Optical properties of soot–water drop agglomerates: An experimental study

Abstract

Black carbon (BC) and organic carbon (OC) are the largest contributors to the aerosol absorption in the atmosphere, yet the absorption cross sections of BC and OC per unit mass are subject to a large uncertainty due to morphology, physicochemical properties, and the mixing state of carbonaceous particles. Theoretical studies suggest the possibility of an enhanced absorption by soot–cloud drop agglomerates; however, the magnitude of the effect has never been measured directly and remains highly uncertain. This study is a laboratory experiment aimed at the modeling of direct radiation forcing due to soot‐water interaction in the presence of glutaric acid, a water‐soluble OC. Specifically, we generate, in the laboratory, hydrophobic soot (acetylene soot) and hydrophilic soot (mixture of acetylene soot and glutaric acid) and investigate the structural and optical properties of hydrophobic and hydrophilic soot particles in dry and water‐saturated air. Hydrophobic soot (HBS) particles do not exhibit any structural or morphological differences under dry and saturated conditions, whereas hydrophilic soot (HLS) particles, i.e., BC with a monolayer of glutaric acid, collapse into globules when relative humidity (RH) is increased to saturation. The optical properties of HBS show very little dependence on RH while HLS scattering and absorption coefficient increase markedly with RH. For the cases considered here, the maximum enhancement in absorption for a soot–water drop mixture was as much as a factor of 3.5, very similar to theoretical predictions. The data provided in this study should advance the treatment of polluted cloud layers in climate models.