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A Cloud Chamber Study of the Effect Which Nonprecipitating Clouds Have on the Aerosol Size Distribution

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When an air parcel in the atmosphere passes through a nonprecipitating cloud cycle, a subset of the aerosol population called cloud condensation nuclei CCN is activated and forms cloud droplets. During the cloud phase, trace gases, such as SO2, are dissolved into the droplets and undergo aqueous-phase chemical reactions, forming low-volatility products, such as sulfates, which remain as residue when the cloud droplets evaporate. The resulting increase in residual mass can have a dramatic effect on the aerosol size distribution, causing the CCN to grow relative 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 environmental chamber, under conditions where the precloud reactants could be carefully controlled. Size distributions taken before and after a cloud cycle showed significant conversion of SO2 to H2SO4 and a dramatic change 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 of 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 formed on a sulfuric acid aerosol particle one-fiftieth its size is about 5. Atmospheric aerosols, Atmospheric chemistry.

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  • Meteorology
  • Optics

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