Nucleation And Growth Of Sub-3nm Particles In The Polluted Urban Atmosphere Of A Megacity In China

Particle size distribution down to 1.4 nm was measured in the urban atmosphere of Nanjing, China, in spring, summer, and winter during 2014-2015. Sub-3 nm particle event, which is equivalent to nucleation event, occurred on 42 out of total 90 observation days, but new particles could grow to cloud condensation nuclei (CCN)-active sizes on only 9 days. In summer, infrequent nucleation was limited by both unfavorable meteorological conditions (high temperature and relative humidity -RH) and reduced anthropogenic precursor availability due to strict emission control measures during the 2014 Youth Olympic Games in Nanjing. The limiting factors for nucleation in winter and spring were meteorological conditions (radiation, temperature, and RH) and condensation sink, but for the further growth of sub3 nm particles to CCN-active sizes, anthropogenic precursors again became limiting factors. Nucleation events were strong in the polluted urban atmosphere. Initial J(1.4) at the onset and peak J 1 : 4 at the noontime could be up to 2.1 X 10(2) and 2. 5 X 10(3) cm(-3) s(-1), respectively, during the eight nucleation events selected from different seasons. Time-dependent J(1.4) usually showed good linear correlations with a sulfuric acid proxy for every single event (R-2 = 0.56-0.86, excluding a day with significant nocturnal nucleation), but the correlation among all eight events deteriorated (R-2 = 0.17) due to temperature or season change. We observed that new particle growth rate (GR) did not increase monotonically with particle size, but had a local maximum up to 25 nm h(-1) between 1 and 3 nm. The existence of local maxima GR in sub3 nm size range, though sensitive to measurement uncertain-ties, gives new insight into cluster dynamics in polluted environments. In this study such growth rate behavior was interpreted as the solvation effect of organic activating vapor in newly formed inorganic nuclei.