Modeling Terrestrial Biogenic Sources of Oxygenated Organic Emissions

Abstract

In recent years, oxygenated volatile organic chemicals (OVOCs) like acetone have been recognized as important atmospheric constituents due to their ability to sequester reactive nitrogen in the form peroxyacetyl nitrate (PAN) and to be a source of hydroxyl radicals (HOx) in critical regions of the atmosphere. The potential biogenic sources of acetone include terrestrial plant canopies, oxidation of dead plant material, harvest of cultivated plants, biomass burning, and the oceans. These sources are poorly constrained at present in budgets of atmospheric chemistry. Based on reported laboratory, field, and satellite observations to date, an approach is presented for a biosphere model to estimate monthly emissions of acetone from the terrestrial surface to the atmosphere. The approach is driven by observed land surface climate and estimates of vegetation leaf area index (LAI), which are generated at 0.5o spatial resolution from the NOAA satellite Advanced Very High Resolution Radiometer (AVHRR). Seasonal changes in LAI are estimated using the Moderate Resolution Imaging Spectroradiometer (MODIS) radiative transfer algorithms to identify the probable times and locations of crop harvest in cultivated areas and leaf fall of newly dead plant material in noncultivated areas. Temperature-dependent emission factors are applied to derive global budgets of acetone fluxes from terrestrial plant canopies, oxidation of dead plant material, and harvest of cropland plants. The predicted global distribution of acetone emissions from live foliage is strongly weighted toward the moist tropical zones, where relatively warm temperatures and high LAI are observed in rain forest areas year-round. Predicted acetone emissions are estimated at between 54 and 172 Tg yr?1 from live foliage sources and between 7 and 22 Tg yr?1 from decay of dead foliage. These flux totals from vegetation are large enough to account for the majority of postulated biogenic acetone sources in models of global atmospheric chemistry, but our model predictions are subject to verification in subsequent flux control experiments using a variety of plant species, particularly those from humid tropical zones.