Changes in Atmospheric Gaseous Elemental Mercury Concentrations and Isotopic Compositions at Mt. Changbai During 2015-2021 and Mt. Ailao During 2017-2021 in China

China is the largest contributor to the global total anthropogenic mercury (Hg) emissions. However, the trend in anthropogenic Hg emissions in recent years in China has not been effectively evaluated due to the lack of long-term atmospheric Hg observations. This study documents the changes in atmospheric gaseous elemental mercury (GEM) concentrations and isotopic compositions at Mt. Changbai (MCB) in northeastern China during 2015-2021 and Mt. Ailao (MAL) in southwestern China during 2017-2021, and explores the potential factors controlling these changes. GEM concentrations showed continuous declines from 2015 to 2021 (-2.1 +/- 0.6% yr(-1)) at MCB and from 2017 to 2021 (-4.0 +/- 1.4% yr(-1)) at MAL. Accompanied with these GEM declines are positive shifts in delta Hg-202 (medians: from 0.42 to 0.46 parts per thousand at MCB and from 0.17 to 0.57 parts per thousand at MAL), and negative shifts in Delta Hg-199 (medians: from -0.17 parts per thousand to -0.21 parts per thousand at MCB and from -0.10 parts per thousand to -0.17 parts per thousand at MAL) and Delta Hg-200 values (medians: from -0.07 parts per thousand to -0.08 parts per thousand at MCB and from -0.03 parts per thousand to -0.05 parts per thousand at MAL) (at significant levels for Delta Hg-199 at MCB and d202Hg and.199Hg at MAL). These changes were mainly caused by the decreases in regional anthropogenic emissions in the study areas. Based on a ternary mixing model with Delta Hg-199 and Delta Hg-200 as input, we estimate decline rates of 5.8 +/- 2.8 and 4.8 +/- 3.0% yr(-1) for the regional anthropogenic GEM emissions in northeastern and southwestern China, respectively. Plain Language Summary Long-term observations of atmospheric mercury (Hg) are important for evaluating the effectiveness of the Minamata Convention on Mercury. China is one of the largest anthropogenic atmospheric Hg emission regions in the world, and the temporal change in Chinese anthropogenic atmospheric Hg emissions plays an important role in global Hg cycling. However, long-term observations of atmospheric Hg in China are very limited, which makes it difficult to constrain the trend in atmospheric Hg concentrations in China and hampers our capability to evaluate the temporal changes in Chinese anthropogenic atmospheric Hg emissions. This study measures the gaseous elemental mercury (GEM) concentrations and isotopic compositions at two rural sites in China during 2015-2021 and 2017-2021, respectively. The results show rapid declines (-4.0%- -2.1% yr(-1)) in GEM concentrations at the two sampling sites over the past 5-7 years, which are companied by systematically consistent increase of delta Hg-202 and decrease of Delta Hg-199 and Delta Hg-200 values. Our observations indicate that the GEM declines are mainly driven by reduced anthropogenic emissions, and the reduction rates of regional anthropogenic emissions are estimated to be in the range of 4.8%-5.8% over the past 5-7 years.