Estimating UV Trends Discussion

Satellite observations of UV and VIS irradiance can be used to distinguish regional and global changes in derived atmospheric properties (e.g., ozone, sulfur dioxide, reflectivity, aerosol distribution, nitrogen dioxide, UV-irradiance) in contrast to purely local observations from ground-based instruments. The estimates are based on a single well-calibrated instrument that is used over extended periods (Herman et al., 1991; Herman et al., 1996). For example, the + 52° side scanning Nimbus-7/TOMS measured six radiance channels (312, 317, 331, 340, 360, and 380 nm, 1-nm wide) producing a full global map in each channel once per day for the period October 1978 to January 1993. As with most satellite instruments, the accuracy and precision were maintained using in-flight earth radiance and solar irradiance data to detect and correct changes from the pre-launch laboratory calibration. The data series obtained by Nimbus-7/TOMS has been extended through the present (2009) using the NOAA SBUV-2 series of spectrometers and the hyperspectral spectrometer OMI (270 nm - 500 nm with a resolution of about 0.5 nm).

Satellite derived geophysical quantities (column ozone amounts, values of aerosol optical depth, scene reflectivities (cloud, aerosol, plus ground), and a ground reflectivity climatology) are used to estimate the solar UV irradiance at the ground. Since the entire process is based on remote sensing, complicated retrieval algorithms, and radiative transfer model calculations, the results must be validated against a small set of well-calibrated ground-based measurements distributed in a variety of regions that observe under different atmospheric and geographic conditions. The main purpose of these validation measurements is to provide timely warning that either the satellite or the ground-based instruments are experiencing calibration drift, or that there is some missing physical parameter in the satellite algorithm that can only be supplied from ground-based ancillary data (e.g., aerosol absorption).

UV radiation reaching the earth's surface varies on all time scales, from seconds to seasons to years. In today's atmosphere, the multi-year UV-B variations are principally controlled by changes in stratospheric ozone, changes in the extent of cloud cover, and other longer-term changes such as in the amount of aerosol and pollution. Day-to-day ozone-caused changes from stratospheric and tropospheric dynamics can be significant, but are usually smaller than changes due to cloud cover, because the stratospheric abundance of ozone usually changes as a moderate percentage (~15%) of its seasonally changing mean value. The broad, seasonally repeatable cloud patterns can also cause changes in daily and monthly time scales as the weather changes. On longer time scales (decadal), most regional changes in cloud cover have been small (Herman et al., 2009b), so that global and zonal average changes in UV-B due to long-term ozone depletion are dominant over cloud-change effects. In some regions (e.g., northern Europe), decadal-term cloud changes are also important. Ultraviolet-A changes are controlled by all of the above factors, except ozone.

Ozone data from Nimbus-7/TOMS, obtained during June for the entire 5° longitudinal zone centered at 40 °N, shows that the day-to-day ozone amount can vary by 50 DU, the approximate mean value of 350 DU, or dQIQ = + 0.14. The day-to-day June ozone variation is obtained from figures similar to those shown in Herman et al. (1995). Using an average noon SZA for June of about 23° and an ozone absorption coefficient for 305 nm a = 4.75 cm-1 yields a typical 305 nm irradiance change dF/F = - dQIQ aQ sec(#) = ± 0.14 x4.75 x 0.35 x 1.09 = ± 0.25. In other words, for clear-sky conditions, the 305 nm irradiance typically varies by ± 25% during June, just from to day-to-day ozone changes. As will be discussed later, the day-to-day variability of clear-sky 40 °N UV June irradiance is much larger than the change caused by the long-term June decreases in ozone from 1979 to 2008.

Other factors, such as Rayleigh scattering and land/ocean surface reflectivity, affect the magnitude of measured or theoretically estimated UV irradiance. However, these factors do not significantly affect the short- or long-term changes in irradiance, since their changes are small. Hourly or daily changes in Rayleigh scattering follow the small changes in atmospheric pressure, which usually are less than 2%. There have been no long-term changes in mean atmospheric pressure. The UV surface reflectivity RG is small (3 RU -10 RU) and almost constant with time, except in regions seasonally or permanently covered with snow or ice. Based on radiative transfer studies, clear-sky atmospheric backscattering to the surface contributes less than 0.2 RG to the measured UV irradiance, which is quite small for most ice/snow-free scenes (Herman et al., 1999).

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