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 pm 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 $ampling 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 wac observed. The calculated annual average precipitation scavenging ratios for sulfate and ammonium ions were observed to lie in the range 100-3,000. INTRODUCTIONRelationships between local atmospheric aerosol properties and precipitation scavenging were investigated for selected storms at Argonne National Laboratory (ANL) near Chicago. Samples of ambient aerosol particles in the 0.3-1.0 pm aerodynamic diameter size range collected before, during, and after 25 precipitation events occurring in a one-year period were analyzed for sulfate, ammonium, and total mass loading. A modified Lundgren cascade impactor was used to collect aerosol samples, and an automatic wet/dry collector was used to obtain precipitation samples. Aerosol particle samples were analyzed by Fourier-transform infrared spectrophotometry. Precipitation samples were analyzed by liquid ion chromatography. The precipitation data were used in conjunction with the aerosol data to compute sulfate and ammonium scavenging ratios.Of particular interest was the relationship observed between the particle concentration behavior before and during precipitation, and the scavenging ratios for sulfate and ammonium ions. Two types of behavior in the 0.3-1.0 pm particle size range were observed during precipitation. In one of these, aerosol sulfate and ammonium ion concentrations decreased during precipitation; in the other, the concentrations of the two ions increased during the precipitation event. Several fac-

Influence of precipitation scavenging on the PM2.5PM10 ratio at the Kennedy locality of Bogotá, Colombia

“…The increase in PM 2.5 during rainy periods may be attributed to an increase in RH. One study [22] found that for particles with an aerodynamic diameter between 0.3 and 1.0 µm, RH can affect the particles' mass more strongly than precipitation, potentially increasing their mass concentration.…”

An Assessment of Severity of Environmental Aerosol Particles During Precipitation

“…A study in Southeast Asia (Ma et al, 2003) showed that the biomass burning plume contributes approximately 35-40% of the fine organic aerosol mass in the Pacific Ocean. The concentration of aerosol particle were observed to decrease during rain (Sisterson et al, 1985) The studies conducted in Gaborone, Botswana (Jayaratna and Verma and Thomas, 2007) also showed that there is an increase in atmospheric particle concentration during winter season due to biomass burning.…”