The impact of megacity pollution on local climate and implications for the regional environment: Mexico City

We present calculations to estimate potential changes to the local climate and photochemistry caused by pollutants (gases and particles) produced in Mexico City, and the implications for the regional scale when pollutants are exported to surrounding regions. Measured aerosol optical properties are used in a 2-stream delta-Eddington radiative transfer model (Slingo and Schrecker, 1982. Quarterly Journal of the Royal Meteorological Society 108, 407-426) to estimate net radiative fluxes and heating rates, while photolysis rates for nitrogen dioxide and ozone are estimated from a much more detailed model (Madronich, 1987. Journal of Geophysical Research 92, 9740-9752). The presence of highly absorbing aerosols in Mexico City leads to a 17.6% reduction in solar radiative flux at the surface when an optical depth of 0.55 is considered. Photolysis rates for nitrogen dioxide and ozone are reduced between 18 and 21% at the surface, while an increase of between 15 and 17% is predicted above the boundary layer, for local noon calculations. The: non-uniform vertical structure of aerosol concentrations observed (Perez Vidal and Raga, 1998. Atmosfera 11, 95-108) plays a significant role in determining localized regions of heating, i.e. stabilization at the top of the boundary layer that results in a temperature increase of 0.4K h(-1) at that level. The presence of a 200 m-deep aerosol layer at the top of the boundary layer results in vertical profiles of the photolysis rates that are significantly different from the case where the aerosols are uniformly distributed in the mixed layer. At the bottom of the aerosol layer (about I km above the surface), the rates are about 28% lower than when there is a uniform aerosol distribution in the boundary layer. Finally, there is also an enhancement of photolysis rates at the top of the boundary layer that may lead to increased ozone production com pared to the non-aerosol case. (C) 2001 Elsevier Science Ltd. All rights reserved.