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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-892
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
10 Oct 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
Impacts of bromine and iodine chemistry on tropospheric OH and HO2: Comparing observations with box and global model perspectives
Daniel Stone1, Tomás Sherwen2, Mathew J. Evans2,3, Stewart Vaughan1, Trevor Ingham1,4, Lisa K. Whalley1,4, Peter M. Edwards2, Katie A. Read2,3, James D. Lee2,3, Sarah J. Moller2,3, Lucy J. Carpenter2,3, Alastair C. Lewis2,3, and Dwayne E. Heard1,4 1School of Chemistry, University of Leeds, Leeds, UK
2Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, UK
3National Centre for Atmospheric Science, University of York, York, UK
4National Centre for Atmospheric Science, University of Leeds, Leeds, UK
Abstract. The chemistry of the halogen species bromine and iodine has a range of impacts on tropospheric composition, and can affect oxidising capacity in a number of ways. However, recent studies disagree on the overall sign of the impacts of halogens on the oxidising capacity of the troposphere. We present simulations of OH and HO2 radicals for comparison with observations made in the remote tropical ocean boundary layer during the Seasonal Oxidant Study at the Cape Verde Atmospheric Observatory in 2009. We use both a constrained box model, using detailed chemistry derived from the Master Chemical Mechanism (v3.2), and the three-dimensional global chemistry transport model GEOS-Chem. Both model approaches reproduce the diurnal trends in OH and HO2. Absolute observed concentrations are well reproduced by the box model but are overpredicted by the global model, potentially owing to incomplete consideration of oceanic sourced radical sinks. The two models, however, differ in the impacts of halogen chemistry. In the box model, halogen chemistry acts to increase OH concentrations (by 9.8 % at midday at Cape Verde), while the global model exhibits a small increase in OH at Cape Verde (by 0.6 % at midday) but overall shows a decrease in the global annual mass weighted mean OH of 4.5 %. These differences reflect the variety of timescales through which the halogens impact the chemical system. On short timescales, photolysis of HOBr and HOI, produced by reactions of HO2 with BrO and IO, respectively, increases the OH concentration. On longer timescales, halogen catalysed ozone destruction cycles lead to lower primary production of OH radicals through ozone photolysis, and thus to lower OH concentrations. The global model includes more of the longer timescale responses than the constrained box model and overall the global impact of the longer timescale response (reduced primary production due to lower O3 concentrations) overwhelms the shorter timescale response (enhanced cycling from HO2 to OH), and thus the global OH concentration decreases. The Earth system contains many such responses on a large range of timescales. This work highlights the care that needs to be taken to understand the full impact of any one process on the system as a whole.

Citation: Stone, D., Sherwen, T., Evans, M. J., Vaughan, S., Ingham, T., Whalley, L. K., Edwards, P. M., Read, K. A., Lee, J. D., Moller, S. J., Carpenter, L. J., Lewis, A. C., and Heard, D. E.: Impacts of bromine and iodine chemistry on tropospheric OH and HO2: Comparing observations with box and global model perspectives, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-892, in review, 2017.
Daniel Stone et al.
Daniel Stone et al.
Daniel Stone et al.

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Short summary
Halogen chemistry in the troposphere impacts oxidising capacity, but model studies assessing the nature of these impacts can vary according to the model framework used. In this work we present simulations of OH and HO2 radicals using both box and global model frameworks, and compare to observations made at the Cape Verde Atmospheric Observatory. We highlight, and rationalise, differences between the model frameworks.
Halogen chemistry in the troposphere impacts oxidising capacity, but model studies assessing the...
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