Role of Sulfates in Regional Cloud-Climate Interactions

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Title: Role of Sulfates in Regional Cloud-Climate Interactions
Author: Menon, Surabi
Advisors: Vinod Saxena, Chair
R.R. Braham, Jr., Co-Chair
S.P. Arya, Member
G.F. Watson, Member
J.J. DeLuisi, Member
Abstract: Aerosols affect the radiation budget of the earth-atmosphere system by directly reflecting or absorbing solar radiation and also indirectly, by altering the cloud albedo through changes in the cloud condensation nuclei concentration (CCN). Increases in CCN concentrations result in an increase in the cloud droplet number concentration (N). Assuming the cloud liquid water content (LWC) stays the same, this will result in smaller cloud droplet sizes. Thus, this will increase cloud reflectance and cloud lifetime as cloud cover also increases. An accurate quantification of the aerosol forcing effect is still not possible due to the complexity involved in understanding aerosol processes and their effects on climate. There has also been a lack of a coordinated effort toward linking surface and in situ observations, as well climate model results and satellite data. Due to the spatial and temporal heterogeneity in aerosol forcing, regional effects are important. In this dissertation, the direct and indirect radiative forcing effects of aerosols - primarily sulfates and to lesser extent soot aerosols at a site located in the southeastern U.S. are investigated by means of surface observations, modeling results and satellite data.During the summers of 1993-96, field experiments were conducted at Mt. Mitchell, North Carolina, at a site representative of the southeastern U.S. to determine the effect of pollutants on the cloud microphysical and optical properties. Analyses of the results from empirical relationships are used to obtain an estimate of the contribution of sulfates to indirect radiative forcing. Concurrent measurements of size resolved chemical concentrations, light scattering and absorption coefficients, aerosol size distribution and optical depth measurements were obtained during the winter of 1997 for cloud-free skies. Data from these measurements are used to investigate the chemical-physical-optical interaction between aerosols and to determine the direct forcing effect of aerosols by means of a column forcing model. Cloud water sulfate concentration is used as a measure of anthropogenic pollution. Back-trajectory analysis is used to identify the source of the air masses classified as polluted continental, continental and marine. The effect of anthropogenic pollution on cloud microphysical properties such as LWC, N, effective radii (Reff), CCN activation spectrum, cloud optical depth and reflectivity are investigated. The relationship between Reff and sulfate for different air masses, as well as the N-sulfate mass relationship, suggests that the counteracting effect of sulfates on greenhouse warming for the southeastern U.S. would be of a magnitude greater than -4.0 W m-2 obtained by previous modeling studies. Acidity variations between cloud droplets of different sizes indicated that on an average, smaller drops are enriched in sulfates, nitrates and ammonium, whereas, larger droplets have higher concentrations of sodium, calcium and magnesium. As part of a closure experiment cloud albedo calculated from in situ measurements was compared to that retrieved from the Advanced Very High Resolution Radiometer data for four years (1993-96). The nonlinear relationships between the cloud microphysical/optical properties and the sulfate content imply the existence of an optimum level for the sulfate concentration that would affect cloud albedo. In terms of the direct forcing effect, wintertime forcing obtained for an internal mixture of sulfate and soot aerosols is much lower than that obtained during summer, due to reduced sulfate concentrations in winter. A quantitative measure of the direct forcing indicates higher magnitudes both for summer and winter than is obtained from previous modeling results. Analyses of the direct and indirect radiative forcing effect of sulfates for the southeastern U.S. indicate that the negative forcing effect is of greater magnitude than is predicted by modeling results. Thus, reduction in sulfate emissions would have a significant impact on climate for the southeastern U.S.
Date: 1998-11-05
Degree: PhD
Discipline: Marine, Earth and Atmospheric Sciences

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