Changes in atmospheric gaseous elemental mercury concentrations and isotopic compositions at Mt. Changbai during 2015-2021 and Mt. Ailao during 2017-2021 in China

This dataset is associated with the manuscript published in 𝐽𝑜𝑢𝑟𝑛𝑎𝑙 𝑜𝑓 𝐺𝑒𝑜𝑝𝒉𝑦𝑠𝑖𝑐𝑎𝑙 𝑅𝑒𝑠𝑒𝑎𝑟𝑐𝒉 - 𝐴𝑡𝑚𝑜𝑠𝑝𝒉𝑒𝑟𝑒𝑠: Changes in atmospheric gaseous elemental mercury concentrations and isotopic compositions at Mt. Changbai during 2015-2021 and Mt. Ailao during 2017-2021 in China. This dataset includes all the measured data used in the paper.Methodology1.     Site locations: Mt. Changbai (MCB, 128.113° E, 42.400° N, 741 m a.s.l.) is located northeastern China. Mt. Ailao (MAL, 101.020° E, 24.533° N, 2450 m a.s.l.) is located in southwestern China. These two sites are regional background sites without strong point mercury sources around the sampling sites.2.     Monthly mean GEM (gaseous elemental mercury) concentrations in the atmosphere: GEM concentrations (in ng m-3) were measured using Tekran 2537 mercury vapor analyzers at an interval of 5 min. 5-min GEM concentrations were averaged to hourly mean concentrations and further used to calculate the monthly mean GEM concentrations, which are used for the discussions throughout the manuscript. 3.     GEM (gaseous elemental mercury) isotopic composition and analytical uncertainty: GEM samples in ambient air were collected using chlorine-impregnated activated carbon (CLC) traps at a flow rate of 2.0-2.5 L min-1. GEM in CLC traps were then pre-concentrated into mixing acid trapping solutions (2HNO3/1HCl, v/v, 5 mL). Isotope ratios of Hg in trapping solutions were measured using cold vapor-multicollector inductively coupled plasma mass spectrometry (CV-MC-ICPMS, Nu Instruments, U.K.). Mass dependent fractionation signatures (δ199Hg, δ200Hg, δ201Hg, δ202Hg, δ204Hg, in ‰) were calculated as follows:δxxxHg = [(xxxHg/198Hg)sample/(xxxHg/198Hg)Nist 3133 - 1] × 1000‰ (1)where xxxHg represents the mercury stable isotopes (199Hg, 200Hg, 201Hg, 202Hg, and 204Hg). NIST 3133 refers to bracketing NIST (National Institute of Standards & Technology) SRM (Standard Reference Material) 3133 mercury standard solution, of which the mercury isotope ratios were measured before and after the sample collection to correct the mass bias of MC-ICPMS. MIF signatures (Δ199Hg, Δ200Hg, Δ201Hg, and Δ204Hg, in ‰) were calculated based on the Kinetic MDF law as follows:ΔxxxHg = δxxxHg - (βxxx ×δ202Hg) (2)where βxxx is 0.252, 0.5024, 0.752, and 1.493 for 199Hg, 200Hg, 201Hg, and 204Hg, respectively. The 2σ are the 2 times analytical uncertainty of the isotopic compositions, which are the larger 2σ value of either repeated measurements of NIST RM 8610 during the whole analytical sessions or the internal repeated measurements of an individual GEM sample.