Seasonal variations of ultra-fine and submicron aerosols in Taipei, Taiwan: implications for particle formation processes in a subtropical urban area
Other literature type
Cheung, H. C.
Chou, C. C.-K.
Lee, C. S. L.
The aim of this study is to investigate the seasonal variations in the
physicochemical properties of atmospheric ultra-fine particles (UFPs, <i>d</i> ≤ 100 nm) and submicron particles (PM<sub>1</sub>, <i>d</i> ≤ 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 PM<sub>1</sub>, 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 PM<sub>1</sub> was elevated during cold
seasons with a peak level of 18.5 µg m<sup>−3</sup> in spring, whereas the
highest concentration of UFPs was measured in summertime with a mean of 1.64 µg m<sup>−3</sup>.
Moreover, chemical analysis revealed that the UFPs and
PM<sub>1</sub> were characterized by distinct composition; UFPs were composed
mostly of organics, whereas ammonium and sulfate were the major constituents
of PM<sub>1</sub>. The seasonal median of total PNCs ranged from 13.9 × 10<sup>3</sup> cm<sup>−3</sup> in autumn to 19.4 × 10<sup>3</sup> cm<sup>−3</sup> in spring.
Median concentrations for respective size distribution modes peaked in
different seasons. The nucleation-mode PNC (<i>N</i><sub>4 − 25</sub>) peaked at
11.6 × 10<sup>3</sup> cm<sup>−3</sup> in winter, whereas the Aitken-mode
(<i>N</i><sub>25 − 100</sub>) and accumulation-mode (<i>N</i><sub>100 − 736</sub>) PNC exhibited summer maxima
at 6.0 × 10<sup>3</sup> and 3.1 × 10<sup>3</sup> cm<sup>−3</sup>,
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<sup>−1</sup> and
1.4 ± 0.8 cm<sup>−3</sup> s<sup>−1</sup>, respectively. The prevalence of new
particle formation (NPF) in summer was suggested as a result of seasonally
enhanced photochemical oxidation of SO<sub>2</sub> that contributed to the
production of H<sub>2</sub>SO<sub>4</sub>, and a low level of PM<sub>10</sub> (<i>d</i> ≤ 10 µm) that served as the condensation sink. Regarding the sources of aerosol
particles, correlation analysis of the PNCs against NO<sub><i>x</i></sub> 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.