Iodine May Rival Bromine as a Mercury Oxidant in the Free Troposphere

C.F. Lee^1,2, T. Elgiar³, L.M. David^3,4, K. Wilmot⁵, M. Reza^1,2, N. Hirshorn^5,6, I.B. McCubbin⁷, V. Shah^8,9, J.C. Lin¹⁰, S.N. Lyman^3,11, A.G. Hallar^10,7, L.E. Gratz^12,13 and R. Volkamer^2,1

¹University of Colorado, Department of Chemistry, Boulder, CO 80309; 661-917-6117, E-mail: christopher.f.lee@colorado.edu
²Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309
³Bingham Research Center, Utah State University, Vernal, UT 84078
⁴Ramboll, Fort Collins, CO 80521
⁵Department of Atmospheric Sciences, University of Utah, Salt Lake City, UT 84112
⁶Ramboll, New York, NY 10119
⁷Storm Peak Laboratory, Desert Research Institute, Steamboat Springs, CO 80488
⁸Harvard University, School of Engineering and Applied Sciences, Cambridge, MA 02138
⁹Science Systems and Applications, Inc. (SSAI), Lanham, MD 20706
¹⁰University of Utah, Salt Lake City, UT 84112
¹¹Department of Chemistry & Biochemistry, Utah State University, Logan, UT 84322
¹²Environmental Studies Program, Colorado College, Colorado Springs, CO 80903
¹³Department of Chemistry and Environmental Studies Program, Reed College, Portland, OR 97202

In June 2020, the World Health Organization named mercury on its list of ten chemicals of public health concern due to its global ubiquity and potent neurotoxicity. In humans, there is no known safe level of mercury exposure. The atmospheric oxidation of gaseous elemental mercury, Hg⁰, is thought to proceed mostly by bromine atoms (Br) or hydroxyl radicals (OH) to form Hg^I species; iodine atoms (I) also warrant consideration in light of the recent realization that Hg^I is more rapidly oxidized than previously thought by ozone (O₃) to form water-soluble and longer-lived Hg^II species. The atmospheric deposition of Hg^II species is the primary source of mercury to aquatic ecosystems, where mercury bioaccumulates in the food chain. Interestingly, particulate mercury in the upper troposphere correlates with iodine for reasons that are currently not understood, and iodine is currently not considered as an oxidant of Hg⁰ in atmospheric models. Here we present co-located measurements of Hg^II and iodine, and show that atmospheric models systematically underestimate the amount of iodine and Hg^II observed in the free troposphere. Ours are the first ground-based observations of tropospheric iodine radicals over the central continental United States. At the observed levels, iodine may rival bromine as an oxidant of Hg⁰, especially at cold temperatures; this may explain previously observed particulate mercury-iodine correlations in the upper troposphere. We briefly discuss the atmospheric implications and future research needs to reduce uncertainties in our understanding of the mercury geochemical cycle.

Figure 1. Bromine atom (Br) and hydroxyl radical (OH) are major oxidants of gaseous elemental mercury throughout the atmosphere. Iodine atom (I) is largely unexplored as a mercury oxidant but may rival bromine in the free troposphere.