1Department of Occupational Safety and Health, China Medical University, 91 Hsueh-Shih Road, Taichung 40402, Taiwan
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
4Graduate Institute of Epidemiology and Preventive Medicine, National Taiwan University, 17 Xu-Zhou Road, Taipei 10020, Taiwan
5Department of Health Risk Management, China Medical University, 91 Hsueh-Shih Road, Taichung 40402, Taiwan
Abstract. We present the first systematic analysis for new particle formation (NPF), growth and shrinkage of new particles observed at four different sites in subtropical Central Taiwan. A total of 14 NPF events were identified during 137 days of ambient measurements during a cold and warm season. The derived nucleation rates of 1 nm particles (J1) and growth rates were in the range of 39.6–252.9 cm−3 s−1 and 6.5–14.5 nm h−1, respectively. The NPF events occurred on days either with low condensation sink (CS), increased morning traffic emissions and the breakup of nocturnal inversion layer (type A), or with high CS, minimum levels of primary traffic emissions and enhanced atmospheric dilution (type B). On non-event days, the particle number concentrations were mostly driven by traffic emissions. We have also observed shrinkage of new particles (type A-S and B-S), reversal of growth, during five out of the 14 NPF events. In intense shrinkage 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 ratios of shrinkage-to-growth rates were mostly in the range of 0.40–0.65, suggesting that a large fraction of the condensable species that contributed to growth were likely semi-volatile. The particle shrinkage was related to air masses with low CS due to atmospheric dilution, high ambient temperature and low relative humidity and such atmospheric conditions may have facilitated the evaporation of semi-volatile species from the particles to the gas phase. Our observations show that the new particle growth may be a~reversible process and the evaporating semi-volatile species are important for the growth of new particles to cloud condensation nuclei sizes.