Glyoxal Retrieval from the Ozone Monitoring Instrument

Abstract:

We have developed an improved algorithm for the retrieval of glyoxal (CHOCHO) from OMI. Glyoxal is a short-lived product of volatile organic carbon (VOC) oxidation. Satellite glyoxal observations may provide a powerful addition to existing formaldehyde products for constraining VOC emissions. However, glyoxal-to-formaldehyde ratios of current retrievals disagree with ground-based observations over anthropogenic and biogenic regions, inhibiting their combined use in top-down studies. To address these discrepancies we perform a series of sensitivity studies on real and simulated OMI spectra. We show that the sensitive fit window dependence found in previous studies is consistent with a combination of errors from reference cross section uncertainties and spectral interference due to the Ring effect. This implies an optimal fit window of 435-461 nm, consistent with previous retrievals. Using simulated spectra we confirm a NO2 interference associated with cross section temperature, that can be corrected using multiple cross sections. We show that cross-track biases in the OMI retrieval can be corrected by deriving offsets over the Sahara, and apply the correction to the OMI data. Average background concentrations are lower in the new product since the Sahara reference sector is free of liquid water interference associated with the oceanic regions used in previous studies. Concentrations measured over Asia during the monsoon season and the tropics are also lower, suggesting that the new retrieval is less sensitive to water vapor abundance. Consequently we do not see significant enhancements over tropical oceans. The glyoxal-to-formaldehyde ratios of the new OMI product are consistent with surface observations.