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

Abstract

[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.Citation: Antón, M., V. E. Cachorro, J. M. Vilaplana, N. A. Krotkov, A. Serrano...

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

“…The relationship between OMI-Brewer differences and AOD at 340 nm presents a great variability, depending on the site or aerosol type (e.g., Kazantzidis et al, 2006;Ialongo et al, 2008;Buchard et al, 2008). Antón et al (2007) found a better correlation for binned data, showing clearly that the OMI bias increases as the AOD increase. Note that this result is relevant it is the same that working with the case of SSA constant as we will discuss later in this paper.…”

“…These two statistical parameters were used according to the methodology used in previous works (Antón et al, 2007(Antón et al, , 2010. For the OMI UV data correction, the formula provided by Krotkov et al (2005) based on the aerosol absorbing correction factor (CAA) was used:…”

“…It is well established by several validation works (Arola et al, 2005;Antón et al, 2007;Kazantzidis et al, 2006Kazantzidis et al, , 2009Arola 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. This positive (satellite overestimation) clear-sky bias varies widely depending on the climatological conditions at validation sites.…”

“…), representative of the SW of Spain. 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.…”

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