Spatiotemporal Variability in and Factors Associated with Wet Deposition of Atmospheric Mercury in China from 2000 to 2022

Abstract

The wet deposition of atmospheric mercury (Hg) transfers Hg to terrestrial and aquatic ecosystems, yet China lacks spatially consistent, long-term monitoring, a critical gap given its high and spatially heterogeneous anthropogenic Hg emissions. We compiled candidate observations from 58 published studies at 120 coordinate sites and used a random forest model to reconstruct the wet Hg deposition from 2000 to 2022. Site-grouped five-fold validation gave an overall coefficient of determination (R2) of 0.596 and root mean square error (RMSE) of 47.80 µg m−2 yr−1; leave-one-region-out validation was weaker (RMSE = 66.29 µg m−2 yr−1). The reconstructed national annual mean flux ranged from 20.06 to 26.19 µg m−2 yr−1, and the derived volume-weighted mean (VWM) Hg concentration in precipitation ranged from 28.02 to 37.12 ng L−1. The concentration estimate was highest in 2022, while flux remained strongly associated with rainfall. The high-rainfall southern regions had lower VWM concentrations and high fluxes; the drier northwestern regions had relatively high concentrations but low fluxes. Transport sector value-added, coal consumption, industrial value-added, and precipitation ranked highest in Hg input importance, as characterized by the SHapley Additive exPlanations (SHAP) method, where the socioeconomic variables were correlated proxies rather than direct measures of Hg emissions. The joint interpretation of concentration, flux, and precipitation is essential for assessing ecosystem Hg loading, although regional transfer and local extremes remain uncertain.