Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 2 2008 10.5194/acpd-8-7149-2008 http://www.atmos-chem-phys-discuss.net/8/7149/2008/ http://www.atmos-chem-phys-discuss.net/8/7149/2008/acpd-8-7149-2008.html http://www.atmos-chem-phys-discuss.net/8/7149/2008/acpd-8-7149-2008.pdf 7149 7188 2008-04-14 Effects of urban pollution on UV spectral irradiances R. L. McKenzie r.mckenzie@niwa.co.nz C. Weinreis P. V. Johnston B. Liley H. Shiona M. Kotkamp D. Smale N. Takegawa Y. Kondo NIWA Lauder, Central Otago, New Zealand University of Hannover, Germany University of Tokyo, Japan 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 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 NO₂ and SO₂ amounts are generally small. However, at times the amount of NO₂ can be 20 times higher than the mean amount, and on these days it can decrease the UV-A irradiance up to 50%. If SO₂ 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 characterising these boundary layer effects are probably needed. Badosa, J., McKenzie, R. 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