Airborne Investigations of Black Carbon Aerosols in Low-level Stratocumulus off the Coast of Southern California

DONGLI WANG, Bradley Ries, Minghao Han, Alexander B. MacDonald, Don Collins, Roya Bahreini, Patrick Chuang, Mason Leandro, Lisa Welp-Smith, Mikael Witte, Sierra Bollinger, Andrew Metcalf, Clemson University

     Abstract Number: 439
     Working Group: Aerosol Processes and Properties in Changing Environments in the Anthropocene

Abstract
Black carbon (BC) aerosols have significant impacts on climate change, including the direct effect on atmospheric absorption and indirect and semidirect effects on clouds. However, aerosol-cloud interactions remain one of the largest uncertainties in estimating atmospheric radiative forcing. To address this uncertainty, comprehensive airborne observations of bulk aerosols and cloud droplet residuals were conducted off the coast of Southern California in June 2023 as part of the Southern California Interactions of Low Cloud and Land Aerosol (SCILLA) campaign. A counter-flow virtual impactor (CVI) was used to sample cloud residuals, while an isokinetic inlet was used to sample both interstitial particles and cloud residuals.

To improve understanding of the role of BC particles as cloud condensation nuclei and to assess impacts of local meteorology on clouds and aerosols, a subset of 7 flights which conducted above, below, and within cloud layer samples were analyzed. Cloud droplet sizes decreased from upwind to downwind of San Clemente Island; the percentage of droplets with diameters larger than 11 µm was approximately 97% upwind but dropped to ~53% and ~15% at two cloud modules downwind. Mass concentrations of BC in cloud droplet residuals and in clear air below clouds increased from upwind to downwind, indicating the effects of pollution gradients and island wake turbulence. BC made up 1.07% to 2.60% of total sub-micron aerosol mass in cloud residuals. The size distributions of BC in cloud residuals were shifted towards larger diameters as compared to the BC size distributions out of cloud in upwind regions, but this shift was less consistent downwind. The vertical evolution of BC indicated efficient aerosol exchanges, including cloud processing and activation. Detailed characterization will help to understand the connection between BC and cloud properties and to evaluate their impact on the marine environment.