Active and widespread halogen chemistry in the tropical and subtropical free troposphere

Siyuan Wang, Johan A. Schmidt, Sunil Baidar, Sean Coburn, Barbara Dix, Theodore K. Koenig, Eric Apel, Dene Bowdalo, Teresa L. Campos, Ed Eloranta, Mathew J. Evans, Joshua P. Di Gangi, Mark Andrew Zondlo, Ru Shan Gao, Julie A. Haggerty, Samuel R. Hall, Rebecca S. Hornbrook, Daniel Jacob, Bruce Morley, Bradley PierceMike Reeves, Pavel Romashkin, Arnout Ter Schure, Rainer Volkamer

Research output: Contribution to journalArticlepeer-review

78 Scopus citations


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 (O3), oxidize mercury, and modify oxidative capacity that is relevant for the lifetime of greenhouse gases. Most of the tropospheric O3 and methane (CH4) 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°N to 40°S), and show that these halogens are responsible for 34% of the column-integrated loss of tropospheric O3. The observed BrO concentrations increase strongly with altitude (∼3.4 pptv at 13.5 km), and are 2-4 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 2.5-6.4 pptv total inorganic bromine (Bry), or model overestimated Bry wet scavenging. Our results highlight the importance of heterogeneous chemistry on ice clouds, and imply an additional Bry 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 halogencatalyzed loss of tropospheric O3 needs to be considered when estimating past and future ozone radiative effects.

Original languageEnglish (US)
Pages (from-to)9281-9286
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number30
StatePublished - Jul 28 2015

All Science Journal Classification (ASJC) codes

  • General


  • Atmospheric chemistry
  • Halogens
  • Heterogeneous chemistry
  • Oxidative capacity
  • UTLS


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