Light‐absorbing material extracted from cloud droplets and its effect on cloud albedo

Abstract

Using the counterflow virtual impactor, a new instrument for sampling cloud droplets, measurable levels of light‐absorbing material were found to exist inside droplets in stratocumulus clouds off the coast of southern California. Eighty percent of the samples of droplet residue material had light absorption coefficients ranging from 6 to 20 × 10−8 m−1 (where m−1 means per meter of distance in the cloud, not in the cloud droplet nor in the dried bulk aerosol material). Calculated soot concentrations were between 23 and 79 ng soot g−1 of cloud water. These values are in general agreement with aerosol, rainwater, and snow measurements from other experiments and suggest that the coastal clouds were influenced by continental air. Aircraft measurements determined that the sampled clouds had average liquid water contents of 0.24–0.31 g m−3, effective droplet radii of 5.0–7.8 μm, and optical thicknesses of 14–28. Radiative transfer calculations indicate that even at the most sensitive wavelength, the maximum amount of light‐absorbing material from cloud air and inside cloud droplets in this experiment would not significantly alter the albedos of the clouds that were measured. The same amount could possibly affect the albedo of much thicker clouds or of snowpacks (which have relatively large particles and optical thicknesses).