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.

Keywords: UTLS; atmospheric chemistry; halogens; heterogeneous chemistry; oxidative capacity.

Fig. 1.

Spectral proof of the detection of (A) BrO, RF01 in the tropical FT, and (C) IO, RF05 in the SH subtropical FT; red dots in (B) map of study area indicate locations along the flight tracks where BrO was detected above detection limit, while blue dots represents those for IO. AMAX-DOAS detection limits are ∼0.3−0.6 pptv for BrO and ∼0.04−0.1 pptv for IO (19) (see Methods). Dashed circles on B refer to the profiling locations. The unique BrO and IO fingerprint absorptions are shown in optical density units (the product of absorption cross-section times slant column density; it is overlaid on top of the instrument noise) for spectra of scattered solar photons collected during limb viewing forward of the aircraft at various altitudes.

Fig. 2.

Vertical profiles of (A) BrO, (B) IO, and (C) O₃ during RF01, RF04, RF05, RF12, and RF14. AMAX-DOAS measured BrO and IO are shown as solid circles, while the campaign-averaged profiles of BrO, IO, and O₃ are shown as open boxes. Solid black line in A is the campaign average GEOS-Chem (Goddard Earth Observation System - Chemistry model) base case simulation (GEOS-5 meteorological field). Also shown in A is the tropospheric BrO background in the tropical latitudes inferred from satellite (gray shading) and the equivalent tropospheric BrO level calculated using AMAX-DOAS data in this work (dashed gray vertical line). C shows campaign averages of GEOS-Chem modeled O₃ using GEOS-4 and GEOS-5 inputs.

Fig. 3.

ΔBrO vs. H₂O/O₃ in subtropical (A) and tropical (B) latitudes, as well as vertically binned ΔBr_y in the subtropics (C) and tropics (D). ΔBrO is defined as the BrO difference between AMAX-DOAS measured and BM calculated, while ΔBr_y is defined as the difference between Br_y inferred from AMAX-DOAS measurements and BM modeled. Black represents GEOS-Chem Br_y redistributed by BM, while blue (in B and D) denotes BM calculations including ice heterogeneous chemistry (ice content calculated by GEOS-5). Blue shading (B) shows sensitivity runs with ice content multiplied by ∼0.1−10. Error bars in A and B are ΔBrO uncertainty propagated from both measurement uncertainty, ∼30% modeling uncertainty and atmospheric variability (among different RFs), while error bars in C and D are ΔBr_y uncertainty propagated from ΔBrO uncertainty. Cyan shadings (A and B) show rough regimes corresponding to lower stratosphere (Strat.) or lower TTL.

Fig. 4.

Ozone loss rate and percentage contribution as calculated by box model. The box model was constrained to the median measured BrO and IO profiles using data from RF01, RF05, RF12, and RF14; profiles in Central Pacific are reproduced from Dix et al. (20) (only IO was measured; BrO is assumed to be ∼0.5 pptv for lack of measurements here). Error bars show 25th/75th percentiles among the five RFs to represent the atmospheric variability.