Investigations of New Particle Formation Mechanisms in North China Plain during the Spring Campaign Based on Observations and Simulations

Abstract

New particle formation (NPF) plays a critical role in atmospheric aerosol loading, yet the mechanisms controlling NPF under high-pollution conditions remain poorly understood. Ground-based measurements and GEOS-Chem-APM simulations were combined to investigate the factors governing NPF in a typical industrial city over North China Plain. During the spring campaign (3 May-2 June 2016), 10 NPF days were identified, each preceded by nighttime northwesterly ventilation and occurring under dry daytime conditions. The model reproduced 6 of 10 observed NPF events, whereas the remaining 4 showed substantially weaker simulated nucleation. Well-simulated events were generally associated with low relative humidity (RH) and condensation sink (CS) environment. NH3 remained in large excess throughout the campaign. In our test, increasing H2SO4 by up to a factor of 3 substantially enhanced nucleation rate (J) but did not fully reproduce the observed particle evolution, while replacing modeled RH and aerosol surface area with observations only moderately adjusted J. The model underestimated growth rates by 51% and 75% on well-simulated and poorly simulated days. These results indicate that nighttime ventilation reduces surface area and CS of pre-existing aerosol loading, favoring NPF, while successful reproduction of observed NPF depends jointly on diverse precursor-driven nucleation, aerosol scavenging, and subsequent particle growth.