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

Abstract

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, that 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. No such limit to the conversion rate of SO₂ in a droplet was observed 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 equilibrium 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 measured size distribution in the chamber. These modeling results predicted 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.