Chemical Compositions and Particle Number Size Distributions of Remote Coastal Atmospheric Aerosol

Autor: CHEN, YU-PING., 陳鈺萍
Rok vydání: 2017
Druh dokumentu: 學位論文 ; thesis
Popis: 105
This study is focused on the chemical composition and the particle number size concentration of atmospheric particulate matter (PM) in the coastal region of Hengchun Peninsula to understand the aerosol chemistry and particle number characteristics in the remote region of the southernmost tip of Taiwan, which is far away from any specific pollution sources. The aerosol chemical characteristics include the composition of inorganic salts, carbohydrates, sugar alcohols, anhydrosugars, and organic acids. The particle number characteristics include the number concentrations and the particle number size distribution. The average mass concentration of PM in Hengchun coastal area is 25.59±12.27 μg/m3. The concentration of species in PM is in the following order: SO42- > Cl- > Na+ > NO3- > NH4+; the corresponding concentrations of these five species are 3.46 ± 2.18 μg/m3, 2.15 ± 0.98 μg/m3, 2.14 ± 1.37 μg/m3, 1.50 ± 0.77 μg/m3 and 1.27 ± 1.15 μg/m3 respectively. The main distributions of the species are the primary coarse mode and the photochemical submicron-droplet mode. This indicates sea spray and photochemical products significantly contribute to remote coastal aerosol. Generally, K+ and levoglucosan are regarded as molecular indicators of biomass burning aerosol, however the particle size peaks of K+ concentration are mainly at 6.2 μm, 1.8 μm, and 0.54 μm, whereas although levoglucosan showed up in the particle size peaks of submicron droplet mode, its concentration peaks differed from those of K+. This indicated that despite the slightly presence of long-range transport of biomass burning aerosols in the coast atmosphere environment during the period of study, there is no prominent burning in close range. Moreover, coarse mode K+ also indicated that K+ in the coastal region came from sea spray and crustal matters. Among the organic acids of remote coastal aerosol, oxalate appeared the highest concentration. Oxalate is the final product of dicarboxylates; it constitutes 35.4 ± 26.7% of the total carboxylates, followed by acetate, the most important monoacid, which accounts for 28.9 ± 21.9% of the total carboxylates. Non-sea salt sulfate (nss-SO42-) accounted for 11.58 ± 7.62% of the PM mass concentration. The correlation coefficient, r, between oxalate and nss-SO42- was as high as 0.919 and the peak concentration of both was generated in the submicron droplet size mode at 0.54 μm. However, in the 0.32 ~ 1.0 μm particle size range, the average oxalate/nss-SO42- ratio is as high as 15.7%, far exceeding the average 5.88% of suburban southern Taiwan. This indicated that the photo-generation potential of carboxylates in the submicron droplet size mode was higher than that of inorganic sulfate. In addition, a peak in aerosol number concentration was observed at dawn and at dusk, occurring respectively at 7 AM and 4 PM. The total number concentrations were 2,571 ±1,440 # cm-3 and 2,399 ±262 # cm-3 respectively. During the day, the ocean breeze brings in fresh ocean air and a higher mixed layer is resulted in, especially at noon. The total number concentration dropped to the daily low of 951 ±453 # cm-3 from noon to 1 PM. The highest number concentration of the day was 2.7 times of the lowest number concentration of the day. The mode diameter of 75.7 nm at 4 PM was close to that at 7 AM, but the corresponding mode concentration was higher. This showed that the contributing sources of the particle number concentration were similar and both were related to the small scale local traffic exhaust transport. An overview of the aerosol chemistry composition and particle number characteristics showed that the main contributing sources of the remote atmospheric particulates in Hengchun area were sea spray and photochemical products, as well as long-range transport of biomass burning.
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