Underestimation of atmospheric oxidized mercury at a mountaintop site by the GEOS-Chem chemical transport model

Abstract. An improved mechanistic model of mercury redox chemistry has recently been implemented in the GEOS-Chem model. In this study, GEOS-Chem simulations were compared to ambient measurements made during a high-oxidized mercury episode that originated in the free troposphere at a mountaintop site in Colorado, USA (40.455° N, -106.744° W, 3220 meters above sea level). Measurements were collected with a dual channel atmospheric oxidized mercury measurement system that has been shown to accurately quantify oxidized mercury compounds in ambient air. The model and observations showed similar temporal trends for elemental and oxidized mercury (R² of 0.54 to 0.79) and similar elemental mercury concentrations (normalized mean square error of 0.04 in the base model). However, the base model only produced 17 % of the maximum oxidized mercury observed in the dual channel system. In sensitivity tests with increased oxidation rates, the model still only produced, at most, 23 % of maximum observed oxidized mercury. In addition to underestimating net mercury oxidation, an analysis of elemental to oxidized mercury slopes indicated the model overestimated oxidized mercury deposition. An analysis of GEOS-Chem results from a separate study confirmed that while GEOS-Chem is able to simulate the range of measured oxidized mercury in low-oxidized mercury episodes and locations it consistently underestimates measured values during high-oxidized mercury periods at surface locations in western USA.