Effects of aerosols on the wavelength dependence of atmospheric transmission in the ultraviolet and visible: 2. Continental and urban aerosols in clear skies
In this study we model the effects of continental and urban aerosols and their variation with humidity on the transmission of ultraviolet and visible radiation to the Earth's surface. Normalizing the transmission to that of an aerosol-free atmosphere, we examine the mechanisms behind two wavelength-dependent aerosol effects. The first is a dip in the normalized transmission at wavelengths below around 320 nm, which is caused by a coupling between multiple scattering by the aerosol particles and absorption by ozone and by the rapidly increasing absorption coefficient of tropospheric water-soluble aerosols below 340 nm, based on limited available refractive index data in the UV. The second effect is an increase in normalized transmission with wavelength from 320 nm through the visible, which is caused by the decrease with wavelength in the Mie scattering coefficients of tropospheric water-soluble, soot, and stratospheric sulfate aerosols. Using our continental aerosol model, at 0% relative humidity we compute aerosol optical depths of 0.72 at 310 nm and 0.35 at 550 nm, which reduce atmospheric transmission by 12.8% at 310 nm and by 7.9% at 550 nm. With our urban aerosol model we compute aerosol optical depths of 1.82 at 310 nm and 0.87 at 550 nm, which reduce transmission by 34.5% at 310 nm and by 21.1% at 550 nm. Absorption by the aerosols is a significant contributor to this, reducing transmission by 20.9% at 310 nm and by 8.6% at 550 nm beyond nonabsorbing aerosols. For average summer humidity conditions our continental aerosol model predicts an increase in optical depth to 1.26 at 310 nm and to 0.65 at 550 nm, leading to a reduction in transmission of 15.2% at 310 nm and 9.7% at 550 nm, as compared with an aerosol-free atmosphere. For average summer humidity conditions our urban aerosol model predicts an increase in optical depth to 3.22 at 310 nm and to 1.65 at 550 nm, leading to a reduction in transmission of 40.0% at 310 nm and 25.3% at 550 nm, as compared with an aerosol-free atmosphere. Comparison with ground-based data indicates that our estimated optical depths are toward the high end of values seen for continental aerosols, being more typical of industrial than rural areas.