Abstract
Abstract. 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 µg m−3 in spring, whereas the highest concentration of UFPs was measured in summertime with a mean of 1.64 µg m−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 nucleation-mode 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.
Highlights
Due to the significant impact of particulate matter on human health and climate change, it is vital to understand the formation process of atmospheric particles (Charlson et al, 1992; Donaldson et al, 1998)
The results suggested that the influences of local vehicle emission on particle number concentration (PNC) were still in place; whereas growth of particles due to secondary production of condensable vapors could have been suppressed, as new particle formation (NPF) was rarely observed during the longrange transport (LRT) events
The mass concentration and chemical composition of ultrafine particles (UFPs) and submicron particles (i.e., PM1) as well as the particle number concentration (PNC) and size distributions (PSD) with size ranging from 4 to 736 nm were measured during four seasonal campaigns in the period from October 2012 to August 2013 at TARO, a subtropical urban aerosol station in Taipei, Taiwan
Summary
Due to the significant impact of particulate matter on human health and climate change, it is vital to understand the formation process of atmospheric particles (Charlson et al, 1992; Donaldson et al, 1998). Investigations were carried out on new particle formation within the urban boundary layer (e.g., Cheung et al, 2013, and references therein), where particle formation was suggested to be mainly influenced by the photooxidation of SO2. Previous investigations have indicated that the air pollutants, in both gaseous and particulate form, associated with the continental outflows of air masses, could have affected a wide region in east Asia and caused severe regional air pollution (e.g., Lin et al, 2004; Wang et al, 2003; Buzorius et al, 2004). The formation processes of ultra-fine particles (UFPs, d ≤ 100 nm) and submicron particles (PM1, d ≤ 1 μm) under the influences of continental outflows are not yet well understood
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