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

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 groundbased 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 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 Correspondence to: V. E. Cachorro (chiqui@goa.uva.es) 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.

Aerosol radiative effects in the ultraviolet, visible, and near-infrared spectral ranges using long-term aerosol data series over the Iberian Peninsula

“…For instance, the minimum values of α obtained for the Granada station during summer months are linked to the higher likelihood of desert dust events because they are sometimes associated with high aerosol loads (Córdoba-Jabonero et al, 2011). These results corroborate the findings obtained by previous studies about desert dust events over the Iberian Peninsula (see, e.g., Lyamani et al, 2005;Toledano et al, 2007b;Cachorro et al, 2008).…”

“…7. Furthermore, the influence of mineral dust aerosol (with high aerosol optical depth) during these months also causes strong radiative effects, as was also reported by previous studies (e.g., Cachorro et al, 2008;Guerrero-Rascado et al, 2009;Antón et al, 2011;Román et al, 2013;García et al, 2014). In addition, the bimodality of the monthly AOD climatology mentioned in Section 4 has its impact on the radiative effects.…”

“…The aerosol radiative effects are stronger during summer months. This can be related to the higher likelihood of desert dust or biomass burn-ing events over the Iberian Peninsula in these months (e.g., Cachorro et al, 2008;Valenzuela et al, 2012), as was mentioned above. This is corroborated by the increase of the differences among the stations during the warm season, likely due to the variability in the impact of the desert dust episodes which strongly depend on the geographical location of each site.…”

“…First of all, as mentioned in the text, most of the data present AOD 440 nm < 0.15 (∼ 70 % for the Palencia, Granada, and Évora sites). For these low values, SSA = 0.9 and g = 0.75 are selected for the representativeness of the local aerosols in the six sites of study (e.g., Cachorro et al, 2010). To analyze possible uncertainties emerging from this choice, the radiative net fluxes are also evaluated for SSA and g values covering the largest variety of aerosols observed in the Iberian Peninsula.…”

Validation of modeled daily erythemal exposure along tropical and subtropical shipping routes by ship‐based and satellite‐based measurements

“…To our knowledge, this is the first comparison between UV forecast data of erythemal exposure, satellite-based data, and ship-based measurements over the oceans. OMI-derived surface UV retrievals were compared earlier with ground-based measurements on continents for several sites [Tanskanen et al, 2007;Buchard et al, 2008;Ialongo et al, 2008;Kazadzis et al, 2009;Anton et al, 2010;Cachorro et al, 2010;Buntoung and Webb, 2010]. OMI-derived daily erythemal exposure showed a slight overestimation of 0 to 10% for snow-free regions with modest absorbing aerosol loads.…”