11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Wednesday, 5 June 2002: 8:45 AM
Estimation of black carbon effect on light scattering and absorption by cloud water droplets
Li Liu, Columbia University and NASA/GISS, New York, NY; and M. I. Mishchenko, S. Menon, A. Macke, and A. A. Lacis
Poster PDF (456.3 kB)

Black carbon (BC) has long been recognized as an important atmospheric pollutant. It plays a significant role in the absorption of solar radiation by atmospheric aerosols and possibly also by clouds. In this paper we address the problem of scattering and absorption of solar radiation by cloud droplets containing BC inclusions using the ray-tracing/Monte Carlo (RT/MC) techniques. In addition, Lorenz-Mie calculations are performed assuming that the same amount of BC particles are mixed with cloud water droplets externally. Both RT/MC and Lorenz-Mie calculations are performed at a visible wavelength of 0.55 m, corresponding to the maximum in the spectral distribution of the solar radiation. The results show that internal mixing enhances absorption compared to external mixing and for a fixed amount of BC internally mixed with cloud droplets, the absorption is maximal when the effective radius of the BC inclusions is about 0.050.06 m. However it is unlikely under normal conditions that BC aerosols can modify scattering and absorption properties of cloud droplets in any significant way except for geographical locations very close to major sources of BC. The differences in the single-scattering co-albedo 1- v and asymmetry parameter g between BC-fraction-equivalent internal and external mixtures are negligibly small for normal black carbon loadings. Taking into account that the majority of BC particles remain outside cloud droplets and that the differences in 1- v and g between the internal and external mixtures are very small, we conclude that irrespective of the actual form of mixing, one can always use the much simpler external mixing scheme in radiative transfer modeling with great confidence.

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