AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Bimodal Aerosol Caused by Cloud Processing
JAMES HUDSON, Stephen Noble, Desert Research Institute
Abstract Number: 508 Working Group: There Must be Something in the Water: Cloud, Fog and Aerosol Aqueous Chemistry for Aerosol Production
Abstract Higher cloud droplet concentrations and less drizzle were found in stratus clouds formed on bimodal CCN spectra than clouds formed on unimodal CCN. Cumulus clouds formed on bimodal CCN had lower droplet concentrations and more drizzle than clouds formed on unimodal CCN. These opposite effects are due to chemical cloud processing dominance making bimodal aerosol in stratus clouds, which are thinner with smaller droplets, compared to thicker cumuli with larger droplets where coalescence is the main cause of aerosol bimodality. Since maritime stratus cause most of the indirect aerosol effects (IAE), cloud processing enhances first IAE (greater cloud albedo due to more droplets) and second IAE (greater cloudiness due to drizzle suppression; greater cloud lifetimes).
Surface aerosol and CCN bimodality measurements that were correlated with greater cloud fractions and lower cloud base altitudes substantiated that cloud processing causes aerosol bimodality, i.e., the accumulation or cloud-processed mode. DMA (SMPS) dry size spectra transposed to critical supersaturation (Sc) spectra compared well with simultaneous CCN spectra in both aircraft and surface measurements. This transposition of size to Sc was done by applying various hygroscopicities, kappa. The kappa that tuned the best fit of these simultaneous spectra was ambient kappa. For the vast majority of cases agreement was good over most of the Sc range, except sometimes at high Sc where some small particles are marginally hygroscopic; i.e., not CCN but detected by the DMA. Kappa differences for the two modes indicate chemical cloud processing whereas the same kappa fitting both modes indicates coalescence cloud processing. Kappa differed between the two modes more often in stratus clouds whereas cumulus clouds had greater incidence of kappa similarity between modes. Thus, kappa differences between cloud type are consistent with cloud microphysics and drizzle responses to CCN modality; chemical dominance in stratus, coalescence dominance in cumuli.