Origin of the high oxidized mercury concentrations observed over the southwestern US during the NOMADSS campaign

Atmospheric mercury occurs primarily in its elemental form (Hg⁰), with a lifetime on the order of months. Oxidized mercury (Hg^II), which can exist in gaseous or particle-bound forms, is highly soluble and makes a larger contribution to the amount of mercury deposited to terrestrial and aquatic ecosystems. Although Hg^II is emitted due to anthropogenic activities, in-situ oxidation of Hg⁰ is the dominant source of Hg^II in much of the global atmosphere. Results from the GEOS-Chem global chemical transport model indicate frequent summertime occurrences of high free tropospheric Hg^II over the southwestern US. Aircraft observations from the Nitrogen, Oxidants, Mercury, Aerosols, Distributions, Sources and Sinks (NOMADSS) campaign during June-July 2013, confirm the presence of high Hg^II concentrations in the free troposphere (2-7 km) over Texas, especially associated with dry air in upper level anticyclones. In this study we investigate the origin of these Hg^II-rich air masses and compare model simulations to observed profiles of Hg^II. We examine the relative influences of local production of Hg^II, transport from the subtropical Pacific High, and transport from the upper troposphere/lower stratosphere. We perform multi-year simulations and tag Hg^II as either direct emissions or in-situ oxidation, divided further based on its region of production.