Contributions of traffic emissions and new particle formation to the ultrafine particle size distribution in the megacity of Beijing

Abstract The important role of traffic emissions in air pollution has been widely recognized. However, the corresponding contributions to the ultrafine particle distribution and new particle formation (NPF) are poorly understood due to the mixed influences of various sources. Measurements of particle evolution and NPF were made at a heavy traffic site near the 5th Ring Road of Beijing from August 2017 to October 2019. A positive matrix factorization receptor model (version 5.0) consisting of the log-normal fit method and the k-means clustering method was used to identify and quantify the major sources of particle number concentrations. Based on the characteristics of NPF, additional analyses were performed to identify formation sources, including back-trajectory clustering and correlations between auxiliary variables and the relative contributions of all resolved factors. Traffic emissions mainly influenced Aitken mode (25–100 nm) particles (>60%), and NPF significantly contributed to the ultrafine particle distribution (47.2%). Additionally, regional transport notably affected the occurrence of NPF at the observation site, with northwest airmass transport influencing the nucleation mode the most and southeast airmass transport influencing the Aitken mode the most.