The measured and calculated values of JNO2 for
Mexico City are in fair agreement if allowance is made
for the scattering and absorption of solar radiation by
boundary layer aerosols. The e!ect of such aerosols is
substantial, leading to reductions of J
NO2 of 10}30%.
Such reductions in JNO2 (and in J values for other photochemical
reactions) have profound impacts on local air
quality. Speci"cally, according to our calculations, surface
O3 concentrations in Mexico City could be higher
by several tens of ppb if the incident solar UV radiation
were not attenuated by aerosols.