Changes in the urban dew chemical composition in Northeast China from 2013 to 2023

The chemical composition of dew is closely related to the air quality. Since 2013, China has promulgated a series of laws and regulations focused on air pollution control, and remarkable results have been achieved over the past decade. As an indicator of near-surface air quality, the chemical composition of dew obviously changes. The dew quality was affected mainly by the local air quality. The mean dew pH was 6.3 ± 0.4 (n = 186) from 2013 to 2023, and the order of ions was SO₄^2-(3180.7 ± 3592.0 µeq/L) > NH₄⁺(2552.3 ± 2971.8 µeq/L) > Ca²⁺ (1006.2 ± 945.5 µeq/L) > NO₃^- (397.2 ± 511.0 µeq/L) > Cl^-(152.3 ± 133.4 µeq/L) > K⁺(149.4 ± 191.6 µeq/L) > F^-(133.0 ± 110.3 µeq/L) > Na⁺(123.8 ± 94.9 µeq/L) > Mg²⁺(83.3 ± 65.6 µeq/L). The dew quality deteriorated on hazy days. There were no haze events during the condensation period after 2020, which caused a significant reduction in the concentration of the main ions in the dew. Over the past decade, electrical conductivity (EC), total dissolved solids (TDS), PM_2.5 and PM₁₀, as well as major ions (SO₄^2-, NO₃^-, NH₄⁺, Ca²⁺, Cl^-, Na⁺, F^-, K⁺ and Mg²⁺) in dew, have all tended to decrease. The annual mean NO₃^- concentration in 2023 was 86.5 % lower than that in 2013, with values of 79.3 % for SO₄^2-, 77.3 % for Ca²⁺, 76.7 % for NH₄⁺, 74.6 % for K⁺, 65.4 % for Mg²⁺, 63.4 % for Na⁺, 61.7 % for Cl^-, and 60.3 % for F^-.