Seasonality of ultrafine and sub-micron aerosols and the inferences on particle formation processes

Abstract

The aim of this study is to investigate the seasonal variations in the physicochemical properties of atmospheric ultrafine particles (UFPs, d ≤ 100nm) and submicron particles (PM 1, 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 the TARO, 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 PM 1, as well as the particle number concentration (PNC) and size distribution (PSD) with size range of 4–736 nm. The results indicate that the mass concentration of PM 1 was elevated during cold seasons with peak level of 18.5 μg m -3 in spring, whereas the highest UFPs concentration was measured in summertime with a seasonal mean of 1.62 μg m -3. Moreover, chemical analysis revealed that the UFPs and PM 1 were characterized by distinct composition; UFPs were composed mostly of organics, whereas ammonium and sulfate were the major constituents in PM 1. The seasonal median of total PNCs ranged from 13.9 × 10 3 cm -3 in autumn to 19.4 × 10 3 cm -3 in spring. The PSD information retrieved from the corresponding PNC measurements indicates that the nucleation mode PNC ( N 4–25) peaked at 11.6 × 10 3 cm -3 in winter, whereas the Aitken mode ( N 25–100) and accumulation mode ( N 100–736) exhibited summer maxima at 6.0 × 10 3 and 3.1 × 10 3 cm -3, respectively. The shift in PSD during summertime is attributed to the enhancement in the photochemical production of condensable organic matter that, in turn, contributes to the growth of aerosol particles in the atmosphere. In addition, remarkable photochemical production of particles was observed in spring and summer seasons, which was characterized with averaged particle growth and formation rates of 4.3 ± 0.8 nm h -1 and 1.6 ± 0.8 cm -3 s -1, respectively. The prevalence of new particle formation (NPF) in summer is suggested as a result of seasonally enhanced photochemical oxidation of SO 2, which contributes to the production of H 2 SO 4, and low level of PM 10 ( d ≤ 10 μm) that serves as the condensation sink. Regarding the sources of aerosol particles, correlation analysis upon the PNCs against NO x revealed that the local vehicular exhaust was the dominant contributor of the UFPs throughout a year. On the contrary, the Asian pollution outbreaks can have significant influence in the PNC of accumulation mode particles during the seasons of winter monsoons. The results of this study underline 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.