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Ž .Fuel Processing Technology 65–66 2000 379–392

www.elsevier.comrlocaterfuproc

An overview of PM sources and control strategies2.5

W. Gene Tucker )

National Risk Management Research Laboratory, U.S. EnÕironmental Protection Agency,

Research Triangle Park, NC 27711 USA

Received 22 February 1999; accepted 30 June 1999

Abstract

The new national ambient air quality standard for particulate matter is summarized, with

emphasis on fine particles, or particulate matter with aerodynamic diameter less than 2.5 mmŽ .PM . Sources of ambient and indoor PM are summarized, followed by an overview of

2.5 2.5

Ž .control strategies for both primary directly emitted particles and gaseous precursors of ambient

particles. The role of future findings on the health effects of particles of different sizes orcompositions is noted, as is the potential importance of controlling indoor exposures for

susceptible people. EPA risk management research on sources and control options is summarized.

The cost-effectiveness of prevention and control technologies for PM has not been well2.5

researched and documented. Optimum control strategies will likely include a variety of control

options. Published by Elsevier Science B.V.

Keywords: Ambient PM; Indoor PM; Control strategies; Prevention

1. Introduction

Because epidemiological associations had been found at particle concentrations below

the existing national ambient air quality standard, a new national standard was promul-w xgated in July 1997 27 . Whereas the former standard applied to particulate matter less

Ž .than 10 mm in aerodynamic diameter PM , the new standard also addresses particles10

Ž .smaller than 2.5 mm PM . The new standard, like the old one, considers only size2.5

)

College of Integrated Science and Technology, James Madison University, MSC 4102, Harrisonburg, VA

22807, USA. Tel.: q1-540-568-8771; fax: q1-540-568-2761; e-mail: [email protected]

0378-3820r00r$ - see front matter Published by Elsevier Science B.V.Ž .P I I : S 0 3 7 8 - 3 8 2 0 9 9 0 0 1 0 5 - 8



( )W.G. Tucker r Fuel Processing Technology 65–66 2000 379–392380

and mass concentration; chemical composition and toxicity of the particles are not

addressed. To meet the new standard, sources of both primary and secondary particles

will need to be controlled. Control of ‘‘primary’’ particles, which are emitted directly

into the air, involves emissions prevention or collection at the source. Control of

‘‘secondary’’ particles, which are formed in the atmosphere, requires reducing emissions

of precursor constituents such as condensible organic compounds, sulfur oxides, nitro-gen oxides, and ammonia.

Fig. 1 shows a typical size distribution of particles in the ambient air, and how PM 10

and PM relate to the total distribution. Particles are generally distributed bimodally by2.5

size in the atmosphere, with the minimum of the distribution between 1 and 3 mm

aerodynamic diameter. Particles in the fine mode are created as primary particles at the

source, mostly by combustion or other high-temperature processes, or as secondary

particles in the atmosphere. Most of the mass of particles in the coarse mode comes

from materials that have been ground down by mechanical processes, although fine-mode

w xparticles can also become attached 26 .Ambient PM is a complex mixture of sizes and types of particles that originate from

many sources. The size, chemical composition, and source of particles may all play a

role in human exposures, and health effects resulting from those exposures. Little

information is available on the specific chemical makeup, especially the metal speciation

and organic components of fine particles. Chemical characterization of PM from a broad

Ž w x.Fig. 1. PM and PM related to a typical size distribution for ambient particles adapted from USEPA 26 .10 2.5



( )W.G. Tucker r Fuel Processing Technology 65–66 2000 379–392 381

range of locations with a variety of source types is needed to better determine the

composition, range of transport, and variability of airborne particles.

The types of sources that contribute to the two modes in the distribution shown in

Fig. 1 are generally known. Particles in the fine mode include primary particles from

high-temperature metallurgical and combustion processes, secondary particles from

atmospheric reactions, and an unknown but theoretically small amount of fine particlesthat have been deposited and resuspended by wind or human activities. Particles in the

coarse mode include coarse windblown and road dust, pollens and spores, and some

industrial particles. PM samples are generally dominated, on a mass basis, by10

coarse-mode particles. PM samples can also have substantial amounts, on a mass2.5

basis, of coarse-mode particles from the left-hand tail of the coarse-mode distributionŽ .about 1–2.5 mm diameter ; this may be especially likely in areas with significant

sources of resuspended dust.

Since the epidemiological studies of PM health effects are based on geographically

Ž .dispersed but mostly urban locations with numerous and varied emission sources, themajor sources of the particles are difficult to ascertain. Researchers most familiar with

these studies have speculated that two types of sources may be particularly important:

primary emissions from combustion sources, and secondary particles formed in the

atmosphere. Table 1 lists the key causal factors that are the basis for current hypotheses

on what aspects of particulate matter are responsible for observed health effects.

Hypotheses regarding acidic particles, particles containing sulfates, ultrafine particlesŽ .smaller than about 0.1 mm , and particles containing transition metals would implicate

w xcombustion sources. See Refs. 28,31 for recent reviews.

There is a lot of uncertainty with respect to ‘‘fugitive’’ particles. Their emission ratesare poorly quantified, but potentially large compared to emission rates of combustion

and other industrial sources. Table 2 summarizes the U.S. national inventory for particle

and gaseous precursor emissions from general categories of sources. Note that approxi-

mately 70% of the total primary PM is from fugitive sources. This is presumably2.5

because the national total PM emissions are so large, and the mass in the left-hand tail10

Ž .of the coarse mode see Fig. 1 is large compared to the mass of particles being created

in the fine mode. Even though the estimates for PM fugitive emissions are being2.5

reevaluated and may be revised downward somewhat, they will still constitute a large

percentage of the national total.Emissions data summarized in Table 2 illustrate the large potential contribution of

w Ž .x wsulfur oxides SO — principally sulfur dioxide SO and nitrogen oxides NO — x 2 x

Table 1ŽFactors or constituents that are currently hypothesized to be significant in fine PM toxicity from Mauderly et

w x.al. 18

PM mass concentration Metals

Particle sizersurface area Acids

Ultrafine PM Biogenic particlesOrganic compounds Peroxides

Sulfate and nitrate salts SootŽ .Cofactors e.g., other pollutants, weather




Table 2

Summary of U.S. national inventory of particle and precursor emissions

3Ž .Approximate national emissions 10 Mgryear

aPM PM SO NO2.5 10 x x

Industrial processes 500 1000 1600 900

Combustion sources

Electric utility 100 300 13,000 6000

Industrial and commercial 500 700 4000 4000

Residential 500 500 10 900

Vehicular 700 900 700 12,000

Managed burningqwildfires 700 800 10 200

Fugitive sources

Roads 2000 13,000

Agriculturalrsilvacultural crops 900 5000

Construction 800 4000

Natural sourcesŽ .Geogenic wind erosion 800 5000

b bŽ .Biogenic from HC precursors unk unk

Rounded totals 8000 30,000 20,000 20,000

a Ž .As NO nitrogen dioxide ; however, most NO is emitted as NO.2 xb Ž .An unknown amount of natural hydrocarbon emissions e.g., terpenes is photochemically converted to

atmospheric aerosols.w xAdapted from USEPA 25 .

Ž .xprincipally nitric oxide NO gaseous precursors to ambient PM concentrations. If their

combined 40 million tonsryear were completely converted to sulfate and nitrate

particles, their emissions would be equivalent to about 60 million tonsryear of primary

PM . However, ambient PM in the U.S. is typically 30–50% sulfate and much less2.5 2.5

nitrate, except in southern California where nitrate may be as much as 45% in somew xwinter periods 26 . This implies a conversion to PM of somewhat less than 102.5

million tonsryear, or somewhat less than 20% of the gaseous precursor emissions.

Fig. 2 shows geographically the areas that do not meet the PM standard of 150103 Ž . 3 Ž .mgrm 24 h average and 50 mgrm annual average . It also indicates the source

types that are thought to be the major causes of non-attainment with the standard. The

number of additional areas of the country that will not meet the new PM require-2.5

ments, and the additional sources needing control, will be determined largely by analysis

of concentration and composition data from the new network of ambient monitoring

stations.

There is further uncertainty about exposures to airborne particles inside buildings.ŽConsidering the high fraction of time that people especially those who may be most

.health-susceptible spend indoors, there is a need to quantify the three major sources of indoor exposure: particles from outdoors, direct emissions from indoor sources, and

Ž .resuspended particles from indoor activities a type of fugitive source . Table 3

summarizes the various types of sources of particles found in buildings. Cigarette




ŽFig. 2. Areas designated non-attainment for PM particles, by emission type adapted from Bachmann et al.10

w x.2 .

smoking, cooking, and penetration of outdoor particles through the building envelope

have all been reported to make significant contributions to indoor concentrations of finew xparticles, but a substantial portion is due to unexplained indoor sources 29 .

Table 3

Sources of exposure to indoor particles

Outdoor origin Indoor origin

Ž .Outdoor air through infiltration, ventilation Smoking, cookingŽ .Tracked-in soil Space heating especially kerosene, wood

Carried-in dusts Ventilation systems, humidifiers

– from industrial workplaces Office machines

– from other places Dust mites, petsŽ .Diseased people bacteria, viruses

Ž .Personal activities e.g., hobbies

Maintenancerrenovation activities

Resuspension of particles of either outdoor or indoor originCleaning activities

Maintenancerrenovation activities

People movements




2. The new national ambient air quality standard for PM 2.5

Concentration limits and the compliance timeline for the new national ambient air

quality standard for PM are shown in Table 4. Approximately 1500 monitoring2.5

stations are being set up across the United States in locations to reflect the populationŽdistribution rather than the distribution of major sources, which has been more of a

.factor in the past .

Compliance with the standard, often referred to as ‘‘in attainment’’, will be deter-

mined by calculations using data from the monitoring stations. The calculations consider

averaging over time and in some cases over more than one station in a geographic area,Žand allow exceedances of the standard for a certain amount of time see details in Ref.

w x.27 .

Areas that are not in attainment will have to have plans, usually developed by States,

that describe strategies to be used for bringing the areas into compliance. These plans,Ž .most often referred to as State Implementation Plans SIPs , are where actual PM2.5

control strategies will be developed.

3. Control strategies for PM 2.5

The major categories of sources of PM and gaseous precursors are listed in Table2.5

5, with approximate annual emissions on a national basis. While any particular area that

is in non-attainment status will have its own distribution of these sources, it is likely thatcontrol strategies will address one or more of these source types. Two general ap-

proaches will be taken to estimate which sources are contributing most to ambient

monitoring stations in the non-attainment area: source inventories and source-orientedŽ .dispersion modeling to estimate impacts on the ambient stations, and chemical

characterization of particles collected at the ambient sites coupled with receptor-oriented

modeling to calculate contributions of various source types.

Table 4Ž .National ambient air quality standard for fine particulate matter PM2.5

Ambient concentration limits

324 h average 65 mgrm3Annual mean 15 mgrm

Timeline

1997 New NAAQS for PM 2.5

1998 –2001 National monitoring network set up

2001–2004 Initial 3-year monitoring data sets available

2002 Periodic 5-year review of standardŽ .2002–2005 Areas designated attainment or non-attainment

2005 –2008 States submit implementation plansŽ .2012–2017 States have up to 10 years to comply 12 years with extensions




In addition to these estimates of mass contributions, there may very well be some

consideration of particle toxicity. By the time the implementation plans are being

developed in the 2002–2008 time period, health researchers may have sufficient

evidence to support one or more of the casual hypotheses for PM toxicity listed in Table

1. Such evidence may influence priorities on which sources to control, even before any

changes are made in the ambient standard. When different source types contribute aboutequally to exceedances of the mass-based standard, sources of the most toxic particles

may be given higher priority for control.

Since many of the currently available estimates of PM emissions are based on data2.5

on PM rather than direct PM measurements, there is great uncertainty in the fine10 2.5

particle emissions inventory. In addition, there is a general lack of data on the chemical

composition of fine particle emissions. The need for emission characterization is greatestŽ .for sources with constituents such as metals and acidic components that are candidates

for causal mechanism studies of respiratory health effects, and for sources having the

largest mass contributions to PM in the environment.2.5

As Table 6 implies, there are few data on the effectiveness and costs of emissions

prevention, emissions reduction, or exposure reduction technologies for fine particlesŽ .i.e., PM . Reliable data on emission prevention for either industrial or indoor sources2.5

are nearly non-existent. Most of the available data on cost-effectiveness of emissionŽ .controls are for industrial sources of total PM and at best, for PM . Although limited10

data are available on the efficiency and cost of air cleaning to remove particles from

indoor air, there are virtually no data on the effectiveness of air cleaning in reducing

indoor exposures to fine particles. Since indoor concentrations of particles approach

outdoor concentrations when outdoor concentrations are high, or are about twice outdoorŽ w x.concentrations when outdoor concentrations are low e.g., Refs. 20,22 , and since

people spend roughly an order of magnitude more time indoors than outdoors, it will be

important to evaluate controls for indoor exposures.

4. EPA research related to PM control strategies2.5

Research related to fine PM control strategies is being conducted by EPA in two

areas: Source Characterization, where the focus is on new knowledge about theemission rates, chemical and physical compositions, and toxicities of particles from

various sources; and Risk Management E Õaluation, where the emphasis is on develop-

ing new information about the effectiveness and costs of prevention and control

technologies for emissions of PM .2.5

Ž .The overall objectives of the source characterization research are to 1 develop new

or improved emission factors for sources of primary fine particles and ammonia, one of Ž .the gaseous precursors of secondary fine particles; 2 develop new information on the

size distribution and composition of primary particles, to improve source profiles

Ž . Ž .fingerprints for source apportionment studies; and 3 chemically characterize andtoxicologically test source particles, to improve understanding of mechanisms of PM

toxicity and to obtain measures of health risk of various types of PM emissions. Current

focus is on field and laboratory studies to develop improved emission factors for



Table 5

Overview of current knowledge on U.S. emissions of fine particles

Numbers in regular type are typical values, selected from the referenced literature; entries in italics are estimates o

Source type Constituents of concern Total U.S. emission rate 3Ž . 10 Mgryear

PM PM2.5 10

a,b,c 1Roads Fine silica and other crustal elements 3300 18,000

plus reentrained carbon, asbestos,

and metal compoundsa,b,c 1Agricultural production Fine silica and other crustal elements 2000 11,100

Ž .including erosiona,b,c 1Construction activities Fine silica and other crustal elements 1700 8500

plus industrial reentrainment of

carbon, asbestos,

and metal compounds1ŽOpen burning including wild-fires, Products of uncontrolled combustion 1130 1320

a,b,c.agricultural burninga,b,c Ž .Residential wood combustion Polycyclic organic matter POM 550 ;600

a,b,cDiesel engine combustion Products of incomplete combustion, 450 500Ž .PM precursor NO x

a,b,cMineral products production Fine silica and other crustal elements 100 200a,dPulverized coal boilers Ar, Cr, Hg, Mn, Ni, Pb, Sb, Se, V, Cl, Unknown 160

Ž .and PM precursors SO , NO x x

a,eHeavy fuel oil combustion Cr, Fe, Ni, Pb, V, POM, Cl, ;30 30Ž .PM precursors SO , NO x x



a Ž .Residential fuel oil combustion POM, PM precursor NO ;20 20 xa,f,g,hWaste incineration As, Be, Cr, Cd, Hg, Ni, Pb, Unknown ;45

PCDDrF, PCBsa,iMetal smelting and refining Cd, Cr, Pb, Zn, SO Unknown 400 x

jAutomobiles V, carbon, organics, ;20 ;20

Ž .PM precursor NO x

Outdoor air introduced into Fine and coarse particles Unknown Unknowthe indoor environment

Tracked-in dust Pb, other heavy metals, Unknown Unknow

pesticidesŽIndoor activities that generate or Metals, pesticides, combustion Unknown Unknow

.resuspend particles aerosol organics

1Estimates of fine particle emissions from these ‘‘fugitive’’ sources, although large compared to other source

confirmed.a w xRef. 25 .b w xRef. 6 .c w xRef. 8 .d w xRefs. 1,3,7,13–15,17,30 .e w xRefs. 4,5,9,12 .f w xRefs. 19,23,24 .g w xRefs. 10,16 .h w xRef. 21 .i w xRef. 11 .

jAverage of 10 mgrmile; 2=1012 vehicle miles traveled.



Table 6Ž Overview of current knowledge on control of fine particles values for efficiencies and costs are estimates or judg

Source type Primary control options, efficiencies for PM Approximate co10

Ž .Roads Vacuum sweeping 0–50% , Dependent on tyŽ .Water flushing and sweeping 0 –96% , frequency of eve

Paving and roadside improvements, Very limited pubCovering trucks,

Speed and traffic reduction

Agricultural production Low tillage, punch planting, crop strips, Dependent on crŽ .including erosion vegetative cover, windbreaks; conditions. Little

Chemical stabilizers, irrigation

Construction activities Wet suppression of unpaved areas, material storage, Dependent on ty

handling and transfer operations; land area of even

Wind fences for windblown dust Very limited pubŽOpen burning including Low wind speed and appropriate wind direction Unknown

wild-fires, agricultural

.burningResidential wood Replace with cleaner burning stoves or furnaces ;US$1000 per

combustion

Diesel engine combustion Combustion modification, improved fuel Very limited pub

characteristics, particle traps

Mineral products Enclosing crushing, transfer areas; Water spray Dependent on ty

production suppression; Chemical stabilization of of equipment. Li

unpaved traffic areas



Pulverized coal boilers ESPs, Fabric Filters Capital cost US$

cost 2–5 millsrk

US$25–50 per m

Heavy fuel oil Cyclones, ESPs Unknown

combustion

Residential fuel oil Proper maintenance, modern furnaces Unknown

combustionWaste incineration Fabric filters, ESPs, venturi scrubbers Total installed c

Metal smelting and ESPs, cyclones Total installed c

refininga Ž .Outdoor air introduced Air cleaners for ventilation air 30 –98% ; Capital cost US$

a Ž .into the indoor Whole-building air cleaners 30–98% ; Capital cost US$aŽ .environment In-room air cleaners 30–98% US$200–800 pe

Ž .Tracked-in dust Cleaning e.g., vacuuming ; No published anŽ .a Whole-building air cleaners 30 –98% ; Capital cost US$

aŽ .In-room air cleaners 30–98% US$200–800 pe

Indoor activities Source control, including maintenance; Highly variable;a Ž Ž .that generate or Whole-building air cleaners 30–98% ; Capital cost US$

a. Ž .resuspend particles In-room air cleaners 30–98% US$200–800 pe

aRange of single-pass efficiency for removing particles. The effectiveness of air cleaners in reducing exposures to

and operating conditions, and is generally less than the single-pass efficiency.




on-the-highway diesel vehicles, wood stoves and fireplaces, oil- and coal-fired boilers,

and open burning of biomass. Other areas of emphasis are validation of instrumentation

for measuring rates and size distributions of emissions from fugitive dust sources, and

ammonia emissions from animal feeding operations. Source priorities are established inŽ .consultation with EPA’s Office of Air Quality Planning and Standards OAQPS .

Field studies of sources are being arranged in connection with one or more of EPA’sambient monitoring ‘‘supersites’’. Chemical profiles of source emission samples and

ambient samples collected at an appropriate supersite monitoring station will be devel-

oped, and source apportionments will be calculated to estimate contributions of the

source to the ambient air at the monitoring site. This will develop improved emission

factors and source profiles that can be used in implementation planning by the states,

using either source- or receptor-oriented modeling. A similar approach will be used to

apportion indoor exposure to fine PM among indoor and outdoor sources.

The overall objective of the risk management evaluation research is to evaluate the

cost and effectiveness of currently available options for reducing emissions of primaryparticles and gaseous precursors of secondary particles, and exposures to ambient and

indoor fine particles. Current focus is on pilot-scale field evaluation of devices for

industrial and utility boilers. A paper study is also underway to develop a framework for

evaluating fine PM control strategies for all types of sources. Laboratory- and house-scale

studies are being conducted to understand how penetration of outdoor particles into

buildings is affected by particle size, composition, environmental conditions, and

building operational conditions.

5. Summary

Growing concerns about the health risks of fine particles have led to the promulgation

of a revised national ambient air quality standard for PM smaller than 2.5 mm inŽ .aerodynamic diameter PM in the U.S. The existing data base on national emissions2.5

shows that sources of both primary and secondary particles make significant contribu-

tions. On a nationwide basis, the greatest mass of primary and precursor emissions come

from combustion, fugitive, and industrial sources, in that order. In specific localities, any

of these source categories can be dominant.It appears that the new emphasis on fine particles will focus risk management

research on large sources and combustion sources of various types. The role of fugitive

sources in creating exposures to fine particles is less clear, and field studies to obtain

better emissions data are needed. The importance of exposures inside buildings is also

unclear; the penetration of outdoor particles into buildings of various types and the role

of indoor sources of particles need further study. Current knowledge of the costs and

effectiveness of control options for PM is limited and poorly documented. Carefully10

documented data on control options for PM are nearly non-existent.2.5

A risk management research program is now in the early stages of developing newinformation to address several questions:

Ø What sources contribute to the fine particle exposures that are of greatest concern to

the health research and regulatory community?




Ø What are the emission rates and physical and chemical characteristics of particles

from these sources?

Ø What are the most cost-effective prevention and control options for these sourcesŽ .e.g., process changes, upgrades of existing controls, application of new technology ?

Risk management researchers are currently emphasizing source characterization

research that produces new information on PM emission factors and chemical profiles2.5

to enable better source apportionment by either source- or receptor-oriented modeling.

They are also extending collaborations with health researchers to increase toxicity

testing of particles for a wide variety of sources. Types of sources include on-the-high-

way diesel trucks, residential wood combustion, oil- and coal-fired boilers, fugitive

dust-generating construction activities, open burning of biomass, and indoor sources of

fine PM.

Research results will enable the U.S. Environmental Protection Agency to assist state

and local regulatory agencies in the development of cost-effective local prevention and

control strategies for reducing exposures to fine particles. These strategies are likely tobe based on a combination of industrial process changes, improved operation of existing

particle control devices, installation of new control equipment on selected sources,

increased control of gaseous precursors to ambient fine particles, and control of particles

in buildings.

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