Effects of urban pollution on UV spectral irradiances

Spectral measurements of UV irradiances at Tokyo are compared with corresponding measurements at a pristine site (Lauder New Zealand) to identify the causes of the reductions in urban UV irradiances, and to quantify their effects. Tropospheric extinctions in Tokyo were found to be up to ∼40% greater than at Lauder. Most of these differences 5 can be explained by differences in cloud and aerosols, but ozone differences are also important in the summer. Examining spectral signatures of tropospheric transmission of both sites shows that reductions due to mean NO 2 and SO 2 amounts are generally small. However, at times the amount of NO 2 can be 20 times higher than the mean amount, and on these days it can decrease the UV-A irradiance up to 50%. If SO 2 10 shows comparable day to day variability, it would contribute to significant reductions in UV-B irradiances. The results indicate that at Tokyo, interactions between the larger burden of tropospheric ozone and aerosols also have a significant effect. These results have important implications for our ability to accurately retrieve surface UV irradiances at polluted sites from satellites that use backscattered UV. Supplementary data char-15 acterising these boundary layer effects are probably needed.