1Department of Occupational Safety and Health, China Medical University, 91 Hsueh-Shih Road, Taichung 40402, Taiwan, Republic of China
2College of Public Health, Kent State University, 750 Hilltop Drive, Kent, OH 44240, USA
3Graduate Institute of Environmental Engineering, National Central University, 300 Jhongda Road, Jhongli City,Taoyuan County 32001, Taiwan, Republic of China
4Graduate Institute of Epidemiology and Preventive Medicine, National Taiwan University, 17 Xu-Zhou Road, Taipei 10020, Taiwan, Republic of China
5Department of Health Risk Management, China Medical University, 91 Hsueh-Shih Road, Taichung 40402, Taiwan, Republic of China
*currently at: Center for Environmental Science and Engineering, Department of Civil Engineering, Indian Institute of Technology, Kanpur-208016, Uttar Pradesh, India
Abstract. We present the first systematic analysis for new particle formation (NPF), growth and shrinkage of new particles at four different sites in subtropical central Taiwan. A total of 14 NPF events were identified from 137 days of ambient measurements during a cold and warm season. The measured formation rates of 10 nm particles (J10) and growth rates were in the range of 4.4–30 cm−3 s−1 and 7.4–24 nm h−1, respectively. The onset of NPF events coincided with decreases of condensation sink (CS) and increases of SO2 under enhanced atmospheric mixing and dilution. However, the lower or comparable SO2 on event days than on non-event days suggests that SO2 was not a limiting factor for NPF. On non-event days, the particle number concentrations were mostly driven by traffic emissions. We also observed shrinkage of new particles, the reversal of growth, during five out of the identified secondary formation. UFP particles events. In intense cases, the grown particles shrank back to the smallest measurable size of ~10 nm, thereby creating a unique "arch-like" shape in the size distribution contour plot. The particle shrinkage rates ranged from −5.1 to −7.6 nm h−1. The corresponding particle volume losses suggest that a notable fraction of the condensable species that contributed to growth was semi-volatile. The particle shrinkage was related to enhanced atmospheric dilution, high ambient temperature and low relative humidity, thus favoring the evaporation of semi-volatile species from the particulate phase to the gas phase. Our observations show that the new particle growth could be a reversible process, in which the evaporating semi-volatile species are important for the growth of new particles to sizes of environmental health concerns.