In Situ Observations of Aerosol Darkening by Clouds: Role of Chemistry, Dynamics and Microphysics

Extensive airborne photo-acoustic measurements of aerosol scattering and absorption were gathered over Texas during the GoMACCS (Gulf of Mexico Atmospheric Composition and Climate Study) campaign in the summer of 2006. In the analysis presented here, particular attention is focused on an event on 14th September 2006 wherein a strongly absorbing aerosol plume layer at 2 km in cloud free and cloudy air was measured. We observed much higher absorption, lower scattering, and substantially lower single scatter albedo (0.4 vs. 0.75) associated with the cloud interstitial particles relative to the surrounding air. Also apparent was a gradient from cloud to near-cloud to cloud-free air. A reduction in single scatter albedo could result from selective scavenging of aerosols such as sulfate and internally mixed black carbon by cloud drops, the reduction in aerosol mean size by cloud processing and other dynamical processes. The concomitant increase in absorption suggests that the in-cloud interstitial aerosol is more absorbing per particle without significant change in mean CPC count. We evaluate the role of cloud chemistry, dynamics and microphysics by utilizing concurrent measurements of aerosol size (PCASP), number (CPC), chemical composition (PILS and AMS) and simple optical models. While chemical data clearly implicates selective scavenging, the microphysical data is being analyzed to determine its contribution to cloud-induced darkening of aerosols. Our goal is to use our data to constrain process level cloud models to determine the additional radiative forcing from aerosol darkening by clouds.