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

Abstract. 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  ∼  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_Γ, 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_Γ are sulfuric acid concentration, the growth enhancement factor characterized by contribution of other gaseous precursors to particle growth, Γ, and A_Fuchs. Different from other atmospheric environments, such as in Boulder and Hyytiälä, the daily-maximum sulfuric acid concentration and Γ 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_Γ in Beijing (R² = 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 µm² cm⁻³. NPF events were suppressed due to the coagulation scavenging when A_Fuchs was greater than 200 µm² cm⁻³. Measured A_Fuchs in Beijing had a good correlation with its PM_2.5 mass concentration (R² = 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 PM_2.5 mass concentration.