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

Abstract

Aerosols (PM_2.0), 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. PM_2.0 (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 PM_2.0 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 NO_X 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.