Investigation of aerosol optical properties on regional climate forcing and Spatial and temporal distributions of aerosol and ozone associated with the Antarctic polar vortex processes
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2002-11-12
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Aerosols influence Earth's heat budget both directly by scattering and absorbing sunlight, and indirectly by acting as nuclei for cloud droplets. To reduce the uncertainties of current estimates of aerosol climate forcing, aerosol optical properties relevant to the computation of direct radiative forcing were measured at a regionally representative site near Mount Mitchell, North Carolina. On the basis of these measurements and model calculations, we have studied (1) the effects of relative humidity (RH) on aerosol optical properties and direct aerosol radiative forcing and (2) the influence of long-range transport on black carbon (BC) concentrations, its seasonal and weekly patterns, and the effects of BC on the regional climate of the southeastern US. The light scattering of aerosol is strongly dependent on RH at which it is measured, due to hygroscopic growth nature of most atmospheric aerosols. In this study, the hygroscopic growth factor (ratio of total scattering coefficient at RH=80% to that at RH=30%) was calculated to be almost constant value of 1.60 +/- 0.01 for polluted, marine, and continental air masses. In addition, it was found that as the RH increased from 30% to 80%, the backscatter fraction decreased by 23%. The patterns of direct radiative climate forcing by aerosols for various values of RH were similar for the three air masses, but the magnitudes of the forcing were larger for polluted air masses than for marine and continental air masses by a factor of nearly 2 due to higher sulfate concentration in polluted air masses. The averaged forcing for all the observed ambient RHs was -2.9 W m^(-2) (the negative forcing of -3.2 by aerosol scattering plus the positive forcing of +0.3 by aerosol absorption) for polluted air masses, -1.4 W m^(-2) (-1.5 plus +0.1) for marine air masses, and -1.5 W m^(-2) (-1.6 plus +0.1) for continental air masses. The BC mass concentration of the southeastern US showed the highest average concentration in polluted air masses and the lowest in marine air masses. During the winter, the overall average BC value was 74.1 ng m^(-3), whereas the overall summer mean BC value was higher by a factor of 3. The main reason for the seasonal difference may be enhanced thermal convection during summer, which increases transport of air pollutants from the planetary boundary layer of the surrounding urban area to this rural site. In the spring of 1998, abnormally high BC concentrations from the continental sector were measured. These concentrations were originating from a biomass burning smoke plume in Mexico. This was confirmed by the observations of the Earth Probe Total Ozone Mapping Spectrometer. The net aerosol radiative forcing (scattering effects plus absorption effects) per unit vertical depth at 2006 m MSL was calculated to be -1.4x10^(-3) W m^(-3) for the southeastern US. The magnitude of direct radiative forcing by aerosol scattering was reduced by 15 +/- 7 % due to the BC absorption.
SAGE II ozone and aerosol measurements and NCEP/NCAR potential vorticity and temperature fields during 1985-1999 were analyzed to study the spatial and temporal distributions of aerosol and ozone associated with the Antarctic polar vortex processes and to investigate the impact of polar stratospheric clouds (PSCs) on aerosol distribution and ozone depletion. During austral spring (Sep/Oct) there exist strong radial gradients in ozone, aerosol, and temperature near the polar vortex edge region and an isolation of materials inside polar vortex, confirming that springtime polar vortex acts effectively in keeping out intrusions of materials from the exterior of the polar vortex. The analysis of vertical profiles of 1020-nm aerosol extinction and 525- to 1020-nm aerosol extinction ratio inside spring polar vortex indicates aerosol enhancement in 9-15km layer and lack of larger particles in 15-20km layer, which is consistent with the effects of gravitational sedimentation and subsequent evaporation of PSCs. For the spherical PSC particles for which the Reynolds number < 1, gravitational sedimentation velocity is estimated to be 0.11, 0.04, and 1.5 km/wk for Type Ia, Ib, II PSCs, respectively. The probabilities of the occurrence of T<195K (threshold temperature of Type Ia PSC formation), T<192K (Type Ib), and T<190K (Type II) at 50mb pressure height are calculated in the region of 60-75 S during austral winter (Jul/Aug). The probabilities of occurrence for the three categories of temperature show increasing trends with time, whereas springtime stratospheric column ozone amount shows decreasing trend. During volcanically unperturbed years, the stratospheric column ozone in the spring is highly correlated with the probability of occurrence of temperature below 195 K in the previous winter (R=-0.88). The empirical relationship between stratospheric column ozone amount (O3) in the spring (Sep/Oct) and the probability (P) of occurrence of T<195K in the previous winter (Jul/Aug) is found to fit the equation of O3 = -360.38 x P + 446.38, which can be used to predict ozone depletion from the previous winter temperature.
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aerosol, hygroscopic growth factor, black carbon, ozone depletion, polar stratospheric clouds
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PhD
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Marine, Earth and Atmospheric Sciences
