Size Resolved Aerosol Composition around Rocky Mountain National Park

Understanding aerosol properties in national parks is important due to their potential impacts on visibility, human health, and sensitive ecosystems. The Rocky Mountain Airborne Nitrogen and Sulfur (RoMANS) study was conducted in 2006 to identify the sources, transport, and speciation of atmospheric gases and aerosols throughout Colorado that influence Rocky Mountain National Park (RMNP). As one component of this study, Micro-Orifice Uniform Deposition Impactor (MOUDI) samples were collected at two sites in the vicinity of RMNP. Samples were taken over a time span of 48 hours during a period of 36 days in the spring (March-April) and summer (July-August). The samples were analyzed by ion chromatography to determine the concentrations of NH₄⁺, Na⁺, Ca²⁺, K⁺, Mg²⁺, Cl^-, NO₂^-, NO₃^-, SO₄^2-, and C₂O₄^2- in the different size bins from >18 μm to <0.18 μm.

In the spring nitrate was observed to be mainly in the accumulation mode while in the summer it was primarily in the coarse mode. The size of nitrate has important implications for nitrogen deposition in RMNP as larger particles have a higher deposition velocity. Ammonium and sulphate were the dominant species in the accumulation mode and on several days the sulphate was sufficient to fully neutralize ammonium. However, there were a significant number of days where the addition of nitrate and oxalate to the ammonium neutralization was not enough for full neutralization. The excess ammonium suggests other organic acids may be an important component of the aerosol in the region. There are few studies that have reported the size distribution of oxalate. The size distribution of oxalate was found to peak in the accumulation mode, specifically between 0.32 and 0.56 μm. While the majority of oxalate was observed in the accumulation mode some was also observed in the coarse mode. We examined the potential for oxalate formation via in-cloud and photo-oxidation processes but it is unclear which pathway is dominant without further measurements.