Solar actinic flux and photolysis frequency determinations by radiometers and a radiative transfer model at Rishiri Island: comparisons, cloud effects, and detection of an aerosol plume from Russian forest fires

A spectroradiometer equipped with a single monochromator/diode array detector and a J(NO2) filter radiometer (both from Meteorologie Consult, GmbH) were deployed at Rishiri Island in June 2000 to determine spectral actinic fluxes and photolysis frequencies of atmospheric chemical species. The measured 2π-sr actinic fluxes for the 320–700 nm wavelength region under clear conditions closely agreed with those calculated by using the TUV radiative transfer model especially with small solar zenith angles (<60°), while the pixels of the diode array for 295–305 nm suffered from a stray light problem and resulted in overestimation. J values of more than 20 chemical species of atmospheric interest including those of NO2 and halogenated species were calculated by convoluting the absorption cross sections and quantum yields with the observed actinic fluxes. The J(NO2) values agreed very well with the 2π component of the J(NO2) values from the filter radiometer even under cloudy conditions. Each J value under clear conditions was approximated by using three parameters. The effects of clouds and aerosols on actinic fluxes were studied as functions of wavelength. The presence of clouds attenuated actinic fluxes at longer wavelengths more efficiently, but the transmission factor was almost constant throughout 320–700 nm. During 3 days, an aerosol plume presumably originating from Russian forest fires heavily attenuated actinic fluxes in the UV region.