest soils before trees and other 

plants can use them to grow. At the same time, acid rain causes the release of substances that are 

toxic to trees and plants, such as aluminum, into the soil. 

While acid rain is not a serious problem for water bodies in the Bay Area, because primary PM and 

PM precursors can travel a considerable distance in the atmosphere before depositing elsewhere, 

pollution emitted in the Bay Area may impact ecosystems in downwind areas including the Sierra 

Nevada. According to a National Parks Service report,12 acid rain and snow is not as serious a 

problem in the Sierra Nevada as in the eastern U.S. or the Colorado Rockies. However, many highelevation Sierra lakes have low buffering capacity (ability to cope with acid), so it is important to 

minimize any future acid deposition. 

12 See http://www.nature.nps.gov/air/Pubs/pdf/techInfoEpaDeposition.pdf 

62 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
According to the National Acid Precipitation Assessment Program Report to Congress (2011) and 

the U.S. Environmental Protection Agency, numerous negative ecosystem effects are attributed to 

increased acid deposition, including:

• Impaired visibility;

• Acidification of lakes and streams, which has a cascading effect onto fish in 

terms of reductions in total population, hardiness of the fish, age distribution, 

and size; 

• Reduction in plankton biodiversity (specific to the western US);

• Reduction in acid neutralizing capacity;

• Decrease in pH (increase in acidity level) which can affect the ability of 

certain plant, insect and aquatic species to survive;

• High levels of nitrates in water which are toxic to aquatic life; and

• Depletion in oxygen levels of the water from accelerated plant life/death.

• Slower growth, injury or death of forests and plant species from altered soil 

chemistry, and/or damage to leaves or plant organs;

• Increases in atmospheric nitrogen deposition which tends to decrease 

species diversity (particularly in alpine plant communities); and

• Degrading effect on built structures and monuments, particularly those made 

of limestone, marble, lime mortars and carbonate-cemented sandstone.

Studies to date have found that the rise in CO2 concentrations in oceans via absorption, which 

causes decreases in ocean pH and alkalinity, is the major issue regarding ocean acidification. 

However, acid rain also contributes to ocean acidification. Approximately one-third of all nitrogen oxide 

emissions end up in the oceans. The contribution of acid rain to ocean acidification is likely greater in 

coastal regions such as the Bay Area, where the acidifying effect of nitrogen oxides can be as high as 

10 to 50 percent of the impact of carbon dioxide (Doney, 2007). Studies have shown that increased 

acidity interferes with the formation of the shells and skeletons in coral, crabs, marine snails, and 

clams (World Wildlife Fund 2011). 

Effects of PM on Water, Aquatic Systems, and Wildlife

As previously discussed, some components of anthropogenic (manmade) PM such as trace metals 

have a particularly damaging effect on ecosystems, including mercury, a significant trace metal 

component of PM that moves readily through ecosystems; as well as pesticides and polyaromatic 

hydrocarbons (PAHs). Once deposited, these pollutants may travel through the snow pack and feed 

into the water system. Deposition of PM containing these compounds has been found in the Sierra 

Nevada mountains in California, the major source of the state’s water supply. 

Some environments produce PM concentrations more toxic than others. In urban areas, motor 

vehicles emit toxic metals and other particles which are deposited on roads and parking lots, 

from where they are washed into the streams and bays, thus degrading nearby water quality. For 

example, particles from tire wear are a significant source of zinc, and brake pad wear is a significant 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 63
source of copper (Stolzenbach 2006). Copper from brake pads is toxic to aquatic organisms such 

as phytoplankton that serve as the foundation of the food chain, thus affecting the health of entire 

ecosystems. Elevated copper levels may also be one of the factors contributing to the decline of 

salmon populations.13

Physiological responses of fish to higher pollutant levels include increased mortality rates, 

chromosomal damage, retarded growth and development, and disruption of normal biological 

functions, including reduced stamina for swimming and maintaining positions in streams. An increase 

in concentrations of certain heavy metals such as aluminum, nickel, cadmium, copper, and mercury 

can poison fish and shellfish, and those who prey upon fish/shellfish.

Deposition of PM on land and water can have a range of negative impacts on ecosystems and 

wildlife from the bottom of the food chain to the top, due to the process of bio-magnification or 

bioaccumulation. For example, in addition to impairing the health of fish populations, air pollutants 

deposited in the aquatic environment can damage the broader ecosystem. To the extent that PM and 

related air pollutants are deposited in water and then absorbed by fish, frogs, snails and other marine 

life, these then travel up the food chain, increasing in concentration with each step up the ladder, to 

fish-eating predators including bald eagles, osprey, otters, pelicans, and grizzly bears. 

PM and Visibility (Haze)

Particulate matter is a major cause of reduced visibility, or haze, in both urban and rural areas. Haze 

is one of the most visible manifestations of air pollution. Reduced visibility is of special concern in 

areas of great natural beauty such as national and state parks and wilderness areas. In addition to 

detracting from the aesthetic enjoyment of vistas and landscapes, haze can have negative economic 

impacts in areas that depend on tourism. The emissions that create haze in parks and wilderness 

areas often originate elsewhere, sometimes from distant urban areas. For example, 33% of the haze 

found in the Grand Canyon is attributed to sources of particulate pollution in California.

To address regional haze problems, the US EPA created a Regional Haze Program and issued 

regulations to improve visibility, particularly in national parks and recreation areas. The original 

regulations, adopted in 1999, required states to develop plans to address the emissions that 

contribute to regional haze. In addition, all 50 states were required to submit a regional haze state 

13 For discussion regarding the impact of copper from brake pads on water bodies, see http://www.suscon.org/bpp/#

64 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
implementation plan by December 2007 to demonstrate each state’s long-term strategy for making 

reasonable progress towards achieving natural visibility conditions. In June 2012, US EPA proposed 

to issue a new urban visibility standard to provide increased protection from particle-induced haze 

in urban areas. As discussed in Section 3-B, the proposed standard would measure visibility on the 

basis of light extinction as expressed in units called deciviews.

Haze is caused when fine particulates in the air scatter and absorb sunlight. Some light is absorbed 

by particles, while other light is scattered away before it reaches the observer. More particles lead to 

greater absorption and scattering of light, reducing visual clarity and color. Some types of particles, 

such as sulfates, scatter more light. Haze-causing particles come from a variety of both manmade 

and natural sources, including windblown dust, wildfires, motor vehicles, electric utility and industrial 

fuel burning, and so on. Some particles which cause haze are produced primarily, while others are 

produced secondarily. 

Visibility is closely tied to wind and weather conditions. Wind affects how pollutants are mixed and 

dispersed. On very windy days, the air is normally clear because particles are well dispersed. On days 

when surface winds are present but weaker, particles usually form a plume which causes reduced 

visibility. When no surface wind is present, haze typically forms near the ground and continues to build 

as long as the stagnant condition persists. These conditions are most conducive to reduced visibility. 

Key sources contributing to the formation of haze include combustion of fossil fuels or biomass 

burning in electric utilities, manufacturing processes, and transportation. Natural sources of haze 

include wildfires, volcanoes and wind-blown dust. As a result of regulations to reduce emissions of 

PM2.5 and PM10, visibility has improved in many US cities and national scenic areas in recent years.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 65
Reduced visibility may also impose monetary costs in terms of reduced property values, and 

negative impacts on tourism. An analysis performed by the South Coast AQMD for its 2007 Air 

Quality Management Plan (AQMP) found that visibility improvements related to achieving compliance 

with federal PM2.5 standards would provide an estimated $3.6 billion benefit per year in terms of 

increase property values in the South Coast air basin. South Coast staff analyzed the sales price of 

owner-occupied single family homes between 1980 and 1995 and found that visibility has an impact 

on property values. Their analysis demonstrated a willingness on the part of home-buyers to pay a 

premium for visibility. (Final Socioeconomic Report for the 2007 AQMP, June 2007, SCAQMD)

Effects of PM on Materials and Property Values

In addition to negative impacts on soil and water quality and ecosystems, PM and other air pollutants 

also damage the man-made built environment. Based on the available evidence, US EPA has 

determined that a causal relationship exists between PM and damage to building materials and 

other surfaces. Exposure to air pollutants can accelerate the natural wear and tear on buildings from 

wind, rain, moisture, and temperature changes, further damaging these surfaces. For example, PM 

deposition on buildings affects the durability of paint finishes and promotes discoloration, chalking, 

loss of gloss, erosion, and causes blistering and peeling of surface material. This requires costly 

cleaning or washing, and potentially re-painting, depending upon the soiled surface. The effect of 

PM deposition on national monuments and other cultural treasures and historical structures is of 

particular concern. Other effects on PM deposition on buildings and surfaces include:

• Enhanced weathering process on stone in 

combination with exposure to PM. Black 

crusts commonly develop from airborne 

particles deposited on stone surfaces; 

• Corrosion of metals and masonry;

• Soiling of motor vehicles and damage to their 

finish;

• Increased building maintenance and repair 

costs; and

• Reduced property values.

Additionally, PM contributes to the formation of acid rain, which has a serious effect on structures 

and monuments, particularly those made of limestone, marble, lime mortars and carbonatecemented sandstone.

The 2007 AQMP prepared by the South Coast AQMD also quantified the damage to wood and 

stucco surfaces of residential properties, as well as the cost of household cleaning, from PM2.5 

emissions at eight locations in southern California. The total benefit of the decrease in costs for 

repainting stucco and wood surfaces, cleaning, and replacing damaged materials is projected to 

be $204 million, on average, every year between 2007 and 2025. Further, this figure is likely to 

be understated, because it takes into account only residential buildings. As previously discussed, 

66 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
damage to historic buildings or monuments, such as statues, cemetery gravestones, and the like, as well as nonresidential buildings, occurs from exposure to PM2.5. Taking into account other building types, especially special 

and historic ones, is likely to greatly increase the benefit from reduced PM2.5 emissions. (Final Socioeconomic 

Report for the 2007 AQMP, June 2007, SCAQMD)

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 67
Section 2: PM

Technical information

This chapter provides technical information about particulate matter, including PM size ranges, 

PM formation and dynamics, the results of PM air quality modeling, PM speciation data as to 

the contribution of key emission sources to ambient concentrations of PM, and the Bay Area PM 

emission inventory.

PM Characteristics

The term particulate matter (PM) encompasses a diverse assortment of microscopic airborne 

particles. Many air pollutants, such as ozone or carbon monoxide, consist of a single molecule or 

compound. PM, by contrast, includes a potpourri of disparate particles that vary greatly in terms 

of their size and mass, physical state, chemical composition, toxicity, and how they behave and 

transform in the atmosphere. 

A variety of chemical & physical processes are involved in PM formation and transformation. 

Because PM is so heterogeneous and dynamic, this presents technical challenges in terms of 

measuring emissions and ambient concentrations, estimating population exposure, determining 

PM health impacts, assessing PM impacts on ecosystems and climate change, and devising 

appropriate control strategies.

PM typically consists of a mixture of solid particles as well as liquid droplets known as aerosols. The 

components of PM include elemental carbon, organic carbon, and trace metals; compounds such 

as nitrates, organics, and sulfates; and complex mixtures such as diesel exhaust, wood smoke, and 

geologic dust. Types of particles include:

• Dusts and fibers generated by handling, grinding, abrasion or cutting of bulk 

materials.

• Mists composed of liquid droplets generated by condensation from a 

gaseous state or breaking up of bulk liquid.

• Smoke produced by incomplete combustion of carbonaceous materials.

• Fumes composed of solid particle generated by condensation of vapors or 

gases from high temperature processes.

• Bio-aerosols composed of solid or liquid particles from biological sources.

Particles emanate from a variety of man-made processes and sources, such as fuel combustion, as 

well as from natural sources, such as wildfires, volcanos, and sea salt. Particulate matter is generated 

both indoors and outdoors. Emission sources that affect ambient (outdoor) air are described in the 

68 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Emissions Inventory section below. Indoor sources of PM emissions, which include cooking, heating, 

fireplaces, appliances, smoking, and consumer products, are discussed in Section 1-B. 

Particle Size

PM is commonly characterized on the basis of particle size. Figure 2-1 shows the various particle 

sizes in comparison to the diameter of a human hair.

• Ultrafine PM (PM0.1) includes the very smallest particles. The term 

generally refers to particles less than 0.1 micron in diameter (one micron 

equals one-millionth of a meter, or m x 10-6). Because ultrafine particles are 

so small, their size is often described in nanometers, or billionths of a meter 

(m x 10-9). By definition, the largest ultrafine particles measure 0.1 microns 

or 100 nanometers in diameter, but many ultrafine particles are as small as 

3 nanometers to 20 nanometers at the time they are emitted.

• Fine PM or PM2.5 consists of particles 2.5 microns or less in diameter 

(including ultrafine PM). 

• Coarse PM refers to particles between 2.5 microns and 10 microns in 

diameter. The term “coarse” particles may be misleading; it should be 

emphasized that even “coarse” particles are still very tiny, many times 

smaller than the diameter of a human hair.

• PM10 consists of particles 10 microns or less in diameter (including 

ultrafine, fine and coarse PM).

• Total suspended particles (TSP) includes particles of all sizes, including 

particles larger than 10 micron in diameter.

Figure 2-1 Comparison of PM10, PM2.5, and Ultrafine PM

Source: California Air Resources Board

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 69
These distinctions based on particle size are important, because the different size ranges 

vary in terms of how the particles within each size range are formed and emitted; how long 

they remain suspended in the atmosphere and how far they travel; how easily they can evade 

the body’s defenses and how deeply they can penetrate into the lungs and key organs; and 

the mechanisms by which the particles are removed from the air. However, although particle 

size is a useful way to categorize PM, it should be emphasized that the particles within each 

size range are by no means homogeneous. The diameter and mass of the particles within 

each of these ranges varies considerably, and there is great variation in terms of the chemical 

composition of the particles.

In terms of composition, coarse PM is generally dominated by geologic particles of dust and 

soil (from farms, quarries, mines, volcanos) and other particles of natural origin (sea salts, 

pollen, mold, spores, etc.). By contrast, fine and ultrafine PM are primarily the product of 

combustion and therefore contain compounds such as black carbon, sulfates, nitrates, acids, 

and metals which are more harmful to health.

Table 2-1 provides a summary of PM characteristics by particle size range.

Table 2-1 Characteristics of Particulate Matter by Size Range

Particle Size

Characteristics 

Key Attributes How Measured? Key Emission Sources

Ultrafine PM: Particles 

less than 0.1 micron 

in diameter 

Mostly primary PM.

Short-lived in atmosphere. 

Local impacts.

Produced by fossil fuel combustion. 

Particles can be inhaled deep into 

the body.

Particle count Motor vehicles, diesel 

engines.

Fine PM: Particles less 

than 2.5 microns in 

diameter

Combination of primary & 

secondary formation. Produced by 

fossil fuel combustion & woodburning. Particles can be inhaled 

deep into the body.

Mass; expressed in 

μg/m3 (micro-grams 

per cubic meter)

Wood-burning, motor 

vehicles, off-road engines 

& equipment, industrial 

processes & combustion.

Coarse PM: Particles 

between 2.5 and 10 

microns in diameter 

Mostly primary PM.

Relatively few particles on a number 

basis, but they account for about 

half of PM10 on a mass basis.

Mass; expressed in 

μg/m3 (micro-grams 

per cubic meter)

Geologic dust, brake and 

tire wear, residential woodburning, motor vehicles.

Relationship between Particle Count, Particle Mass, and Surface Area

The particles that comprise PM vary in both size (diameter) and mass (weight). Larger particles are 

much heavier than small ones; a single coarse particle may weigh more than thousands of ultrafine 

particles. Although larger particles account for most of PM on a mass basis, they represent only a 

70 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
small percentage of the total number of particles. Conversely, smaller particles contribute less PM 

mass, but dominate in terms of the number of particles. Ultrafine particles account for the greatest 

number of particles in PM, but only a small proportion of the total mass of PM. Ultrafine particles 

account for roughly 90% of the total number of particles (Wu et al. 2008). There tends to be little 

correlation between the number of particles (most of which are in the ultrafine and fine size range) 

in a given air sample and the mass of PM2.5 or PM10 in that sample. So measuring the mass of PM 

inthe air generally reveals little as to the number of particles that make up that mass. 

As shown in Figure 2-2, as particle size increases, particle mass (volume) increases much faster than 

the surface area. So a given mass of ultrafine PM will have a much greater particle number and total 

surface compared to an equal mass of fine PM or coarse PM. The huge number of fine and ultrafine 

particles suspended in the air collectively presents a great deal of surface area relative to their small 

mass. Surface area is a concern for two reasons. First, greater surface area means more surface 

to which microscopic airborne toxics can adhere. Fine and ultrafine particles coated with toxics can 

penetrate deep into the lungs when they are inhaled. Second, because of their relatively large surface 

area, once these fine and ultrafine particles enter the respiratory system, they interact with a large 

area of lung tissue; this means that they can do greater damage to the lungs (or other organs with 

which they come into contact).

Figure 2-2 Ratio of surface area to volume

Source: http://cikgurozaini.blogspot.com/2010/09/gas-exchange-in-animals.html

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 71
Primary PM versus Secondary PM

In addition to size ranges, PM is also categorized on the basis of how the particles are formed 

and emitted. Primary PM refers to particles that are directly emitted in solid or aerosol form. 

Secondary PM refers to particles that are formed in the atmosphere through chemical reactions 

among different pollutants. 

Primary PM includes black carbon (soot) and fugitive dust from a wide variety of sources, including 

cars, trucks, buses, industrial facilities, cooking, power plants, construction sites, tilled fields, paved and 

unpaved roads, rock quarries, and burning wood. Some primary particles are emitted directly from a 

tailpipe or smokestack in particle form. However, primary PM also includes condensable PM (discussed 

below) which is formed when organic compounds that are emitted as hot gases condense into particles 

upon exposure to cooler ambient air. Organic carbon is the largest directly-emitted constituent of Bay 

Area PM2.5; its main sources are wood-burning, fossil fuel combustion, and cooking.

Secondary PM describes particles formed indirectly via chemical processes when precursor 

pollutants, such as sulfur oxides (SOx), nitrogen oxides (NOx), volatile organic compounds (VOC), 

and ammonia (NH3), react in the presence of sunlight and water vapor. These precursor pollutants 

are emitted from fuel combustion, industrial processes, household activities, agriculture, natural 

vegetation, and other sources. Combustion of fossil fuels produces NOx, which converts to nitric 

acid (NO3) and combines with ammonia (NH3) in the atmosphere to form ammonium nitrate, as well 

as sulfur dioxide (SO2), which converts to sulfuric acid (H2SO4) and combines with ammonia to form 

ammonium sulfate. In determining whether ammonia is a significant contributor to PM formation, the 

key question is which pollutant – ammonia, or NOx (in the form of nitric acid) - is the limiting factor in 

ammonium nitrate formation. 

These secondary compounds account for roughly one-third of Bay Area PM2.5 on an annual-average 

basis and approximately 40-45% during winter peak periods. Ammonium nitrate, which is stable in 

solid form only during the cooler winter months, contributes an average of about 40% of total PM2.5 

under peak PM conditions. The contribution of ammonium sulfate to Bay Area PM2.5 is relatively low, 

accounting for approximately 10% of total PM2.5 on an annual-average basis. 

The distinction between primary PM and secondary PM is important for understanding and analyzing 

how the various emissions sources contribute to ambient PM concentrations. However, in ambient 

air where particles are constantly interacting and transforming, most individual particles are actually 

composed of a mix of primary and secondary PM. An individual primary particle typically has a core of 

carbonaceous material, often containing trace metals and other toxic materials. Layers of organic and 

inorganic compounds are then deposited onto the core particle. Depending on the composition of the 

material deposited on the core particle, the particle may become more toxic as it grows in size. 

Physical Processes that Affect PM Formation

In addition to chemical processes in which precursor compounds react to form secondary PM, 

several physical processes also play an important role in determining how particles interact and 

transform while suspended in the air. The processes that affect PM vary depending on particle size. 

72 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Key physical processes that affect the formation of fine and ultrafine particles at the micro scale 

include condensation, nucleation, and coagulation. These processes occur very rapidly, especially 

in the initial seconds after a plume of emissions is released by a combustion process. During “plume 

processing”, the hot particles and gases produced by combustion interact vigorously upon exposure 

to cooler ambient air. As a result of these processes, particle count and particle size distribution 

can change very rapidly. These physical processes are especially potent among ultrafine particles; 

because ultrafine particles are so numerous and have a great deal of surface area, they interact and 

agglomerate more rapidly than other types of PM. Therefore, ultrafine PM is very dynamic and shortlived in the atmosphere. 

Condensation and nucleation are related processes. Combustion processes emit a great variety 

of organic compounds in gaseous form. Upon exposure to cooler ambient air, these hot gases 

seek to condense. When the hot gases condense by adhering to existing particles, this is called 

condensation. When the hot gases condense by forming new particles, this is called nucleation. 

There is competition between condensation and nucleation.14 If the ambient air already contains an 

abundance of fine particles, then the hot gases will generally condense on to the existing particles. 

However, if the supply of existing particles is limited, then in the absence of existing particles on 

which to condense, the gaseous emissions will nucleate to form new particles, primarily in the 

ultrafine size range. The number of particles produced as hot gases condense thus depends in large 

part upon the supply of pre-existing particles in the air.

Because the presence of existing particles promotes condensation (instead of nucleation which 

forms new particles), this means that not only is PM2.5 mass concentration a poor surrogate for 

ultrafine particle count, but that PM2.5 mass and ultrafine particle count may actually be negatively 

correlated. The number of new ultrafine particles produced as a by-product of combustion will 

generally be low when the existing PM2.5 mass concentration is high, and vice versa. This helps to 

explain measurements showing that when particulate filters are installed on diesel engines, the mass 

of PM2.5 emitted by diesel combustion is greatly reduced, but the number of ultrafine particles in the 

diesel exhaust may actually increase (Van Setten et al. 2001). 

Coagulation occurs when two or more existing particles join to form a larger particle. Coagulation is 

very prevalent among ultrafine particles, but tends to decrease as particles grow to a larger, more 

stable size. Ultrafine particles produced by combustion coagulate very rapidly to form larger particles 

upon exposure to ambient air. Coagulation reduces particle number and increases particle size, but 

does not affect overall PM2.5 particle mass. Due to the processes of coagulation and condensation, 

the number of ultrafine particles tends to drop off rapidly as distance from the emission source 

increases, whereas PM2.5 mass is more stable.

PM is removed from the air through processes such as diffusion, coagulation, and deposition. 

Because ultrafine particles are so small that they are only weakly affected by the force of gravity, 

they are removed mainly by diffusion, in which their random thermal motion (known as “Brownian 

motion”) causes the particles either to adhere to man-made or natural surfaces or to adhere to other 

14 For purposes of developing PM emissions inventories, the particles formed by both the condensation and the nucleation processes are 

referred to as “condensable” emissions.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 73
particles (coagulation). As particles grow larger and heavier via coagulation, they are eventually 

deposited to the earth’s surface by means of gravity through the processes of dry deposition or wet 

deposition (rain and snow). Fine and ultrafine particles often provide a nucleus that facilitates the 

condensation of water vapor in the atmosphere, thus forming water droplets; so these particles can 

also be removed from the air via wet deposition. Although beneficial for purposes of clearing the air, 

PM deposition may have negative impacts on soil, water, flora and fauna, as discussed in Section 1-D. 

Spatial Variation in PM Concentrations (Concentration 

Relative to Distance from Emission Source)

The ambient concentration of a directly-emitted air pollutant, such as primary PM, generally 

decreases rapidly via dispersion as distance from the emission source increases. This means that 

concentrations of primary pollutants will vary considerably on a spatial basis. By contrast, ambient 

concentrations of pollutants that are formed by means of chemical processes in the atmosphere, 

such as ozone and secondary PM, are not so directly related to distance from the emission source; 

these secondary pollutants tend to be more broadly and evenly distributed on a spatial basis.

Since ultrafine PM is composed mainly of primary PM, the number of ultrafine particles typically 

decreases rapidly as distance from the emission source increases. Fine PM (PM2.5) and coarse PM, on 

the other hand, include a mix of both primary and secondary particles. So concentrations of primary PM 

from emission sources such as engine combustion and wood-burning can vary greatly at the local scale, 

whereas the distribution of secondary PM such as ammonium nitrate and ammonium sulfate tends to 

be more uniform across a region.

Relationship between PM and Toxic Air Contaminants

Air pollutants are generally regulated either as criteria air pollutants or as

toxic air contaminants (TACs). Criteria pollutants are generally controlled on 

a regional scale in an effort to attain air quality standards which are based 

on ambient concentrations in the atmosphere. TACs are generally present 

in the atmosphere only in very low concentrations. But because of their high 

toxicity, TACs are regulated at the emissions source so as to limit individual 

exposure on the basis of risk-based standards; for example, a maximum 

cancer risk no greater than 10 in one million. Although PM is categorized 

and regulated as a criteria air pollutant, PM displays some characteristics 

of a TAC to the extent that it acts as a local air pollutant. Areas of overlap 

between fine PM and toxic air contaminants (TACs) include the following:

• In the case of both fine PM and TACs, exposure to even small 

amounts of the pollutant can cause negative health effects;

• PM and TACs share common emissions sources, such as 

combustion of fossil fuels and biomass;

• Diesel PM has been identified as a TAC by the California Air 

Resources Board; 

74 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
• Some air toxics may be emitted in particle form, such as cadmium emitted 

from combustion of fossil fuels;

• TACs and PM are fellow travelers; air toxics frequently adhere to fine 

particles and then enter the lungs when these particles are inhaled. 

PM Formation & Dynamics in the Bay Area

The basic chemical and physical processes described above that govern PM formation and 

transformation at the micro level hold true for the Bay Area. However, local meteorology and climate, 

the specific mix of PM sources and their geographical distribution within the region, and air exchange 

with neighboring air basins all influence PM formation and dynamics in the Bay Area.

Temporal & Seasonal Variation in Bay Area PM Levels

Ambient PM in the Bay Area varies considerably both in composition and spatial distribution on a 

day-to-day basis and on a seasonal basis, due to changes in emissions and weather. Changes in 

meteorological conditions are the most important factor in explaining the day-to-day and seasonal 

variation in PM concentrations. The Bay Area experiences its highest PM concentrations in the winter; 

exceedances of the 24-hour national PM2.5 standard almost always occur from November through 

February. High PM2.5 episodes are typically regional in scale, impacting multiple Bay Area locations. 

During other seasons, by contrast, Bay Area PM2.5 levels tend to be relatively low, due largely to the 

region’s natural ventilation system. Thus, on an annual-average basis, PM2.5 levels in the Bay Area 

are among the lowest measured in major U.S. metropolitan areas.

Meteorological factors are the main reason that the Bay Area experiences its highest PM levels 

in winter months. However, it is important to note that winter is also the season when the most 

residential wood-burning occurs; in some parts of the Bay Area, wood smoke accounts for the 

majority of airborne PM2.5 during high PM episodes. In addition to higher wood smoke emissions, 

secondary PM2.5 levels are also elevated during the winter months. Cool weather is conducive to 

the formation of ammonium nitrate which contributes an average of about 40% of total PM2.5 under 

peak PM conditions. 

Studies in Southern California have found that seasonal variation in meteorological conditions 

also affect emissions and concentrations of ultrafine particle. Similar to PM2.5, ultrafine particle 

numbers are higher in winter compared to spring and summer. This is likely due to fact that lower 

temperatures promote particulate formation; condensable organics emitted as hot vapors from 

tailpipes and other combustion sources quickly cool and condense to form particles. One study 

(Zhang et al. 2005) found that the ultrafine particle formation rate from vehicle exhaust is higher in 

winter than in summer because average particle size is smaller in winter (~ 10 nanometers) than in 

summer (~ 60 nanometers).

While PM emissions and concentrations tend to be highest in winter for the reasons described above, 

meteorological conditions primarily determine whether the concentrations will build up to levels that 

exceed the national 24-hour PM2.5 standard. Horizontal mixing (i.e., surface winds) and vertical mixing 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 75
(which occurs when air temperatures increase with height) are the key to dispersing particulates in the 

atmosphere, and thus keeping ambient concentrations below the PM standards. Winter is associated 

with decreased atmospheric mixing height and more stagnant, less windy weather. 

Weather & Wind Patterns Conducive to High PM Concentrations

The Central Valley (comprised of the Sacramento and San Joaquin valleys) borders the Bay Area 

to the east, and there is considerable air exchange between them. In summer, the typical pattern 

is westerly winds blowing from the Bay Area into the Central Valley. In the winter, during periods of 

stagnation when PM concentrations are high, the pattern tends to reverse with easterly flow draining 

from the Central Valley through the Bay Area and into the Pacific Ocean. 

Using a technique called cluster analysis to identify correlations between weather conditions and 

PM concentrations in historical data, Air District staff collaborated with UC Davis staff to analyze 

how meteorological conditions impact Bay Area PM levels during winter months. Weather systems 

conducive to high PM2.5 levels have shallow flows through the complex terrain that form stagnating 

cold pools in valleys that trap pollutants. Consecutive stagnant and rainless winter days are typically 

prerequisites for development of elevated PM2.5 episodes. 

A single weather pattern was found to account for approximately 80% of all 

Bay Area PM2.5 exceedances. This pattern is characterized by a ridge of 

high pressure moving over the Bay Area during a period of multiple days. 

This system leads to calm conditions within the Central Valley, coupled with 

persistent easterly winds from the Central Valley into the Bay Area. The lower 

levels of solar radiation (sunlight) in the winter lead to stronger temperature 

inversions; these inversions prevent vertical mixing and are therefore 

conducive to the buildup of PM in ambient air near ground level. 

During this weather pattern, PM levels in the Central Valley can be about 

2-3 times higher than in the Bay Area. This is largely because meteorological conditions are more 

conducive to the transformation of NOx to nitric acid (needed for the formation of ammonium nitrate) 

in the Central Valley than for coastal locations. Conditions that enhance daytime and nighttime 

conversion of NOx to nitric acid include low wind speeds combined with abundant sunlight during 

the day and high humidity at night. Nitric acid then rapidly reacts with ammonia emissions, mostly 

from dairy activities, which are especially concentrated in the northern San Joaquin Valley, to form 

ammonium nitrate. The easterly winds that prevail during this pattern transport both primary and 

secondary PM from the Central Valley into the Bay Area, as further discussed in the modeling section 

below. 

Although the weather pattern described above accounted for over 80 percent of all Bay Area 

exceedances, it should be noted that only around one in three days belonging to this pattern resulted 

in an exceedance. Therefore, this weather pattern constitutes a necessary, but not sufficient, 

condition for an exceedance to occur. Days with this pattern were further analyzed to distinguish the 

characteristics of days that result in exceedances of the 24-hour PM2.5 standard. Exceedance days 

could be defined in terms of a number of simultaneous meteorological characteristics: a ridge of high 

76 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
pressure over the Bay Area from strongly stable conditions aloft providing a weak surface pressure 

gradient over Central California; persistent shallow flows from the east through the Bay Area; winds 

channeled by terrain; enhanced nocturnal cooling under clear-sky conditions leading to enhanced 

overnight drainage flows off the Central California slopes; and at least two consecutive days of these 

listed conditions.

PM Photochemical Modeling Results 

The Air District performs photochemical modeling to better understand the complex relationship 

between emissions, ambient concentrations, and population exposure to air pollutants. Air District 

staff has employed EPA’s Community Multiscale Air Quality (CMAQ) model to simulate PM2.5 

formation and dynamics in the Bay Area. PM2.5 simulations were performed with the CMAQ model 

for four months (December-January, 2000-01 and 2006-07). The modeling domain included the 

Bay Area and the entire Central Valley to account for the impact of inter-basin transport. The model 

was applied on 4-km horizontal grids. The results of the PM modeling have been summarized in the 

October 2009 report entitled Fine Particulate Matter Data Analysis and Modeling in the Bay Area.

Figure 2-3 shows the spatial distribution of simulated primary and secondary PM2.5 concentrations 

around the Bay Area. These results were averaged across the 52 simulated days for which measured 

Bay Area 24-hour PM2.5 levels exceeded 35 μg/m3. For most of these episodic days, light winds 

flowed through the Bay Area from the east, and Central Valley conditions were near calm. Primary 

PM2.5 levels were elevated mainly in and around major Bay Area cities, including Oakland, 

San Francisco and San Jose; near industrial facilities and highways along the Carquinez Strait; 

at Travis AFB; and Santa Rosa. Secondary PM2.5, present mostly as ammonium nitrate, was not 

localized near the sources of its precursor emissions, NOx and ammonia. Rather, secondary PM2.5 

was regionally elevated. A sharp gradient existed, with very high secondary PM2.5 levels in the 

Central Valley decreasing westward through the Bay Area. Around San Francisco and San Jose, PM2.5 

levels were dominated by primary (directly-emitted) PM. For other areas affected by PM episodes, 

such as the eastern, northern, and southern Bay Area and also the Delta, primary and secondary 

PM2.5 levels were comparable. Both primary and secondary build-up were required for exceedances 

to occur in these locations.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 77
Figure 2-3 Spatial distribution of simulated 24-hr primary and secondary 

PM2.5 levels averaged across the 52 simulated days when 

measured Bay Area 24-hr PM2.5 level exceeded 35 μg/m3

(Bay Area counties and the California coastline are drawn using thick black lines. 

City limits for Sacramento and Stockton are drawn using thin black lines.)

As noted above, analysis of meteorological patterns found that more than 80% of Bay Area 

PM exceedances occur when easterly winds blow into the Bay Area from the Central Valley. 

Therefore, Air District staff also performed photochemical modeling to estimate the contribution 

of PM transport from the Central Valley during elevated PM episodes in the Bay Area. Transport 

impacts were evaluated for 55 days (from 2000-01 and 2006-07) having simulated base-case 

PM2.5 concentrations of 35 μg/m3 or higher. Anthropogenic Bay Area emissions were eliminated 

to estimate the cumulative transport impacts from all sources outside of the Bay Area. These 

simulations found that significant amounts of both primary and secondary PM2.5 in the form of 

ammonium nitrate were transported into the Bay Area. On days when the Bay Area exceeded the 

24-hour PM2.5 standard, modeling indicated that transported primary PM2.5 levels averaged 

as high as 8 μg/m3 and transported secondary PM2.5 levels averaged as high as 13 μg/m3. The 

largest transport impacts for both primary and secondary PM2.5 occurred along the eastern 

boundary of the Bay Area. 

The modeling also examined the sensitivity of ambient PM concentrations in response to 

hypothetical reductions in Bay Area emissions of primary PM2.5, as well as reductions in precursor 

pollutants (ROG, NOx, SOx, and NH3) individually and in combination. Reducing Bay Area primary 

(directly-emitted) PM2.5 emissions provided far greater reductions in ambient Bay Area PM2.5 

levels than reducing Bay Area secondary PM2.5 precursor emissions. Of the precursor emissions 

reductions simulated, Bay Area ammonia reductions were most effective. Reducing ammonia 

emissions by 20% was found to decrease PM2.5 concentrations by approximately 0.5 to 1.0 μg/

m3. The ammonia emissions reductions lowered the ammonium nitrate component of PM2.5 only 

for relatively cold winter days favoring ammonium nitrate buildup. (Ammonium nitrate PM2.5 tends 

to evaporate faster than it forms at temperatures above around 60 degrees Fahrenheit.) Combined 

78 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
NOx and ROG emissions reductions for the Bay Area were relatively ineffective. NOx emissions 

reductions were relatively ineffective because ammonium nitrate PM2.5 formation involves the 

relatively slow and incomplete conversion of NOx to nitric acid. Reducing Bay Area sulfur-containing 

PM precursor emissions typically had a small impact on Bay Area ambient PM2.5 levels. Under 

certain conditions, however, reducing Bay Area sulfur-containing emissions did decrease ambient 

Bay Area PM2.5 levels by approximately 1 μg/m3. 

The Air District also modeled wood smoke to estimate the impact of efforts to reduce residential 

wood-burning during the winter season. The modeling period included 8 of the 11 Spare the 

Air periods during the winter of 2008-09. Bay Area wood-smoke levels were simulated with and 

without wood-burning restrictions during these periods. Without burning restrictions during these 

Spare the Air periods, the simulations indicated that peak wood-smoke levels of up to 10-20 μg/

m3 would have occurred over the areas that generally have high wood-burning emissions. For 

many of the remaining populated locations within the Bay Area, wood-smoke levels would have 

been approximately 5 μg/m3. Peak benefits of the wood-smoke rule were about 10 mg/m3 of 

reduced wood smoke. Maximum simulated benefits of the wood-smoke rule occurred for areas that 

generally have the highest wood-smoke levels. Often, the areas most heavily impacted by wood 

smoke are away from the Air District’s PM monitoring locations. Therefore, reductions of population 

exposure to wood smoke resulting from the rule may be significantly greater than indicated by the 

monitoring data.

Source Contributions to Ambient PM Concentrations

Determining the relative contributions of various sources of direct PM2.5 emissions and PM2.5 precursors 

to total PM concentrations is complex. The Air District collects PM on filters at four monitoring sites 

(Livermore, West Oakland, San Jose, and Vallejo) for PM speciation purposes. The filters are then analyzed 

to estimate the contribution of various emission sources to the total ambient concentration of PM2.5 using 

a technique known as chemical mass balance (CMB).15 To estimate the overall contribution of various 

sources, the results of the source apportionment analysis are combined with emissions data from the 

2010 Emissions Inventory. The CMB analysis presented here updates a previous CMB analysis prepared 

in 200816 and includes data from July 2009 through December 2011. The goal was to represent the mix 

of PM2.5 sources as of 2010, including the reduced contributions from ships following the effective date 

of the ARB rule to require ocean-going ships to use low-sulfur fuel within 24 miles of the coast.

Figure 2-4 shows the estimated contributions to Bay Area annual PM2.5 concentrations using the 

average of the results from the four sites. Note that this includes contributions to secondary PM2.5; 

i.e., ammonium nitrate formed from NOx and ammonia, and ammonium sulfate formed from SO2 and 

ammonia. The source apportionment analysis shows that combustion of both fossil fuels and biomass 

are major PM2.5 contributors for all seasons. The largest contributor to annual PM2.5 is wood-burning, 

15 Chemical mass balance (CMB) analysis is a methodology in which a computer model is used to apportion ambient PM2.5 collected on 

filters over 24-hour periods at monitoring sites around the Bay Area to a set of source categories. Each filter was analyzed for a range of 

chemical species. The same species were measured in special studies of emissions from various sources, such as motor vehicles and 

wood burning. The CMB model finds the mix of these source measurements that best matches the ambient sample, chemical species by 

chemical species.

16 See report entitled Sources of Bay Area Fine Particles, April 2008 at:

www.baaqmd.gov/Divisions/Planning-and-Research/Research-and-Modeling/Publications/Reports.aspx

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 79
contributing about a quarter of the total. This is mainly from residential wood burning, but also includes 

wood smoke from controlled burns and wildfires. Nearly another quarter comes from on-road motor 

vehicles, 14% from gasoline and 8% from diesel. 

Figure 2-4 Estimated Source Contributions to Annual PM2.5 Concentrations

Cooking, petroleum refining, and domestic sources are also sizeable contributors, along with the 

naturally occurring sea salt from marine air. Domestic sources, for purposes of Figures 2-4 and 

2-5, include emissions from use of natural gas for residential heating and cooking, as well as the 

contribution to formation of secondary PM2.5 as a result of ammonia emissions from pet waste and 

human perspiration and respiration Contributions from ships have been cut by two-thirds or more, 

thanks to the ARB rule mandating low sulfur fuel near ports. The estimate for construction equipment 

has also dropped substantially because of a revision in ARB’s estimation method. Note that 

several sources – soils, livestock, and landfill – appear because of their ammonia emissions, which 

contribute to formation of ammonium nitrate and ammonium sulfate.

80 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Figure 2.5 shows the sources contributing to peak PM2.5 concentrations, specifically, the 10 samples 

at each monitoring site with the highest PM2.5 concentrations. Almost all the highest Bay Area 

PM2.5 concentrations occur in winter, so the wood-burning contribution is higher than the annual, 

representing almost 30% of the peak total. The biomass combustion contribution to peak PM2.5 

levels is about 3-4 times higher in winter than the other seasons, as confirmed by isotopic carbon 

(

14C) analysis, due to increased levels of wood burning during the winter season. Motor vehicles 

are also a larger contributor during winter months because their NOx emissions contribute most to 

the formation of secondary PM in the form of ammonium nitrate. Similarly, domestic sources are 

another large source of secondary PM because of their ammonia emissions. Marine air is a smaller 

contributor in the winter than on an annual-average basis, because winter winds are frequently 

offshore, unlike the typical on-shore pattern the rest of the year.

Figure 2-5 Estimated Source Contributions to Peak PM2.5 Concentrations

PM Emissions Inventory

An emissions inventory is an essential tool for air quality planning. The inventory contains a detailed 

breakdown of the estimated emissions from each source category, thus providing information as 

to the source, magnitude, and location of emissions. Emissions inventories are used to perform 

air quality modeling, to identify source categories where there may be opportunities for additional 

emission reductions, and to predict trends in terms of future air quality. 

This section presents the Air District’s latest Bay Area inventory for primary emissions (directlyemitted particles) of both PM2.5 and PM10, as well as precursors that combine via chemical 

reactions to form secondary PM. These precursors include reactive organic gases (ROG), oxides 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 81
of nitrogen (NOx), sulfur oxides (SOx), and ammonia (NH3). The Air District released it’s first-ever 

emissions inventory for ultrafine PM in August 201217.

The inventory includes annual-average emissions, as well as emissions during the winter season 

when the Bay Area normally experiences its highest PM concentrations. In addition to the inventory 

for base year 2010, projected emissions for future years out to 2030 are also provided. Tables in 

Appendix A provide the detailed inventory showing estimated emissions by source category. 

This inventory does not include “condensable” emissions that also form PM. Condensables are 

emissions which are emitted in a gaseous phase, but then condense to form particles upon 

exposure to cooler ambient air, as discussed in the section above entitled Physical Processes 

that Affect PM Formation. Depending on the emission source and combustion conditions, 

this condensable component may account for a significant share of overall PM emissions for 

certain emission sources. Recent US EPA guidelines call on air quality agencies to consider the 

condensable component of PM in developing PM emission inventories and control strategies. 

EPA has published a source test method to be used for source testing stationary sources (with 

a few minor exceptions). However, test methods and methodologies to estimate condensable 

emissions are not available at the current time for certain emission source categories. Therefore, 

in the interest of methodological consistency, the Air District has chosen to exclude condensable 

emissions in the current inventory. The Air District will work with ARB and other partners to develop 

estimates of condensable emissions for future PM inventories.

Methodologies Used to Estimate Emissions

The estimated emissions provided in the inventory are based upon data from source tests, published 

emission factors, and engineering calculations. Emission inventories are revised and improved on a 

regular basis. The PM inventory provided in this report differs significantly from the inventory provided 

in the Bay Area 2010 Clean Air Plan (2010 CAP). This is due to the fact that, since the release of the 

2010 CAP, ARB and Air District staff reviewed and improved the methodologies used to estimate 

emissions from several of the most important PM source categories, as explained below. 

Stationary (Point) Sources

Actual 2010 reported emissions from permitted facilities are included in this inventory. Examples 

of stationary sources that emit primary PM and/or PM precursors including oil refineries, metal 

smelters, charbroilers, and back-up generators used to supply emergency power at many facilities. 

Residential Fuel Combustion

This category includes residential combustion of both wood and natural gas for space-heating, 

water-heating, and cooking. Most homes rely primarily (or completely) on natural gas (or electricity) 

for these purposes. Nonetheless, although only a minority of Bay Area households burn wood, 

PM from wood smoke is the largest single source of winter-time PM emissions in the Bay Area, 

and greatly exceeds the PM emissions from (the much more common) residential natural gas 

17 See the report entitled, Ultrafine Emissions Inventory for the San Francisco Bay Area, BAAQMD August 2012

82 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
combustion. Previous estimates of the amount and location of residential wood burning were 

developed based on a Bay Area 2005-2006 telephone survey on wood burning. In 2008 the Air 

District adopted Regulation 6, Rule 3 to limit emissions of particulate matter (PM) and visible 

emissions from wood-burning devices, as described in Section 4. The District also implemented 

a robust outreach effort to inform Bay Area residents about “no-burn days” and the detrimental 

health effects of wood smoke through its winter Spare the Air program. Annual surveys of Bay Area 

households, as well as chemical mass balance analysis of PM captured on filters, both indicate 

that wood-burning has decreased approximately 40% since 2008 when the Air District’s wood 

burning rule went into effect. Emissions from residential fuel combustion in this inventory have 

been revised to reflect this decrease in wood-burning.

Commercial Cooking

Previous estimates for this source category included condensable PM. The reason for this was that 

source testing conducted as part of the regulatory process to control emissions from this source 

category (via Regulation 6, Rule 2) included condensable PM. However, because methods to estimate 

condensable emissions for other source categories are not yet available, as discussed above, the 

condensable emissions for the Commercial Cooking source category have been omitted here for 

purposes of methodological consistency. 

On-Road Motor Vehicles 

Estimates for on-road motor vehicle emissions are based on ARB’s latest emissions factor 

model, EMFAC2011. In preparing EMFAC2011, ARB staff conducted major research to determine 

the population and compositions (e.g. construction trucks, port trucks, in-state trucks, etc.) of 

medium and heavy duty diesel truck fleets (>14,000 pounds Gross Vehicle Weight), as well as age 

distribution and vehicle miles traveled for these vehicles. Also, emission factors for these vehicles 

have been updated to reflect the major benefits of recent ARB regulations to reduce emissions 

from diesel trucks and buses. The EMFAC2011 model also includes the impact of the recent 

economic recession on both diesel and gasoline vehicle activity. 

The on-road category includes PM emissions from both diesel and gasoline engines. PM 

emissions from late-model light-duty gasoline vehicles are extremely low on a grams-per-mile 

basis. However, emissions from gasoline vehicles are significant on an aggregate basis because 

gasoline vehicles account for approximately 95% of the 175 million miles driven on an average 

day in the Bay Area. 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 83
Motor vehicle emissions that contribute to PM include tailpipe emissions of both primary PM 

and PM precursors from fuel combustion, lubricating oil, and particles generated by brake and 

tire wear. PM from tire wear and brake wear are included for on-road motor vehicles. There is 

insufficient information to produce estimates for tire-wear and brake-wear for off-road equipment; 

however, these emissions are expected to be very low as they are related to vehicle miles traveled 

(VMT) and most of the off-road equipment with tires have very low VMT.

A breakdown of annual average motor vehicle emissions in the current inventory is shown in 

Table 2-2.

Table 2-2 Bay Area Annual Average Primary PM2.5 Emissions – 

Year 2010: On-Road and Off-Road Motor Vehicles

On-Road Motor Vehicle tons/day %

Gasoline Exhaust 0.8 10%

Diesel Exhaust 2.7 37%

Tire/Brake-Wear (Gasoline Vehicles) 3.2 44%

Tire/Brake-Wear (Diesel Vehicles) 0.6 8%

Total On-Road Motor Vehicles 7.3 100%

Off-Road Motor Vehicles tons/day %

Gasoline Exhaust 1.7 37%

Diesel Exhaust 2.9 63%

Total Off-Road Motor Vehicles 4.6 100%

On-Road and Off-Road Motor Vehicles Combined

All Motor Vehicles tons/day %

Total Gasoline Exhaust 2.5 21%

Total Diesel Exhaust 5.6 47%

Tire/Brake Wear 3.8 32%

Grand Total 12.0 100%

The new emission factors show a reduction in tailpipe emissions from both gasoline and diesel 

engines. However, the decrease in tailpipe exhaust has been offset to a considerable extent by a 

major increase in estimated PM emissions from brake wear. Brake-wear emissions of PM2.5 in 

84 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
EMFAC2011 are three times the estimates produced by EMFAC2007. (It should be noted, however, 

that chemical mass balance analysis performed by Air District staff shows better agreement with the 

EMFAC2007 factors for brake and tire wear. Therefore, Air District staff will work with ARB staff to 

continue to investigate and improve PM emission factors for brake wear.) 

Whereas brake wear and tire wear made up a relatively small portion of PM2.5 from on-road vehicles 

using the previous EMFAC2007 emission factors, they now account for half of total PM2.5 emissions 

from on-road vehicles, according to the EMFAC2011 model. Although PM2.5 exhaust (tailpipe) 

emissions from diesel vehicles are much greater than from gasoline vehicles, when the new brake 

wear factors are included, gasoline vehicles account for 54% of total PM2.5 from on-road vehicles, 

compared to 45% for diesel vehicles. 

It is important to note that, in addition to their direct emissions of primary PM, motor vehicles are 

the major source of precursor pollutants that combine to form secondary PM. For example, on-road 

vehicles account for 37% of ROG, 57% of NOx, and 18% of ammonia emissions in the Bay Area 

inventory for year 2010, as shown in Figures 2-10, 2-11, and 2-12, respectively.

Although the Air District does not yet have an ultrafine PM inventory, emissions testing indicates that 

both gasoline and diesel engines are a major source of ultrafine particles. Tests indicate that gasoline 

vehicles emit especially large amounts of ultrafine particles when in a hard acceleration mode. 

Research also suggests that engine lubricating oil may be an important source of ultrafine particles. 

It is likely that when an ultrafine PM inventory for the Bay Area becomes available, it will show that 

combustion from both diesel and gasoline vehicles account for significant shares of UFPM emissions 

in the region.

Construction, Industrial and Airport Ground Support Equipment

Emissions estimates for off-road mobile sources (such as construction, cargo handling at ports, 

and airport ground support equipment) are taken from ARB’s latest emissions factor model, 

OFFROAD2011. Emissions for this category have decreased significantly compared to the estimates 

in the previous inventory derived from ARB’s OFFROAD2007 model. Research on fuel sales for 

off-road equipment showed that fuel usage, and hence emissions from these vehicles, had been 

substantially over-estimated in the OFFROAD2007 model. 

In response, ARB staff improved the inventory for this category by revising equipment population 

estimates based on historical equipment sales data, and incorporating data from industry regarding 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 85
hours of equipment operation and load factor, a measure of how intensively the equipment is being 

used. When coupled with the impact of the 2007 recession, the new OFFROAD2011 model estimated 

PM2.5 emissions from this category to be just one-fifth of the estimate produced by EMFAC2007 that 

was used in the 2010 CAP.

Ships and Commercial Boats

ARB recently revised the methodology 

to calculate emissions from ships and 

commercial boats. For ships, the new 

inventory increases the specificity 

of the earlier inventory by including 

vessel-specific characteristics and 

activity for each port. Emissions 

were calculated by estimating ship 

emissions on a ship-by-ship and a 

port-by-port basis, using actual ship 

engine power, speeds, and berthing 

times where possible. Projected 

emissions for future years were 

estimated using a set of growth factors specific to each port and each ship type. For commercial 

boats, emissions were calculated based on data collected from ARB’s 2004 Statewide Commercial 

Harbor Craft Survey. This survey collected information from boat owners as to vessel type, home port, 

engine characteristics, hours of operation, annual fuel usage, etc. This information along with other 

studies, such as emission inventories developed for the Ports of Los Angeles and Long Beach, were 

used to estimate emissions for boats. Cruise ship berthing emissions were taken from the 2005 Bay 

Area Seaports Air Emissions Inventory Report.

Another major difference in the current estimate of ship emissions, compared to the estimate 

provided in the 2010 CAP, is that ship emissions reported in this inventory are based on ship activity 

within three nautical miles of the coastline, whereas the 2010 CAP reported emissions for ship 

activity up to 100 nautical miles. Based on direction from ARB, using a limit of three nautical miles is 

consistent with the limit being used by other air districts that are in the process of preparing PM2.5 

SIP submittals.

Aircraft

Aircraft emissions are based on actual 2010 

activity data (landing and take-off data, taxi times 

between the runway and the terminal, etc.) at Bay 

Area airports, based on the current aircraft fleet 

mix and the latest emissions factors for PM, ROG, 

NOx, and SO2. Aircraft emissions are decreasing 

on a per-passenger mile basis, due to the shift 

toward larger planes and the development of 

86 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
more fuel efficient engines. As a result of new aircraft engine technological changes that rely on 

higher combustion temperatures to improve fuel economy, ROG emissions are decreasing, but NOx 

emissions are increasing per landing and take-off.

Paved and Unpaved Road Dust (Re-entrained Dust)

Previous paved road dust emissions, including those reported in the 2010 CAP, were believed to 

be over-estimated when compared to observed ambient concentrations and source apportionment 

(chemical mass balance) analysis. A new methodology from US EPA was used to estimate PM 

emissions from vehicular travel on paved roads.18 This methodology results in significantly lower 

estimates of PM10 and PM2.5 emissions compared with previous estimates. The revised estimates 

of road dust emissions from paved roads for PM10 and PM2.5 are only about one-third of the 

estimated emissions reported in the 2010 CAP. 

Unpaved road dust emissions were also updated to reflect the latest information from Caltrans on 

Bay Area unpaved road miles and vehicle miles traveled on these roads. As with the paved road dust 

estimates, the revised estimates of road dust emissions from unpaved roads for PM10 and PM2.5 

are only about one-third of the estimated emissions reported in the 2010 CAP. 

2010 Annual Average Primary PM2.5 Emissions

Estimated annual average emissions of primary PM2.5 have decreased significantly in the current 

inventory compared to the estimates provided in the 2010 CAP. Whereas the 2010 CAP reported 

47 tons per day of primary PM2.5 for year 2009, the current inventory shows an estimated 87 tons 

per day of primary PM2.5 for year 2010, a decline of 46%. The revised inventory includes significant 

reductions in several major source categories, such as residential wood-burning, commercial cooking, 

off-road vehicles (“Other Mobile Sources”), and road dust (included in “Geological dust”). This 

decrease in the inventory is based in part on real emission reductions due to factors such as turnover 

in the vehicle fleet and the impact of new regulations that had not been accounted for in previous 

emission factors. However, changes in methodologies used to estimate emissions, as discussed 

above, also account for a significant portion of the decrease in the inventory. Figure 2-6 shows the 

annual average emissions of PM2.5 for year 2010 broken down by major source categories.

18 The new methodology for road dust emissions is set forth in two documents:

1) US EPA guidance, January 2011: www.epa.gov/ttn/chief/ap42/ch13/final/c13s0201.pdf and

2) Air Resources Board guidance re: Draft Entrained Paved Road Travel Paved Road Dust, (Section 7.9) 6/15/11

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 87
Figure 2-6 2010 Annual Primary Emissions of PM2.5, 47 tons/day

2010 Winter PM2.5 Emissions

The breakdown of primary PM2.5 emissions by source category differs for the winter PM2.5 inventory 

compared to the annual inventory. The key difference is that PM2.5 emissions from residential fuel 

combustion account for 38% of the winter inventory versus 24% in the case of the annual inventory. 

PM2.5 emissions from residential fuel combustion, which is dominated by wood-burning in fireplaces 

and wood stoves, are 65% higher (an additional 7 tons per day) in the winter months than the annual 

average emissions for this category. However, emissions from several source categories are lower in 

winter. For example, winter emissions from off-road equipment, as well as from the industrial sector, 

are lower than annual average emissions due to reduced activity during the winter months. Accidental 

fires and geological dust emissions are also lower in the winter than the annual average. Overall, 

however, winter-time PM2.5 emissions are 4% higher (an additional 2 tons per day) than annualaverage emissions. Figure 2-7 shows the winter emissions of PM2.5 for year 2010 broken down by 

major source categories.

Industrial/

Commercial1

Processes1

7%1

Commercial1Cooking1

4%1

Residen9al1Combus9on1

38%1

Industrial1

Combus9on1

11%1

On?Road1Motor1Vehicles1

15%1

Other1Mobile1Sources1

9%1

Geological1Dust11

12%1

Animal1Waste1

2%1

Accidental1Fires/Smoking1

2%1

20101Winter1Emissions1

Par9culate1MaNer11<12.51microns1

491tons/day1

88 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Figure 2-7 2010 Winter Primary Emissions of PM2.5, 49 tons/day

2010 Annual Average Primary PM10 Emissions

PM2.5 emissions are a sub-set of PM10 emissions. For some sources, such as the various types 

of engine combustion, virtually all PM10 actually consists of fine particles less than 2.5 microns in 

diameter. For example, 97% of diesel PM10 is PM2.5, meaning that diesel particulate is essentially 

all PM2.5. But for other sources, only a fraction of PM10 is made up of particles less than 2.5 

microns in diameter, and the rest consists of coarse particles between 2.5 and 10 microns in 

diameter. For example, only 15% of PM10 from road dust is PM2.5; the remaining 85% is coarse 

particles. For the emissions inventory as a whole, on a mass basis, roughly half of PM10 is composed 

of fine particles less than 2.5 microns in diameter and half is composed of coarse particles between 

2.5 and 10 microns in diameter. In Figures 2-8 and 2-9 showing annual and winter emissions of 

PM10, source categories that emit coarse particles become more prominent, thus contributing more 

to PM10 emissions compared to Figures 2-6 and 2-7 for PM2.5. In particular, geological dust, which 

includes dust from construction and farming operations, re-entrained road dust from paved and 

unpaved roads, and wind-blown dust, accounts for a much greater percentage of PM10 (43% on an 

annual average basis) than PM2.5 (13% on an annual average basis).

Industrial/Commercial1

Processes1

9%1

Commercial1Cooking1

4%1

Residen9al1Combus9on1

24%1

Industrial1Combus9on1

12%1

On>Road1Motor1Vehicles1

16%1

Other1Mobile1Sources1

11%1

Geological1Dust11

13%1

Animal1Waste1

2%1

Accidental1Fires/Smoking1

9%1

20101Annual1Emissions1

Par9culate1MaLer11<12.51microns1

471tons/day1

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 89
Figure 2-8 2010 Annual Average Primary Emissions of PM10, 106 tons/day

Industrial/

Commercial1

Processes1

7%1

Commercial1Cooking1

2%1

Residen9al1Combus9on1

11%1

Industrial1

Combus9on1

5%1

On>Road1Motor1Vehicles1

13%1

Other1Mobile1Sources1

5%1

Geological1Dust11

43%1

Animal1Waste1

9%1

Accidental1Fires/Smoking1

5%1

20101Annual1Emissions1

Par9culate1MaMer11<1101microns1

1061tons/day1

2010 Winter Primary PM10 Emissions

The relationship between annual average PM10 and winter PM10 (i.e., the relative contributions 

from various source categories) is similar to that for PM2.5. The main difference is that emissions 

for residential fuel combustion (primarily wood-burning) increase from 11% of the annual PM10 

inventory to 18% of the winter PM10 inventory, while emissions from accidental fires (primarily 

wildfires) decrease from 5% of the annual PM10 inventory to 1% of the winter PM10 inventory. Figure 

2-9 shows the winter emissions of PM10 for year 2010 broken down by major source categories.

Figure 2-9 2010 Winter Primary Emissions of PM10, 104 tons/day

Industrial/Commercial1

Processes1

7%1

Commercial1Cooking1

3%1

Residen9al1Combus9on1

19%1

Industrial1

Combus9on1

5%1

On?Road1Motor1Vehicles1

13%1

Other1Mobile1Sources1

4%1

Geological1Dust11

41%1

Animal1Waste1

7%1

Accidental1Fires/Smoking1

1%1

20101Winter1Emissions1

Par9culate1MaMer11<1101microns1

1041tons/day1

90 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Sources of Precursors Pollutants That Form Secondary PM

Precursor pollutants that combine via chemical processes to form secondary PM include reactive 

organic gases (ROG), oxides of nitrogen (NOx), sulfur dioxide (SO2), and ammonia (NH3). The sources 

that produce these pollutants are described below, with pie charts depicting emissions during the 

winter season when secondary PM formation is greatest.

Reactive Organic Gases (ROG)

As shown in Figure 2-10, on-road motor vehicles (37%) and off-road vehicles (14%) together produce 

approximately 50% of the winter-time ROG emissions. Evaporation from petroleum products 

(including those from refineries and fuels distribution) and solvents (such as those from structures 

coating, adhesives, and sealants) are the second largest contributors to ROG emissions, accounting 

for approximately 24% of winter emissions. Emissions from consumer products contribute another 

15% of the winter-time ROG emissions.

Figure 2-10 2010 Winter Emissions Reactive Organic Gases (ROG) 294 tons/day

Industrial/ Commercial 

Processes

4%

Petroluem Products/Solvent 

Evaporation

24%

Commercial 

Cooking

0%

Residential Combustion

3%

Industrial Combustion

2%

On-Road Motor Vehicles

37%

Other Mobile Sources

14%

Consumer Products

15%

Animal Waste

1%

2010 Winter Emissions

Reactive Organic Gases (ROG)

294 tons/day

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 91
Oxides of Nitrogen (NOx)

As shown in Figure 2-11, on-road motor vehicles are the single largest source of NOx emissions in 

the Bay Area. Together with off-road mobile sources, they produce over 80% of the winter-time NOx 

emissions. Industrial combustion and residential fuel combustion (including wood-burning) produce 

10% and 6% of the winter-time NOx inventory, respectively. 

Figure 2-11 2010 Winter Emissions Oxides of Nitrogen (NOx), 347 tons/day

!Industrial/!Commercial!

Processes!

1%! Residen6al!Combus6on!

6%!

Industrial!Combus6on!

10%!

On;Road!Motor!Vehicles!

57%!

Other!Mobile!Sources!

26%!

2010!Winter!Emissions!

Oxides!of!Nitrogen!(NOx)!

347!tons/day!

92 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Sulfur Dioxide (SO2)

Figure 2-12 provides a breakdown of winter SO2 emissions for year 2010. The majority of SO2 

emissions in the Bay Area are from combustion at industrial facilities; the Industrial Combustion 

wedge (62%) in Figure 2-12 includes SO2 emissions from refineries (50%) and other industrial 

facilities (12%). Industrial and commercial processes employed in the production of pharmaceuticals, 

cosmetics, inks, and resins, as well as those used at refineries, create another 24% of SO2 emissions 

in winter. Off-road and on-road motor vehicles produce 13% of the Bay Area winter emissions. 

Although most of the SO2 in the Bay Area (over 50%) is emitted during refinery operations, it is worth 

noting that the refinery products are used to fuel motor vehicles in the Bay Area and throughout 

California. This suggests that SO2 levels can be reduced both by further controlling emissions at the 

refineries, as well as by reducing motor vehicle use.

Figure 2-12 2010 Winter Emissions Sulfur Dioxides (SO2), 29 tons/day

!Industrial/!Commercial!

Processes!

24%!

Residen7al!Combus7on!

1%!

Industrial!Combus7on!

62%!

On<Road!Motor!Vehicles!

3%! Other!Mobile!

Sources!

10%!

2010!Winter!Emissions!

Sulfur!Dioxides!(SO2)!

!29!tons/day!

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 93
Predicting Future Emissions Trends

Predicting future trends in emissions is challenging, since many factors come into play, such as changes 

in control technologies; emission standards and fuel formulations for mobile sources; population growth 

and household formation; economic growth rates; land use decisions; and changes in transportation 

infrastructure and travel mode choice which affect motor vehicle travel. The inventory projections 

presented here include anticipated changes in population and economic activity, and well as emission 

reductions from ARB and Air District regulations that have already been adopted and turnover in 

the motor vehicle fleet (whereby older, dirtier vehicles are replaced by newer, cleaner vehicles)ARB. 

Implementation of ARB regulations on mobile sources is typically phased in, so the entire benefit of these 

adopted regulations will not be realized until they have been fully implemented over the next 10-15 years. 

At that point, emissions are projected to slowly increase in response to population and economic growth, 

if no additional regulations are adopted. 

The projected emissions are based on a conservative “business as usual” assumption that no additional 

regulation or polices will be adopted to reduce emissions in the future. However, past experience suggests 

that it is likely that future measures will in fact be adopted and implemented to provide additional 

emission reductions. Past experience also suggests that the projected inventory may underestimate the 

future reduction in emissions. For example, whereas previous PM emissions inventories for the Bay Area 

predicted that overall emissions would increase over the past 10-15 years, monitoring data and CMB 

analysis shows that PM emissions and ambient concentrations actually declined substantially during 

that period. Given the fact that previous inventories under-predicted the emission reductions that were 

actually achieved, it is possible that this could occur again over the coming years.

Assumptions and methods used to estimate future emissions for key source categories are briefly 

discussed below.

Industrial and Commercial Processes and Combustions

PM emissions from industrial and commercial sources are projected to increase at a rate of around 1% 

per year based on previous observed growth and regulations adopted to date. 

Commercial Cooking 

PM emissions from commercial cooking are also projected to increase at a rate of around 1% per year 

based on previous observed growth and regulations adopted to date.

Residential Fuel Combustion

As noted above, estimated PM emissions from residential wood-burning were reduced for the 2010 

inventory to reflect recent progress in response to the Air District’s wood-burning regulation and its winter 

Spare the Air program. The projected inventory assumes that residential wood-burning rates will hold 

steady for the foreseeable future, with a slight increase based upon growth in population and households. 

Although it is possible that future emissions may decrease if the District is successful in increasing 

compliance with the wood-burning regulation and further educating Bay Area residents as to the health 

risks from wood smoke, Air District staff has opted to hold the wood-burning emissions steady, pending 

94 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
development of better data to use in estimating future wood-burning trends, so as to provide a 

conservative estimate. Modest increases of emissions are expected in other residential combustion 

categories, mainly for space heating. This is assumed to grow in-line with population growth resulting 

in modest increases in overall residential combustion emissions.

On-Road and Off-Road Motor Vehicles

Emissions from on-road and off-road motor vehicles are expected to decline until 2020 due to 

aggressive regulations on diesel engines; retirement of older, dirtier vehicles; and penetration of 

cleaner gasoline vehicles into the fleet. After 2020, vehicle emissions are expected to increase 

by less than 1% per year until 2030. The projected increase in vehicle emissions rests upon two 

assumptions: 

1. An increase in vehicular activity following recovery from the economic 

recession. ARB staff examined various economic recovery scenarios; the 

projected rate of growth in vehicle activity is based on an average between a 

slow-recovery scenario and a fast-recovery scenario. 

2. No additional regulation apart from those already adopted by ARB. (The 

assumption that no future regulations will be adopted is unlikely, however. 

Although it will be technically challenging to further reduce motor vehicle 

emissions rates below the stringent standards already adopted in California, 

experience to date suggests motor vehicle emission standards will be further 

tightened in coming years as needed to continue progress toward attainment 

of air quality standards in the major urban areas of the state.) 

It is important to note that the projected trends for diesel and gasoline vehicles differ. Whereas diesel 

PM emissions are projected to sharply decrease over the next decade in response ARB regulations, 

PM emissions from gasoline engines are expected to hold steady. For year 2010, diesel vehicles 

account for about half of the primary PM emissions from on-road vehicles, including both tailpipe 

exhaust and brake and tire wear. However, by 2030, this figure is projected to decrease to about 27%. 

It should be noted, however, that the EMFAC2011 model does not include the potential reduction 

in PM from light- and medium-duty vehicles that may occur in response to ARB’s LEV III program, as 

described in Section 4. Efforts to reduce PM from mobile sources in recent years have focused on 

heavy-duty, diesel-powered vehicles. However, the fact that gasoline vehicles are projected to account 

for an increasing share of the PM from motor vehicles in coming years suggests that future efforts 

to reduce PM emissions from on-road vehicles will need to focus on reducing tailpipe emissions and 

emissions due to brake and tire wear from light-duty vehicles.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 95
Projected Trends for Primary PM and Precursors To Secondary PM

Projected trends for emissions of primary PM and PM precursors are presented in summary form in 

the bar charts below. A more detailed breakdown of projected future emissions by source category 

is provided in the tables in Appendix A. As shown in Figures 2-13 and 2-14, the overall inventory for 

emissions of primary PM2.5 and PM10 is projected to decrease thru 2020, then to begin to rise 

slowly through 2030 in tandem with population and economic growth. The same trend is projected for 

emissions from on-road motor vehicles, as well as other mobile sources. 

Figure 2-13 Bay Area Winter Primary PM2.5 Emissions Trends

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The trend for winter emissions of primary PM10 is similar to that for PM2.5, as shown in Figure 2-14.

Figure 2-14 Bay Area Winter Primary PM10 Emissions Trends

Emissions projections for key precursors to formation of secondary PM are mixed. Similar to primary 

PM, ROG emissions are projected to decrease through 2020 in response to already-adopted control 

measures, then to begin to increase slowly through 2030, as shown in Figure 2-15. NOx emissions 

are projected to decrease steadily and substantially through 2030 in response to already-adopted 

control measures, as shown in Figure 2-16. SO2 emissions, by contrast, are projected to increase 

slowly but steadily through 2030 in the absence of additional regulations or controls, as shown in 

Figure 2-17.

Reactive Organic Gases (ROG)

As shown in Figure 2-15, overall ROG emissions are expected to decline until 2025. ROG emissions 

from on-road motor vehicles are expected to decline due to fleet turnover, despite increases in vehicle 

population and VMT. Emissions from off-road mobile sources will continue to decline until 2020 

due to implementation of already-adopted regulations. After 2020, a projected increase in off-road 

vehicular activity is expected to lead to increases in ROG emissions from off-road mobile sources. 

For the inventory as a whole, ROG emissions are projected to increase slightly after 2025 due to 

increased population and economic activity, in the absence of future regulatory measures.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 97
Figure 2-15 Bay Area Winter ROG Emissions Trends

Oxides of Nitrogen (NOx)

NOx emissions from on-road motor vehicles are expected to decline due to fleet turnover despite 

increases in vehicle population and VMT, as shown in Figure 2-16. Emissions from other mobile 

sources will continue to decline until 2025 due to aggressive regulations on diesel vehicles. After 

2025, projected increase in off-road vehicular activity is expected to lead to increases in NOx 

emissions from off-road mobile sources. Other major contributors to NOx emissions are expected to 

increase due to population increase unless new regulations are introduced. Overall NOx emissions 

are expected to decline until 2025 and then increase slightly. However, it is expected that with 

introduction of new rules on major sources of NOx emissions in the future, further reduction in NOx 

emissions is likely to occur. 

98 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Figure 2-16 Bay Area Winter NOx Emission Trends

Sulfur Dioxide (SO2)

After decreasing substantially in the past few years in response to regulations on sulfur content 

used in ships and commercial boats, SO2 emissions are projected to increase slowly in future years, 

as shown in Figure 2-17, primarily due to projected expansion in industrial activity. Overall SO2 

emissions are expected to increase in line with SO2 emissions increases mentioned above. The rate 

of increase is less than 1% per year until 2030.

Figure 2-17 Bay Area Winter SO2 Emission Trends

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 99
Section 3: PM Air 

quality standards 

and trends

Section 3-A: PM Monitoring Program

Regional PM Monitoring Network

The Air District operates a network of monitoring stations to measure ambient concentrations of 

particulate matter in the Bay Area. The Air District’s PM monitoring network has evolved in tandem 

with the evolution of the PM air quality standards. Because the original PM standards issued in 

response to the Clean Air Act of 1970 were based on total suspended particles (TSP), the initial 

PM monitoring stations measured TSP. When US EPA adopted standards for PM10 in 1987, the 

District established monitoring sites to measure ambient PM10 concentrations. Likewise, when US 

EPA issued standards for PM2.5 in 1997, the District established a network of monitoring sites to 

measure ambient PM2.5 concentrations. Therefore, we can track progress in reducing TSP in the Bay 

Area back to the 1970’s, PM10 to the late 1980’s, and PM2.5 to the late 1990’s.

Until recently, all PM measurements were performed by collecting particles on filters, and PM 

concentrations were estimated by weighing the filters before and after collecting the particles. A filter 

is pre-weighed, and then placed in a sampler that draws air through the filter, typically for 24 hours. 

The PM concentration is estimated by comparing the before and after weight difference of the filter, 

divided by the total air flow, yielding a measurement of ambient PM expressed in micrograms per 

cubic meter (μg/m3). Filter-based methods that meet specific operational requirements are called 

Federal Reference Method (FRM) and have been used for determining compliance with the national 

air quality standards. Because this process is labor-intensive, measurements have not been made 

every day at every station. At most stations, they were made either on a 1-in-3 or 1-in-6 day schedule. 

More recently, instruments have been developed to measure PM continuously; the Air District 

employs a continuous method known as a Beta-Attenuation Monitor (BAM). 

The Air District began measuring PM10 concentrations at a number of sites in 1989 on a 1-in-6 day 

schedule. In addition to total PM10 concentrations, a set of ions has been measured: nitrate, sulfate, 

ammonium, and chloride. Potassium was added in 1995, and elemental and organic carbon were 

added in 2004. The District began measuring PM2.5 in 1999. In recent years, the number of sites 

measuring PM10 has been reduced, as US EPA guidelines have placed greater emphasis on the need 

to monitor PM2.5.The Bay Area PM monitoring network meets and exceeds both state and US EPA 

requirements. The network provides data to measure regional PM levels relative to state and national 

standards. The network includes 8 sites which measure PM10 and 13 sites which measure PM2.5. 

There are three categories of PM monitors: 

100 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
• Filter-based Federal Reference Method (FRM);

• Real-time Beta-Attenuation Monitor (BAM) monitors designated as Federal 

Equivalent Method (hereafter referred to as FEM/BAM); and

• Ordinary Beta-Attenuation Monitors that are not designated FEM (hereafter 

referred to as BAM).

Ten PM2.5 monitoring sites provide data to 

determine whether the Bay Area meets national 

PM2.5 standards. This includes eight FEM/BAM 

sites that monitor PM2.5 on a continuous basis, as 

well as two FRM (Federal Reference Method) sites 

that use filters to measure PM2.5 on a schedule 

based on the location’s PM2.5 level relative to the 

national standard.1 The PM2.5 network includes 

four additional sites with BAM monitors that are 

used (in conjunction with the FEM and FRM sites) 

in determining whether the Bay Area attains State 

PM2.5 standards. (These four BAM sites are not 

included for purposes of determining compliance 

with federal PM2.5 standards.) 

In addition to the sites used to determine 

compliance with State and federal PM2.5 

standards, the Air District also operates SASS (Speciation Air Sampler System) instruments at four 

sites (San Jose, Vallejo, Livermore, and West Oakland) that provide PM2.5 speciation data; these 

speciation data are used to analyze PM by chemical type and emissions source category in order 

to refine the Air District’s PM emissions inventory and to help identify emission source categories 

that may warrant additional control measures. The speciation data provided by these four monitors 

are the source for the information provided in Figures 2-4 and 2-5 in Section 2. The Air District also 

operates one mobile sampling van, as well as one re-locatable trailer-mounted unit that can be 

deployed to monitor local concentrations for special purpose monitoring studies lasting a minimum 

of one year. For example, the mobile van provided measurements used in the 2010 West Oakland 

Monitoring Study described below.

The Air District’s current PM monitoring sites are shown in Table 3-1. 

1 The schedule can be every day, once every three days, or once every six days depending on the site and season.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 101
Table 3-1 Bay Area PM Monitoring Sites

Location PM10 PM2.5 Type of Monitor

Livermore √ FEM/BAM & Speciation Sampler

Oakland √ FEM/BAM

Concord √ √ FRM 

Bethel Island √

San Rafael √ √ FEM/BAM

San Francisco √ √ FEM/BAM

Redwood City √ FEM/BAM 

Gilroy √ FEM/BAM

San Jose √ √ FRM, BAM & Speciation Sampler

San Pablo √

Vallejo √ FEM/BAM & Speciation Sampler

Santa Rosa √ FEM/BAM

Cupertino √ √ BAM

Napa √ √ BAM

West Oakland √ BAM & Speciation Sampler

Measuring PM Concentrations at the Local Scale

The primary objective of the Bay Area air quality monitoring network for PM is to measure ambient 

PM levels at the regional scale in comparison to State and federal PM standards. The network is 

not intended to measure ambient concentrations of PM and other air pollutants at a fine-grain 

local scale. Also, while the network can measure PM impacts from larger-scale incidents such 

as wildfires, it is not designed to measure localized PM impacts from short-term incidents and 

episodes at a specific facility or source. However, concentrations of pollutants such as PM, carbon 

monoxide, and air toxics can vary greatly at the local scale. Among its limitations, for example, the 

existing regional monitoring network cannot accurately measure the local impacts of residential 

wood-burning on ambient PM concentrations due to the highly localized and variable nature of this 

activity. 

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The Air District recognizes that information about local PM levels is needed in order to identify impacted 

areas and develop strategies to reduce PM concentrations in such areas. Since it is neither technically 

nor financially feasible to install and operate PM monitors in every neighborhood, the Air District has 

been working to estimate PM concentrations and population exposure at the local scale by means of 

computerized photochemical modeling. This approach has been used to develop local estimates of 

PM concentrations and population exposure in support of the District’s Community Air Risk Evaluation 

(CARE) program, as well as two pilot Community Risk Reductions Plans (CRRPs) that are under 

development in partnership with the cities of San Francisco and San Jose. 

The Air District has also performed special monitoring 

to develop better measurements of PM and other 

pollutants in various locations with specific air quality 

issues or concerns. Monitoring was performed in 

Berkeley from December 2007 through December 

2009 in proximity to Pacific Steel Casting; in Benicia 

from April 2007 through December 2008 in proximity 

to the Valero oil refinery; and in Cupertino beginning in 

September 2010 (scheduled to run through December 

2012) in proximity to the Lehigh cement plant, and 

in areas, such as West Oakland, using its mobile 

van. Several examples of localized studies of PM that 

were extremely helpful in quantifying PM levelsin 

communities believed to be significantly impacted by 

PM are briefly described below.

The Air District sponsored Desert Research Institute to 

assist with the West Oakland Monitoring Study (WOMS) 1 developing and analyzing air quality monitoring 

data for the West Oakland area during two seasonal periods of four weeks in summer 2009 and winter 

2009/10. The WOMS data were used by the District to evaluate local-scale dispersion modeling of 

diesel emissions and other toxic air contaminants for the area within and around the Port of Oakland. 

The monitoring data showed spatial patterns of higher pollutant concentrations that were generally 

consistent with proximity to vehicle traffic. Concentrations of directly-emitted pollutants were highest on 

heavily traveled roads with consistently lower concentrations away from the roadways.

The Air District also measured PM2.5 and PM10 and analyzed concentrations of certain metals and 

other chemical species in the ambient air of West Oakland from August 2009 through July 2010. The 

goals were to look for how these concentrations vary spatially within West Oakland and specifically 

in the neighborhood of Custom Alloy Scrap Sales (CASS), and to look for evidence of elevated metals 

concentrations in the West Oakland area. PM2.5 filters were collected at 7 sites as part of the West 

Oakland Monitoring Study for a month in the summer of 2009 and the winter of 2009-10. PM10 filters 

were collected at four sites near CASS from August 2009 through July 2010. The monitoring did not 

reveal a clear signature from CASS. The estimated cancer risk from measured metals concentrations 

1 West Oakland Monitoring Study, Desert Research Institute, 2010. 

 See: www.baaqmd.gov/Divisions/Planning-and-Research/CARE-Program/CARE-Documents.aspx

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 103
was less than 10 in a million, considerably less than from diesel exhaust. The concentrations are 

all within the corresponding reference exposure levels (RELs) for morbidity effects.

Air District staff is currently working to identify several sites to monitor near-roadway levels of 

nitrogen dioxide (NO2), as required by EPA regulations. NO2 monitors are expected to be installed 

in near-roadway environments in San Francisco, San Jose, and the I-80 or I-880 corridor in 

Alameda County. The Air District may install PM2.5 and/or ultrafine PM monitors at one or more of 

these sites.

To expand the Bay Area monitoring network, one possibility may be to deploy smaller and less 

expensive monitoring units to supplement the PM monitors that comprise the Air District’s official 

PM2.5 monitors, provided that equipment and personnel funds can be secured to purchase and 

operate such units.

Measuring Ultrafine PM

As noted in Section 1-A, evidence suggests that ultrafine particles may be especially harmful to 

public health. However, measuring ultrafine particles (UFPM) presents unique challenges. Due 

to their extremely small size, conventional technologies are not well-suited to measuring ultrafine 

particles. PM2.5 and PM10 monitors measure the mass of particles in a given volume of air; 

however, UFPM is negligible on a mass basis. Therefore, UFPM measurements usually count the 

number of particles rather than the particle mass. Measuring UFPM is especially difficult because 

many of the particles are actually smaller than the wave length of light. Only in recent years has 

measurement technology progressed such that the size distribution of nanometer-size particles 

can be measured in the atmosphere. Current methods to measure UFPM typically expose the 

particles to water vapor to make them grow large enough that they can be counted. Although 

several UFPM monitoring devices are currently available, technologies are still evolving, equipment 

and maintenance costs are relatively high, and accuracy and dependability of the devices can be 

an issue.

There are currently no State or national requirements for monitoring ambient concentrations of 

ultrafine PM. Most of the ultrafine particle UFPM monitoring performed to date has occurred in the 

Los Angeles area; UFPM monitoring in the LA area has focused on measuring ultrafine particles in 

close proximity to major roadways. (See discussion of near-roadway measurements in Section 1-B.)

In spring 2012 the Air District purchased and installed UFPM particle counters (TSI EPC 3783) at 

three sites in Santa Rosa, Redwood City and Livermore. An additional UFPM counter on loan to 

the District has been in operation in San Pablo; the District is in the process of purchasing this 

monitor, so that it will continue to provide monitoring data for the San Pablo area. The Air District 

also plans to install an ultrafine particle counter in conjunction with the near-roadway NO2 monitor 

described above, once the location for that monitor has been finalized. These UFPM monitors will 

provide data on ambient concentrations of UFPM at the regional scale and on a near-roadway 

basis. The data from these monitors will be used to track progress in reducing ultrafine particle 

concentrations in the Bay Area and to inform the District’s future UFPM computer modeling efforts. 

104 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
In addition, measurements of ambient levels of UFPM levels for the Bay Area (and many other urban 

areas) is needed to provide data that epidemiologists can use to study the health effects of exposure to 

UFPM.

Although the monitors described above should be useful for purposes of determining baseline 

concentrations of ambient UFPM in the Bay Area, using traditional air quality monitoring networks to 

measure UFPM may be of limited value, given the great spatial and temporal variability exhibited by 

UFPM. To adequately measure the great variability in UFPM levels, air quality agencies may need to 

explore the use of smaller, cheaper devices (perhaps including personal monitors) that can be deployed 

more densely on a neighborhood scale. 

Challenges with PM Monitoring: Air quality monitoring stations are expensive to build and 

maintain. Due to budgetary constraints in recent years, the Air District is currently hard-pressed to 

operate its existing air quality monitoring network. Expanding the monitoring network would entail 

securing funds for the initial capital cost to purchase monitoring equipment, finding good locations 

that meet applicable criteria, and deploying human resources to operate and maintain the sites on 

an on-going basis. As the Air District continues its technical work to develop a better understanding 

of the dynamics and distribution of PM in the atmosphere by means of computer modeling and 

special studies, this should enable the Air District to deploy its limited monitoring resources so 

as to measure ambient concentrations and population exposure to PM in the most cost-effective 

manner.

As mentioned above, the PM monitoring network is not designed to measure localized PM impacts 

from short-term incidents and episodes at a specific facility or source. Therefore, the Air District is 

investigating the possibility of augmenting the current network with incident response capabilities 

that would allow for accurate, real-time, mobile measurement of localized PM impacts from shortterm episodes.

See Section 5 for additional discussion regarding future directions in monitoring ambient PM 

concentrations.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 105
Section 3-B: PM Standards and 

PM Planning Requirements

To protect public health and welfare, US EPA and the California Air Resources Board have 

both adopted ambient air quality standards for particulate matter. The federal Clean Air Act 

requires the US EPA Administrator to adopt standards for six “criteria pollutants”, including 

PM, with an “adequate margin of safety to protect public health.” EPA is charged with reviewing 

the standards every five years based on the latest scientific research on health and welfare 

effects, and considering recommendations provided by an expert panel called the Clean Air 

Scientific Advisory Committee (CASAC). PM standards have evolved and become more stringent 

over the past several decades in response to better understanding of the negative effects of 

PM on public health. In addition to primary standards which are designed to protect public 

health, U.S. EPA also issues secondary standards for PM to protect “public welfare”, including 

visibility (clarity of the air), flora, fauna, and the built environment. The national secondary 

standards for PM are currently set at the same level as the primary standards.

PM Standards

Ambient air quality standards are based on three key elements: the averaging time period 

(e.g., 24-hour or annual); the form of the standard; and the level of the standard. 

Annual average standards are intended to protect public health from chronic (long-term) 

health impacts related to PM. EPA adopted an annual average PM2.5 standard of 15 μg/m3

in 1997. In 2002, the State of California adopted a (more protective) annual average standard 

of 12 μg/m3 in 2002. In June 2102, EPA proposed to lower the national annual standard to a 

value in the range of 12 to 13 μg/m3, as discussed below. 

24-hour standards are intended to protect public health from acute (short-term) health 

impacts related to PM. In 2006, EPA significantly tightened the 24-hour PM2.5 NAAQS from 

65 μg/m3 to 35 μg/m3. The State of California has not yet adopted a short-term 24-hour PM 

standard.

For criteria pollutants, the level of the standard is generally defined in terms of the ambient 

concentration of a pollutant in outdoor air, as expressed in terms of either a parts per million

ratio (e.g., the state 8-hour ozone standard is 0.070 parts per million) or a mass per volume

basis. For example, the national 24-hour PM2.5 standard is 35 mg/m3, or micrograms per 

cubic meter (one microgram equals one-millionth of a gram). State and national PM standards 

for PM2.5 and PM10 are based on the mass (i.e., the total weight), rather than the number, of 

particles suspended in the air. 

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Design Value: Determining whether an air basin attains a given air quality standard requires 

comparing ambient pollutant levels with the standard to calculate the region’s design value.

2 (For 

purposes of State standards, the term designation value is used.) The design value is calculated 

for each station in the official monitoring network (See Section 3-A for a description of the Bay Area 

PM monitoring network). A region meets the standard only if the design value for each and every 

official monitoring site does not exceed the standard. The stringency of an air quality standard 

depends upon (1) the numerical threshold and (2) the form of the standard which specifies the 

method and statistical protocol used to calculate the design value. The form of a standard may 

allow each region to exceed the standard on a limited number of occasions over a given time 

period. For example, the design value for the national 24-hour PM2.5 standard for any site is 

defined as the site’s annual 98th percentile PM measurements averaged over a three-year period; 

thus, a site may exceed the standard on a limited number of days and still attain the standard. The 

basic steps to calculate the design value for the national PM2.5 standards are shown in Table 3-2.

The PM standards established by the State of California are more difficult to attain than the 

national standards; not only are the State standards set at lower numerical thresholds, but also 

they have a more stringent form of the standard. The State 24-hour PM10 standard allows fewer 

exceedances in order to remain in attainment. The State annual standard for PM2.5 is more 

stringent because it is based on the maximum of three annual averages, rather than the average

of three annual averages). The design values that determine whether the Bay Area attains the 

various PM standards are calculated using measurements of ambient PM concentrations from the 

regional monitoring network described in Section 3-A. 

Table 3-2 Basic Design Value Calculation Method for National PM2.5 Standards

Averaging 

Period 

Ambient 

Concentration

Calculation Method – 

performed for each official monitoring station

24-hour 35 μg/m3

Step 1: Determine the 98th percentile value for each year over

 a consecutive three year period. (In practice this means 

 that the seven highest values per year are excluded.)

Step 2: Average the three 98th percentile values.

Step 3: Round the resulting value to the nearest 1.0 μg/m3.

Step 4: Compare the result to the standard.

Annual 15.0 μg/m3

Step 1: Calculate the average of each quarter of each year over

 a three year period.

Step 2: Average the four quarters in a calendar year to determine the 

average for each year.

Step 3: Average the three annual values.

Step 4: Round the resulting value to the nearest 0.1 μg/m3.

Step 5: Compare the result to the standard.

2 Details on how design values are calculated are provided in 40 Code of Federal Regulations (CFR) Part 50 Appendix N, and the April 1999 EPA 

document Guideline on Data Handling Conventions for the PM NAAQS. See http://epa.gov/ttncaaa1/t1/memoranda/pmfinal.pdf.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 107
Bay Area Attainment Status for Current PM Standards 

The current State and national standards for PM2.5 and PM10, the Bay Area’s attainment status 

relative to those standards, and the region’s design value for each standard, are summarized in Table 

3-3. PM standards recommended by the World Health Organization (WHO) in 2005 are also shown 

for purposes of comparison; the national PM standards issued by US EPA are less stringent than the 

recommended WHO guidelines. Despite increasing concern about the health impacts of ultra-fine 

particles, as yet there are no State or national ambient air quality standards for ultra-fine PM.

The Bay Area attainment status shown in Table 3-3 is based on the current formal designation by 

US EPA or ARB. However, although the Bay Area is formally designated as non-attainment for these 

standards, monitoring data shows that the region currently meets the national 24-hour PM2.5 

standard, the State annual PM2.5 standard, and the State annual PM10 standard, as indicated by 

the fact that the design value is less than or equal to the standard in each case. For purposes of 

attainment status, although monitoring data for an air basin may show that it meets a standard, 

once a region has been designated as non-attainment, it is still formally designated as nonattainment until such time as the region submits a redesignation request and maintenance plan 

which is approved by EPA.

National standards: The Bay Area attains the national 24-hour PM10 standard and the national 

annual PM2.5 standard. The region’s design value for both these standards is well below the 

threshold. The national 24hour PM2.5 standard was tightened to 35 mg/m3 in 2006. The Bay Area’s 

attainment status for this standard is explained below in Federal PM Planning Requirements for the 

Bay Area. 

State standards: The California Air Resources Board has adopted PM standards that are more 

stringent (health-protective) that the national standards. The most recent monitoring data (through 

2011) demonstrates that the Bay Area continues to meet the State annual average PM2.5 standard. 

Recent data also shows that the Bay Area has attained the State annual PM10 standard for the first 

time ever, based on data for the 2009-2011 period. The Bay Area does not attain the State 24-hour 

PM10 standard; however, the region’s design value for this standard has been decreasing in recent 

years, a sign that we are making progress toward attaining this standard.

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Table 3-3 PM Standards, Bay Area Attainment Status, and Design Values 

Averaging 

Time

National /

California Standard Bay Area Design Value * Attainment Status 

World Health 

Organization 

PM Guidelines

Pollutant: PM2.5

24-hour National 35 μg/m3 30 μg/m3 Non-attainment 25 μg/m3

Annual

National 15 μg/m3 10.3 μg/m3 Attainment

10 μg/m3

California 12 μg/m3 10.4 μg/m3 Non-attainment

Pollutant: PM10

24-hour

National 150 μg/m3 72 μg/m3 Unclassified

50 μg/m3

California 50 μg/m3 70 μg/m3 Non-attainment

Annual California 20 μg/m3 20 μg/m3 Non-attainment 20 μg/m3

* Design values are calculated based on PM monitoring data thru year 2011.

Federal PM Planning Requirements for the Bay Area

Any state or region that fails to attain the national standard for any criteria pollutant is required to 

submit a State Implementation Plan (SIP) to U.S EPA to demonstrate how it will reduce ambient 

concentrations in order to attain the national standard. U.S. EPA designated the Bay Area as “nonattainment” for the revised 24-hour PM2.5 national standard in December 2009, based on air 

quality monitoring data for the three-year period 2006-2008. Areas designated as non-attainment 

for the revised standard, including the Bay Area, are required to submit a PM2.5 SIP to U.S. EPA by 

December 2012 to show how they will attain the standard by December 2014. 

Although the Bay Area was designated as non-attainment for the national 24-hour PM2.5 standard 

based on monitoring data for the 2006-2008 period, the region exceeded the standard by only a 

slight margin. Since then, Bay Area PM2.5 levels have continued to decline. Air quality data from 

the regional monitoring network shows that the Bay Area met the national 24-hour PM2.5 standard 

during the three-year period from 2008 through 2010, as well as the three-year period from 2009 

through 2011. 

Under US EPA guidelines, a region with monitoring data showing that it currently attains an air quality 

standard can submit a “redesignation request” and a “maintenance plan” in lieu of a SIP attainment 

plan. However, the Air District believes that it would be premature to submit a PM2.5 redesignation 

request for the Bay Area at this time. Instead, the Air District is pursuing another option provided 

by US EPA guidelines for areas with monitoring data showing that they currently meet the PM2.5 

standard. In December 2011, the Air Resources Board submitted a “clean data finding” request on 

behalf of the Bay Area. This request is currently under review by EPA. If EPA verifies that monitoring 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 109
data shows that the Bay Area currently meets the standard (i.e., has “clean data”), then EPA 

will suspend the SIP provisions that apply to preparing an attainment plan to demonstrate how 

the region will attain the standard by the specified target date. These SIP provisions will remain 

suspended as long as Bay Area monitoring data continues to show compliance with the standard. 

Although the SIP requirements related to demonstrating attainment would be suspended, the 

region will still be required to submit a “clean data” SIP consisting of the following elements: 

• Amendments to the Air District’s New Source Review (NSR) regulations to 

address PM2.5; and 

• An emissions inventory for PM2.5 for the attainment year: i.e. the year in 

which monitoring data shows that the Bay Area first achieved attainment. 

For the Bay Area, the attainment year is 2010. The inventory must include 

both primary PM2.5 emissions by source category, as well as precursors to 

secondary PM formation.

The Air District is preparing these required SIP elements for submittal to US EPA in fall 2012.

State PM Planning Requirements

The California Clean Air Act of 1988, the primary legislation that defines State air quality planning 

requirements, is focused primarily on reducing ground-level ozone. The California Clean Air Act 

does not require that local air districts prepare plans to reduce PM. In response to concern about 

the health impacts of PM, in 2003 the State legislature enacted SB 656 (codified as Health 

& Safety Code Section 39614); this legislation required ARB and local air districts to evaluate 

potential PM control measures and to develop a PM implementation schedule for appropriate PMreduction measures. The Air District complied with this legislation; staff developed a Particulate 

Matter Implementation Schedule that was adopted by the Air District’s Board of Directors in 

November 2005. The three measures identified in the PM Implementation Schedule have been 

adopted and implemented: stationary internal combustion engines (Regulation 9-8); commercial 

charbroiling operations (Regulation 6-2); and the residential wood-burning rule (Regulation 6-3) 

which is further described in Section 4.

The SB 656 legislation sunset on January 1, 2011 and is therefore no longer in effect. Thus, 

despite the fact that State PM standards are more stringent than the national standards, formal 

PM planning efforts in California are governed primarily by the national standards and the SIP 

process describe above.

Proposed Revisions to National PM Standards

As noted above, EPA is required to review the national standards for PM and other criteria 

pollutants every five years based on the latest scientific research on health and welfare effects. 

After reviewing the evidence, EPA issued a draft proposal on the national PM standards for public 

comment on June 14, 2012. EPA proposes to: 

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• Strengthen the annual health standard for fine particles (PM2.5) by setting 

the standard at a level within the range of 12 μg/m3 to 13 μg/m3.

• Retain the existing 24–hour PM2.5 standard of 35 μg/m3.

• Set a separate PM standard to improve visibility in urban areas, as 

discussed below.

• Retain existing secondary standards for PM2.5 and PM10 identical to 

primary standards to provide protection against other effects, such as 

ecological effects, effects on materials, and climate impacts.

• Retain the existing 24-hour standard for PM10 of 150 μg/m3; this standard 

has been in place since 1987. 

The proposed urban visibility standard would provide increased protection from particle-induced 

haze. The standard would measure visibility on the basis of light extinction as expressed in units 

called deciviews. Each deciview represents a constant change in visual air quality, with zero 

deciviews representing the most pristine conditions. EPA is proposing a 24-hour averaging time, a 

90th percentile form averaged over 3 years, and a visibility level set at either 28 or 30 deciviews. 

Because monitors to directly measure visibility are not currently available, EPA proposes to use 

data on speciated PM2.5 mass concentrations as well as relative humidity, in conjunction with an 

algorithm, to calculate PM2.5 light extinction.

The Air District provided comments at a public hearing on the proposed standards in Sacramento 

on July 19, 2012, as well as written comments, urging EPA to adopt the most health-protective PM 

standards.

After reviewing public comments on the proposed revisions to the PM standards summarized 

above, EPA will issue final standards by December 14, 2012. Revisions to the current standards 

will trigger a process to evaluate monitoring data and issue new attainment designations for 

air basins throughout the nation. EPA expects to make attainment designations based on the 

revised standards by December 2014. A preliminary, unofficial review of Bay Area monitoring 

data for years 2008 through 2011 indicates that the region is likely to attain a more stringent 

annual standard set at either the 12 μg/m3 or the 13 μg/m3 level, provided that recent ambient 

PM2.5 concentrations prevail in future years.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 111
Section 3-C: Trends in Ambient 

PM Concentrations

Trends in ambient PM concentrations can be analyzed in terms of design value (as explained in 

Section 3-B), as well as the number of days the region exceeds the PM standard. The Bay Area has 

been making progress in reducing PM levels as measured by both of these metrics. The graphs in 

Figures 3-1, 3-2, and 3-3 below show trends for ambient concentrations of PM2.5 and PM10. Trends 

in ambient PM10 concentrations are shown since the late 1980’s, and trends in PM2.5 are shown 

since 1999, because these are the years for which monitoring data are available. The information 

presented here is derived from a more detailed report entitled Trends in Bay Area Ambient 

Particulates (BAAQMD, November 2011). The November 2011 Trends report also provides analysis 

showing trends in reducing several specific PM components, including nitrate, sulfate, potassium, 

elemental carbon, and organic carbon.

PM2.5 Trends

Figure 3-1 shows the trend in the District’s design value relative to the national annual PM2.5 

standard of 15 μg/m3. The annual design value is the maximum of design values from individual 

PM2.5 sites. The annual design value for a particular site is the 3-year average of its quarterly 

averaged annual mean PM2.5 concentrations. The design values are marked at the third year of 

three year averages. The District’s annual design value decreased from 14 μg/m3 for 1999-2001 to 

10 μg/m3 for 2009-2011, a 28% reduction. As can be seen, the Bay Area met the national annual 

PM2.5 standard during the entire period.

Figure 3-1 Bay Area PM2.5 Annual Design Value 1999-2001 through 2009-2011

Figure 3-2 shows the District’s design values relative to the national 24-hour PM2.5 standard. For 

the earliest period available, 1999-2001, the design value was 57 μg/m3. By 2009-2011, it had 

112 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
declined to 30 μg/m3, well below the 35 μg/m3 standard. The reduction from 1999-2001 to 2009-

2011 was approximately 46%.

Figure 3-2 Bay Area Design Values for the 24-Hour Standard

1999-2001 through 2009-2011

Figure 3-3 shows an overall downward trend in the number of days that Bay Area PM2.5 levels 

exceeded the 35 μg/m3 standard for each winter from 1999-2000 through 2011-2012. (Although the 

35 μg/m3 standard did not take effect until 2006, the number of exceedance days per year is shown 

as if the 35 μg/m3 standard had been in effect for the entire period shown.) The overall downward 

trend reflects the reduction in PM emissions in response to ARB and Air District control measures, 

whereas the sawtooth pattern in the number of exceedances is primarily due to year-to-year variation 

in meteorology, rather than short-term changes in emissions.

Figure 3-3 Bay Area PM2.5 Exceedances by Winter Number of days 

exceeding the 24-hour NAAQS, November 15 – February 15

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Not only is the Bay Area experiencing fewer exceedance days per year, but when exceedances 

do occur, they are generally less severe. Table 3-4 shows the number of exceedances per 

year relative to the national 24-hour PM2.5 standard adopted in 2006. Both the number of 

exceedance days per year has declined, as well as the average value of the exceedances that 

occur in a given year. The average PM2.5 exceedance value has decreased from over 50 μg/m3

in the 1999-2002 period to approximately 40 μg/m3 in the 2009-2011 period. The combination 

of fewer exceedance days and lower peak values on the days when an exceedance of the 

standard does occur translates into reduced population exposure to unhealthy PM2.5 levels for 

Bay Area residents.

Table 3-4 Exceedances of 24-Hour PM2.5 (35 μg/m3)

 3

Year Number of Exceedances Mean Exceedance Value (μg/m3)

1999 29 52.2

2000 28 50.8

2001 17 61.1

2002 28 51.5

2003 16 42.0

2004 20 43.0

2005 21 41.9

2006 10 46.8

2007 14 44.1

2008 12 42.4

2009 11 38.2

2010 6 40.3

2011 8 40.5

3 Please note that the data shown in Table 3-4 is reported on a calendar year basis, whereas the data presented in Figures 3-3 and 3-4 is 

based on the winter season.

114 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Relationship Between Meteorology and Exceedances of the 24-Hour PM2.5 Standard

Many factors affect PM ambient concentrations. Although emissions of primary PM and the precursor 

pollutants that contribute to secondary PM formation appear to be declining, meteorological factors 

(temperature, humidity, wind speed and direction) that affect PM build-up and transport have a 

strong impact on PM levels on a day-to-day basis. For example, analysis shows that winter periods of 

three or more days with light winds and no rain are conducive to build-up of PM, as discussed more 

fully in Section 2. Figure 3-4 shows that there is a clear correlation between the number of “PMconducive” days (defined here as winter days with light wind and no rain) and the number of days that 

the Bay Area exceeds the national 24-hour PM2.5 standard in a given year.

Figure 3-4 Bay Area PM2.5 Exceedances and Conducive Days by Winter

Comparison with PM2.5 Trends in the Central Valley

Since there is considerable air flow between the Bay Area and the Central Valley – and vice versa 

– it is instructive to compare the current PM levels and design value trends among the three major 

central California districts: that is, Bay Area, Sacramento, and San Joaquin Valley. Figure 3-5 shows 

the trend in Bay Area design values for the national annual PM2.5 standard of 15 μg/m3 compared 

with trends in the Central Valley. The trends in the Bay Area and Sacramento are similar, with an 

average reduction of 2.8% per year for the Bay Area and 1.9% per year for Sacramento. There is no 

clear trend for the San Joaquin Valley.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 115
Figure 3-5 Design Values for Annual PM2.5 Standard for 3 Central California Air Basins

Figure 3-6 shows the design values for the national 24-hour PM2.5 standard of 35 μg/m3 for the 

three air basins. The Bay Area’s design value has decreased 5.8% per year from 2000 to 2010, and 

met the standard for 2008-2010. The design value in the San Joaquin Valley shows a decrease of 

3.8% per year. Sacramento’s design value shows no decrease, partly because of the 2008 wildfires. 

Excluding the wildfire months of June and July 2008, the decrease is 3.3% per year.

Figure 3-6 Design Values* for 24-Hour PM2.5 Standard for 3 Central California Air Basins

116 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
PM10 Trend

Bay Area PM10 levels have decreased significantly since 1990; peak concentrations have declined 

by approximately half and annual average values have declined by about one-third. Figure 3-7 shows 

quarterly-averaged annual PM10 concentrations from 1989 through 2011 relative to the State 

annual standard. (There is no national annual PM10 standard.) The solid line shows the average, 

based upon data from nine PM10 monitoring sites. The reductions were approximately 3%

per year in the 1990s and 2% per year from 2000 through 2011. The average decreased from 

33 μg/m3 in 1989-1991 to 17 μg/m3 in 2009-2011, a reduction of 46%.

Figure 3-7 Bay Area Annual Mean PM10

In summary, we have made substantial progress in reducing PM levels in the Bay Area, but further 

reductions in PM would provide additional benefit by reducing the negative health impacts of PM 

described in Section 1-A.

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Section 4: Summary 

of PM Control 

Program 

This chapter summarizes the existing regulations, policies, and programs that the Air 

District, the California Air Resources Board, and the US Environmental Protection Agency are 

implementing to reduce emissions of primary PM and PM precursors, and to reduce population 

exposure to PM. 

Because primary PM and PM precursors are emitted by a wide range of stationary and mobile 

sources, a comprehensive and multi-faceted effort is needed to reduce ambient PM levels. As 

described in Chapter Section 1-A, there are negative health effects caused by both acute (shortterm) and chronic (long-term) exposure to PM. Therefore, PM control programs aim to reduce both 

short-term (peak) and long-term (annual average) PM concentrations in order to protect public 

health. 

As discussed in Sections 2 and 3, emissions inventory data and air quality monitoring data 

demonstrate that PM emissions and ambient PM concentrations have been greatly reduced in 

the Bay Area in recent years. So even though the Air District has never been required to prepare a 

formal PM SIP attainment plan to date, this data offers tangible evidence that the PM control efforts 

implemented by the Air District and other agencies summarized in this chapter have been effective 

in reducing PM and related public health effects in the Bay Area.

Although the Bay Area has made substantial progress toward meeting State and national PM 

standards, the Air District recognizes that some communities are exposed to above average levels 

of PM and that some individuals are especially vulnerable to the negative health effects of PM. 

Therefore, in addition to reducing emissions of PM and its precursors at the regional scale, the PM 

control effort also focuses on the need to reduce population exposure to PM, especially in the most 

heavily impacted communities and among the most sensitive populations. 

ARB PM Reduction Program

The California Air Resources Board (ARB) has played a vital role in reducing PM by regulating 

emissions of primary PM and PM precursors from most mobile sources, such as on-road cars 

and trucks as well as off-road equipment. ARB has pursued an aggressive program to reduce PM 

emissions from mobile sources throughout California over the past 15 years. ARB classified diesel 

PM as a toxic air contaminant (TAC) in 1998. In September 2000, ARB adopted a Diesel Risk 

Reduction Plan with a goal of 75 percent PM reduction by 2010 and 85 percent by 2020. ARB has 

adopted a comprehensive set of regulations to implement the Diesel Risk Reduction Plan (DRRP) 

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and reduce population exposure to diesel PM. The ARB program to reduce emissions of diesel PM 

includes four key components:

• Regulations to reduce tailpipe emissions of primary PM and PM precursors 

from on-road and off-road mobile sources;

• Cleaner fuel, especially the requirement for ultra-low sulfur diesel;

• Restrictions on vehicle use, such as idling restrictions on diesel engines, and;

• Grants and incentives to encourage emission reductions over and above the 

regulatory requirements.

As summarized in Table 4-1, the heart of the DRRP is a set of regulations called Air Toxics 

Control Measures (ATCMs) to reduce diesel PM emissions from on-road trucks and buses; offroad equipment, including construction, farm and port equipment; harbor craft, and ocean-going 

ships. ARB has phased in the effective dates of these regulations in order to provide time for fleet 

owners to prepare and comply. These regulations will provide increasing benefit as they come into 

full effectiveness over the next 5-10 years. In addition to reducing emissions of primary PM, ARB 

regulations will reduce emissions of precursors that contribute to formation of secondary PM, such as 

ROG and NOx.

Table 4-1 ARB Diesel Air Toxic Control Measures for HeavyDuty Vehicles, Equipment and Ships

Trucks and Buses

Since 2008, idling limited to 5 minutes

By 2016, all trucks meet equivalent of 2007/2010 PM standard

By 2023, all trucks meet equivalent of 2010 NOx standard

Drayage Trucks

By 2010, pre-MY 1994 trucks banned

By 2010, MY 1994-2003 trucks meet 2007/2010 PM standard

By 2014, all trucks meet 2007/2010 PM and 2007 NOx standards

By 2023, all trucks meet 2010 NOx standard

Public Fleet Vehicles By 2012, all vehicles meet equivalent of 2007/2010 PM standard

Garbage Trucks By 2011, all vehicles have installed Best Available Control Technology (BACT)

Transit Buses

By 2003, met a NOx fleet average of 4.8 g/bhp-hr

By 2007, PM emissions reduced by 85% from 2002 baseline

For fleets in the Bay Area with 200+ buses, 15% of new buses purchased from 

2011-2026 must be zero emissions. (May be amended in 2012.)

Truck Refrigeration 

Units By 2020, engines must meet Ultra-Low Emission standard

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 119
Insert text here.

Locomotives

In 2007, begin using 15 ppm Sulfur fuel in California-based locomotives

By 2008, conduct health risk assessments for major rail yards

By 2009, install idling reduction devices on California based locomotives

Construction 

Equipment

Since June 2008, idling limited to 5 minutes

Between 2014 and 2023, fleets with more than 5,000 total hp must meet fleet 

average NOx targets or turnover/replace 4.6-10% of fleet hp

Between 2017 and 2023, fleets with 2,501 to 5,000 total hp must meet fleet 

average NOx targets or turnover/replace 4.6-10% of fleet hp

Between 2019 and 2029, fleets with less than 2,501 total hp must meet fleet 

average NOx targets or turnover/replace 4.6-10% of fleet hp

Cargo Handling 

Equipment

By 2007, new equipment meets equivalent of Tier 4 off-road engine standards or 

2007 PM/NOx on-road engine standards

By 2015, pre-2007 yard trucks meet equivalent of Tier 4 off-road or 2007 PM/

NOx on-road standards

By 2017, all other pre-2007 equipment must meet equivalent of Tier 4 off-road 

or 2007 PM/NOx on-road standards

Harbor Craft

Beginning 2009, engines for new vessels or repowers meet Tier 2 or Tier 3 offroad standards; new ferries must be 85% below Tier 2 standards

By 2016, pre-2000 engines meet Tier 2,3 or 4 off-road standards

By 2022, all marine engines must meet Tier 2,3 or 4 off-road standards

Ships

In 2009, ships began using Marine Diesel Oil (MDO) with 0.5% sulfur or Marine 

Gas Oil (MGO) with 1.5% sulfur. By August 2014, ships begin using MDO or MGO 

with 0.1% sulfur.

By 2014, 50% reduction in auxiliary engine use during 50% of visits by cruise 

and container ships (shore power)

By 2017, 70% reduction in auxiliary engine use during 70% of visits by cruise 

and container ships (shore power)

By 2020, 80% reduction in auxiliary engine use during 80% of visits by cruise 

and container ships (shore power)

Back-Up Generators 

(BUGs) By 2008, PM emissions for BUG’s reduced by 85% in new engines

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Light-Duty Vehicle Emission & Fuel Economy Standards 

Although ARB’s program to reduce PM emissions from diesel engines may have stolen the spotlight, 

its efforts to control emission from light-duty vehicles (LDVs) and medium-duty vehicles (e.g. vans 

and pick-up trucks) have also provided important PM reduction benefits. These vehicles account for 

the vast majority of the 175 million miles that Bay Area residents drive every day. So even though PM 

emissions from gasoline-powered LDVs are very low on a per-mile basis, the combined emissions of 

primary PM from light-duty vehicles are significant. In addition, LDVs account for a major portion of 

ROG and NOx emissions, which are important precursors to the formation of secondary PM. 

ARB’s Low Emission Vehicle (LEV) program is the backbone of its effort to reduce emissions from 

light-duty vehicles. The LEV program has greatly reduced emissions of ROG, NOx, and PM from LDVs 

throughout the state. In its initial phase, LEV I regulations reduced emissions in model year 1994-

2003 vehicles. The more stringent LEV II program, which took effect in model year 2004, continues to 

provide major air quality benefits. As discussed in Section 5, ARB is in the process of adopting a new 

iteration of the LEV program – LEV III – to further reduce pollution from LDVs.

ARB is in the process of finalizing proposed amendments to California’s Low Emissions Vehicle (LEV) 

regulations to strengthen the LEV program. One element of the LEV III proposal is more stringent 

PM standards for passenger cars and light-duty trucks. Although PM emissions from new light-duty 

vehicles are already very low, ARB staff is aware that California and federal emission requirements 

to reduce greenhouse gas emissions have fostered development of advanced internal combustion 

technology such as gasoline direct injection engines (GDI). To encourage the continued development 

of GDI engines that emit PM at the same low levels as port fuel injection engines, the LEV III 

standards would reduce the PM standard from 0.010 grams per mile for passenger cars and lightduty trucks. These standards would be phased in from 2017 through 2021. The LEV III amendments 

propose to further reduce the PM standard to 0.001 grams per mile (one milligram per mile) to be 

phased in during the 2025 and 2028 period. At this 1 mg/mile emission rate, a car would emit a 

total of 150g or 1/3 of a pound of particulate matter over a typical lifetime of 150,000 miles. The 

LEV III provisions should help offset the potential increase in PM emissions from light-duty vehicles 

that would occur if vehicle miles of travel (VMT) in the Bay Area rise in response to population growth 

or other factors.

In addition to the LEV tailpipe emission standards which reduce emissions on a per-mile basis, ARB 

has also adopted fuel economy standards to implement the Pavley legislation4. Although the impetus 

for improved fuel economy is primarily to reduce carbon dioxide emissions that contribute to climate 

change, by reducing fossil fuel combustion these standards also provide important benefits in 

reducing criteria air pollutants and air toxics. 

The ARB regulations summarized above are primarily responsible for the statewide reductions in PM 

emissions from mobile sources to date. These regulations are expected to provide continued reductions 

in emissions of primary PM and PM precursors from mobile sources over the next two decades.

4 State legislation enacted in 2002 (Assembly Bill 1493, Pavley) directed the ARB to adopt regulations to reduce greenhouse gases from 

passenger vehicles. For additional information, see www.arb.ca.gov/cc/ccms/ccms.htm

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 121
EPA Actions to Reduce PM from “Federal Sources”

In addition to establishing ambient air quality standards for PM, US EPA is responsible for establishing 

emission standards for aircraft engines, new locomotive engines and new non-road engines less 

than 175 horsepower used in construction or farm equipment. US EPA regulations help to reduce PM 

emissions from trucks, locomotives, and marine engines that operate across state boundaries when 

these vehicles or engines operate within California. These EPA actions complement the ARB mobile 

source regulations described above. A summary of US EPA programs and actions to control PM and 

PM precursors is available at US EPA at www.epa.gov/pm/links.html.

 In 1998, US EPA adopted more stringent “Tier 2” and “Tier 3” emission standards for ROG, NOx, 

and PM from new non-road diesel engines. This program established the first emission standards 

for non-road diesel engines less than 50 horse-power (hp), including marine engines in this size 

range. The Tier 2 standards were phased in for all engine sizes from 2001 to 2006. More stringent 

Tier 3 standards for engines between 50 and 750 hp were phased in from 2006 to 2008. The 

Clean Air Non-Road Diesel: Tier 4 rule was adopted to provide a comprehensive program to reduce 

emissions from future non-road diesel engines. The Tier 4 standards require engine manufacturers 

to produce new engines with advanced emission control technologies similar to those already 

mandated for on-road trucks and buses. Emissions from these engines are expected to decrease 

by more than 90 percent as a result of this rule. In addition to requiring new locomotives to meet 

stringent standards, US EPA regulations also mandate that old locomotives must be rebuilt to 

comply with cleaner standards. 

Pursuant to Annex VI to the International Convention on the Prevention of Pollution from Ships 

(MARPOL), the US and Canada have collaborated to establish a North American Emissions Control 

Area (ECA) effective August 1, 2012. The ECA will require the use of low-sulfur fuel in ships operating 

within 200 miles of the coast, effective in 2015. The reduction in sulfur will reduce emissions of SO2 

which combine with ammonia to form ammonium sulfate, a type of secondary PM.

BAAQMD PM Reduction Program

The Air District has developed a comprehensive program to reduce PM in the Bay Area. This includes 

measures to reduce emissions and ambient concentrations of PM, as well as population exposure 

to PM. The Air District implements a number of regulations and programs to reduce PM emissions. 

These include rules limiting primary PM emissions from open burning of agricultural and nonagricultural waste; limiting emissions from combustion sources such as boilers, cement kilns and 

furnaces; controlling dust from earth-moving and construction/demolition operations; regulating 

residential wood-burning during the winter season; and reducing PM from activities that generate 

dust or smoke.

The Air District’s Community Air Risk Evaluation (CARE) program has identified communities in the 

Bay Area that are disproportionately impacted by local emission sources. The CARE program, which 

is further discussed later in this chapter, serves as the foundation for the District’s efforts to reduce 

population exposure to toxic air contaminants (TACs), including diesel PM.

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PM Reductions from Control Strategy in the Bay Area 2010 Clean Air Plan

In fall 2010, the Air District adopted the Bay Area 2010 Clean Air Plan (2010 CAP). The legal impetus 

for the 2010 CAP was to update the region’s plan to control ground-level ozone as required by 

the State Health & Safety Code. However, the Air District took the initiative to expand the scope 

of this plan by developing a multi-pollutant air quality plan. The 2010 CAP laid out an integrated 

control strategy to reduce four types of air pollutants: ground-level ozone; primary PM as well as 

PM precursors; toxic air contaminants (TACs); and greenhouse gases, such as carbon dioxide and 

methane, that contribute to climate change. The 2010 CAP control strategy included a total of 55 

control measures in five categories, including:

• Stationary Source Measures (SSMs): The control strategy includes 18 

measures to reduce emissions from stationary and area sources, as further 

described below.

• Mobile Source Measures (MSMs): The control strategy includes 10 

measures reduce emissions by promoting the use of advanced-technology 

vehicles and cleaner fuels that reduce emissions of criteria pollutants 

and/or greenhouse gases, as well as accelerating 

the replacement or repair of older vehicles with high 

emission rates.

• Transportation Control Measures (TCMs): The 

control strategy includes 17 measures to reduce 

motor vehicle emissions by decreasing vehicle use, 

vehicle idling, or traffic congestion by improving transit 

service; encouraging walking, bicycling, and transit 

use; improving the efficiency of the regional transit 

and roadway systems; supporting focused growth; and 

developing and implementing transportation pricing 

strategies.

• Land Use and Local Impact Measures (LUMs): The control strategy 

includes six measures to promote mixed-use, compact development to 

reduce motor vehicle travel and emissions, and to ensure that we plan for 

focused growth in a way that protects people from exposure to air pollution 

from stationary and mobile sources of emissions.

• Energy and Climate Measures (ECMs): The control strategy includes four 

measures designed to protect air quality and the climate by promoting energy 

conservation and energy efficiency; promoting renewable forms of energy 

production; reducing “urban heat island” effects; and promoting the planting 

of shade trees in order to lower air temperatures, provide shading to reduce 

energy use, and absorb CO2 and other air pollutants. 

In developing the 2010 CAP control strategy, the Air District sought to maximize reductions of primary 

PM as well as PM precursors, and to prioritize measures to reduce PM in the implementation 

The control strategy 

defined in the 2010 Clean 

Air Plan is the backbone 

of the Air District’s current 

PM control program.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 123
schedule for the control strategy. The control strategy defined in the 2010 CAP is the backbone of 

the Air District’s current PM control program. Emissions of primary PM and PM precursors will be 

reduced as the Air District adopts and implements the measures in the 2010 CAP.

Reducing PM from Stationary Sources

Controlling emissions from stationary sources (factories, refineries, gas stations, etc.) is the Air 

District’s core regulatory function. The Air District has a long history of controlling PM emissions 

by means of (1) regulations that apply to certain categories of facilities or sources, and (2) permit 

conditions imposed on individual facilities. Permit conditions vary depending upon the size of the 

facility and/or magnitude of emissions that it generates and the type of permit required.

In addition to controlling emissions of primary PM from stationary sources, the Air Districts also 

adopts and enforces regulations to reduce emissions of PM precursors such as NOx and SO2 from 

power plants, industrial facilities, and other combustion sources, as well as reactive organic gases 

(ROG) from oil refineries, coatings and solvents, fuel storage, transfer and dispensing activities, and 

many other industrial and commercial facilities and processes.

The Air District already controls PM10 emissions from facilities subject to its New Source Review 

(NSR) program, and is in the process of amending the NSR requirements to include PM2.5 as 

well. Major PM emission sources are required to implement Best Available Control Technology 

(BACT) for PM in permit conditions when new sources are constructed or existing sources are 

modified. Three types of control equipment are commonly used to abate particulate emissions 

from industrial facilities:

• Wet mechanical scrubbers and/or cyclones

• Baghouses

• Electrostatic precipitators

The Air District has adopted five regulations that directly address primary PM:

• Regulation 5: Open Burning: Generally prohibits open burning, but also 

allows for exemptions such as agricultural burning, disposal of hazardous 

materials, fire training, and range, forest, and wildlife management.

• Regulation 6: Particulate Matter, Rule 1: General Requirements: Limits 

PM emissions from stationary sources by controlling emission rates, 

concentration, visible emissions and opacity.

• Regulation 6: Particulate Matter, Rule 2: Commercial Cooking Equipment: 

Regulates emissions from commercial charbroilers in restaurants.

• Regulation 6: Particulate Matter, Rule 3: Wood Burning Devices: 

Regulates emissions from residential wood-burning devices (fireplaces and 

woodstoves)

• Regulation 12: Miscellaneous Standards of Performance, Rule 4: Sand 

Blasting

124 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
The estimated reductions in emissions of primary PM and PM precursors (NOx and SO2) from Air 

District regulations are shown in Table 4-2.

Table 4-2 Estimated Reductions in Primary PM & PM Precursors 

from BAAQMD Regulations (tons per day)

Category Rule(s) Description

Dates of 

RuleMaking

Primary PM 

Reduced 

tons/day

NOx 

Reduced 

tons/day

Wood-Burning & 

Cooking

6-2

Commercial 

Cooking 

(Charbroiling)

2007 0.6 0

6-3 Wood-burning 

Devices (annual) 2008 0.7 0

6-3

Wood-burning 

Devices (during 

peak season)

2008 6.0 0

Refinery & 

Chemical Plant 

Processing

12-11, 

12-12

Flare Monitoring 

and Minimization 

(SO2 reduction)

2003

2005 

2006

(SO2) 6.3

Combustion of 

Fuels (Nitrogen 

Oxides controls)

9-7, 10

Boilers, Steam 

Generators & 

Process Heaters

1992, 

1994, 

2008, 

2010

0 41.3

9-8, 11-17

Internal 

Combustion 

Engines

1993, 

2007, 

2011

0 27.6

9-11

Electric Power 

Generating 

Boilers

1994, 

2000 0 17.5

9-9 Stationary Gas 

Turbines

1993, 

2006 0 7.4

9-6 Gas-Fired Water 

Heaters 2007 0 2.5

-- All Others n/a 0 5.7

Total 7.3 NOx: 102.0

SO2: 6.3

In developing the control strategy for the Bay Area 2010 Clean Air Plan, the Air District performed 

a thorough review of its stationary source regulations, as well as regulations from other regions 

throughout the United States, and identified several new or amended rules to further reduce 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 125
emissions of primary PM as well as precursors to secondary PM. Stationary Source Measures in 

the 2010 CAP to reduce emissions of primary PM and PM precursors are shown in Table 4-3.

Table 4-3 Stationary Source Measures to Reduce PM & PM

Precursors in Bay Area 2010 Clean Air Plan

Stationary Source 

Measure (SSM) Description Status Completion Date

SSM #1 - Metal Melting 

Facilities

Limit emissions of organic compounds, fine 

particulates, toxic compounds, and odors 

from foundry operations and metal melting 

facilities.

Initial workshops, 

July 2011. Second 

workshops July 2012. 

Fall 2012

SSM #2 Digital Printing

Establish VOC limits or control requirements 

for inkjet, electro-photographic and other 

digital printing technologies.

Not yet initiated. TBD*

 SSM #3 - Livestock 

Waste

Establish management practices to reduce 

ROG, ammonia, PM, GHG. Not yet initiated. TBD

 SSM #4 - Natural 

Gas Production and 

Processing

Reduce emissions of VOCs and methane from 

natural gas production facilities. Not yet initiated. TBD

SSM #5 - Vacuum 

Trucks

Require carbon or other control technology on 

vacuum trucks to reduce emissions of VOCs.

Adopted April 18, 

2012. April 2012

SSM #6 - General 

Particulate Matter 

Emission Limitation

Reduce particulate weight limitation as a 

function of exhaust gas volume and/or as a 

function of process weight rate.

Rulemaking initiated 

May, 2011. TBD

SSM #7 - Opening 

Burning

Further limit agricultural burning of some 

crops to be burned on a given day to reduce 

VOCs, NOx, and PM.

Rulemaking not yet 

initiated. TBD

SSM #8 - Sulfur Dioxide 

from Petroleum Code 

Calcining

Reduce SOx emissions from coke calcining. Initiated April 2012. TBD

SSM#9 – Cement Kilns Further limit NOx and PM from cement 

production and reduce toxic emissions.

Workshop was held 

in December 2011. 

Public hearing 

expected Sept, 2012.

Expected 

Sept 2012

SSM #10 - Refinery 

Boilers and Heaters

Further reduce NOx emissions from refinery 

boilers, heaters, and steam generators.

Adopted December 

15, 2010. 

December 

2010

SSM #11 - Residential 

Fan Type Furnaces

Reduce allowable NOx limits for residential 

furnaces. Not yet initiated. TBD

126 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Stationary Source 

Measure (SSM) Description Status Completion Date

SSM #12 - Large 

Residential and 

Commercial Space 

Heating 

Establish NOx limits for industrial and 

commercial space heating. Not yet initiated. TBD

SSM #13 - Dryers, 

Ovens, and Kilns

Establish NOx limits for industrial dryers, 

ovens, and kilns. Not yet initiated. TBD

SSM #14 - Glass 

Furnaces Reduce NOx limits for glass furnaces. Not yet initiated. TBD

SSM #16 - New Source 

Review Addressing PM 

2.5

Amend Reg. 2, Rule 2 to address the District’s 

anticipated non-attainment status of the 

24-hour PM2.5 National Ambient Air Quality 

Standard. 

Workshop March, 

2012. Board hearing 

anticipated Sept/Oct 

2012.

Fall 2012

SSM #17 - New Source 

Review for Toxic 

Contaminants

Implement more health-protective permitting 

requirements in Regulation 2, Rule 5, New 

Source Review of Toxic Air Contaminants 

based on revisions to OEHHA risk factors and 

method. 

Adopted January 6, 

2010. January, 2010

SSM #18 – Revisions 

to Air Toxic Hotspots 

Program

Revise the District’s Air Toxics Hot Spots 

program to incorporate more stringent risk 

reduction requirements from existing sources.

Awaiting OEHHA 

revisions to exposure 

assessment 

guidelines expected 

2012.

TBD

*To be determined

Pursuant to SSM #6 in the 2010 CAP, Air District staff has embarked upon a thorough review of the 

general PM rule (Regulation 6, Rule 1) with the objective of imposing more stringent emissions limits 

based upon the latest control technologies, as discussed in Section 5. 

Reducing PM from Mobile Sources

The California Air Resources Board has primary legal authority to regulate emissions from mobile 

sources, as described above. However, recognizing that on-road and off-road vehicles are major 

sources of primary PM emissions and PM precursors in the Bay Area, the Air District also works to 

reduce emissions from mobile sources. The Air District seeks to reduce PM emissions from mobile 

sources by means of grants and incentives, targeted enforcement of ARB regulations, partnerships, 

and public education.

In the case of heavy-duty vehicles, the Air District focuses its efforts on ensuring compliance with 

ARB’s diesel regulations in the Bay Area; incentivizing early compliance with ARB regulations; 

and promoting the use of alternative fuels and technologies. The Air District’s efforts to reduce 

emissions from passenger vehicles include accelerating the repair or replacement of old vehicles 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 127
with high emission rates; promoting the use of alternative fuels and technologies, such as zero 

emissions vehicles; and reducing motor vehicle use by promoting transit use, ridesharing, bicycling, 

walking, and telecommuting.

Reducing Emissions from Seaports and Goods Movement

Movement of goods and freight is a major source of particulate matter emissions and other air 

pollutants in major freeway corridors, in ports and rail yards, and in the disproportionately-impacted 

communities identified by the Air District’s Community Air Risk Evaluation (CARE) program. Therefore, 

reducing emissions from seaports and the goods movement sector has been a major focus of Air 

District efforts in recent years. To provide a technical foundation, the Air District has developed 

detailed emissions inventories for each of the five Bay Area seaports (Oakland, Richmond, Redwood 

City, Benicia, and San Francisco). Much of the emission reduction effort has been directed at the Port 

of Oakland, since this port handles by far the greatest volume of goods and is located in the impacted 

western Alameda County area identified by the CARE program. 

To develop a comprehensive approach to reducing emissions from port operations, the Port of 

Oakland, in partnership with the Air District and other stakeholders, developed the Maritime Air 

Quality Improvement Plan (MAQIP) in 2009, with the overall goal of protecting the local residents 

and workers by reducing their exposure to diesel PM. The Air District and the Port of Oakland have 

developed a joint work program that includes outreach to the regulated community to ensure 

compliance with state and federal regulations; and cooperating to identify and implement projects 

to reduce emissions, such as replacement and retrofit of drayage trucks; shore power (dockside 

electrification) for ships; vessel speed reduction; and development of a “marine highway” between 

the ports of Oakland, West Sacramento, and Stockton to help reduce on-road truck traffic between 

these ports. 

Significant emission reductions at the Port of Oakland have already been achieved through a 

combination of grants and regulations. For example, over the past several years, ships have been 

required to switch to low-sulfur fuel. In addition, significant grant funding has been provided to equip 

drayage trucks that serve the Port with diesel particulate filters or with new cleaner engines, as 

discussed further below. Additional benefits will be achieved by 2015 as engines in cargo-handling 

equipment and harbor craft are either replaced or retrofitted, and ships begin using shore power 

while berthed.

In addition to these efforts to reduce emissions, the Air District also collaborated with ARB, the Port 

of Oakland, and Union Pacific Railroad in performing the 2008 West Oakland Health Risk Assessment 

(HRA). The HRA was performed to help understand the potential public health impacts from diesel PM 

emissions on the West Oakland community. The study addressed the health impacts from maritime 

activities at the Port, as well as locomotives, non-Port marine vessels and trucks and other significant 

sources of diesel PM emissions in and around the West Oakland community.

128 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Mobile Source Compliance Plan 

As noted above, ARB has adopted a comprehensive set of regulations to reduce emissions of 

PM from diesel engines. These regulations provide an option for local air districts to play a role in 

enforcing these regulations within their boundaries. Recognizing that effective enforcement of the 

diesel regulations is essential to protect the health of Bay Area residents, the Air District executed 

a Memorandum of Understanding (MOU) with ARB in fall 2009 and established a Mobile Source 

Compliance Plan (MSCP) which sets forth a comprehensive strategy to enforce specified ARB 

regulations, including regulations that apply to drayage trucks, commercial vehicle idling, transport 

refrigeration units, and off-road vehicles. 

The goal of the MSCP is to reduce diesel PM health risk in CARE impacted communities through a 

robust enforcement and compliance assistance program. The initial focus of the MSCP was to provide 

a strong presence at the Port of Oakland to enforce ARB’s 2010 Drayage Truck Rule. As of the first 

quarter of 2012, MSCP resources have been focused on preventing illegal “drayoffs” whereby drayage 

trucks switch loads from compliant to dirty trucks. By helping to ensure compliance with ARB’s truck 

rule, the Air District’s enforcement efforts have contributed to a major reduction in emissions from 

drayage trucks in the West Oakland area, as discussed below. 

Table 4-4 summarizes MSCP enforcement efforts for calendar years 2010 and 2011. 

Table 4-4 Mobile Source Enforcement Summary for Port of Oakland: 

1/1/2010-12/31/2011

Inspection Type # Inspections # Violations Compliance 

Rate

Heavy-Duty Drayage Trucks 3,581 29 99%

Port Truck Terminal Idling 34 0 100%

Commercial Vehicle/Sleeper Berth Idling 267 12 96%

Railroads: Statewide MOU & BAAQMD Protocol 8 0 100%

Off-Road (Construction) Diesel Equipment 7 0 100%

Portable Equipment Registration Program 600 0 100%

Transport Refrigeration Units 434 8 98%

Commercial Harbor Craft 4 0 100%

Oceangoing Vessels: Fuel-Sulfur Limits 41 0 100%

Oceangoing Ships: Onboard Incineration Limits 41 0 100%

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 129
Grant and Incentive Programs

To complement ARB’s regulations to limit emissions from mobile sources, the Air District provides grants 

and incentives for projects to reduce emissions from both on-road and off-road vehicles. The purpose 

of these grant programs is to achieve “surplus” emissions reductions (i.e., over and above regulatory 

requirements) in order to complement ARB regulations. The Air District awarded a total of $308 million in 

external grants during a five-year period covering FY 06/07 through FY 11/12. In aggregate, these projects 

are estimated to reduce emission of over 1,300 tons of PM; 21,000 tons of NOx; 3,100 tons of ROG; and 

1,325,000 tons of CO2 over their lifetime. Table 4-5 summarizes key grant programs, projects funded from 

2007 through 2011, and the emission benefits of these projects over their lifetime. 

Table 4-5 Air District Grant Programs: Projects Funded from 2007 through 2011

Eligible Equipment/Projects # Projects $$ Awarded PM 

(Tons Reduced)

NOx 

(Tons Reduced)

Grant Program: Transportation Fund for Clean Air (TFCA)

Shuttles

Ride-Sharing

Bicycle Facilities 

Smart Growth

Arterial Management 

Clean Vehicles 

Alternative Fuel Infrastructure

499 $111,000,000 470 940

Grant Programs: Carl Moyer Program (CMP), Mobile Source Incentive Fund (MSIF)

On-road Heavy-Duty Vehicles

Off-Road

Marine Engines

Shore power for Ports/ Ships 

Locomotives

Agricultural Equipment

1,158 $79,373,112 604 13,440

Grant Program: (Vehicle buyback)

Light-duty scrappage 21,673 $18,094,880 8 565

Grant Program: Goods Movement Emission Reduction Program

Drayage Trucks

Other Trucks

Shore power for Ports/ Ships 

Cargo Handling Equipment

Locomotives

Marine Engines

1,901 $72,138,878 292 6,606

As discussed later in this chapter, the Air District has developed its Community Air Risk Evaluation (CARE) 

program to identify communities disproportionately impacted by air pollutants and reduce emissions and 

130 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
health risks in these areas. One of the most direct ways to improve air quality in CARE communities is to 

replace or retrofit dirty engines and vehicles that operate in these communities. The Air District has made 

a commitment to target its grant funds to projects in impacted communities. Figure 4-1 summarizes the 

Air Districts grant funded project allocations over the last five years by project type. Figure 4-2 summarizes 

the funding awarded for projects in CARE communities in 2011 when approximately $60 million in Air 

District grant funds were directed to these communities. 

Figure 4-1 Air District Grants Awarded by Project Type from 2007 

through 2011 (Total value = $308 million)

Figure 4-2 Grant Funds Awarded to Projects in Impacted 

Communities in 2011 (Total value = $60 million)

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 131
Key projects funded by the Air District to reduce emissions from mobile sources in recent years are 

briefly described below.

Drayage trucks

The term “drayage trucks” refers to heavy-duty trucks that handle freight at seaports and intermodal 

rail yards. The Port of Oakland is served by 2,000-3,000 drayage trucks, many of which were 

equipped with old uncontrolled engines. Collectively, these trucks were a major source of emissions 

of diesel PM and other pollutants that endangered the health of people who live or work in West 

Oakland and surrounding areas 

included in the western Alameda 

County CARE community. The Air 

District and partner agencies therefore 

prioritized the need to reduce diesel 

PM emissions from the drayage truck 

fleet that serves the Port of Oakland. 

In 2008, the Air District accepted 

applications for drayage truck retrofit 

and replacement projects as part of its 

port truck upgrade program. Through 

this program the Air District received 

and awarded a total of $25.8 million 

in funding from Air District, State, 

and federal sources. These funds 

were used to upgrade 1,522 trucks operating at the Port of Oakland, including 1,319 truck retrofits 

and 203 truck replacements. This program reduces over 14 tons of diesel PM per year at the Port of 

Oakland. This program reduces approximately 0.3 tons of diesel particulate emissions daily at the 

Port of Oakland and over 14 tons of diesel PM on an annual basis. 

In March 2012 the Air District Board of Directors approved a new initiative that will provide additional 

grant funds to replace drayage trucks with newer trucks that meet the stringent 2007 engine 

emission standards. Grants of up to $10,000 will be available to eligible Bay Area truck owners 

toward the purchase of a truck with a cleaner 2007 model year engine or newer. The Air District 

Board of Directors committed an intial $1.9 million in funding for this initiative, and Alameda County 

has committed an additional $1.4 million. In addition, $25 million in State grant funding has been 

awarded for drayage truck replacement in the Bay Area. This new drayage truck initiative is expected 

to reduce 1.5 tons of PM and 3,401 tons of NOx over the life of the project. This effort will protect 

public health in communities adjacent to the Port, help local port truck drivers comply with the ARB 

drayage truck regulation ahead of schedule, and help maintain the economic vitality of the Port.

There is evidence that the efforts to reduce emissions from goods movement have already improved 

air quality in the West Oakland area. A recent study performed by UC Berkeley (Dallmann et al. 2011) 

found substantial reductions in exhaust emissions of black carbon (the primary constituent of diesel 

PM) and NOx from trucks operating in the vicinity of the Port of Oakland as a result of drayage truck 

retrofit and replacement projects implemented to date. The average black carbon emission factor for 

132 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
this drayage truck fleet decreased by approximately 50%, while the average NOx emission factor 

was reduced by roughly 40%. Emission reductions for black carbon were driven by the retrofit of 

trucks with diesel particulate filter systems and the replacement of older model year trucks with 

newer vehicles; reductions in NOx emissions were mainly the result of truck replacement. 

On-Road Trucks

Since 2008 the Air District has issued two major solicitations for grant applications for on-road 

trucks used to transport goods and freight. The first solicitation resulted in the expenditure of 

nearly $10 million on 211 truck projects (primarily for truck replacements) and reduced more 

than 109 tons of PM and more than 2,300 tons of PM. The Air District received applications for 

over 880 eligible on-road emission reduction projects for the second solicitation, and is currently 

in the process of contracting with truck owners. As part of the second solicitation, the Air District 

will allocate approximately $15 million to truck owners to assist in replacing existing trucks. The 

trucks funded from the second solicitation should be on the road by mid-2013, providing estimated 

emission reductions of more than 55 tons of PM and 1,400 tons of NOx.

Shore Power

Since 2008 the Air District has invested over $31 million in shore power projects in the Bay Area. 

By eliminating the need for ships to run their engines while docked, these projects will provide 

significant reductions in the communities adjacent to the ports of Oakland and San Francisco. The 

shore power installations at the Port of San Francisco (Pier 27) and at the three berths at the APL 

terminal at the Port of Oakland have been completed. The installation of shore power at twelve 

more berths at the Port of Oakland are expected to be completed by the end of 2013. Combined, 

these shore power projects will reduce over 75 tons of PM and 4,000 tons of NOx over their lifetime.

Cleaner School Buses

As noted in Section 1-B, children who go to school in diesel school buses may be exposed to 

emissions from the bus they ride in, especially if the buses are old models that lack emission 

controls. The Lower-Emission School Bus Program is one of the most effective ways to reduce 

exposure of children to diesel 

PM. Using a combination 

of funds from its own grant 

programs, as well as funding 

provided by the California 

Air Resources Board, the Air 

District has allocated over 

$47 million since the year 

2000 for projects to reduce 

emissions from over 1,100 

school buses throughout the 

Bay Area. This includes $36.5 

million to replace old buses 

with new ones; $10 million to 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 133
retrofit 694 buses with diesel particulate control devices, and $676,000 to replace the engines in 

25 buses. 

Reducing Emissions from Light-Duty Vehicles

Reducing population exposure to PM emissions from motor vehicles requires reducing emissions 

from light-duty passenger vehicles as well as heavy-duty diesel engines. Although PM emissions from 

light- and medium-duty gasoline vehicles are very low on a per-mile basis, PM emissions from these 

vehicles are significant on an aggregate basis because light-duty vehicles account for roughly 95% of 

total motor vehicle travel in the Bay Area. The Air District is working to reduce emissions from lightduty vehicles. Several of these efforts are described below.

Smoking Vehicle Program

High-emitting vehicles, often called “smoking vehicles”, make up a small percentage of the vehicle 

fleet; however, they account for a much bigger share of total emissions of PM and other pollutants. 

Fortunately, the number of smoking vehicles has declined in recent years, due to turnover in the 

vehicle fleet as older, dirtier vehicles are replaced by newer, cleaner vehicles that achieve stringent 

State emission standards. Retirement of older, high-emitting vehicles has been accelerated by 

programs to purchase and scrap old vehicles. The Air District administered a successful vehiclescrappage program from 1996 through 2010, which retired over 55,000 old vehicles from Bay Area 

roads during this period. Cumulatively, the program 

reduced over 4,600 tons of ROG, over 2,500 tons 

of NOx, and over 32 tons of PM. The Air District 

phased out its program, but Bay Area residents can 

still participate in the statewide Consumer Assistance 

Program to scrap old vehicles which is administered by 

the California Bureau of Auto Repair.

Although their numbers have been reduced, smoking 

vehicles are still a problem, exposing both the driver 

and members of the public to harmful pollutants. To 

help identify these vehicles, the Air District established 

a smoking vehicle assistance program in the early 

1990’s. Smoking vehicles can be reported via the 

1-800-EXHAUST line, or online at www.800exhaust.org, 

or via an app for iPhones and Android devices. When 

smoking vehicle reports are received, the Air District 

sends an informational letter to the owner describing 

the harmful effects of smoking vehicles and options for vehicle repair or retirement, and requesting 

that the owner take appropriate action to rectify the problem. In the two-year period 2010-2011, 

more than 13,000 smoking vehicle reports were submitted to the Air District.

134 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Electric Vehicles

The long-term solution to improving air quality and reducing emissions of greenhouse gases is 

to transition to zero-emission vehicles (ZEV), such as battery electric vehicles. Therefore, the 

Air District is playing a key role in funding projects to accelerate the adoption of battery electric 

vehicles (EVs) in the Bay Area, with the goal of an adoption of 10,000 ZEVs and 100,000 plug-in 

hybrid electric vehicles in the Bay Area by 2020. Much of this effort is directed at installing a robust 

EV-charging network throughout the Bay Area. In FY 2009/10, the Air District allocated $1.3 million 

for projects to install publicly-available Level 2 chargers in up to 250 locations around the region, 

six direct current (DC) fast chargers to serve taxi fleets, and a Battery-switch station to test the 

viability of this advanced technology as a pilot project with taxi fleets. In FY 2010/11, the Air District 

allocated an additional $5 million to expand this effort, with a goal to install up to 3,000 Level 

2 chargers in the Bay Area, and up to 50 DC fast chargers by the end of 2013. Looking forward, 

the Air District is considering other actions to expand the use of electric vehicles, such as offering 

grants to encourage cities and counties to expand the use of EVs in their fleets, as well as funds 

to incentivize the early adoption of electric vehicles in medium- and heavy-duty vehicles used in 

delivery fleets and similar applications, thus reducing emissions of PM and other pollutants from 

diesel engines.

Bicycle-Sharing

In recent years, major cities in Europe, Asia, and North America have implemented publicly-available 

bicycle-sharing programs to reduce traffic and air pollution in the urban core. The Air District is 

leading a partnership to implement a regional bicycle sharing pilot project in five Bay Area cities, 

in collaboration with transportation agencies in the counties of Santa Clara, San Mateo, and 

San Francisco. This project will deploy 1,000 bicycles in the cities of San Jose, Palo Alto, Mountain 

View, Redwood City and San Francisco for a period of at least 12 months. The goal of the pilot is to 

test and develop a self-funded regional Bike Share System to complement existing transportation 

options by providing a convenient option for residents, commuters, and visitors making short trips 

to and from transit facilities, places of employment and residence, and social and recreational 

destinations. The current schedule is to launch the project by the end of 2012. If successful, the 

project may be expanded to additional communities within the Bay Area.

Reducing PM from Wood Smoke

Wood smoke from residential wood-burning is a major component of PM in the Bay Area, especially 

on winter days when high PM concentrations that exceed the 24-hour PM2.5 standard are most likely 

to occur. At the local scale, in neighborhoods where wood-burning is prevalent during winter months, 

wood smoke can expose people to high PM levels, especially if topographical features (e.g., a valley 

or canyon) and/or weather conditions (an inversion) prevent dispersion of the smoke. Therefore, 

reducing emissions from wood-burning is a key component of the Air District’s efforts to reduce PM 

levels and protect public health in the Bay Area.

The Air District’s efforts to reduce residential wood-burning have evolved over the past two decades. 

Public education and voluntary compliance were the focus of this effort in the 1990’s. The Air District 

began implementing a voluntary Winter Spare the Air program in 1991, requesting that Bay Area 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 135
residents voluntarily curtail wood-burning on days when an 

exceedance of PM standards was forecast. 

In the mid-1990’s, the Air District developed a model wood 

smoke ordinance as a guidance document for cities and 

counties that wished to regulate sources of particulate matter 

in their communities. Along with requesting that residents 

curtail the use of fireplaces and woodstoves in response to 

Winter Spare the Air alerts, this model ordinance promoted the 

use of cleaner technologies to reduce wood smoke pollution. 

Air District staff worked with health agencies and interested 

residents throughout the Bay Area to promote adoption of 

the ordinance. To date, 49 Bay Area cities and counties have 

adopted wood smoke ordinances.

In 2006, US EPA significantly strengthened the national 

24-hour PM2.5 standard, lowering the threshold from 

65 to 35 mg/m3. The Air District recognized that further 

reductions in PM emissions from wood smoke would be 

needed to achieve the new PM2.5 standard, especially on 

days when meteorological conditions are conducive to high 

PM concentrations. Therefore, in 2008 the Air District adopted a stringent wood-burning rule 

(Regulation 6-3), and amended another rule which regulates open burning (Regulation 5). The Air 

District also substantially expanded its public outreach and education program for wood smoke 

reduction.

Summary of Wood-Burning Rule

Key provisions of the wood smoke rule (Regulation 6, Rule 3: Wood-burning Devices) include the 

following:

• Prohibits operation of any indoor fireplace, fire pit, wood or pellet stove or fireplace 

insert on specific winter days when the Air District forecasts that PM2.5 levels may 

exceed the national 24-hour PM2.5 standard. (Regulation 5: Open Burning prohibits 

outdoor recreational fires during the same periods when elevated PM2.5 levels are 

forecast.) 

• Prohibits excess visible emissions from wood-burning devices.

• Requires cleaner burning technology (EPA-Phase II certified wood-burning device or 

pellet stove) when wood-burning devices are sold, resold or installed.

• Requires cleaner burning technology if wood-burning devices are permitted for 

installation in new building construction and remodels. (Installation of new woodburning fireplaces is prohibited).

• Prohibits burning of garbage, non-seasoned wood, plastics and other inappropriate 

materials.

136 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
• Requires labeling of moisture content for wood sold for use within Air District 

boundaries, including instructions on how to dry wood that has moisture content greater 

than 20 percent. 

• Requires a label on packages of wood and other solid fuels (such as pressed logs and 

pellets) instructing the user to check local air quality status before burning these products.

Promoting Compliance with the Wood-Burning Rule

The Air District relies upon both public education and enforcement to promote compliance with the 

wood-burning rule. In addition to preventing exceedances of the national 24-hour PM2.5 standard, 

this effort is intended to reduce wood-burning over the long term by educating the public about the 

hazards of wood smoke.

The public outreach campaign is intended to educate the public as to the requirements of the rule, 

ensure that people are aware that they need to check air quality status before burning, and explain 

the public health benefits from reduced wood smoke pollution. The Winter Spare the Air Alert 

outreach campaign utilizes a wide variety of media and multiple languages to reach the diverse Bay 

Area population and notify the public when a Winter Spare the Air Alert has been called. 

Recognizing that certain areas experience localized impacts of wood smoke, the Air District has 

conducted targeted mailings, with information about the wood smoke rule and the negative 

health effects of wood burning, to neighborhoods with high levels of wood smoke complaints and/

or burning. In addition, the Air District recently developed a new wood smoke model ordinance 

that offers local governments a menu of more stringent and innovative options to choose from to 

reduce neighborhood wood smoke within their jurisdictions. The Air District provided the new model 

ordinance to all cities and counties in the Bay Area in April 2012.

The Air District is planning to take the following actions to enhance enforcement of the wood-burning 

rule for the winter 2012-2013 season:

• Use more conservative forecasting to call Winter Spare the Air alerts that 

trigger the no-burn provision of Regulation 6, Rule 3;

• Add a requirement for first-time violators to complete “wood smoke 

awareness school” or pay a $100 fine; and

• Increase fines for repeated violations of Regulation 6, Rule 3 for secondtime violators, increase the fine to $500, with progressive penalties for 

subsequent violations.

Survey data and air quality monitoring data both indicate that the Air District’s efforts have helped to 

reduce residential wood-burning and avoid exceedances of the 24-hour PM2.5 standard. Responses 

to surveys that the Air District performs to monitor residential wood-burning suggest that Bay Area 

residents are burning less wood and are burning less often. Monitoring data shows that the number 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 137
and severity of high PM days during the winter have decreased, and chemical mass balance data 

indicate that PM2.5 from wood smoke has decreased by approximately 40% in the past several years. 

Reducing Population Exposure to PM

The Air District recognizes that protecting public health means more than just attaining air quality 

standards at the regional scale. Local concentrations of directly-emitted air pollutants, such as 

primary PM, may be elevated in proximity to emissions sources such as major roadways, ports and 

freight distribution hubs, refineries and industrial facilities, airports, and large construction sites. 

To protect public health, we need to analyze population exposure to air pollution, identify those 

communities and populations that are most heavily exposed to air pollutants, and develop strategies 

to reduce population exposure among people who live or work in the most impacted areas. 

The Air District’s efforts to identify and protect impacted communities have been bundled together 

under the banner of the multi-faceted Clean Air Communities Initiative (CACI). Key elements of the 

CACI include the following:

• Implementation of the Community Air Risk Evaluation (CARE) program to identify areas 

in the Bay Area that are disproportionately impacted from transportation and stationary 

sources.

• New or amended regulations to control emissions from stationary sources that impose 

disproportionate impacts in CARE communities (e.g., SSM 1, the metal melting rule).

• Implementation of the control strategy in the Bay Area 2010 Clean Air Plan which 

includes Mobile Source Measures to reduce vehicular emissions; Transportation Control 

Measures to reduce motor vehicle use, and Land Use and Local Impact Measures to 

focus on reducing population exposure in impacted areas.

• Performing special monitoring studies to measure ambient concentrations and/or health 

risks related to PM and/or toxic air contaminants, such as the West Oakland Monitoring 

Study, the Custom Alloy Scrap Sales (CASS) metals study in West Oakland, and the UC 

Berkeley study of truck emissions in West Oakland.

• Enforcement of ARB regulations to reduce emissions from diesel engines, via the Mobile 

Source Compliance Plan described above.

• Providing grants and incentives for projects targeted to reduce emissions within CARE 

communities, as described above.

• Public education and outreach to encourage compliance with the Air District’s wood 

smoke rule.

• Collaboration with local governments to develop Community Risk Reduction Plans, as 

described below.

• Collaboration with regional agency partners at MTC, ABAG and BCDC to coordinate 

regional efforts to promote focused development in a health-protective way via the Air 

Quality/Priority Development Area working group, as described below.

• Development of on-line analytical tools to help local government agencies to identify and 

address air quality issues and impacts in their communities. 

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• Development of a set of standard mitigation strategies to address potential impacts from 

siting new sensitive receptors near sources of TACs, as described below.

• Reviewing and commenting on air quality analyses in CEQA documents prepared for key 

plans and projects. 

• Development of guidance documents and technical tools to help Bay Area cities and 

counties address air quality in their General Plans, as described below.

Figure 4-3 BAAQMD Clean Air Communities Initiative

CARE Program to Identify Impacted Communities

Recognizing that certain neighborhoods and communities in the Bay Area are disproportionately 

impacted by local emission sources, the Air District launched the Community Air Risk Evaluation 

(CARE) program in 2004. The CARE program was initially focused on identifying risks related to 

Toxic Air Contaminants (TACs). An analysis of the various TACs in the Bay Area found that diesel PM 

(identified by ARB as a TAC in 1998) is the TAC that poses by far the greatest health risk in the Bay 

Area. Based on a combination of major emission sources, high population exposures, and sensitive 

populations, the CARE program identified six impacted areas as impacted communities: Concord; 

Richmond/San Pablo; western Alameda County; San Jose; Redwood City/East Palo Alto; and eastern 

San Francisco. In recent years, recognizing that fine PM of all types is harmful to public health, the 

scope of the CARE program has been expanded to include PM2.5 as well as TACs. (The Air District is 

also considering adding other air pollutants, such as ozone, for purposes of identifying and defining 

impacted communities.)

Community Risk Reduction Plans

Addressing air quality issues in local land use and transportation planning also can help reduce 

exposure to air pollution. The Air District, in cooperation with Bay Area planning and health agencies, 

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is developing a new planning tool, known as a Community Risk Reduction Plan (CRRP), to help 

local jurisdictions identify, evaluate, and reduce risks from local sources of toxic air contaminants 

(TAC) and fine PM. The Air District is encouraging cities, especially those that have been identified 

by the Air District’s CARE program as disproportionately impacted by local pollutants, to prepare a 

CRRP. The basic approach to develop a CRRP includes several key steps: (1) developing an inventory 

of TAC and fine PM emissions within a planning area, which may be a whole city or part of a city; (2) 

using dispersion modeling to map ambient concentrations and risks from local pollutants within the 

planning area; (3) developing specific goals and objectives to reduce health risks; and 4) defining 

implementation actions, such as policies and mitigation measures, in order to achieve the goals and 

objectives. The Air District is providing financial and technical support in a pilot program to assist the 

City and County of San Francisco and the City of San Jose in developing CRRPs. 

The San Francisco Department of Public Health and the SF Planning Department have collaborated 

with the Air District to develop a city-wide CRRP that encompasses the impacted areas of eastern 

San Francisco, with the goal of reducing air pollution exposures and associated health risk on a 

city-wide basis. City staff worked with the Air District to develop a detailed emissions inventory and 

in applying local-scale dispersion modeling to identify areas with increased risk from air pollution 

and to produce maps of TAC risks and PM2.5 concentrations from all emission sources. City staff is 

developing a range of potential policies and programs to reduce residents’ exposure to air pollution, 

such as expanding current air filtration requirements (Article 38 in the San Francisco Health Code), 

limiting construction emissions, and more. 

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Insert text here.

San Francisco Health Code Article 38

Model Ordinance for Cities: San Francisco is the first jurisdiction in the country to create a law, 

known as Article 38, to protect future residents from exposure to roadway air pollution. The law will 

prevent avoidable lung disease and premature death in residents living near busy roadways, as well 

as prevent avoidable health care spending, for example, on hospital charges for prevented asthma 

attacks.

San Francisco Health Code Article 38, adopted in 2008, requires residential projects with more than 

10 units located in “Potential Roadway Exposure Zones” (as defined according to maps provided 

by the San Francisco Department of Public Health) to prepare an air quality assessment, using 

modeling tools, to determine whether residents would be exposed to unhealthy levels of PM2.5. The 

Department of Public Health has defined “unhealthy” levels of PM2.5 as roadway concentrations 

greater than 0.2 μg/m3. If the air quality assessment indicates that the roadway-attributable PM2.5 

would be less than 0.2 μg/m3, then no further action is required. If the air quality assessment for 

the residential project indicates that concentrations would be unhealthy, then the project is required 

to mitigate the traffic-related PM2.5 pollutants, using available technology and design features, to 

reduce or remove at least 80% of the ambient PM2.5 from indoor spaces. 

Meeting the performance standard can be accomplished in several ways, including:

1. Designating lower floors for commercial use and upper for residential use;

2. Setback of buildings from roadway air pollution sources;

3. Locating the intake for fresh air ventilation sources at a non-polluted site;

4. Filtration of fresh air ventilation sources; and/or

5. Recirculation and filtration of indoor air.

Economic Impacts: The City/County of San Francisco’s Office of the Controller has determined 

that the economic impacts of Article 38 on the San Francisco economy, the development 

community, and future residents of the City are neutral to positive. Although there is a cost 

associated with implementation of the mitigation measures described above, Article 38 will 

also prevent avoidable health care spending (for example, hospital charges for emergency room 

visits for asthma attack) and help to prevent premature mortality associated with exposure 

to PM. If using a filtration system, the City estimates that costs to install and maintain the 

system will range from approximately $50-700 per year per unit, while the monetary benefit 

of the reduction of premature death is estimated to be approximately $2,100 per unit per 

year. On the basis of this analysis, if installation of a filtration system is required in order 

to comply with the requirements of Article 38, then the Controller has determined that the 

net economic benefit of Article 38 would be approximately $1,400 per unit per year. 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 141
The Air District has also been providing technical assistance to help the City of San Jose develop 

a CRRP. The Air District is preparing city-wide emissions inventory for on-road mobile sources on 

freeways and surface streets, permitted stationary sources, and railroads, airports, and construction 

projects. Initial air dispersion modeling is underway. The City has also engaged in public outreach, in 

partnership with the Air District. As a first step on the policy side, the City included several policies 

in its 2011 General Plan update to analyze and mitigate population exposure from major emissions 

sources. For example, the air quality section of the General Plan includes policies which (1) require 

completion of air quality modeling for sensitive land uses such as new residential developments 

located near emission sources such as freeways and industrial uses; (2) require new residential 

development projects and projects characterized as sensitive receptors to incorporate effective 

mitigation into project designs or to be located an adequate distance from sources of toxic air 

contaminants to avoid significant health risks; and (3) require projects that would emit toxic air 

contaminants to prepare health risk assessments as part of environmental review and employ 

effective mitigation to reduce possible health risks to a less-than-significant level. In addition, the 

General Plan policies mentioned above encourage the use of air filtration devices in existing schools, 

houses and other sensitive land uses; re-designation of truck routes; and the use of vegetative 

buffers between emission sources and sensitive receptors.

Promoting Healthy Focused Development

Continued growth in motor vehicle travel could erode the air quality benefits from the ARB and Air 

District programs described above. We need to better integrate land use, transportation, and air 

quality planning in order to constrain future increases in vehicle travel and emissions. Therefore, 

the Air District supports the effort to focus future development in the Bay Area in areas where public 

transit, biking and walking are viable transportation options. At the same time, however, many of 

the areas identified as good sites for focused growth already experience high concentrations of air 

pollutants due to emissions from existing local sources. In fact, a comparison of areas that have been 

designated as Priority Development Areas (PDAs) to date and the impacted communities identified 

by the Air District’s CARE program shows that there is considerable overlap. This emphasizes that 

we need to plan for focused growth in a way that protects people from exposure to air pollutants, 

especially local pollutants such as PM and air toxics. To address this issue, the Air District is 

committing its resources to help planning agencies (cities, counties, MTC, and ABAG) identify, 

evaluate and mitigate these impacts through the planning and design processes.

The Air District is working actively with partners at both regional and local agencies to support 

focused development to reduce motor vehicle emissions, while ensuring that development is planned 

and designed so as to minimize public exposure to air pollutants and protect public health. 

At the regional scale, the Air District is engaged with its regional agency partners in the effort to 

develop Plan Bay Area. Plan Bay Area, scheduled for adoption in 2013, will update the Regional 

Transportation Plan (RTP) and incorporate a Sustainable Communities Strategy to better integrate 

land use and transportation planning, in response to the requirements of Senate Bill (SB) 375. 

Although SB 375 requirements focus on the need to reduce emissions of greenhouse gases, the Air 

District worked with its regional agency partners to make sure that the performance targets for Plan 

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Bay Area include targets to reduce exposure to particulate matter emissions from motor vehicles and 

to achieve greater reductions of PM in impacted communities. 

In addition, Air District staff is participating in the development and environmental review of Station 

Area Plans where most of the future high-density, transit-oriented development in the Bay Area is 

projected to occur. The Air District is working to identify potential PM2.5 and TAC impacts and develop 

plan-level approaches on how to mitigate these impacts, so that subsequent project-level development 

that conforms to the Station Area Plans is not burdened with costly environmental review.

Since local governments are responsible for land use planning, the Air District is working to develop 

partnerships and provide technical assistance to the nine counties and 101 cities that comprise the 

Bay Area to ensure that air quality considerations are addressed in local land use decisions.

Analytical tools: The Air District developed a set of on-line analytical tools to identify and assess 

the potential impacts from stationary sources, freeways and major roadways in close proximity to a 

development project (or throughout a plan-area). The stationary source tools can be used to identify 

all stationary sources permitted by the Air District and to estimate local PM2.5 concentrations 

associated with each permitted source. The highway tool can be used to identify all highways 

throughout the Bay Area and to estimate local PM2.5 concentrations associated with each highway. 

The roadway tool is a set of tables which show the estimated PM2.5 concentrations associated with 

each major roadway (defined as 10,000 AADT and above) throughout the entire Bay Area. All of the 

tools are county-specific, meaning the information used in the development of the tool has been 

customized for each Bay Area county. 

Mitigation Strategies: The Air District has also developed a set of recommended mitigation 

strategies to reduce exposure to PM2.5 concentrations. These mitigation strategies include measures 

such as: 

• Requiring installation of HEPA filtration systems (rated at MERV 16 or higher);

• Designing the project site to minimize population exposure to air pollutants;

• Limiting residential uses on the ground floor;

• Planting trees to buffer and absorb pollutants;

• Reducing emissions, where applicable via measures such as replacing or 

retrofitting diesel back-up generators; installing electrical hook-ups for diesel 

trucks; requiring trucks to use transportation refrigeration units that meet 

EPA Tier 4 emission standards; requiring advanced drive trains or alternative 

fuels in heavy-duty trucks; and establishing alternative truck routes.

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General Plan Guidelines: The Air District is developing a guidance document to help local 

governments address air quality in their general plans. The general plan is a critically important 

document for local community planning, because local decisions related to growth and development 

must be consistent with the policies and objectives contained in the general plan. In addition to 

elements required by State law, local governments may elect to include additional elements in their 

general plan, such as health or air quality, to better guide their community’s development. The Air 

District’s guidance document provides local agencies with a comprehensive set of air quality-related 

“model policies” that may be used to build an optional air quality element within their general plan. 

The guidance document also offers policy recommendations and implementation strategies for 

community education and outreach, reducing wood burning, improving indoor air quality, and “green” 

building and contracting practices. The Air District’s guidance document should facilitate land use 

planning to improve air quality and protect public health at both the local and regional scale.

Organizational Changes to Focus on Exposure Assessment & Protecting Public Health: To 

further its commitment to analyzing air pollution and minimizing population exposure in impacted 

communities, the Air District reorganized its Planning Division in 2012. The “Special Projects 

Section”, which previously focused on developing emission inventories, was expanded and renamed 

the “Exposure Assessment and Emission Inventory Section”. In addition, the Air District created a 

Health and Science Officer position in July 2012; this position will bring additional expertise to inform 

development of Air District policies to protect public health, as well as strengthen the Air District’s 

partnerships with the public health community in the Bay Area. These changes emphasize that the 

Air District will place a high priority on improving its capabilities in terms of analyzing and reducing 

population exposure to air pollutants in the coming years.

PM Reductions in the Central Valley: As discussed in Section 2, technical analysis performed by the 

Air District indicates that transport of PM from the Central Valley contributes significantly to elevated 

Bay Area PM concentrations during the winter weather pattern typically associated with elevated PM 

levels in the Bay Area. The San Joaquin Valley Unified Air Pollution Control District is in the process 

of developing a PM2.5 SIP submittal to demonstrate how it will reduce emissions of primary PM 

and PM precursors in order to attain the national 24-hour PM2.5 standard. The San Joaquin Valley 

attainment plan must be submitted to US EPA by December 2012. Monitoring data indicates that the 

Sacramento air basin recently met the national 24-hour PM2.5 standard. The Sacramento Air Quality 

Management District plans to develop a re-designation request and maintenance plan to show how 

it will continue to attain the national 24-hour PM2.5 standard over the next decade. In addition to 

improving air quality in the Central Valley, implementation of the control strategies set forth in the San 

Joaquin and Sacramento PM2.5 plans should also help to reduce ambient PM concentrations in the 

Bay Area during weather patterns that facilitate transport of PM from the Central Valley to the Bay 

Area.

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Section 5: Looking 

Forward

This final section provides a conceptual framework to guide future Air District efforts to reduce 

PM in order to protect public health, the climate and the environment. This section identifies 

several challenges; suggests policy guidelines to inform the development of potential future 

measures to reduce PM; describes areas where further study and technical enhancements are 

needed; and provides ideas for how Bay Area residents can reduce their exposure to PM.

As discussed in the preceding sections of this report, the current control programs being 

implemented by the Air District, the California Air Resources Board, and other partners have reduced 

substantially ambient PM concentrations in the Bay Area over the past 15-20 years. The Bay Area 

currently meets the national PM standards and is close to meeting state PM standards. Continued 

implementation of these programs described in Section 4 is expected to further reduce emissions of 

primary PM and PM precursors over the next decade. In addition, new initiatives in the final stages 

of development such as ARB’s Low Emission Vehicle (LEV) III standards, should lead to additional 

PM reductions from targeted emission sectors.

Why It’s Important to 

Continue Reducing PM

The fact that the Bay Area has made tangible progress in reducing PM levels does not mean that we 

can rest easy, however. There are compelling reasons why it is important to continue and enhance 

our efforts to reduce PM.

• Researchers have not been able to establish a safe threshold for population 

exposure to PM. A robust and growing body of research shows that there 

are health impacts associated with exposure to PM even below the current 

standards.

• As new information about the health effects of PM becomes available, the 

US EPA and/or the ARB may issue more stringent standards in the future.

• Even at the relatively low PM levels that currently prevail in the Bay Area, 

PM is the air pollutant most harmful to public health, including premature 

mortality, heart attacks, chronic bronchitis and other key health effects.

• PM levels - and population exposure to PM - can vary significantly at 

the local scale. Even though the Bay Area currently meets national PM 

standards (based on the measurements from the regional PM monitoring 

network), some communities and individuals are exposed to higher 

concentrations of PM.

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• In addition to its detrimental impacts on public health, PM also plays a role in 

climate change and has negative impacts on ecosystems and visibility.

Challenges We Face

Although further PM reductions would benefit the health of Bay Area residents, reducing PM presents 

challenges, several of which are described below.

• The Air District’s authority is limited. Emission sources are diverse, and 

the Air District’s authority is limited to a defined set of sources. For example, 

indoor exposure to ultrafine and fine PM accounts for a significant share of 

total exposure for many people, but the sources and conditions contributing 

to indoor PM excposure are complex and the Air District has limited authority 

to regulate emissions from most indoor sources that contribute to indoor 

exposures.5

• Low-hanging fruit is sparse. As described in Section 4, policies and 

regulations have already been implemented to reduce emissions of 

primary PM and PM precursors from key sources, such as diesel engines 

and residential wood burning. It will be a challenge to identify new control 

measures that achieve significant PM emissions on a cost-effective basis.

• Wood smoke remains a problem. The Air District has made a major effort 

to reduce PM from residential wood burning, as described in Section 4. The 

evidence indicates that this effort has been effective in reducing PM levels 

at the regional scale. However, wood smoke is still a significant source 

of emissions, accounting for roughly one-third of PM2.5 during winter 

months when the Bay Area experiences its highest PM levels. Moreover, in 

neighborhoods where wood burning is prevalent during winter months, wood 

smoke can expose residents to high PM levels, especially if topographical 

features and/or weather conditions prevent dispersion of the smoke. 

Achieving further reductions in residential wood burning will be a challenge, 

however, because the sources are widely dispersed and compliance depends 

upon public education and cooperation and large-scale behavioral change.

• Transport of PM from outside the Bay Area. Analysis shows that on the 

winter days when the Bay Area experiences its highest PM concentrations, 

transport of PM by easterly winds from the Central Valley contributes to PM 

levels in the Bay Area.

• Coordination challenges. Many public agencies and other stakeholders 

have a role to play in reducing PM and protecting public health. The diversity 

of players and perspectives provides opportunities for collaboration and 

5 The Air DIstrict does have authority to regulate emissions from certain indoor sources, including wood-burning cooking, water-heaters and 

furnaces.

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partnership, but also highlights the need to coordinate efforts, identify areas 

of mutual interest, and reconcile competing objectives.

• Regional and community needs. Because certain communities are 

disproportionately impacted by PM and other air pollutants, the Air District 

needs to reduce PM at both the regional scale and in the communities most 

impacted by PM.

• Protecting public health at the regional and community scale. Monitoring 

data show that the Bay Area currently meets the national PM standards. 

However, health studies show that there are health effects even below these 

standards, and we know that certain communities and individuals may be 

exposed to higher levels of PM. GIven this context, how do we determine 

the appropriate objectives in terms of reducing exposure to PM at both the 

regional and the community scale?

Policy Guidelines to Inform 

Future PM Planning

The discussion in this section provides a conceptual framework to guide the Air District’s future 

efforts relating to PM, based on the best available information to date. This is intended to be a 

“working” framwork; the Air District recognizes that it may need to be revised as new information is 

available.

• The Air DIstrict will continue to pursue a multi-faceted approach which combines 

regulations and control measures to reduce emissions of PM and PM precursors 

from sources under its jursidiction, targeted enforcement of ARB regulations on 

sources under ARB control, grants and incentives to achieve emission reductions 

above and beyond regulatory requirements, efforts to reduce population 

exposure, partnerships with the health community and other stakeholders, and 

public education.

• Pursuant to the Bay Area 2010 Clean Air Plan, potential future measures to 

reduce PM will be evaluated on a multi-pollutant basis, to maximize their overall 

air quality, health and climate protection benefit.

• Since science has not yet determined precisely which components of PM are the 

most harmful to public health, the Air District will continue its efforts to reduce 

PM across the board, including all sizes and types of particles. However, the Air 

District will continue to monitor the latest research on PM health effects to inform 

its PM reduction efforts, and will collaborate with the health community to provide 

information on PM emissions and exposure in the Bay Area.

• Results from the Air District’s PM modeling for the Bay Area indicate that reducing 

emissions of primary PM offers the most direct means to reducing ambient PM 

concentrations. This is especially true in terms of reducing local PM “hot spots” 

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which are caused mainly by exposure to emissions of primary PM from motor 

vehicles, residential wood burning, or major point sources.

• The Air District and its partners will also continue efforts to reduce emissions of 

precursor pollutants that contribute to the formation of secondary PM, especially to 

help avoid exceedances of the 24-hour PM2.5 standard during winter months when 

ammonium nitrate is a major component of ambient PM.

• The evidence indicates that reducing emissions of black carbon particles should 

provide both health and climate protection benefits. Therefore, the Air District 

will evaluate potential measures to further reduce combustion of fossil fuels and 

biomass (wood) in order to decrease emissions of black carbon.

• The Air District will continue to enhance its efforts to reduce emissions from 

sources, such as motor vehicles, that account for high population to exposure to 

fine and ultrafine particles.

• The Air District will continue its efforts to better understand local PM 

concentrations and exposures by, for example, expanding monitoring near 

roadways and major emission sources, undertaking special studies, and improving 

methods to estimate population exposure to PM.

• The Air District work to strengthen its partnerships with county health 

departments, local planning agencies, community groups, academic institutions, 

and other stakeholders with an itnerest in improving air quality and protecting 

public health to address the challenges and policy issues identified in this report.

Areas for Further Evaluation

To inform the Air DIstrict’s future efforts to control PM and reduce the negative impacts described in 

previous sections, we need:

• Better information as to which types and sizes of PM are most harmful to 

public health.

• More precise analysis of personal exposure to PM in various microenvironments to determine which types of exposures pose the highest risk to 

health.

• Better understanding as to the interplay between PM and climate change, 

and how the various types of particles promote or inhibit climate heating.

Several specific areas merit further evaluation: (1) ultrafine PM, (2) the role of ammonia as a 

precursor to secondary PM formation, and (3) the climate change impacts of black carbon. The Air 

District will consider each of these areas, as discussed below.

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Ultrafine PM

As described previously, ultrafine particles (UFPM) present special challenges since they are hard 

to measure and extremely dynamic, and because ultrafine PM concentrations tend to be elevated 

and localized in close proximity to emission sources. The current understanding of ultrafine PM in 

the scientific community is still very limited. Although evidence suggests that exposure to ultrafine 

PM may be especially harmful to public health because the particles can penetrate deep into 

body organs and tissues, there are as yet no ambient air quality standards for ultrafine PM nor 

requirements for monitoring of ambient ultrafine PM concentrations. Because of the gaps in our 

understanding of ultrafine PM, efforts to characterize these particles and develop appropriate control 

strategies are still in the early stages. Key constraints in terms of ultrafine PM include the following:

Emissions inventory: As discussed below, the Air District prepared its first ultrafine PM emissions 

inventory in August 2012. However, further work will be needed to refine the inventory as emission 

factors are developed and improved for various many source categories. 

Ambient concentrations: The technology to measure ambient concentrations (particle numbers) of 

ultrafine PM is still evolving. Several ultrafine PM monitors have recently been installed in the Bay 

Area, but we do not yet have a comprehensive ultrafine PM monitoring network.

Modeling: Given its localized and dynamic nature, modeling ultrafine PM may prove especially 

valuable. Developing good estimates to where and when ultrafine PM concentrations are high will 

help identify the types of conditions that are conducive to acute human exposure.

Health studies: Research is on-going to analyze the health effects of ultrafine PM and to determine 

whether there are distinct health effects related to ultrafine PM as opposed to fine particles (PM2.5). 

However, the lack of ultrafine PM monitoring networks in most urban areas, and the consequent 

absence of data on ultrafine particle concentrations, makes it difficult to perform epidemiological 

studies to better elucidate the health effects of ultrafine PM. 

Ambient air quality standards: Despite growing concerns about the health effects of ultrafine PM, there 

are as yet no State or national ambient air quality standards for UFPM. Nor are such standards likely 

to be adopted until the gaps described above have been addressed. In the meantime, PM2.5 serves 

as the closest surrogate for ultrafine PM, despite the fact that evidence indicates that the correlation 

between PM2.5 mass and ultrafine PM numbers is tenuous at best, and may actually be negatively 

correlated.

To address the gaps in our understanding of ultrafine PM, the Air District will undertake initiatives in 

regard to the emissions inventory, monitoring and modeling for ultrafine PM, as summarized in the 

Technical Enhancements section below.

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BAAQMD Advisory Council Recommendations

The Air District’s Advisory Council has been studying ultrafine PM over the past two years and 

receiving presentations from leading experts on ultrafine PM measurement and its health effects. In 

December 2011, the Advisory Council presented recommendations on ultrafine PM to the Board of 

Directors. Key Advisory Council recommendations to the Board include the following:

• Integrate ultrafine PM considerations into PM2.5 planning.

• Continue its efforts to characterize ultrafine PM sources, chemical 

composition, and ambient air levels in the Bay Area. 

• Consider development of a ultrafine PM emission inventory and monitoring 

strategy.

• Consider conducting short-term intensive ultrafine PM monitoring to 

characterize ambient ultrafine PM levels and speciation at selected key 

locations (e.g., near heavily traveled roadways), possibly integrating those 

efforts with upcoming near-roadway continuous NO2 monitoring required 

pursuant to the 1-hour NO2 national standard.

• Maintain a focus on PM2.5 and ultrafine PM emissions from fuel-burning 

vehicles, with particular attention to PM emissions from unburned and 

partially burned fuel and lube oil.

• Continue to investigate and evaluate measures to reduce personal exposure 

to PM2.5 and ultrafine PM.

• Collaborate with the Association of Bay Area Governments and the 

Metropolitan Transportation Commission to educate the public on the public 

health effects associated with on-road and near road ultrafine PM and with 

PM2.5 exposures; and promote strategies to reduce vehicle miles traveled.

• Collaborate with other agencies (e.g., ARB, South Coast Air Quality 

Management District) in studying ultrafine PM measurements, and health 

impacts, fashioning effective public policy strategies and focusing policy 

development on vulnerable populations and highly impacted areas.

PM and Climate Change (Black Carbon)

As discussed in Section 1-C, the relationship between PM and climate change is complex. A variety 

of physical processes are involved, and different types of particles have differing impacts on climate. 

Although it is difficult to determine the net effect of PM on climate, there is strong evidence that black 

carbon (soot) may be a significant contributor to climate heating. Policy actions to reduce heating 

effects of PM on the climate should focus on reducing black carbon, as well as brown carbon. Such 

actions would also yield important health benefits by further reducing exposure to fine particles. 

Future actions may include the following:

• Develop a Bay Area black carbon emissions inventory; work with the Air 

Resources Board to obtain black carbon emissions factors for motor vehicles 

and other mobile sources.

150 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
• Collaborate with U.S. EPA, ARB, and other agencies as appropriate, to 

develop a Global Warming Potential (or a substitute metric) value that places 

proper weight on the near-term climate protection benefits of reducing black 

carbon.

• Investigate methods to identify and speciate the types of fine particles that 

will be reduced by potential control measures in order to target the particles 

that contribute most to climate heating. 

• Consider developing additional control measures to reduce black carbon 

emissions in the next update of the Bay Area Clean Air Plan.

• In developing measures to reduce particulate emissions, prioritize controls 

on emission sources that have a high ratio of black carbon compared to 

organic carbon, sulfates, and nitrates.

• Continue to enhance the Air District’s efforts to reduce residential woodburning and to educate Bay Area residents about the detrimental health and 

climate effects of wood-burning. 

• Encourage Bay Area cities (and other local agencies) to address black carbon 

in their climate protection plans, and provide technical support to assist in 

this effort.

• Work with local governments and other stakeholders to estimate the public 

health benefits of climate protection measures to reduce greenhouse gases, 

as well as short-term climate forcers such as black carbon.

• Monitor on-going research into the climate impacts of PM, and consider how 

new findings should be reflected in Air District policies and climate protection 

efforts.

• Facilitate communications between policy-makers and the scientific 

community to identify research needs regarding fine particles and climate 

impacts in order to reduce uncertainties in the policy arena.

Ammonia

As noted in Section 2 of this report, ammonia is a precursor to the formation of ammonium nitrate, 

a compound which accounts for a significant share of PM concentrations in the Bay Area, especially 

in winter months when PM concentrations are highest. NOx is converted to nitric acid, which then 

combines with ammonia to form ammonium nitrate. Ammonia is also a precursor to ammonium 

sulfate, which accounts for over 10% of Bay Area PM2.5 on an annual basis. For purposes of PM2.5 

SIP planning requirements, US EPA guidelines6 state that certain precursors (e.g., SO2 and NOx) are 

presumed to be significant contributors to the formation of secondary PM2.5, whereas others, such 

as volatile organic compounds (VOC) and ammonia are presumed not to be significant contributors 

unless the State makes a finding that the precursor contributes significantly to the PM2.5 nonattainment in the air basin or in downwind areas. 

6 See US EPA Clean Air Fine Particle Implementation Rule, April 25, 2007: www.gpo.gov/fdsys/pkg/FR-2007-04-25/pdf/E7-6347.

pdf#page=1 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 151
In determining whether ammonia is a significant contributor to PM formation, the key question is 

which pollutant – ammonia, or NOx (in the form of nitric acid) - is the limiting factor in ammonium 

nitrate formation. Based on the ratio of ammonia to NOx and the dynamics of ammonium nitrate and 

ammonium sulfate formation in the Bay Area, PM2.5 modeling performed by Air District staff found 

that reducing ammonia would be the most effective PM precursor to reduce in order to decrease 

PM2.5 concentrations, as described in Section 2.7

To date, PM2.5 SIP plans prepared by other air districts in the state, including South Coast, San 

Joaquin Valley, and Sacramento, have not treated ammonia as a significant precursor to PM and 

have not proposed any regulations or policies to control ammonia emissions. It should also be 

noted that, even if there is a solid technical rationale for controlling ammonia emissions in the 

Bay Area, there would be challenges in terms of how to control ammonia emissions. The source 

categories that account for the bulk of ammonia emissions are very different than for primary PM 

or the other PM precursors. The menu of available control measures appears to be limited, and 

some of the major sources (e.g. human perspiration and respiration, domestic animal waste) do 

not lend themselves to regulation.

To better inform a future decision as to whether to consider ammonia a significant contribute to 

secondary PM formation and pursue measures to reduce ammonia emissions, Air District staff will 

analyze future PM2.5 modeling results, seek to improve the Bay Area ammonia emissions inventory, 

and monitor on-going research and policy guidelines that shed light on this issue.

Technical Enhancements Needed to 

Support the Air District’s Efforts

As discussed in the preceding sections of this report, the Air District has been building a solid 

technical and analytical foundation for its PM control efforts in recent years. The Air District 

has been performing cutting edge work in its efforts to measure, analyze, and characterize PM 

emissions, concentrations, population exposure, and health effects. However, because PM is such a 

complex pollutant, there are a number of gaps to be filled. Opportunities to enhance our technical 

understanding and capabilities in regard to PM include:

• Better measurements and/or estimates of localized PM concentrations;

• Better information as to population exposure to PM; and

• Technical data to characterize ultrafine PM emissions, concentrations, and 

population exposure in the Bay Area.

7 This is the opposite of results from the Central Valley, where modeling found that the Valley is so rich in ammonia, primarily from 

agricultural and livestock operations, that reducing ammonia emissions would have little or no effect on decreasing ammonium nitrate 

formation.

152 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Emissions Inventory

The Air District has a detailed inventory of both primary PM emissions of PM2.5 and PM10, as well as 

the precursors that contribute to formation of secondary PM. Several potential enhancements to the 

existing inventory for PM2.5 and PM10 are described below.

Domestic cooking: The Air District PM inventory, like other PM inventories throughout California, 

currently does not include PM from domestic (residential) cooking. This category would include PM 

emitted by the both natural gas and electric stoves and ovens, as well as barbeque grills. It is possible 

that PM from domestic cooking could prove to be a significant source of emissions, given the number 

of stoves and ovens used in Bay Area homes, as well as the volume of outdoor cooking in summer 

months. As noted in Section 1-B, indoor PM levels spike when stoves and ovens are in use, and some 

fraction of this PM finds its way outdoors either by means of stove hood fans, windows, or other types 

of dispersion. Air District staff will investigate PM emissions from domestic cooking with the objective 

of developing emissions estimates for this category. 

Projections of future wood smoke emissions: As discussed previously, information from 

speciation of PM collected on filters, as well as Bay Area wood-burning survey results, indicates 

that the Air District’s program has been successful in reducing wood-burning during the winter 

months. The projections in the PM inventory currently assume that wood-burning emissions will 

remain constant in future years. However, it seems plausible that PM emissions from residential 

wood-burning will decrease further in future years in response to continued public education, the 

gradual phasing out of housing with uncertified wood stoves, conversion of fireplaces to natural 

gas inserts, and other factors. In order to better understand future wood-burning trends, Air District 

staff will analyze and research the various factors that influence residential wood-burning, such as 

the compliance rate during Spare the Air Alerts, the rate at which fireplaces and woodstoves are 

removed, upgraded, or converted to natural gas inserts, etc. Staff may revise the wood-burning 

survey to ensure that it provides information to track compliance with all provisions of the woodburning regulation and overall progress reducing residential wood-burning.

Condensable PM emissions: As mentioned in the emissions inventory discussion, the inventory does 

not include gaseous emissions that condense to form particles when combustion exhaust cools upon 

exposure to ambient air. US EPA guidance for PM2.5 planning states that planning efforts should 

“consider” condensable emissions. This makes sense conceptually; the challenge is to develop test 

methods to accurately estimate condensable emissions for all the various emission source categories. 

Ammonia inventory: As discussed previously, ammonia is one of the precursors to formation of 

ammonium nitrate, a key component of secondary PM. The Air District has developed an ammonia 

emissions inventory for purposes of photochemical modeling of PM2.5 in the Bay Area. ARB has 

developed a separate ammonia inventory for the Bay Area and for other air districts to use in 

preparing PM2.5 SIP submittals that are due to US EPA by December 2012. There are significant 

differences between the Air District’s ammonia emissions inventory and the one provided by ARB. Air 

District staff will work to improve the Air District’s ammonia inventory and to reconcile the differences 

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 153
with the ARB ammonia inventory, as methodologies to estimate ammonia emissions from the various 

source categories are improved and refined.

In addition to improving the existing inventory, the Air District will pursue new initiatives for the 

emissions inventory in regard to ultrafine PM and black carbon.

Regional ultrafine PM emissions inventory: The Air District released its first-ever emissions 

inventory for ultrafine PM in August 2012. The ultrafine PM inventory will be an important tool to 

help in analyzing key emission sources, performing photochemical modeling for PM, and laying the 

groundwork for a potential control strategy to reduce ultrafube PM. Comparing the breakdown of 

emission sources for ultrafine PM versus PM2.5 will be very instructive for purposes of informing 

future control efforts. The Air District will refine and revise the ultrafine PM inventory as new emission 

factors become available.

Develop black carbon inventory: As noted in the section on PM and Climate, the component of PM 

known as black carbon is a potent climate heating agent produced primarily by combustion of fossil 

fuels and biomass. Air District staff will consider the utility and feasibility of developing a black carbon 

inventory in order to inform future efforts to reduce black carbon emissions.

PM Monitoring / Ambient Concentrations

Ultrafine PM Monitoring: The Air District recently installed several ultrafine PM monitors to provide 

continuous measurements in various Bay Area locations, as described in Section 3-A. The monitors 

will serve as the preliminary foundation for the District’s effort to develop data as to ultrafine particle 

number concentrations. Looking forward, the District will consider expanding its UFPM monitoring 

network, subject to the availability of resources to purchase and operate the monitors.

Monitoring Localized and Episodic Concentrations: The Air District will also pursue new 

technologies, opportunities, and partnerships to increase the density of PM2.5 measurements 

in the region, especially in most impacted communities, and near freeways and other major 

emission sources where PM “hot spots” are most likely to occur. In addition, the Air District will 

also investigate whether the network can be augmented with accurate, real-time, mobile PM 

measurement capabilities to determine impacts from short-term episodes and to provide better 

understanding as to how PM concentrations vary over space and time.

PM Photochemical Modeling

Additional PM2.5 Modeling: The Air District performed initial photochemical modeling for PM2.5 

in 2009, as described in Section 2. The 2009 modeling was performed on a 4 km by 4 km grid 

scale using emissions and meteorological data based upon several high PM episodes in winter 

months. The Air District is currently performing additional regional-scale PM2.5 modeling to 

simulate PM formation fat the regional scale for a full year, using emissions and meteorological 

data from all four seasons in order to be able to simulate PM formation for the entire year. Since 

most epidemiological studies are based on annual average exposure, this will enable the Air 

154 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
District to more accurately estimate the health effects related to PM in the Bay Area. In addition, 

it will provide a better understanding of secondary PM formation, and how the formation of 

secondary PM contributes to ambient PM concentrations for the entire year, rather than just 

the winter months. This modeling will also analyze potential impacts of reductions in emissions 

from key PM sources, such as wood smoke. As resources permit, the Air District may consider 

performing additional PM modeling using a finer grid resolution in order to better identify local 

areas with the potential for high PM formation.

Ultrafine PM Modeling: Information as to ultrafine particle concentrations in the Bay Area is 

currently very limited. The Air District will perform its first-ever ultrafine PM modeling on a regionwide basis on a 4 km by 4 km grid scale. The UFPM modeling will simulate concentrations to 

identify potential hot spot areas in the Bay Area. This modeling will also establish a relationship 

between UFPM levels and meteorological conditions.

Analyzing Population Exposure to PM

Reducing population exposure to PM in the Bay Area is the key to reducing its health effects. 

This can be accomplished both by reducing emissions and by avoiding exposures. However, 

as noted in Section 1-B, analyzing population exposure to PM requires accurate data as to (1) 

ambient concentrations of PM at a fine-grained spatial scale throughout the region, (2) personal 

activity patterns; that is, where and when people are exposed to PM, and (3) estimates of PM 

concentrations in various micro-environments such as homes, schools, cars, and sidewalks, and 

how these concentrations are related to ambient levels.

The Air District will consider how to improve its PM population exposure assessment capabilities. Improved 

exposure assessment methods could potentially be used for any or all of the following purposes: 

• To evaluate total population exposure to ambient PM in the Bay Area, and 

to track progress in reducing population exposure at the regional scale or 

among defined population sub-groups.

• To estimate the PM exposure from outdoor versus indoor environments, as 

well as the contribution of key micro-environments, such as in-vehicle, nearroadway, etc.

• To inform future efforts to target and implement the Air District’s CARE 

program and Clean Air Communities Initiative (CACI).

• To inform analyses and policy decisions at the regional and local level to 

promote focused growth, including the development of station area plans, 

general plans, specific plans, community risk reduction plans, etc. 

• To help Bay Area residents understand the magnitude and the key sources or 

their personal exposure to PM, and how they reduce their exposure.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 155
Analyzing Health & Climate Impacts

Air District staff will continue to monitor the latest research as to the health and climate effects of PM 

and incorporate this information into its PM reduction program. In terms of estimating the impact of 

PM on public health in the Bay Area, the Air District will:

• Update the findings of its September 2011 report on the morbidity and 

mortality impacts of PM2.5 in Bay Area, and the economic cost of those 

impacts, based upon the new round of PM2.5 modeling; 

• Update the District’s Multi-Pollutant Evaluation Method (MPEM) to reflect 

results of full-year PM2.5 modeling, the latest health risk factors, and the 

latest information as to the climate impacts of greenhouse gases and the 

value of reducing GHG emissions; and

• Develop estimates as to the health and economic impacts of ultrafine PM, 

based on the results of ultrafine PM modeling and health studies.

How Bay Area Residences Can Reduce 

Their Personal Exposure to PM

In concert with the Air District’s efforts, Bay Area residents may decrease their personal exposure to 

PM by taking simple steps including the following:

• Minimize time spent driving on, or in close proximity to, busy roadways, 

especially those that carry a high volume of heavy-duty diesel vehicles.

• Avoid opening vents and windows while driving on busy roadways.

• Avoid smoke from tobacco products, incense and candles.

• Avoid exposure to wood smoke. Don’t burn wood in a fireplace or stove. 

Avoid campfires, bonfires, and charcoal fires. Replace wood-burning 

fireplaces with a natural gas insert.

• Reduce exposure to PM from cooking by ventilating the kitchen when 

cooking, and switching to electric pilot lights.

• Change filters in furnaces and range hoods on a regular bsais.

• Reduce exposure to PM and other air pollutants from cleaning products by 

ventilating work areas while cleaning and disposing of used rags promptly.

156 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
Conclusion

The Air District is committed to analyzing and reducing PM to protect public health, the climate, 

and the environment. To maintain progress in reducing Bay Area PM levels in the coming years, 

the Air District will continue to monitor the latest research on PM impacts to public health and the 

environment, and to enhance its technical capabilities to measure and analyze PM. In addition, the 

Air District will maintain its efforts to reduce PM by implementing the control measures described in 

the Bay Area 2010 Clean Air Plan and by considering potential additional measures (to be determined 

at a future date) in the course of developing future air quality plans.

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 157
Insert text here.

PM10 PM10 PM10 PM10 PM10 PM2.5 PM2.5 PM2.5 PM2.5 PM2.5 ROG ROG ROG ROG ROG NOx NOx NOx NOx NOx SO2 SO2 SO2 SO2 SO2

2010 2015 2020 2025 2030 2010 2015 2020 2025 2030 2010 2015 2020 2025 2030 2010 2015 2020 2025 2030 2010 2015 2020 2025 2030

INDUSTRIAL/COMMERCIAL PROCESSESBasic Refining Processes 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.2 0.2 0.2 -- -- -- -- -- 0.2 0.2 0.2 0.3 0.3Wastewater (Oil-Water) Separators -- -- -- -- -- -- -- -- -- -- 1.2 1.2 1.3 1.4 1.4 -- -- -- -- -- -- -- -- -- --Wastewater Treatment Facilities -- -- -- -- -- -- -- -- -- -- 0.5 0.5 0.5 0.5 0.6 -- -- -- -- -- -- -- -- -- --Cooling Towers -- -- -- -- -- -- -- -- -- -- 0.9 0.9 1.0 1.0 1.1 -- -- -- -- -- -- -- -- -- --Flares & Blowdown Systems -- -- 0.1 0.1 0.1 -- -- -- -- -- 0.8 0.8 0.9 0.9 1.0 1.2 1.3 1.3 1.4 1.5 0.6 0.7 0.7 0.7 0.8Other Refining Processes -- -- -- -- -- -- -- -- -- -- 0.3 0.3 0.3 0.3 0.4 -- -- -- -- -- -- -- -- -- --Fugitives -- -- -- -- -- -- -- -- -- -- 0.5 0.5 0.6 0.6 0.6 -- -- -- -- -- -- -- -- -- --Subtotal 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.2 4.3 4.5 4.7 5.0 5.2 1.2 1.3 1.4 1.4 1.5 0.9 0.9 1.0 1.0 1.1CHEMICAL MANUFACTURING FACILITIESCoating, Inks, Resins & Other Facilities 0.1 0.1 0.1 0.1 0.1 -- 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 2.7 2.9 3.0 3.2 3.3Pharmaceuticals & Cosmetics 0.4 0.4 0.4 0.5 0.5 0.4 0.4 0.4 0.4 0.5 0.8 0.8 0.9 0.9 1.0 1.4 1.4 1.5 1.6 1.7 3.2 3.3 3.5 3.7 3.9Fugitives - Valves & Flanges -- -- -- -- -- -- -- -- -- -- 0.6 0.7 0.5 0.6 0.6 -- -- -- -- -- -- -- -- -- --Subtotal 0.5 0.5 0.5 0.5 0.6 0.4 0.5 0.5 0.5 0.5 1.5 1.6 1.5 1.6 1.7 1.5 1.5 1.6 1.7 1.8 5.9 6.2 6.5 6.8 7.2OTHER INDUSTRIAL / COMMERCIAL PROCESSESBakeries -- -- -- -- -- -- -- -- -- -- 0.8 0.9 0.9 1.0 1.0 -- -- -- -- -- -- -- -- -- --Cooking(Charbroiling) 2.0 1.8 2.0 2.2 2.4 1.5 1.4 1.5 1.7 1.9 0.9 1.0 1.1 1.2 1.3 -- -- -- -- -- -- -- -- -- --Cooking(Deep Fat Frying) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 -- -- -- -- -- -- -- -- -- --Cooking(Griddles) 0.6 0.7 0.7 0.8 0.9 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.2 -- -- -- -- -- -- -- -- -- --Wineries & Other Food & Agr. Processes 0.5 0.6 0.6 0.7 0.7 0.3 0.3 0.4 0.4 0.4 1.1 1.1 1.2 1.2 1.2 -- -- -- -- -- -- -- 0.1 0.1 0.1Metallurgical & Minerals 2.6 2.8 3.0 3.3 3.5 1.8 1.9 2.1 2.2 2.4 0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.3Waste Management 2.1 2.1 2.2 2.2 2.2 0.6 0.6 0.6 0.6 0.6 3.1 3.1 3.2 3.2 3.2 -- -- -- -- -- -- -- -- -- --Semiconductor Mfg. -- -- -- -- -- -- -- -- -- -- 0.2 0.2 0.3 0.3 0.3 -- -- -- -- -- -- -- -- -- --Fiberglass Products Mfg. -- -- -- -- -- -- -- -- -- -- 0.1 0.2 0.2 0.2 0.2 -- -- -- -- -- -- -- -- -- --Rubber & Plastic Products -- -- -- -- -- -- -- -- -- -- 0.3 0.3 0.3 0.3 0.3 -- -- -- -- -- -- -- -- -- --Contaminated Soil Aeration -- -- -- -- -- -- -- -- -- -- 0.1 0.1 0.1 0.1 0.1 -- -- -- -- -- -- -- -- -- --Other Industrial Commercial 0.8 0.9 1.0 1.0 1.1 0.4 0.5 0.5 0.5 0.6 0.9 1.0 1.1 1.1 1.2 -- -- -- -- -- -- -- -- -- --Subtotal 8.8 9.0 9.6 10.3 11.0 4.9 5.0 5.3 5.8 6.2 7.9 8.3 8.6 9.1 9.5 0.3 0.3 0.3 0.4 0.4 0.3 0.3 0.3 0.3 0.4PETROLEUM PRODUCT / SOLVENT EVAPORATIONPETROLEUM REFINERY EVAPORATIONStorage Tanks -- -- -- -- -- -- -- -- -- -- 3.3 3.5 3.6 3.8 4.0 -- -- -- -- -- -- -- -- -- --Loading Operations 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.2 0.2 -- -- -- -- -- -- -- -- -- --Subtotal 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 3.4 3.6 3.8 4.0 4.2 -- -- -- -- -- -- -- -- -- --FUELS DISTRIBUTIONNatural Gas Distribution -- -- -- -- -- -- -- -- -- -- 0.7 0.8 0.8 0.8 0.8 -- -- -- -- -- -- -- -- -- --Bulk Plants & Terminals -- -- -- -- -- -- -- -- -- -- 0.7 0.6 0.6 0.6 0.6 -- -- -- -- -- -- -- -- -- --Trucking -- -- -- -- -- -- -- -- -- -- 2.6 2.6 2.5 2.5 2.5 -- -- -- -- -- -- -- -- -- --Gasoline Filling Stations -- -- -- -- -- -- -- -- -- -- 5.2 4.9 4.8 4.6 4.5 -- -- -- -- -- -- -- -- -- --Aircraft Fueling -- -- -- -- -- -- -- -- -- -- 1.1 1.0 1.0 1.0 1.0 -- -- -- -- -- -- -- -- -- --Recreational Boat Fueling -- -- -- -- -- -- -- -- -- -- 0.9 0.9 1.0 1.0 1.0 -- -- -- -- -- -- -- -- -- --Portable Fuel Container Spillage -- -- -- -- -- -- -- -- -- -- 3.8 2.8 2.3 2.0 1.8 -- -- -- -- -- -- -- -- -- --Other Fueling -- -- -- -- -- -- -- -- -- -- 0.4 0.4 0.4 0.5 0.5 -- -- -- -- -- -- -- -- -- --

SOURCE CATEGORY

Winter Emissions tons/day

Appendix A

Bay Area Winter Emissions Inventory for Primary PM + PM Precursors: 2010-2030

Appendix A - 

Table 1: Bay Area Winter Emissions Inventory for PM & PM Precursors 2010-2030

158 understanding Particulate matter | 2012 | Bay Area Air Quality Management District
PM10 PM10 PM10 PM10 PM10 PM2.5 PM2.5 PM2.5 PM2.5 PM2.5 ROG ROG ROG ROG ROG NOx NOx NOx NOx NOx SO2 SO2 SO2 SO2 SO2

2010 2015 2020 2025 2030 2010 2015 2020 2025 2030 2010 2015 2020 2025 2030 2010 2015 2020 2025 2030 2010 2015 2020 2025 2030

SOURCE CATEGORY

Winter Emissions tons/day

Appendix A

Bay Area Winter Emissions Inventory for Primary PM + PM Precursors: 2010-2030

Subtotal -- -- -- -- -- -- -- -- -- -- 15.3 14.0 13.4 13.0 12.8 -- -- -- -- -- -- -- -- -- --OTHER ORGANIC COMPOUNDS Cold Cleaning -- -- -- -- -- -- -- -- -- -- 2.7 2.9 3.1 3.3 3.5 -- -- -- -- -- -- -- -- -- --Vapor Degreasing -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --Handwiping -- -- -- -- -- -- -- -- -- -- 2.3 2.5 2.7 2.8 3.0 -- -- -- -- -- -- -- -- -- --Dry Cleaners -- -- -- -- -- -- -- -- -- -- 0.6 0.7 0.7 0.7 0.8 -- -- -- -- -- -- -- -- -- --Printing -- -- -- -- -- -- -- -- -- -- 4.1 4.1 4.2 4.4 4.5 -- -- -- -- -- -- -- -- -- --Adhesives & Sealants -- -- -- -- -- -- -- -- -- -- 9.7 10.6 11.5 12.8 14.0 -- -- -- -- -- -- -- -- -- --Structures Coating -- -- -- -- -- -- -- -- -- -- 21.2 16.3 17.1 17.8 18.6 -- -- -- -- -- -- -- -- -- --Industrial/Commercial Coating -- -- -- -- -- -- -- -- -- -- 8.1 6.8 7.3 7.6 8.1 -- -- -- -- -- -- -- -- -- --Storage Tanks -- -- -- -- -- -- -- -- -- -- 0.8 0.8 0.9 0.9 0.9 -- -- -- -- -- -- -- -- -- --Lightering & Ballsting -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --Other Organics Evaporation -- -- -- -- -- -- -- -- -- -- 1.5 1.6 1.7 1.8 2.0 -- -- -- -- -- -- -- -- -- --Subtotal -- -- -- -- -- -- -- -- -- -- 51.0 46.2 49.0 52.2 55.4 -- -- -- -- -- -- -- -- -- --COMBUSTION - STATIONARY SOURCESFUELS COMBUSTIONNatural Gas(Space/Water Heating/Cooking) 1.7 1.8 1.9 2.0 2.1 1.7 1.8 1.9 2.0 2.1 1.0 1.1 1.2 1.2 1.3 18.3 19.8 20.9 22.1 23.3 0.1 0.1 0.2 0.2 0.2LPG and Liquid Fuel -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 0.5 0.5 0.5 0.5 0.5 0.3 0.3 0.2 0.2 0.2Solid Fuel (Woodstoves) 4.0 4.0 4.0 4.0 4.0 3.7 3.7 3.7 3.7 3.7 2.8 2.8 2.8 2.8 2.8 0.5 0.5 0.5 0.5 0.5 0.1 0.1 0.1 0.1 0.1Solid Fuel (Fireplaces) 13.9 13.9 13.9 13.9 13.9 13.1 13.1 13.1 13.1 13.1 5.3 5.3 5.3 5.3 5.3 1.1 1.1 1.1 1.1 1.1 0.2 0.2 0.2 0.2 0.2Cogeneration 0.7 0.7 0.8 0.8 0.9 0.7 0.7 0.8 0.8 0.9 1.9 2.0 2.1 2.2 2.3 3.9 4.1 4.3 4.6 4.9 0.3 0.4 0.4 0.4 0.4Power Plants 0.3 0.3 0.3 0.4 0.4 0.3 0.3 0.3 0.4 0.4 0.1 0.1 0.1 0.1 0.1 1.0 1.1 1.2 1.3 1.4 0.1 0.1 0.1 0.1 0.1Oil Refineries External Combustion 2.5 2.6 2.7 2.8 3.0 2.4 2.5 2.7 2.8 2.9 1.0 1.1 1.1 1.2 1.2 10.5 11.0 11.6 12.2 12.8 14.0 14.7 15.5 16.2 17.1Glass Melting Furnaces - Natural Gas -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 0.9 0.9 0.9 1.0 1.0 -- -- -- -- --Reciprocating Engines 0.2 0.1 0.1 0.1 0.2 0.2 0.1 0.1 0.1 0.2 0.5 0.5 0.5 0.5 0.6 2.9 2.7 2.6 2.7 2.9 0.1 0.1 0.1 0.1 0.1Turbines 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 -- -- -- -- -- 0.8 0.8 0.9 0.9 1.0 -- -- -- -- --Landfill/Cement Plant Combustion 1.6 1.6 1.7 1.7 1.8 1.6 1.6 1.7 1.7 1.8 1.4 1.5 1.5 1.6 1.6 15.4 16.2 17.1 17.8 18.7 3.0 3.2 3.4 3.6 3.8Subtotal 24.8 25.2 25.5 25.9 26.3 23.8 24.1 24.5 24.8 25.2 14.0 14.3 14.6 15.0 15.3 55.6 58.7 61.4 64.6 67.9 18.2 19.1 20.1 21.1 22.1BURNING OF WASTE MATERIALIncineration 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.2 0.2 0.3 -- -- -- -- --Planned Fires 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 0.2 -- -- -- -- --Subtotal 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 -- -- -- -- 0.1Banked Emissions -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --Alternative Compliance Allowance -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --Subtotal 34.9 35.5 36.5 37.7 38.8 29.8 30.3 31.0 31.9 32.8 97.9 92.9 96.1 100.2 104.5 59.0 62.2 65.1 68.4 72.0 25.3 26.5 27.9 29.3 30.8COMBUSTION - MOBILE SOURCESON-ROAD MOTOR VEHICLESPassenger Cars 5.1 5.2 5.4 5.6 5.8 2.3 2.2 2.3 2.4 2.5 42.1 21.5 12.4 10.1 9.3 35.5 18.8 11.6 9.8 9.2 0.4 0.4 0.4 0.4 0.4Light Duty Trucks<6000lbs 2.3 2.3 2.3 2.4 2.5 1.0 1.0 1.0 1.0 1.1 23.4 15.8 10.9 9.0 7.8 27.1 16.2 9.9 7.1 5.8 0.2 0.2 0.2 0.2 0.2Medium Duty Trucks 6001-8500 lbs 1.2 1.2 1.2 1.2 1.3 0.5 0.5 0.5 0.5 0.5 10.1 9.2 7.9 6.6 5.9 17.9 13.0 9.1 6.5 5.0 0.2 0.2 0.2 0.2 0.2Light Heavy Duty Trucks 8501-14000lbs 0.7 0.6 0.6 0.6 0.6 0.4 0.3 0.3 0.3 0.3 6.2 5.1 4.2 3.4 2.8 23.1 18.0 13.8 10.6 8.5 0.1 0.1 0.1 0.1 0.1Medium Heavy Duty Trucks 14001-33000lbs 1.1 0.7 0.6 0.6 0.6 0.8 0.5 0.3 0.3 0.3 2.4 1.3 0.8 0.7 0.7 21.6 13.8 6.6 3.9 4.0 -- -- -- -- --

Understanding particulate matter | 2012 | Bay Area Air Quality Management District 159
Insert text here. PM10 PM10 PM10 PM10 PM10 PM2.5 PM2.5 PM2.5 PM2.5 PM2.5 ROG ROG ROG ROG ROG NOx NOx NOx NOx NOx SO2 SO2 SO2 SO2 SO2 2010 2015 2020 2025 2030 2010 2015 2020 2025 2030 2010 2015 2020 2025 2030 2010 2015 2020 2025 2030 2010 2015 2020 2025 2030 SOURCE CATEGORY Winter Emissions tons/day

Appendix A

Bay Area Winter Emissions Inventory for Primary PM + PM Precursors: 2010-2030

Heavy Heavy Duty Trucks>33000 lbs 2.0 1.0 0.9 1.0 1.1 1.6 0.7 0.5 0.6 0.6 2.8 1.8 1.8 1.9 2.1 51.7 36.5 23.8 15.6 16.5 0.1 0.1 0.1 0.1 0.1School/Urban Buses 1.0 0.8 0.8 0.8 0.8 0.6 0.5 0.4 0.4 0.4 1.4 1.0 0.9 0.8 0.6 16.9 14.0 11.3 10.2 8.2 -- -- -- -- --Motor-Homes -- -- -- -- -- -- -- -- -- -- 1.4 1.2 0.8 0.5 0.4 1.4 1.0 0.8 0.6 0.5 -- -- -- -- --Motorcycles 0.1 0.1 0.1 0.1 0.1 -- -- -- -- -- 17.5 13.7 13.0 13.3 13.8 2.3 2.4 2.5 2.6 2.7 -- -- -- -- --Subtotal 13.5 11.9 11.9 12.3 12.8 7.3 5.6 5.4 5.6 5.8 107.4 70.6 52.6 46.1 43.3 197.6 133.7 89.5 66.8 60.5 0.9 0.9 1.0 1.0 1.1OFF-HIGHWAY MOBILE SOURCESLawn and Garden Equipment 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 13.6 11.8 10.8 10.8 11.0 2.1 2.0 2.0 2.1 2.2 -- -- -- -- --Transportation Refrigeration Units 0.6 0.3 0.1 0.1 0.1 0.6 0.3 0.1 0.1 0.1 1.9 0.9 0.9 1.2 1.5 7.7 7.5 7.1 8.4 10.4 -- -- -- -- --Agricultural Equipment 0.3 0.2 0.1 0.1 -- 0.3 0.2 0.1 0.1 -- 1.0 0.7 0.4 0.3 0.3 5.9 4.1 2.7 1.7 1.2 -- -- -- -- --Construction and Mining Equipment 0.6 0.5 0.3 0.3 0.3 0.6 0.5 0.3 0.3 0.3 1.6 1.4 1.0 1.0 1.1 11.7 9.6 5.2 4.2 3.8 -- -- -- -- --Industrial Equipment 0.2 0.2 0.1 0.1 0.1 0.2 0.2 0.1 0.1 0.1 1.1 0.9 0.9 0.9 0.9 6.5 5.4 5.0 5.0 5.1 -- -- -- -- --Light Duty Commercial Equipment 0.6 0.5 0.4 0.4 0.3 0.6 0.5 0.4 0.3 0.3 4.5 3.4 2.8 2.7 2.8 7.4 6.3 4.9 4.1 3.8 -- -- -- -- --Trains 0.2 0.2 0.2 0.3 0.3 0.2 0.2 0.2 0.3 0.3 0.9 0.9 0.9 0.9 1.0 10.7 12.3 13.0 13.8 14.9 -- -- -- -- --Off Road Recreational Vehicles -- -- -- -- -- -- -- -- -- -- 1.3 1.2 1.3 1.3 1.4 -- -- -- -- -- -- -- -- -- --Ships 0.2 0.2 0.2 0.2 0.3 0.2 0.2 0.2 0.2 0.3 0.4 0.5 0.6 0.7 0.7 8.3 8.5 7.5 6.1 5.0 1.8 0.7 0.8 0.8 0.9Commercial Harborcraft 0.6 0.4 0.3 0.2 0.2 0.6 0.4 0.3 0.2 0.2 1.2 1.1 1.1 1.1 1.1 16.1 11.1 9.2 8.9 8.9 -- -- -- -- --Recreational Boats 0.5 0.7 0.9 1.1 1.5 0.5 0.7 0.9 1.1 1.5 8.5 8.0 7.9 8.3 9.5 2.3 2.3 2.4 2.4 2.7 -- -- -- -- --Subtotal 4.1 3.4 2.8 2.9 3.3 4.0 3.3 2.8 2.9 3.2 35.9 30.7 28.6 29.3 31.3 78.7 69.2 59.0 56.8 58.0 1.9 0.7 0.8 0.9 1.0AIRCRAFTCommercial Aircraft 0.1 0.2 0.2 0.2 0.2 0.1 0.2 0.2 0.2 0.2 2.4 3.1 3.7 4.0 4.3 9.1 11.1 14.6 16.6 17.5 0.8 0.9 1.0 1.1 1.3General Aviation 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.5 0.5 0.6 0.6 0.6 0.1 0.2 0.2 0.2 0.2 -- -- -- -- --Military Aircraft 0.1 0.2 0.2 0.2 0.2 0.1 0.2 0.2 0.2 0.2 0.7 1.2 1.2 1.2 1.3 1.7 4.0 4.0 4.1 4.1 0.1 0.2 0.2 0.2 0.2Airport Ground Support Equipment -- -- -- -- -- -- -- -- -- -- 0.1 0.1 0.1 -- 0.1 0.8 0.7 0.5 0.3 0.6 -- -- -- 0.1 --Subtotal 0.4 0.4 0.4 0.5 0.5 0.3 0.4 0.4 0.4 0.5 3.7 4.9 5.5 5.8 6.3 11.7 15.8 19.2 21.2 22.3 0.9 1.2 1.3 1.5 1.5MISCELLANEOUS OTHER SOURCESConstruction Operations 10.1 11.2 12.2 13.6 14.8 1.0 1.1 1.2 1.4 1.5 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --Farming Operations 1.5 1.4 1.4 1.4 1.3 0.2 0.2 0.2 0.2 0.2 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --Entrained Road Dust-Paved Roads 23.5 23.5 23.5 23.5 23.5 3.5 3.5 3.5 3.5 3.5 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --Entrained Road Dust-Unpaved Roads 3.8 3.8 3.8 3.8 3.8 0.4 0.4 0.4 0.4 0.4 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --Wind Blown Dust 3.9 3.9 3.9 3.9 3.9 0.7 0.7 0.7 0.7 0.7 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --Animal Waste 7.3 7.4 7.5 7.5 7.6 0.9 0.9 0.9 0.9 1.0 3.9 3.9 3.9 4.0 4.0 -- -- -- -- -- -- -- -- -- --Agricultural Pesticides -- -- -- -- -- -- -- -- -- -- 0.6 0.6 0.6 0.6 0.6 -- -- -- -- -- -- -- -- -- --Non-Agricultural Pesticides -- -- -- -- -- -- -- -- -- -- 0.3 0.3 0.3 0.3 0.3 -- -- -- -- -- -- -- -- -- --Consumer Products(Excluding Pesticides) -- -- -- -- -- -- -- -- -- -- 44.1 45.3 46.5 47.5 48.6 -- -- -- -- -- -- -- -- -- --Accidental Fires and Cigarette Smoking 1.3 1.2 1.1 1.2 1.2 1.2 1.1 1.1 1.1 1.1 0.4 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 0.2 -- -- -- -- --Subtotal 51.3 52.4 53.3 54.8 56.1 7.9 8.0 8.0 8.2 8.3 49.2 50.5 51.8 52.8 53.9 0.2 0.2 0.2 0.2 0.2 -- -- -- -- --GRAND TOTAL EMISSIONS 104 104 105 108 111 49 48 48 49 51 294 250 235 234 239 347 281 233 213 213 29 29 31 33 34

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Area Air Quality Management District
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