On the formation of an elevated ozone peak in Mexico City

High ozone concentrations have been a major pollution problem in the Mexico City basin for at least 10 years, exceeding 110 ppbv (Mexican standard) mo re than 300 days per year. Aircraft observations obtained during February 1 991 showed for the first time that vertical profiles of aerosol particles a nd ozone presented peak concentrations close to the top of the mixed layer. We hypothesize that the interplay between aerosol particles, solar radiati on and photochemical processes is the key to the development of the elevate d ozone peak. This peak is reproduced when a layer of highly absorbing part icles, representative of Mexico City aerosols, is included in a 1-D turbule nt transport and chemistry model of the boundary layer. The results suggest that the presence of large concentrations of absorbing particles within th e mixed layer is likely inhibiting ozone formation that would otherwise rea ch even higher levels. (C) 2000 Elsevier Science Ltd. All rights reserved.