Total ozone mapping spectrometer retrievals of noon erythemal-CIE ultraviolet irradiance compared with Brewer ground-based measurements at El Arenosillo (southwestern Spain)

[1] During the last decade, methods have been developed for estimating ultraviolet (UV) irradiance reaching the Earth's surface using satellite-measured backscattered UV radiances. The aim of this work is to compare UV products ( version 8), noon erythemal Commission Internationale de l'Eclairage (CIE)-UV irradiance ( and daily CIE UV doses) from NASA Total Ozone Mapping Spectrometer ( TOMS), with ground-based measurements from a well-calibrated Brewer spectrophotometer. This system is installed at the ESAt-"El Arenosillo'' (Huelva) station, located in southwest Spain, near the Gulf of Cadiz, an area that is exposed to terrestrial Atlantic-Mediterranean air masses, with a high frequency ( about 50%) of cloud-free days. The period analyzed was from 2000 to 2004, separated into two periods, 2000 - 2002 and 2003 - 2004, for comparative analysis. The reason for the two periods is the known calibration problem of Earth Probe-TOMS that became more pronounced by the end of 2002. The calibration errors are most important in the reflectivity and Aerosol Index TOMS data. Although the comparison results are slightly better for the first period, we do not observe major discrepancies during the second period; thus both periods were combined for final conclusions. Four different atmospheric conditions are tested in order to analyze the effects of clouds and aerosols on the differences between TOMS and Brewer data: all-sky, cloud-free with any aerosols, as well as with low and high aerosol loads. In general, under all sky conditions, TOMS overestimates the noon CIE irradiance by 7.5 +/- 0.7% ( daily CIE doses 8.3 +/- 0.6%), but the most relevant is that the bias becomes negative for high values of TOMS reflectivity ( thick clouds or high cloud optical depth) with a significant noise increase. Therefore the TOMS bias is higher for cloud-free days, 11.8 +/- 0.2%, increasing with the aerosol optical depth, as measured by a colocated Cimel-AERONET Sun photometer. A high correlation between TOMS and Brewer CIE noon irradiances was observed when aerosol-binned data are considered, reaching R-2 = 0.8. For low aerosol load [ aerosol optical thickness (AOT) < 0.1], the TOMS bias of noon CIE data decreases to 8.2 +/- 0.4% while for high load ( AOT > 0.25 and alpha-Angstrom < 0.8; mostly due to desert dust events), the bias increases to 15.1 +/- 0.6%. This is due to the fact that absorbing aerosols in the boundary layer are currently not adequately modeled in the operational TOMS UV algorithm. TOMS data can be corrected off-line if the absorption part of the aerosol optical thickness (AAOT) is known at the site. However, currently there are no standard methods of measuring AAOT ( or aerosol single-scattering albedo) in the UV wavelengths even from the ground. The new AAOT product from Ozone Monitoring Instrument on board of NASA EOS Aura satellite ( launched in July 2004) could be used to reduce the bias along with other improvements. This is currently a subject of ongoing research.