ATM-9422993 Johnston In this project, two areas that are relevant to gas-particle transport in tropospheric air pollution will be investigated. The first hypothesis is that surface coatings of organic compounds limit condensation and evaporation of particles. In particular, respiratory and atmospheric neutralization of sulfuric acid particles by ammonia may be inhibited by surface coatings of atmospheric organic matter. To test this hypothesis, sulfuric acid particles containing surface active organics will be exposed to ammonia vapor. The accommodation coefficient of ammonia onto sulfuric acid will be determined as a function of the composition and thickness of the organic coating. The second hypothesis is that particle morphology, namely formation of solid phases as an aqueous particle dries, may inhibit evaporation or condensation of volatile components. In particular, ammonium nitrate/ammonium sulfate particles relevant to polluted urban environments will be studied. If an ammonium sulfate surface coating forms upon drying, then evaporation of ammonium nitrate will be inhibited. To test this hypothesis, aqueous particles containing ammonium nitrate and ammonium sulfate will be dried and the morphology of the dry particles will be characterized. The evaporation rates of particles generated and characterized in this manner will be measured. Most of this research will be performed with rapid-single particle mass spectrometry (RSMS), a new method for on-line chemical analysis of single aerosol particles that was developed with previous NSF funding. The sample particles are detected by light scattering from a continuous laser beam and then ablated by a pulsed excimer beam. Ions produced by laser desorption/ionization are detected by time-of-flight mass spectrometry and the chemical composition of the particles is inferred by the distribution of ions o`served.
