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) more than 300 days per year. Aircraft observations obtained during February 1991 showed for the first time that vertical profiles of aerosol particles and ozone presented peak concentrations close to the top of the mixed layer. We hypothesize that the interplay between aerosol particles, solar radiation and photochemical processes is the key to the development of the elevated ozone peak. This peak is reproduced when a layer of highly absorbing particles, representative of Mexico City aerosols, is included in a 1-D turbulent transport and chemistry model of the boundary layer. The results suggest that the presence of large concentrations of absorbing particles within the mixed layer is likely inhibiting ozone formation that would otherwise reach even higher levels.