A puzzle of new particles: Atmospheric particulates can be produced by emissions or form de novo. New particle formation usually occurs in relatively clean air. This is because preexisting particles in the atmosphere will scavenge the precursors of new particles and suppress their formation. However, observations in some heavily polluted megacities have revealed substantial rates of new particle formation despite the heavy loads of ambient aerosols. Yao et al. investigated new particle formation in Shanghai and describe the conditions that make this process possible. The findings will help inform policy decisions about how to reduce air pollution in these types of environments. Science , this issue p. 278

Based on our comprehensive observations in Beijing, we show that 80–90% of PM _2.5 was formed via atmospheric reactions during haze days and over 65% of the number concentration of haze particles resulted from new particle formation.

Intense and frequent new particle formation (NPF) events have been observed in polluted urban environments, yet the dominant mechanisms are still under debate. To understand the key species and governing processes of NPF in polluted urban environments, we conducted comprehensive measurements in downtown Beijing during January–March, 2018. We performed detailed analyses on sulfuric acid cluster composition and budget, as well as the chemical and physical properties of oxidized organic molecules (OOMs). Our results demonstrate that the fast clustering of sulfuric acid (H₂SO₄) and base molecules triggered the NPF events, and OOMs further helped grow the newly formed particles toward climate‐ and health‐relevant sizes. This synergistic role of H₂SO₄, base species, and OOMs in NPF is likely representative of polluted urban environments where abundant H₂SO₄ and base species usually co‐exist, and OOMs are with moderately low volatility when produced under high NO_x concentrations.

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 ( 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.