This study presents continued development of the Nenes and Seinfeld (2003) cloud droplet activation parameterization. First, we expanded the formulation to i) allow for a lognormal representation of aerosol size distribution, and, ii) include a size-dependant mass transfer coefficient for the growth of water droplets to accommodate the effect of size (and potentially organic films) on the droplet growth rate. The performance of the new scheme is evaluated by comparing the parameterized cloud droplet number concentration with that of a detailed numerical activation cloud parcel model. The resulting modified parameterization robustly and closely tracks the parcel model simulations, even for low values of the accommodation coefficient (average error 4.11.3%). The modifications to include the effect of accommodation coefficient do not increase the computational cost but substantially improves the parameterization performance. This work offers a robust, computationally efficient and first-principles approach for directly linking complex chemical effects (e.g., surface tension depression, changes in water vapor accommodation, solute contribution from partial solubility) on aerosol activation within a global climate modeling framework.

ABSTRACT Visibility degradation has become one of the major problems of public concern in Kaohsiung City as well as in most of the urban areas of Taiwan in recent years due to the increasing severity of smog. This study investigated the different aspects of atmospheric visibility degradation problems in metro Kaohsiung. First of all, both the long-term and short-term variation trends of atmospheric visibility were interpreted by analyzing the past data of prevailing visibility. Secondly, the correlation of atmospheric visibility with its major causative factors (i.e. meteorological and pollutant parameters) was established. Thirdly, the relationship between visibility degradation and aerosol mass/composition was derived by using multiple linear regression techniques based on in-situ field measurements of ambient aerosols and light extinction coefficient. Finally, the effective strategies for improving the visual air quality of metro Kaohsiung were proposed based on the results of the receptor-oriented modeling. In metro Kaohsiung, the seasonal variation of atmospheric visibility from the highest to the lowest were found to be in the sequence of summer, spring, autumn and winter, with mean values of 9.1, 8.2, 5.4, and 3.4 km, respectively. A diurnal variation of visibility was observed and showed that the visibility was generally lower in the morning and higher in the afternoon. A mass light scattering efficiency of 3.6 m2 g-1 for PM2.5 and a much lower value (0.3 m2 g-1) for PM2.5-10 indicated that the visible light was mainly scattered by the fine aerosol particles. The derived multiple linear regression model of light scattering coefficient yielded the mass scattering efficiencies of 4.6 m2 g-1 for (NH4)2SO4, 6.7 m2 g-1 for NH4NO3, 3.3 m2 g-1 for total carbon, and 3.2 m2 g-1 for PM2.5-remainder with an R2 of 0.97. On average, the percentage contributions of the visibility-degrading species to the light scattering coefficient were 29% for sulfates, 28% for nitrates, 22% for total carbon, and 21% for PM2.5-remainder, respectively. Furthermore, the major component of light extinction coefficient (bext) was the scattering of light by particles (75% of bext), followed by the absorption of light by particles (20%), while the remaining 5% of bext was attributed to gases. An empirical regression model of visibility based on sulfates, nitrates, PM2.5-remainder, and relative humidity was developed. The results showed that the variation of sulfate in PM2.5 aerosols was most sensitive to visibility change among the parameters. In terms of visibility degradation sources, source apportionment results indicated that the major contributors to fine particles were motor vehicle exhaust and secondary aerosols, which contributed more than half of the visibility degradation in metro Kaohsiung. Meanwhile, the second largest contributor was secondary aerosols containing ammonium sulfate and ammonium nitrate. Additionally, the contribution of soil dust increased markedly from normally only 4% to 25%, owing to an impact of continental dust storm from Mainland China. The results strongly indicated that soil dust blown from the desert areas of Northern China could be transported across the Yellow Sea and the East China Sea and evidently deposited in metro Kaohsiung. Results of the correlation analysis between atmospheric visibility and emission sources revealed a similarity between the source contribution pattern for visibility impairment and the source apportionment of fine particles. It showed ammonium sulfate contributed approximately 46% of the logarithm of atmospheric visibility, while the ¡§remainders¡¨, ammonium nitrate, and elemental carbons contributed about 20%, 17%, and 17%, respectively. Accordingly, this study concluded that the most effective strategy for improving atmospheric visibility in metro Kaohsiung was to prevent the formation of secondary fine particles containing ammonium sulfate and ammonium nitrate.

Global warming theories continue to overestimate their predictions of increased mean global temperatures (Hudson 1991). This would imply that some other influence is counteracting the global warming influences; i.e. a cooling effect. Cloud albedo characteristics are currently being researched to determine the impact clouds have on the net cooling of the atmosphere in relation to the global warming theory. These characteristics are influenced by the type, size, composition, and abundance of aerosol particles that act as cloud condensation nuclei. This study employs Tandem Differential Mobility Analyzer (TDMA) data collected in the vicinity of Japan during the Asian Aerosol Characterization Experiment (ACE-Asia) to investigate the influence of aerosol concentration and composition on the light scattering properties of clouds. Measurements of particle size (Dp), particle growth factor (GF), and relative humidity (RH) yield critical supersaturations (Sc) with the assumption that the soluble part of the particle is composed primarily of one substance. This indirect composition analysis allows us to determine whether the aerosol was internally mixed (particles have uniform composition and yield a single-peak distribution or similar growth factors) or externally mixed (different particles have different compositions yielding multiple peaks in the distribution). Through the use of calculated supersaturations, we can gain insight into cloud droplet activation properties of the samples for various aerosol types, which ultimately allows us to look at the influence of these particles on albedo characteristics of clouds formed by these particles.

Thesis (Ph. D.)--University of Washington, 2002. / Vita. Includes bibliographical references (leaves 184-197).

Thesis (Ph. D.)--University of Washington, 2001. / Vita. Includes bibliographical references (leaves 236-248).

Thesis (M.S.)--West Virginia University, 2003. / Title from document title page. Document formatted into pages; contains vii, 112 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 109-112).

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An optical microscope coupled to a flow cell was used to investigate ice nucleation on five atmospherically relevant mineral dusts at temperatures ranging from 233 to 247 K. Kaolinite and muscovite particles were found to be efficient ice nuclei in the deposition mode, requiring relative humidities with respect to ice (RHi) below 112% in order to initiate ice crystal formation. Quartz and calcite particles, by contrast, were poor ice nuclei, requiring relative humidities close to water saturation before ice crystals would form. Montmorillonite particles were efficient ice nuclei at temperatures below 241 K, but poor ice nuclei at higher temperatures. In several cases, there was a lack of quantitative agreement between these data and previously published work. This can be explained by several factors including mineral source, particle size, observation time and surface area available for nucleation. Heterogeneous nucleation rates (Jhet) were calculated from the onset data. Jhet values ranged from 60 to 1100 cm-²s-¹ for the five minerals studied. These values were then used to calculate contact angles (θ) for each mineral according to classical nucleation theory. The contact angles measured for kaolinite and muscovite ranged from 6 to 12º; for quartz and calcite the contact angles were much higher, ranging from 25 to 27º. The contact angles measured for montmorillonite were less than 15º at temperatures below 241 K, and above 20º at higher temperatures. The reported Jhet and θ values may allow for a more direct comparison between laboratory studies and can be used when modeling ice cloud formation in the atmosphere. The roles of H₂SO₄ and (NH4)₂SO₄ coatings on the ice nucleating properties of kaolinite were also investigated. Onset data was collected for H₂SO₄ coated and (NH4)₂SO₄ coated kaolinite particles at temperatures ranging from 233 to 247 K. In contrast to uncoated kaolinite particles, which were effective ice nuclei, H₂SO₄ coated particles were found to be poor ice nuclei, requiring relative humidities close to water saturation before nucleating ice at all temperatures studied. (NH4)₂SO₄ coated particles were poor ice nuclei at 245 K, but effective ice nuclei at 236 K.

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