Abstract
Abstract. We present an algorithm for the retrieval of glyoxal from backscattered solar radiation, and apply it to spectra measured by the Ozone Monitoring Instrument (OMI). The algorithm is based on direct spectrum fitting, and adopts a two-step fitting routine to account for liquid water absorption. Previous studies have shown that glyoxal retrieval algorithms are highly sensitive to the position of the spectral fit window. This dependence was systematically tested on real and simulated OMI spectra. We find that a combination of errors resulting from uncertainties in reference cross sections and spectral features associated with the Ring effect are consistent with the fit-window dependence observed in real spectra. This implies an optimal fitting window of 435–461 nm, consistent with previous satellite glyoxal retrievals. The results from the retrieval of simulated spectra also support previous findings that have suggested that glyoxal is sensitive to NO2 cross-section temperature. The retrieval window limits of the liquid water retrieval are also tested. A retrieval window 385–470 nm reduces interference with strong spectral features associated with sand. We show that cross-track dependent offsets (stripes) present in OMI can be corrected using offsets derived from retrieved slant columns over the Sahara, and apply the correction to OMI data. Average glyoxal columns are on average lower than those of previous studies likely owing to the choice of reference sector for offset correction. OMI VCDs (vertical column densities)are lower compared to other satellites over the tropics and Asia during the monsoon season, suggesting that the new retrieval is less sensitive to water vapour abundance. Consequently we do not see significant glyoxal enhancements over tropical oceans. OMI-derived glyoxal-to-formaldehyde ratios over biogenic and anthropogenic source regions are consistent with surface observations.
Highlights
The oxidation of non-methane volatile organic compounds (NMVOC)s is an important atmospheric process for both air quality and climate (Lippmann, 1989; Kanakidou et al, 2005; Lelieveld and Dentener, 2000; Forster et al, 2007)
Emission estimates for biogenic isoprene, the largest NMVOC source on a global scale (Guenther et al, 2006) are uncertain by almost a factor of 2 (Pfister et al, 2008)
Anthropogenic NMVOC emissions are highly uncertain (Borbon et al, 2013; Parrish et al, 2012), especially in developing regions that rely on potentially unrepresentative foreign emissions data (Klimont et al, 2002; Streets et al, 2003; Wei et al, 2008)
Summary
The oxidation of non-methane volatile organic compounds (NMVOC)s is an important atmospheric process for both air quality and climate (Lippmann, 1989; Kanakidou et al, 2005; Lelieveld and Dentener, 2000; Forster et al, 2007). Emission estimates for biogenic isoprene, the largest NMVOC source on a global scale (Guenther et al, 2006) are uncertain by almost a factor of 2 (Pfister et al, 2008). Glyoxal (CHO-CHO) is a short-lived product of NMVOC atmospheric oxidation observable from space (Wittrock et al, 2006; Vrekoussis et al, 2010; Lerot et al, 2010; Chance, 2006). Satellite observations of formaldehyde (HCHO), another product of NMVOC atmospheric oxidation, have proven to be a valuable constraint on NMVOC emissions estimates (Abbot et al, 2003; Palmer et al, 2006; Millet et al, 2008; Marais et al, 2012; Fu et al, 2007; Curci et al, 2010). Satellite observations of glyoxal are made in a longer wavelength range
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