Biogenic carbon and anthropogenic pollutants combine to form a cooling haze over the southeastern United States

Abstract

Remote sensing data over North America document the ubiquity of secondary aerosols resulting from a combination of primary biogenic and anthropogenic emissions. The spatial and temporal distribution of aerosol optical thickness (AOT) over the southeastern United States cannot be explained by anthropogenic aerosols alone, but is consistent with the spatial distribution, seasonal distribution, and temperature dependence of natural biogenic volatile organic compound (BVOC) emissions. These patterns, together with observations of organic aerosol in this region being dominated by modern 14 C and BVOC oxidation products with summer maxima, indicate nonfossil fuel origins and strongly suggest that the dominant summer AOT signal is caused by secondary aerosol formed from BVOC oxidation. A link between anthropogenic and biogenic emissions forming secondary aerosols that dominate the regional AOT is supported by reports of chemicals in aerosols formed by BVOC oxidation in a NO x-and sulfate-rich environment. Even though ground-based measurements from the IMPROVE network suggest higher sulfate than organic concentrations near the surface in this region, we infer that much of the secondary organic aerosol in the Southeast must occur above the surface layer, consistent with reported observations of the organic fraction of the total aerosol increasing with height and models of the expected vertical distribution of secondary organic aerosols from isoprene oxidation. The observed AOT is large enough in summer to provide regional cooling; thus we conclude that this secondary aerosol source is climatically relevant with significant potential for a regional negative climate feedback as BVOC emissions increase with temperature.aerosol ͉ biogenic volatile organic compound ͉ climate ͉ remote sensing ͉ secondary organic aerosols T he importance of biogenic volatile organic compounds (BVOC) reacting with anthropogenic oxides of nitrogen (NO x ) to cause high regional ozone concentrations during summer in the southeastern United States (SE U.S.) was first explained by Chameides et al. (1). This work fundamentally changed the understanding of regulatory approaches for controlling ozone pollution by focusing attention on the need to reduce NO x emissions. We present evidence that similar interactions control secondary aerosol in this region in summer, when concentrations are highest, suggesting that appropriate control strategies focused on the key anthropogenic factors will be required if effects on climate and visibility are to be reduced.The largest uncertainties in projections of radiative forcing involve the direct effects of aerosols on the Earth's radiation budget and the indirect effects through its influence on clouds (2). Atmospheric aerosols come from a combination of anthropogenic and natural sources and include both primary aerosols emitted directly to the atmosphere and secondary aerosols condensing from gaseous precursors. Aerosols above industrialized countries, such as the United States, until recently were generally assu...

Epoxide as a precursor to secondary organic aerosol formation from isoprene photooxidation in the presence of nitrogen oxides

“…Furthermore, the distribution of the predicted mean gaseous MAE (Fig. 5A) matches the spatial patterns of enhanced summertime aerosol optical thickness over the southeastern United States observed by Goldstein et al (43) (see SI Text for spatial distribution of other isoprene-derived high-NO x products), likely linking our observations with the dominant summertime regional aerosol and the importance of biogenic volatile organic compound-anthropogenic interactions in radiative forcing in climate models. Taken together with the recent detection of isoprene epoxydiols under low-NO conditions (9-11), identification of MAE also implies epoxides derived from the oxidation of isoprene play a major role as SOA precursors.…”

Aerosol loading in the Southeastern United States: reconciling surface and satellite observations

“…This theory is supported by the strong winter-to-summer seasonality in satellite-derived aerosol optical depth (AOD) that spatially and temporally matches biogenic volatile organic compound (BVOC) emissions in the region (Goldstein et al, 2009). A potential source of this summertime aerosol could be enhanced production of secondary organic aerosols (SOA) formed by the oxidation of volatile organic compounds (VOC) emitted from vegetation in the presence of anthropogenic pollutants from urban areas (Volkamer et al, 2006;Hoyle et al, 2011).…”

Observational Insights into Aerosol Formation from Isoprene

“…This notation is supported by (i) a recent aircraft study that observed a substantial enhancement in isoprene SOA aloft in the presence of anthropogenic NO x , 3 and (ii) discrepancies between satellite retrievals of aerosol optical thickness and surface organic aerosol measurements in the southeast US that imply there must be an altitudinal dependence in the SOA source. 4 …”