The effect of precipitation on gaseous oxidized and elemental mercury concentrations as quantified by two types of atmospheric mercury measurement systems

Abstract

Gaseous and particulate-bound oxidized mercury (Hg) compounds (Hg^II) have high solubility in precipitation compared to gaseous elemental Hg (Hg⁰). Wet and dry deposition are the primary routes of entry for atmospheric Hg^II into aquatic ecosystems. Information on how much Hg^II is removed from the atmosphere to the landscape during precipitation is lacking. In this study, oxidized Hg^II concentrations were measured with a dual-channel system (DCS) at two sites in the United States, Storm Peak Laboratory (SPL), in Colorado (2021–2022), and Beltsville (MD99) in Maryland (2022–2024), and compared with data from 16 co-located Atmospheric Mercury Network (AMNet) and Mercury Deposition Network (MDN) sites that used a KCl denuder method. At the two DCS sites, gaseous oxidized Hg concentrations were segregated by wet and dry periods from the nearest precipitation gauge to determine values for median dry Hg^II and median wet Hg^II concentrations (dry-wet = “Hg^II washout”) for each site. SPL had higher median ambient Hg^II and higher median Hg^II washout (90 pg m⁻³ and 22 pg m⁻³, respectively) compared to that for MD99 (43 pg m⁻³ and 7 pg m⁻³). This difference could be due to site elevation (3161 vs. 77 m) as there is generally more Hg^II higher in the atmosphere. In contrast, the ambient Hg^II/washout Hg^II ratios were more similar, 4.1 for SPL and 5.8 at MD99. The mean ambient Hg^II/washout Hg^II ratio for the 16 AMNet sites was 1.8 ± 0.1. The AMNet Hg^II data are known to be biased low due to issues with the KCl-denuder method, and this low bias appears to result in lower ambient Hg^II/washout Hg^II ratio observed for the AMNet sites. Correction factors for AMNet data using Hg^II measurements from DCS instruments were found to range from 2–3 and could be used to improve the accuracy of older data.