In this work, the spatial extent of new particle formation (NPF) events and the relative probability of observing particles originating from different spatial origins around three rural sites in eastern China were investigated using the NanoMap method, using particle number size distribution (PNSD) data and air mass back trajectories. The length of the datasets used were 7, 1.5, and 3 years at rural sites Shangdianzi (SDZ) in the North China Plain (NCP), Mt. Tai (TS) in central eastern China, and Lin'an (LAN) in the Yangtze River Delta region in eastern China, respectively. Regional NPF events were observed to occur with the horizontal extent larger than 500 km at SDZ and TS, favoured by the fast transport of northwesterly air masses. At LAN, however, the spatial footprint of NPF events was mostly observed around the site within 100–200 km. Difference in the horizontal spatial distribution of new particle source areas at different sites was connected to typical meteorological conditions at the sites. Consecutive large-scale regional NPF events were observed at SDZ and TS simultaneously and were associated with a high surface pressure system dominating over this area. Simultaneous NPF events at SDZ and LAN were seldom observed. At SDZ the polluted air masses arriving over the NCP were associated with higher particle growth rate (GR) and new particle formation rate ( J ) than air masses from Inner Mongolia (IM). At TS the same phenomenon was observed for J , but GR was somewhat lower in air masses arriving over the NCP compared to those arriving from IM. The capability of NanoMap to capture the NPF occurrence probability depends on the length of the dataset of PNSD measurement but also on topography around the measurement site and typical air mass advection speed during NPF events. Thus the long-term measurements of PNSD in the planetary boundary layer are necessary in the further study of spatial extent and the probability of NPF events. The spatial extent, relative probability of occurrence, and typical evolution of PNSD during NPF events presented in this study provide valuable information to further understand the climate and air quality effects of new particle formation.

A heavy pollution episode (HPE) that lasted for seven days occurred over the North China Plain in December 2016. An in situ Ambient Ion Monitor was applied to analyze the chemical composition of PM_2.5 (fine particulate matter with diameters less than 2.5 µm) and gaseous HONO concentration during that event. A representative explosive growth in the pollution cumulative stage was selected to investigate the pollution mechanism during the HPE in Beijing. PM_2.5 cumulative explosive growth processes were observed to occur commonly under high relative humidity (RH) condition. Our results demonstrated that the aqueous-phase oxidation of SO₂ by NO₂ to sulfate could contribute to the cumulative explosive growth. Nitrate produced by secondary formation was another factor in the growth of PM_2.5. Depending on the relative humidity, temperature, and chemical species, the deliquescence relative humidity was calculated to 82%, 81%, and 83% for (NH₄)₂SO₄, NH₄NO₃, and NH₄Cl, respectively. The preexisting PM_2.5 surface changed from solid to liquid when RH > 81%. Coincidentally, both the sulfur oxidation ratio (SOR) and reaction product HONO displayed an evident exponential relationship with RH and increased more quickly when RH was larger than 80%. In addition, sufficiently excessive NO₂ made the aqueous-phase oxidation of SO₂ efficiently proceed even at relative low SO₂ concentrations (below 15 µg m⁻³). Potential H⁺ in the reactions was neutralized by NH₃, resulting in fully neutralized PM_2.5 during HPE. The chemical evolution of these reactions was discussed in detail in this study.