2.2	PM AIR QUALITY CHARACTERIZATION

This chapter defines the various subclasses of particulate matter (PM) and then briefly discusses some of the available information on recent PM concentrations and trends. Materials for this section are extracted form the EPA PM Criteria Document (EPA, 1996), EPA PM staff paper (EPA, 1996b) and the EPA Trends Report on Air Quality (EPA, 1995). The most comprehensive discussion on PM air quality characterizations is available in Chapter 6 of the PM Criteria Document.

2.2.1	Overview of particle composition and size distributions

PM consists of discrete particles in the condensed (liquid or solid) phase spanning several orders of magnitude in size, ranging from clusters of 0.005 m in diameter to coarse particles on the order of 100 m.1 In addition to characterizations by size, particles can be described by their formation mechanism or origin, chemical composition, physical properties, and by sampling/measurement technique. This latter point on monitoring is critically important, given: (1) the complexities of measuring multicomponent, multiphase properties of aerosols, (2) different requirements between health/exposure based monitoring and comprehensive (space/time/chemical composition) air quality characterizations, and (3) difficulty in identifying causative components associated with adverse health.

A variety of man-made and natural activities contribute to PM concentrations, including fuel combustion (from vehicles, power generation, and industrial facilities), residential fireplaces, agricultural/forest burning, construction and demolition activities, wind blown dust, road dust, fugitive emissions, biogenic precursors. All of these source generation activities are coupled with "natural/prisitine" background levels, atmospheric transformations, deposition/removal and meteorological processes to produce a given "state". The major chemical constituents of PM are sulfates, nitrates, carbonaceous compounds (both elemental and organic carbon compounds), acids, ammonium ions, metals, water, and crustal materials. The relative fractions of these components vary from place to place and over time.

2.2.2	Multi-modal Size Distributions

Particles typically exhibit a multi-modal distribution, which varies with location and time. Based on particle size and formation mechanism, particles can be classified into two fundamental modes (fine and coarse). An idealized mass distribution of the fine and coarse modes (Figure 1) can be subdivided further into nuclei (≤ 0.1 m) and accumulation (0.3 to 0.7 m) modes. Nuclei or ultrafine particles tend to exist as disaggregated particles for very short periods of time (minutes) and rapidly coagulate into accumulation mode particles.

A cut point distinguishing fine and coarse modes lies between 1.0 m to 3.0 m where minimum mass occurs between the two modes. A clear choice of cut point does not exist, given the overlap that occurs between the modes. Most ambient measurements of fine particle mass in the U.S. have used instruments with cut points2 of 2.5 or 2.1 m. Table 1 provides size-cutoff related terminolgy used to describe PM.

Table 1. Particle Size Fraction Terminology

Fraction	Description

Fine particles	Fine mode particles which are generally formed through chemical reaction, nucleation, condensation of gases, and coagulation of smaller particles; contains most numerous particles and represents most surface

PM2.5	Particles with an upper 50 percent cut point of aerodynamic diameters1 less than 2.5 m, a measurable approximation for fine particles2

Coarse particles, generally	Coarse mode particles which are mostly mechanically generated through crushing or grinding

Coarse fraction of PM10	Particles with an upper 50 percent cut point of aerodynamic diameters between 2.5 m and 10 m

PM10	Particles with an upper 50 percent cut point of aerodynamic diameters less than 10 m, including fine fractions and part of the general coarse mode

Total Suspended Particles (TSP)	Particles with an upper 50 percent cut point of aerodynamic diameters less than approximately 50 m; highly wind speed dependent

1Aerodynamic diameter is defined as the diameter of a spherical particle with equal settling velocity but a material density of 1 g/cm3. This normalizes particles of different shapes and densities.

2PMX indicates an upper cut point with a 50 percent cut point of X m diameter. Because samplers have a collection efficiency that varies around the 50 percent cut point, not all particles less than X m diameter will be collected and some particles greater than X m diameter will be collected.

2.2.3	Composition of Fine and Coarse Fraction Particles

Fine and coarse mode particles generally have distinct chemical composition, solubility, and acidity. Fine mode PM is composed of varying proportions sulfates, acids, nitrates, elemental carbon, organic carbon compounds, trace metals, and water. Coarse fraction constituents primarily are composed crustal elements; Si, Al, Fe, and K (small amounts of Fe and K reflect ???). Biological materials (bacteria, pollen, spores) also appear in the coarse mode. The chemical composition of PM10, the sum of both modes, is more heterogeneous than either mode alone.

Figure 2 aggregates the available published data on the chemical composition of PM2.5 and coarse fraction particles in U.S. cities by region. In addition to the larger relative shares of crustal materials in the West, total concentrations of coarse fraction particles are generally higher in the arid areas of the Western and Southwestern U.S. The fraction of PM2.5 due to sulfate is greater in the East, and the nitrate fraction is much larger in the West. Isopleths drawn from the IMPROVE data for selected metals (Figure 3) illustrate the elevated sulfate levels in the East (Cahill...or Malm find source). Tables 1 and 2 summarize composition data (from IMPROVE) by region and season, respectively.

Figure 2. Major constituents of particles measured at sites in the eastern and western United States (left to right) (source PM CD).

TABLE 1.	ANNUAL AVERAGE CONCENTRATIONS AND CHEMICAL COMPOSITION FROM IMPROVE MONITORING SITES

Annual Average Concentrations, g/m3 and Composition

Region	No. of Total Sites

PM 2.5	(NH4)2SO4/ % PM 2.5	Organics/ % PM 2.5	PM Coarse

PM 10
Northwesta	15	3.55	0.88/25	1.63/46	4.46	8.0
Southwestb	5	3.91	1.28/33	1.38/35	5.62	9.5
California Coastal Mountainsc	3	4.99	1.41/28	1.95/39	8.85	13.8
Transitional Regiond	5	5.15	1.97/38	2.01/39	6.54	11.7
Appalachian Mountainsc	2	10.91	6.33/58	2.97/27	6.24	17.2

aCascades (1), central Rocky Mt. (5), Great Basin (1), N. Rocky Mt. (1), Sierra Nevada (1), Sierra Humboldt (2), and Colorado Plateau (4)

bColorado Plateau (3), Sonora Desert (2)

cSame as subregion

dWestern Texas (2), northern Great Plains (1), Boundary Waters (2).

TABLE 2.	ANNUAL SUMMER AND WINTER CONCENTRATIONS FROM IMPROVE MONITORING SITESa

Subregion

Region of U.S.	No of Sites	Seasons of the Year

PM2.5

(NH4)2SO4

Organics	PM Coarse
Central Rockies	NW	5	annual

summer

winter	3 .3

4 .8

2 .0	0 .8

1 .0

0 .5	1 .5

2 .4

0 .9	4 .8

7 .5

3 .0

Colorado Plateau	NW-SW	7	annual

summer

winter	3 .4

4 .1

2 .9	1 .1

1 .3

0 .9	1 .2

1 .6

1 .1	4 .7

6 .4

3 .2

Coastal Mountains	NW	3	annual

summer

winter	5 .0

4 .5

5 .6	1 .4

1 .9

0 .9	1 .9

1 .4

2 .3	8 .9

10 .7

7 .7

Sonora Desert	SW	2	annual

summer

winter	4 .4

5 .6

3 .2	1 .5

2 .1

1 .2	1 .5

1 .8

1 .1	6 .0

7 .6

3 .3

West Texas	Transitional to east	2	annual

summer

winter	5 .4

6 .6

3 .6	2 .1

2 .5

1 .5	1 .5

1 .7

1 .1	7 .5

7 .4

5 .1

Northern Great Plains	Transitional to east	1	annual

summer

winter	4 .5

5 .6

3 .4	1 .5

1 .8

1 .2	1 .5

2 .2

1 .1	6 .3

9 .7

3 .9

Boundary Waters	Transitional to east	2	annual

summer

winter	5 .3

6 .2

5 .2	2 .0

2 .2

2 .0	2 .1

3 .1

1 .4	5 .7

8 .2

3 .2

Appalachian Mountains	Eastern U.S.	2	annual

summer

winter	10 .9

16 .6

6 .5	6 .3

10 .5

3 .0	3 .0

4 .4

2 .0	6 .2

11 .2

3 .1

a From Malm et al., 1994.

2.2.4	PM Concentrations

2.2.4.1	Correlations between PM2.5 and Coarse Fraction Mass

Ambient daily fine and coarse fraction mass concentrations generally are not well correlated, plausible considering differences between both modes in sources and chemical/physical attributes. Current analyses (SAI, 1996) based on various data sets show R2 between daily PM2.5 and PM10-2.5 mass of 0.13 for all non-rural sites and 0.21 with rural sites included. Fine and coarse fractions should correlate in certain instances where the fine mode fraction is the major fraction of PM10.

2.2.4.2	PM10 Concentrations and Trends

State and local air pollution control agencies have been collecting PM10 mass data and reporting to EPA's AIRS database since mid-1987. A geographic distribution (Figure 4) of the 83 PM10 nonattainment areas (as of September, 1994) shows an overwhelming number of rural western nonattainment areas, relative to more populated eastern areas. Many of the highest values occur in western areas with fugitive dust sources and in mountain valleys impacted by wood smoke during winter inversions.

National trends from 1988 to 1993 trend and show site-to-site variability in the composite annual mean and the ninetieth percentile of 24-hour PM10 concentrations (Figures 5 and 6). The composite annual mean trend shows a steady decline of 20 percent; and the ninetieth percentile decreases roughly 19 percent over the same period (U.S. EPA, 1993). Annual average PM10 concentrations ranged from 25 to 35 g/m3 for most U.S. regions by 1994.

2.2.4.3	Fine Particle Concentrations and Trends

PM2.5 concentration data are more limited than for PM10. From 1983 to 1993, less than 50 sites reported data to AIRS in any given year. Quarterly U.S. maps (Figure 7) of fine PM based on aggregations IMPROVE, Northeast States Coordinated Air Use Management (NESCAUM) and airport visibility data show the regional character of elevated fine particle levels in the Eastern U.S. and California as well as a strong seasonality. Higher levels prevail in the East during summer; high wintertime levels exist in southern California. These data generally do not include urban concentrations and only reflect regional non-urban concentrations; hence much urban area spatial detail is not available and care should be exerted when interpreting such diagrams.

National PM2.5 trends are not available because of the limited number of sites measuring PM2.5 and the sampling period at most sites is restricted to a few years. The development of national trends is further hindered because a variety of sampling frequencies and techniques have been used for PM2.5. Visibility data can has been used as a surrogate to estimate fine particle trends because the extinction coefficient (Bext) is directly related to fine particle mass (EPA staff paper). Sufficient visibility data are available to produce national trends from 137 U.S. sites (principally airports) since 1948. In the northeastern states, winter haze shows a 25 percent decrease while in the southeastern states, there is a 40 percent increase in winter haze (NAPAP, 1991).2 The summer haziness in the Northeast shows an increase up to the mid-1970s followed by a decline. In the Southeast, there was an 80 percent increase in summer haziness, mainly occurring in the 1950s and 1960s (NAPAP, 1991).

Visibility has been monitored with more precision by the IMPROVE network from 1987 to present. In eastern remote locations, air quality data from 1982 to 1992 showed roughly a 3 percent increase in sulfate mass concentration during the summer and a smaller negative (although not statistically significant) trend in the winter (Eldred and Cahill 1994). Western visibility monitoring through the IMPROVE network has not shown any trends for the period.

2.2.4.4	Background Levels

Natural sources contribute to both fine and coarse particles in the atmosphere. Background PM is defined as the distribution of PM concentrations that would be observed in the U.S. in the absence of anthropogenic emissions of PM and precursor emissions of volatile organic compounds (VOCs), NOx, and SOx in North America.

Background levels of PM vary by geographic location and season. The natural component of the background arises from physical processes of the atmosphere that entrain fine particles of crustal material (i.e., soil) as well as emissions of organic particles resulting from natural combustion sources such as wildfire. In addition, certain vegetation can emit fine organic aerosols as well as their precursors. The exact magnitude of the natural portion of PM for a given geographic location can not be precisely determined because they are difficult to separate from the long-range transport of anthropogenic particles or precursors. Only broad estimates for longer averaging times can be developed at this time, as shown in Table 3.

TABLE 3. PM10 AND PM2.5 REGIONAL BACKGROUND LEVELS

Western U.S. ( g/m3)	Eastern U.S. ( g/m3)
PM10 , annual average	4 - 8	5 - 11
PM2.5 , annual average	1 - 4	2 - 5

Source: CD, page 6-44. The lower bounds of the above ranges are based on compilations of natural versus human-made emission levels, ambient measurements in remote areas, and regression studies using human-made and/or natural tracers (NAPAP, 1991; Trijonis, 1982). The upper bounds are derived from the multi-year annual averages of the "clean" remote monitoring sites in the IMPROVE network (Malm et al., 1994). It is important to note, however, that IMPROVE data used here reflect the effects of background and anthropogenic emissions from within North America and therefore provide conservative estimates of the upper bounds.

As noted in the estimates, there is a definite geographic trend to these levels with the lower value applicable to the Western U.S. and the somewhat higher value applicable to the Eastern U.S. The Eastern U.S. is estimated to have more natural organic fine particles and more water associated with hygroscopic fine particles than the West.

The range of background concentrations can be higher on an episodic basis. Specific natural events such as wildfires, volcanic eruptions, and dust storms can lead to very high levels of PM. Disregarding such large and unique events, an estimate of range of "typical" background on a daily basis can be obtained from reviewing various multi-year data as well as special field studies. On very clean days, IMPROVE daily measurements are less than 1 g/m3 of PM2.5. On some days atmospheric conditions are more conducive to accumulation and formation of PM from both natural and anthropogenic emissions sources. Upper bound estimates of daily background under these conditions can be made using field data, such as short-term special studies. One study measured daily concentrations of up to approximately 12 g/m3 PM10 in remote "clean" areas of the Eastern U.S. (Wolff et al., 1983).