AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Surface Aerosol Bimodality Due to Continental Cloud Processing and Photochemical Particle Production
JAMES HUDSON, Stephen Noble, Desert Research Institute
Abstract Number: 148 Working Group: Aerosols, Clouds and Climate
Abstract Relationships between clouds and surface aerosol were investigated at the Oklahoma ARM site. Cloud chemical transformations, coalescence, and Brownian capture increase material within cloud droplets. Then when droplets evaporate their residuals are larger than unnucleated particles. Bimodal aerosol is thus created by moving Aitken particles to the accumulation mode. Clouds also block solar radiation that causes photochemical production of small particles that grow into the Aitken mode. Highly significant positive correlations of remotely-sensed cloud fractions (cf) with time-lagged Aitken and accumulation mode mean particle diameters (mpd) isolated the effects of cloud processing on both modes. Positive correlations of cf with accumulation concentrations and negative correlations of cf with Aitken concentrations provided further evidence of cloud processing. Photochemical production under clear daylight skies worked together with cloud processing to further enhance Aitken mpd and concentration correlations with cf. Cloud-processed aerosol was evident only during daylight and only when the boundary layer mixing height exceeded cloud base altitude. Greater cf, especially consecutive high cf hours increased accumulation and Aitken mpd and accumulation concentrations while it decreased Aitken concentrations. Lower cf, especially consecutive hours of no clouds, decreased overall mpd and did not enhance the accumulation mode. These results implicated clouds as the source of the accumulation mode. Thus, clouds explained bimodal aerosol over the mid North American continent.
Unlike previous bimodal aerosol observations, the degree of bimodality here is directly linked to the extent of cloudiness to thus better substantiate that cloud processing is the source of the accumulation mode. Augmentation of the accumulation mode was absent when there were few clouds or no clouds. The fact that Aitken particles nucleated cloud droplets in the presence of high accumulation particle concentrations indicated high cloud supersaturations in continental air. This indicates that CCN are not confined to the accumulation mode.