Seasonal Characteristics and Source Apportionment of Water-Soluble Inorganic Ions of PM2.5 in a County-Level City of Jing–Jin–Ji Region

Water-soluble inorganic ions (WSIIs) are major components of PM_2.5 and play a prominent role in atmospheric acidification. Previous studies have mainly focused on urban areas, whereas research pertaining to county-level cities remains comparatively limited. To fill this gap, PM_2.5 samples were collected from March 2018 to February 2019 in Botou, a county-level city in the Jing-Jin-Ji region. Seasonal variation of WSII were studied, and their sources was apportioned by Positive Matrix Factorization (PMF) model. Annual PM_2.5 concentrations were 79.15 ± 48.44 mg/m³, which is 2.26 times of the Level II standard limit specified the National Ambient Air Quality Standard. Nitrate (NO₃^-) was the most abundant ion, followed by ammonium (NH₄⁺) and sulfate (SO₄^2-). The secondary inorganic aerosols (SIA, i.e., SO₄^2-, NO₃^-, and NH₄⁺) constituted 35.1± 4.7% of PM_2.5 mass. PM_2.5 mass, SO₄^2-, NO₃^-, NH₄⁺, K⁺, and Cl^- showed highest concentrations in winter. Ammonium salts were existed as ammonium sulfate ((NH₄)₂SO₄) and ammonium nitrate (NH₄NO₃) in spring, summer, and autumn, while it also can be existed as ammonium chloride (NH₄Cl) in winter. PMF analysis shows that the sources of WSIIs dominated by secondary source and followed by biomass burning. These results highlight the need for improved controls on gaseous precursors (NH₃, NO₂ and SO₂) and biomass burning to effectively reduce PM_2.5.