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We present the new version (v14) of the BIRA-IASB algorithm for the retrievalof formaldehyde (H2CO) columns from spaceborne UV–visible sensors.Applied to OMI measurements from Aura and to GOME-2 measurements from MetOp-Aand MetOp-B, this algorithm is used to produce global distributions ofH2CO representative of mid-morning and early afternoon conditions. Itsmain features include (1) a new iterative DOAS scheme involving three fittingintervals to better account for the O2–O2 absorption, (2) the useof earthshine radiances averaged in the equatorial Pacific as referencespectra, and (3) a destriping correction and background normalisationresolved in the across-swath position. For the air mass factor calculation, apriori vertical profiles calculated by the IMAGES chemistry transport modelat 09:30 and 13:30 LT are used. Although theresulting GOME-2 and OMI H2CO vertical columns are found to be highlycorrelated, some systematic differences are observed. Afternoon columns aregenerally larger than morning ones, especially in mid-latitude regions. Incontrast, over tropical rainforests, morning H2CO columns significantlyexceed those observed in the afternoon. These differences are discussed interms of the H2CO column variation between mid-morning and earlyafternoon, using ground-based MAX-DOAS measurements available from sevenstations in Europe, China and Africa. Validation results confirm the capacityof the combined satellite measurements to resolve diurnal variations inH2CO columns. Furthermore, vertical profiles derived from MAX-DOASmeasurements in the Beijing area and in Bujumbura are used for a moredetailed validation exercise. In both regions, we find an agreement betterthan 15 % when MAX-DOAS profiles are used as a priori for the satelliteretrievals. Finally, regional trends in H2CO columns are estimated forthe 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 adecrease in the eastern US and Europe. We find that the higher horizontalresolution of OMI combined with a better sampling and a more favourableillumination at midday allow for more significant trend estimates, especiallyover Europe and North America. Importantly, in some parts of the Amazonianforest, we observe with both time series a significant downward trend inH2CO columns, spatially correlated with areas affected by deforestation

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