2.0	Explain the Current Environmental State

2.A	Ozone Air Quality Characterization

This section provides an overview of ozone air quality in the continental U.S., covering to the extent practicable diurnal and seasonal time frames, rural and urban environments and recent trends. Materials were extracted and condensed from EPA's Criteria Document for Ozone ( EPA, 1993), the EPA Air Quality Trends Report (EPA, 1996), and the EPA Staff Paper on ozone (EPA, 1996).

Surface Ozone Concentrations

Ozone is a naturally occurring, trace constituent of the atmosphere. Estimates of the natural component of O3 vary widely in the scientific literature, and there is no standardized terminology regarding the concept of O3 background concentrations.

"Natural" O3 background is a multidimensional and complex concept. Background O3 concentrations vary by geographic location, altitude and season. For the purposes of this document, background ozone is defined as the ozone concentrations that would be observed in the U.S. in the absence of anthropogenic precursor emissions of volatile organic compounds (VOCs) and oxides of nitrogen (NOx) in North America. During the summertime ozone season, in the U.S., daily 1-hour maximum background ozone concentrations are typically between 0.03 to 0.05 parts per million (ppm). Part of this background is due to natural sources and part of it is due to long-range transport of anthropogenic precursor emissions.

The natural component of the background originates from three sources: stratospheric ozone, which is transported down to the troposphere; ozone formed from the photochemically- initiated oxidation of biogenic and geogenic methane and carbon monoxide; and, the photochemically-initiated oxidation of biogenic VOCs. The magnitude of this natural part cannot be precisely determined for two reasons. First, the part due to long-range transport of anthropogenic precursor emissions is not known. Second, NOx plays an important role in the oxidation of methane, carbon monoxide and biogenic VOCs, and it is not possible to determine the amount of ozone that would have been formed due soley to natural NOx emissions. However, some estimates can be made.

On the basis of O3 data, from isolated monitoring sites (EPA, 1993), an estimate of the annual average O3 background concentration near sea level in the U.S. is 0.020 to 0.035 ppm. This estimate includes a 0.005 to 0.015 ppm contribution (averaged over time) from stratospheric intrusions into the troposphere and a 0.01 ppm contribution from photochemically-initiated oxidation of methane and carbon monoxide. The remainder is due to the photochemically- initiated oxidation of biogenic VOCs and long-range transport.

Similarly, an estimate of the background O3 concentration near sea level in the U.S. for a 1-hour daily maximum during the summer is usually in the range of 0.03 to 0.05 ppm. At clean sites in the Western U.S., the maximum annual hourly values are in the range of 0.060 to 0.075 ppm. Such elevated O3 levels may be occurring at higher altitudes due to stratospheric O3 intrusion. An estimate for background 8-hour daily maximum O3 concentrations during the summer is also in the range of 0.03 to 0.05 ppm.

Hourly Ozone Air Quality Distributions

Figure 2.1.1 presents histograms of the hourly O3 concentrations for the 3-month peak summer period at sites in varying geographic locations and source-receptor environments: an urban site in Chicago; a site downwind of Chicago; a site downwind of Atlanta; and, a site at higher elevation in Albuquerque. All hourly concentrations are displayed. These charts show that there are distinct differences among these sites, with downwind sites exhibiting a greater frequency of higher concentrations.

Figure 2.1.2 shows the typical relationship between hourly ozone concentrations and the corresponding running 8-hour averages. The individual bars correspond to hourly values and the line shows the corresponding 8-hour averages. Each 8-hour average is associated with the start hour of the 8-hour period. For example, the 8-hour average from 4 p.m. to midnight is plotted at 4 p.m. It should be noted that the next 8-hour period, from 5 p.m. to 1 a.m., is plotted at 5 p.m. but it contains hourly values from two different days. If the 8-hour daily maximum is selected from all twenty-four 8-hour averages starting within the day, it is possible to have daily maximums from adjacent days that have some hourly values in common. This is unlikely for the typical urban diurnal pattern shown in Figure 2.1.2, but it may occur for sites with less pronounced diurnal patterns, such as rural, higher elevation sites.

Another point to note with 8-hour averages is that the 8-hour daily maximum for a particular day can actually be higher than the 1-hour maximum. Again, this would be uncommon but can happen because the daily maximum 8-hour average could contain up to seven hourly values from an adjacent day.

Ozone Air Quality Trends

The interpretation of recent O3 trends is difficult due to the large temporal variation that results from the confounding factors of meteorology and emissions changes. Peak O3 concentrations typically occur during hot, dry, stagnant summertime conditions. Thus, Summer 1988, as the third hottest summer on record since 1931, was highly conducive to O3 formation with peak O3 levels comparable to those recorded in the earlier peak year of 1983 (Heim, 1988; EPA, 1990). Meteorological conditions in 1991 and 1993 were also highly conducive to O3 formation, especially in the eastern half of the country, although the magnitude and frequency of exceedances were significantly less than those recorded in 1988 (Brown,1993; EPA, 1994). In contrast, the years 1989 and 1992 saw meteorological conditions that were generally not as conducive to O3 formation. These changes in meteorological conditions have led to large year-to- year differences in peak O3 concentrations (Brown, 1992). In response to the National Academy of Sciences recommendations (NAS, 1991), EPA has developed a statistical model that adjusts for meteorological variability to detect the underlying O3 trend (Cox and Chu, 1994). Figure 2.1.3 presents the meteorologically adjusted, and unadjusted, ten year O3 concentration trends in 43 metropolitan areas. The 99th percentile daily maximum 1-hour concentration declined 1 percent per year or 12 percent since 1984. The national trend in the composite mean of the annual second highest daily maximum 1-hour concentration at 509 sites is shown for comparison, which also declined by 12 percent between 1984 and 1993.

Spatial Distributions of Air Quality Concentrations

This section provides a brief overview of how O3 concentrations vary across the country with respect to alternative standards. Figure 2.1.4 displays a map of those counties with 1-hour daily maximum, 1 expected exceedance O3 design values greater than 0.12 ppm based on 1991- 1993 air quality monitoring data. Figure 2.1.5 shows the spatial distribution of counties with average annual fifth highest 8-hour daily maximum O3 design values greater than 0.08 ppm, based on 1991-1993 data. Figure 2.1.6 depicts those counties with a 3-month SUM06 O3 exposure index value greater than 25 ppm-hours in 1990. These SUM06 values are based only on the daylight hours, 8:00 am - 8:00 pm Local Standard Time (LST). In each of these maps, the county air quality status was determined by the peak design value site in each county. For the current standard, the design value is simply the fourth highest daily maximum 1-hour concentration measured during 1991-1993, since if the fourth highest value is reduced to the level of the standard, there will be only three days above the level of the standard, or 1 exceedance per year. Because both the proposed primary and secondary standards are concentration based standards, the design value is identical to the standard test statistic.

Ozone Air Quality Status with Respect to Standards

Ambient air quality data from 1991-93 have been used to estimate the air quality status with respect to the current and the proposed primary and secondary standards. Table 2.1.1 presents the air quality status of the original 98 areas designated nonattainment for O3 under the Clean Air Act Amendments of 1990 with respect to these alternative standards.

Table 2.1.1	Number of original nonattainment areas not meeting selected standards based on 1991-93 air quality monitoring design values.

OZONE AIR QUALITY STANDARD Number of Non- Compliant Areas Primary standard

1-hour, 1 exceedance, 0.12 ppm

39

8-hour, avg annual 2nd max, 0.08 ppm

avg annual 5th max, 0.08 ppm

Secondary standard 85

64

3-month SUM06 > 25 ppm-hours

(8:00 am-8:00 pm LST) 70	Number of Areas with Monitors 97

Ozone Air Quality Relationship Among Standards

Although previous sections have shown that there are significant differences in air quality distributions among monitoring sites with differing environments, several patterns emerge from examining typical relationships. For example, the average ratio of 8-hour to 1-hour design values for a 1 exceedance per year standard is 0.86 based on 1991–93 data. This ratio has been fairly stable over time increasing slightly from 0.81 in 1980 to 0.86 in 1993. The ratio is also fairly consistent across EPA Regions, with the median ratio ranging from 0.80 to 0.88.

Design value comparisons based on 1991-93 data show that, on average, ozone design values for an 8-hour, 0.08 ppm average annual 5th highest daily maximum standard are 25% lower than the current 1-hour standard. On average, design values for an average annual 2nd highest daily maximum 8-hour standard are 18% lower than the current 1-hour standard.

In terms of exceedances (i.e., the number of days with daily maximum 8-hour average concentrations greater than 0.08 ppm), sites meeting an average annual 2nd highest daily maximum 0.08 ppm standard average, 1.2 exceedances per year and 2.3 exceedances in the worst year of three. Sites meeting an average annual 5th highest daily maximum 0.08 ppm standard average 3.0 exceedances per year and 5.4 exceedances in the worst year of three. In the worst year of three, 95 % of sites meeting an average annual 2nd highest daily maximum 0.08 ppm standard have seven or fewer exceedances, while 95 % of sites meeting an average annual 5th highest daily maximum 0.08 ppm standard have 12 or fewer exceedances.

2.B	Particulate Matter Air Quality Characterization

This section 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, 1996). The most comprehensive discussion on PM air quality characterizations is available in Chapter 6 of the PM Criteria Document.

Overview of Particle Composition and Size Distributions

Particulate matter 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. 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 and 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, and 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.

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 7) 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 points of 2.5 or 2.1 m. Table 1 provides size-cutoff related terminolgy used to describe particulate matter (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

PM-2.5	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 PM-10	Particles with an upper 50 percent cut point of aerodynamic diameters between 2.5 m and 10 m

PM-10	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.

2PM-X 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.

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 of sulfates, acids, nitrates, elemental carbon, organic carbon compounds, trace metals, and water. Coarse fraction constituents are primarily composed of crustal elements: silicon (Si), aluminum (Al), iron (Fe), and potassium (K). Biological materials (bacteria, pollen, and spores) also appear in the coarse mode. The chemical composition of PM-10, the sum of both modes, is more heterogeneous than either mode alone.

Figure 2 combines the available published data on the chemical composition of PM-2.5 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 PM-2.5 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 (see Figure 3) illustrate the elevated sulfate levels in the East (Cahill...or Malm find source). Tables 1 and 2 summarize composition data from the IMPROVE database by region and season, respectively.

A material balance on the chemical composition of atmospheric fine particle samples typically shows that the fine particle mass consists of a combination of organic compounds, elemental (black) carbon, sulfate ion, nitrate ion, ammonium ion, sea salt, soil dust and water. These chemically distinct particle constituents provide a very strong indication of the sources from which the fine particle mass originated. In addition, small quantities of trace metals and hydrogen ion may be present in fine particles at concentrations too low to account for most of the mass but at concentrations high enough to produce concern over the potential for undesirable effects (e.g. strong acid aerosols; heavy metals toxicity).

Direct emissions of particle-phase material from sources are referred to as primary emissions in order to distinguish them from the secondary fine particles that are produced from gas-phase precursors by atmospheric chemical reactions. Primary fine particles typically arise from combustion, from specialized industrial processes, and from sources of geological dust. Black elemental carbon particles are entirely due to primary emissions from combustion sources. In many cases, elemental carbon particle concentrations are dominated by diesel soot and wood smoke. Elemental carbon particles often act as a good indicator of atmospheric dilution and transport because they are not produced by atmospheric chemical reactions and are generated at particles sizes (typically about 0.2 m particle diameter) that do not deposit out of the atmosphere easily but instead disperse in much the same way as carbon monoxide (CO). Primary organic carbon-containing particles are emitted directly from more than 70 different types of mobile, stationary and fugitive source types. Prominent sources of fine organic aerosols include motor vehicle exhaust, food cooking (e.g. meat charbroliers), wood combustion, the organic carbon content of paved road dust, plus smaller contributions from airborne plant fragments, stationary oil and gas combustion, aircraft engines, fine particle tire dust, brake dust and cigarette smoke, to name but a few of the many possible sources. Fine particle organic compounds plus elemental carbon typically contribute 25-40% of the fine particle mass concentrations measured in cities.

A further contribution to primary fine particle concentrations consists of the fine particle fraction of the geological dust emissions to the atmosphere. While most of the mass of soil and road dust emissions to the atmosphere is found in coarse particles having aerodynamic diameter greater than 2.5 m, a few percent of the crustal dust emissions occur in the fine particle size range. It is not uncommon to find that perhaps 10% of an airborne fine particle sample is composed of geological material, with fugitive paved road dust being the most important source of this type in cities.

Sulfate-containing particles can be emitted directly from combustion sources; typically about 3% of the sulfur content of fuels is emitted directly as primary particulate sulfate. However, the majority of the sulfates in the atmosphere arise from gas-to-particle conversion processes in which SO2 gas produced by combustion of sulfur bearing fuels is oxidized in the atmosphere to form sulfuric acid, ammonium bisulfate or ammonium sulfate particles. SO2 oxidation can occur by both homogeneous gas phase chemical reactions and heterogeneous processes within cloud or fog droplets. The gas phase homogeneous oxidation of SO2 occurs primarily due to reaction between SO2 and the hydroxyl radical, with additional contributions from ozone-olefin reactions products. The heterogeneous conversion of SO2 to form sulfates with liquid water droplets can occur due to reaction of dissolved SO2 with dissolved hydrogen peroxide, or ozone. In the presence of catalytic metals (Fe, Mn, and others), dissolved SO2 can be oxidized at appreciable rates by dissolved oxygen. Which of the various SO2 oxidation pathways dominates depends on the details of the local accumulation of co-pollutants such as ozone, hydrogen peroxide, hydrocarbons, ammonia, and catalytic metals. The important point to observe is that, given so many competitive pathways, SO2 oxidation usually is ongoing due to at least one or more of the available routes, generally producing overall conversion rates of a fraction of a percent per hour to 10% per hour or more. Over a period of several days, much of the SO2 in the atmosphere will be converted to sulfate if it is not removed first by wet or dry deposition at the earth's surface.

The reaction products of SO2 in the atmosphere are non-volatile and will not return to the gas-phase once formed. The speciation of the particulate sulfates as sulfuric acid, ammonium bisulfate, ammonium sulfate, or some combination of the above, is largely determined by the amount of ambient ammonia available to neutralize the acid sulfate species. In the Eastern United Stated, unneutralized acid sulfate particles are frequently reported, while in California and in other western locations the sulfate aerosols are commonly found to be fully neutralized, with excess ammonia remaining in the gas phase.

Aerosol nitrates are produced by reactions involving atmospheric nitric acid vapor. Nitric acid is formed in the atmosphere as a further reaction product of NO2. During the daytime, NO2 oxidation occurs by reaction with the hydroxyl radical. At night, NO2 can react to form nitric acid by pathways based on formation of the nitrate radical, NO3, followed by production of N2O5, which then can hydrolyze to produce nitric acid. The nitrate radical can also react with certain organic species to produce nitric acid. As nitric acid concentrations build-up in the atmosphere in the presence free ammonia gas, a critical point is passed at which the product of the nitric acid vapor concentration times the ammonia concentration in the gas phase exceeds the value of the equilibrium dissociation constant for ammonium nitrate aerosol. Since ammonium nitrate is a solid or liquid phase species, gas-to-particle conversion occurs at this point. The value of the equilibrium dissociation constant for ammonium nitrate is extremely sensitive to temperature and humidity, with more ammonium nitrate formation from the same quantity of ammonia and nitric acid vapor expected at low temperature and at high relative humidity. While heavy photochemical smog episodes greatly favor rapid nitric acid vapor production, such photochemical smog episodes often occur on days with very high temperatures that do not necessarily favor ammonium nitrate production form that nitric acid vapor. Instead at high temperatures the nitric acid vapor may well stay in the gas phase where it is available for relatively rapid deletion by dry deposition at the ground. Fine particle nitrate concentrations in the vicinity of 100 m/m3 over 24-hour averaging times have been observed in the eastern end of the South Coast Air Basin that surrounds Los Angeles during late October, a time of year with moderate photochemical smog but generally lower values of the equilibrium dissociation constant for ammonium nitrate. Just as ammonium nitrate formation can be encouraged by high nitric acid levels, high ammonia levels, low temperatures, and high relative humidity, ammonium nitrate aerosols once formed can be destroyed (returned to the vapor phase) if one or more of the key factors driving ammonium nitrate formation is removed from an air parcel. Thus ammonium nitrate contained in air parcels transported into the desert downwind of Los Angeles could be expected to evaporate if the gas phase nitric acid or ammonia that surrounds the particles is removed by dry deposition, or if temperatures increase or relative humidities decline, all of which are likely to occur in a desert environment. If the aerosol studied is a mixed salt composed of various proportions of sulfate, nitrate, ammonium, chloride, etc., all found in the same particles, then the above discussion of ammonium nitrate equilibrium with nitric acid and ammonia only needs to be replaced by a more complex description based on multi-component aerosol thermodynamics.

A second common route for aerosol nitrate formation involves nitric acid (and possible NO2) attach on the NaCl in sea salt particles to form NaNO3 aerosol and to release gaseous chloride species to the atmosphere. While most sea salt particles are in the coarse particle mode and are larger than 2.5 m in diameter, one should still look for the formation of some fine particle NaNO3. Further, the corresponding release of gaseous chloride species from the aerosol is another transformation route that can affect fine particle concentration and composition.

The final major route for formation of fine particles by atmospheric chemical reactions involves secondary organic aerosol production. Secondary organic aerosol production is the subject of much ongoing research. Two routes for secondary organics formation seem possible. The traditional view has been that secondary organic aerosol formation will occur when atmospheric chemical reactions produce organic compounds such as aliphatic dicarboxylic acids that have extremely low vapor pressures. Once these low vapor pressure products saturate the gas phase at the level of a few parts per thousand (ppt), any further chemical production will result in aerosol production. More recently, it has been realized that secondary organic aerosol production also can occur if organic gases dissolve into an existing organic liquid phase in the atmospheric particle complex. This organic liquid phase could be present due to primary organic aerosol emissions or due to previous secondary organic aerosol production. Because of the difficulties of distinguishing primary organic aerosols from secondary organic aerosols in an atmospheric sample, and due to the general lack of much organic aerosol data in the Eastern U.S., much less is known about the relative importance of secondary organic aerosol formation than is known about primary particle emissions and sulfate plus nitrate formation. In the Los Angeles area where data are available that can be used to draw some preliminary conclusions, it appears that secondary organic aerosol accounts for 15-30% of the total organic aerosol (hence 5-10% of the entire fine particle mass concentration) on an annual average basis, but can account for the majority of the organic aerosol present over short periods during severe photochemical smog episodes.

TABLE 1.	Annual Average Concentrations and Chemical Composition from the IMPROVE Monitoring Sites

Annual Average Concentrations, g/m3 and Composition Region Total Sites PM-2.5 (NH4)2SO4/% PM-2.5 Organics/% PM-2.5 PM Coarse PM-10

PM10

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 the 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).

Particulate Matter Concentrations

Correlations Between PM2.5 and Coarse Fraction Mass

Ambient daily fine and coarse fraction mass concentrations generally are not well correlated. This is plausible considering differences between both modes in sources and chemical/physical attributes. Current analyses (SAI, 1996) based on various data sets show R2 values between daily PM-2.5 and PM-10-2.5 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 PM-10.

PM-10 Concentrations and Trends

State and local air pollution control agencies have been collecting PM-10 mass data and reporting to EPA's AIRS database since mid-1987. A geographic distribution (Figure 4) of the 83 PM-10 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 ninetieth percentile of 24-hour PM-10 concentrations and the composite annual mean (Figures 5 and 6, respectively). The ninetieth percentile decreases roughly 19 percent and the composite annual mean trend shows a steady decline of 20 percent over the same period (U.S. EPA, 1993). Annual average PM-10 concentrations ranged from 25 to 35 g/m3 for most U.S. regions by 1994.

Fine Particle Concentrations and Trends

PM-2.5 concentration data are more limited than for PM-10. 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 PM-2.5 trends are not available because of the limited number of sites measuring PM-2.5 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 PM-2.5. Visibility data have 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). 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.

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. PM-10 and PM-2.5 Regional Background Levels

Western U.S. ( g/m3) Eastern U.S. ( g/m3) PM-10 , annual average 4 - 8 5 - 11 PM-2.5 , 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 PM-2.5. 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 PM-10 in remote "clean" areas of the Eastern U.S. (Wolff et al., 1983).

2.C	Visibility/Regional Haze Air Quality Characterization

Overview of Current Visibility Conditions

Annual average visibility conditions vary regionally across the U.S. The rural East generally has higher levels of impairment than remote sites in the West, with the exception of the San Gorgonio Wilderness, Point Reyes National Seashore, and Mount Rainier, which have annual average levels comparable to rural sites in the Northeast. Higher averages in the East are due to generally higher concentrations of anthropogenic fine particles and precursors, higher background levels of fine particles, and higher average relative humidity levels. Visibility conditions also vary significantly by season of the year. With the exception of remote sites in the northwestern U.S., visibility is typically worse in the summer months. This is particularly true in the Appalachian region, where average extinction in the summer exceeds the annual average by 40% (Sisler, 1996).

Figure 15 presents 1990 quarterly averages (75th percentile) of monitored visibility levels based on a fusion of IMPROVE, NESCAUM and Federal Aviation Authority (FAA) data (Husar, 1996) nationally. Clear regional and seasonal patterns are evident. The eastern U.S. is strongly affected by summertime visibility degradation. Western (southern California) visibility degradation is apparent in summer and winter seasons.

Estimated Background Levels of Fine Particles and Associated Light Extinction

Total light extinction is determined by the combined effects of fine particles from background (i.e., geogenic and biogenic) sources, Rayleigh scattering (i.e., the degree of light extinction that would be found even in a particle-free atmosphere), and fine particles from anthropogenic sources. Chapter 6 of the Criteria Document identifies several alternative definitions of "background" levels of fine particles. For the purposes of this document, 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 VOC, NOx, and SOx in North America. Table IV-4 (missing) describes the range for annual average regional background fine PM mass in the East as 2-5 g/m3, and in the West 1-4 g/m3. The lower bounds of these ranges, taken from estimates in the 1990 report of the National Acid Precipitation Assessment Program, 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 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. Table VIII-2 (missing) from the NAPAP report includes estimates of annual average background levels of fine particles by aerosol constituent, as well as their related contributions to light extinction, expressed in inverse megameters (Mm-1) (NAPAP, 1991). On an hourly or daily basis, however, background concentrations will vary considerably depending on seasonal, meteorological, and geographic factors.

Role of Humidity in Light Extinction

As mentioned previously, humidity plays a significant role in the impairment of visibility by fine particles, particularly in the East, where annual average relative humidity levels are 70-80% as compared to 50-60% in the West (Sisler et al., 1993). Table VIII-2 (missing) accounts for relative humidity effects by assigning an extinction efficiency for water associated with aerosols, while extinction efficiencies found in Table VIII-3 (missing) are modified by a relative humidity adjustment factor in calculating total extinction. The adjustment factor represents (1) the hygroscopic nature of the aerosol constituent, and (2) the average annual humidity for the relevant location (Sisler et al., 1993).

Because annual average relative humidity is higher in the East, the same ambient concentration of sulfate, for example, will on average lead to greater light extinction in an eastern location rather than a western one. The top map in Figure VIII-9 (missing) illustrates the regional variability of annual mean relative humidity nationwide. The bottom map depicts the variability of the relative humidity correction factor used for sulfates in the IMPROVE analysis (Sisler et al., 1993). For example, when corrected for humidity, the overall extinction efficiency for sulfates in the East may exceed 11-12 m2/g, whereas the extinction efficiency for sulfate in the West may be one-third to one-half of that.

Rayleigh Scattering

Rayleigh scattering represents the degree of light extinction found in a particle-free atmosphere caused by the gas molecules that make up "blue sky" (e.g., N2, 02, CO2 )(U.S. EPA, 1979). The concept of Rayleigh scattering can be used to establish a theoretical maximum horizontal visual range in the earth's atmosphere, absent visibility-impairing particles. At sea level, this maximum visual range would be approximately 330 kilometers (equivalent to light extinction of 10-12 Mm-1). Table VIII-2 (missing) shows the contribution to total light extinction from "Rayleigh scattering." While Rayleigh scattering can be used in calculating this maximum visual range, it is analagous to a baseline or boundary condition and should not be considered to contribute to visibility impairment. Rather, only fine and coarse particles from natural and anthropogenic sources should be treated as factors in determining visibility impairment.

Table VIII-2 (missing) illustrates that estimated extinction contributions from Rayleigh scattering plus background levels of fine and coarse particles, in the absence of anthropogenic emissions of visibility-impairing particles, are 26 plus or minus 7 Mm-1 in the East, and 17 plus or minus 2.5 Mm-1 in the West. These equate to a visual range in the East of 150 plus or minus 45 kilometers and 230 plus or minus 40 kilometers in the West. Excluding light extinction due to Rayleigh scatter, annual average background levels of fine and coarse particles are estimated to account for 14 Mm-1 in the East and about 6 Mm-1 in the West. Major contributors that reduce visibility from the Rayleigh maximum (approximately 330 km visual range) to the ranges noted above are naturally-occurring organics, water, and suspended dust (including coarse particles) in the East, and suspended dust, organics, and water in the West. In these ranges of fine particle concentrations, small changes have a large effect on total extinction. Thus, one can see from Table VIII-2 (missing) that higher levels of background fine particles and associated humidity in the East result in a fairly significant difference between estimates of naturally-occurring visual range in the East and West.