Case Study: Conceptual Model of Air Pollutant Transport and Fate in the Study Area

Technical studies have been performed in the [study area] to characterize the emission, transport, transformation, and removal processes that govern the behavior of chemical pollutants. This section presents our understanding of the major processes that govern the transport and fate of pollutants in the study area.

Emissions

In preparing emission estimates of sources that contribute to ozone, PM-10, visibility degradation, and acid deposition, manmade (anthropogenic) and natural (biogenic and geogenic) emissions must be considered. Anthropogenic emissions include point, area, and mobile sources. Point sources are defined as a facility with a fixed and identifiable location that emits more than a specified rate of at least one pollutant. Area sources are stationary, and mobile sources (not including on-road motor vehicles) that are too numerous to be treated individually are significant in the aggregate and are usually estimated by subcategories on the basis of common characteristics. In addition, some pollutants are not directly emitted, but occur as a result of atmospheric transformation processes. For example, ozone concentration levels are due to emissions of NO_xand volatile organic compounds (VOCs). To properly characterize ozone production, NO_x and VOC source strengths must be estimated. Since the individual organic chemicals that comprise VOC have significantly different contributions to ozone production, it is important to resolve VOC into its constituent reactive organic species. Similar high resolution of particle-size distribution is equally important to properly characterize emission source contributions to visibility degradation.

There are 10 major source categories of air pollutant emissions in the study region.

Point sources include fossil-fuel power plants, cement plants, resource recovery facilities, tire-burning facilities, and petroleum refineries. This category produces NO_x, VOCs, particulate matter, and SO₂.
On-road motor vehicles include automobiles, light- and heavy-duty trucks, and buses. These vehicles may be with or without catalytic converters and may use gasoline or diesel fuel. This category produces NO_x, VOC, particulate matter, and CO.
Oil field refinery operations include sources such as internal combustion engines, oil heaters and boilers, and steam generators. Oil field and refinery operations also lead to fugitive hydrocarbon emissions from storage tanks, spills, wells, valves, and flanges. This category produces NO_x, VOC, and particulate matter.
During summer, agricultural and forest burning is extensive in the study area, and includes
prescribed burning of fields and forests and unprescribed forest fires. This category produces NO_x, VOC, and particulate matter.
Agricultural operations that may lead to pollutant emissions include fertilizing, pesticide and herbicide spraying (which may result in VOC emissions from the solvent used as the transfer medium), and animal husbandry. The latter is a major source of NH₃ emissions.
Small sources include domestic space heating, domestic solvent use, architectural coating, dry cleaning, gasoline/diesel distribution, cooking, printing, degreasing, paving, and domestic animals. This category produces NO_x, VOC, particulate matter, and NH3.
Biogenic emissions are produced by agricultural crops and natural vegetation. This category produces NO_x, VOC, and particulate matter. Irrigation water may also be a source of hydrocarbons and reduced sulfur.
Geogenic emissions include seepage of natural gas and oil from the earth's surface either on dry land or on the sea floor.
Off-road mobile sources include off-road vehicles (e.g., construction and farming vehicles), aircraft, trains, and ships. This category produces NO_x, VOC, particulate matter, and CO.
Fugitive dust results from vehicle traffic on unpaved roads, fields, and recreation areas; agricultural and construction activities; and wind erosion of soils. This category produces particulate matter.

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There are three basic types of emission sources: area, point, and biogenic. Although biogenic emissions are a type of area source, they are separated from other area sources because they may represent over 50% of the VOC emissions in the [study area], and procedures used to estimate biogenic emission rates are different from those used to estimate anthropogenic area emissions. Emission estimates for anthropogenic sources are derived by multiplying an emission factor by an activity level (process rate). Biogenic emission rates are estimated by multiplying vegetative emission factors by biomass factors for the study region.

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Meteorology and Transport

... During the summer, synoptic wind flows present a northwesterly trend along the California coast. This trend persists inland, although wind flows are also significantly affected by the high terrain, particularly the Coast Range and the Sierra Nevada. Upvalley mesoscale flows predominate (about 70% of the time), while southerly flows are nearly nonexistent (about 2% of the time). At night, downvalley drainage or calm conditions are sometimes observed. Localized air flows make it difficult to treat air pollutant transport. These air flows include land-sea breeze effects (e.g., in the San Francisco Bay Area), mountain-valley winds that lead to drainage flow, and wind patterns such as the Fresno eddy (wind flows that turn at the southern part of the [study area] and lead to a southerly wind flow along the eastern part of the valley).

During summer, little precipitation occurs in the [study area]. However, in the mountains, precipitation in summer may occur under conditions that lead to the advection of warm, moist air from the southeast. Along the coast, advection fog and stratus clouds are generally present in the summer as moist, marine air flows over cold ocean currents.

Chemical and Physical Transformations

Although significant gaps in our understanding of atmospheric chemistry still exist (e.g., aromatic chemistry), the basic processes of ozone and PM-10 formation, visibility degradation, and acid deposition are well established. In the presence of sunlight, NO_x and VOCs (anthropogenic, biogenic, and geogenic) are involved in reactions that allow the continuous conversion of NO to NO₂and the subsequent formation of ozone through NO₂ photolysis. Particulate matter is directly emitted into the atmosphere and is created when organic and inorganic compounds form, primarily through oxidation, condensible species. These particles and NO₂contribute to visibility degradation. Size is a major parameter of the optical properties of particles, and the size distribution of particulate matter must, therefore, be determined to correctly predict visibility degradation. Strong inorganic acids such as sulfuric acid (H₂SO₄) and nitric acid (HNO₃) and weak organic acids such as formic acid (HCOOH) and acetic acid (CH₃COOH) are formed in the atmosphere and lead to acid deposition through dry and wet removal processes. The atmospheric chemistry of California does not fundamentally differ from that of other regions. However, some air pollution characteristics that are specific to California include:

Ambient ozone levels tend to be higher than in other parts of the United States.
NO_x emissions are more important than SO₂ emissions. The NO_x/SO₂ emission mole ratio is about 6 in California, and about 0.7 in the eastern United States. Consequently, the chemistry of Nox reactions and nitrate formation are emphasized.
The chemistry of acetaldehyde and acetic acid is as important as the chemistry of formaldehyde and formic acid. This is due to the fact that aldehyde concentrations in California primarily consist of formaldehyde and acetaldehyde in comparable amounts.
The influx of ocean air leads to the presence of sea salt aerosols in the California atmosphere. Sea salt contains sodium, potassium, chloride, and sulfate ions. Thses species have significant effects on droplet and aerosol chemistry. For example, chloride ions affect the formation of sulfate and nitrate in droplets and aerosols.
Soil dust is alkaline; it may contain sulfate (e.g., gypsum) and affect nitrate aerosol formation due to the presence of carbonates. The presence of iron and manganese in soil dust accelerates SO₂ oxidation in aqueous aerosols and droplets.

A photochemical kinetic mechanism (i.e., a chemical kinetic mechanism that describes ozone formation) is a fundamental part of a comprehensive air quality model for the simulation of photochemical oxidant formation. The treatment of photochemical oxidants and aerosols should emphasize SO₂ oxidation, NH₃ emissions, chemical composition of primary aerosols, and the formation of condensable organics. The treatment of fog and cloud droplet chemistry requires some knowledge of the aqueous inorganic and organic chemistry of acid formation, and information on the chemical composition of the air parcels advected from the ocean to the coast.

Removal Processes

Removal processes physically remove mass from the atmosphere (chemical reactions lead to the removal of some species, but lead to the formation of other species). Removal processes include dry and wet deposition. In summer, dry deposition predominates in the study area. Wet deposition is limited to fog settling, stratus cloud impaction on coastal mountains along the Pacific Ocean, and scattered convective activity in the Sierra Nevada. Little is known about the effect of complex terrain on dry deposition; this removal process must be investigated carefully because of the presence of several mountain ranges in the study area and the ecological sensitivity of some of these areas to air quality and chemical deposition.

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