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Abstract The aim of this study was to chemically characterize the fine particulate matter (PM 2.5) at a subtropical forest in East Asia under the influences of anthropogenic and biogenic sources and a complex topographic setting. Four seasonal campaigns were conducted at the Xitou Experimental Forest in central Taiwan from the winter of 2013 to the autumn of 2014. The results indicated that the ambient levels and chemical features of PM 2.5 exhibited pronounced seasonal variations. Non-sea-salt sulfate (nss-SO 4 2-) constituted the major component of PM 2.5 , followed by ammonium (NH 4 +) and nitrate (NO 3 −) during winter, summer and autumn. However, it was revealed that the mass fraction of NO 3 − increased to be comparable with that of nss-SO 4 2- in springtime. The mass contribution of secondary organic carbon (SOC) to PM 2.5 peaked in summer (13.2%), inferring the importance of enhanced photo-oxidation reactions in SOC formation. Diurnal variations of O 3 and SO 2 coincided with each other, suggesting the transport of aged pollutants from distant sources, whereas CO and NO x were shown to be under the influences of both local and regional sources. Notably high sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) were observed, which were 0.93 ± 0.05 and 0.39 ± 0.20, respectively. Precursor gases (i.e. SO 2 and NO x) could be converted to sulfate and nitrate during the transport by the uphill winds. Furthermore, due to the high relative humidity at Xitou, enhanced aqueous-phase and/or heterogeneous reactions could further contribute to the formation of sulfate and nitrate at the site. This study demonstrated the significant transport of urban pollutants to a subtropical forest by the mountain-valley circulations as well as the long-range transport from regional sources, whereas the implications of which for regional climate change necessitated further investigation. Graphical abstract Image 1 Highlights • Distinct seasonal variations are observed in PM 2.5 composition at forest site. • Notably high nitrate concentrations (6.19 μg/m3) are found in spring. • Significant secondary organic carbon (up to13.2% in summer) in PM 2.5 • The sulfur and nitrogen oxidation ratios are 0.93 and 0.39 Four measurement campaigns were conducted at Xitou Experimental Forest in central Taiwan to characterize the fine particulate matter (PM 2.5) at a high-elevation subtropical forest in East Asia where pollutants were effectively transported by mountain-valley circulations, and long-range transport from regional sources. [ABSTRACT FROM AUTHOR]

The aim of this study is to investigate the seasonal variations in the physicochemical properties of atmospheric ultra-fine particles (UFPs, d ≤ 100 nm) and submicron particles (PM1, d ≤ 1 μm) in an east Asian urban area, which are hypothesized to be affected by the interchange of summer and winter monsoons. An observation experiment was conducted at TARO (Taipei Aerosol and Radiation Observatory), an urban aerosol station in Taipei, Taiwan, from October 2012 to August 2013. The measurements included the mass concentration and chemical composition of UFPs and PM1, as well as the particle number concentration (PNC) and the particle number size distribution (PSD) with size range of 4-736 nm. The results indicated that the mass concentration of PM1 was elevated during cold seasons with a peak level of 18.5 μgm-3 in spring, whereas the highest concentration of UFPs was measured in summertime with a mean of 1.64 μgm-3. Moreover, chemical analysis revealed that the UFPs and PM1 were characterized by distinct composition; UFPs were composed mostly of organics, whereas ammonium and sulfate were the major constituents of PM1. The seasonal median of total PNCs ranged from 13.9×10³ cm-3 in autumn to 19.4×103 cm-3 in spring. Median concentrations for respective size distribution modes peaked in different seasons. The nucleationmode PNC (N4-25) peaked at 11.6×10³ cm-3 in winter, whereas the Aitken-mode (N25-100) and accumulation-mode (N100-736) PNC exhibited summer maxima at 6.0×10³ and 3.1×10³ cm-3, respectively. The change in PSD during summertime was attributed to the enhancement in the photochemical production of condensable organic matter that, in turn, contributed to the growth of aerosol particles in the atmosphere. In addition, clear photochemical production of particles was observed, mostly in the summer season, which was characterized by average particle growth and formation rates of 4.0×1.1 nm h-1 and 1.4×0.8 cm-3 s-1, respectively. The prevalence of new particle formation (NPF) in summer was suggested as a result of seasonally enhanced photochemical oxidation of SO2 that contributed to the production of H2SO4, and a low level of PM10 (d ≤ 10 μm) that served as the condensation sink. Regarding the sources of aerosol particles, correlation analysis of the PNCs against NOx revealed that the local vehicular exhaust was the dominant contributor of the UFPs throughout the year. Conversely, the Asian pollution outbreaks had significant influence in the PNC of accumulation-mode particles during the seasons of winter monsoons. The results of this study implied the significance of secondary organic aerosols in the seasonal variations of UFPs and the influences of continental pollution outbreaks in the downwind areas of Asian outflows. [ABSTRACT FROM AUTHOR]