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 diameter/dry particle diameter) and chemical composition monitoring were conducted during a 3 month period in the winter of 1990 at the South Rim of Grand Canyon National Park, AZ as part of the Navajo Generating Station Visibility Study. Particle growth data are from a Tandem Differential Mobility Analyzer (TDMA). Typically for relative humidities above 75%, the TDMA-measured moist particle distribution is distinctly bimodal, indicating two aerosol fractions based on growth and providing direct evidence of an external mixture of soluble and insoluble constituents. In this study both particle fractions grew in size, thus the terms "more hygroscopic" and "less hygroscopic" were used to distinguish 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 developed to partition the overall volume fraction of the soluble material as determined from the MOUDI composition data, ε, to the two growth fractions obtained from the TDMA data (i.e. ε is partitioned between ε_m and ε₁). The model calculates ε_m and ε₁ for each TDMA measurement as well as growth capacity of the soluble material at the measurement relative humidity, G_s, which 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 hygroscopic fraction is dominated by insoluble material (about 85%).