No abstract

Secondary aerosols formed from anthropogenic pollutants and natural emissions have substantial impacts on human health, air quality, and the Earth's climate. New particle formation (NPF) contributes up to 70% of the global production of cloud condensation nuclei (CCN), but the effects of biogenic volatile organic compounds (BVOCs) and their oxidation products on NPF processes in forests are poorly understood. Observations show that isoprene, the most abundant BVOC, suppresses NPF in forests. But the previously proposed chemical mechanism underlying this suppression process contradicts atmospheric observations. By reviewing observations made in other forests, it is clear that NPF rarely takes place during the summer when emissions of isoprene are high, even though there are sufficient concentrations of monoterpenes. But at present it is not clear how isoprene and its oxidation products may change the oxidation chemistry of terpenes and how NOx and other atmospheric key species affect NPF in forest environments. Future laboratory experiments with chemical speciation of gas phase nucleation precursors and clusters and chemical composition of particles smaller than 10 nm are required to understand the role of isoprene in NPF. Our results show that climate models can overpredict aerosol's first indirect effect when not considering the absence of NPF in the southeastern U.S. forests during the summer using the current nucleation algorithm that includes only sulfuric acid and total concentrations of low‐volatility organic compounds. This highlights the importance of understanding NPF processes as function of temperature, relative humidity, and BVOC compositions to make valid predictions of NPF and CCN at a wide range of atmospheric conditions.

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.