Seasonal variations of ultra-fine and submicron aerosols in Taipei, Taiwan: implications for particle formation processes in a subtropical urban area

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 (N_4 − 25) peaked at 11.6  ×  10³ cm⁻³ in winter, whereas the Aitken-mode (N_{25 − 100}) and accumulation-mode (N_{100 − 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 NO_x 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.