Measurement Report: Wintertime new particle formation in the rural area of North China Plain: influencing factors and possible formation mechanism

Abstract. The high concentration of fine particles as well as gaseous pollutants makes polluted areas, such as the urban setting of North China Plain (NCP) of China, a different environment for NPF compared to many clean regions. Such conditions also hold for other polluted environments in this region, for instance, the rural area of NCP, yet the underlying mechanisms for NPF remain less understood owing to the limited observations of particles in the sub-3nm range. Comprehensive measurements, particularly covering the particle number size distribution down to 1.34 nm, were conducted at a rural background site of Gucheng (GC) in the North China Plain (NCP) from 12 November to 24 December in 2018. Five NPF events during the 39 effective days of measurements for the campaign were identified, with the mean particle nucleation rate (J_1.34) and growth rate (GR_1.34-2.4) were 29.1 cm^-3·s^-1 and 0.54 nm·h^-1, respectively. During these five days, NPF concurrently occurred in an urban site in Beijing, indicating that NPF events during these days in this region might be a regional phenomena. This implies that H2SO4-amine nucleation, concluded for urban Beijing there, could also be the dominating mechanism for NPF at our rural site. The condensation sink or coagulation sink for the survival of newly-formed and small clusters are the dominating factor controlling the occurrence of NPF under current atmosphere, whereas the contribution from the available H2SO4 cannot be neglected, either. This feature is slightly different from that of urban Beijing, where CS mainly determines whether NPF takes place or not.