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Glyoxal and formaldehyde from space: tracing pollution and biogenic emissions over the greater Mediterranean Region

Final Activity Report Summary - GlyoTrapTeam (Glyoxal and formaldehyde from space: Tracing pollution and biogenic emissions over the Greater Mediterranean Region)

The emissions of volatile organic compounds (VOCs) play a significant role in several important environmental issues such as photochemical smog, the production of ozone (O3) and the secondary organic aerosol (SOA) formation, thus impacting the quality of life and human health. The VOC family consists of tens of thousands of gas-phase species containing carbon. Studying the VOCs is therefore a difficult task as these organic compounds are numerous and their chemistry is complex. Once emitted into the atmosphere, they react rapidly with radicals and oxidants creating a variety of products with different chemical and physical properties.

To overcome these difficulties it was proposed, in the frame of the GLYOTRAPTEAM project, to study two of the most important VOC tracers, namely the formaldehyde, HCHO, and the glyoxal, CHO.CHO molecules. These species were surrogates for VOCs and were produced form the oxidation of both anthropogenic and biogenic emissions.

The oxidation of the VOCs gave different yields of these two oxygenated products and ultimately carbon monoxide (CO). The project focussed on the observations of both species at not only a local scale but also at regional scale, such as the Mediterranean basin, and, finally, at global scale. The main objectives of GLYOTRAPTEAM were to retrieve both species from satellite-based measurements of backscattered solar radiation and investigate their seasonal and temporal variation on regional and global scales. The global observations of these VOC tracers were the key for understanding the role of VOCs, in general, in controlling the abovementioned atmospheric processes.

For the retrievals of HCHO and CHO.CHO two different satellite instruments were used, namely GOME, on board of the ESR-2 satellite, which obtained HCHO data, and Sciamachy on board of Envisat, which obtained data for both substances, covering an extended period of 14 years of measurements. It was found that the highest levels for both species were observed over the tropical and sub-tropical regions, associated with high biological activity and the plumes from vegetation fires. The regions with enhanced amounts of HCHO and CHO.CHO or else the photochemical active hot-spots, were characterised be a well-defined seasonality with the highest values being observed during the warm and dry periods. This was another indication of their main source, i.e. the biogenic emissions of primarily isoprene, and biomass burning from natural and man-made fires. Interestingly, regions influenced by strong anthropogenic pollution also encountered enhanced amounts of formaldehyde and glyoxal and, notably, an increase in glyoxal values was recorded over China for the period from 2003 to 2007, as mentioned in Vrekoussis et al., 2009.

The satellite results were used in two independent modeling studies (refer to Fu et al., 2008, and Myriokefalitakis et al., 2008, for further information) to quantify the sources and sinks of glyoxal in the atmosphere. It was found that 55 % of overall glyoxal was produced from the various biogenic precursors, 20 % from biomass burning, 17 % from biofuel use and 8 % from other anthropogenic emissions. The dominant sink for CHO.CHO was the photolysis, accountaing for 60 % of its total quantity, followed by comparable losses to oxidation and SOA formation, contributing of 15 % each, and wet and dry deposition, equal to 10 %. However, both modelling studies underestimated the global budget of glyoxal as observed from space. A third modelling study, as seen in Stavrakou et al., 2009, found that this mismatch between the observations and the simulation potentially originated from an unknown secondary source of glyoxal which was currently not accounted for in the model simulations.

Overall, during the GLYOTRAPTEAM project, an extensive data set of formaldehyde, covering the period from 1996 to 2008, and glyoxal, covering the period from 2003 to 2009, column amounts was computed from untreated level-zero satellite spectral data. It was envisaged that GLYOTRAPTEAM would be a useful tool for future strategies aiming at reducing ozone and aerosol levels, especially above hot spot areas controlled by enhanced biogenic emissions, anthropogenic activities and biomass burning.