Chemical characteristics and source apportionment of water-soluble ions in precipitation in Harbin, China

Precipitation can effectively scavenge atmospheric pollutants, making precipitation chemistry a crucial indicator for tracing anthropogenic impact on air quality. In this study, we collected a total of 99 precipitation samples in Harbin, a megacity in Northeast China, spanning the period from November 2021 to October 2023, and analyzed precipitation pH as well as concentrations of elemental carbon (EC) and major water-soluble ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, NH₄⁺, NO₃⁻, SO₄²⁻, Cl⁻, and F⁻). Our findings revealed that the equivalent concentrations of total water-soluble ions ranged from 40.0 to 1225.2 μeq·L⁻¹, with a volume weighted mean (VWM) concentration of 264.9 μeq·L⁻¹. NO₃⁻, Ca²⁺, NH₄⁺, and SO₄²⁻ were identified the dominant ions in precipitation, collectively accounting for 86.7 % of the total water-soluble ions. The concentrations of most ions in precipitation were elevated by a factor of 1.2–3.6 during the heating period compared to those in the non-heating period. The sources of water-soluble ions were determined through combined analyses involving enrichment factors, positive matrix factorization (PMF), and backward trajectories, which revealed that Ca²⁺, K⁺, and Mg²⁺ were primarily originated from crustal sources, Na⁺ and Cl⁻ from marine sources, and SO₄²⁻ and NO₃⁻ from anthropogenic sources. Furthermore, contributions to water-soluble ion levels in precipitation were attributed to crustal dust deposition (29.8 %), biomass combustion and agricultural activities (26.0 %), as well as fossil fuel combustion (22.0 %). The potential source regions of water-soluble ions in precipitation were identified as cities and regions located northwest, southwest, and south of Harbin. The knowledge gained from this study provides critical information necessary for formulating future pollution control policies.