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Several validation studies have shown a notable overestimation of the clearsky ultraviolet (UV) irradiance at the Earth's surface derived fromsatellite sensors such as the Total Ozone Mapping Spectrometer (TOMS) andthe Ozone Monitoring Instrument (OMI) with respect to ground-based UV dataat many locations. Most of this positive bias is attributed to boundarylayer aerosol absorption that is not accounted for in the TOMS/OMIoperational UV algorithm. Therefore, the main objective of this study is toanalyse the aerosol effect on the bias between OMI erythemal UV irradiance(UVER) and spectral UV (305 nm, 310 nm and 324 nm) surface irradiances andground-based Brewer spectroradiometer measurements from October 2004 toDecember 2008 at El Arenosillo station (37.1° N, 6.7° W,20 m a.s.l.), with meteorological conditions representative of the South-West ofSpain.  The effects of other factors as clouds, ozone and the solar elevation overthis intercomparison were analysed in detail in a companion paper (Antónet al., 2010). In that paper the aerosol effects were studied making only arough evaluation based on aerosol optical depth (AOD) information at 440 nmwavelength (visible range) without applying any correction. We have used theprecise information given by single scattering albedo (SSA) from AERONET forthe determination of absorbing aerosols which has allowed the correction ofthe OMI UV data.  An aerosol correction expression was applied to the OMI operational UV datausing two approaches to estimate the UV absorption aerosol optical depth,AAOD. The first approach was based on an assumption of constant SSA value of0.91. This approach reduces the OMI UVER bias against the reference Brewerdata from 13.4% to 8.4%. Second approach uses daily AERONET SSA valuesreducing the bias only to 11.6%. Therefore we have obtained a 37% and12% of improvement respectively. For the spectral irradiance at 324 nm,the OMI bias is reduced from 10.5% to 6.98% for constant SSA and to9.03% for variable SSA. Similar results were obtained for spectralirradiances at 305 nm, and 310 nm.  Contrary to what was expected, the constant SSA approach has a greater biasreduction than variable SSA, but this is a reasonable result according tothe discussion about the reliability of SSA values. Our results reflect thelevel of accuracy that may be reached at the present time in this type ofcomparison, which may be considered as satisfactory taking into account theremaining dependence on other factors. Nevertheless, improvements must beaccomplished to determine reliable absorbing aerosol properties, whichappear as a limiting factor for improving OMI retrievals

atmospheric chemistry and physics

Comparison of UV irradiances from Aura/Ozone Monitoring Instrument (OMI) with Brewer measurements at El Arenosillo (Spain) – Part 2: Analysis of site aerosol influence