Abstract
Abstract. Several validation studies have shown a notable overestimation of the clear sky ultraviolet (UV) irradiance at the Earth's surface derived from satellite sensors such as the Total Ozone Mapping Spectrometer (TOMS) and the Ozone Monitoring Instrument (OMI) with respect to ground-based UV data at many locations. Most of this positive bias is attributed to boundary layer aerosol absorption that is not accounted for in the TOMS/OMI operational UV algorithm. Therefore, the main objective of this study is to analyse the aerosol effect on the bias between OMI erythemal UV irradiance (UVER) and spectral UV (305 nm, 310 nm and 324 nm) surface irradiances and ground-based Brewer spectroradiometer measurements from October 2004 to December 2008 at El Arenosillo station (37.1° N, 6.7° W, 20 m a.s.l.), with meteorological conditions representative of the South-West of Spain. The effects of other factors as clouds, ozone and the solar elevation over this intercomparison were analysed in detail in a companion paper (Antón et al., 2010). In that paper the aerosol effects were studied making only a rough evaluation based on aerosol optical depth (AOD) information at 440 nm wavelength (visible range) without applying any correction. We have used the precise information given by single scattering albedo (SSA) from AERONET for the determination of absorbing aerosols which has allowed the correction of the OMI UV data. An aerosol correction expression was applied to the OMI operational UV data using two approaches to estimate the UV absorption aerosol optical depth, AAOD. The first approach was based on an assumption of constant SSA value of 0.91. This approach reduces the OMI UVER bias against the reference Brewer data from 13.4% to 8.4%. Second approach uses daily AERONET SSA values reducing the bias only to 11.6%. Therefore we have obtained a 37% and 12% 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 to 9.03% for variable SSA. Similar results were obtained for spectral irradiances at 305 nm, and 310 nm. Contrary to what was expected, the constant SSA approach has a greater bias reduction than variable SSA, but this is a reasonable result according to the discussion about the reliability of SSA values. Our results reflect the level of accuracy that may be reached at the present time in this type of comparison, which may be considered as satisfactory taking into account the remaining dependence on other factors. Nevertheless, improvements must be accomplished to determine reliable absorbing aerosol properties, which appear as a limiting factor for improving OMI retrievals.
Highlights
Cachorro et al.: Part 2: Analysis of site aerosol influence. It is well established by several validation works (Arola et al, 2005; Anton et al, 2007; Kazantzidis et al, 2006, 2009; Arola et al, 2009) that UV irradiance at the ground given by satellite instruments such as TOMS (Total Ozone Mapping Spectrometer) and OMI (Ozone Monitoring Instrument) (Krotkov et al, 2002) is overestimated compared with ground based UV measurements at many polluted locations
TOMS UV algorithm has an Aerosol Index-based correction for the absorbing aerosol which is not included in the current version of OMI UV algorithm (Krotkov et al, 2005; Arola et al, 2009)
The validation results showed a good agreement between OMI-derived daily erythemal doses and the daily doses calculated from the groundbased spectral UV measurements from 18 reference instruments
Summary
It is well established by several validation works (Arola et al, 2005; Anton et al, 2007; Kazantzidis et al, 2006, 2009; Arola et al, 2009) that UV irradiance at the ground given by satellite instruments such as TOMS (Total Ozone Mapping Spectrometer) and OMI (Ozone Monitoring Instrument) (Krotkov et al, 2002) is overestimated compared with ground based UV measurements at many polluted locations. Under cloud-free and low aerosol load conditions (AOD(440 nm) < 0.1) the OMI bias was smaller ∼11% for UVER, and similar spectral results These studies had not attempted correcting OMI UV data for aerosol absorption, because no information about aerosol absorption properties at UV wavelengths was available at our site. The number of selected data is slightly lower, 633, and reduced to 583 (45% of the total days) in order to use the available SSA values, as we explain below This high number of values (as was shown in Arola et al, 2009) reflects the good weather conditions of El Arenosillo for solar radiation and aerosol studies, currently providing of one of the best ten AERONET long-continuous data-set, suitable to analyse the OMI bias due to aerosols
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