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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, D21208, doi:10.1029/2004JD004999, 2004

Evidence that the spectral dependence of light absorption by aerosols is affected by organic carbon

Thomas W. Kirchstetter

Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA


T. Novakov

Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA


Peter V. Hobbs

Atmospheric Sciences Department, University of Washington, Seattle, Washington, USA


Abstract

The wavelength dependence of light absorption by aerosols collected on filters is investigated throughout the near-ultraviolet to near-infrared spectral region. Measurements were made using an optical transmission method. Aerosols produced by biomass combustion, including wood and savanna burning, and by motor vehicles, including diesel trucks, are included in the analysis. These aerosol types were distinguished by different wavelength (λ) dependences in light absorption. Light absorption by the motor vehicle aerosols exhibited relatively weak wavelength dependence; absorption varied approximately as λ−1, indicating that black carbon (BC) was the dominant absorbing aerosol component. By contrast, the biomass smoke aerosols had much stronger wavelength dependence, approximately λ−2. The stronger spectral dependence was the result of enhanced light absorption at wavelengths shorter than 600 nm and was largely reduced when much of the sample organic carbon (OC) was extracted by dissolution in acetone. This indicates that OC in addition to BC in the biomass smoke aerosols contributed significantly to measured light absorption in the ultraviolet and visible spectral regions and that OC in biomass burning aerosols may appreciably absorb solar radiation. Estimated absorption efficiencies and imaginary refractive indices are presented for the OC extracted from biomass burning samples and the BC in motor vehicle-dominated aerosol samples. The uncertainty of these constants is discussed. Overall, results of this investigation show that low-temperature, incomplete combustion processes, including biomass burning, can produce light-absorbing aerosols that exhibit much stronger spectral dependence than high-temperature combustion processes, such as diesel combustion.

Received 10 May 2004; accepted 19 August 2004; published 12 November 2004.

Keywords: aerosol light absorption; biomass burning; organic carbon.

Index Terms: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 1610 Global Change: Atmosphere (0315, 0325).

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Citation: Kirchstetter, T. W., T. Novakov, and P. V. Hobbs (2004), Evidence that the spectral dependence of light absorption by aerosols is affected by organic carbon, J. Geophys. Res., 109, D21208, doi:10.1029/2004JD004999.