The influence of aerosols on photochemical smog in Mexico City

Aerosols in the Mexico City atmosphere can have a non-negligible effect on the ultraviolet radiation field and hence on the formation of photochemical smog. We used estimates of aerosol optical depths from sun photometer observations in a detailed radiative transfer model, to calculate photolysis rate coefficients (J(NO2)) for the key reaction NO2 + hv --> NO + O (lambda < 430 nm). The calculated values are in good agreement with previously published measurements of J(NO2) at two sites in Mexico City: Palacio de Mineria (19 25'59N, 99 07'58 W, 3233masl), and IMP (19 degrees 28'48 N, 99 degrees 11'07 W, 2277masl) and in Tres Marias, a town near Mexico City (19 03'N, 99 14'W, 2810masl). In particular, the model reproduces very well the contrast between the two urban sites and the evidently much cleaner Tres Marias site. For the measurement days, reductions in surface J(NO2) by 10-30% could be attributed to the presence of aerosols, with considerable uncertainty due largely to lack of detailed data on aerosol optical properties at ultraviolet wavelengths (esp. the single scattering albedo). The potential impact of such large reductions in photolysis rates on surface ozone concentrations is illustrated with a simple zero-dimensional photochemical model, (C) 2001 Elsevier Science Ltd. All rights reserved.