Volatile Organic Compounds and Secondary Organic

							            Volatile Organic Compounds and Secondary Organic Aerosol

                                     Joost de Gouw
                     NOAA Earth System Research Laboratory &
       Cooperative Institute for Research in Environmental Sciences, CU Boulder


Volatile Organic Compounds (VOCs) are emitted from a wide variety of natural and
man-made sources. In the atmosphere, the photo-oxidation of VOCs leads to the
formation of ozone and secondary organic aerosol (SOA) that are both important for air
quality and climate forcing. In this talk, I will give an overview of our recent studies on
the emissions and chemical transformation of VOCs.
        We studied the direct emissions and secondary formation of VOCs at a sub-urban
ground site in Mexico City and compared the results with previous studies in the U.S.
VOC emission ratios relative to combustion tracers such as CO and CO2 are higher in
Mexico City than in the U.S. The formation of SOA was found to be significant in the
afternoon and much higher than can be explained from the measured precursors and their
particulate mass yields determined from smog chambers.
        In an effort to quantify the contribution from biogenic VOCs to the formation of
SOA, we studied if measurements of carbonyl sulfide (COS) can be used as an inert,
inverted tracer of biogenic emissions. COS is a long-lived trace gas in the atmosphere
with an oceanic source and a surface sink through the uptake by vegetation and soils. We
demonstrate that air masses with reduced COS were, on average, enriched in biogenic
VOCs and their oxidation products. The enrichment was only small for water-soluble
organic carbon (WSOC), however, suggesting that biogenic VOCs only contributed a
small amount of SOA.
        We summarize the recent insights into the emissions and formation of organic
aerosol. Different studies of SOA formation in urban air consistently reveal a strong
growth in the first day after emission. The results on SOA formation in biomass burning
plumes are less clear. We combined the evidence to construct a zonally averaged
distribution of organic aerosol sources. The analysis suggests that biomass burning and
biogenic SOA dominate in the tropics, but that SOA formation from urban VOCs may be
a significant source of aerosol at northern mid-latitudes.