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Deposition of acidic air pollutants occurs via precipitation, fog, and dry deposition. Each of these processes is important in California. Both the condition of the atmosphere and that of the earth's surface control the deposition rate. Studies of geochemical cycling of air pollutants indicates the importance of both wet and dry deposition mechanisms. However, the characteristics of wet deposition are likely to be considerably different from those of dry deposition. Rainfall concentrates pollutants from large quantities of air and delivers them in irregular doses and in a manner that permits relatively simple sampling. Dry deposition, on the other hand, is a slow, continuous process. The key factors influencing dry deposition rates are the characteristics of the atmosphere, the nature of the surface, the properties of the deposition species, and the concentration of the pollutants in the air.
The calculation of deposition rates at any one location requires appropriate monitoring methods and, in the case of dry deposition, an appropriate model to be applied to the monitoring data. As part of Atmospheric Acidity Protection Program, the California Acid Deposition Monitoring Program was established in 1989 to provide information about the concentrations and deposition rates of acidic species delivered by precipitation and dry deposition.
This study was performed to estimate the deposition rates of certain pollutant species (such as nitric acid, nitrates, ammonium, and sulfates) at the ARB's 25 wet and 10 dry deposition monitoring locations, to generalize the estimated deposition rates to larger regions where possible, and to quantify the relative significance of wet and dry deposition in California. |
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For wet deposition, precipitation chemistry data from the California Acid Deposition Program (25 sites), the National Atmospheric Deposition Program/ National Trends Network (8 California sites), and a 10-site wet-deposition network in the Sierra Nevada were used to estimate regional-scale rates of wet-deposited nitrate, sulfate, ammonium, calcium, and hydrogen ion for nine years -- 1985 through 1994. Each species' mean seasonal concentration was multiplied by the total precipitation recorded at the site during a specified interval. Deposition rates, associated error terms, and uncertainties were interpolated for 40 km x 40 km cells throughout the state. (It should be noted that these estimates do not include inputs from cloudwater and fog, which can provide a significant fraction of the wet deposition in some areas.)
Dry deposition rates were calculated for ozone, sulfur dioxide, nitric acid, nitrogen dioxide, particulate sulfate, particulate nitrate, and the reduced nitrogen species ammonia and particulate ammonium from early 1988 through April 1994. The rate of deposition of a particulate species is calculated as the product of its ambient concentration and its deposition velocity. Because a limited amount of data was available from the dry-deposition network, estimates of dry-deposition fluxes were generated only for the monitoring locations and were not generalized to broader regions. Estimates of the total magnitude of deposition were generated for the ten locations having both wet- and dry-deposition data. |
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Wet deposition rates of sulfate and nitrate for California were found to be less than 12 kg/ha/yr in all years (1985 through 1994) at all sites, and those of ammonium less than 5 kg/ha/yr. In comparison, wet sulfate and nitrate deposition rates in portions of eastern North America exceed 25 and 15 kg/ha/yr, and rates of ammonium and calcium deposition are less than 4 and 2.5 kg/ha/yr in almost all parts of eastern North America.
For most years, wet nitrate deposition estimates were greater in the South Coast Air Basin (SoCAB) and the southern Sierra Nevada than in other parts of California. In some areas where wet sulfate deposition is highest, such as the northwestern coast, much of the sulfate has its origin as sea salt.
The range of uncertainties for wet-deposition calculations is associated with the density of monitor distribution. Rates based on data obtained using a reasonably dense network of stations are subject to potentially small uncertainties. Uncertainties of wet-deposition calculations are less than or equal to 20 percent in the SoCAB, which has a large number of monitors. In some other areas, uncertainties are as large as 40 percent (southern California) and 60 percent (northern California) for sulfate, sulfate not including sea salt, and nitrate.
During the period May 1988 through April 1994, the estimated dry deposition of nitric acid ranged from 1 to 86 kg/ha/yr. At the urban sites, nitric acid deposition accounts for 30 to 80 percent of the dry deposition of oxidized nitrogen species and 20 to 70 percent of the total nitrogen dry deposition. NOx is the precursor to nitric acid formation; the comparison of deposition rates and emissions estimates shows that the calculated deposition rates of oxidized nitrogen species at the SoCAB stations range from 16 to 37 percent of the NOx emission rate within the SoCAB. The estimated nitrogen deposition rates at Bakersfield and Sacramento are about 76 and 32 percent, respectively, of the NOx emission rates in Kern County and the area around Sacramento County. Transport of NOx from upwind areas could account in part for the relatively large deposition-to-emissions ratio at Bakersfield.
The sum of wet- and dry- deposition rates for nitrogen at the Fremont sampling site in the San Francisco Bay Area air basin is about 11 percent of the NOx emission rate occurring within that basin. Comparison of wet- and dry-deposition data for three nonurban sites (Gasquet, Yosemite, and Sequoia) indicates that the wet deposition rates of nitrate and sulfate equal or slightly exceed the dry deposition rates of oxidized nitrogen and sulfur species. In contrast, rates of dry deposition of sulfur at the seven urban sites were one to three times greater than wet deposition rates. Dry deposition rates for oxidized nitrogen species at the urban sites ranged from about 10 to about 35 times wet deposition rates for nitrate. At all sites, dry deposition rates for reduced nitrogen species (ammonia and particulate ammonium) were greater than those for wet deposition of ammonium by about a factor of two.
Conditions in California are different from those of locations where acidic deposition is principally due to wet sulfate deposition. Across the state, the deposition of nitrogen-derived acidic gases and particles provides the main portion of atmospheric acidity and nitrogen to urban landscapes, and possibly to mid-elevation forests (elevation 1000 to 2000 m) in southern California. |
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This study has increased our understanding of acidic deposition rates in California. The results of this project will help guide the ARB in setting atmospheric acidity and/or deposition standards, should they become necessary. |
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The ARB has sponsored these related projects (ARB contract number in parentheses): Dry Deposition onto Aerodynamic Surfaces and Vegetation (A6-186-32), Measurements of Dry Deposition Parameters for the California Acid Deposition Monitoring Program (A6-076-32), Mathematical Modeling and Control of Dry Deposition Fluxes of Nitrogen-Containing Air Pollutants (A6-188-32). |
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