RELATIONSHIP BETWEEN MEASURED WATER-VAPOR GROWTH AND CHEMISTRY OF ATMOSPHERIC AEROSOL FOR GRAND-CANYON, ARIZONA, IN WINTER 1990

Size-resolved aerosol growth measurements (growth = moist particle dia meter/dry particle diameter) and chemical composition monitoring were conducted during a 3 month period in the winter of 1990 at the South R im of Grand Canyon National Park, AZ as part of the Navajo Generating Station Visibility Study. Particle growth data are from a Tandem Diffe rential Mobility Analyzer (TDMA). Typically for relative humidities ab ove 75%, the TDMA-measured moist particle distribution is distinctly b imodal, indicating two aerosol fractions based on growth and providing direct evidence of an external mixture of soluble and insoluble const ituents. In this study both particle fractions grew in size, thus the terms ''more hygroscopic'' and ''less hygroscopic'' were used to disti nguish them. Micro-Orifice Uniform Deposit Impactors (MOUDI) collected size-segregated 24 h duration samples for subsequent analysis by XRF, ion chromatography, and by thermo-optical analysis (i.e. for carbon). A model that synthesizes growth and compositional information was dev eloped to partition the overall volume fraction of the soluble materia l as determined from the MOUDI composition data, epsilon, to the two g rowth fractions obtained from the TDMA data (i.e. epsilon is partition ed between epsilon(m) and epsilon(l)). The model calculates epsilon(m) and epsilon(l) for each TDMA measurement as well as growth capacity o f the soluble material at the measurement relative humidity, G(s), whi ch is assumed to be the same for both growth fractions. Model results indicate that on average, the more hygroscopic particles are composed of equal volumes of soluble and insoluble materials, while the less hy groscopic fraction is dominated by insoluble material (about 85%).