Comparison of UV irradiances from Aura/Ozone Monitoring Instrument (OMI) with Brewer measurements at El Arenosillo (Spain) – Part 1: Analysis of parameter influence

Abstract

Abstract. The main objective of this study is to compare the erythemal UV irradiance (UVER) and spectral UV irradiances (at 305, 310 and 324 nm) from the Ozone Monitoring Instrument (OMI) onboard NASA EOS/Aura polar sun-synchronous satellite (launched in July 2004, local equator crossing time 01:45 p.m.) with ground-based measurements from the Brewer spectrophotometer #150 located at El Arenosillo (South of Spain). The analyzed period comprises more than four years, from October 2004 to December 2008. The effects of several factors (clouds, aerosols and the solar elevation) on OMI-Brewer comparisons were analyzed. The proxies used for each factor were: OMI Lambertian Equivalent Reflectivity (LER) at 360 nm (clouds), the aerosol optical depth (AOD) at 440 nm measured from the ground-based Cimel sun-photometer (http://aeronet.gsfc.nasa.gov), and solar zenith angle (SZA) at OMI overpass time. The comparison for all sky conditions reveals positive biases (OMI higher than Brewer) 12.3% for UVER, 14.2% for UV irradiance at 305 nm, 10.6% for 310 nm and 8.7% for 324 nm. The OMI-Brewer root mean square error (RMSE) is reduced when cloudy cases are removed from the analysis, (e.g., RMSE∼20% for all sky conditions and RMSE smaller than Correspondence to: V. E. Cachorro (chiqui@goa.uva.es) 10% for cloud-free conditions). However, the biases remain and even become more significant for the cloud-free cases with respect to all sky conditions. The mentioned overestimation is partially due to aerosol extinction influence. In addition, the differences OMI-Brewer typically decrease with SZA except days with high aerosol loading, when the bias is near constant. The seasonal dependence of the OMI-Brewer difference for cloud-free conditions is driven by aerosol climatology.To account for the aerosol effect, a first evaluation in order to compare with previous TOMS results ( Antón et al., 2007) was performed. This comparison shows that the OMI bias is between +14% and +19% for UVER and spectral UV irradiances for moderately-high aerosol load (AOD>0.25). The OMI bias is decreased by a factor of 2 (the typical bias varies from +8% to +12%) under cloud-free and low aerosol load conditions (AOD<0.1). More detailed analysis of absorbing aerosols influence on OMI bias at our station is presented in a companion paper (Cachorro et al., 2010).

Measurements of the total ozone column using a Brewer spectrophotometer and TOMS and OMI satellite instruments over the Southern Space Observatory in Brazil

“…This is consistent with the results obtained by Antón et al (2009Antón et al ( , 2010a in their study conducted for the Iberian Peninsula. This reduction, along with the reduced standard deviation, indicates that the spread between the datasets decreased due to launching more modern satellite instruments and the repair of the algorithms of the satellite instruments, performed over time (Bhartia and Wellemeyer, 2002), in addition to the substitutions of the ground equipment that occurred during these periods.…”

“…Figure 2 shows the scatterplot plotted for Brewer spectrophotometer data and satellite data. (Viatte et al, 2011;Antón et al, 2009Antón et al, , 2010a; Rome, Italy (Ialongo et al, 2008); and at other monitoring stations around the planet (Hendrick et al, 2011;Keckhut et al, 2010).…”

“…Such studies have been systematically carried out, resulting in TOC differences of approximately 3 % (Fioletov, 2008;Antón et al, 2009Antón et al, , 2010aHendrick et al, 2011).…”

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

“…In a companion paper (Antón et al, 2010) the effect of several factors as clouds, aerosols, ozone and the solar elevation on OMI-Brewer UV bias were analysed and compared with previous TOMS-Brewer results (Antón et al, 2007). In the previous works, as in Antón et al (2010), the aerosol effect was studied in terms of aerosol extinction optical depth (AOD) measured at visible 440 nm wavelength. The comparison results showed that under moderate-high aerosol load (AOD(440 nm) > 0.25) the OMI bias is about 19-15% for UVER and spectral UV irradiances.…”

“…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.…”