PNNL

Abstract

Shallow convective clouds are ubiquitous in many regions of the world. Currently, aerosol-cloud interactions parameterizations for convective clouds are a major source of uncertainty in global climate model predictions of radiative forcing, highlighting the need for in-situ characterization of the size, composition, and mixing state of activated particles. Here, aircraft-based single particle measurements reveal that cloud residuals of shallow cumuli were enriched in larger and sulfate-rich particles. Using single-particle size and composition measurements, we provide field evidence for a strong dependence of the size, composition, and mixing state of below-cloud aerosol on the droplet activation fraction for continental shallow cumuli. Results show that treating the aerosol as internally mixed yielded calculated supersaturations a factor of 2 larger than required for activation, highlighting the importance of accurately describing the aerosol mixing state in models.