Exploring the missing source of glyoxal (CHOCHO) over China

Recent comparisons between satellite observed and global model simulated glyoxal (CHOCHO) have consistently revealed a large unknown source of CHOCHO over China. We examine this missing CHOCHO source by analyzing SCIAMACHY observed CHOCHO vertical column densities (VCDs) using a Regional chEmical trAnsport Model (REAM). This missing source is first quantified by the difference between SCIAMACHY observed and REAM simulated CHOCHO VCDs (Delta C-CHOCHO), which have little overlap with high biogenic isoprene emissions but are collocated with dense population and high anthropogenic NOx and VOC emissions. We then apply inverse modeling to constrain CHOCHO precursor emissions based on SCIAMACHY CHOCHO and find that this missing source is most likely caused by substantially underestimated aromatics emissions (by a factor of 4-10, varying spatially) in the VOC emission inventories over China used in current regional and global models. Comparison with in situ observations in Beijing, Shanghai, and a site in the Pearl River Delta shows that the large model biases in aromatics concentrations are greatly reduced after the inversion. The top-down estimated aromatics emission is 13.4 Tg yr(-1) in total, about 6 times the bottom-up estimate (2.4 Tg yr(-1)). The resulting impact on regional oxidant levels is large (e. g., similar to 100% increase of PAN in the afternoon). Furthermore, since aromatics are important precursors of secondary organic aerosol (SOA), such an increase of aromatics could lead to similar to 50% increase of global aromatic SOA production and thereby help to reduce the low bias of simulated organic aerosols over the region in previous modeling studies. Citation: Liu, Z., et al. (2012), Exploring the missing source of glyoxal (CHOCHO) over China, Geophys. Res. Lett., 39, L10812, doi:10.1029/2012GL051645.