New particle formation (NPF) events, initiated by the nucleation from gas-phase pollutants such as SO2 and volatile organic compounds (VOCs), followed by subsequent growth, are considered one of the most significant source of atmospheric ultrafine particles. Understanding these particles is crucial for addressing climate change, mitigating harmful health effects, and developing air pollution control strategies. However, the factors driving the occurrence and development of NPF events, along with the underlying chemical mechanisms, remain inadequately explained. 

The Guanzhong Plain, located to the north of the Loess Plateau in a semi-arid region, is highly susceptible to dust transport. Additionally, emissions from industrial sources, motor vehicles, and rural biomass combustion are concentrated in this basin and are difficult to disperse due to its topography. As a result, the Guanzhong Plain consistently ranks among the most severely polluted areas in China regarding atmospheric particulate matter. 

Despite this, few studies have simultaneously analyzed particle number concentration and NPF events in both urban polluted area and alpine atmosphere within this region, limiting our understanding of how meteorological and anthropogenic conditions influence NPF occurrences.

The researchers find that the average particle number concentration in Xi’an is significantly higher than that at Mt. Hua. The diurnal variation of total particle number concentration differs between the two sites, highlighting distinct influencing factors. In Xi’an, size distributions vary across different timescales and weather conditions, while Mt. Hua shows minimal variation, likely due to its cleaner atmospheric background and the consistent influx of aging particles with larger diameters transported from the free atmosphere. In both areas, geometric mean diameters (GMDs) are inversely proportional to particle number concentrations, suggesting that increase in particle numbers were primarily driven by the generation of smaller particles. 

The factors governing NPF events differ between the urban and mountainous stations. In Xi’an, strong local emissions from stationary and mobile sources drive the growth of newly formed nanoparticles, with ozone-oxidized condensable vapors serving as key precursors. In contrast, at Mt. Hua, NPF process are significantly influenced by anthropogenic precursors from long-range transport and locally emitted biogenic organics. 

This work, published in Science of The Total Environment on Sep. 28. 2024, will contribute to advancing the understanding of NPF mechanisms and aerosol particle growth in both urban and alpine areas in Northwest China.