Improved retrieval of global tropospheric formaldehyde columns from GOME-2/MetOp-A addressing noise reduction and instrumental degradation issues

Abstract

Abstract. We present a new dataset of formaldehyde vertical columns retrieved from observations of GOME-2 on board the EUMETSAT MetOp-A platform between 2007 and 2011. The new retrieval scheme, which has been optimised for GOME-2, includes a two-step fitting procedure that strongly reduces the impact of spectral interferences between H 2 CO and BrO, and a modified DOAS approach that better handles ozone absorption effects at moderately low sun elevations. Owing to these new features, the noise in the H 2 CO slant columns is reduced by up to 40 % in comparison to baseline retrieval settings used operationally. Also, the previously reported underestimation of the H 2 CO columns in tropical and mid-latitude regions has been largely eliminated, improving the agreement with coincident SCIAMACHY observations. To compensate for the drift of the GOME-2 slit function and to mitigate the instrumental degradation effects on H 2 CO retrievals, an asymmetric Gaussian line-shape is fitted during the irradiance calibration. Additionally, external parameters used in the tropospheric air mass factor computation (surface reflectances, cloud parameters and a priori profile shapes of H 2 CO) have been updated using most recent databases. Similar updates were also applied to the historical datasets of GOME and SCIAMACHY, leading to the generation of a consistent multi-mission H 2 CO data record covering the time period from 1997 until 2011. Comparing the resulting time series of monthly averaged H 2 CO vertical columns in 12 large regions worldwide, the correlation coefficient between SCIAMACHY and GOME-2 columns is generally higher than 0.8 in the overlap period, and linear regression slopes differ by less than 10 % from unity in most of the regions. In comparison to SCIAMACHY, the largely improved spatial sampling of GOME-2 allows for a better characterisation of formaldehyde distribution at the regional scale and/or at shorter timescales, leading to a better identification of the emission sources of non-methane volatile organic compounds.

Diurnal, seasonal and long-term variations of global formaldehyde columns inferred from combined OMI and GOME-2 observations

“…In those figures, the mid-morning time series (in green) combine SCIAMACHY (2003-2011), GOME-2A (2007 and GOME-2B (from 2013) measurements, while early afternoon columns (in red) are derived from OMI measurements. The good agreement between the different morning observations can be noted (see also De Smedt et al, 2012), as well as the previously described differences and similarities between afternoon and morning time series. In particular, the higher morning columns over tropical forest are also observed in the SCIAMACHY time series.…”

“…They have been calculated in an ozone-containing atmosphere for low and high SZA (solar zenith angle) using LIDORT RRS (Spurr et al, 2008). Two additional terms (called O 3 L and O 3 O 3 in Table 1), resulting from the Taylor expansion of the O 3 absorption as a function of the wavelength, are included in order to better cope with strong O 3 absorption effects (Puķīte et al;De Smedt et al, 2012). Introduced in the previous version 12, a second (larger) retrieval interval is used to pre-fit the BrO slant columns (Fig.…”

“…Multiple scattering calculations are required for the determination of the air mass factors, and the vertical distribution of the absorber has to be assumed a priori. In the case of optically thin absorbers, the formulation of Palmer et al (2001) is used and has been described for the TEMIS H 2 CO retrievals in De Smedt et al (2012). It decouples the vertical sensitivity of the measurements (the scattering weighting functions, derived with radiative transfer model calculations) from the vertical profile shape of the species of interest (vertical shape factors, taken from an atmospheric chemistry transport model or from some other prior knowledge of the vertical distribution of the absorber).…”

“…The absorption cross sections of the different trace gases are also convolved with the instrumental slit function. In the case of GOME-2, the slit function shape is fitted during the calibration procedure, in order to take into account its changes with time (Dikty and Richter, 2011;De Smedt et al, 2012). In the case of OMI, pre-flight measured slit functions are used, and the calibration is performed for each binned spectrum of the detector array (60 rows).…”

Total ozone trends from 1979 to 2016 derived from five merged observational datasets – the emergence into ozone recovery

“…The most recent modification was in the GOME-2A data record. In the WFDOAS retrieval, the change in the GOME-2A instrument function with time (De Smedt et al, 2012) was accounted for by convolving ozone cross-section data with instrument function derived from daily spectral solar observations with the same instrument. Without such a correction, a drift of about +1.5 % decade −1 becomes apparent.…”