Effects of aerosols on tropospheric photolysis rates in clear and cloudy atmospheres

Abstract

The effect of aerosols on 14 tropospheric photolysis reactions is examined under noncloudy and cloudy sky conditions by using a detailed one-dimensional radiative transfer model. Pure (NH_4)_2SO_4, pure soot, and internal and external mixtures of the two aerosols, as well as mineral dust aerosol, are considered. Nonabsorbing aerosol generally enhances photolysis rates above and in the upper part of the aerosol layer in both noncloudy and cloudy atmospheres, with the enhancement effect reduced in the presence of clouds. In contrast, soot aerosol reduces photolysis rates under both noncloudy and cloudy sky conditions, with the reduction accentuated by a cloud layer. Mixtures of absorbing and nonabsorbing aerosols may produce enhancement or reduction in photolysis rates under clear sky conditions, whereas they generally reduce rates when a cloud is present. In the absence of cloud, sulfate aerosol at urban levels enhances tropospheric average photolysis rates from 11 to 18% for the 14 reactions studied; soot aerosol decreases tropospheric average rates from 6 to 11%. In the presence of a 500-m-thick stratus cloud, sulfate aerosol enhances each of 14 tropospheric average photolysis rates by about 5%; soot aerosol decreases tropospheric average photolysis rates from 9 to 19%.