title: Active and widespread halogen chemistry in the tropical and subtropical free troposphere.
abstract: 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 halogen-catalyzed loss of tropospheric O3 needs to be considered when estimating past and future ozone radiative effects.
authors.affiliation: Department of Chemistry & Biochemistry, University of Colorado, Boulder, CO 80309-0215; Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309-0216;
authors.forename: Siyuan
authors.initials: S
authors.lastname: Wang
authors.affiliation: School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138; Department of Chemistry, University of Copenhagen, 2100 Copenhagen, Denmark;
authors.forename: Johan A
authors.initials: JA
authors.lastname: Schmidt
authors.affiliation: Department of Chemistry & Biochemistry, University of Colorado, Boulder, CO 80309-0215; Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309-0216;
authors.forename: Sunil
authors.initials: S
authors.lastname: Baidar
authors.affiliation: Department of Chemistry & Biochemistry, University of Colorado, Boulder, CO 80309-0215; Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309-0216;
authors.forename: Sean
authors.initials: S
authors.lastname: Coburn
authors.affiliation: Department of Chemistry & Biochemistry, University of Colorado, Boulder, CO 80309-0215;
authors.forename: Barbara
authors.initials: B
authors.lastname: Dix
authors.affiliation: Department of Chemistry & Biochemistry, University of Colorado, Boulder, CO 80309-0215; Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309-0216;
authors.forename: Theodore K
authors.initials: TK
authors.lastname: Koenig
authors.affiliation: Earth System Laboratory, Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO 80307;
authors.forename: Eric
authors.initials: E
authors.lastname: Apel
authors.affiliation: Department of Chemistry, University of York, York YO10 5DD, United Kingdom;
authors.forename: Dene
authors.initials: D
authors.lastname: Bowdalo
authors.affiliation: Earth System Laboratory, Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO 80307;
authors.forename: Teresa L
authors.initials: TL
authors.lastname: Campos
authors.affiliation: Space Science and Engineering Center, University of Wisconsin, Madison, WI 02138;
authors.forename: Ed
authors.initials: E
authors.lastname: Eloranta
authors.affiliation: Department of Chemistry, University of York, York YO10 5DD, United Kingdom;
authors.forename: Mathew J
authors.initials: MJ
authors.lastname: Evans
authors.affiliation: Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544;
authors.forename: Joshua P
authors.initials: JP
authors.lastname: DiGangi
authors.affiliation: Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544;
authors.forename: Mark A
authors.initials: MA
authors.lastname: Zondlo
authors.affiliation: Earth System Research Laboratory, Chemical Science Division, National Oceanic and Atmospheric Administration, Boulder, CO 80305;
authors.forename: Ru-Shan
authors.initials: RS
authors.lastname: Gao
authors.affiliation: Earth Observing Laboratory, Research Aviation Facility, National Center for Atmospheric Research, Boulder, CO 80307;
authors.forename: Julie A
authors.initials: JA
authors.lastname: Haggerty
authors.affiliation: Earth System Laboratory, Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO 80307;
authors.forename: Samuel R
authors.initials: SR
authors.lastname: Hall
authors.affiliation: Earth System Laboratory, Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO 80307;
authors.forename: Rebecca S
authors.initials: RS
authors.lastname: Hornbrook
authors.affiliation: School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138;
authors.forename: Daniel
authors.initials: D
authors.lastname: Jacob
authors.affiliation: Earth Observing Laboratory, Research Aviation Facility, National Center for Atmospheric Research, Boulder, CO 80307;
authors.forename: Bruce
authors.initials: B
authors.lastname: Morley
authors.affiliation: The National Environmental Satellite, Data, and Information Service, National Oceanic and Atmospheric Administration, Madison, WI 53706;
authors.forename: Bradley
authors.initials: B
authors.lastname: Pierce
authors.affiliation: Earth Observing Laboratory, Research Aviation Facility, National Center for Atmospheric Research, Boulder, CO 80307;
authors.forename: Mike
authors.initials: M
authors.lastname: Reeves
authors.affiliation: Earth Observing Laboratory, Research Aviation Facility, National Center for Atmospheric Research, Boulder, CO 80307;
authors.forename: Pavel
authors.initials: P
authors.lastname: Romashkin
authors.affiliation: Electric Power Research Institute, Palo Alto, CA 94304.
authors.forename: Arnout
authors.initials: A
authors.lastname: Ter Schure
authors.affiliation: Department of Chemistry & Biochemistry, University of Colorado, Boulder, CO 80309-0215; Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309-0216; rainer.volkamer@colorado.edu.
authors.forename: Rainer
authors.initials: R
authors.lastname: Volkamer
keywords: UTLS; atmospheric chemistry; halogens; heterogeneous chemistry; oxidative capacity