Atmospheric observations of new particle growth and shrinkage

Abstract

In subtropical central Taiwan, a total of fourteen new particle formation (NPF) events were identified at four sites that represent urban, coastal, mountain and downwind area, respectively. Among them, there were five particle shrinkage events showing the grown particles shrank back to the smallest measureable 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 strong 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.