High Concentration of Atmospheric Sub‐3 nm Particles in Polluted Environment of Eastern China: New Particle Formation and Traffic Emission.

Observations of atmospheric sub‐3 nm particles are essential for understanding the initial stages of new particle formation (NPF) and the origin of aerosol particles. In this study, 3 years (2018–2020) of measurements of sub‐3 nm particles were conducted in the Yangtze River Delta (YRD) of eastern China. High concentrations of sub‐3 nm particles were observed, with number concentration in the range from 103 to 106 cm−3. During the daytime, the sub‐3 nm particle concentration was found to peak at around the noon, indicating strong photochemical nucleation processes. The formation rates of sub‐3 nm particles were high during the NPF event days, with an average value of 86 cm−3 s−1, and this rate was related to the sulfuric acid (SA) concentration. The particle growth rates below 3 nm were about 1–2 nm h−1, much lower than the growth rates of larger particles. At nighttime, sub‐3 nm particle concentrations remained relatively high (3 × 103 to 1.2 × 104 cm−3) and were related to the NOx concentration, suggesting traffic emission to be a significant source. The sub‐3 nm proxy was developed to estimate the contributions of NPF and traffic emission. During the daytime, 74.8% and 12.4% of the sub‐3 nm particles were estimated to originate from SA‐driven NPF and traffic emissions, respectively. However, other sources were estimated to contribute 61.8% of sub‐3 nm particles at nighttime, suggesting the formation mechanisms of sub‐3 nm particles are still unclear in this environment. Our study sheds more light on the characteristics and sources of sub‐3 nm particles in polluted environments. Plain Language Summary: Sub‐3 nm particles play a vital role in the initial stage of new particle formation (NPF) as well as haze formation. We performed long‐term measurements of the sub‐3 nm particles and supporting atmospheric parameters from 2018 to 2020 in the Yangtze River Delta of eastern China. High number concentrations of sub‐3 nm particles were observed, with obvious diurnal and seasonal variations. NPF and traffic emissions were found to be two major sources of sub‐3 nm particles. The formation rate of sub‐3 nm particles was high and related to the sulfuric acid (SA) concentration during the NPF events. Sub‐3 nm particle concentrations remained relatively high at nighttime and were related to the NOx concentration. By developing the proxy of sub‐3 nm particle concentration, we estimated SA‐driven NPF accounted for over 70% of the sub‐3 nm particles during the daytime. At nighttime, over 30% of the sub‐3 nm particles were emitted from traffic sources, while about 60% remain unknown. This study sheds more light on the characteristics and sources of sub‐3 nm particles in polluted environments. Key Points: High number concentration of sub‐3 nm particles was observed in eastern China, with obvious diurnal and seasonal variationsSulfuric acid‐driven new particle formation is the dominant source of sub‐3 nm particles during the daytimeTraffic emission contributes over 30% of sub‐3 nm particles, while the origin of 62% of sub‐3 nm particles remains unclear during the night [ABSTRACT FROM AUTHOR]