Wet deposition fluxes of cosmogenic 32P, 33P and its utility in tracing air mass mixing in coast of east China sea

Abstract

Despite its proven utility of cosmogenic phosphorus (P) radionuclides to trace atmospheric processes and oceanic P biodynamic, atmospheric cosmogenic ³²P and ³³P have been rarely measured and quantified along the East Asian coast. This study reports the activities of ³²P and ³³P in individual precipitation events for the first time on the coast of the East China Sea, Shanghai, China (31°1′35.5″N, 121°26′49.3″E) from March 2021 to April 2023. The average specific activity of ³²P and ³³P was 1.56 ± 0.14 dpm L⁻¹ and 1.55 ± 0.17 dpm L⁻¹, respectively, and exhibited to decrease with increasing rainfall (n = 83). The ³³P/³²P activity ratios ranged from 0.70 to 1.22 (average, 0.96 ± 0.13), showing significant seasonal variations with high in spring/winter and low in summer/autumn. The higher ³³P/³²P activity ratios (>1.0) in typhoons and thunderstorms precipitation, suggested the intrusion of stratospheric air masses associated with those extreme weather events. The average irradiation period of tropospheric air masses and aerosols residence time was estimated to be 47 ± 32 d and 72 ± 119 d based on the ³²P and ³³P mass balance model, respectively. The wet deposition fluxes of ³²P and ³³P ranged from 0.94 to 84.46 dpm m⁻² d⁻¹ and 0.84 to 91.03 dpm m⁻² d⁻¹, respectively, and higher deposition fluxes were found in spring and summer. The relationship between individual rainfall event wet deposition fluxes and rainfall throughout 2021 to 2023 appears to follow a power law, though it showed high correlation in spring/winter. Moreover, a significant correlation with precipitation intensity indicated that precipitation intensity played a more dominant influence on the scavenging of ³²P and ³³P.