Seasonal Variations and Sources of Water-Soluble Inorganic Anions in Atmospheric Particulate Matter (PM_2.5 and PM₁₀) of Suzhou Urban Area

To investigate the temporal and chemical characteristics of atmospheric particulate matter (PM_2.5 and PM₁₀) in the urban area of Suzhou, China, we conducted a comprehensive study involving sample collection over a three-month period. This research analyzed five water-soluble inorganic anions, fluoride (F⁻), chloride (Cl⁻), nitrite (NO₂⁻), nitrate (NO₃⁻), and sulfate (SO₄²⁻) using ion chromatography on PM_2.5 and PM₁₀ samples collected during two representative seasons: summer and winter of 2021 and 2022. Our results indicated that PM_2.5 concentrations exceeded national grade II standard limits by 27.6%, while PM₁₀ concentrations exceeded these limits by 11.9% during the winter months compared to summer. The concentrations of the measured anions followed the order of magnitude: SO₄²⁻ > NO₃⁻ > Cl⁻ > NO₂⁻ > F⁻, with significant seasonal variations observed. Notably, SO₄²⁻ exhibited a strong correlation with NO₃⁻ and Cl⁻, with correlation coefficients reaching 0.89 for PM_2.5 and a minimum of 0.81 for PM₁₀, suggesting the presence of ammonium salts. The NO₃⁻/SO₄²⁻ ratios in PM_2.5 were 0.36 in summer and 0.72 in winter, demonstrating that Suzhou was strongly influenced by stationary sources in summer and mobile sources contribute more in winter. The outcomes of the principal component analysis apportioned the sources of water-soluble inorganic anions in PM_2.5 to secondary transformation and combustion sources. These results underscore the significant seasonal variations in particulate matter composition and the influence of varying emission sources in urban environments.