Project at a Glance
Project Status: complete
Title: Mathematical modeling & control of the dry deposition flux of nitrogen-containing air pollutants.
Principal Investigator / Author(s): Russell, Armistead G
Contractor: Carnegie-Mellon University
Contract Number: A6-188-32
Research Program Area: Atmospheric Processes
Topic Areas: Acid Deposition, Ecosystem Impacts
The largest fluxes of acidic air pollutants to the vegetation, materials and soils of the South Coast Air Basin are believed to be from the nitrogen-containing acid gases, including NO, NO₂ and nitric acid, along with the aerosol nitrates. The goal of this research project is to determine the effects that alternative emission control strategies would have on the dry flux of NO, NO₂, PAN, HNO₃, aerosol nitrate and NH₃. An Eulerian grid-based air quality model, constructed to predict the atmospheric concentration of those pollutants, has been modified to include a resistance-based dry deposition code in order to permit the calculation of the magnitude and spatial distribution of the dry flux of nitrogen-containing pollutants to different surfaces. The resistance to deposition includes atmospheric transport processes that convey the pollutants to the vicinity of the earth's surface plus the resistance to dry deposition due to chemical interaction at the surface. Surface resistances are specified as a function of land cover type (e.g., cropland, forests, suburban residential), season of the year, and solar radiation intensity. The dry deposition model has been applied using existing aerometric and emissions data from August, 1982, to calculate the dry flux of nitrogen-containing air pollutants to the surface of the central portion of the South Coast Air Basin. The land cover for the air basin was described spatially using 31 different land use categories, each with its own surface resistance and surface roughness characteristics. The dry deposition flux of nitrogen-containing pollutants to the surface of the modeling region was calculated for 1982 base case conditions (in metric tons N/day) as follows: NO (4.8); NO₂ (49.1); HNO₃ (101.4); PAN (7.2); NH₃ (58.7); and ammonium nitrate (25.9). NOₓ emissions accounted for 175.5 tons/day of the total 247.1 tons/day of deposited nitrogen, while the remainder originated from ammonia emissions. That 175.5 tons/day of nitrogen is equivalent to 577 metric tons/day of NOₓ emissions if stated at the molecular weight of NO₂, which corresponds to more than half of the NOₓ daily emissions to the local atmosphere. Gas phase species delivered 90% of the deposited nitrogen. Model verification studies show that atmospheric ozone and inorganic nitrate concentrations are over-predicted when the new dry deposition code is used in place of the former Caltech airshed model dry deposition code. The new dry deposition code requires data on the surface resistance to dry deposition as a function of land use type for a very large number of surface types. For pollutants other than SO₂, the experimental data simply do not exist that accurately specify the surface resistance values needed to perform such detailed calculations with high precision. In particular, the scientific literature contains nothing more than educated estimates of the surface resistance values for large urban areas. This is a critical matter when evaluating the South Coast Air Basin, where most of the surface of the center of the air basin consists of a giant urban area. A program of experiments is needed to measure the surface resistance parameters required by the present model. An experimental determination of the effect of the surface area of buildings and other roughness elements on the effective surface area for removal of pollutants also is needed. The revised dry deposition model was employed to examine the nature of the effects that would occur if emission controls were applied to the NOₓ and hydrocarbon sources in the South Coast Air Basin as they existed in 1982. At the highest level of control studied (37% reactive hydrocarbon reduction, 61% NOₓ reduction), the nitrogen dry flux would be reduced from 247 tons per day N in the pre-control Base Case to 174 tons/day N after control. Of that 174 tons/day N, 87 tons/day N would be derived from deposition of acid gases plus PAN, while 75 tons/day N would be deposited as NH₃. The remaining 12 tons/day N would be deposited as ammonium nitrate. In general, as emission controls are applied to reactive hydrocarbons and NOₓ the dry flux of acid gases declines while the dry flux of NH₃ increases (due to greater NH₃ emissions and due to higher NH; concentrations that result from lowered aerosol nitrate formation).
For questions regarding this research project, including available data and progress status, contact: Heather Choi at (916) 322-3893
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