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. 2012 Jul 17;109(29):11624-9.
doi: 10.1073/pnas.1203707109. Epub 2012 Jul 2.

Observationally constrained estimates of carbonaceous aerosol radiative forcing

Chul E Chung  1 , V Ramanathan, Damien Decremer
Affiliations

Observationally constrained estimates of carbonaceous aerosol radiative forcing

Chul E Chung et al. Proc Natl Acad Sci U S A. .

Abstract

Carbonaceous aerosols (CA) emitted by fossil and biomass fuels consist of black carbon (BC), a strong absorber of solar radiation, and organic matter (OM). OM scatters as well as absorbs solar radiation. The absorbing component of OM, which is ignored in most climate models, is referred to as brown carbon (BrC). Model estimates of the global CA radiative forcing range from 0 to 0.7 Wm(-2), to be compared with the Intergovernmental Panel on Climate Change's estimate for the pre-Industrial to the present net radiative forcing of about 1.6 Wm(-2). This study provides a model-independent, observationally based estimate of the CA direct radiative forcing. Ground-based aerosol network data is integrated with field data and satellite-based aerosol observations to provide a decadal (2001 through 2009) global view of the CA optical properties and direct radiative forcing. The estimated global CA direct radiative effect is about 0.75 Wm(-2) (0.5 to 1.0). This study identifies the global importance of BrC, which is shown to contribute about 20% to 550-nm CA solar absorption globally. Because of the inclusion of BrC, the net effect of OM is close to zero and the CA forcing is nearly equal to that of BC. The CA direct radiative forcing is estimated to be about 0.65 (0.5 to about 0.8) Wm(-2), thus comparable to or exceeding that by methane. Caused in part by BrC absorption, CAs have a net warming effect even over open biomass-burning regions in Africa and the Amazon.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
AOD (τ) and AAOD (τa) as a function of wavelength, as calculated with 2001–2009 AERONET data. AOD and AAOD are shown as the ratio to the 440-nm values. Fossil fuel combustion-dominated aerosols: Each line representing a calendar month averaged (and 2001–2009 averaged) over northeastern United States (85–70° west and 38–44° north; between March and October), eastern Asia (114–145° east and 31–42° north; between July and August), or western Europe (5° west–17° east and 44–55° north; between April and May). Biomass burning–dominated aerosols: southern Africa (20–35° east and 28–15° south; between July and October) or Amazon (70–40° west and 20–5° south; between July and October). Dust-dominated aerosols: Saharan dust area (10° west–10° east and 15–30° north; between May and July) or Saudi Arabia region (40–60° east and 15–40° north; between April and May).
Fig. 2.
Fig. 2.
Estimated 2001–2009 annual mean 550-nm AOD and AAOD. (A) CA (BC + OM) AAOD; (B) dust AAOD; (C) BC AOD; and (D) OM. Baseline run is shown.
Fig. 3.
Fig. 3.
Estimated annual mean aerosol DRE (direct radiative effect) at the TOA in units of Wm-2. (A) CA; (B) BC; and (C) OM (BrC + scattering OM). Baseline run is shown.
See this image and copyright information in PMC

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