Atmospheric transmission in the ultraviolet and visible: Aerosols in cloudy atmospheres

This study considers the effects of aerosols in cloudy atmospheres on the wavelength dependence of atmospheric transmission in the ultraviolet and visible parts of the solar spectrum. Normalizing the transmission to that of a cloud- and aerosol-free atmosphere, we examine the competing influences of clouds and aerosols on the shape of the transmission function when clouds and aerosols are mixed; while pure water clouds cause the normalized transmission to decrease with wavelength from around 320 nm through the visible, aerosol particles cause the normalized transmission to increase with wavelength from 320 nm through the visible. The results show that when clouds are superimposed on an aerosol profile with the cloud drops and aerosol particles externally mixed, the shape of the normalized transmission spectrum is dominated by the effect of the cloud drops, unless the optical depth of the aerosols begins to approach the optical depth of the cloud. This is the case for an optically thin stratus cloud and an urban aerosol profile. When cloud drops and aerosol particles are internally mixed through coagulation, the shape of the normalized transmission spectrum is again dominated by the effect of the cloud drops, unless there is an unrealistically high volume fraction of strongly absorbing aerosols embedded in the droplets. While measurements of the mass fraction of absorbing aerosols such as soot in cloud and rainwater range from 3.0 × 10^-9 to 6.9 × 10^-6, a soot volume fraction of 1 × 10^-4 is necessary to cause the normalized transmission to increase with wavelength from 320 nm through the visible. The model results are also shown to be consistent with Brewer spectrophotometer irradiance measurements under cloudy and hazy conditions.