Particulate Matter Science

for Policy Makers

A NARSTO Assessment

February 2003

Part 1
EXECUTIVE SUMMARY

Particulate Matter Science for Policy Makers: A

NARSTO Assessment was commissioned by

NARSTO, a cooperative public-private sector

organization of Canada, Mexico and the United

States. It is a concise and comprehensive discussion

of the current understanding of airborne particulate

matter (PM) among atmospheric scientists.1

 Its goal

is to provide policy makers who implement airquality standards with this relevant and needed

scientific information. Policy development and

revision is an ongoing process, and scientific

understanding is continually improving. This

assessment describes current science in a manner that

reflects the needs of policy makers in addressing

current and anticipated standards.

This assessment is organized using the following

considerations for dealing with air-quality issues:

• Perspective for Managing PM

• Health Effects Context

• Atmospheric Aerosol Processes

• Emission Characterization

• Particle and Gas Measurements

• Spatial and Temporal Characterization of PM

• Receptor Methods

• Chemical Transport Models

• Visibility and Radiative Balance Effects.

Conceptual models of PM for nine geographic

regions in North America illustrate the application

of these considerations for developing management

strategies.

This Executive Summary condenses the current

understanding of the PM problem using key topics:

• PM2.5 responses to changes in emissions

• Local, regional, and continental management

• Predictive capabilities

• Co-pollutant interactions

• Improved measurement tools

• Links with health effects and climate change

studies

• Tracking success of air-quality management.

The Synthesis following this Executive Summary

answers specific policy questions, developed through

interviews with policy makers, with summary

information from this assessment.

PM2.5 MASS AND COMPOSITION

RESPONSES TO CHANGING

EMISSIONS

A mbient PM2.5 results from direct particleemissions (carbon and soil dust, for example)

and secondary particles (generated by atmospheric

reactions of precursor gas emissions). Ambient PM

is also influenced by meteorology. The major

precursor gases are sulfur dioxide, nitrogen oxides,

ammonia, and volatile organic compounds (VOCs).

Ambient PM mass is thus a mixture composed mostly

of sulfate, nitrate, ammonium, organic carbon, black

carbon and soil dust.

The relative mass fractions of secondary PM can

change in a non-linear manner with changing

precursors owing to the complex chemistry of

secondary PM formation. As sulfur dioxide

emissions decline, so will particulate sulfate

concentrations. The presence of ammonia allows the

formation of ammonium nitrate and ammonium

sulfates. As sulfate is removed, more ammonium

nitrate can be formed, provided sufficient ammonia

and nitric acid are present. Consequently, the

response of particulate nitrate to changing nitrogen

oxide or VOCs is less clear, as it depends upon the

amount of ammonium and sulfate present in particles

in addition to VOC or nitrogen oxide limiting

processes. In most locations, insufficient information

exists on both emissions and ambient levels of

ammonia to predict how particle mass and

composition would change in response to changing

ammonia emissions.

The black carbon fraction will decrease in direct

response to reductions in source emissions from, for

example, diesel combustion. The organic carbon

fraction stems from both direct emissions of organic

_________

1

 The major emphasis of this assessment is on fine particles (PM2.5), although larger particle sizes (PM10 and PM10-2.5) are discussed.
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EXECUTIVE SUMMARY

PM and the oxidation and polymerization of VOC

emissions. Consequently, the extent to which the

organic fraction may be managed depends upon both

direct organic carbon emissions and the attendant

occurrence of oxidants and acids. As the particleforming VOCs, acids, and oxidants decrease, so

would the secondary organic carbon fraction.

Considerable uncertainty exists about the chemical

composition of organic carbon particles, both at the

source and in the atmosphere. Routine measurements

of organic carbon species do not yet exist. Thus,

explaining the response of the organic carbon fraction

of PM to changing precursor emissions is in its very

early stages.

What we know and expect...

Based upon current understanding of secondary

particle formation, it is anticipated that the

existing management strategies in North America

focused on the reduction of sulfur dioxide will

reduce PM2.5 mass concentration, as will

reductions in direct particle emissions, notably

of black and organic carbon. The benefits of

reducing nitrogen oxides or VOCs are uncertain.

LOCAL, REGIONAL, AND

CONTINENTAL MANAGEMENT OF

PM2.5

Particles remain in the atmosphere for days to afew weeks, depending on their size and the rates

at which they are removed from the atmosphere, for

example, by precipitation. Particles in any given area

may originate locally or from sources hundreds to

thousands of kilometers away, or be formed during

atmospheric transport from precursor gases

originating from sources locally or far away.

Prevailing meteorology, topography and seasonal

influences also alter the bulk PM2.5 mass

concentration and composition of particles.

Intercontinental transport of dust from both Asia and

Africa occurs. It does not contribute significantly to

annual average concentrations in North America. It

may occasionally contribute significantly to 24-hour

concentrations. Forest fires and biomass burning both

contribute locally and regionally during certain

seasons. Satellite imagery and PM composition

measurements are used to identify the influence of

these large-scale events.

Both local and regional emissions underlie local

concentrations in many urban areas. Regional

contributions from sources distant to eastern North

American urban areas can account for 50% to 75%

of the total observed PM2.5 mass concentration within

a specific urban area.

What we know and expect ...

PM2.5 management strategies need to include the

impact of both local and distant sources, within

the context of prevailing meteorology and

seasonal variability. Consequently, a

management approach for one region or airshed

may not be applicable in another. Each needs to

be assessed individually to develop the most

effective management approach.

PREDICTIVE CAPABILITIES

Several strategy development tools are available

utilizing analysis (e.g., receptor modeling) and

simulation (e.g., chemical transport modeling).

Receptor models and chemical transport models can

be used in a complementary fashion to develop advice

for policy makers, as part of a corroborative approach

to providing guidance based on the best scientific

understanding available. Receptor models are useful

in selecting scenarios and identifying contributing

sources and/or source types.

Current chemical transport models are one useful tool

for guiding policy as part of the collective scientific

analysis, being most informative regarding the

inorganic fraction (sulfate, nitrate and ammonium)

on regional and episodic (days to weeks) scales.

What we know and expect...

Source-specific options to reduce PM

concentrations are best approached through

corroborative analyses using emission

inventories, ambient concentration

measurements, and air-quality modeling.

Capabilities of chemical transport models, as

well as being bound by current understanding
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PARTICULATE MATTER SCIENCE FOR POLICY MAKERS

of atmospheric processes, are limited by the type

and quality of the ambient, meteorological, and

emission information used to evaluate and run

them. Evaluated, policy-ready models that can

be applied routinely will become available

gradually as these data needs are met; such

models can be applied with growing confidence

over the next five to ten years.

CO-POLLUTANT INTERACTIONS

P M2.5 and ground-level ozone, with other aircontaminants, are closely related through

common precursors, sources, physicochemical

pathways, and meteorological processes. Thus,

changes in the emissions of one pollutant can lead to

changes in the concentrations of other pollutants. All

fine particles, including those composed of

carbonaceous materials, sulfates, and nitrates, scatter

incoming and, lesser so, outgoing radiation. Black

carbon and other dark particles absorb radiative

energy. Coarse particles and cloud droplets formed

by the condensation of water vapor on particles also

have radiative effects. These radiative effects of PM

can have local and global impacts on photochemistry

and climate change. The scattering and absorption

of visible radiation (light) can also obscure visibility

and result in regional haze.

What we know and expect...

The current understanding of atmospheric

processes shows that PM2.5 problems are related

to ground-level ozone, acid rain, and climate

issues and share many of the same sources. This

recognition provides the impetus for integrated

and optimized management strategies that

accommodate different atmospheric responses

for each pollutant.

NEW INSIGHTS FROM IMPROVED

MEASUREMENTS AND MONITORING

Considerable advances have been achieved in the

real-time measurement of particle size and

composition. Many of the new measurements are

still research activities, but they demonstrate how

more detailed measurements can provide insights to

the fundamental atmospheric chemical processes and

source contributions, and be used in chemical

transport model development and evaluation. For

example, size-resolved composition measurements

can help differentiate between local and regional

source contributions, primary vs. secondary particle

components, and natural vs. anthropogenic

contributions.

Insufficient ambient data are available to examine

fully all of the hypothesized causal elements for

health-effects, either due to lack of data or lack of

suitable measurement technology. As the

measurement techniques evolve toward more robust

methods, suitable for mid- to long-term monitoring,

it will become possible to include many more aspects

of PM composition and characteristics in studies of

population exposure and health outcomes.

What we know and expect...

Health impacts based on epidemiology can be

attributed only to PM characteristics for which

ambient measurements are available, and effects

can be assigned only to those specific source

types represented in the ambient concentration

measurements. Linking of health effects to

sources is being explored by way of source

apportionment and toxicological studies. The

rapid evolution in ambient and source

monitoring methods will improve the capability

to assign exposure and health links to sources.

LINKAGES BETWEEN THE HEALTH

AND ATMOSPHERIC SCIENCE

COMMUNITIES

A considerable and growing body of evidenceshows an association between adverse health

outcomes, especially of the cardio-respiratory system,

and short- and long-term exposures to ambient PM,

especially PM2.5. Evidence, from both opportunistic

studies of significant reductions in ambient PM over

short time periods as well as long-term studies, also

suggests that health improves when the ambient

concentration of PM is reduced.

Hypotheses that explain how the various chemical

and physical characteristics of PM cause specific
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EXECUTIVE SUMMARY

adverse health impacts have been proposed and

partially tested. Continuing to test these hypotheses

will require ongoing close collaboration between the

atmospheric and health-science communities.

What we know and expect...

Stronger linkages between the health, exposure,

and atmospheric science communities will

strengthen our understanding. Examples include

joint planning of field studies and projects,

development of coordinated long-term research

strategies that take into account the major areas

of focus in each community, presentations/

lectures at one another’s conferences, and

inclusion of contextual information on human

health, exposure and atmospheric science within

all major assessments/reports.

TRACKING PROGRESS AND SUCCESS

OF AIR-QUALITY MANAGEMENT

Long-term data sets of a basic suite of

measurements (PM mass concentration and

composition) are needed to track the impact of

changing emissions and management actions. The

information from such long-term monitoring,

combined with model predictions, will provide the

iterative guidance needed to revise and focus

management strategies.

At present, considerable PM data for North America

exist in many different data sets (both short term and

long term), obtained from different networks or

studies, using several measurement techniques.

However, major gaps in method consistency and data

continuity have limited investigation of PM

formation and transport on regional and continental

scales, and assessment of trends.

What we know and expect...

Sulfate particle loadings have responded to sulfur

dioxide emission reductions in both eastern North

America and California. It is anticipated that

more responses to currently planned changes in

emission rates will appear in the next five to ten

years. This information will be needed to revise

and optimize PM management approaches.

SCIENTIFIC UNCERTAINTIES AND

LOOKING FORWARD

The physics and chemistry of aerosols have beenstudied since before the 20th century. The indepth study of aerosols in the context of air pollution

is much more recent. The science community has a

fundamental understanding of PM formation and

transport, yet there remain several significant areas

of uncertainty.

Qualitative uncertainties have been identified in

emission inventory information, monitoring and

chemical transport models. Pollutant emissions from

point sources which have been of management

interest the longest are best characterized, while those

from dispersed area sources for chemically complex

pollutants or pollutants of recent interest, such as

VOCs and ammonia, are much more uncertain. The

confidence in the measurements of bulk PM mass

concentration is defined by the measurement method.

The confidence in characterization of the carbon

fraction is low due to as yet unresolved artifacts when

collecting PM and determining the black and organic

carbon contents in the sample.

The current chemical transport models for PM

provide conceptually acceptable, though unevaluated,

simulations for the inorganic particulate species over

episodic periods and regional scales.

What we know and expect...

Policy makers are currently benefiting from

research initiated five to ten years ago, or longer.

This research provides a basic understanding of

PM formation, transport and its major

contributing sources. It characterizes the areas

of North America where PM concentrations,

visibility reduction, and potential population

exposure are the greatest. Despite considerable

scientific uncertainties, sufficient scientific

confidence exists to devise management actions

likely to improve air quality.

Policy makers are faced with the need to make

decisions based on imperfect understanding of the

atmosphere and human health impacts. Therefore,

this assessment concludes with research

recommendations that will substantially improve the

ability of the atmospheric science community to
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PARTICULATE MATTER SCIENCE FOR POLICY MAKERS

provide guidance for improving plans to implement

air-quality standards and goals. Furthermore,

continuing research will reduce the risk of erroneous

decisions. Each recommendation addresses a

consideration that is critical for improved

understanding of the atmospheric environment.

Because the individual recommendations cover areas

that are respectively supportive they should progress

simultaneously. The result will be better science tools

and stronger atmospheric science analysis. The

recommendations focus on:

• Improving the understanding of the carbonaceous

fraction

• Performing long-term monitoring of PM,

gaseous precursors and co-pollutants

• Performing further evaluation and development

of chemical transport models

• Developing improved emission estimates

(including chemical speciation)

• Making a commitment to the analysis of ambient

data and fostering interactions between

atmospheric, climate, and health science

communities

• Developing more systematic approaches for

integrating diverse types of knowledge to guide

development of PM management practices and

tracking progress toward protecting health.

The atmospheric science community has a good

understanding of particle formation and transport that

supports current directions for achieving air-quality

standards and goals. The new science that is on the

horizon will help refine the management actions for

more effective approaches to address PM air-quality

problems.
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EXECUTIVE SUMMARY