Cloud Droplet Residual Particles in Biomass-burning Influenced Stratocumulus clouds

At the end of the dry season in southern Africa, extensive biomass burning produces a thick plume of smoke that propagates westward over the southeast Atlantic, where it can interact with the stratocumulus deck off the west coast of the continent.

During the NASA ORACLES expedition in August 2017, a suite of aerosol instruments were sampled from a Counterflow Virtual Impactor (CVI) to examine size and composition of aerosol particles that activated in stratocumulus clouds impacted by biomass burning smoke over the southeast Atlantic ocean. The aerosol instruments included an Aerodyne Aerosol Mass Spectrometer, a DMT Single Particle Soot Photometer, a DMT Ultra High Sensitivity Aerosol Spectrometer, TSI 31010 CN counters, and briefly a TSI radial scanning mobility particle sizer.

Both composition and particle size played a prominent role in droplet residual population behavior. In broken, thin clouds, residual particles were relatively large and much more hygroscopic than in thicker clouds. It may be that the larger, more hygroscopic particles were preferentially removed by drizzle. This resulted in an inverse relationship between mean diameter of cloud droplets and mean diameter of the residual particles. In one case, inorganic salts were up to 50% of the mass of activated particles in tenuous, broken cloud, but only 15% in thicker cloud. Soot was less common in cloud droplets than in the aerosol below cloud.

The SMPS found particles below 30 nm diameter, implying that supersaturations in cloud at least occasionally reached about 1%, well above the mean supersaturations expected in such thin clouds.