The Influence of Humidity on Fine-Particle Aerosol Dynamics and Precipitation Scavenging

Abstract

Precipitation chemistry and the chemical composition and mass loading of atmospheric fine-particle aerosols were examined for 25 individual precipitation events during a 12-month period. Aerosol and precipitation data were used (a) to determine the influence of the precipitation event on the mass loading and the chemical composition of the fine-particle aerosol, and (b) to calculate the apparent scavenging ratios for sulfate and ammonium ions. Two distinct types of aerosol behavior were observed. In one, the concentration of aerosol particles were observed to decrease during the rain but to recover quickly after the event. In the other, the aerosol particle concentration in the 0.3–1.0 μm size range was found to increase either during or just after the rain. The meteorology associated with each precipitation event was examined in detail to determine the factors responsible for the observed behavior of the aerosol particles. Particle mass concentration decrease in the first type of behavior was attributed to local precipitation scavenging of the particles, with the subsequent recovery caused by advection of other portions of the same air mass to the sampling location. The second type of behavior, an increase in fine-particle concentration during or after the precipitation, was apparently caused by an increase in relative humidity from less than 70% to greater than 90% (generally almost 100%) at the time the increase was observed. The calculated annual average precipitation scavenging ratios for sulfate and ammonium ions were observed to lie in the range 100–3,000.