Abstract
Abstract. Formaldehyde (CH2O) is an important tracer of tropospheric photochemistry, whose slant column abundance can be retrieved from satellite measurements of solar backscattered UV radiation, using differential absorption retrieval techniques. In this work a spectral fitting sensitivity analysis is conducted on CH2O slant columns retrieved from the Global Ozone Monitoring Experiment 2 (GOME-2) instrument. Despite quite different spectral fitting approaches, the retrieved CH2O slant columns have geographic distributions that generally match expected CH2O sources, though the slant column magnitudes and corresponding uncertainties are particularly sensitive to the retrieval set-up. The choice of spectral fitting window, polynomial order, I0 correction, and inclusion of minor absorbers tend to result in the largest modulations of retrieved slant column magnitude and fit quality. However, application of a reference sector correction using observations over the remote Pacific Ocean is shown to largely homogenise the resulting CH2O vertical columns obtained with different retrieval settings, thereby largely reducing any systematic error sources from spectral fitting.
Highlights
Despite CH2O’s high atmospheric abundance, its detection remains inherently difficult due to its extremely weakFormaldehyde (CH2O) is an important atmospheric trace gas found throughout the troposphere, produced from the oxidation of volatile organic compounds
The differential optical absorption spectroscopy (DOAS) retrieval itself is performed using the QDOAS analysis software (Fayt et al, 2011), prior to which trace gas cross sections listed in Table 1 are corrected to vacuum wavelengths, and convolved with the GOME2 slit function measured pre-launch (Siddans et al, 2006), using the daily Solar Mean Reference (SMR) wavelength grid taken from the relevant L1B orbit file
Test 2d applies the suggested Ozone Monitoring Instrument (OMI) retrieval window (327.5–356.6 nm) on the GOME-2 data, leading to a global decrease in fit χ 2 of −2.43 × 10−7 compared to the reference retrieval, whilst we observe continental CH2O slant column decreases of −0.26 and −0.16 × 1016 molecules cm−2 for the Amazon rainforest (AMA) and southeast United States (SEUS) regions
Summary
Despite CH2O’s high atmospheric abundance, its detection remains inherently difficult due to its extremely weak. W. Hewson et al.: Characterisation of GOME-2 formaldehyde retrieval sensitivity (2006) and De Smedt et al (2008), using spectral measurements from SCIAMACHY on-board ENVISAT, retrieved CH2O to a precision of about 1.0 × 1016 molecules cm−2 for a single observation. CH2O columns have been observed from Ozone Monitoring Instrument (OMI), a CCD spectrometer aboard NASA’s Aura satellite (launched 2004), through a non-linear direct fitting of radiances developed by Chance (2002) and Kurosu et al (2004); reported OMI slant column uncertainties are about 40–100 %. Despite a heritage of CH2O monitoring from space, the CH2O column retrievals still remain less well characterised when compared to major absorbers such as ozone Increased accuracy of the CH2O column product improves confidence in its use for constraining surface emissions
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