Attenuation of spectral actinic flux and photolysis frequencies at the surface through homogenous cloud fields

Abstract

The effect of clouds on derived photolysis rates has been investigated using actinic flux spectra measured from a 2π sr diode‐array spectral radiometer deployed in a series of differing field campaigns. Homogenously cloudy and clear days were selected, and ratios of cloudy‐to‐clear actinic flux spectra were used to derive a cloud transmittance slope (CTS), which is calculated as the gradient of the cloudy/clear ratio plot at 340 < λ < 420 nm. CTS is a measure of the wavelength dependence of actinic flux attenuation by clouds and shows an approximately linear dependence with solar zenith angle. When the solar zenith angle is high, the CTS is largest because of the enhanced path length of radiation through the scattering medium. The dependence of measured CTS under a range of atmospheric conditions is explored; the slopes of the CT being rationalized in terms of the cloud optical depth and single scattering albedo of the cloud. The appearance of a short‐wave enhancement in CT in a multiple scattering environment is noted in agreement with theory. The photolysis frequency coefficients j(O1D) and j(NO2), calculated from the measured actinic fluxes, are compared to observe the effect of wavelength‐dependent attenuation of actinic flux on photochemical reactions. After the subtraction of the effect of changing overhead ozone column, the effect of cloud was quite variable in that j(NO2) was found to be reduced to a greater extent than j(O1D) by some clouds or vice versa depending on the individual case. The results clearly demonstrate that photolysis frequencies in the UVB and UVA do not vary linearly under different atmospheric conditions in a cloudy field.