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Abstract Title: Global Climate Change Offsetting the Descending Trend of Mercury Wet Deposition in the Western United States
Presenter Name: Xia Liu
Company/Institution: Nanjing University
Session: Special Session - Impacts of Climate Change on Global Mercury Cycling
Co-Authors: Xia Liu,Lei Zhang,Weikang Zhang,Peisheng Zhou,Yang Zhang,Daniel Jaffe

Abstract Information :

Wet deposition is a crucial pathway for mercury (Hg) to enter terrestrial ecosystems. Based on the observation data from the Mercury Deposition Network (MDN), Hg wet deposition exhibited a significant descending trend in the eastern US from 1996 till now, while there was no evident trend observed in the western US. Limited contribution from anthropogenic sources to the Hg wet deposition in the western US lead to a more prominent impact from climate change. In this study, a generalized additive model (GAM) was used to qualify the impacts of meteorological factors on Hg wet deposition in the western US, based on the datasets from European Centre for Medium-Range Weather Forecast (ECMWF) and MDN during 1996-2020. To prevent the interference from the aggressive impact of the precipitation amount on Hg wet deposition, Hg concentration in precipitation (Conc, in logarithmic form) was instead considered as the dependent variable in GAM, and 6 meteorological and environmental factors were chosen to be the predictors (through sensitivity analysis), including the amount of precipitation (Prec), temperature (Temp), relative humidity (RH), total cloud coverage (TCC), wind speed (WS), and elevation (Elev). The performance of GAM was promising with an adjusted R2 of 0.53. Temp, Prec, TCC, and Elev contributed 39.04%, 31.49%, 12.06%, and 11.20% to the variance of the dependent variable log(Conc), respectively. The influencing patterns of each predictor on the dependent variable were examined. With the increase of Prec, log(Conc) drops dramatically and then gradually levels off, indicating that precipitation has a dilution effect on Hg concentration in precipitation. With this pattern considered, Hg wet deposition increases steadily with Prec but grows rapidly when Prec exceeds 200 mm. Another key predictor Temp has a positive correlation with log(Conc). The reason for this pattern is probably that higher temperature accelerates the Hg oxidation process and the yielded HgII could be easily scavenged by precipitation. The pattern for Elev is similar to Temp, which is also a result of faster Hg oxidation with abundant halogen radicals at higher elevation. TCC acts as a proxy for solar radiation. With the increase of TCC, log(Conc) increases steadily, implying that solar radiation has a negative effect which is likely related to the photoreduction of Hg in precipitation. Based on the above results from GAM, we conclude that the significant ascending trends of both precipitation and summertime temperature could be the main driving forces of the offsetting effect for the descending trend of Hg wet deposition in the western US.

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