Atmos. Chem. Phys. Discuss., 13, 8985-9016, 2013
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Characterization of ultrafine particle number concentration and new particle formation in urban environment of Taipei, Taiwan
H. C. Cheung, C. C.-K. Chou, W.-R. Huang, and C.-Y. Tsai
Research Center for Environmental Changes, Academia Sinica, Taipei 11529, Taiwan

Abstract. An intensive aerosol characterization experiment was performed at the Taipei Aerosol and Radiation Observatory (TARO, 25.02° N, 121.53° E) in the urban area of Taipei, Taiwan during July 2012. Number concentration and size distribution of aerosol particles were measured continuously, which were accompanied by concurrent measurements of mass concentration of submicron particles, PM (d ≤ 1 μm), and photolysis rate of ozone, J(O1D). The averaged number concentrations of total (Ntotal), accumulation mode (Nacu), Aitken mode (Ntotal), and nucleation mode (Nnuc) particles were 7.6 × 103 cm−3, 1.2 × 103 cm−3, 4.4 × 103 cm−3, and 1.9 × 103 cm−3, respectively. Accordingly, the ultrafine particles (UFPs, d ≤ 100 nm) accounted for 83% of the total number concentration of particles measured in this study (10 ≤ d ≤ 429 nm), indicating the importance of UFPs to the air quality and radiation budget in Taipei and its surrounding areas. An averaged Nnuc/NOx ratio of ~60 cm−3 ppbv−1 was derived from nighttime measurements, which was suggested to be the characteristic of vehicle emissions that contributed to the "urban background" of nucleation mode particles throughout a day. On the contrary, it was found that the number concentration of nucleation mode particles was independent of NOx and could be elevated up to 10 times the "urban background" levels during daytime, suggesting a substantial amount of nucleation mode particles produced from photochemical processes. Consistency in the time series of the nucleation mode particle concentration and the proxy of H2SO4 production, UVB·SO2, for new particle formation (NPF) events showed that photo-oxidation of SO2 was responsible for the formation of new particles in our study area. Moreover, analysis upon the diameter growth rate, GR, and formation rate of nucleation mode particles, J10−25, found that the values of GR (8.5 ± 6.8 nm h−1) in Taipei were comparable to other urban areas, whereas the values of J10−25 (2.2 ± 1.2 cm−3 s−1) observed in this study were around the low end of the range of new particle formation rate reported by previous investigations. It was revealed that the particle growth rate correlated exponentially with the photolysis of ozone, suggesting the condensable vapors were produced mostly from photo-oxidation reactions. In addition, this study also revealed that both GR and J10−25 exhibited quadratic relationship with the number concentration of particles. The quadratic relationship was inferred as a result of aerosol dynamics and featured NPF process in urban areas.

Citation: Cheung, H. C., Chou, C. C.-K., Huang, W.-R., and Tsai, C.-Y.: Characterization of ultrafine particle number concentration and new particle formation in urban environment of Taipei, Taiwan, Atmos. Chem. Phys. Discuss., 13, 8985-9016, doi:10.5194/acpd-13-8985-2013, 2013.
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