Evidence against Rapid Mercury Oxidation in Photochemical Smog

Abstract

Mercury pollution is primarily emitted to the atmosphere, and atmospheric transport and chemical processes determine its fate in the environment, but scientific understanding of atmospheric mercury chemistry is clouded in uncertainty. Mercury oxidation by atomic bromine in the Arctic and the upper atmosphere is well established, but less is understood about oxidation pathways in conditions of anthropogenic photochemical smog. Many have observed rapid increases in oxidized mercury under polluted conditions, but it has not been clearly demonstrated that these increases are the result of local mercury oxidation. We measured elemental and oxidized mercury in an area that experienced abundant photochemical activity (ozone >100 ppb) during winter inversion (i.e., cold air pools) conditions that restricted entrainment of air from the oxidized mercury-rich upper atmosphere. Under these conditions, oxidized mercury concentrations decreased day-uponday, even as ozone and other pollutants increased dramatically. A box model that incorporated rapid kinetics for reactions of elemental mercury with ozone and OH radical overestimated observed oxidized mercury, while incorporation of slower, more widely accepted reaction rates did not. Our results show that rapid gas-phase mercury oxidation by ozone and OH in photochemical smog is unlikely.

Traceable Calibration of Atmospheric Oxidized Mercury Measurements

“…This may have occurred due to (1) rapid changes in ambient Hg 0 since total Hg and Hg 0 were not measured simultaneously or (2) contamination in the Hg 0 channel. When operating earlier versions of the dual-channel system, ,, persistent negative Hg II measurements occasionally occurred and were rectified by changing the cation exchange membranes or cleaning the channel. These negative Hg II values were thus presumed to be caused by contamination of the cation exchange membrane channel.…”

“…Ambient Hg 0 and Hg II were measured at 10 min time resolution using a dual-channel Hg measurement system developed at Utah State University (Figure S1) and based on systems used in several previous studies. ,, The dual-channel system pulled air through a heated inlet (110 °C) at a total flow of 9 L min –1 . The inlet contained an elutriator and quartz particle impactor with a 50% cut point at 2.5 μm aerodynamic diameter.…”

“…A similar dual-channel system has been used in previous studies. ,, The dual-channel system used in this study differed in a few significant ways. First, the inlet line of the dual-channel system was shortened to less than 1 m to minimize loss of Hg II .…”

Observations of the chemistry and concentrations of reactive Hg at locations with different ambient air chemistry

“…The average coefficient of determination (R 2 ) for the model used to interpolate the thermal desorption profiles (cf. Dunham-Cheatham et al, 2022) to determine the chemistry of the compounds was 0.993 ± 0.011, with the lowest R 2 values occurring when the Hg concentrations were lowest. A gaussian distribution was used, because it best reflects the profile of desorption of Hg compounds.…”

Determining sources of reactive mercury compounds in Reno, Nevada, United States

“…In a recent paper, Lyman et al (2022) did not observe a daily buildup of Hg II concurrent with ozone build up during photochemical smog episodes in a remote location affected by oil and gas recovery, casting doubt on the current theory of gas-phase Hg oxidation kinetics and mechanisms. During an inversion, oxide-Hg II compounds disappeared, leading to the conclusion that oxide-Hg II compounds were sourced from the free troposphere.…”