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

We present calculations to estimate potential changes to the local climate and photochemistry caused by pollutants (gases and particles) produced in M exico City, and the implications for the regional scale when pollutants are exported to surrounding regions. Measured aerosol optical properties are u sed in a 2-stream delta-Eddington radiative transfer model (Slingo and Schr ecker, 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 detaile d model (Madronich, 1987. Journal of Geophysical Research 92, 9740-9752). T he presence of highly absorbing aerosols in Mexico City leads to a 17.6% re duction in solar radiative flux at the surface when an optical depth of 0.5 5 is considered. Photolysis rates for nitrogen dioxide and ozone are reduce d 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 Vi dal and Raga, 1998. Atmosfera 11, 95-108) plays a significant role in deter mining localized regions of heating, i.e. stabilization at the top of the b oundary 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 boundar y layer results in vertical profiles of the photolysis rates that are signi ficantly different from the case where the aerosols are uniformly distribut ed 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 a erosol distribution in the boundary layer. Finally, there is also an enhanc ement 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 Els evier Science Ltd. All rights reserved.