On the Physicochemical Differences between Cloud Droplet Residual Particles and Below-Cloud Particles over the Northwest Atlantic

Abstract

Cloud droplet residual particles provide insight into cloud formation and processes controlling cloud radiative effects and precipitation. Aerosol particle size distribution and composition were measured in situ from an aircraft with a laser aerosol spectrometer (LAS) and an aerosol mass spectrometer (AMS), respectively, both of which switched between an isokinetic inlet when flying below cloud base (BCB) and a counterflow virtual impactor (CVI) when flying in cloud (above cloud base, ACB). Pairs of data linking below-cloud and in-cloud measurements reveal that the particle effective radius is up to a factor of 2 higher for in-cloud droplet residuals compared to below-cloud aerosol, suggesting some possible mix of preferential activation of larger particles, cloud processing that promoted larger droplet residuals, or other effects. Droplet residuals also show higher mass fractions of organics (and f44) and chloride, with the latter correlated with factors suggestive of an increased sea salt influence. Case studies of African dust transported to Bermuda did not show a clear signature of the coarse aerosol influence in droplet residual measurements, in contrast to sea salt cases. These results leverage a unique airborne data set from ACTIVATE over the northwest Atlantic, providing insight into preferential particle size and compositions describing the subset of particles serving as cloud condensation nuclei (CCN).