Distribution of UV radiation at the Earth's surface from TOMS-measured UV-backscattered radiances

Daily global maps of monthly integrated UV-erythemal irradiance (290-400 nm ) at the Earth's surface are estimated using the ozone amount, cloud transm ittance, aerosol amounts, and surface reflectivity from the solar UV radiat ion backscattered from the Earth's atmosphere as measured by the total ozon e mapping spectrometer (TOMS) and independently measured values of the extr aterrestrial solar irradiance. The daily irradiance values at a given locat ion show that short-term variability (daily to annual) in the amount of UV radiation, 290-400 nm, reaching the Earth's surface is caused by (1) partia lly reflecting cloud cover, (2) haze and absorbing aerosols (dust and smoke ), and (3) ozone, The reductions of UV irradiance estimated from TOMS data can exceed 50 +/- 12% underneath the absorbing aerosol plumes in Africa and South America (desert dust and smoke from biomass burning) and exceeded 70 +/- 12% during the Indonesian fires in September 1997 and again during Mar ch 1998. Recent biomass burning in Mexico and Guatemala have caused large s moke plumes extending into Canada with UV reductions of 50% in Mexico and 2 0% in Florida, Louisiana, and Texas. Where available, ground-based Sun phot ometer data show similar UV irradiance reductions caused by absorbing aeros ol plumes of Just and smoke. Even though terrain height is a major factor i n increasing the amount of UV exposure compared to sea level, the presence of prolonged clear-sky conditions can lead to UV exposures at sea level riv aling those at cloudier higher altitudes. In the equatorial regions, +/-20 degrees, the UV exposures during the March equinox are larger than during t he September equinox because of increased cloudiness during September, Exte nded land areas with the largest erythemal exposure are in Australia and So uth Africa where there is a larger proportion of clear-sky days. The large short-term variations in ozone amount which occur at high latitudes in the range +/-65 degrees cause changes in UV irradiance comparable to clouds and aerosols for wavelengths between 280 nm and 300 nm that are strongly absor bed by ozone. The absolute accuracy of the TOMS monthly erythemal exposure estimates over a TOMS field of view is within +/-6%, except under UV-absorb ing aerosol plumes (dust and smoke) where the accuracy is within +/-12%. Th e error caused by aerosols can be reduced if the height of the aerosol plum e is more accurately known. The TOMS estimated irradiances are compared wit h ground-based Brewer spectroradiometer data obtained at Toronto, Canada. T he Brewer irradiances are systematically 20% smaller than TOMS irradiance e stimates during the summer months. An accounting of systematic errors bring s the Brewer and TOMS irradiances into approximate agreement within the est imated instrumental uncertainties for both instruments.