Variations and Source Apportionment of Chemical Ions in Atmospheric Precipitation in the Pearl River Delta

Determining the chemical characteristics of atmospheric precipitation and its source contributions is critical for improving atmospheric environmental quality and water security. In this study, we analyzed the pH, Electrical Conductivity (EC), and major ion concentrations of precipitation from 2000 to 2021 at 29 monitoring stations in the Pearl River Delta (PRD), which typically experiences acid rain. Correlation analysis, the Positive Matrix Factorization (PMF) model, and other statistical methods were used to analyze the correlations between the major ion concentrations and their source contributions. The results indicate that the pH, EC, and Fractional Acidity (FA) of precipitation from 2000 to 2021 ranged from 4.66 ± 0.18 to 5.91 ± 0.45, from 1.63 ± 0.59 to 5.49 ± 1.62 mS/m, and from 0.05 ± 0.06 to 0.19 ± 0.13, respectively. The pH exhibited interannual variations (first decreasing and then increasing), whereas EC and FA exhibited the opposite trend. The total ion concentrations in precipitation ranged from 166.63 ± 46.56 to 631.48 ± 212.83 μeq/L, which first increased and then decreased. The weighted equivalent ion concentrations were in order of SO42- > Ca2+ > NH4+ > NO3- > Cl- > Na+ > K+ > Mg2+ > F-. SO42-, NO3-, Ca2+, and NH4+ were the main ions, accounting for 72.98% of the total concentration. The ratio of the SO42- concentration to the total concentration decreased from 32.71% in 2000 to 14.33% in 2021, whereas NO3- increased from 9.03% to 13.74% during the same period. The ratio of SO42- to NO3- concentrations (SO42-/NO3-) decreased by 65.95% from 2000 to 2021. After 2008, the SO42-/NO3- values ranged from 0.5 to 3, and precipitation shifted from a sulfuric acid type to a mixed sulfuric-nitric acid type. This change indicates that increased NOx emissions in the PRD under urbanization and agricultural intensification altered the structure of precipitation pollution and further increased the complexity of regional air pollution. The ions were highly significantly positively correlated, of which NO3- and SO42- had the strongest positive correlation (r=0.69‒0.74). Generally, NOx and SO2 are emitted together and enter precipitation via overlapping pathways. Of the six ion sources, coal combustion had the highest contribution rates (a>60%) to SO42- and F-, while secondary inorganic salts contributed 82.30% of the NO3-. Agriculture, sea salt, biomass combustion, and dust contributed > 70% of the NH4+, Na+, K+, and Ca2+. The contribution rates of secondary inorganic salts to SO42-, Cl-, and NH4+ were also greater than 25%. Owing to the increase in motor vehicles driven by urbanization, the impacts of secondary inorganic salt sources on ions in precipitation have become more extensive, and controlling NOx emissions to effectively control secondary pollution sources is urgently required.