A CLOUD CHAMBER STUDY OF THE EFFECT THAT NONPRECIPITATING WATER CLOUDS HAVE ON THE AEROSOL-SIZE DISTRIBUTION

When an air parcel in the atmosphere passes through a nonprecipitating cloud cycle, a subset of the aerosol population called cloud condensa tion nuclei (CCN) is activated and forms cloud droplets. During the cl oud phase, trace gases, such as SO2, are dissolved into the droplets a nd undergo aqueous phase chemical reactions, forming low-volatility pr oducts, such as sulfates, that remain as residue when the cloud drople ts evaporate. The resulting increase in residual mass can have a drama tic effect on the aerosol size distribution, causing the CCN to grow r elative to the smaller particles (interstitial aerosol) which were not activated in the cloud. This process was graphically demonstrated in a series of experiments carried out in the Calspan 600-m3 environmenta l chamber, under conditions where the precloud reactants could be care fully controlled. Size distributions taken before and after a cloud cy cle showed significant conversion of SO2 to H2SO4 and a dramatic chang e in the aerosol size distribution. Subsequent cloud cycles (with the same expansion rate and trace gas concentrations) exhibited very small mass conversion rates. The decreased conversion rate is explained by the increased acidity of the cloud droplet due to the increased mass o f the CCN. The terminal size of the resulting CCN was on the order of one-fiftieth the size of the cloud droplets. The pH of a droplet forme d on a sulfuric acid aerosol particle one-fiftieth its size is about 5 . No such limit to the conversion rate of SO2 in a droplet was observe d when H2O2 was used as the oxidant or when gaseous NH3 was present in sufficient concentration to neutralize the acid. Growth laws for the increase in the equivalent dry mass of CCN during the time the CCN was within the cloud droplet were derived from the rate of SO2 conversion in bulk water when the gaseous reactants are in Henry's law equilibri um with the bulk solution. These growth laws were incorporated into a microphysical cloud model which simulated cloud droplet formation and growth processes in the chamber. The model was initialized using the m easured size distribution in the chamber. These modeling results predi cted the double-peaked character of the size distribution observed in the experiment, but the observed conversion was much greater than that predicted for the case of SO2 oxidation by O3.