Cloud processing and photoparticle production from cloud remote sensing and surface aerosol size spectra at SGP

Description

Clouds modify atmospheric particles largely by aqueous oxidation of dissolved gases. This increases the mass of particles that activated droplets (CCN) while unactivated particles are unaffected. This results in bimodal particle size distributions, which divide the Aitken (25-100 nm diameter) and accumulation (100-800 nm diameter) modes. Aitken particles that nucleate cloud droplets often move to the accumulation mode while some smaller Aitken particles are incorporated into accumulation particles by Brownian capture. Some cloud-processed Aitken particles may remain within the Aitken mode at larger sizes. Cloud-processed aerosol was evident at the SGP surface only during daylight hours and only when the boundary layer mixing height (MH) exceeded the remotely-sensed cloud base altitude (cba). MH was determined from analysis of balloon soundings (Delle Monache et al. 2004). Cloud processing did not seem to happen at night probably because of the absence of the oxidizers for the cloud chemical reactions, which seem to require sunlight, photochemistry. Clouds also block solar radiation that causes photochemical reactions that produce small particles that can grow into the Aitken mode by coagulation and condensation. Photochemistry thus increases Aitken concentrations and decreases Aitken mean particle diameter (mpd) because of the influx of very small Aitken particles. Fig. 1 shows how daytime consecutive hours of clouds and clear sky affect the two particle modes. Increasing accumulation concentrations (black) and mpd (blue) of Fig. 1a indicate cloud processing enhancement of the accumulation mode. Decreasing Aitken concentrations (red) of Fig. 1b indicate conversion of Aitken particles to accumulation particles by cloud processing. Increasing Aitken mpd (green) in Fig. 1b indicate that cloud processing increases the sizes of Aitken particles. Fig. 1d red indicates photochemical production of small particles during clear skies when actinic flux is maximum. The decrease of Aitken mpd (green) is also consistent with production of particles that are smaller than Aitken until they coagulate and receive further condensed vapors that put them into the bottom of the Aitken range. This decreased Aitken mpd for 4 hours after which mpd increased. Fig. 1c shows that without clouds the accumulation mode is not enhanced either in terms of concentration (black) or mpd (blue). Thus, clouds seem to be required for enhancement of the accumulation mode. These results implicate clouds as the source of the accumulation mode and thus clouds seem to explain bimodal aerosol over SGP in May, 2003. Delle Monache, L., Perry, K.D., Cederwall, R.T., & Ogren, J.A., 2004: In situ aerosol profiles over the Southern Great Plains cloud and radiation test bed site: 2. Effects of mixing height on aerosol properties. Journal of Geophysical Research, 109, D06209.