Effects of Continental Clouds on Surface Aitken and Accumulation Modes

摘要

Relationships between clouds and surface aerosol are investigated at the Oklahoma Atmospheric Radiation Measurements Southern Great Plains site. Cloud chemical transformations, coalescence, and Brownian capture increase material within cloud droplets. Then when cloud droplets evaporate, as they often do, their residuals are larger than unnucleated particles. This creates a bimodal particle size distribution by moving some Aitken mode (25- to 100-nm diameter) particles to the accumulation mode (100- to 825-nm diameter). Clouds also block solar radiation that causes photochemical reactions that produce small particles that grow by coagulation and condensation into the Aitken mode. Highly significant positive correlations of remotely sensed cloud fraction (cf) with time-lagged Aitken and accumulation mode mean particle diameter (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 also provided evidence of cloud processing. Photochemical production of very small particles under clear daylight skies worked together with cloud processing to enhance Aitken mpd and concentration correlations with cf. Cloud-processed aerosol was evident only during daylight and when the boundary layer mixing height exceeded the remotely sensed 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. All of these results implicated clouds as a source of the accumulation mode, and thus, clouds seemed to explain bimodal aerosol over the mid-North American continent.