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IMPROVED REPRESENTATION OF THE REACTION OF N205 ON AEROSOLS FOR MESOSCALE AIR QUALITY MODELS
Citation:
Pleim, J., J.K.S. Ching, F.S. Binkowski, R. Dennis, AND N. Gillani. IMPROVED REPRESENTATION OF THE REACTION OF N205 ON AEROSOLS FOR MESOSCALE AIR QUALITY MODELS. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/A-94/004 (NTIS PB94140803).
Description:
An important mechanism for the conversion of NO2 to HNO3, and hence the termination of photochemically active NO3, involves nighttime reactions of NO3 and N2O3. ey step in this chemical pathway is the heterogeneous reaction of N2O3 with H2O to form HNO3. he rate of this reaction has been found to be proportional to the surface area of ambient wetted aerosols. his paper describes a simple technique for estimating the rate of this reaction using sulfate concentration and relative humidity to estimate aerosol surface area. ulfate mass is apportioned into two log normal size distribution modes, nucleation and accumulation. he aerosols are allowed to grow or shrink according to thermodynamic equilibrium with the ambient RH. seudo-first order reaction rate is computed as a function of total aerosol surface area, the rate of diffusion of N2O5 to the aerosols, and the reaction probability.This scheme for computing the N2O5 rate coefficient is tested in the Regional Acid Deposition Model (RADM). urrently, RADM uses a two step function based on RH to represent this rate coefficient such that the rate is set to zero when RH < 70%. omparisons between the modified RADM and the standard RADM are presented for a 4 1/2 day simulation period. oth coarse grid ( x = 80 km) and nested grid ( x = 80/3 = 26.7 km) simulations were made and compared. he results show the greatest differences in HNO3 and NO2 concentrations (increases and decreases, respectively), up to 7 ppb, in areas where RH < 70%. here are also more subtle differences where relatively high sulfate concentrations coincide with near saturated RH values.