Seasonal variation and secondary formation of size-segregated aerosol water-soluble inorganic ions in a coast megacity of North China Plain

The aerosol samples of water-soluble inorganic ions (WSIs), including SO₄²⁻, NO₃⁻, NH₄⁺, Cl⁻, K⁺, Na⁺, Ca²⁺, and Mg²⁺ in size-segregated particulate matter (PM), were collected by an Anderson sampler (with 8 nominal cut-sizes ranged from 0.43 to 9.0 μm) in urban Tianjin during 2013–2014. The results showed that particulate matters in the fine mode (PM₂.₁, Dp < 2.1 μm) comprised large part of mass concentrations of aerosols, and the water-soluble ionic species in the fine mode were 47.07 ± 14.29 μg m⁻³ (spring), 67.87 ± 28.74 μg m⁻³ (summer), 86.60 ± 48.53 μg m⁻³ (autumn), and 104.16 ± 51.76 μg m⁻³ (winter), respectively, which accounted for 59.5%, 63.3%, 71.9%, and 71.4% of the PM₂.₁ mass concentrations. Secondary pollutants of SO₄²⁻, NO₃⁻, and NH₄⁺ (SNA) were the dominant contributors of WSIs, which showed a bimodal size distribution in each season, with the larger peak appeared in the size fraction of 0.65–1.1 μm and the smaller one in 3.3–5.8 μm fraction. SNA concentrations in lightly polluted days (LPD) and heavily polluted days (HPD) were observably higher than non-polluted days (NPD), especially in the fine mode, with the peak diameter moving from 0.43–0.65 μm on NPD to 0.65–1.1 μm on LPD and HPD. The correlation analysis between NH₄⁺, NO₃⁻, and SO₄²⁻ suggested that almost all SO₄²⁻ and NO₃⁻ for fine particles had been completely neutralized by NH₄⁺, and primarily existed in the forms of (NH₄)₂SO₄ and NH₄NO₃. The sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) on LPD and HPD in fine mode were observably higher than those on NPD, especially in the range of 0.65–1.1 μm and 1.1–2.1 μm. Furthermore, SOR and NOR values in the size fraction of 0.43–3.3 μm increase as the RH elevated, especially in 0.43–2.1 μm, where RH was significantly positive correlated with SOR and NOR, indicating the significant contributions of heterogeneous processes to the secondary formation of SO₄²⁻ and NO₃⁻. These results suggested an enhanced formation ability of secondary pollutants under high RH in the coast city. Therefore, controlling the precursors of SNA, such as SO₂ and NOx, would be more effective to reduce the fine particulate pollution in the coast megacity of Tianjin.