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, 106 (22), 8835-40

Biogenic Carbon and Anthropogenic Pollutants Combine to Form a Cooling Haze Over the Southeastern United States

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Biogenic Carbon and Anthropogenic Pollutants Combine to Form a Cooling Haze Over the Southeastern United States

Allen H Goldstein et al. Proc Natl Acad Sci U S A.

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.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Aerosol optical thickness (AOT) observed from 3 platforms. (A) MISR annual mean AOT map. (B) Map of difference between mean summer (JJA) and winter (DJF) AOT from MISR instrument. (C) same as B but for MODIS-TERRA instrument. (D) Time series of mean AOT over the SE U.S. from MISR (red line), MODIS-TERRA and MODIS-AQUA (black lines). Also shown are AERONET observations from the Walker Branch site (dots).
Fig. 2.
Fig. 2.
Emission maps for the United States. (A) Annual total anthropogenic SO2 emission. (B) Annual anthropogenic VOC emissions. (C) Annual total biogenic emissions of isoprene and monoterpenes. Anthropogenic Emissions are from U.S. Environmental Protection Agency National Emissions Inventory version 3 (NEI-99) (18), regridded from 4 km to 1° resolution.
Fig. 3.
Fig. 3.
Comparison of Walker Branch AERONET AOT (440 nm) vs. corresponding NCEP SE mean surface temperature reconstructions. Curve is exponential following the temperature dependence of biogenic VOC emission (28): γ = exp[β(TTs)], where β is an empirical coefficient of exponential dependence and Ts is the base temperature of standard conditions (303 K). The best-fit curve shown above (R2 = 0.55) has a β of 0.11, in good agreement with BVOC emission observations.
Fig. 4.
Fig. 4.
R2 correlation for the best-fit exponential curve of AOT vs. the corresponding NCEP mean temperature reconstruction, as in Fig. 3, for AERONET stations.
Fig. 5.
Fig. 5.
Interannual anomalies in summertime southeast regional mean temperature vs. aerosol optical thickness, for years 2000–2005. Temperature is from Goddard Institute for Space Studies Surface Temperature (GISTEMP) analysis (55), AOT is from MISR.

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