New tracer for anthropogenic pollution in the atmosphere: Stable potassium isotopes in rainwater

Rainwater dissolves urban air contaminants that threaten sustainable urbanization and human health. Tracing rainwater contamination supports urban management and environmental protection. Potassium isotopes (delta K-41) have become new tracers in recent years, but K isotopes in urban rainwater and their application in tracing anthropogenic contamination have not yet been reported. Therefore, novel data on rainwater from Beijing, the capital of China, were presented in this study (delta K-41: -1.16 parts per thousand to -0.17 parts per thousand). Rainwater delta K-41 values showed a good nonlinear correlation with NO3-/K+ (R = 0.71; p < 0.05). Nitrogen (N) and K isotopes data were applied in three-end-member mixing model, and the three anthropogenic sources are biomass burning (delta N-15: -1.3 +/- 4.3 parts per thousand, delta K-41: -1.16 +/- 0.06 parts per thousand), fertilizer (delta N-15: 1.6 +/- 1.9 parts per thousand, delta K-41: 0.14 +/- 0.09 parts per thousand), and traffic emission (delta N-15: -8.0 +/- 4.5 parts per thousand, delta K-41: -0.17 +/- 0.06 parts per thousand). The MixSIAR model and backward trajectory with fire maps indicated that biomass burning and traffic emission were the primary contributors, with most biomass burning coming from southwest and southeast Beijing. This study expands rainwater K isotope composition data and first shows its potential to trace anthropogenic contamination. With K isotope ratios effectively tracing pollution sources in Beijing rainwater, it will be widely applied in environmental research.