Abstract
Abstract. We present the new version (v14) of the BIRA-IASB algorithm for the retrieval of formaldehyde (H2CO) columns from spaceborne UV–visible sensors. Applied to OMI measurements from Aura and to GOME-2 measurements from MetOp-A and MetOp-B, this algorithm is used to produce global distributions of H2CO representative of mid-morning and early afternoon conditions. Its main features include (1) a new iterative DOAS scheme involving three fitting intervals to better account for the O2–O2 absorption, (2) the use of earthshine radiances averaged in the equatorial Pacific as reference spectra, and (3) a destriping correction and background normalisation resolved in the across-swath position. For the air mass factor calculation, a priori vertical profiles calculated by the IMAGES chemistry transport model at 09:30 and 13:30 LT are used. Although the resulting GOME-2 and OMI H2CO vertical columns are found to be highly correlated, some systematic differences are observed. Afternoon columns are generally larger than morning ones, especially in mid-latitude regions. In contrast, over tropical rainforests, morning H2CO columns significantly exceed those observed in the afternoon. These differences are discussed in terms of the H2CO column variation between mid-morning and early afternoon, using ground-based MAX-DOAS measurements available from seven stations in Europe, China and Africa. Validation results confirm the capacity of the combined satellite measurements to resolve diurnal variations in H2CO columns. Furthermore, vertical profiles derived from MAX-DOAS measurements in the Beijing area and in Bujumbura are used for a more detailed validation exercise. In both regions, we find an agreement better than 15 % when MAX-DOAS profiles are used as a priori for the satellite retrievals. Finally, regional trends in H2CO columns are estimated for the 2004–2014 period using SCIAMACHY and GOME-2 data for morning conditions, and OMI for early afternoon conditions. Consistent features are observed, such as an increase of the columns in India and central–eastern China, and a decrease in the eastern US and Europe. We find that the higher horizontal resolution of OMI combined with a better sampling and a more favourable illumination at midday allow for more significant trend estimates, especially over Europe and North America. Importantly, in some parts of the Amazonian forest, we observe with both time series a significant downward trend in H2CO columns, spatially correlated with areas affected by deforestation.
Highlights
Atmospheric formaldehyde (H2CO) is an intermediate product common to the degradation of many volatile organic compounds (VOCs)
This study focuses on tropospheric formaldehyde retrievals from OMI, using an algorithm historically developed within the TEMIS (Tropospheric Emission Monitoring Internet Service) framework and applied to morning observations from the GOME, SCIAMACHY and GOME-2 sensors
Three sets of monthly averages are used for the satellites, in order to evaluate uncertainties related to the AMF calculation: (1) the vertical columns calculated using the IMAGES a priori profile shapes and no cloud correction, (2) same but applying the IPA to correct for cloud radiative effects, and (3) the vertical columns calculated using as a priori the H2CO vertical profile shapes retrieved from the MAX-DOAS measurements and the IPA cloud correction
Summary
Atmospheric formaldehyde (H2CO) is an intermediate product common to the degradation of many volatile organic compounds (VOCs). De Smedt et al.: Diurnal, seasonal and long-term variations of H2CO inferred from GOME-2 and OMI tropospheric ozone, a key actor in air quality and climate change, and of the hydroxyl radical OH and secondary organic aerosols For these reasons, H2CO satellite observations have been increasingly used in combination with tropospheric chemistry transport models to constrain NMVOC emissions Very little attention has been paid to the diurnal variations of the H2CO columns and to their local dependencies, which are a complex blend of local NMVOC emission variations, H2CO production and loss via oxidation and photolysis depending on local chemical regimes and season In this regard, the latest generation of MAX-DOAS instruments and retrieval algorithms offer new perspectives for the validation of tropospheric trace gas concentrations and aerosol optical densities (Clémer et al, 2010; Pinardi et al, 2013; Vlemmix et al, 2014; Wang et al, 2014).
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