Gb. Raga et al., The impact of megacity pollution on local climate and implications for theregional environment: Mexico City, ATMOS ENVIR, 35(10), 2001, pp. 1805-1811
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.