Source contributions and mechanisms underlying reduced precipitation acidity in the Lingang New Area of Shanghai

Dissolved sulfur dioxide (SO₂), nitrogen oxides (NO_x), and organic acids in precipitation can lower rainwater pH and affect ecosystems and infrastructure. Although precipitation acidity and acid rain frequency have declined in Shanghai in recent years, the factors driving this change remain insufficiently understood, especially in rapidly urbanizing coastal areas. In this study, precipitation samples were collected throughout 2024 in the Lingang New Area of Shanghai to investigate rainwater chemistry and the processes associated with reduced acidity. Major ions, dissolved organic carbon (DOC), and stable hydrogen and oxygen isotopes were analyzed, and enrichment factor (EF) analysis, together with positive matrix factorization (PMF), was applied to identify major sources and influencing processes. The volume-weighted mean pH was 5.65 (n = 52), and the acid rain frequency was 21.8%. Sulfate (SO₄²⁻) remained the dominant acidic ion, with a higher equivalent concentration than nitrate (NO₃^-). PMF results showed different dominant source associations for sulfate and nitrate: nitrate was mainly associated with secondary aerosols linked to anthropogenic NOₓ emissions, whereas sulfate was more strongly associated with a processed marine aerosol factor, especially during humid periods. At the same time, neutralization by Ca²⁺-rich dust and NH₃-related inputs helped alleviate precipitation acidity. By combining source apportionment with isotope-based information on precipitation processes, this study provides new evidence that the reduction in precipitation acidity in the Lingang New Area during 2024 was jointly influenced by changes in anthropogenic emissions, coastal meteorological conditions, and neutralizing inputs.