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.

The Guanzhong Plain, located to the north of the Loess Plateau in a semi-arid region, consistently ranks among the most severely polluted areas in China regarding atmospheric particulate matter.

A group of Chinese scientists recently compared number concentrations and size distributions of atmospheric ultrafine particles at Xi'an (urban area) and the summit of Mt. Hua (alpine region) in summer to investigate the NPF mechanism and particle growth in both clean and polluted areas of the Guanzhong Plain.

"Understanding these NPF particles is crucial for addressing climate change, mitigating harmful health effects, and developing air pollution control strategies," said Professro LI Jianjun, leading researcher and also corresponding author of the study from Institute of Earth Environment (IEE) of the Chinese Academy of Sciences.

The Guanzhong Plain is highly susceptible to dust transport. Emissions from industrial sources, motor vehicles, and rural biomass combustion concentrate in this basin and are difficult to disperse due to its topography.

In their study, the IEE researchers found 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.

The rapid increase in ultrafine particle concentrations primarily poses serious health risks and degrades air quality in urban areas, while also contributing to climate-related effects in alpine regions, the study found.

This work will contribute to advancing the understanding of NPF mechanisms and aerosol particle growth in both urban and alpine areas in Northwest China.

Results of the study were published in Science of The Total Environment on Sep. 28. 2024.