Understanding the atmospheric new particle formation (NPF) process within the global range is important for revealing the budget of atmospheric aerosols and their impacts. We investigated the seasonal characteristics of NPF in the urban environment of Beijing. Aerosol size distributions down to ∼1∼1 nm and H 2 SO 4 concentration were measured during 2018-2019.2018-2019. The observed formation rate of 1.5 nm particles (J 1.5 ) is significantly higher than those in the clean environment, e.g., Hyytiala;Hyytiala, whereas the growth rate is relatively lower. Both J 1.5 and NPF frequency in urban Beijing showed a clear seasonal variation with maxima in winter and minima in summer, while the observed growth rates were generally within the same range around the year. We show that ambient temperature is a governing factor driving the seasonal variation of J 1.5 . In contrast, the condensation sink showed no significant seasonal variation during the NPF periods and the daily maximum H 2 SO 4 concentration was slightly higher in summer than that in winter. In all four seasons, condensation of H 2 SO 4 and (H 2 SO 4 ) nn (amine) nn clusters contributes significantly to the growth rates in the sub-3 nm size range, whereas it is less important for the observed growth rates of particles above 3 nm. Therefore, other species are always needed for the growth of larger particles.

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 , 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 are sulfuric acid concentration, the growth enhancement factor characterized by contribution of other gaseous precursors to particle growth, , and A Fuchs . Different from other atmospheric environments, such as in Boulder and Hyytiälä, the dailymaximum sulfuric acid concentration and 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 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 µm 2 cm −3 . NPF events were suppressed due to the coagulation scavenging when A Fuchs was greater than 200 µm 2 cm −3 . Measured A Fuchs in Beijing had a good correlation with its PM 2.5 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 PM 2.5 mass concentration.

Gas-phase oxygenated organic molecules (OOMs) can contribute substantially to the growth of newly formed particles. However, the characteristics of OOMs and their contributions to particle growth rate are not well understood in urban area, which has complex anthropogenic emissions and atmospheric conditions. We performed long-term measurement of gas-phase OOMs in urban Beijing during 2018-2019 using nitrate-based chemical ionization mass spectrometry. OOM concentrations showed clear seasonal variations, with the highest in summer and the lowest in winter. Correspondingly, calculated particle growth rates due to OOM condensation were highest in summer, followed by spring, autumn, and winter. One prominent feature of OOMs in this urban environment was a high fraction (~75%) of nitrogen-containing OOMs. These nitrogen-containing OOMs contributed only 50%-60% of the total growth rate led by OOM condensation, owing to their slightly higher volatility than non-nitrate OOMs. By comparing the calculated condensation growth rates and the observed particle growth rates, we showed that sulfuric acid and its clusters are the main contributors to the growth of sub-3 nm particles, with OOMs significantly promoting the growth of 3-25 nm particles. In winter Beijing, there are missing contributors to the growth of particles above 3 nm, which remain to be further investigated.