10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Seasonal Variations of Mass Concentration and Chemical Composition of Fine Particulate Matter in a High-Elevation Subtropical Forest in East Asia – Impact of Anthropogenic and Biogenic Emissions
CELINE SIU LAN LEE, Charles C.K. Chou, Chien-Cheng Jung, Joe Hing Cho Cheung, Chao-Yang Tsai, RCEC, Academia Sinica, Taiwan, R.O.C.
Abstract Number: 482 Working Group: Aerosol Transport and Transformation
Abstract The aim of this study was to characterize the fine particulate matter (PM2.5) in a subtropical forest in East Asia under the influences of anthropogenic and biogenic sources and a complex topographic setting. A one-year measurement campaign was conducted in Xitou forest in central Taiwan to measure the mass concentrations of PM2.5 and its components, including ionic (Ca2+, Na+, NH4+, K+, Mg2+, Cl-, NO3- and SO42-) and carbonaceous (OC and EC) species. The aerosol chemical composition exhibited distinct seasonal variation. Non-sea-salt sulfate (nss-sulfate) constituted the major component of PM2.5, followed by NH4+ and NO3- during winter, summer and autumn, while nss-SO42- and NO3- contributed in similar mass fraction to PM2.5 in spring season. The results showed that (NH4)2SO4 was preferably formed, but a larger fraction of NH4NO3 was formed due to existence of strong NOx sources in spring, which could be originated from local traffic exhaust or industrial sources. Higher secondary organic carbon was observed in summer, due to the enhanced photo-oxidation of biogenic precursors during summertime. Significant amount of K+ was present which could be attributed to biomass burnings of straws and/or municipal waste incineration. The transport of pollutants from the urban sources was found to be critically driven by the mountain-valley circulation, where polluted air mass was uplifted and transported by the northwesterly winds (valley breezes) during the daytime, and relatively clean air mass was transported by the southeasterly winds (mountain breezes) during the nighttime. The diurnal variations of O3 and SO2 coincided with each other, suggesting that the pollutants are aged which were transported from distant sources, whereas CO and NOx were shown to be under the influences of both local and regional emission sources. The sulfur oxidation ratio (SOR) for PM2.5 measured in this study (0.878-0.952) was notably higher compared to that obtained in other urban sites (e.g. 0.12–0.39 in Beijing, 0.19 in Tianjin, and 0.22-0.62 in Italy). It is suggested that the aqueous-phase oxidation of SO2 could be strongly enhanced under high RH condition (average RH of 87.1-92.1% for different seasons) of subtropical climate at the study site to form sulfate and hence attributed to the exceptionally high sulfur oxidation state. The nitrogen oxidation ratio (NOR) observed in this study (0.058-0.481) was also found to be higher than that observed in other urban sites (e.g. 0.04-0.08 in Beijing, 0.19 in Tianjin and 0.01-0.08 in Italy), indicating significant amount of NO2 is converted to NO3-. This study exemplified the significant impact of anthropogenic pollutants from a megacity (with population over 2 million) and its suburbs, where intense industrial and biomass burning activities have occurred, on a high-elevation subtropical forest which itself is an important source of biogenic VOCs. The results showed that the interaction of both anthropogenic and biogenic pollutants have yielded significant amounts of secondary aerosols, mostly in the form of sulfate, followed by nitrate and other secondary organics products. The anthropogenic pollutants transported to the site have altered the atmospheric processing involved in particle formation in the forest, resulting in significant pollution which requires attention and further investigation.