Chemical characteristics of summer rainwater at an urban site in South Korea

In this study, we investigated the chemical characteristics of rainwater and evaluated the correlation among rainwater quality factors for seven precipitation events from June 2020 to August 2020. Rainwater samples (n = 84) were collected every 50 mL at Pukyong National University, Busan, South Korea. Values of pH and electrical conductivity (EC) were measured in the field, and concentrations of water-soluble cations (Na⁺, Mg²⁺, K⁺, Ca²⁺, and NH₄⁺) and anions (Cl^-, NO₃^-, and SO₄^2-) were determined using ion chromatography. For all rainwater samples, the pH ranged from 3.63 to 5.59, with mean pH = 4.78, and the measured mean EC was 30.54 µS/cm, indicating that the precipitation was acidified in Busan, South Korea. A strong negative correlation (r = -0.83) was found between the pH and EC values. The major ionic components of rainwater were SO₄^2- > NH₄⁺ > NO₃^-, which are predominantly attributed to anthropogenic forces in the study area, such as emissions from vessels and fossil fuels. Anion concentrations of rainwater samples were SO₄^2- (average concentration: 2.15 mg/L) > NO₃^- (1.43 mg/L) > Cl^- (1.04 mg/L) and showed a strong positive correlation with EC values (r = 0.81) and a negative correlation with pH values (r = -0.72) of rainwater samples. The average concentrations of cations (NH₄⁺ (1.56 mg/L) > Ca²⁺ (1.31 mg/L) > Na⁺ (0.63 mg/L) > K⁺ (0.57 mg/L) > Mg²⁺(0.29 mg/L)) were relatively lower than those of anions. Cation concentrations showed no significant correlation with the values of EC (r = 0.29) and pH (r = -0.21). The result of this study indicates that acidic precipitation occurs even in summer with relatively low concentrations of air pollution and strong rainfall intensity.