A Decade of Aerosol and Gas Precursor Chemical Characterization at Mt. Lemmon, Arizona (1992 to 2002)

摘要

Aerosols (PM2.0), and associated precursor gases have been continuously monitored at Mt. Lemmon (2791 m ASL), Arizona, since September 1992. Month-long samples are collected on filters and chemically analyzed resulting in a decade-long record with over 100 data points for each species—among the longest such records currently available. The species determined include SO42−, NO3−, Cl−, NH4+, Ca2+, Mg2+, K+, Na+, elemental carbon (EC), organic carbon (OC), NH3(g), SO2(g), HCI(g) and HNO3(g). The data reveal long-term trends, seasonal variations, and correlations between species. PM2.0 (1.48 μg m−3, annual mean) is mainly comprised of SO42− (49% w/w), NHI4+ (16%), EC (11%) and OC (22%). The mean SO42−/NH4− equivalent ratio is 1:1 suggesting complete neutralization. Median PM2.0 was 1.33 μg m−3 (range = 0.17-4.32 μg m−3). Median EC was 0.14 μg m−3 (0.01-0.76), and median OC was O.29 μg m−3 (0.03-1.33). The annual mean trends of all species, with the exception of SO42−, SO2(g), NH4+ and NH3(g), appear to be increasing, but some trends may not be statistically significant. Long-term decreasing trends in SO2(g) and SO42−, reflect source controls implemented over the past decade, whereas HNO3(g) has been increasing, possibly due to increased NOX emissions associated with population growth in the region. The associated conversion of agricultural land to urban use might be leading to a decrease in NH3(g). Annual trends for EC (5.2 ± 2.7 ng m−3 y−1) and EC/OC ((1.5 ± 0.75) × 10−2 y−1) appear to be positive and significant, but there is no significant annual OC trend. There appears to be a significant secondary source of OC, presumably derived from photoxidation of biogenic hydrocarbons. There is no significant trend in the calculated annual mean extinction coefficient but the calculated single scattering albedo (ω) may be decreasing (−1.5 ± 1.1 × 10−3 y−1), possibly caused by increasing EC associated with forest fires and/or fossil fuel combustion. Depending on the value of the critical single scattering albedo, the aerosol might already be a net absorber, or it might only become so by the end of the century if current trends continue.