To date, few comprehensive field observations of new particle formation?(NPF) have been carried out at mountaintop sites in China. In this study, simultaneous measurements of particle size distribution, trace gases, meteorological parameters, and mass concentration and chemical composition of PM sub2.5/sub were performed at the summit of Mt.?Tai (1534?m?a.s.l.) from 25?July to 24?August?2014 (Phase?I), 21?September to 9?December?2014 (Phase?II), and 16?June to 7?August?2015 (Phase?III) to investigate characteristics and favorable conditions of NPF in a relatively clean mountaintop environment. The NPF events were identified based on particle size distribution measured by the neutral cluster and air ion spectrometer?(NAIS), and 66?such events were observed during a period of 164?days – corresponding to an occurrence frequency of 40?%. The formation rates of 3?nm particles?( Jsub3/sub ) and growth rates were in the ranges of 0.82–25.04?cm sup?3/sup s sup?1/sup and 0.58–7.76?nm?h sup?1/sup , respectively. On average, the condensation sink?(CS), O sub3/sub concentration, air temperature, and relative humidity were lower, whereas the SO sub2/sub concentration was higher on NPF days than that on non-NPF days. The CS on Mt. Tai was at a low level and lower CS was critical for NPF. NPF events were common when wind came from the east-southeast and west-southwest, which was probably associated with relatively lower CS in the east-southeast and higher SO sub2/sub concentration in the west-southwest. O sub3/sub was not a governing factor for NPF in this study, and a high level of NO subx/sub concentration might be responsible for the decreased O sub3/sub concentration on NPF days. Three categories of backward trajectories were classified, among which the continental air mass was the majority. The continental air mass passing through more polluted areas (denoted as Type?I) favored NPF because of enhanced SO sub2/sub concentration and potential ammonia with it. An in-depth analysis of SO sub2/sub indicated that sulfuric acid was a dominant precursor on Mt.?Tai; meanwhile, biogenic organics released from ambient forests in warm seasons and anthropogenic volatile organic compounds emitted from domestic heating in cold seasons also promoted?NPF.
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