The Chemistry of Mercury in the Stratosphere. Issue 12 (15th June 2022)

• Record Type: Journal Article
• Authors: Saiz‐Lopez, Alfonso; Acuña, A. Ulises; Mahajan, Anoop S.; Dávalos, Juan Z.; Feng, Wuhu; Roca‐Sanjuán, Daniel; Carmona‐García, Javier; Cuevas, Carlos A.; Kinnison, Douglas E.; Gómez Martín, Juan Carlos; Francisco, Joseph S.; Plane, John M. C.
• Is Part Of: Geophysical research letters. Volume 49:Issue 12(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-06-15
• Subjects: mercury -- chemistry -- stratosphere; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2022GL097953
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Mercury, a global contaminant, enters the stratosphere through convective uplift, but its chemical cycling in the stratosphere is unknown. We report the first model of stratospheric mercury chemistry based on a novel photosensitized oxidation mechanism. We find two very distinct Hg chemical regimes in the stratosphere: in the upper stratosphere, above the ozone maximum concentration, Hg 0 oxidation is initiated by photosensitized reactions, followed by second‐step chlorine chemistry. In the lower stratosphere, ground‐state Hg 0 is oxidized by thermal reactions at much slower rates. This dichotomy arises due to the coincidence of the mercury absorption at 253.7 nm with the ozone Hartley band maximum at 254 nm. We also find that stratospheric Hg oxidation, controlled by chlorine and hydroxyl radicals, is much faster than previously assumed, but moderated by efficient photo‐reduction of mercury compounds. Mercury lifetime shows a steep increase from hours in the upper‐middle stratosphere to years in the lower stratosphere. Plain Language Summary: Mercury is a toxic pollutant that is released to the atmosphere as a result of human activities and from natural sources. Due to its long lifetime, mercury can travel around the world and also can be injected to the stratosphere via convective uplift in the tropical regions. Aircraft observations have reported significant concentrations of different forms of mercury in the stratosphere, however, the chemistry of mercury in this region of the atmosphere remains unexplored. This work reports a new chemical framework that sheds light on the chemical cycling of stratospheric mercury. The results reveal the stratosphere as a unique environment for mercury chemistry that has potential implications for the global transport of this metal. Key Points: A new chemical mechanism for photosensitized mercury oxidation is proposed Above the UV‐absorbing ozone layer, elemental mercury is quickly oxidized by photosensitized reactions In the lower stratosphere, the photosensitized oxidation is replaced by much slower thermal oxidation driven by Cl, OH, and Br radicals