Atmospheric Science

Violet Rain

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Science  12 Sep 2003:
Vol. 301, Issue 5639, pp. 1447
DOI: 10.1126/science.301.5639.1447a

Most analyses of the amount of solar absorption by clouds assume that cloud droplets are composed of pure water. This assumption is particularly important for climate models, because they tend to underestimate the amount of solar energy absorbed by the troposphere. However, if cloud droplets were not pure water, they could absorb more radiation, which might help explain one apparent shortcoming of cloud parameterization. Gelencsér et al. report laboratory experimental results that demonstrate that aromatic hydroxyacids, species that are produced by biomass burning and are commonly found in fine continental aerosols, can react in rain droplets with hydroxyl radicals to form colored organic species that absorb solar radiation in the ultraviolet and visible spectra. These chromophoric droplets could absorb six times as much solar radiation as pure water droplets. Although it is unlikely that this phenomenon can resolve the discrepancy between models and observations of solar absorption by clouds, it could be an important contributing factor and adds to the understanding of the effects of polar organic compounds in natural systems. — HJS

J. Atmos. Chem. 45, 25 (2003).

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