Investigating the contribution of grown new particles to cloud condensation nuclei with largely varying pre-existing particles – Part 2: Modeling chemical drivers and 3-D NPF occurrence

Abstract

Abstract. In this study, we utilized a 20-bin WRF-Chem (Weather Research and Forecast coupled with Chemistry regional model) to investigate the contributions of chemical drivers to the growth of newly formed particles, as well as to simulate the three-dimensional dynamics of new particle formation (NPF) events over the North China Plain during a summer campaign in 2019, which was reported in the accompanying paper. The model demonstrated good performance in replicating the occurrence of NPF, the growth pattern of newly formed particles, and the number concentration of particles in the size range of 10–40 nm in five events between June 29 and July 6. This period was characterized by a high frequency of NPF occurrence (>60 %). During this time, the model was also able to accurately reproduce the levels of organics in PM1.0 relative to observations, and to reasonably replicate the levels of SO42- and NH4+ in PM1.0, as well as PM2.5 mass concentrations. Therefore, we further analyzed three NPF events with distinct particle growth characteristics: Case 1, featuring observable growth of newly formed particles to cloud condensation nuclei (CCN) size on July 1–2; Case 2, characterized by continuous growth of new particles for several hours without any net contribution to CCN on July 3; and Case 3, where no detectable continuous growth of newly formed particles was observed on July 6. In these instances, the model tended to overpredict the condensation of H2SO4 vapor during daytime and the formation of NH4NO3 during nighttime, resulting in an overestimation of the hygroscopicity parameter of nanometer particles. Nevertheless, the model was able to reasonably reproduce the CCN (cloud condensation nuclei) at a super saturation (SS) of 0.4 % on days with NPF, compared to the observations. This was because the overestimation effect caused by inorganics was offset by the model's underestimation of CCN originating from submicron-sized particles. Additionally, three-dimensional simulations of NPF events have demonstrated some key findings. Firstly, NPF consistently initiates at the upper fraction of the planetary boundary layer (PBL) before expanding. Secondly, during daytime growth of newly formed particles in the PBL, organics play a dominant role, whereas the primary chemical drivers shift to inorganic species in the free troposphere. However, to confirm these findings, vertical observations are required.