Remote-sensed formaldehyde measurements from the ESA GOME instrument

Formaldehyde (HCHO) is an important indicator of photochemical activity and hydrocarbon reactivity in the troposphere. In the continental boundary layer it reflects hydrocarbon production from biogenic activity, including forest production of isoprenes. In the remote marine troposphere, where methane oxidation dominates its production, HCHO is an important proxy for tropospheric OH, which cannot be measured from satellites by any currently envisaged technique. We present vertical column abundances of HCHO retrieved from the Global Ozone Monitoring Experiment (GOME) spectrometer aboard the ERS-2 satellite. Previous GOME studies have shown that HCHO can be measured from space. The present study provides the sensitivity to measure HCHO globally for the first time. We determine slant columns of HCHO to better than < 4 × 10¹⁵ molecules cm^-2 (0.15) DU sensitivity by directly fitting GOME radiances using a nonlinear least-squares algorithm. This method has been quantitatively compared with DOAS fitting and found to give superior results for the fitting of trace gases in GOME spectra. The fitting includes correction for interfering species, the Ring effect, and the spectrum undersampling by GOME. Improved wavelength calibration using cross-correlation with a Fraunhofer reference spectrum, which was produced as part of the SAO Ring effect determination for GOME, was also implemented. We transform the slant columns to total vertical columns by correcting for vertical profile variability in the presence of Rayleigh and aerosol scattering. This transformation is achieved using the SAO LIDORT multiple scattering radiative transfer model to determine scattering weights for HCHO versus height in conjunction with vertical profile shape factors derived from the GEOS-CHEM global 3-D model of tropospheric chemistry and transport. Total HCHO columns over North America in July 1996 determined in this manner show significant biogenic activity (isoprene production) over the southeastern United States. These HCHO determinations are compared with in-situ surface and aircraft measurements and with the GEOS-CHEM model predictions. Selected examples of HCHO from biomass burning in South America, Asia, and Africa are also presented.