Aerosol surface area concentration: a governing factor in new particle formation in Beijing

The predominating role of aerosol Fuchs surface area, A(Fuchs), in determining the occurrence of new particle formation (NPF) events in Beijing was elucidated in this study. The analysis was based on a field campaign from 12 March to 6 April 2016 in Beijing, during which aerosol size distributions down to similar to 1 nm and sulfuric acid concentrations were simultaneously monitored. The 26 days were classified into 11 typical NPF days, 2 undefined days, and 13 non-event days. A dimensionless factor, L-Gamma, characterized by the relative ratio of the coagulation scavenging rate over the condensational growth rate (Kuang et al., 2010), was applied in this work to reveal the governing factors for NPF events in Beijing. The three parameters determining L-Gamma are sulfuric acid concentration, the growth enhancement factor characterized by contribution of other gaseous precursors to particle growth, Gamma, and A(Fuchs). Different from other atmospheric environments, such as in Boulder and Hyytiala, the daily-maximum sulfuric acid concentration and Gamma in Beijing varied in a narrow range with geometric standard deviations of 1.40 and 1.31, respectively. A positive correlation between the estimated new particle formation rate, J(1.5), and sulfuric acid concentration was found with a mean fitted exponent of 2.4. However, the maximum sulfuric acid concentrations on NPF days were not significantly higher (even lower, sometimes) than those on non-event days, indicating that the abundance of sulfuric acid in Beijing was high enough to initiate nucleation, but may not necessarily lead to NPF events. Instead, A Fuchs in Beijing varied greatly among days with a geometric standard deviation of 2.56, whereas the variabilities of A Fuchs in Tecamac, Atlanta, and Boulder were reported to be much smaller. In addition, there was a good correlation between A Fuchs and L-Gamma in Beijing (R-2 = 0.88). Therefore, it was A Fuchs that fundamentally determined the occurrence of NPF events. Among 11 observed NPF events, 10 events occurred when A(Fuchs) was smaller than 200 mu m(2) cm(-3). NPF events were suppressed due to the coagulation scavenging when A(Fuchs) was greater than 200 mu m(2) cm(-3). Measured A(Fuchs) in Beijing had a good correlation with its PM2.5 mass concentration (R-2 = 0.85) since A Fuchs in Beijing was mainly determined by particles in the size range of 50-500 nm that also contribute to the PM2.5 mass concentration.