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

The high concentration of fine particles and gaseous pollutants makes polluted areas, such as the urban setting of North China Plain (NCP) of China, a different environment for new particle formation (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-3 nm range. Comprehensive measurements, particularly covering the particle number size distribution down to 1.3 nm, were conducted at a rural background site of Gucheng (GC) in the North China Plain (NCP) from 12 November to 24 December 2018. In total, five NPF events during the 39 effective days of measurements for the campaign were identified, with the mean particle nucleation rate (J(1.3)) and growth rate (GR(1.3-2.4)) being 22.0 cm(-3) s(-1) and 3.9 nm h(-1), respectively. During these 5 d, NPF concurrently occurred at an urban site in Beijing. Sharing similar sources and transport paths of air masses arriving at our site to that of urban Beijing, we hypothesize that NPF events during these days in this region might be a regional phenomenon. The simultaneous occurrence of NPF in both places implies that H2SO4-amine nucleation, concluded for urban Beijing there, could probably be the dominating mechanism for NPF at our rural site. The higher concentration of sulfuric acid during many non-event days compared to that of event days indicates that the content of sulfuric acid may not necessarily lead to NPF events under current atmosphere. Only when the condensation sink or coagulation sink was significantly lowered, atmospheric NPF occurred, implying that condensation sinks (CSs) and coagulation sinks (CoagSs) are the dominating factors controlling the occurrence of NPF for the present rural environment of the NCP, which is quite similar to the feature seen in urban Beijing.