San Joaquin Valley Unified Air Pollution Control District December 20, 2012

Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

2012 PM2.5 Plan

SJVUAPCD

Appendix A

Ambient PM2.5 

Data Analysis
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

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San Joaquin Valley Unified Air Pollution Control District December 20, 2012

Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Contents

A.1 PM2.5 CONCENTRATIONS—MEASUREMENT AND INFLUENCES ...............A-1

A.1.1 PM2.5 Monitor Types..............................................................................A-2

A.1.2 Meteorological Influences on PM2.5 Concentrations..............................A-2

A.1.3 Exceptional Events Influences on PM2.5 Concentrations.......................A-5

A.1.3.1 PM2.5 Exceptional Event Documentation Submitted to EPA .....A-5

A.1.3.2 High-Wind Events Effects on PM2.5 Data ..................................A-7

A.2 ATTAINMENT DEMONSTRATION—DESIGN VALUES ....................................A-8

A.3 AMBIENT PM2.5 CONCENTRATION DATA TRENDS ....................................A-17

A.3.1 Days Over the 24-Hour PM2.5 NAAQS ................................................A-17

A.3.2 Seasonal Trends—First- and Fourth-Quarter Averages .......................A-19

A.3.3 Annual Trends of Diurnal PM2.5 Concentration Profiles.......................A-28

A.3.4 PM2.5 Driven Air Quality Index Analysis...............................................A-33

A.3.5 PM2.5 Concentration Frequency Distributions......................................A-38

A.3.6 PM2.5 Concentration Distributions .......................................................A-41

A.3.7 PM2.5 Concentration by Day of Week..................................................A-46

A.4 METEOROLOGY, PM2.5 SPECIATION, AND RULE EFFECTIVENESS ........A-49

A.4.1 Meteorologically Adjusted Trends.........................................................A-49

A.4.2 Trends in PM2.5 Species......................................................................A-51

A.4.3 Effectiveness of District Rule 4901 .......................................................A-55
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

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San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-1 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Appendix A: Ambient PM2.5 Data Analysis

The concentration of ambient PM2.5 at any given location in the San Joaquin Valley 

(Valley) is a function of meteorology, the natural environment, atmospheric chemistry, 

and PM2.5 emissions from regulated and unregulated sources. The San Joaquin Valley 

Air Pollution Control District (District), the California Air Resources Board (ARB), and 

other agencies1 monitor PM2.5 concentrations throughout the Valley, as detailed in the 

2011 Air Monitoring Network Plan,

2

using filter-based monitoring (starting in 1999) and 

real-time concentration monitoring (starting in 2002). The U.S. Environmental Protection 

Agency (EPA) serves as the official repository of ambient PM2.5 data and analysis.3

The District uses the collected data to show air quality improvement through the 

standardized design value calculations, using EPA protocols to document basin-wide 

improvement and attainment of the National Ambient Air Quality Standards (NAAQS). 

As shown in this appendix, the design value data show steady, long-term air quality 

improvement that will lead to the attainment of the 2006 PM2.5 NAAQS. 

The District also uses the data to evaluate the impact of changing daily, quarterly, and 

annual PM2.5 concentrations on public health. These trend analyses provide the 

District with critical information about how to develop control measures and incentive 

programs that provide the most impact to public health improvements.

This appendix provides the technical details used to evaluate and analyze the District’s 

PM2.5 concentration data as summarized in Chapters 3 and 4 of the 2012 PM2.5 Plan. 

It also shows the multiple factors that affect ambient PM2.5 concentrations in the Valley 

(e.g. meteorology, exceptional events) and the evidence for air quality improvement 

through District regulatory actions, including the District’s highly successful Rule 4901 

(Wood Burning Fireplaces and Wood Burning Heaters).

A.1 PM2.5 CONCENTRATIONS—MEASUREMENT AND INFLUENCES

The District, ARB, and other agencies manage an extensive air monitoring network 

throughout the Valley. The information obtained from the PM2.5 monitors within this 

network provide the District with necessary information for demonstrating attainment of 

the NAAQS and valuable information for protecting public health throughout the year. 

The monitoring network captures the spatial, seasonal, daily, weekly, and annual 

variations in PM2.5 concentrations throughout the Valley that result from changing 

meteorology, the occurrence of exceptional events (e.g. high winds and wild fires), and 

PM2.5 emissions from regulated and unregulated sources. 

 

1 Other agencies include the Tachi Yokut Tribe and the National Park Service.

2

San Joaquin Valley Air Pollution Control District [SJVAPCD]. (2011). 2011 Air Monitoring Network Plan. Fresno, CA: 

June 30, 2011 submittal to EPA. Available at 

http://www.valleyair.org/aqinfo/Docs/2011/1_2011AirMonitoringNetworkPlanandAppendixA_Final2.pdf

3 U.S. Environmental Protection Agency: Technology Transfer Network (TTN), Air Quality System (AQS): AQS Web 

Application. (2010). Available at http://www.epa.gov/ttn/airs/airsaqs/aqsweb/
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-2 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

A.1.1 PM2.5 Monitor Types

The District and ARB use three types of PM2.5 monitors in the Valley: 

 Filter-based Federal Reference Method (FRM) monitors, defined as the 

standard for data collection; 

 Real-time beta-attenuation method (BAM) monitors designated as federal 

equivalent method (FEM) monitors, and hereafter referred to as BAM/FEM

monitors; 

 Ordinary BAMs, not designated FEM, and hereafter referred to as BAM; and

 Filter-based speciation monitors, similar to FRM monitors.

Only FRM and BAM/FEM monitors produce data that is suitable for comparison with the 

NAAQS, and are therefore used for design value calculations. Real-time monitors 

(BAM/FEM and BAM) produce hourly measurements that the District uses every day to 

produce daily air quality forecasts, wood burning declarations, public health 

notifications, and Real-time Air Advisory Network (RAAN) notifications for schools. 

The filter-based speciation monitors operate similarly to the standard FRM monitors; 

however, because of the specific analysis requirements for the different PM2.5 species 

(e.g. metals, silicon, chlorine, organics) multiple filter media are required, hence a multifilter collection system. The evaluation and analysis of multiple PM2.5 species is critical 

to the development of an effective attainment strategy.

A.1.2 Meteorological Influences on PM2.5 Concentrations

Particulates in the atmosphere are dispersed by horizontal and vertical mixing within an 

air mass. Wind flow (horizontal mixing) and temperature instability (decreasing 

temperature with height leading to vertical mixing) provides the strongest mechanisms 

for dispersing pollutants. Wind speed can greatly influence the pollutant concentrations

by horizontally mixing and dispersing pollutants over a large area. Generally, the higher 

the wind speed the lower the PM2.5 concentrations; however, in some cases, excessive 

winds may cause elevated PM2.5 levels as high winds entrain PM10 as well as PM2.5.

Vertical mixing of the air mass can result from atmospheric instability. A temperature 

inversion, or increasing temperature with increasing height, can shut down the vertical 

mixing of an air mass, thus creating a situation in which pollutants are trapped near the 

surface. Prolonged periods of high pressure and stable conditions with low-wind speeds 

can cause stagnant conditions that trap pollutants near the surface. PM2.5 

concentrations increase during these poor dispersion periods. During low-pressure 

events, unstable conditions and stronger wind speeds occur. PM2.5 concentrations can 

decrease or increase depending on the strength and characteristics of the low pressure 

system. 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-3 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Hemispheric weather patterns affect Valley meteorology and PM2.5 concentrations. As 

an example, during the 2011–12 wood-burning season (November 2011 through

February 2012), the Valley experienced a strong stagnation episode from December 

2011 to January 2012 that resulted in many days above the PM2.5 24-hour NAAQS of 

35 μg/m3. Unusual climate conditions caused by the La Niña weather pattern resulted 

in historically dry and poor air quality conditions in the San Joaquin Valley and 

throughout the state of California. During this event the District issued an

unprecedented number of wood-burning prohibitions.

A La Niña is caused by a buildup of cooler-than-normal subsurface waters in the tropical 

Pacific. Eastward-moving atmospheric and oceanic waves help bring the cold water to 

the surface resulting in drier-than-normal precipitation and stagnant weather conditions 

not only in the Valley, but throughout most of California. December 2011 tied 

December 1989 (another strong La Niña year) as the driest December on record for the 

Valley. For the Valley, unusually cold overnight temperatures and warm air aloft during 

the 2011–2012 wood-burning season created strong surface-based temperature 

inversions (ranging from 500 feet to 2,500 feet) that trapped particulate pollution within a 

very small volume of air. Because of the La Niña weather pattern, the Valley 

experienced four times as many days at or below freezing during the 2011–2012 winter 

season compared to the previous winter. Combined with clear skies and afternoon 

sunlight, secondary particulate aerosol formation occurred, contributing to higher PM2.5 

levels.

Such extreme, prolonged poor dispersion conditions have not occurred since the 1999–

2000 and 2000–2001 La Niña years. Figures A-1 and A-2 show the PM2.5 

concentration comparison between the three La Niña years for the Fresno-First and 

Bakersfield-California air monitoring sites, respectively.4The 1999–2000 and 2000–

2001 PM2.5 concentrations reflect filter-based data, whereas, 2011–2012 PM2.5 

(preliminary) concentrations reflect measurements from real-time monitors. The 

duration of the PM2.5 events were similar amongst the three seasons, however PM2.5

concentrations were lower during the 2011–2012 season. This reduction in PM2.5 

concentrations under very similar stagnant weather patterns suggests that efforts 

related to District Rule 4901 (Wood Burning Fireplaces and Wood Burning Heaters) and 

other controls that have been implemented have resulted in a reduction in winter-time 

emissions.

 

4 Note: In 2012, the Fresno-First site was moved from the cross streets of First and Shields to First and Garland. The 

data from both Fresno site locations are shown together in Figure A-1.
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-4 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figure A-1 Stagnation Episodes Comparison at Fresno-First

Figure A-2 Stagnation Episodes Comparison at Bakersfield-California
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-5 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

A.1.3 Exceptional Events Influences on PM2.5 Concentrations

In addition to local, regional, and hemispheric weather patterns, Valley PM2.5 

concentrations are also affected by exceptional events such as wildfires, high winds 

events, and fireworks. These unforeseen events (with the exception of fireworks), can 

result in PM2.5 concentration peaks or even extended high-concentration episodes (in 

the case of summertime wildfires). 

With proper documentation and EPA concurrence, data influenced by exceptional 

events can be excluded from official attainment demonstration calculations. Such 

documentation is extensive and requires significant District resources.5 But since 

exceptional events are not reasonably preventable or controllable, it is inappropriate to 

use data influenced by these events without recognition of these circumstances.

EPA generally reviews only those requests that will directly affect an area’s attainment 

status. Although not every event results in a formal submittal to EPA, the District tracks 

these events and their impact on attainment as part of its ongoing air quality analysis. 

These ongoing efforts help the District to more accurately characterize ambient PM2.5 

concentrations and attainment progress.

A.1.3.1 PM2.5 Exceptional Event Documentation Submitted to EPA

The District submitted documentation for the July 4 and 5, 2007 fireworks exceptional 

event and ARB submitted documentation for the summer 2008 wildfires exceptional 

event. If EPA approves this documentation, data from these events will be excluded 

from official attainment demonstration calculations. 

On July 4th and 5th, 2007, elevated hourly PM2.5 concentrations at Fresno-First and 

Bakersfield-California coincided with fireworks activity. Table A-1 summarizes the 

effects of removing this data from the annual mean and 24-hour mean concentration 

calculations. The main effect of removing the data was lowering the PM2.5 annual 

mean values at Fresno-First and Bakersfield-California by 0.4 and 0.2 μg/m3, 

respectively.

In the summer of 2008, just months after adoption of the 2008 PM2.5 Plan, California 

experienced a record number of wildfires, burning more than one million acres. The 

resulting emissions, mostly from outside of the Valley, caused serious public health 

impacts and unprecedented levels of summertime PM2.5 and ozone in the Valley and 

throughout the state. Valley PM2.5 and ozone concentrations were elevated for a 

number of days during this period. 

These exceptional events caused the Valley’s PM2.5 design values to be higher than 

normal. Table A-1 also summarizes the effects of removing this data from official 

attainment demonstration calculations. The prolonged 2008 wildfire event had a 

 

5

Treatment of Air Quality Monitoring Data Influenced by Exceptional Events, 72 Fed. Reg. 55, pp. 13560–13581. 

(2007, March 22). (to be codified in 40 C.F.R. pts. 50 and 51)
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-6 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

noticeable impact, especially for monitoring sites closest to the wildfires in the northern 

portion of the Valley. The largest difference occurred at the Stockton air monitoring site, 

where the 24-hour value was 61.6 μg/m3, with the exceptional event data included, and 

48.2 μg/m3, with that data removed. Excluding days that were impacted by smoke 

reduces the PM2.5 annual mean value by 1.5 μg/m3. 

The EPA has a policy of acting only upon those exceptional event documents that have 

a direct impact on an area’s attainment status. As such, the EPA reviews and makes 

decisions on the concurrence or non-concurrence of the District’s PM10 exceptional 

event documents, but has not yet made a decision on the submitted PM2.5 documents 

mentioned above since that decision would not change the District’s attainment status 

of the PM2.5 NAAQS.

Since the timeframe discussed in the above analysis is outside of the 2009–2011 

period, these exceptional events would not affect the 2011 attainment demonstration 

calculations. However, this analysis is illustrative of how these events can influence 

such calculations, and possibly whether an area is able to achieve attainment of the 

PM2.5 NAAQS.

Subsequent fireworks and wildland fire use exceptional events have occurred since the 

2007 and 2008 documentation that was submitted to EPA. On July 4th, 2010, elevated 

PM2.5 concentrations at Bakersfield-Planz, Fresno-First, and Bakersfield-California 

coincided with fireworks activity. On September 25–30, 2010, Corcoran and MaderaCity experienced smoke impacts and elevated PM2.5 levels from a wildland fire in the 

Sierra Nevadas. Since the District has not submitted official documentation of these 

events to EPA, the impact on the 24-hour and annual mean PM2.5 values are not 

shown. 

Table A-1 2007 and 2008 Exceptional Events Impact on 24-hour and Annual PM2.5 

Values

Year Site

Annual Mean 

Before EE 

Concurrence

Annual Mean 

with EPA EE 

Concurrence

Difference 

μg/m3

24-hour Mean 

Before EE 

Concurrence

24-hour Mean 

with EPA EE 

Concurrence

Difference

μg/m3

2008 Stockton 14.4 12.9 -1.5 61.6 48.2 -13.4

Modesto 16.0 14.7 -1.3 53.3 49.5 -3.8

Merced 14.9 13.9 -1.0 51.1 45.2 -5.9

Clovis 16.0 14.8 -1.2 49.0 49.0 0.0

FresnoFirst 17.4 16.1 -1.3 57.4 54.4 -3.0

FresnoWinery 16.5 15.6 -0.9 44.5 44.5 0.0

Visalia 21.0 19.5 -1.5 62.1 55.5 -6.6

BAK-CA 21.9 20.0 -1.9 64.5 63.4 -1.1

BAK-Planz 23.5 22.8 -0.7 72.3 72.3 0.0

2007 Fresno -

First 18.8 18.4 -0.4 67.0 66.0 -1.0

BAK-CA 22.0 21.8 -0.2 73.0 73.0 0.0
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-7 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

A.1.3.2 High-Wind Events Effects on PM2.5 Data

Valley high-wind events can result in significant increases in PM10 concentrations. 

Geologic particulates are the primary component of PM10 measured during these 

events. The District routinely submits formal PM10 exceptional event documentation for 

such events. 

The District has observed similarities in hourly increases in PM10 and PM2.5 during 

certain high-wind events. High-wind events were recorded at the Bakerfield-Planz 

monitoring site on January 4, 2008 and October 13, 2009, and corresponding PM2.5

measurements were unusually high—100.3 μg/m3and 167.7 μg/m3, respectively. Table 

A-2 summarizes an example of the impact of the 2008 and 2009 high-wind events on 

Bakersfield-Planz attainment calculations (design values represent a three-year 

average of mean 24-hour and annual pollutant concentrations and are discussed fully in 

Section A.2). The District has not submitted PM2.5 exceptional event documentation for 

either of these events. 

Although not included in Table A-2, another high-wind event occurred on April 11, 2010. 

The Bakersfield Planz monitor recorded a PM2.5 concentration of 107.8 μg/m3. 

Similarly, the District submitted documentation to EPA in regards to a PM10 exceptional 

event affecting Bakersfield-Planz that occurred on April 11, 2010, but has not submitted 

PM2.5 documentation

Table A-2 Example of High-Wind Event Influence on Bakersfield–Planz Design Value

High-Wind Events

24-hour Design Values (μg/m3) Annual Design Values (μg/m3)

Year Included Excluded Included Excluded

2007* 72.2 72.2 21.8 21.8

2008 72.3 61.0 23.5 22.6

2009 65.5 65.4 22.5 21.4

2007-09 DV 70 66 22.6 21.9

Difference 4 0.7

* - No high-wind events were captured in the 2007 data set.
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-8 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

A.2 ATTAINMENT DEMONSTRATION—DESIGN VALUES

Design values represent the official metric for assessing air quality improvements and 

attainment of the NAAQS per the Federal Clean Air Act and EPA regulations. Design 

value calculations are three-year averages that follow EPA protocols for rounding, 

averaging conventions, data completeness, sampling frequency, data substitutions, and 

data validity. The results provide consistency and transparency to determine basinwide attainment for both components of the 2006 PM2.5 NAAQS, including the 24-hour 

PM2.5 standard of 35 μg/m3 and the annual PM2.5 standard of 15.0 μg/m3. If any 

monitoring site within the air basin has either a 24-hour or annual PM2.5 design value 

higher than the respective standard, then the entire air basin is designated 

nonattainment.

Table A-3 provides the generalized descriptions of how the 24-hour average and annual 

average design values are calculated for PM2.5. EPA provides detailed guidelines and 

standards for the calculation6and data handling7 methodologies.

Table A-3 General PM2.5 Design Value Calculation Methods

Averaging Period Level Calculation Method

24-hour 35 μg/m3

Step 1: Determine the 98th percentile value for each year 

over a consecutive three year period.

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.

Tables A-4 through A-7 show the trend of the 24-hour average and annual average 

values for each PM2.5 monitoring site in the Valley by year as well as the three-year 

average design values for these metrics through the year 2011.

Table A-4 shows 24-hour single-year 98th-percentile averages, which are used to 

generate the three-year average 24-hour design values in Table A-5. Table A-6 shows 

single-year average PM2.5 concentrations, which are used to generate the three-year 

average annual design values in Table A-7. These data are also shown graphically in 

Figures A-3.1 through A-3.20 for a number of monitoring sites in the Valley. Monitoring 

 

6

Interpretation of the National Ambient Air Quality Standards for PM2.5, 40 C.F.R. Pt. 50 Appendix N (2012).

Available at http://ecfr.gpoaccess.gov/cgi/t/text/textidx?c=ecfr&sid=9bdb7a34dcb75892aef9ee60b74da642&rgn=div9&view=text&node=40:2.0.1.1.1.0.1.18.15&idno=40

7

Environmental Protection Agency [EPA]: Office of Air Quality Planning and Standards. (1999, April). Guideline on 

Data Handling Conventions for the PM NAAQS (EPA-454/R-99-008). Retrieved from 

http://www.epa.gov/ttn/oarpg/t1/memoranda/pmfinal.pdf
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-9 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

sites with a brief PM2.5 monitoring history do not have line charts displayed. Note that 

the sites at Manteca, Madera-City, Tranquillity, and Hanford have limited PM2.5 data 

available; therefore, there is no three-year average. Design values for these sites will 

be able to be calculated in the future as their PM2.5 data records continue. The 

Bakersfield-Golden site was closed in early 2010, and the Corcoran site has been 

unavailable since the middle of 2011.

Average ambient PM2.5 concentrations vary by monitoring site within the Valley. In 

general, monitoring sites in the northern part of the Valley record the lowest ambient

PM2.5 concentrations. Currently more Valley air monitoring sites meet the 1997 24-

hour average standard of 65 μg/m3than the annual average standard of 15.0 μg/m3. 

Although the single-year 98th-percentile and annual average values were higher in 2011 

compared to the last few years, the 2009–2011 design values for some of the sites are 

showing a downward trend, including the peak Valley design value, while others have 

shown an increase. A downward trend will need to occur for all of the sites in the region 

as the Valley progresses towards attainment of the federal standard.
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-10 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Table A-4 Single Year 24-hour Average PM2.5 98th Percentile Values (μg/m3)

SJV Monitoring Site 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Stockton 79.0 55.0 58.0 50.0 41.0 36.0 44.0 42.0 48.0 61.6 40.4 29.7 44.8

Manteca 38.9

Modesto 100.0 71.0 69.0 69.0 47.0 45.0 55.0 52.0 57.4 53.9 54.5 37.3 54.7

Turlock 53.1 43.5 57.4

Merced-Coffee 39.9 47.4

Merced-M 91.9 60.0 49.3 55.1 44.2 43.0 48.3 43.8 52.7 54.0 45.2 35.5 35.4

Madera-City 59.1

Fresno-1st 120.0 90.0 75.0 75.0 56.0 52.0 71.0 51.0 67.0 57.4 55.8 48.8 69.5

Fresno-Winery 64.8 61.5 71.9 49.7 49.4 71.2 55.0 57.4 44.5 48.2 37.0 59.6

Clovis 59.2 72.5 71.5 53.2 48.1 52.4 63.0 51.3 60.9 49.0 49.0 44.3 68.5

Tranquillity 27.7 27.5

Corcoran 53.0 55.1 89.5 65.1 42.2 49.4 74.5 50.1 57.9 47.9 53.4 46.8

Hanford 64.6

Visalia 114.0 103.0 96.0 70.0 47.0 54.0 65.0 50.0 59.7 62.1 53.9 36.3 50.7

Bakersfield-Golden 95.3 93.9 95.9 80.4 51.9 53.9 74.9 64.4 67.7 60.8 68.6

Bakersfield-California 97.4 92.7 94.9 73.0 48.3 61.5 63.2 60.5 73.0 64.5 66.7 53.3 65.5

Bakersfield-Planz 76.5 90.6 66.8 47.5 47.6 66.4 64.7 72.2 72.3 65.5 56.2 43.2

Table A-5 24-hour Average PM2.5 Design Values (Three-Year Averages, μg/m

3

)

SJV Monitoring Site 1999-

2001

2000-

2002

2001-

2003

2002-

2004

2003-

2005

2004-

2006

2005-

2007

2006-

2008

2007-

2009

2008-

2010

2009-

2011

Stockton 64 54 50 42 40 41 45 51 50 44 38

Manteca ^

Modesto 80 70 62 54 49 51 55 54 55 49 49

Turlock 53 48 51

Merced-Coffee 44**

Merced-M 67 55 50 47 45 45 48 50 51 45 39

Madera-City ^

Fresno-1st 95 80 69 61 60 58 63 58 60 54 58

Fresno-Winery 63 66 61 57 57 59 61 52 50 43 48

Clovis 68 66 58 51 55 56 58 54 53 47 54

Tranquillity 28**

Corcoran 66 70 66 52 55 58 61 52 53 49

Hanford ^

Visalia 104 90 71 57 55 56 58 57 59 51 47

Bakersfield-Golden 95 90 76 62 60 64 69 64 66

Bakersfield-California 95 87 72 61 58 62 66 66 68 62 62

Bakersfield-Planz 84 78 68 54 54 60 68 70 70 65 55

Notes for Tables A-2 and A-3

 Empty cell: No data or insufficient data

 Asterisk (*): Values do not meet completeness criteria

 Double asterisk (**): Value based on 2-year average of 2010-2011, 2009 had minimal sampling

 ^ : Site does not have enough data to calculate a 3-year average, see text for details.
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-11 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Table A-6 Single Year Annual Mean PM2.5 Concentrations (μg/m3)

SJV Monitoring Site 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Stockton 19.7 15.5 13.9 16.7 13.6 13.2 12.5 13.1 12.9 14.4 11.3 10.6 11.3

Manteca 10.7

Modesto 24.9 18.7 15.6 18.7 14.5 13.6 13.9 14.8 15.0 16.0 13.0 12.1 14.7

Turlock 16.1 12.7^^ 17.1

Merced-Coffee 16.3 15.6

Merced-M 22.6 16.7 14.5* 18.7 15.7 15.2 14.1 14.8 15.2 14.9* 13.6 11.2 10.4

Madera-City 20.4

Fresno-1st 27.6 24.5 19.8 21.5 17.8 16.3 16.7 16.8 18.8 17.4 15.1 13.0 15.5

Fresno-Winery 18.4 18.6 21.3 17.8 17.0 16.9 17.6 16.8 16.5 14.6 13.4 15.4

Clovis 19.8 16.3 18.0 16.2 18.5* 16.4 16.3 16.4 16.4 16.2 18.3 14.7 17.9

Tranquillity 8.8 8.2

Corcoran 14.3* 16.4 19.2 21.5 16.2 17.4 17.5 16.9 18.4 15.8 17.7 17.9

Hanford 18.0

Visalia 27.6 23.9 22.5 23.2 18.2 17.0 18.8 18.8 20.4 19.8 16.0 13.6 16.1

Bakersfield-Golden 26.2 22.6 21.8 24.1 19.6 18.2 19.1 18.6 19.9 17.9 20.0

Bakersfield-California 23.8 22.5 21.2 22.7 17.1 18.9 18.0 18.7 22.0 21.9 19.0 14.2 16.2

Bakersfield-Planz 20.3 20.8 23.5 17.8 17.4 19.8 19.3 21.8 23.5 22.5 17.6 14.5

Table A-7 Annual PM2.5 Design Values (Three-Year Averages, μg/m3)

SJV Monitoring Site 1999-

2001

2000-

2002

2001-

2003

2002-

2004

2003-

2005

2004-

2006

2005-

2007

2006-

2008

2007-

2009

2008-

2010

2009-

2011

Stockton 16.4 15.3 14.7 14.5 13.1 12.9 12.8 13.5 12.9 12.1 11.1

Manteca ^

Modesto 19.7 17.7 16.2 15.6 14.0 14.1 14.6 15.3 14.7 13.7 13.3

Turlock 15.3^^

Merced-Coffee 16.0**

Merced-M 17.9 16.6 16.3 16.5 15.0 14.7 14.7 15.0 14.6 13.2 11.7

Madera-City ^

Fresno-1st 24.0 21.9 19.7 18.6 16.9 16.6 17.4 17.7 17.1 15.2 14.5

Fresno-Winery 18.5 19.4 19.2 18.7 17.2 17.2 17.1 17.0 16.0 14.8 14.5

Clovis 18.0 16.8 17.6 17.0 17.1 16.4 16.4 16.3 17.0 16.4 17.0

Tranquillity 8.5**

Corcoran 19.0 19.0 18.4 17.0 17.2 17.6 17.0 17.3 17.1

Hanford ^

Visalia 24.7 23.2 21.3 19.5 18.0 18.2 19.3 19.7 18.8 16.5 15.2

Bakersfield-Golden 23.6 22.8 21.8 20.6 19.0 18.6 19.2 18.8 19.3

Bakersfield-California 22.5 22.1 20.3 19.6 18.0 18.5 19.6 20.9 21.0 18.4 16.5

Bakersfield-Planz 21.5 20.7 19.6 18.4 18.9 20.3 21.5 22.6 21.2 18.2

Notes for Tables A-4 and A-5

 Empty cell: No data or insufficient data

 Asterisk (*): Values do not meet completeness criteria

 Double asterisk (**): Value based on 2-year average of 2010-2011, 2009 had minimal sampling

 ^ : Site does not have enough data to calculate a 3-year average, see text for details. 

 ^^ : Data incomplete in 2010, however high PM2.5 season was still captured.
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-12 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-3.1 through A-3.4 24-hour and Annual Design Value Trends at Stockton-Hazelton and Modesto

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-13 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-3.5 through A-3.8 24-hour and Annual Design Value Trends at Merced-M and Clovis

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-14 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-3.9 through A-3.12 24-hour and Annual Design Value Trends at Fresno-First and Fresno-Winery

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-15 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-3.13 through A-3.16 24-hour and Annual Design Value Trends at Corcoran and Visalia

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-16 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-3.17 through A-3.20 24-hour and Annual Design Value Trends at Bakersfield-CA and Bakersfield-Planz 

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-17 Appendix A: Ambient PM2.5 Data Analysis 

 2012 PM2.5 Plan 

A.3 AMBIENT PM2.5 CONCENTRATION DATA TRENDS

Design values summarize data from a monitoring site with just two concentration values

representing a three-year time period: an annual average and a value representing 24-

hour peaks. These parameters are required for attainment demonstrations, but design 

values alone do not reveal the hourly, daily, weekly, seasonal, and regional PM2.5 

effects on public health, nor do they track air quality improvements within such

parameters. The District uses data from air monitoring sites to analyze air quality trends 

to provide a deeper understanding of changes in ambient PM2.5 concentrations as they 

relate to the implementation of District programs and to inform the attainment planning 

process and Risk-based Strategy.

A.3.1 Days Over the 24-Hour PM2.5 NAAQS

The number of days over the PM2.5 NAAQS is another indicator of air quality progress. 

Focusing on historical air monitoring sites from the northern, central, and southern 

portions of the Valley, Figures A-4 and A-5 show the trend of the number of days above 

both the 1997 and 2006 24-hour PM2.5 NAAQS (65 μg/m3 and 35 μg/m3, respectively)

at the Modesto, Fresno-First, and Bakersfield-California monitoring sites. These counts 

have been estimated and normalized to account for the varying sampling schedules of 

the Valley’s 1-in-6-day, 1-in-3-day, and daily PM2.5 monitors. Design value calculations 

for the 24-hour NAAQS use the 98th-percentile concentration value from each 

monitoring site (higher values in the 99th and 100th percentiles are not used to account 

for extreme outliers). Because of this, a region may experience a limited number of 

days over the standard, but still be considered in attainment.

Figure A-4 Trend in Days over the 1997 24-Hour PM2.5 NAAQS

Note: Years and sites with no data (colored bars) represent zero exceedances.
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-18 Appendix A: Ambient PM2.5 Data Analysis 

 2012 PM2.5 Plan 

Figure A-5 Trend in Days over the 2006 24-Hour PM2.5 NAAQS

As shown in Figure A-4, the District has experienced a significant drop in the number of 

exceedances of the 65 μg/m3 standard. In 1999, approximately 103 exceedances of 

this standard occurred between the sites of Modesto, Fresno-First, and BakersfieldCalifornia. Comparing this to the 17 exceedances that occurred in 2011, this represents 

an 83% decrease in the number of violations among these sites.

Figure A-5 shows that significant progress has been made towards the 35 μg/m3

standard despite the fact that sites recorded 35 to 40 exceedances in 2011. The 

number of exceedances of the 2006 PM2.5 NAAQS has decreased by 46%. The 

District’s emissions reduction strategy, the investment from the regulated industry in 

control technology, and the public’s willingness to make a change for cleaner air have 

all played key roles in the reduction of concentrations over this time period. 

Despite this notable progress, the Valley still experiences many exceedance days over

the 2006 24-hour PM2.5 NAAQS (35 μg/m3) during the winter months. During the 

winter, with unfavorable meteorology, as experienced during the 2011–2012 winter 

season, the number of exceedances spiked relative to seasons with favorable 

meteorology under identical regulatory controls. The values for 2011 in Figure A-5 show 

this resulting spike as compared to the years 2009 and 2010, when meteorology was 

more favorable. Similar poor dispersion conditions were experienced during the winter 

of 1999–2000; however, under those similar conditions, the number of exceedances in 

2011 was markedly less than the number of exceedances in 1999, which strongly 

suggest a real reduction in emissions.
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-19 Appendix A: Ambient PM2.5 Data Analysis 

 2012 PM2.5 Plan 

A.3.2 Seasonal Trends—First- and Fourth-Quarter Averages

The Valley experiences the highest PM2.5 concentrations during the fall and winter 

months, when residential wood burning is at its highest. The District evaluates ambient 

concentration trends during this time period (October through March) to document 

changes and patterns in seasonal PM2.5 peak concentrations. 

A review of historical 24-hour PM2.5 filter data shows a general trend of ambient 

concentration reductions in both the average concentration and the magnitude of the 

concentration. Specifically, the District looked at PM2.5 filter data from 1999 through 

2011, focusing on the first and fourth quarters at six sites in the Valley that tend to have 

the highest concentrations, including Clovis, Fresno-First, Corcoran, Visalia,

Bakersfield-California, and Bakersfield-Planz.

The Bakersfield-California site is typical of the trend in improvement for these six sites: 

first quarter (January through March) ambient PM2.5 concentrations show greater 

reduction rates than fourth quarter (October through December) measurements. At the 

Bakersfield-California site the average PM2.5 concentration showed a downward trend 

of 0.74 μg/m3 per year for the first quarter and 1.15 μg/m3 per year for the fourth quarter, 

as shown in Figures A-6 and A-7, respectively. 

Figure A-6 1st Quarter Average Trend at Bakersfield-California
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-20 Appendix A: Ambient PM2.5 Data Analysis 

 2012 PM2.5 Plan 

Figure A-7 4th Quarter Average Trend at Bakersfield-California

Using the same data, but focusing only on the five worst days of each quarter, the 

District was able to determine that not only are the average concentrations decreasing 

in each quarter, but the severity of the worst days (highest concentrations) are 

decreasing at a more rapid rate. Using the five worst days from the Visalia air 

monitoring site data (Figures A-9.13 through A-9.16), over the same time period as 

above, shows a 3.30 μg/m3 per year reduction rate for the first quarter compared to a 

1.22 μg/m3 per year reduction rate for the same quarter when considering the average 

of all the first-quarter data. This data gives indication that the severity of PM2.5 

episodes is decreasing over time, and supports the effectiveness of District woodburning controls, controls that reduce public exposure to extremely high concentrations 

of PM2.5.

While most Valley air monitoring sites showed a similar pattern of peak reductions as 

with the Visalia site, evaluation of the Clovis site did not show the same pattern. As 

shown in Figure A-8, evaluation of the five worst days indicates a slight increase in the 

peak ambient concentration. However, such an increase may reflect random variation 

in the data and does not conclusively indicate an overall increase in the average PM2.5 

concentration. While this anomaly occurs in the first-quarter data for the Clovis site, the 

fourth-quarter data is consistent with the overall decreasing trend in ambient PM2.5 

concentrations.
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-21 Appendix A: Ambient PM2.5 Data Analysis 

 2012 PM2.5 Plan 

Figure A-8 1st Quarter Average Trend at Clovis

The following graphs, Figures A-9.1 through A-9.24, show the first- and fourth-quarter

24-hour PM2.5 averages, along with the average of the top five values within each of 

these quarters for the six monitoring sites included in this analysis.
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-22 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-9.1 through A-9.4 1st and 4th Quarter PM2.5 Average Trends at Clovis

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-23 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-9.5 through A-9.8 1st and 4th Quarter PM2.5 Average Trends at Fresno-First

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-24 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-9.9 through A-9.12 1st and 4th Quarter PM2.5 Average Trends at Corcoran

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-25 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-9.13 through A-9.16 1st and 4th Quarter PM2.5 Average Trends at Visalia

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-26 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-9.17 through A-9.20 1st and 4th Quarter PM2.5 Average Trends at Bakersfield-California

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-27 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-9.21 through A-9.24 1st and 4th Quarter PM2.5 Average Trends at Bakersfield-Planz

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-28 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

A.3.3 Annual Trends of Diurnal PM2.5 Concentration Profiles

The District collects hourly PM2.5 concentration data from 17 of the 34 air monitoring 

stations in the Valley using real-time PM2.5 monitors. The District uses this data every 

day to produce daily air quality forecasting, wood burning declarations, public health 

notifications, and Real-time Air Advisory Network (RAAN) notifications for schools. 

Based on historical hourly data, the District has compiled long-term diurnal profiles to 

evaluate how PM2.5 concentration vary throughout the day at each of the Valley 

monitoring sites that measure such data. An understanding of such profiles helps to 

develop control strategies and programs that target activities during times of peak 

concentrations.

Calculating and comparing annual and 24-hour averages for PM2.5 can be helpful in 

their own right; however, these metrics can often mask the trend in hourly 

concentrations throughout the day. An hourly analysis of PM2.5 measurements can 

show what portions of the day tend to have the highest concentrations and which 

portions of the day have the lowest. Comparing the diurnal (or daily) profiles over time 

shows how this curve has changed from year to year.

The District compares relative changes in hourly PM2.5 concentrations from year to 

year at each monitoring site to better understand the implications and effectiveness of 

PM2.5 control measures, especially the use of wood-burning prohibitions. Such 

prohibitions became mandatory prior to the 2003–2004 winter season and were 

strengthened prior to the 2008–2009 winter season.

Figures A-10.1 through A-10.14 show the yearly average diurnal profiles of most of the 

real-time monitoring sites in the Valley. Sites profiled here are those with the most 

complete data record, in which a comparison could be made with previous years.

As shown in these profiles, the year 2011 (represented by triangles in all of the charts) 

tended to experience higher hourly PM2.5 concentrations when compared to 2010. In 

the larger metropolitan areas like Bakersfield and Fresno, this difference between 2011 

and 2010 was more pronounced. Although concentrations were higher in 2011, partly 

because of unfavorable meteorology, the evening peaks were “flatter” (less 

pronounced) than in years past. Focusing on the Bakersfield-California and FresnoFirst sites, the higher evening peaks in the year 2002 can be observed. Comparing this 

to the evening peaks in recent years, including 2011, one can see that the peak is not 

as pronounced. This could be attributable to more wood-burning prohibitions, which 

became mandatory during the winter of 2003-04.
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-29 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-10.1 through A-10.4 PM2.5 Diurnal Profiles: Stockton-Hazelton, Tracy, Modesto, Turlock

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-30 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-10.5 through A-10.8 PM2.5 Diurnal Profiles: Merced-Coffee, Clovis, Fresno-First, Tranquillity

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-31 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-10.9 through A-10.12 PM2.5 Diurnal Profiles: Hanford, Corcoran, Visalia, Porterville

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-32 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-10.13 through A-10.14 PM2.5 Diurnal Profiles: Ash Mountain, Bakersfield-California

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-33 Appendix A: Ambient PM2.5 Data Analysis

2012 PM2.5 Plan 

A.3.4 PM2.5 Driven Air Quality Index Analysis

The EPA and the District use the Air Quality Index (AQI) to provide daily information 

about the Valley's air quality, to inform the public about how unhealthy air may affect 

them, and educate the public about how they can protect their health. AQI scales exist 

for all of the criteria pollutants regulated by the Clean Air Act, including PM2.5. The 

current 24-hour average PM2.5 AQI scale is defined in Table A-8.

Table A-8 24-Hour PM2.5 AQI Scale

Concentration 

(μg/m3)

AQI Category AQI Color AQI Range

0 - 15.4 Good Green 0-50

15.5 - 40.4 Moderate Yellow 51-100

40.5 - 65.4 Unhealthy for Sensitive Groups Orange 101-150

65.5 - 150.4 Unhealthy Red 151-200

150.5 - 250.4 Very Unhealthy Purple 201-300

250.5+ Hazardous Maroon 301+

The District analyzed the trends in the PM2.5 data from the sites with at least two years 

of daily AQI observations based on real-time data. For this analysis, the AQI reflects 

only PM2.5 data and not ozone or PM10. By excluding other pollutants, the District is 

able to isolate the change in air quality related to PM2.5 only. 

For the majority of the Valley sites, the observed AQI data for the 2008–2010 timeframe 

shows an improvement in PM2.5 air quality. Over these three years, the frequency of 

Good AQI days increased sharply, coupled with a decrease in the frequency of the 

Moderate and Unhealthy-for-Sensitive-Groups (USG) categories. For example, at the 

Fresno-First site, the number of Good days increased from 155 in 2008, to 205 in 2009, 

and to 227 in 2010. At the same time, the USG days at the Bakersfield-California site 

decreased from 61 in 2008, to 34 in 2009, and to 16 in 2010. This trend shows a 

progressive “shift in improvement” of the AQI and air quality—as air quality improves 

there will more AQI days falling within the Good and Moderate categories and fewer in 

the USG category. 

Although the improvement over the 2008–2010 timeframe is partly attributable to

favorable meteorology, emissions reductions were also occurring over these three 

years. The District’s Rule 4901 (Wood Burning Fireplaces and Wood Burning Heaters) 

was strengthened just before the 2008–2009 winter season, lowering the curtailment 

threshold from 65 μg/m3to 30 μg/m3. The sharp improvement in PM2.5 air quality 

began as the amended wood-burning rule took effect, which supports the effectiveness 

of Rule 4901.

In 2011, the PM2.5 air quality declined throughout the majority of the District as

compared to previous years. Abnormally stagnant meteorology during the 2011–2012 

winter season contributed greatly to this deterioration. Despite the overall air quality 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-34 Appendix A: Ambient PM2.5 Data Analysis

2012 PM2.5 Plan 

decline there were still more Good AQI days and fewer USG AQI days than in previous

years. For example, in 2011 the Modesto site observed 252 Good AQI days and 16 

USG AQI days, compared to 2006 with 233 Good AQI days and 22 USG days. 

Although the air quality for the year 2011 did not continue the favorable trend from

2008–2010, it was still not as severe as in years prior to 2008.

Figure A-11 is shown as a reference for interpreting Figures A-12.1 through A-12.12.

The stacked bars represent the number of days within each year that fell within each of 

the AQI categories (totaling 365 days8). Within each stacked bar, the categories are 

ordered as Good, Moderate, etc. from the bottom up.

Figure A-11 Air Quality Index (AQI) Categories

 

8 Note: Because of regular maintenance or repairs, monitors may be non-operational for a day or longer. For years 

with “missing” days, proportional adjustments are made to estimate the missing days so as to provide a full year’s 

data to display.
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-35 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-12.1 through A-12.4 Number of Days per AQI Category per Year: Stockton-Hazelton, Tracy, Modesto, and Turlock

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-36 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-12.5 through A-12.8 Number of Days per AQI Category per Year; Merced, Clovis, Fresno-First, and Hanford

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-37 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-12.9 through A-12.12 Number of Days per AQI Category per Year; Corcoran, Visalia, Ash Mountain, and 

Bakersfield-California

 

 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-38 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

A.3.5 PM2.5 Concentration Frequency Distributions

The District analyzed filter-based PM2.5 data from various sites throughout the Valley to 

form histograms based on the distribution of concentrations over the time periods of 

1999–2003 and 2008–2011. In this analysis, the concentrations were separated into 

the concentration categories of 0–9.9 μg/m3, 10–19.9 μg/m3, 20–35.4 μg/m3, 35.5–65.4 

μg/m3, and greater than 65.4 μg/m3. For each air monitoring site, the observations for 

each time period were grouped into the appropriate categories depending upon their 

concentration. The frequency of the observations within each category were converted 

to a percentage of the total time period and displayed as a bar chart comparing the 

distribution of 1999–2002 with 2008–2011. The air monitoring sites included in this 

analysis were Modesto, Fresno-First, Corcoran, Visalia, and Bakersfield-Planz because 

of these sites had a robust set of measurements beginning in 1999, except BakersfieldPlanz, which began operation in2000.

The data, as represented in Figure A-13 for the Fresno-First site, shows that in the most 

recent four years (2008–2011) there has been a significant increase in the percentage 

of days with low PM2.5 concentrations (under 10 μg/m3) when compared to 1999–2002. 

This increase is observed among all of the sites in this analysis, as seen in Figures A-14 

through A-17. Data from all the sites reveals a dramatic decrease in the percentage of 

days that exceed the 1997 24-hour PM2.5 standard of 65 μg/m3. 

Figure A-13 Histogram Comparison for Fresno-First

While the Corcoran, Visalia, and Bakersfield-Planz sites show an increase in the 

frequency of measurements in the 20–35.4 μg/m3category, as progress continues to be 

made in reducing PM2.5, the curve of the overall distribution will become more sharply 
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-39 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

pushed to the left as higher concentrations become less frequent and lower 

concentrations become more frequent. 

Figure A-14 Histogram Comparison for Modesto

Figure A-15 Histogram Comparison for Corcoran9

 

9

The Corcoran site was non-operational during 2011, thus data is only represented through 2010
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-40 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figure A-16 Histogram Comparison for Visalia

Figure A-17 Histogram Comparison for Bakersfield-Planz
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-41 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

A.3.6 PM2.5 Concentration Distributions

While evaluating changes in peak PM2.5 concentrations increases our understanding of 

air quality and informs the attainment planning process, evaluation of non-peak 

concentrations can also be useful in providing a wider perspective on the progress of air 

quality improvement.

To accomplish such an evaluation, the District constructed box-and-whisker plots for a 

number of air monitoring sites in Valley using data collected from filter-based PM2.5

monitors. Figure A-18 diagrams the use of the box-and-whisker plots as follows: the

box-and-whisker diagram for each year is a representation of the 25th (Q1), 50th (Q2), 

and 75th (Q3) percentile values in the PM2.5 concentration dataset. The “whiskers”

extending from each end of the box represent the outer ends of the dataset

(approximately the top and bottom 25% of the values), where any point outside of these 

boundaries is considered an outlier for this analysis method. The difference between 

Q3 and Q1 is called the interquartile range (IQR). For ease of viewing, the outlier 

values are not displayed in these plots. 

Figure A-18 Box-and-Whisker Plot Interpretation
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-42 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Focusing on the Fresno-First plot in Figure A-19, air quality improvement is evident 

when comparing the 1999 and 2000 box-and-whisker diagrams to those of recent years. 

Not only has the IQR been reduced, but the top 25% of the values has decreased 

sharply. This shows that the entire dataset of PM2.5 concentrations has been shifting 

downward in addition to the reduction of peak values. Since the winter of 2011–2012 

experienced meteorology conducive to the formation of high PM2.5 concentrations, an 

increase in the IQR, Q3, and top 25% values is evident when comparing 2011 to 2010. 

This increase in 2011 is observed among most of the sites in the Valley.

Figure A-19 Box-and-Whisker Plot of PM2.5 at Fresno-First

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

100

80

60

40

20

0

PM2.5 Concentration (ug/m3)

Distribution of PM2.5 at Fresno-First

An exception to the increase in 2011 was at the Bakersfield-Planz air monitoring site, 

where a steady downward slope among most of the components of the plot has 

occurred from 2008 through 2011, as seen in Figure A-20. Since Bakersfield-Planz has 

historically been one of the highest PM2.5 sites in the Valley, this improvement is 

important and needs to continue to occur not only at this site but all other sites in order 

for the region to attain current and future PM2.5 standards.
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-43 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figure A-20 Box-and-Whisker Plot of PM2.5 at Bakersfield-Planz

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

100

80

60

40

20

0

PM2.5 Concentration (ug/m3)

Distribution of PM2.5 at Bakersfield-Planz

The northern most air monitoring sites (Stockton-Hazelton and Modesto) tend to have 

smaller IQRs than the sites in the central and southern portions of the San Joaquin 

Valley. This shows that PM2.5 in the northern part of the Valley tends to have a tighter 

dataset, where less variance occurs. Since the highest concentrations of PM2.5 usually 

occur in the central and southern portions of the Valley, the IQR values for the sites in 

these regions are higher, showing greater variance.
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-44 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-21.1 through A-21.4 PM2.5 Distributions for Stockton-Hazelton, Modesto, Merced-M, and Clovis

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

100

80

60

40

20

0

PM2.5 Concentration (ug/m3)

Distribution of PM2.5 at Stockton-Hazelton

 

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

100

80

60

40

20

0

PM2.5 Distribution (ug/m3)

Distribution of PM2.5 at Modesto

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

100

80

60

40

20

0

PM2.5 Concentration (ug/m3)

Distribution of PM2.5 at Merced-M

 

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

100

80

60

40

20

0

PM2.5 Concentration (ug/m3)

Distribution of PM2.5 at Clovis

PM2.5 filter sampling ended in mid2010.

PM2.5 filter sampling ended in late 

2009.
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-45 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

Figures A-21.5 through A-21.8 PM2.5 Distributions for Fresno-Winery, Corcoran, Visalia, and Bakersfield-California

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

100

80

60

40

20

0

PM2.5 Concentration (ug/m3)

Distribution of PM2.5 at Fresno-Winery

 

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

100

80

60

40

20

0

PM2.5 Concentration (ug/m3)

Distribution of PM2.5 at Corcoran

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

100

80

60

40

20

0

PM2.5 Concentration (ug/m3)

Distribution of PM2.5 at Visalia

 

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

100

80

60

40

20

0

PM2.5 Concentration (ug/m3)

Distribution of PM2.5 at Bakersfield-California

Station temporarily unavailable since mid2011.
San Joaquin Valley Unified Air Pollution Control District December 20, 2012

A-46 Appendix A: Ambient PM2.5 Data Analysis 

2012 PM2.5 Plan 

A.3.7 PM2.5 Concentration by Day of Week

Just as public activity varies throughout the week, so do the daily average PM2.5 

concentrations. The District evaluated real-time 24-hour average concentrations from a 

number of monitoring sites in the Valley to quantify such variance from three woodburning seasons (November through February): 2008–2009, 2009–2010, and 2010–

2011. The air monitoring sites included in this analysis were Modesto, Fresno-First, 

Corcoran, Visalia, and Bakersfield-California.

Figures A-22 through A-26 show the results of the District’s analysis for the five air 

monitoring sites. In general, Thursday and Friday recorded the highest 24-hour average 

PM2.5 concentrations over the seven-day week. This suggests a build-up of emission 

beginning on Monday and progressing toward the end of the work week. This pattern 

would be more pronounced during stagnation episodes, where emissions would not 

have a chance to disperse.

Figure A-22 Day of Week PM2.5 Concentrations at Modesto
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Figure A-23 Day of Week PM2.5 Concentrations at Fresno-First

Figure A-24 Day of Week PM2.5 Concentrations at Corcoran
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Figure A-25 Day of Week PM2.5 Concentrations at Visalia

Figure A-26 Day of Week PM2.5 Concentrations at Bakersfield-California
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The analysis also shows a declining average PM2.5 concentration from Saturday into 

Monday, perhaps as a result of the variation in vehicle activity when comparing the 

weekend to the weekday. The typical weekday will have significant vehicle emissions in 

the morning as people commute to work and school, followed by lighter traffic during the 

day, and heavy activity again at the end of the day for the commute home. 

Contrastingly, the weekend activity is spread more uniformly throughout the day. This 

temporal difference in emissions activity may contribute to lower PM2.5 emissions, 

which tend to carry over into the early days of the next week before the next build-up 

begins.

Comparing the trends between the air monitoring sites included in this analysis reveals 

that the Fresno-First and Bakersfield-California sites (the largest urban centers in the 

central and southern valley) tend to have the highest PM2.5 concentrations on 

Thursday and Friday, ranging from 30 to 35 μg/m3. The difference between the highest 

concentration and the lowest concentration among the days of the week is about 5 

μg/m3 for most of the sites; however this difference at Bakersfield-California is more 

pronounced.

As the San Joaquin Valley faces the challenges of future PM2.5 standards, having this 

understanding of what days of the week tend to have the highest concentrations may 

aid in developing a successful attainment strategy. Targeting emissions that contribute 

to the build-up as the week progresses may help in reducing the peaks at the end of the 

week.

A.4 METEOROLOGY, PM2.5 SPECIATION, AND RULE EFFECTIVENESS

The District takes full advantage of the robust data set produced by the extensive air 

monitoring network in the Valley, as seen in the previous sections of this appendix. 

However, there is other information that adds to the comprehensive understanding of 

PM2.5 concentrations and is critical in all aspects of developing and implementing a 

successful attainment plan, including meteorology, PM2.5 speciation, and the overall 

effectiveness of previous control measures.

A.4.1 Meteorologically Adjusted Trends

In order to understand the effectiveness of emission control strategies and regulations 

on ambient air pollution levels, it is important to first understand and be able to delineate 

the effect of meteorology versus changes in emissions as a response to control 

measures. The strong linkage between meteorological conditions and air pollutant 

levels can obscure the effects of the change of emission levels over time resulting from

a regulatory program. Therefore, the meteorological effects need to be removed so that 

the emissions-related trends may be studied more effectively.

The District used the Classification and Regression Trees (CART) method to define the 

relationship between PM2.5 concentrations and meteorological conditions in both the 
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Bakersfield and Fresno areas of the Valley. Three years (2004–2006) were selected as 

base years to define these relationships. The CART model was able to explain 

approximately 75–80% of the variation in daily PM2.5 concentrations during these years 

based on the local meteorological conditions. Based on the CART-defined 

relationships, daily PM2.5 concentrations were predicted for all the other years using 

the observed meteorological data and assuming the emissions stayed constant (i.e. the 

predicted concentrations only represent the PM2.5 conduciveness of meteorology). 

The measured PM2.5 concentrations were then corrected for the influences of 

meteorology to estimate the meteorologically adjusted trends. For example, in a year 

with meteorology conditions that were more conducive to PM2.5 formation, PM2.5 

concentrations were adjusted downward. Conversely, PM2.5 concentrations were 

adjusted upward in years with meteorological conditions that were less conducive.

As shown in Figures A-27 and A-28, the meteorologically adjusted trend at Bakersfield 

indicates a greater decline than the unadjusted trend, while the two trends are generally 

similar at Fresno. Overall, the meteorologically adjusted trends indicate that the PM2.5 

annual averages decreased about 40–50% in both the Bakersfield and Fresno areas 

from 1999 to 2010, with an average rate of decrease of approximately 0.8 μg/m3 per 

year. These meteorologically adjusted trends provide a more robust indicator of the 

impacts of emission reductions from on-going control programs.

Figures A-27 and A-28 show the trend of observed PM2.5 represented as a solid line, 

and the trend of meteorologically adjusted PM2.5 is represented by a dashed line.

Figure A-27 Meteorologically Adjusted PM2.5 Trend for Bakersfield
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Figure A-28 Meteorologically Adjusted PM2.5 Trend for Fresno

A.4.2 Trends in PM2.5 Species

Analyzing the trends among the species of PM2.5 is often more telling than focusing on 

the PM2.5 mass alone. Valley PM2.5 concentrations have been decreasing over time,

but some components, or species, of PM2.5 may be decreasing more rapidly than 

others. The results of a speciation analysis can show which species of PM2.5 are most 

dominant for an area, and therefore guide a more targeted control strategy for reducing 

the overall mass concentration. The following analysis shows the relative contribution 

and temporal change in the key species of PM2.5 at a number of sites in the Valley.

Figures A-29 and A-30 shows the contribution of various species to the overall PM2.5 

mass concentration in the Fresno and Bakersfield areas, respectively. Figure A-29, for 

Fresno, shows that on a peak PM2.5 day the concentration consists of about 51% 

ammonium nitrate and 33% organic carbon. In comparison, a peak PM2.5 day in 

Bakersfield (Figure A-30) is comprised of 67% ammonium nitrate and 16% organic 

carbon. The understanding of this difference can help reveal what sources of pollution 

are contributing to PM2.5 in each area of Valley, which will ultimately aid in developing 

an effective control strategy.
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Figure A-29 Species Contribution to PM2.5 Mass in Fresno

Figure A-30 Species Contribution to PM2.5 Mass in Bakersfield
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Chemical speciation data is regularly collected at four sites in the Valley: Modesto, 

Fresno-First, Visalia, and Bakersfield-California. As previously mentioned, ammonium 

nitrate, ammonium sulfate, and carbon compounds are the major constituents of PM2.5. 

In the following trend analysis, concentrations at each site from each of the PM2.5 

species were averaged over each year from 2002–2010. On an annual average, 

concentrations of these key constituents have all shown significant decreases. 

Ammonium nitrate concentrations in the basin declined about 50% between 2002 and 

2010. During the same time frame, concentrations of ammonium sulfate and carbon 

compounds declined about 30%. The most significant declines occurred between 2002 

and 2003, and again between 2007 and 2010.

The decline in ammonium nitrate provides evidence of a successful NOx control 

strategy in the Valley, which in turn has been effective in reducing PM2.5 mass. As 

ammonium nitrate concentrations further decline into the future, and as more stringent 

federal PM2.5 health standards are established, the results of species trends analyses 

will grow in importance as targeted control strategies will need to be developed.

Note that between 2007 and 2009, the carbon collection and analysis method was 

changed to improve comparability with rural Interagency Monitoring of Protected Visual 

Environments (IMPROVE) PM2.5 carbon data. Since the change was implemented 

mid-year, there are gaps in the carbon data for years with partial data from the old and 

new method.

Figures A-31 through A-34 display the species trends for the four speciation sites in the 

Valley.

Figure A-31 PM2.5 Species Trends at Modesto
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Figure A-32 PM2.5 Species Trends at Fresno-First

Figure A-33 PM2.5 Species Trends at Visalia
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Figure A-34 PM2.5 Species Trends at Bakersfield-California

A.4.3 Effectiveness of District Rule 4901

Emissions from residential wood-burning have historically been one of the greatest 

sources of directly emitted PM2.5 in the Valley. Before residential wood-burning 

curtailments became mandatory during episodes of high PM2.5 concentrations, the air 

quality during the wintertime was often unhealthy in large part due to extensive woodburning. Through the establishment of District Rule 4901, residential wood-burning is 

not allowed on days when high concentrations of PM2.5 are predicted. This reduction 

in wood-burning emissions through the rule has greatly reduced the potential for high 

PM2.5 concentrations. The following analysis displays the effect that Rule 4901 has 

had on PM2.5 in the Fresno area.

To conduct this analysis, a statistical model was developed to quantify PM2.5 

reductions and evaluate air quality improvements attributable to the 2003 and 2008 

amendments to Rule 4901. This statistical model was developed through generalized 

linear model techniques with logarithmic transformations based on the relationships 

between meteorology and PM2.5 concentrations that existed prior to the 2003 Rule 

4901 amendments. Daily and hourly observed PM2.5 concentrations served as 

dependent variables with meteorological parameters such as wind speed, temperature, 

and stability serving as independent variables.

Since this model was developed with data before wood-burning curtailments came into 

effect, its output shows a prediction of what PM2.5 concentrations would be like if Rule 

4901 were not in place. Comparing these predicted concentrations against what was 
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observed, a difference can be calculated that represents the emissions reductions that 

have occurred since wood-burning curtailments began. As shown in Figure A-35, a 

consistent pattern of model-predicted values being higher than what was actually 

observed provides compelling statistical evidence that a control measure, i.e., woodburning curtailments, was responsible for the discrepancy. Reductions in emissions 

from other sources may have also attributed to the differences, but these reductions 

would be minor when compared to the reductions from residential wood-burning.

Note that the 2003 amendment to the rule established a curtailment threshold at 65 

μg/m3of PM2.5, and the 2008 amendment lowered this threshold to 30 μg/m3of PM2.5 

or 135 μg/m3of PM10. This modeling indicates that as of the 2011–2012 wood-burning 

season, there has been a 41% (21 μg/m3) improvement in the 24-hour average PM2.5 

in Fresno since the 2003 and 2008 amendments to Rule 4901. This improvement is 

exemplified in PM2.5 concentrations measured during the evening hours of 8:00 p.m. to 

12:00 a.m. The average evening PM2.5 concentrations have improved by 50 percent 

(42 μg/m3) over the same time period. As shown in this analysis, the 2008 amendment

to Rule 4901 has approximately doubled the seasonal improvements attributable to the 

2003 amendment. 

Figure A-35 Effect of Rule 4901 on Winter (November through February) PM2.5 

Concentrations in Fresno
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Overall, the Valley’s PM2.5 concentrations have dramatically decreased since the 2003 

and 2008 Rule 4901 amendments. Without this further analysis, it would have been 

unclear if decreases in PM2.5 concentrations could be attributed to reductions in 

residential wood-burning or changes in seasonal weather patterns. Rule 4901 will 

continue to play an important role in reducing PM2.5 concentrations throughout the San 

Joaquin Valley both within and beyond the timeframe of this plan.
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