THESIS
1998
xiii, 121 leaves : ill. ; 30 cm
Abstract
The chemical compositions and size distributions of ambient aerosols are important factors in understanding their formation, transformation, removal processes and impacts to human health and environment. Inorganic species such as sulfate and nitrate account for a significant portion of ambient particles (more than 35% of the Respiratory Suspended Particles in Hong Kong). In this study, concentrations and size distributions of the water soluble inorganic ions (SO
42-, NO
3-, Cl
-, NH
4+, Na
+, K
+, Ca
2+ and Mg
2+) in ambient aerosols were investigated at a coastal site in Hong Kong, based on 33 days (1996 - 1997) of sampling experiments using a MOUDI cascade impactor. The effects of aerosol formation processes, interactions of the inorganic aerosols with gaseous species (HNO
3, H
2SO
4 and SO
2), a...[
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The chemical compositions and size distributions of ambient aerosols are important factors in understanding their formation, transformation, removal processes and impacts to human health and environment. Inorganic species such as sulfate and nitrate account for a significant portion of ambient particles (more than 35% of the Respiratory Suspended Particles in Hong Kong). In this study, concentrations and size distributions of the water soluble inorganic ions (SO
42-, NO
3-, Cl
-, NH
4+, Na
+, K
+, Ca
2+ and Mg
2+) in ambient aerosols were investigated at a coastal site in Hong Kong, based on 33 days (1996 - 1997) of sampling experiments using a MOUDI cascade impactor. The effects of aerosol formation processes, interactions of the inorganic aerosols with gaseous species (HNO
3, H
2SO
4 and SO
2), and the meteorological conditions (temperature, humidity and prevailing wind direction) on the size distributions were also studied.
Sulfate was mainly distributed in the fine mode (Dp [is less than to] 2 μm), in coexistence with arnmonium. Two fine modes have been found: the condensation mode and the droplet mode of mode diameter of 0.2 and 0.6 μm, respectively. It was found that high concentrations of droplet mode sulfate was related to the weather conditions of high humidity and the existence of low clouds. Transport of aerosols from China by the northeasterly monsoon also increased the sulfate concentration in Hong Kong.
Nitrate was mainly distributed in the coarse particles with mode diameter of 3.95 ± 0.69 μm. Non-seasalt sulfate was also found in the coarse mode. Other major species in the coarse mode were Na
+, Cl
- and Ca
2+ which are attributed to the seasalt and soil particles. Chloride depletion from seasalt aerosols was found in all the samples as a consequence of the reaction of acidic species with seasalt particles, which is also the major path for the formation of coarse mode nitrate and non-seasalt sulfate at this site. Chloride was depleted more efficiently in small particles, which supports the surface reaction mechanism of the chloride depletion reaction.
Since Hong Kong is influenced by both marine and continental aerosols, coarse mode nitrate and sulfate distributions depend on the weather conditions. When Hong Kong was under prevailing easterly wind and with high relative humidity, Na
+ was the dominant species and significant chloride depletion (74% to 88%) from seasalt aerosols was observed. HNO
3 was the major reactant resulting in chloride depletion under this condition. As a result, nitrate distributions were similar to Na
+. However, significant sulfate was also found to be associated with seasalt particles and accounted for 11% to 29% of the chloride depletion in the heavy pollution period with high concentration of acidic sulfate. When Hong Kong was influenced by a northesterly monsoon, Ca
2+ was the dominant species in the coarse mode, and most of the nitrate was found to be associated with Ca
2+ in soil particles.
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