Chemical and optical properties of lower tropospheric aerosols measured at Mt.Lemmon in Arizona

Atmospheric concentrations of mass and chemical species of aerosols less than 2 μm in diameter, and gaseous SO₂, NH₃, HNO ₃ and HCI were measured at the top of Mt.Lemmon (at a height of 2791 m) near Tucson, Arizona from September 1992 through September 1994. Monthly mean concentration of the aerosols ranged from 0.64 to 3.49 μ gm^-3, which increased in spring and summer, and decreased late in fall and in winter. Monthly mean concentrations of the aerosol species ranged as follows: elemental carbon (0.07-0.21 μ gCm^-3), organics (0.15-0.81 μ gm ^-3), SO^2-₄ (0.29-1.56 μ gm^-3), CI^- (0.00-0.03 μ gm^-3), NO^-₃ (0.00-0.02 μ gm^-3), NH⁺₄ (0.10-0.54 μ gm^-3), sea-salt cation (0.005-0.10 μ gm^-3) and soil (0.02-0.47 μ gm^-3). The lower tropospheric aerosols in Arizona were made up of above eight species. Monthly mean concentrations of gaseous species in ppbv were SO₂ (0.14-0.51), NH₃ (0.06-0.77), HNO₃ (0.05-0.29) and HCI (0.04-0.27). Monthly mean single scattering albedo co and volume extinction coefficient σ _ext of the aerosols were calculated based on the measurement results. The values of ω and σ _ext for external mixture aerosol ranged from 0.90 to 0.96 and from 3.6 x 10^-8 to 1.5 x 10-7^-7cm^-1, respectively. On the other hand, in case of internal half mixture aerosol which contained mixed nuclei formed with elemental carbon and sulfate, ω decreased to be 0.85-0.93. The albedo effect of aerosols on climate depends strongly on the value of ω as well as the albedo of the underlying earth surface. In our method of estimation of the optical properties, then, it is important to determine concentration of the mixed nuclei in the atmosphere.