Yearly variations of water-soluble ions over Xi'an, China: Insight into the importance contribution of nitrate to PM_2.5

Water-soluble inorganic ions (WSIIs) in PM_2.5 play an important role in the formation of air pollution, which in turn affects climate change and human health. The formation pathways and factors influencing WSIIs have received extensive attention. Here, we analyzed the contents of nine WSIIs in PM_2.5, collected from 2015 to 2021 in Xi'an City, China, with the aim of investigating long-term atmospheric pollution changes. Sulfate (SO₄²⁻), nitrate (NO₃⁻), and ammonium (NH₄⁺) together contributed to 66.8%–88.1% and Ca²⁺ accounted for 5.1%–13.1% of total WSIIs. The relative content of SO₄²⁻ exhibited a gradually decreasing trend (from 49.80% in 2015 to 29.98% in 2021), whereas NO₃⁻ was increased in the same time period (from 13.96% in 2015 to 29.92% in 2021). In addition, the nitrogen oxidation rate showed an annual increase in this period, whereas the sulfur oxidation rate decreased, and their fitted curves intersected in 2019. The key finding of this study is that the air pollution pattern in Xi'an has changed from sulfate-dominated to nitrate-dominated particles, as evidenced by the feature importance results of the random forest model. We propose that more attention should be paid to vehicle emissions and road dust as pollution sources. Overall, the findings of this study serve as a useful reference to aid relevant authorities in devising more effective policies for controlling PM_2.5 pollution at its source.