Formaldehyde (HCHO) and Glyoxal (CHOCHO)
In the presence of nitrogen oxides, the photochemical degradation of volatile organic compounds (VOC) leads to secondary gaseous and particulate products, such as ozone and secondary organic aerosols (SOA). Both are important contributors to air pollution with severe impacts on human health, ecosystems, and regional climate. Two important intermediate products in the oxidation of volatile organic compounds are HCHO and CHOCHO. Since both trace gases have short lifetimes, their distribution mainly resembles the source areas and can be used as indicators of tropospheric VOC emissions. SCIAMACHY’s capability to measure HCHO and CHOCHO from space on a global scale was demonstrated by Wittrock et al. (2006). Since then, several studies have been carried out using these data to compare directly with the output from chemical transport models (CTM) and thereby substantially improving the models’ accuracy and reliability. In addition, VOC emission strengths have been derived for biospheric, pyrogenic and anthropogenic sources, respectively.
A consistent dataset of global tropospheric HCHO (fig. 3-10) has been created by De Smedt et al. (2008) using GOME and SCIAMACHY data covering more than a decade. This dataset has been utilised by Stavrakou et al. (2009) to evaluate the performance of pyrogenic and biogenic emission inventories and to investigate trends in HCHO over Asia and large cities worldwide (De Smedt et al. 2010). More regional aspects on the different emissions strengths of VOC based on HCHO data were investigated for Europe (Dufour et al. 2009) and Amazonia (Barkley et al. 2009). In general, for regions with high biogenic emissions like tropical rainforests, a reasonable agreement was found between modelled and measured HCHO columns. Other areas having more variable or less emissions of VOC reveal higher discrepancies. Recently, Marbach et al. (2009) even succeeded in determining HCHO emissions from ships in the Indian Ocean.
Myriokefalitakis et al. (2008) adapted a global chemistry-transport model to simulate the temporal and spatial distribution of CHOCHO columns in the global troposphere focussing on the anthropogenic contribution. They found indication for a missing CHOCHO source of about 20 Tg/ year or an overestimate of its sinks by the model. In addition, Fu et al. (2008) examined the potential of CHOCHO as a source of secondary organic aerosol. Apparently, irreversible uptake of CHOCHO by aqueous aerosols and clouds could make a significant contribution to the global SOA production. The long SCIAMACHY time series also revealed seasonal and year-to-year variability above several photo-chemical hot spots. This has been studied by Vrekoussis et al. (2009) for CHOCHO (fig. 3-11). For the period 2002-2007, a significant annual increase in CHOCHO in addition to a seasonal cycle was reported over north-eastern Asia. In general, the regions influenced by anthropogenic pollution encounter enhanced amounts of CHOCHO.