Photoreduction and origin of dissolved and particulate mercury in cloud water: Insights from stable mercury isotopes.

The photoreduction of mercury (Hg) in clouds is crucial for determining global Hg cycling. The recently-developed isotope approach provides new insight into the fate of atmospheric Hg, however, limited data have been reported on the dynamics of Hg isotopes in clouds. This study presented the isotopic compositions of dissolved mercury (DHg) and particulate mercury (PHg) in cloud water collected at Mt. Tai (1545 m a.s.l.) in eastern China during summer 2021. Both DHg and PHg exhibited positive mass-independent fractionation of odd isotopes (odd-MIF, denoted as Δ199Hg), with averaged Δ199Hg values of 0.83 ± 0.34‰ and 0.20 ± 0.11‰, respectively. This high odd-MIF likely resulted from aqueous photoreduction in clouds, with DHg being more susceptible to photolysis than PHg. Our findings indicated that the photoreduction was promoted by sunlight and influenced by the chemical compositions of cloud water that controlled the Hg(II) speciation. The isotope mixing model estimation revealed that particulate-bound Hg and reactive gaseous Hg constituted the principal sources of Hg in cloud water, accounting for 55% to 99% of the total, while gaseous element Hg also made a notable contribution. Additionally, cloud water samples with faster reduction rates of Hg(II) were located outside of the isotope mixing models, which indicated an enhanced photoreduction process in cloud water.

Changed mercury speciation in clouds driven by changing cloud water chemistry and impacts on photoreduction: Field evidence at Mt. Tai in eastern China.

Chemical speciation of mercury (Hg) in clouds largely determines the photochemistry of Hg in the atmosphere and consequently influences Hg deposition on the surface through precipitation. Cloud water chemistry has notably changed over the last decade in response to global changes, however, the effects on Hg speciation remain poorly understood. During summer 2021, we collected sixty cloud water samples at Mt. Tai in eastern China and compared the cloud chemistry and Hg speciation with our previous findings during summer 2015. The results showed that although there were no statistically significant differences in the concentrations of total Hg (THg), dissolved Hg (DHg), and particulate Hg (PHg), there was a distinct shift in DHg species from the predominated Hg-DOM (78.6% in 2015 campaign) to the more homogeneously distributed Hg(OH)2 (28.4% in 2021 campaign), HgBr2 (26.5%), Hg-DOM (17.3%) and HgBrOH (17.0%). Changes in cloud water chemistry, particularly the significant increase in pH values to 6.49 ± 0.27 and unexpectedly high levels of bromide ions (Br-, 0.19 ± 0.22 mg L-1), were found to drive the changing of Hg speciation by enhancing Hg(II) hydrolysis and binding by Br-. Elevated Br- originating primarily from the continent likely caused noticeable differences in the dominating DHg species between cloud water sourced from marine and continental regions. The changes in chemical speciation of DHg were estimated to result in a 2.6-fold decrease in Hg(II) photoreduction rate between 2015 and 2021 campaigns (0.178 ± 0.054 h-1 vs. 0.067 ± 0.027 h-1), implying a shortened lifetime of atmospheric Hg and increased ecological risks associated with Hg wet deposition.

CFCs measurements at high altitudes in northern China during 2017-2018: Concentrations and potential emission source regions.

Northern China was simulated as the main contributor to global chlorofluorocarbon (CFCs) that slowed down the recovery of stratospheric ozone layer in most recent studies. An atmospheric campaign was carried out from June 2017 to April 2018 to register the concentrations of typical chlorofluorocarbons (CFCs) (i.e., CFC-11, CFC-12, CFC-113, and CFC-114) at the top of Mount Tai, northern China. The mixing ratios of CFC-11 CFC-12, CFC-113, and CFC-114 were 257, 577, 80, and 18 pptv, respectively. These values are similar to the reported data 10 years ago at Mount Tai. CFC concentrations correlated well with those of benzene (an anthropogenic tracer) and were not affected by either humidity, temperature, or solar radiation. However, CFC concentrations were considerably influenced by regional transport: their backward trajectory and the PSCF (potential source contribution function) analysis suggested that higher concentrations (CFC-12, CFC-113 and CFC-114) were detected under the influence of air mass from the industrial regions in mid-eastern China and CFC-11 was through long-range transport from northwestern (i.e., from the higher atmosphere in western China) air masses. Overall, the findings of this study suggested that CFCs still have emissions in China, but no significant increase in recent years. Mid-eastern China might be responsible for the CFC emissions. The conclusions also highlight the need for the enforcement of effective control policies and the management of emissions, in order to avoid increasingly severe scenarios.