@article{1221,
  author = {Siyuan Wang and Johan Schmidt and Sunil Baidar and Sean Coburn and Barbara Dix and Theodore Koenig and Eric Apel and Dene Bowdalo and Teresa Campos and Ed Eloranta and Mathew Evans and Joshua DiGangi and Mark Zondlo and Ru-Shan Gao and Julie Haggerty and Samuel Hall and Rebecca Hornbrook and Daniel Jacob and Bruce Morley and Bradley Pierce and Mike Reeves and Pavel Romashkin and Arnout Schure and Rainer Volkamer},
  title = {Active and widespread halogen chemistry in the tropical and subtropical free troposphere},
  abstract = {Significance Our measurements show that tropospheric halogen chemistry has a larger capacity to destroy O 3 and oxidize atmospheric mercury than previously recognized. Halogen chemistry is currently missing in most global and climate models, and is effective at removing O 3 at altitudes where intercontinental O 3 transport occurs. It further helps explain the low O 3 levels in preindustrial times. Public health concerns arise from bioaccumulation of the neurotoxin mercury in fish. Our results emphasize that bromine chemistry in the free troposphere oxidizes mercury at a faster rate, and makes water-soluble mercury available for scavenging by thunderstorms. Naturally occurring bromine in air aloft illustrates global interconnectedness between energy choices affecting mercury emissions in developing nations and mercury deposition in, e.g., Nevada, or the southeastern United States. , Halogens in the troposphere are increasingly recognized as playing an important role for atmospheric chemistry, and possibly climate. Bromine and iodine react catalytically to destroy ozone (O 3 ), oxidize mercury, and modify oxidative capacity that is relevant for the lifetime of greenhouse gases. Most of the tropospheric O 3 and methane (CH 4 ) loss occurs at tropical latitudes. Here we report simultaneous measurements of vertical profiles of bromine oxide (BrO) and iodine oxide (IO) in the tropical and subtropical free troposphere (10{\textdegree}N to 40{\textdegree}S), and show that these halogens are responsible for 34\% of the column-integrated loss of tropospheric O 3 . The observed BrO concentrations increase strongly with altitude (\~{}3.4 pptv at 13.5 km), and are 2{\textendash}4 times higher than predicted in the tropical free troposphere. BrO resembles model predictions more closely in stratospheric air. The largest model low bias is observed in the lower tropical transition layer (TTL) over the tropical eastern Pacific Ocean, and may reflect a missing inorganic bromine source supplying an additional 2.5{\textendash}6.4 pptv total inorganic bromine (Br y ), or model overestimated Br y wet scavenging. Our results highlight the importance of heterogeneous chemistry on ice clouds, and imply an additional Br y source from the debromination of sea salt residue in the lower TTL. The observed levels of bromine oxidize mercury up to 3.5 times faster than models predict, possibly increasing mercury deposition to the ocean. The halogen-catalyzed loss of tropospheric O 3 needs to be considered when estimating past and future ozone radiative effects.},
  year = {2015},
  journal = {Proceedings of the National Academy of Sciences},
  volume = {112},
  number = {30},
  pages = {9281{\textendash}9286},
  month = {jul},
  issn = {0027-8424, 1091-6490},
  url = {https://pnas.org/doi/full/10.1073/pnas.1505142112},
  doi = {10.1073/pnas.1505142112},
  language = {en},
}