3.0	Processes: How is the State Created, Sustained and Maintained?

This section discusses pollutant emissions, atmospheric conditions, and chemical reactions that contribute to the formation and persistence of ozone, fine particulate matter, and regional haze in the atmosphere. These processes can be very complex and may lead to controversy over the most effective strategies to control each type of pollutant.

3.A	Primary and Secondary Emissions Processes

The formation of ozone, fine particulate matter, and regional haze are dependent on both direct emissions and secondary interactions. Ozone is produced through a series of complicated interactions between HC and NOx in the presence of sunlight. The dominant component of the fine particle mass is formed from secondary reactions and condensation processes. Secondary pollutants, such as sulfate aerosols and nitrate aerosols are also important in the formation of regional haze. The following section discusses the sources of these emissions and their contribution to the formation of ozone, fine particulate matter, and regional haze.

Ozone Emission Inventory

Ozone emission inventory methodology can be summarized by its major components: point, area, mobile and biogenic sources. Point sources are large, individually inventoried emission sources. Point source emissions were estimated using two methods. Emissions from fossil fuel steam electric utility units were estimated by applying an emission factor (pounds of pollutant emitted per ton of fuel consumed) to the fuel consumed by each utility unit in the United States based on Department of Energy utility fuel consumption data. Other point sources were estimated from the 1985 National Acid Precipitation Assessment Program (NAPAP) emission inventory, which used bottom up methodologies to estimate emissions. These point source emissions were projected from 1985 to 1990 using the Bureau of Economic Analysis (BEA) growth factors for each industrial segment.

Area sources are emission sources that are too small or too numerous to be individually inventoried. As with point sources, some of these emissions were projected from the 1985 NAPAP inventory using BEA growth factors for each industrial segment. Two segments of area source emissions were estimated using different methods. Solvent VOC emissions were estimated from the total quantity of solvents consumed in the United States by assuming that all solvents were eventually emitted to the atmosphere. These emissions were apportioned to each county based on county employment for specific industry groups using County Business Patterns. Area source emissions from residential wood combustion were estimated using EPA's wood burning model. This model estimates consumption for all counties in the United States based on survey data from the northeast and several western states.

Emissions from on-road vehicles were estimated by applying emissions factors to an activity level. This activity level is the total mileage driven or vehicle miles traveled (VMT) (one VMT = one mile driven by one vehicle). The annual VMT data for each State were obtained from the Federal Highway Administration's Highway Performance Monitoring System data base. State VMT totals were apportioned to the counties based on population. County CO, VOC and NOx mobile source emission factors were obtained from EPA's MOBILE5a model from the Office of Mobile Sources (OMS). This model includes both evaporative and exhaust emissions. Non-road mobile sources include commercial marine vessels, lawn and garden equipment, and construction equipment. Emissions from these non-road mobile sources were estimated from a detailed 1990 inventory prepared for 27 nonattainment areas by the OMS. These inventories were combined, classified by equipment and engine type, and distributed to the appropriate non-road source categories to create national county-level emissions. Aircraft and railroad non-road mobile source emissions were projected from the 1985 NAPAP inventory.

Natural sources also contribute significant amounts of VOC and some NOx to the emission inventory. Various plant species and the soil produce these emissions, estimated using EPA's Biogenic Emissions Inventory System - Version 2 (BEIS2). County land use and meteorological data were input to this model, producing estimated biogenic emissions by county. This inventory is being used by the EPA in national computer modeling simulations to analyze various options for a revised ozone National Ambient Air Quality Standard (NAAQS). States prepared bottom up inventories, required by the Clean Air Act (CAA), for all ozone nonattainment areas for the year 1990. These State Implementation Plan (SIP) inventories have two applications. First, because the CAA requires specific VOC emission reductions, the SIP inventories and follow-up periodic inventories are used to determine if the required VOC reductions were achieved. Second, the States also used the SIP inventories for some nonattainment areas as input to photochemical grid models to predict future-year ground-level ozone concentrations and to evaluate various control strategies needed to attain the ozone NAAQS.

Emission estimation methodologies varied among the 33 states and the District of Columbia. The most significant differences occurred in point source emission calculations. EPA guidance required all VOC sources emitting 10 tons/year and NOx and CO sources emitting 100 tons/year to be inventoried individually as point sources. These point sources were located either in the nonattainment area or in a surrounding 25 mile wide buffer zone. Emission estimation methods ranged from CEMS (Continuous Emissions Monitoring System) emissions data to the application of emission factors and activity levels determined from survey responses. Although EPA required states to use the EPA MOBILE model to estimate on-road mobile source emissions, application of this model significantly varied, because portions of the MOBILE model permit the user to either accept built-in default values or to provide location-specific values. Some states conducted extensive studies to develop specific inputs while others relied primarily on the default values. Non-road mobile and biogenic emissions were primarily estimated using a top down methodology. Area sources were estimated by area source category for nonattainment areas. Estimation methodologies varied, but often were based on a per capita or per employee emission factor. The activity level ( e.g., population or number of employees) could be locally determined or obtained from national census data. SIP inventories were submitted to the EPA Regional Offices (RO), which used a detailed checklist to review each inventory using a set of minimum requirements. The States submitted 97 ozone nonattainment inventories, including 357 of the total 3141 United States counties.

In the eastern United States, transport from one nonattainment area to other areas has been recognized as a significant problem. The strategy of treating each nonattainment area independently, therefore, has been unsuccessful. The Ozone Transport Assessment Group (OTAG), consisting of 38 eastern states, was formed to address this problem. OTAG's approach is to construct a regional emission inventory, input it to a photochemical grid model, and develop a region-wide attainment strategy. The emission inventory, based on 1990 emissions, will be a blend of EPA's national top down inventory and SIP inventories for VOC, NOx and CO. Because SIP data are presumed to be more accurate than EPA's top down results, the SIP data will be used where available. For those portions of the 38 states not included in the SIP inventories, EPA's top down inventory will be used.

Particulate Matter and Regional Haze Inventories

Emission inventory requirements are the same for both particulate matter (PM) and regional haze (RH). The atmospheric constituents that are important to the formation of both PM and RH are: VOC, NOx, SO2, NH3, PM-10 and PM-2.5. Much of the top down inventory methodology for VOC and NOx applies to SO2, PM-10 and PM-2.5. The main differences are for on-road mobile sources, fugitive dust sources and ammonia sources. Emissions of PM-10, PM-2.5 and SO2 from on-road mobile sources were estimated using EPA/OMS' PART5 model. PART5 estimated emission factors from vehicle exhaust, brake wear, tire wear and re-entrained road dust from paved and unpaved roads. These emission factors were then used with VMT to estimate on-road mobile source emissions. Unique PM source categories included in this inventory are wind erosion, agricultural tilling, and construction activities. Wind erosion emissions from agricultural land were estimated using Department of Agriculture crop land data, meteorological data, and a wind erosion model. Agricultural tillage emissions were estimated based on the area planted, crop type, soil parameters, and the number of tillings per year. Construction emissions were estimated based on an emission factor, the area of land under construction (determined from the cost of construction), and the average duration of construction activity. Ammonia emissions are dominated by animal husbandry activities. Emission factors here were applied to animal populations from the Department of Agriculture's agriculture census.

Bottom up SIP inventories, also required by the CAA, were prepared for PM-10 nonattainment areas. One difference between PM-10 and ozone SIP inventories is the relative size of the nonattainment areas. PM-10 nonattainment areas are generally much smaller than the ozone areas. Also, some PM-10 areas are dominated by a single source category (e.g., residential wood combustion or fugitive dust). Therefore, the extent and scope of these inventories are more limited than the ozone inventories. Because PM-10 inventories were seldom input to computer modeling simulations, no additional quality assurance work associated with modeling occurred. Therefore, EPA regards PM-10 SIP inventories to be less rigorous than ozone SIP inventories. The States have submitted a total of 70 PM-10 nonattainment inventories.

The Grand Canyon Visibility Transport Commission developed an emission inventory for the 11 Western State Region to evaluate regional haze. This emission inventory combines both top down and bottom up methodologies. This inventory is based on EPA's top down ozone inventory. PM and visibility pollutants have been added using a top down methodology. Certain source categories (e.g., prescribed burning) received special emphasis in the preparation of the inventory. Each of the 11 states were participants in the inventory and had to agree with the inventory values for their state. California and Oregon supplied complete data sets so that none of the original top down values were used. Most of the other states supplied primarily point source data.

Emission Trends Appendix

Summaries of emission trends extracted from the recent EPA Emission Trends report (EPA, 1995) are provided in Figures ES-1, ES-2 and Table ES-1.

Table ES-1. Summary of National Emissions

(thousand short tons, 1.1 million short tons equals 1 million metric tons)

Year Carbon Monoxide Nitrogen Oxides Volatile Organic Compounds Sulfur Dioxide Particulate Matter (PM-10) w/o fugitive dust Fugitive Dust (PM-10)* Lead

(short tons) 1900** NA*** 2,611 8,503 9,988 NA NA NA 1905** NA 3,314 8,850 13,959 NA NA NA 1910** NA 4,102 9,117 17,275 NA NA NA 1915** NA 4,672 9,769 20,290 NA NA NA 1920** NA 5,159 10,004 21,144 NA NA NA 1925** NA 7,302 14,257 23,264 NA NA NA 1930** NA 8,018 19,451 21,106 NA NA NA 1935** NA 6,639 17,208 16,978 NA NA NA 1940 93,615 7,374 17,161 19,953 15,956 NA NA 1945**** 98,112 9,332 18,140 26,373 16,545 NA NA 1950 102,609 10,093 20,936 22,358 17,133 NA NA 1955**** 106,177 11,667 23,249 21,453 16,346 NA NA 1960 109,745 14,140 24,459 22,227 15,558 NA NA 1965**** 118,912 17,424 30,247 26,380 14,198 NA NA 1970***** 128,079 20,625 30,646 31,161 13,044 NA 219,471 1975 115,110 21,889 25,677 28,011 7,617 NA 158,541 1980 115,625 23,281 25,893 25,905 7,050 NA 74,956 1984 114,262 23,172 25,572 23,470 6,220 NA 42,217 1985****** 114,690 22,860 25,798 23,230 4,094 40,889 20,124 1986 109,199 22,348 24,991 22,442 3,890 46,582 7,296 1987 108,012 22,403 24,778 22,204 3,931 38,041 6,857 1988 115,849 23,618 25,719 22,647 4,750 55,851 6,513 1989 103,144 23,222 23,935 22,785 3,927 48,650 6,034 1990******* 100,650 23,038 23,599 22,433 3,882 39,451 5,666 1991******* 97,376 22,672 22,877 22,068 3,594 45,310 5,279 1992******* 94,043 22,847 22,420 21,836 3,485 40,233 4,899 1993******* 94,133 23,276 22,575 21,517 3,409 39,139 4,938 1994******* 98,017 23,615 23,174 21,118 3,705 41,726 4,956 Note(s):	*	Fugitive dust emissions not estimated prior to 1985. They include miscellaneous-agriculture & forestry, miscellaneous- fugitive dust, and natural sources-wind erosion.

**	NAPAP historical emissions3,4

***	NA denotes not available.

****	Combination of revised transportation values and NAPAP historical emissions.

*****	There is a change in methodology for determining on-road vehicle and non-road sources emissions (see chapter 6).

******	There is a change in methodology in all sources except on-road vehicles and non-road sources and all pollutants except lead, as reflected by the dotted line.

*******	1990 through 1994 estimates are preliminary. The emissions can be converted to metric tons by multiplying the values by 0.9072.

3.B	Atmospheric Chemistry

Complex chemical reactions dominate the formation of ozone, fine particulate matter, and regional haze. The following section discusses the formation of ozone near the earth's surface, primary particles as well as secondary fine particulate matter and visibility impairing compounds. In addition, a brief discussion of the fundamentals of visibility is presented.

Ozone

Ozone is both a natural component of the earth's atmosphere and an air pollutant. Ozone is formed in the stratosphere from molecular oxygen in air and intense ultraviolet solar radiation. Its formation consumes almost all radiation below 300 nm and thus prevents harmful radiation from reaching the earth's surface. Ozone is slowly transported from the stratosphere to the earth's surface. Thus, a natural background of O3 exists near the earth's surface that has been estimated to be near 20 ppb due solely to stratospheric sources. Current background is about 40 ppb and the increase has been attributed to global pollution including fossil fuel use and agriculturally related biomass burning. It may be increasing slowly. Continental O3 backgrounds (i.e., Eastern US and Western Europe) may be much higher due to region-wide, slow chemical production processes (similar to those in urban areas and dependent upon widely dispersed NOx) as well as due to direct transport of urban produced O3.

The general explanation for the formation of ozone in or near urban areas is that combustion sources that use air as an oxidizer will produce oxides of nitrogen (NO, NO2) when temperatures are above 2500°K. In addition, incomplete combustion results in the emission of raw components and oxygenated organic components from the fuels. In sunlight these are sources of free radicals (e.g., .OH, HO2., RO., RO2.) that oxidize "emitted" volatile organic compounds to carbonyls, CO, and CO2, while simultaneously oxidizing NO to NO2 and recreating the free radical. Each free radical is cycled up to five times. The NO2 reacts with sunlight to recreate NO and to produce O3. After the first oxidation of NO to NO2, every subsequent operation of the cycle produces an O3 with an efficiency of greater than 90%. In current chemical reaction mechanisms, a typical nitrogen is cycled 3 to 5 times. Some of the O3 produced reacts with organics and with sunlight to produce more free radicals to maintain the cyclic oxidation process. This represents a powerful positive feedback process on the formation of more O3, given available NOx.

The carbonyls produced in the organic oxidation also react with sunlight to produce more free radicals. As the cycle operates, NO2 is converted into inorganic and organic nitrates and this form of nitrogen can not cycle. This also removes free radicals. A system that converts all nitrogen oxides to such products cannot create any more O3. NO2 reacts rapidly with free radicals and in situations that have a limited supply of radicals, NO2 can compete with the VOCs for the limited free radicals. This results in a virtually no production of O3. Thus, different mixtures of VOCs and NOx can result in different ozone levels such that the total system is non-linear. That is, large amounts of VOC and small amounts of NOx make O3 rapidly but are quickly limited by removal of the NOx. VOC reductions, under these circumstances, show little effect on O3. Large amounts of NO and small amounts of VOC (which usually implies smaller radical source strengths) result in the formation of inorganic nitrates, but little O3. In these cases, reduction of NOx results in an increase in O3. Some combination of VOC and NOx is optimum at producing O3.

The proceeding is a static description. In reality, physical processes compete with chemical processes and change the outcomes in complex ways. The existence of feed back and non-linearity in the transformation system confounds the description. Competing processes determine the ambient concentration and there are an infinite set of process magnitudes that can give rise to the same ambient concentrations and changes in concentrations. Lack of any direct measurement of process magnitudes results in the need to use inferential methods to confirm any one explanation of a particular O3 concentration.

Primary Particles

Unlike gaseous criteria pollutants (SO2, NO2, CO, O3), which are well defined chemical entities, atmospheric particles comprise a complex mixture of chemical constituents. Because of this fact, sources of each constituent of the atmospheric aerosol must be considered in turn. Since particulate matter (PM) is composed of both primary and secondary constituents, emissions of both the primary components and the gaseous precursors must be considered.

Tables 3.1 and 3.2 summarize anthropogenic and natural sources for the major primary and secondary aerosol constituents of fine and coarse particles. Anthropogenic sources can be further divided into stationary and mobile sources. Stationary sources include fuel combustion for electrical utilities and industrial processes; construction and demolition; metals, minerals, petrochemicals and wood products processing; mills and elevators used in agriculture; erosion from tilled lands; waste disposal and recycling; and, fugitive dust from paved and unpaved roads. Mobile (e.g. transportation-related) sources include direct emissions of primary PM and secondary PM precursors from highway and off-highway vehicles and non-road sources. Also shown are sources for precursor gases whose oxidation forms secondary particulate matter. The latter topic will be discussed in greater detail in the following section. In general, the nature of sources of particulate matter shown in Table 3.1 is very different from that for particulate matter shown in Table 3.2. A large fraction of the mass in the fine size fraction is derived from material that has been volatilized in combustion chambers and then recondensed to form primary fine PM, or has been formed in the atmosphere from precursor gases as secondary PM. Since precursor gases and fine particulate matter are capable of traveling great distances, it is difficult to identify individual sources of the constituents shown in Table 3.1. The PM constituents shown in Table 3.2 have shorter residence times in the atmosphere, so their impacts tend to be more localized. Only major sources for each constituent are listed in these tables.

Natural sources of primary PM include windblown dust from undisturbed land, sea spray and plant and insect debris. The oxidation of a fraction of terpenes emitted by vegetation and

Table 3.1.	Constituents of Atmospheric Fine Particles (< 2.5 m) and Their Major Sources

Sources Primary Secondary Aerosol

Species Natural Anthropogenic Natural Anthropogenic SO4= Sea Spray Fossil Fuel

Combustion Oxidation of Reduced Sulfur Gases Emitted by the Oceans and Wetlands; and, SO2 and H2S Emitted by Volcanism and Forest Fires Oxidation of SO2 Emitted from Fossil Fuel Combustion NO3- N/A Motor Vehicle

Exhaust Oxidation of NOx Produced by Soils, Forest Fires, and Lightning Oxidation of NOx Emitted from Fossil fuel Combustion; and Motor Vehicle Exhaust Minerals Erosion,

Re-entrainment Fugitive Dust; Paved, Unpaved Roads; Agriculture and Forestry N/A N/A NH4+ N/A Motor Vehicle

Exhaust Emissions of NH3 from Wild Animals and Undisturbed Soils Emissions of NH3 from Animal Husbandry; Sewage; and, Fertilized Land Organic

Carbon (OC) Wild Fires Open Burning; Wood Burning; Cooking; Motor Vehicle Exhaust; and, Tire Wear Oxidation of Hydrocarbons Emitted by Vegetation (e.g. terpenes, waxes); and, Wild Fires Oxidation of Hydrocarbons Emitted by Motor Vehicles; Open Burning; and, Wood Burning Elemental

Carbon Wild Fires Motor Vehicle

Exhaust; Wood Burning; and, Cooking N/A N/A Metals Volcanic

Activity Fossil Fuel Combustion; Smelting; and Brake Wear N/A N/A Bioaerosols Viruses,

Bacteria N/A N/A N/A

Table 3.2.	Constituents of Atmospheric Coarse Particles (> 2.5 m) and Their Major Sources

Sources Primary Secondary Aerosol

Species Natural Anthropogenic Natural Anthropogenic Minerals Erosion,

Re-entrainment Fugitive Dust; Paved, Unpaved Roads; Agriculture and Forestry N/A N/A Miscellaneous Ions Sea Spray Road Salting N/A N/A Organic

Carbon (OC) N/A Tire Wear and Asphalt Wear N/A N/A Metals Erosion; Re- entrainment; and, Organic Debris N/A N/A N/A Organic

Debris Plant and Insect Fragments N/A N/A N/A Bioaerosols Pollen; Fungal Spores; Bacterial Agglomerates N/A N/A N/A

reduced sulfur species from anaerobic environments leads to secondary PM formation. Ammonium (NH4+) ions which are crucial for regulating the pH of particles are derived from emissions of ammonia (NH3) gas. Source categories for NH3 have been divided into emissions from undisturbed soils (natural) and emissions which are related to human activities (e.g. fertilized lands, domestic and farm animal waste). Clearly, fuel wood burning is an anthropogenic source of PM, whereas wildfires would be a natural source. Forest fires have been included as a natural source, because of the lack of information on the amount of prescribed burning or accidental fires caused by humans.

Secondary Fine PM

The generation, transformation, transport and removal of particles involves a complex interaction of numerous mechanical, biological, physical and chemical processes. Although large particles (diameters > 2.5 ) share some attributes with fine particles, there are several fundamental differences between fine and large particles that allow for separate treatments. Perhaps the most important difference between fine and large particles is that the major fraction of large particle mass is generated from primary emissions of materials, whereas the dominant component of fine particle-mass is formed via secondary reactions and condensation processes. While this simple picture appears to reflect Eastern U.S. sulfate laden aerosols, both the complexity of aerosols and the scarcity of particle mass, speciated data are causes for concern regarding our ability to formulate basic process understandings of aerosols.

Organic Aerosol Formation

The understanding of secondary-formed organic aerosols is less clear relative to inorganic aerosols. The principal available secondary organic aerosol model (Pandis et al., 1992) assumes that organic compounds with relatively low vapor pressures (semi-volatile organic compounds, SVOC) condense or nucleate when their concentrations exceed saturation concentrations. This gas-particle partitioning is typically described by equilibrium relationships of a solute between aqueous and vapor phases (e.g., Henry's law). Secondary organic aerosols are derived mostly from aromatics (toluene, xylene, benzene compounds) and various C7 and greater olefins, especially the biogenic terpenes (e.g., Smoky mountain haze). These volatile organics initially react with hydroxyl radicals, and subsequently form lower vapor pressure condensible products. Estimates of aerosol yields of individual VOC species (emissions) have been developed (Grosjean and Seinfeld, 1989) and are used to parameterize organic aerosol formation in models. Primary organic aerosols generally dominate the total organic aerosols fraction, although summertime secondary organic formation could be significant. Measurement analyses typically provide estimates of total organics, and therefore do not distinguish primary and secondary organic aerosol components.

Sulfate Formation and Sulfate-Nitrate-Ammonia System

Sulfur dioxide, emitted from many of the same combustion related sources releasing NOx, is oxidized by OH to form sulfuric acid (H2SO4), assuming an ammonia free environment. Intermediate steps within this oxidation generate hydroperoxy radicals, thus continuing the cycling of radicals (i.e., this is not a termination step). In addition to the gas phase oxidation of sulfur dioxide, significant oxidation of SOx occurs in clouds, fogs and available water droplets. Sulfur dioxide is soluble in water and exists in equilibrium with various reduced-form sulfur species, including bisulfite (HSO3+) which is oxidized to sulfate in the presence of dissolved hydrogen peroxide (H2O2) or ozone (in typical acid environments, oxidation through H2O2 is favored) or oxygen, catalyzed by dissolved iron and manganese. On an annual basis, gas and aqueous oxidation pathways produce comparable sulfate yields; gas phase oxidation is enhanced during summer months.

An equilibrium system exists among several gas- and liquid-phase nitrogen and sulfur species where ammonia plays a potentially important role in formation of ammonium nitrate particles. Gaseous ammonia is highly soluble and can act as a neutralizing agent creating a spectrum of sulfate based ions/compounds ranging in acidity from sulfuric acid to neutralized ammonium nitrate. Ammonia preferentially reacts with sulfates and stoichiometrically requires 2 equivalents, versus 1 for nitric acid, for neutralization. Excess ammonia would be available to establish equilibria among ammonia and nitric acid gases and ammonium nitrate (solid/liquid phases). Ammonium nitrate is relatively unstable, and reforms nitric acid and ammonia gases at elevated temperatures (e.g. > 10°C). Because of the abundance of sulfate and temperature effects, the Eastern U.S. is believed to be relatively nitrate free in the summer. Ammonium nitrate typically is a winter time issue; it can be the dominant winter time aerosol component in southern California. Unfortunately, this instability creates substantial measurement problems. For reasons of consistency, filter samples often are equilibrated at room temperatures and humidity before weighing, resulting in loss of excess water (a dominant component) and semi-volatile compounds (e.g., organic aerosols, ammonium nitrate). Additional losses occur during sample transport and storage.

The dominance of sulfate aerosols in the Eastern U.S. suggests a limiting effect on ammonium nitrate formation due to ammonia, although nitrate hot spots probably exist in areas not monitored with local high ammonia and NOx emissions. NOx emissions could lead to increasing nitrate aerosol levels if greater relative reductions of SOx emissions (to NOx) occur in future years. Southern California aerosols are dominated by nitrates and organics (relative to sulfates), consistent with larger NOx/SOx inventories and available ammonia sources.

Fine particle growth often is affected by available water. Sulfate and nitrate-based particles are strongly hygroscopic and much of the particle bound mass, especially at high (e.g., > 95%) relative humidities, consists of water. The ammonium-nitrate-sulfate system is deliquescent. Deliquescent particles undergo a rapid growth phase after transitioning from solid to liquid phases at high relative humidities. The combination of high sulfate levels and relative humidity collectively act to impair visibility more in the Eastern than the Western U.S. (i.e. water accretion shifts particle size distributions toward more effective light scattering regimes). Fine particles often are highly water soluble, providing an eventual pathway for removal through precipitation. In addition, many atmospheric gases (e.g., nitric acid and hydrogen peroxide) which have important roles in particle formation are extremely soluble and play significant roles in aqueous phase oxidation reactions that generate particles.

Regional Haze/Visibility

Light scattering and absorption by particles and gases contribute to visibility extinction. Particles are the major cause of visual impairment in polluted atmospheres, with minor contributions due to light absorption by NO2. Particle scattering dominates (compared to absorption), especially in rural environments. However, light absorption by soot is extremely efficient and a significant component of the light extinction budget in urban environments and is usually associated with diesel fuel combustion. Absorption and scattering are functions of particle composition (refractive index), size distribution and concentration. Light extinction is strongly dependent on particle size distribution, with peak efficiencies, depending on species, in the .1 to 1 range (similar to the visible light spectrum, .4 - .7 ). Thus, characterization of particle size distributions by component (i.e., sulfate, nitrate, organic carbon, elemental carbon) provides the basis for developing light extinction budgets. Modeled visibility calculations typically are developed through post-processing of simulated particle fields. Sulfate, nitrate and organic carbon aerosols effectively scatter light and have similar scattering (mass basis) efficiencies, although organic aerosols appear to be somewhat more efficient on a mass unit basis. Water droplets are very effective light scatterers but, with the exception of fogs, most "pure" water is in the vapor phase. Hygroscopic aerosols (sulfates and ammonium nitrate), which accrete water, change both the aerosol size distribution and refractive index which affects scattering, particularly at relative humidities greater than 70%. As particles accrete water, their mean diameters shift upward, often into very efficient light scattering ranges. Thus, it is the impact of water on the total aerosol, creating "haze" conditions, rather than homogeneous water droplets acting alone. Relative humidity exhibits large temporal variations with typically stronger early morning (high relative humidity) impacts. However, secondary formation peaks in the afternoon, during a diurnal low in the relative humidity. Hence, it is more reasonable to view humidity effects on an average basis (i.e., high humidity days impart greater visual impairment).

Fundamental understanding of visibility extinction is based on Rayleigh and Mie scattering theories, both of which involve relationships between particle (and gas) dimensions and incident wavelengths. Rayleigh scattering occurs in all directions (forward and backward) and is limited to atmospheric gases (mostly N2) and very small particles, with diameters considerably less than the spectral range of visible light (~ .4 - .7 ). Clean, background conditions are referred to as "Rayleigh" atmospheres with visibility up to 300 km (i.e., some natural extinction exists). Blue skies result from dominant Rayleigh scattering of wavelengths in the blue region (~ .5 ) by atmospheric N2, which comprises nearly 80% of all atmospheric gases. Very small particles (<

.1 ) preferentially scatter blue light, leaving larger wavelength red hues relatively unaffected from a line-of-sight perspective during clean condition (very small particles) sunset/rise periods.

Mie scattering (forward and sidewards) by particles with diameters similar to the visible light spectrum is the dominant visibility impairment mechanism. Thus, most efficient scattering is associated with particles on the order of .1 .

3.C	Meteorology

Meteorological processes are among the strongest forcing functions affecting air quality at all atmospheric temporal and spatial scales, including urban and regional ozone, fine particles, and visibility impairment. The strength of the various meteorological processes can change significantly from day to day, imposing a confounding signal on the effects of emissions changes on air quality. It is important to understand these processes as they strongly affect the chemical reactions and rates that can lead to a deterioration in air quality. They are also primarily responsible for the transport and dilution of directly emitted pollutants. Ultimately, to ascribe improvements in air quality to emissions reductions over the long term, the meteorological signal must be removed from the air quality trend. This requires an understanding of how the meteorological processes affect that signal.

Historically only local scales of meteorological processes were considered to be important, as air quality deterioration was thought to be a local problem. In fact this philosophy governed the incentive to build taller stacks and chimneys on industrial and power plants in the 1960's and 1970's, to emit air pollutants at higher levels in faster moving air streams and remove the material more quickly from the local area. Later studies showed that pollutant lifetimes were sufficiently long that long-range transport could carry pollution hundreds of miles from the source area and still have an impact on ground level air quality. The "tall stack movement" was seen to have improved local air quality but at the expense of regional air pollutant transport and air quality. Today we better understand that meteorological processes on the larger scale help organize the smaller scale processes, and that all scales are important to overall air quality issues.

The largest scale of atmospheric motion is the global scale. Exchange between the northern and southern hemispheres is on seasonal or longer timescales. The transport of material around the northern hemisphere was first studied quantitatively using a tracer of opportunity, fallout from nuclear tests. This timescale ranges from weeks to months depending on the latitude and season. The most important scale for regional transport, say over North America, is the scale of the typical high and low pressure system, which is seen on daily weather maps. The spatial extent of these systems is on the order of 1000 km or larger. This is known by meteorologists as the synoptic scale. Smaller scale weather systems are classified under the general term, mesoscale, which can cover systems from a few kilometers in horizontal extent to a few hundred kilometers. The most dramatic examples of mesoscale systems range from the size of an individual tornado to the much larger hurricane. Finally, the smallest scales of relevance here are the scales associated with local features such as small obstacles (hills, buildings, etc.), which cause perturbations in the local flow regime. All of these systems are involved in the transport of pollutants. Physical features associated with these systems can often influence the chemical transformations of species in the atmosphere.

Boundary Layer Processes

The lowest 1-2 km of the troposphere is known as the planetary boundary layer (PBL), where much of the air pollutant burden resides. This layer is also called the mixing layer or mixed layer. In this layer both horizontal and vertical transport of pollutants occurs. The height to which pollutants are actively mixed has a strong diurnal pattern superimposed upon the changes due to the passages of synoptic scale or mesoscale weather systems. Under cloud-free conditions, the ground is heated by sunlight during the day causing bubbles of warm air (thermals) to rise. The result of the rising buoyant thermals is a turbulent flow which mixes pollutants upward from surface-based sources or downward from elevated sources. Another form of turbulent mixing occurs because of wind shear, the rate at which wind speed and direction change with height. This change of wind with height is caused by frictional drag of the earth's surface on the wind. Under conditions of weak solar heating and strong winds this latter form of turbulence dominates. With weak winds and strong heating the former form of turbulence dominates. On any given day, a mixture of the two forms of turbulence exists. At night, again under clear skies, the ground cools and a temperature inversion (temperature increases with height) develops. Shear driven turbulence is then opposed by negatively buoyant air resulting in a shallower mixing layer with much weaker mixing. This type of layer is referred to as a surface-based stable layer.

Most sources of pollutants emit within the mixed layer. Their ground level concentrations are inversely proportional to this height. For secondary species such as ozone and fine particles, their precursor chemicals are directly affected by the mixing height. Diurnal variations in thermal heating and turbulence lead to diurnal variations in the mixing height. It can vary considerably over space and time. "Typical" values of the mixing height range from 50-100 m at night to 1200-2000 m in the afternoon. This diurnal cycling effects a higher concentration of air pollutants emitted near the ground at night and a dilution during the day. However, the rising mixing height also entrains air pollutants from aloft that may have been emitted there by taller stacks or carried there by long range transport. The concentrations of secondary pollutant species typically increase during the day in the deeper mixed layer, as photochemical reactions occur. In areas where the mixing height does not grow very deep during the day, such as when upper level inversions restrict the growth, the potential for much deteriorated air quality is high (e.g. this is the classic Los Angeles smog setup). As the sun sets and the thermal energy source for the ground is cut off, convective motions decrease and a more stable temperature structure develops near the ground. The air near the ground becomes "decoupled" from the air several hundred meters above the ground after sunset. Often a jet of higher speed winds forms above the surface-based stable layer. This jet can carry air pollutants long distances downwind during the

overnight period, allowing entrainment at a different location the next day (Stull, 1988 and Garratt, 1994).

Synoptic Scale Processes

Smaller scale atmospheric processes act within the context of the larger, or synoptic scale (1000 km or greater). When the atmosphere is dynamically active on the synoptic scale, with the frequent passage of frontal zones and air mass exchanges, dilution is strong and air quality is usually good. When air masses slow down, or stagnate, as sometimes happens during the warm season, synoptic scale subsidence (sinking of the air) can occur, increasing the potential for air pollution episodes by decreasing the mixing height. The "Bermuda high" situation in eastern North America refers to the semi-permanent ridge of high pressure just east of the North American continent in the warm season. When this ridge grows stronger, as it occasionally does, it moves onto eastern North America, bringing warmer temperatures, sinking air motions, and southwesterly winds, all inducements for greater air pollution potential in the region. The southwesterly winds also align the major urban source areas of the East Coast increasing the transport aspect of air pollution.

Other synoptic scale processes are also relevant. The barrier between the troposphere and the stratosphere is known as the tropopause. The strong temperature inversion here usually prevents much air exchange across the tropopause, except along the path of the jet stream where tropopause folding generally occurs. In this region, intrusions of ozone-rich stratospheric air into the troposphere take place. Also, seasonal and latitudinal variations in the stratospheric ozone column can affect the amount of ultraviolet (UV) radiation received near the earth's surface. The less ozone in the column the more radiation available to drive photochemical reactions. This effect, however, is marginal compared to the impact of clouds on UV radiation.

Complex Regimes

There are a number of meteorological regimes where the interaction of boundary layer and larger scale processes are quite complex because of the local environment. One of these regimes is the land/water interface near coastal regions or the shores of very large lakes. In these areas a mesoscale (100-500 km) circulation can develop, driven by the relatively large air/water temperature contrasts. In the warm season the daytime air temperature over the land is greater than the water, allowing for water-to-land winds and rising air over the land. This circulation may enhance or retard the synoptic scale flow features. Occasionally it will locally overcome the synoptic influences. The internal mesoscale boundary layer may have restricted mixing, vertical wind shears, and temperature, wind, and moisture discontinuities with the synoptic scale regime above it. At night, the effect lessens and the land temperature becomes cooler than the water, allowing for weak land-to-water transport, and causing local recirculation to occur. Some of the major urban areas of the United States with air pollution problems are located in areas affected by these type circulations, including Los Angeles, Houston-Galveston, Chicago, and New York.

These mesoscale processes must be diagnosed carefully when sources areas are embedded in land-water circulations, since the effects on transport and dilution can be quite complex.

Another complex regime exists in areas of complex, varying terrain: mountains and valleys. Here, the local thermal and mechanical forcing of winds can change the flow patterns from the overall synoptic scale. Mountain/valley circulations can preferentially channel air along certain trajectories and through certain passes. Diurnal variations can evolve from upslope flows during the daytime and downslope at night, forced from thermal heating and temperature contrasts. The scale of these motions can be quite small or rather large, depending upon the extent of the terrain variations. Again, sources of pollution caught up in these circulations can be greatly affected. Urban areas such as Los Angeles, Denver, and Phoenix or rural areas where power plants or industrial facilities are located can set up situations for this meteorological and emissions interaction. Also, downwind from the Appalachian mountains in the northeastern U.S., mechanically-driven perturbations often develop within a given synoptic regime, critically influencing the low-level wind fields in determining the magnitude and location of maximum ozone concentrations.

Clouds

Clouds encompass several meteorological processes that are relevant to air quality. First, clouds are accompanied by rising air motions that contribute to their sustenance. These vertical motions add to the dilution of the PBL, and indeed, when clouds are convectively active and grow in vertical structure themselves they are effective at "venting" the boundary layer, and can introduce polluted air to levels above the PBL where they may remain for extended time periods and can be transported by upper level winds. Clouds affect the solar radiation available to drive meteorological, biological, and chemical processes relevant to local and regional air quality. Direct impacts of reduced solar radiation are lower photolysis rates to drive photochemical reactions (UV-spectrum) and lower emissions of isoprene, a naturally-occurring reactive organic compound and ozone precursor (visible-spectrum). Reduced solar radiation impinging on the ground will also affect the energy balance, resulting in changes in temperature, moisture, and winds. In some cases of spotty summertime cumulus clouds, reflections between clouds may lead to increased UV radiation available for photochemical reactions. The other major effect from clouds is the moisture source for in-cloud aqueous chemical reactions, such as the transformation of sulfur dioxide gas to sulfate aerosol particles. Pollutant removal occurs by washout of material within the cloud, and rainout of material below the cloud during precipitation events. Rain and its associated downward vertical air motions can be a very effective cleansing phenomenon for the PBL. Unfortunately, this cleansing of the air also deposits the pollutants to the ground as in acid deposition.

Interactions of Meteorology and Other Processes

Temperature directly affects some source emissions processes, including evaporative organic emissions from automobiles and other mobile sources, solvent evaporation, and increased natural emissions of isoprene and monoterpenes, which contribute to the ozone and fine particle problems. Temperature can indirectly affect emissions also by leading to increased use of automobiles, and residential and commercial air conditioning on hot days. Temperature and moisture are key factors in many chemical rate constants that determine the speed of photochemical reactions. Solar radiation, as discussed above, strongly influences emissions and chemical processes.

Meteorological processes also act as the context for all other atmospheric processes that may lead to air quality deterioration. That is to say, it is the forces of motion and energy in the atmosphere that cause molecules to come together to participate in atmospheric chemistry or cause sufficient dilution to minimize the impact on air quality. Meteorology also determines the course and lifetimes of pollutants emitted from many major sources into the atmosphere. Understanding the fate of air pollution is not possible without a thorough understanding of meteorological processes.

Data and Analyses

The U.S. National Weather Service operates a monitoring network of surface and upper air meteorological observations. Hourly surface measurements are obtained at several hundred sites, and twice daily upper air "soundings" are taken at a subset of these sites. An increasing array of radar wind profilers are supplementing the balloon-borne sounding information. Data from weather radars and satellites provide additional information and aid in interpreting the surface and upper air data. This data network is necessary for characterizing the synoptic scale meteorological state of the atmosphere at any given time. There are still gaps in the network (typical distances between upper air sites may be 400-600 km) such that accurate determination of hourly resolved upper-air flow fields and the spatial variability of the mixed layer is not completely possible. Special field studies that support increased meteorological measurements aloft help alleviate these data gaps. Another problem area relevant to air quality is the stagnation case, where wind speeds become light and variable through a great depth of the troposphere. In this case, the synoptic observing network may not indicate enough of a coherent flow to make trajectory calculations meaningful. Here, a stochastic approach to meteorological analysis may be helpful.

For the complex meteorological regimes discussed above there are rarely sufficient data to fully characterize the atmospheric state. In these cases meteorologists rely more heavily on complex numerical models that solve the basic equations of motion (e.g. hydrostatic and non- hydrostatic) to obtain a reasonable atmospheric state. Models however tend to accumulate small errors that become significant at timescales beyond 48-hours into a model simulation. The use of

data assimilation (e.g. Four Dimensional Data Assimilation, FDDA), in which observations are dynamically ingested by the model during simulation, helps correct these errors.

Concluding Remarks

The main thrust of this discussion was to point out the importance of meteorology in determining air quality. Meteorology determines how primary precursor emissions are mixed and transported as well as sets the conditions for chemical reactions to occur. The rate of conversion of precursor species into ozone and fine particles and the further distribution of these two pollutants is primarily set by meteorological conditions such as cloud cover and wind flow.

3.D	Deposition/Removal

Deposition processes, while reducing the concentration of airborne pollutants, cause the introduction of these materials into the terrestrial and aquatic ecosystems.	Deposition of pollutants (both particles and gases) during transport can occur through dry or wet processes. Dry processes include retention of airborne pollutants by the Earth's surface or uptake by vegetation, both of which are enhanced by gravitational settling and impaction due to turbulent mixing. Wet processes include washout of pollutants due to all forms of precipitation (e.g., rain, fog, and cloud impaction). All forms of deposition will deplete airborne concentrations of pollutants and increase the exposure of human and natural resources and materials to these pollutants.

Models must include a theoretical treatment of these deposition processes if they are to be well-based assessment tools. Most models approximate deposition by making use of a derived quantity, the deposition velocity, or, in the case of washout by rain, the washout coefficient. The deposition velocity is defined as the ratio of the deposition rate to the airborne concentration, and the washout coefficient is a function of rainfall rate and the terminal velocity of the airborne pollutant. Therefore these deposition parameters must be independently determined.

Correctly approximating the rate of deposition is a particularly difficult challenge for model developers. Limited studies are available from which one can parameterize the deposition process. Terrain effects and site-specific canopy uptake data (dependent on ambient conditions) are frequently not available. For wet deposition, knowledge of the spatial and temporal occurrence of precipitation events and clouds is required.

3.E	Linearity/Nonlinearity

The relationship between changes in sulfur dioxide emissions and the resulting change in ambient sulfate concentration and deposition is not necessarily one-to-one. NAPAP sponsored research indicates that the formation and deposition of secondarily formed sulfate particulates from primary sulfur dioxide emissions is governed by both linear and nonlinear processes. It appears that the clear air (gas phase) conversion of sulfur dioxide to sulfate is probably linear. However, aqueous phase conversion associated with clouds and precipitation scavenging can be nonlinear. The nonlinear cloud-based conversion is particularly important in the winter when concentrations of hydrogen peroxide (necessary for the formation of sulfate) are low, thereby inhibiting the sulfate formation. Similarly, sulfur dioxide saturation of cloud droplets in other seasons can diminish wet deposition of sulfur. NAPAP was less certain about the possible reduction of dry deposition if the surface had become acidic due to previous deposition.

The above processes affect the concentration of secondarily formed sulfate particles. There are also effects, related to the relative humidity and the composition (e.g., ammonium sulfate or sulfuric acid) and characteristics (externally or internally mixed) of the chemical species that make up the sulfate particles. These have significant implications for the interpretation and predictions of concomitant effects on visibility. Hygroscopic particles (e.g., sulfates) will grow in the presence of water vapor to particle sizes that cause greater extinction (visibility impairment) than their "dry" counterparts. The relationship between relative humidity and extinction is highly nonlinear at relative humidities greater than about 70%. Internally and externally mixed particles have different extinction efficiencies and will react differently to the removal of one of its constituents.

The net result of these processes and characteristics is that one can not assume that a given reduction in pollutant emissions will necessarily result in a like change (linearly related) in concentration, deposition, or visibility impairment.

3.F	Commonalities, Disconnects and Integrable Elements

EPA is pursuing a joint implementation strategy for a new ozone and fine particulate matter NAAQS and a regional haze rule. While there are connections among source categories and precursor emissions there are also significant differences in the processes responsible for the formation of fine particulates and ozone and subsequent differences in

ozone and fine particulate matter episodes. This section discusses the commonalities, differences, and the elements needed for a successful integrated implementation program.

Commonalities Among Ozone, Fine Particles, and Haze

Common "Direct" Precursors

NOx and certain VOC species can be oxidized to lead to formation of nitric acid (a precursor for particulate nitrate) and organic aerosols.

Interdependence on Radical Balances

The formation of ozone and secondarily formed particles is preceded by oxidation of their respective precursors. The oxidation is carried out through hydroxyl and peroxy radicals. Thus

any radical initiation, propagation or termination process potentially affects formation of ozone and fine particles.

Common "Indirect" Precursors

Because of the radical interdependence, potentially all precursors are common precursors. In other words, SO2 could be an ozone precursor; and the majority of VOC species, whose daughter products do not eventually form organic aerosols, could be fine particle precursors. This "universal" pool of precursors does not infer "common" response behavior. A reduction of a precursor that reduces a component of fine particulates, potentially can result in increased ozone or an increase in another fine particle component. Multiple nonlinearities and positive and negative feedbacks exist (analogous to NOx-ozone-VOC control). Thus, integrated implementation is far from a straightforward exercise, and the tools available for such assessment are yet to be evaluated.

Common Source Categories

Based on a commonality of precursors, a particular source emitting one precursor can affect ozone and fine particles, and a single source emitting multiple precursors can affect ozone and fine particles.

Co-episodicity

Assuming the above, co-episodes of high ozone and high fine particles can occur. Co-episodic events are a subset shared processes. That is, some of the processes which explain ozone or fine particle formation always operate, whereas concurrent "episodes" may occur rarely, sometimes or often. Most likely, far more single pollutant episodes exist than co-episodic (ozone and secondary PM) events. Examples of single-pollutant episodes include all of the non-summertime secondary PM events, often dominated by ammonium nitrate formation, which is favored by cool temperatures (< 6O°F) over gaseous ammonia and nitric acid. Of course, the "primary" particle episodes rarely (if ever) coincide with an ozone episode. Research exploring the frequency and characterization of co-episodic and uni-episodic events would yield further insight into underlying causes of events and provide direction for integrated implementation opportunities.

Differences Among Ozone, Fine Particles, and Haze

Process Level Differences

Those processes generating the primary component of fine particles can be thought of being emissions based, and often are separated from the secondary chemistry responsible for ozone formation. Emissions based processes include a host of mechanically driven agricultural dust,

fugitive and roadway dust events, as well the generation of carbon soot from wood and diesel fuel combustion.

Episodic Differences

Most of the emissions-based process differences lend themselves almost directly to episodic differences between ozone and particles. For example, the summertime wind-generated dust events in the southwest do not correlate well with high ozone events. This lack of coincidence is

expected given that the high-wind meteorological conditions required to generate significant dust loadings are not favorable for developing high ozone.

Many secondary-driven high particle events do not coincide with ozone. In many episodes where ammonium nitrate is a significant particle component, the meteorology will not favor ozone formation since ammonium nitrate is favored at relatively low temperatures and volatilizes to nitric acid and ammonia gases at higher temperatures. Thus, wintertime nitrate events do not coincide with ozone. Despite these episodic differences, one could argue that nitrate and ozone share common components - the basic radical chemistry cycles and NOx precursors. On balance, the frequency of co-episodic events is probably far less than single event episodes.

Integration

Programmatic

What does integration mean from an implementation perspective? The connections among source categories and precursor emissions (the independent variables in a control program) can be illustrated using an air quality modeling approach. The emission bases underlying most current ozone modeling efforts include nearly all of the source categories for aerosol formation, with certain notable exceptions in agricultural and natural emissions areas. The most typical example is utility boilers which emit both SOx and NOx. From a precursor perspective, NOx and certain VOC categories are obvious potential precursors for ozone and aerosols. But any oxidant precursor (including all VOC and CO) needs to be considered as potentially affecting aerosols, given the interplay between pollutants due to free radical chemistry cycles. In formulating control strategies, one might start with the ozone management strategies developed as part of ongoing regional analyses, and attempt to quantify the future impact on secondary aerosols due to combined CAA provisions for acid precipitation (Title 4) and ozone (Titles 1 and 2). The result of this exercise would produce the residual aerosol (and haze) related air quality benefits from an ozone precursor control oriented approach. This current exercise is planned with the RADM/RPM modeling complex. Although this example does not represent "full" integration given the unidirectional information flow (ozone to particles), it does acknowledge similarities among programs, and avoids mistakes and inefficiencies incurred from independent analysis. Ideally, fully integrated modeling systems with sophisticated feedback treatment would be used to "integrate" multi-pollutant control design efforts. In the meantime, an understanding of pollutant linkages should reinforce the development of data systems (emissions, meteorological and air quality data) and model interfaces with similar attributes to both improve and economize the analysis efforts.