Organic and Inorganic Aerosol Below-Cloud Scavenging by Suburban New Jersey Precipitation

Abstract

Ambient aerosol size distributions and chemical composition in Princeton, NJ, were measured during a 10-day period in August 2003. Ten precipitation events during the sampling period maintained low aerosol concentrations, with an average gravimetric PM1.0 of 8.2 +/- 1.6 microg m(-3) and an average Fourier transform infrared (FTIR) spectroscopy-measured PM1.0 of 8.6 +/- 0.8 microg m(-3). A constrained factor analysis shows that the measured aerosol composition data are consistent with coal combustion and motor vehicle emissions. FTIR spectroscopy shows that the alkene fraction of organic mass (OM) was larger in the aerosol samples dominated by motor vehicle emissions than in the samples dominated by coal combustion, but the solvent-rinsing behavior was unaffected by source type. The aerosol OM was hydrophilic throughout the sampling period, with an average of 52% +/- 10% of the identified OM removed in the water-rinsing stage of the FTIR analysis. Measurements before and after rain events showed changes in particle composition and number distribution that were used to characterize the rate and chemical selectivity of particle removal processes associated with precipitation (or, in general terms, scavenging). The changes in ambient particle distributions showed an average PM1.0 below-cloud scavenging coefficient of 7 x 10(-5) +/- 3 x 10(-5) s(-1), with variability associated with chemical composition. The fraction of the aerosol OM removed in water rinses decreased during rain events, typically 55% at the start and 30% at the end of the observed rain events. These measured chemically dependent removal rates are consistent with other field measurements and can be used to improve the description of aerosol lifetimes in global models.