Spatial and Temporal Variations in Rainwater Chemistry in a Rapid Urbanization Area of Shenzhen, China

Studying chemical constituents in rainwater can provide insights into the origin, characteristics, concentration, and processes involved in clearing atmospheric pollutants. This study investigated the spatial–temporal variations in rainwater chemistry characteristics in a rapidly urbanizing area of Shenzhen from 2007 to 2022. The mean pH during 2007–2022 was 5.12 ± 0.45, significantly higher than the 4.51 recorded in 2006. The electrical conductivity (EC) of rainfall in Shenzhen was 15.79 ± 2.63 μS/cm and showed a progressive decrease over the years. Human activities influenced the trends of SO42−, which decreased, while NO3− and Cl− increased over time. The cations Ca2+ and NH4+ decreased, and Mg2+, Na+, and K+ increased. In recent years, Na+ and Cl−, typical sea-salt ions, became dominant with a continually rising contribution. The study also revealed that Shenzhen has been affected by both sulfuric and nitric acid rain, formally ushering in the nitric acid-type era. Spatial variations in rainfall chemicals were observed, especially for pH and conductivity, attributed to urban growth, distance from the coast, and industrial development. Moreover, the total ion concentration of rainwater demonstrated high values in the spring and low values in the summer due to the significant effect of precipitation.