Abstract
Abstract. 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 10 times higher than the mean amount, and on these days it can decrease the UVA irradiance up to 40%. If SO2 shows comparable day to day variability, it would contribute to significant reductions in UVB 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.
Highlights
Previous studies have clearly demonstrated that UV irradiances at the surface are strongly influenced by tropospheric extinctions (Bais et al, 1993)
At small solar zenith angle (SZA), the presence of aerosols tends to increase the ratio of actinic flux to irradiance, whereas at larger SZAs it tends to decrease the ratio. These differences are relatively small at the SZAs that were used to deduce the correction, which was achieved by comparing irradiances and actinic fluxes measured at SZA 60–65◦
To avoid effects of intensity changes during the scans, we initially used our criterion for an ozone retrieval; SZA
Summary
Previous studies have clearly demonstrated that UV irradiances at the surface are strongly influenced by tropospheric extinctions (Bais et al, 1993). An understanding of the causes of pollution effects is needed to improve estimates of geographical differences in UV, which are usually derived from satellite-borne sensors that make use of solar UV radiation that is backscattered to the satellite sensor from the Earth’s atmosphere. The mean backscattering altitude is located several kilometres above the Earth’s surface, so assumptions must be made about the radiative transfer through the lower troposphere, including the boundary layer which can be polluted, especially over heavily populated areas. These satellite sensors show a significant positive bias at polluted locations, including Tokyo (Tanskanen et al, 2005). SpectraPro DSP-275 Double Monochromator, Additive Dispersion 275 f/3.8 Plane Holographic 3600 0.5 1.0, 1.5, 1.0 20 Shaped PTFE diffuser (in house design) Fibre Bundle (1.95 m×20 mm×1.2 mm − GigaHertz) Hamamatsu R1527 285–450 nm 0.6 nm 0.2 nm 272 s for forward + reverse scan (3 speeds are used in each scan, with slower scanning at shorter λ)
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