Importance of reactive halogens in the tropical marine atmosphere: a regional modelling study using WRF-Chem
Authors: Badia Alba , Reeves Claire E. , Baker Alex R. , Saiz-Lopez Alfonso , Volkamer Rainer , Koenig Theodore K. , Apel Eric C. , Hornbrook Rebecca S. , Carpenter Lucy J. , Andrews Stephen J. , Sherwen Tomás , von Glasow Roland 
Published: March 2019

Abstract. This study investigates the impact of reactive halogen species (RHS,
containing chlorine (Cl), bromine (Br) or iodine (I)) on atmospheric
chemistry in the tropical troposphere and explores the sensitivity to
uncertainties in the fluxes of RHS to the atmosphere and their chemical
processing. To do this, the regional chemistry transport model WRF-Chem has
been extended to include Br and I, as well as Cl chemistry for the first
time, including heterogeneous recycling reactions involving sea-salt aerosol
and other particles, reactions of Br and Cl with volatile organic compounds
(VOCs), along with oceanic emissions of halocarbons, VOCs and inorganic
iodine. The study focuses on the tropical east Pacific using field
observations from the Tropical Ocean tRoposphere Exchange of Reactive halogen
species and Oxygenated VOC (TORERO) campaign (January–February 2012) to
evaluate the model performance. Including all the new processes, the model does a reasonable job reproducing
the observed mixing ratios of bromine oxide (BrO) and iodine oxide (IO),
albeit with some discrepancies, some of which can be attributed to
difficulties in the model's ability to reproduce the observed halocarbons.
This is somewhat expected given the large uncertainties in the air–sea
fluxes of the halocarbons in a region where there are few observations of
their seawater concentrations. We see a considerable impact on the inorganic bromine (Bry)
partitioning when heterogeneous chemistry is included, with a greater
proportion of the Bry in active forms such as BrO, HOBr and
dihalogens. Including debromination of sea salt increases BrO slightly
throughout the free troposphere, but in the tropical marine boundary layer,
where the sea-salt particles are plentiful and relatively acidic,
debromination leads to overestimation of the observed BrO. However, it should
be noted that the modelled BrO was extremely sensitive to the inclusion of
reactions between Br and the oxygenated VOCs (OVOCs), which convert Br to
HBr, a far less reactive form of Bry. Excluding these
reactions leads to modelled BrO mixing ratios greater than observed. The
reactions between Br and aldehydes were found to be particularly important,
despite the model underestimating the amount of aldehydes observed in the
atmosphere. There are only small changes to the inorganic iodine
(Iy) partitioning and IO when the heterogeneous reactions,
primarily on sea salt, are included. Our model results show that tropospheric Ox loss due to
halogens ranges between 25 % and 60 %. Uncertainties in the
heterogeneous chemistry accounted for a small proportion of this range
(25 % to 31 %). This range is in good agreement with other estimates
from state-of-the-art atmospheric chemistry models. The upper bound is found
when reactions between Br and Cl with VOCs are not included and,
consequently, Ox loss by BrOx,
ClOx and IOx cycles is high (60 %).
With the inclusion of halogens in the troposphere, O3 is reduced by
7 ppbv on average. However, when reactions between Br and Cl with VOCs are
not included, O3 is much lower than observed. Therefore, the
tropospheric Ox budget is highly sensitive to the inclusion
of halogen reactions with VOCs and to the uncertainties in current
understanding of these reactions and the abundance of VOCs in the remote
marine atmosphere.