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.
Introduction
Reactive halogen species
(RHS) cause ozone (O3) destruction, change the HOx
(HO2+OH) and NOx (NO2+NO) partitioning,
affect the oxidation of volatile organic compounds (VOCs) and mercury, and
take part in new particle formation
. Moreover, reactive chlorine
reduces the lifetime of methane (CH4). Halogen species are known to
play an important role in the oxidizing capacity of the troposphere. The
atmospheric oxidation capacity is to a large extent determined by budgets of
the hydroxyl radical (OH) and O3; globally, most tropospheric OH is
found in the tropics . Therefore, a quantitative
understanding of the composition and chemistry of the tropical marine
atmosphere is essential to examine the atmospheric oxidative capacity and
climate forcing.
In the troposphere, reactive halogen species catalyse ozone destruction cycles:
O3+X→XO+O2HO2+XO→HOX+O2HOX+hv→OH+X,
where X=Cl, Br, I.
In the past, tropospheric halogen chemistry has been studied using a number
of box models and 1-D models
.
Currently, there are several global models that have been used to study
tropospheric halogens
.
Numerical models predict that reactive halogen compounds account for 30 %
of O3 destruction in the marine boundary layer (MBL)
and
5 %–20 % globally
. Up to
34 % of O3 loss is calculated to be due to I and Br combined in
the tropical east Pacific .
However, there are only a few regional models that have studied tropospheric
halogens. Chlorine chemistry was implemented into the WRF-Chem model
and into the Community Multi-scale Air Quality (CMAQ) model
to study the formation of nitryl chloride
(ClNO2) from the uptake of dinitrogen pentoxide (N2O5) on
aerosols containing chloride. Moreover, bromine and iodine chemistry was
implemented in CMAQ in and
, where the impact of iodide-mediated O3
deposition on surface ozone concentrations was studied, and in the recent
work of , which concluded that oceanic halogens
and dimethyl sulfide (DMS) emissions need to be included into the regional
models to accurately reproduce the air quality in coastal cities.
Oceanic emissions provide a source of very-short-lived halocarbons (VSLHs) to
the atmosphere, defined as trace gases with chemical lifetimes generally
under 6 months, mainly in the form of bromoform (CHBr3),
dibromomethane (CH2Br2) and methyl iodide (CH3I). Once in
the atmosphere, VSLHs (and their degradation products) can ascend into the
lower stratosphere (LS), where they can contribute to the Bry
and lead to ozone depletion. Several emissions inventories for the VSLHs have
been evaluated at a global scale
.
Recent measurements constrain the stratospheric injection of bromine from
VSLHs as ∼5 pptv Bry, confirming
recent WMO estimates. About 40 %–50 % of the bromine (2.1–2.6 pptv
Bry) is injected into stratosphere as product gases
. presents a
comparison of two simulations using the chemistry climate model EMAC. The
first simulation computes the oceanic emissions online, mainly driven by the
surface water concentrations and modelled meteorological variables, and the
second uses prescribed emissions. These results reveal that calculating the
air–sea fluxes online leads, in most cases, to more accurate atmospheric
mixing ratios in the model in comparison with the simulation using prescribed
emissions. Emissions of inorganic iodine compounds (HOI and I2) have
been recognized as a significant source required to reproduce iodine oxide
(IO) measurements over the open ocean
and have been included in some
global models .
There are indications that the chemistry of reactive halogens and that of oxygenated
VOCs (OVOCs) in the tropics are interrelated. Model calculations suggest
aldehydes are an important sink for bromine atoms and hence compete with the
formation of bromine oxide (BrO) (Br+O3→BrO). This
illustrates a link between the cycles of halogens and OVOCs in the marine
atmosphere .
Recent studies have highlighted the key role that heterogeneous chemistry
plays in explaining observations of BrO and IO abundances in the tropical
troposphere. Cycling of Br and I through HOBr, BrNO3,
HOI and INO3 is very slow in the gas phase, making it
necessary to include heterogeneous reactions involving reactive halogen
species to reproduce observed BrO and IO abundances
.
Another source of reactive inorganic bromine in the troposphere is the
release of bromide (Br-) from sea-salt aerosols into the gas phase.
This is known as debromination and occurs through the uptake of a gaseous
species in sea salt and the subsequent reaction with Br-.
Debromination has been included as a source of gas-phase bromine in several
atmospheric models
.
However, this process is poorly understood and its inclusion into the models
can cause inconsistent high levels of bromine species .
Halogen chemistry in atmospheric models remains largely untested due to a lack
of field observations of halogen species. However, during the last few years,
there have been four campaigns that provided vertically resolved measurements
of halogen radicals: the Tropical Ocean tRoposphere Exchange of Reactive
halogen species and Oxygenated VOC
TORERO;, the
CONvective TRansport of Active Species in the Tropics
CONTRAST;, the
Coordinated Airborne Studies in the Tropics
CAST; and the Airborne Tropical
TRopopause EXperiment ATTREX;.
The main objective of this study is to investigate the atmospheric chemistry
in the tropical East Pacific with a focus on reactive halogens using the
Weather Research and Forecasting model coupled with Chemistry
WRF-Chem; and field data from the TORERO campaign
. Our reaction mechanism in WRF-Chem is
based on the Model for OZone and Related chemical Tracers version 4 (MOZART-4) mechanism
and has been extended to include halogen chemistry. Heterogeneous recycling
reactions involving halogens have been included into the model, along with
oceanic emissions of relevant VOCs and halocarbons. The observational data
are described in Sect. . Model developments are described
in Sect. . The model setup and the description of
different sensitivity runs are in Sect. . The results of the
model performance are discussed in Sect. . The last
section summarizes the conclusions of this work.
Observational data
The TORERO campaign , from 15 January to
1 March 2012, was used to evaluate the model. Data on halocarbons are
available from the TORERO ship cruise and flights of
the NSF/NCAR GV aircraft, whilst
observations of O3, BrO, IO and OVOCs are available from the flights.
The TORERO cruise aboard the NOAA RV Ka'imimoana (KA-12-01) took
place from Honolulu, HI, to Puntarenas, Costa Rica, between 27 January and
1 March 2012. Air samples from the TORERO ship cruise were taken from a 10 m
bow mast and surface water samples were taken from the underway supply.
Halocarbons in air and water phases were measured using two automated online
gas chromatography–mass spectrometry (GC-MS) systems
and calibrated using NOAA standard SX-3570. Ozone was measured by UV
absorption , OVOCs were measured by the Trace Organic
Gas Analyzer (TOGA) , and BrO and IO radicals were measured
by the University of Colorado Airborne Multi-AXis Differential Optical
Absorption Spectroscopy (CU AMAX-DOAS) instrument with typical detection
limits of 0.5 pptv for BrO and 0.05 pptv for IO
. A total of 13 flights provide
O3 data and 16 flights provide BrO and IO data.
Figure displays the location of all the observational data
with an orange line for the cruise track, red lines for the flights in the
tropics and green lines for the flights in the subtropics.
Flight and cruise tracks from the TORERO campaign (January–February
2012). Cruise track is represented by a light orange line. Flights are
grouped by the following regions: tropical (red lines) and subtropical (green
lines). Two different domains were defined: domain to evaluate the cruises
(dark orange square) and domain to evaluate aircrafts (green
square).
Model description
WRF-Chem is a highly flexible community model for
atmospheric research where aerosol–radiation–cloud feedback processes are
taken into account. Version 3.7.1 is used in this study.
The sources of halogen atoms considered in this study are inorganic I from
the ocean (HOI and I2), oceanic source of organic halogens
(CHBr3, CH2Br2, CH3I, CH2BrCl,
CHBrCl2, CHBr2Cl, CH2I2, CH2IBr and
CH2ICl) and debromination (Br2, IBr). These sources
are explained in detail below in the following sections.
Oceanic fluxes
The oceanic emission of inorganic iodine (HOI and I2) follows the
deposition of O3 to the surface ocean and reaction with iodide
(I-) . We use Eqs. (19) and (20) in
for the calculation of these emissions. Ocean surface
I- is parameterized using (see Fig. S1 in
the Supplement). Figure shows the average oceanic emission for
inorganic iodine (I2 in Fig. a and HOI in Fig. b)
during January and February 2012. Higher emissions for inorganic iodine
occur in the tropics with HOI being the dominant species.
Mean oceanic surface fluxes for inorganic iodine:
I2(a) and HOI (b). The column-integrated fluxes
for inorganic bromine (Br2, c) from the debromination
process during January and February 2012 are also shown. Values are given in
106 molec cm-2 s-1.
Two different approaches for the marine emissions of the halocarbons
(CHBr3, CH2Br2, CH3I, CH2BrCl,
CHBrCl2, CHBr2Cl, CH2I2, CH2IBr and
CH2ICl) are examined in this model. The first approach uses
prescribed monthly average oceanic fluxes from and
the second computes the oceanic fluxes online. Prescribed monthly average
oceanic fluxes from were calculated using 6-hourly
means of wind speed and sea surface temperature from the ERA-Interim
meteorological assimilation database for the years
1989–2011 (1∘×1∘). Computing the emissions online
accounts for an interaction between the modelled atmosphere and the ocean at
each time step. Thus, this approach can respond to changes in meteorological
parameters, like surface temperature and surface wind speed. A two-layer
model is used to calculate the halocarbons air–sea
fluxes:
F=-Ka⋅Cg-KH⋅Cl,
where Ka is the transfer velocity of the gas (s-1),
Cg (ppm) and Cl (nM) are the bulk gas and
liquid-phase concentrations, and KH is Henry's law constant.
Ka is parameterized following which is
mainly a function of wind speed and sea surface temperature (SST) taken from
the model at each time step. Cg is also taken from the model.
Halocarbon seawater concentrations Cl are taken from
. Figure shows the average air–sea
fluxes for CHBr3, CH2Br2 and CH3I during January and
February 2012 for the two approaches. Note that the online calculation could
increase, decrease or even reverse the fluxes in comparison with the
prescribed emissions. This is the case for the online fluxes of CHBr3
over the tropics where the model calculates negative fluxes, whereas the
prescribed fluxes are positive.
Mean oceanic fluxes for halocarbons (CHBr3, CH2Br2
and CH3I) during January and February 2012. Prescribed fluxes are
shown on the top (a, b, c) and online fluxes on the bottom
(d, e, f). Values are given in
106 molec cm-2 s-1.
Recent studies suggest that the ocean is an important source of OVOCs such as
acetaldehyde, ethanol and methanol
that models do not generally consider or are not able to capture
. Oceanic fluxes of several VOCs
have been included into WRF-Chem as part if this study. For the three
OVOCs (acetaldehyde (CH3CHO), ethanol (C2H6O) and methanol
(CH3OH)), the same online approach as for the VSLHs is used to calculate
the marine fluxes where their seawater concentrations are taken from
.
Emissions for alkenes and alkanes (C2H4, C3H6,
C2H6, C3H8) are prescribed and based on the POET
global inventory.
Deposition over the ocean for the halocarbons and OVOCs is included in the
air–sea fluxes described above. For the rest of the species, dry deposition
is calculated with the Wesely scheme , which is used over land
for several species. Washout of gases by precipitation is simulated using the
scheme included in WRF-Chem which was
modified to include Henry's law constants for the RHS shown in
Table .
Henry's law constant for relevant halogen species implemented in
WRF-Chem. INO2 Henry's law constant is assumed equal to that of
BrNO2. Iodine oxide (I2Ox) Henry's law constants
are assumed to be infinity by analogy with INO3. Virtually infinity
solubility is represented by using a very large number (2.69×1015).
SpeciesHenry's law constant (H)d(lnH)d(1/T)Referenceat 298 K (M atm-1)(K)ClNO3∞–BrNO3∞–INO3∞–HOCl6.5×1025900HOBr1.9×103–HOI4.5×102–HCl*7.1×10155900HBr*7.5×101310 200, HI*7.4×10133190, BrCl9.0×10-15600IBr2.4×101–ICl1.1×102–BrNO23.0×10-1–ClNO24.0×10-2–INO23.0×10-1–See caption textI22.6×1004600Br28.0×10-14000I2O2∞–See caption textI2O3∞–See caption textI2O4∞–See caption text
* Effective Henry's law of HX is calculated for acid
conditions (pH =4.5): KH*(T)=KH(T)×1+Ka[H+], where X =Cl, I or Br
and Ka=1×109 M is the acid dissociation constant
.
The sea-salt aerosol emissions parameterization used in this study is
described in . This parameterization is mainly a
function of wind speed from the model and uses the emissions scheme from
for particles with dry diameters of 0.45 nm or more, and
for smaller particles, it uses .
Gas-phase chemistry scheme
Our reaction mechanism is based on the MOZART-4 mechanism
. This mechanism has been
extended to include bromine, chlorine and iodine chemistry and has been
coupled with the Model for Simulating Aerosol Interactions and Chemistry
(MOSAIC) four-bin aerosol module . A total of 48
species and 159 halogen reactions have been included (see
Tables ,
and for details). Inorganic, organic and inter-halogen
reactions come from the 1-D model MISTRA .
Production and loss reactions of the higher order of iodine oxides
(I2Ox, where x=2,3,4)
reactions have been included into the model. Photochemistry of
I2Ox species is still an area of high uncertainty in
atmospheric iodine chemistry .
Chemical loss of VSLHs through oxidation by the hydroxyl radical (OH) and by
photolysis is included using data from .
Bimolecular and thermal decomposition halogen reactions included in
WRF-Chem. These reactions are given in the Arrhenius form with the rate equal
to A×e-EaRT.
Termolecular reactions for halogens species included in WRF-Chem.
The lower pressure limit rate (K0) is given by A0×T300a. The high pressure limit (K∞) is
given by B0×300Tb. Fc describes the fall
of curve of the reaction described by . Then the
reaction rate (k) is defined as
K0[M]/1+K0[M]K∞×Fcn and n as
1+log10K0[M]K∞2-1.
A schematic representation of the main bromine and iodine chemistry
implemented in the model is shown in Fig. . Chlorine
chemistry is also included into the model; however, since our results are
mainly focused on reactive bromine and iodine, we do not include chlorine
chemistry in Fig. .
Schematic representation
of the implemented iodine and bromine chemistry in WRF-Chem. Chlorine
chemistry has been included into the model; since our results are mainly
focused on reactive bromine and iodine, we decided not to include chlorine
chemistry in this figure. Red lines represent photolytic reactions, dark blue
lines gas-phase pathways, light blue lines fluxes, green lines deposition and
purple curved lines heterogeneous pathways.
Photolysis reactions included in the mechanism are listed in
Table . To compute the photolysis rates the fast
Tropospheric Ultraviolet–Visible (FTUV) online scheme is used. The quantum yields
and cross section for the photolytic reactions of halogens are from the Jet
Propulsion Laboratory (JPL) 10-6 and have been
linearly interpolated onto the 17 wavelength bins used by FTUV. For
I2Ox, we use the quantum yield and cross-section data from
.
Halogen and VOC reactions
Reactions between halogens and VOCs can be important for regulating reactive
halogen chemistry in the MBL by promoting the conversion of Cl and Br atoms
into HCl and HBr or more stable organic halogenated intermediates. The
oxidation of methane (CH4), formaldehyde (CH2O), acetaldehyde
(CH3CHO), methanol (CH3OH), methyl hydroperoxide
(CH3OOH), methylperoxy (CH3O2), ethane (C2H6),
ethene (C3H6) and propene (C3H6) by Cl is included in the
chemical mechanism. In addition, the oxidation of CH2O,
CH3CHO, C3H6 and C3H6 by bromine is also included
in the chemical mechanism with a simplified version of the chemical scheme
presented in used for reactions of bromine with
alkenes:
Br+C2H4+O2→BrRO2Br+C3H6+O2→BrRO2,
where BrRO2 is a brominated peroxy radical.
The loss of BrRO2 is represented by the following reactions:
BrRO2+NO→xHBr+(1-x)Br+CH3CO3+NO2+0.5CH2O+HO2BrRO2+CH3O2→xHBr+(1-x)Br+CH3CO3+HO2+CH2OBrRO2+HO2→BrOR+H2O.
The loss of BrOR is represented by the following reactions:
BrOR+OH→0.5(xHBr+(1-x)Br)+0.5BrRO2+0.5OH+0.5CH3CHOBrOR+hv→xHBr+(1-x)BrOH+HO2+CH3CO3+0.5CH2O,
where BrOR is a brominated organic specie and x is a number between 0 and
1.
Reaction rates for these reactions and deposition velocities are taken from
. Kinetic data for these reactions are poor, and the
partitioning of the products (HBr:Br) is not
clear. Based on the description, it is assumed that
x=0.2 such that the partitioning for HBr:Br
is 1:4 (Kenjiro Toyota, personal communication, 2017).
Heterogeneous chemistry
Heterogeneous reactions on particle surfaces involving halogens are
summarized in Table . The heterogeneous chemistry is assumed
to take place between a gas-phase species and an adsorbed species. The bulk
aqueous-phase chemistry in sea-salt aerosols is not treated. Uptake
coefficients are used to calculate first-order rate constants for
heterogeneous loss of the gas phase to the adsorbing surface
. This follows the approach used by
, which assumes a free molecular transfer regime
approximation. The reaction rate constants, K (s-1), are given by
K=γ4⋅S⋅A,
where γ is the uptake coefficient, S is the root mean square
molecular speed (m s-1) and A is the total available aerosol surface
area density (cm2 cm-3). Equation () does not take account
of any diffusion limitation (i.e. the rate at which gases can diffuse towards
the aerosol surface).
Halogen heterogeneous reactions added to WRF-Chem in this
study.
Heterogeneous reactionsNoteUptake coefficientINO3→0.5IBr+0.5ICl+HNO3Sea salt only if pH <5.50.01INO3→0.5I2+HNO3Sea salt only if pH >5.50.01INO2→0.5IBr+0.5ICl+HNO3Sea salt only if pH <5.50.02INO2→0.5I2+HNO3Sea salt only if pH >5.50.02HOI→0.5IBr+0.5IClSea salt only if pH <5.50.06HOI→0.5I2Sea salt only if pH >5.50.06BrNO3→0.6Br2+HNO3Sea salt only if pH <5.50.08BrNO2→0.6Br2+HNO3Sea salt only if pH <5.50.04HOBr→0.6Br2Sea salt only if pH <5.50.1I2O2→I(aerosol)0.02I2O3→I(aerosol)0.02I2O4→I(aerosol)0.02ClNO3→HOCl+HNO3Hydrolysis0.001a/0.01bBrNO3→HOBr+HNO3Hydrolysis0.03a/0.8bClNO3+HCl→Cl2+HNO30.1ClNO3+HBr→BrCl+HNO30.1HOBr+HBr→Br2+H2O0.1HOBr+HCl→BrCl+H2O0.1
a Uptake coefficient for moderate temperature.
b Uptake coefficient for cold temperatures.
We test the sensitivity of our results by adding the diffusion term,
Dg (cm2 s-1), following ,
where the reaction rate constant, K (s-1), is given by
K=∑i=1nbin4γ+riDg⋅Ai,
where nbin is the number of particle-size bins, ri is the
particle radius for bin i (cm), and Ai is the available aerosol surface
area density for bin i (cm2 cm-3). K is integrated over the
aerosol size distribution in order to resolve the dependence of the rate
constant for the particle radius. Following ,
Dg is given by
Dg=1.53×10181mg+1mair12⋅T12na,
where mg is the mean molecular mass of the gas-phase specie
(g mol-1), mair is the molecular mass of air
(g mol-1), na is the total air number density
(molecules cm-3), and T is the temperature (K).
Second-order reaction rate constants are calculated by dividing the
first-order rate constant by the concentrations of the adsorbed species.
Heterogeneous halogen activation is very efficient under cold or
stratospheric conditions as compared to moderate temperatures. For this
reason, we have made a distinction between moderate (>243.15 K) and cold
temperatures (<243.15 K) in some reactions. Uptake coefficients for
reactions in Table are based on literature values where
available
.
There are six reactions implemented for sea-salt particles. The sea-salt
surface area is calculated in the following way: (1) using the mass of Na and
Cl and the associated H2O for each bin and the individual dry
densities (for Na, Cl and H2O), the total volume of those particles for each bin is calculated and
then, (2) assuming that sea-salt aerosols are spheres, the total surface area
is calculated for each bin using this volume and the radius of aerosols in
each bin.
It is known that the chemistry involving the release of bromine from the
sea-salt aerosol (debromination) is strongly pH dependent, being more
efficient for acidified aerosol especially with a pH <5.5. Therefore, the pH value of the aerosol particles is
calculated in the model for each size bin (see for
further description of the pH calculation). We then apply a pH dependence to
the heterogeneous reactions that occur on the surface of the sea salt. When
the pH <5.5 debromination reactions occur with the release of Br2 and
IBr resulting from the uptake of BrNO3, BrNO2, HOBr,
INO3, INO2 and HOI (Reactions R11–R16). When the pH >5.5
no debromination reactions occur, although uptake of INO3,
INO2 and HOI on the sea salt still occurs (Reactions R17–R19)
leading to a change in iodine speciation but no release of Br. See also
Table .
If the pH <5.5,
BrNO3→0.6Br2+HNO3BrNO2→0.6Br2+HNO3HOBr→0.6Br2INO3→0.5IBr+0.5ICl+HNO3INO2→0.5IBr+0.5ICl+HNO3HOI→0.5IBr+0.5ICl.
If the pH >5.5,
INO3→0.5I2+HNO3INO2→0.5I2+HNO3HOI→0.5I2.
Due to the high uncertainty in the debromination process, the fraction of
Br2 formed by Reactions (R11)–(R13) was chosen arbitrarily in order
to add an extra bromine source in a simple way. A value of 0.6 was chosen.
Figure shows the column-integrated fluxes for inorganic bromine
(Br2; Fig. c) during January and February 2012.
In addition, the heterogeneous uptake of N2O5 onto aerosol particles
that contain Cl- to form ClNO2 is considered in the model.
After uptake, N2O5 is taken up onto the particle; it reacts
reversibly with liquid water to form protonated nitric acid intermediate
(H2ONO+2). This then reacts with either liquid water, to form
aqueous nitric acid (HNO3), or with chloride ions to form
ClNO2. See for further description of this
chemistry. In , ClNO2 was considered as an
inert specie; however, in our study, ClNO2 is not treated as an inert
specie but is broken down via photolysis and reaction with OH (see
Tables and ).
Model setup
The model is set up with a horizontal grid spacing of 30km×30km and 30 vertical layers up to 50 hPa. Simulations that study
the oxidation of VOCs by Br over the tropical area (described in
Sect. ) are performed with more vertical layers than the
standard case in order to capture the vertical mixing in this area. Thus, 52
vertical layers up to 50 hPa are used in this case. The meteorological
initial and lateral boundary conditions were determined using the ERA-Interim
data, and the meteorology was reinitialized every 3 days to
reproduce the observed transport. Chemical initial and boundary conditions
(ICs/BCs) are from the global atmospheric model GEOS-Chem described in
. We conducted WRF-Chem simulations for January and
February 2012 covering the TORERO domain (see Fig. ). We
performed a spin-up of 20 days. Table describes the main
configuration of the model.
Model details and experiment configuration.
Chemistry Chemical mechanismMOZART-4 Halogen chemical mechanismMISTRA Photolysis schemeFTUV Dry depositionWet depositionBiogenic emissionsMEGAN Halocarbons and OVOCs air–sea fluxesOnline calculation Alkene and alkane oceanic emissionsPOET Sea-salt emissionsseas_opt=4; N2O5 heterogeneous chemistryn2o5_hetchem=2; Resolution and initial conditions Horizontal resolution30 km × 30 kmVertical layers30 or 52Top of the atmosphere50 hPaChemical initial conditionGEOS-Chem Meteorological initial conditionERA-Interim Chemistry spin-up20 daysSensitivity studies
A total of 11 different simulations were performed
in this study. Our base simulation, WRF-DEBROM, considered all main processes
involving halogen chemistry (sea-salt debromination, heterogeneous chemistry
and reactions between halogens and VOCs) and computes the oceanic halocarbon
fluxes online. The WRF-ZIS simulation is the same as WRF-DEBROM but uses
prescribed oceanic emissions for the halocarbons. To test the sensitivity to
the heterogeneous reaction rate constants, two runs were performed: the first
one, in which the values for the uptake coefficient (γ from
Eq. ) from Table have been divided by 2,
WRF-GAMMADV2, and the second one, where Eq. () that has a diffusion
term, is used, WRF-DIFF. To account for the importance of the debromination
in sea-salt particles, we performed the simulation WRF-NODEBROM which is the
same as the WRF-DEBROM simulation but without debromination. The WRF-NOHET
simulation is the same as WRF-NODEBROM but without heterogeneous chemistry. A
simulation with no halogen chemistry, WRF-NOHAL, is performed to study the
effect of halogens on the tropospheric chemistry. All simulations except
WRF-NOHAL use ICs/BCs from the GEOS-Chem model that include halogens. The
WRF-NOHAL simulation uses ICs/BCs from the GEOS-Chem model with no halogen
chemistry. Finally, to study the oxidation of VOCs by halogens, four
simulations have been performed: (1) a simulation without the reactions of
bromine reactions with alkenes (WRF-NOBRALKE), (2) a simulation without the
reactions of bromine with aldehydes (WRF-NOBRALD), (3) a simulation without
the reactions of bromine with VOCs, therefore neither alkenes nor aldehydes
(WRF-NOBRVOCS), and (4) a simulation without reactions of bromine and
chlorine with VOCs (WRF-NOHALVOCS). See Table for a
summary of all these simulations.
Summary of all the simulations to investigate the main processes
involving reactions between halogen chemistry.
Simulation nameOceanicDebrominationHeterogeneousBrBrCl VOCsHalogensfluxesalkenesaldehydesWRF-DEBROMOnline√√√√√√WRF-GAMMADV2Online√ (γ divided by 2)√ (γ divided by 2)√√√√WRF-DIFFOnline√ (uses Eq. )√ (uses Eq. )√√√√WRF-ZISPrescribed√√√√√√WRF-NODEBROMOnline√√√√√WRF-NOHETOnline√√√√WRF-NOBRALKEOnline√√√√√WRF-NOBRALDOnline√√√√√WRF-NOBRVOCSOnline√√√√WRF-NOHALVOCSOnline√√√WRF-NOHAL–Model results
This section presents the model evaluation with observations of relevant
trace gases. The model output is sampled at the nearest timestamp and grid
box to the measurements. An ocean mask neglecting grid boxes above land was
applied to compute all model results.
Oceanic emissions: halocarbons
Figure shows the time series of CHBr3
(Fig. a), CH2Br2 (Fig. b) and
CH3I (Fig. c) mixing ratios (in pptv) for the
WRF-ZIS (green line) and WRF-DEBROM (black line) runs. In addition, the
modelled wind speed (black line) is also shown in Fig.
(Fig. d). Measurements for the halocarbons and wind speed
are represented by the solid red lines. Figure
presents the time series of CHBr3, CH2Br2 and CH3I
water concentration (in pmol L-1) from the measurements (dashed red
lines) and from the climatology (dashed blue lines)
used to compute both the prescribed and online fluxes.
Time series of CHBr3(a),
CH2Br2(b) and CH3I(c) mixing rations (in
pptv) for the WRF-ZIS (green line) and WRF-DEBROM (black line) runs during
the period of the TORERO campaign in 2012. In panel (d), the wind
speed (m s-1) of the model is shown with a black line. Measurements
during the TORERO campaign are depicted with red lines.
Time series of measured CHBr3(a),
CH2Br2(b) and CH3I(c) water
concentration (in pmol L-1) during the TORERO campaign (red dashed
line) and from the climatology (blue dashed
line).
In general, both simulations reproduce the concentrations of the halocarbons
to the right order of magnitude, although there are specific periods with a
negative bias. We see a tendency to underestimate CHBr3 for both
model simulations during most of the period. This result is similar to the
study of , who compared 11 global models using
different emissions inventories. The majority of the models do not reproduce
the observed concentrations in the tropical marine boundary layer. Over the
tropics, high emissions observed are associated with tropical upwelling and
active planktonic production . One reason for
low CHBr3 concentrations in our model simulations might be that the
seawater concentrations are too low in this area (see
Fig. for CHBr3). The fluxes are also low (see
Fig. S2). Note that used only a very limited amount
of data to derive the seawater concentration for the halocarbons in our
domain, which leads to uncertainty in the calculated fluxes. The modelled
CHBr3 is underestimated throughout the troposphere when it is compared
with aircraft observations (see Fig. S3). Atmospheric concentrations of
CH2Br2 are in good agreement with the observations although the
model underestimates the observed values by ∼0.5 pptv during the
periods 6–10 and 22–25 February. Bromocarbon concentrations agree better
with the measurements when the oceanic fluxes are calculated online
(WRF-DEBROM); in particular, the underestimation is less for specific periods
(e.g. 20 February for CHBr3 and 10 and 22 February for
CH2Br2) in comparison with WRF-ZIS. Moreover, the correlation
coefficients between the observations and the simulations are better for the
WRF-DEBROM compared to WRF-ZIS: 0.48 and 0.3 for CH2Br2 and
CHBr3, respectively, in the case of WRF-ZIS, and 0.65 and 0.43 for
CH2Br2 and CHBr3 in the case of WRF-DEBROM. Modelled
CH3I concentrations show a similar trend to the observations,
although, like the bromocarbons, both simulations underestimate the
observations during specific periods (6–10 and 18–28 February). This
underestimation is more prominent in the WRF-DEBROM simulation. One reason
for that could be that the wind speed from WRF-Chem is lower than the wind
speed used to calculate the prescribed emissions, producing lower online
fluxes. Nevertheless, the correlation coefficients between the observed and
simulated CH3I atmospheric concentrations are better for WRF-DEBROM
than for WRF-ZIS: 0.19 is calculated for WRF-ZIS and 0.40 for the
WRF-DEBROM simulation.
Specific periods of negative bias for both simulations demand further
attention. A possible explanation for the underestimation in halocarbon
atmospheric concentrations might be due to the input data (e.g. wind speed,
SST, seawater concentration) that we used to compute these fluxes. In the
case of the online fluxes, between 6 and 8 February, the model underestimates
wind speed and this is directly accompanied by an underestimation for all
three halocarbons' atmospheric concentrations.
demonstrate that changes in the input parameters, especially wind speed and
SST, affect the fluxes calculation. The same study suggests that CH3I
emissions are mainly influenced by variations of the wind speed. Moreover,
the study of , that uses the same seawater
concentration as our study, suggests that the negative bias in the modelled
atmospheric concentrations could indicate regions where the seawater
concentration from the climatologies lacks hotspots; thus, it is missing an oceanic
source region. This is clearly seen for the seawater concentrations of
CHBr3 (during most of the period), CH2Br2 (peaks around
15 February) and CH3I (peaks around 20 February) used in this study
that seem to be too low in comparison with the observations (see
Fig. ). More data on the seawater concentrations of
these halocarbons in this region are required to better constrain the oceanic
flux data sets available to models and so to improve the representation of
these gases in the atmosphere.
Gas-phase and heterogeneous chemistry: bromine and iodine partitioning
Figure compares model results sampled along 16 flight tracks
with the observations for BrO (pptv) separating tropical from subtropical
flights for the five simulations: WRF-NOHET, WRF-NODEBROM, WRF-GAMMADV2,
WRF-DIFF and WRF-DEBROM. Results indicate that there is an improvement of the
modelled BrO throughout the troposphere in both the tropics and subtropics
when the heterogeneous chemistry is included in both tropics and subtropics.
Mean vertical profile of BrO (pptv) over the subtropics (a)
and tropics (b). An average over 16 flights of the TORERO campaign
(red line) are compared to the five different WRF-Chem simulations: WRF-NOHET
(blue line), WRF-NODEBROM (green line), WRF-DEBROM (black line), WRF-GAMMADV2
(pink line) and WRF-DIFF (yellow line). Orange and grey horizontal bars
indicate the 25th–75th quartile interval for the observations of the TORERO
campaign and WRF-DEBROM simulation, respectively. Values are considered in
0.5 km bins and the number of aircraft measurement points for each altitude
is given on the right side of each plot.
In the subtropics, higher values of BrO are found in the altitude range
11–13 km due to the lower altitude of the tropopause. Some data points in
this altitude range will be in the lower stratosphere. There is really good
agreement with the observations particularly in the middle and upper
troposphere where the model is able to capture the higher values of BrO.
Within the model, aerosols over the subtropical area tend to be alkaline;
thus, BrO does not increase in this area when sea-salt debromination is
included. Over the tropics, where the aerosol is more acidic and where the
sea-salt aerosols are mostly located (see emissions of Br2 in
Fig. ), elevated BrO is seen with the inclusion of the
debromination (WRF-DEBROM) in the MBL. Debromination improves the simulation
of BrO concentrations in the middle troposphere although it excessively
increases BrO levels up to 1 pptv in the MBL. Higher values are also seen in
other modelling studies that include this process .
In areas, such as the tropics, where debromination dominates, the impact of
halving gamma (WRF-GAMMADV2 run) is approximately half of the impact of
including heterogeneous chemistry (i.e. the difference between the WRF-DEBROM
run and the WRF-NOHET run) at least for the lower troposphere. Very little
impact is seen in the upper troposphere (UT), a slight decrease in BrO, when gamma is halved
(WRF-GAMMADV2). The simulation in which diffusion limitation is considered in
the heterogeneous reactions (WRF-DIFF) gives values of BrO that are generally
between the results from the WRF-GAMMADV2 and WRF-DEBROM simulations. They
are similar to the WRF-GAMMADV2 values in the MBL, but over the subtropics,
where debromination is lower, WRF-DIFF is very close to WRF-DEBROM values.
Significant uncertainties still exist in the sea-salt debromination processes
and the parameterizations used here might be too simple to represent them.
In addition, the conversion of BrO to HBr is dominated by the reaction
between Br and OVOCs, such that the BrO overestimation seen in the MBL could
be reduced if the modelled aldehyde concentrations were increased (discussed
in Sect. ). However, a reduction in the debromination would
also reduce BrO concentrations. Thus, in order to capture the BrO
concentrations in the MBL, the right balance between these two chemical
processes is needed.
BrO is underestimated in the model by 1 pptv in the upper troposphere over
the tropics. The breakdown of bromocarbons, such as CHBr3,
contributes to BrO concentrations in the UT; thus, a good representation of
bromocarbons is needed. CHBr3 is underestimated in the middle and
upper troposphere especially over the tropics (see Fig. S3). The reason for
that could be a combination of different factors: underestimation of the
boundary conditions used in this study for CHBr3, underestimation in
the oceanic fluxes (see Fig. S2) and overestimation of the loss rates.
Moreover, an underestimation in the heterogeneous chemistry or uncertainties
in the reactions between the halogens and VOCs (discussed in
Sect. ) can also contribute to the underestimation of BrO
in the UT over the tropics.
Figure shows the vertical profile distribution for inorganic
bromine (Bry in pptv) for the three simulations – WRF-NOHET
(Fig. a, d), WRF-NODEBROM (Fig. b, e) and
WRF-DEBROM (Fig. c, f) – over the subtropics
(Fig. a–c) and tropics (Fig. d–f). Inorganic
bromine concentrations increase with altitude with a maximum of 8 pptv at
14 km in the subtropical area for all three simulations. This reflects the
lifetime of the bromocarbon species that breakdown and release Br in the UT
and LS. Over the tropical area, inorganic bromine concentrations have a peak
in the middle troposphere at 6 km, then decrease until 12 km and then start
to increase again. A big impact on the vertical Bry
partitioning is seen between the three simulations. With the inclusion of the
heterogeneous chemistry, there is a decrease of HBr and an increase of more
reactive species: dihalogens (BrCl, Br2 and BrI) and BrO. HOBr
increases and BrNO3 decreases in the UT due to BrNO3
hydrolysis. Over the tropics, Bry increases in the MBL
(∼4 pptv) when debromination is included (WRF-DEBROM). This
enhancement is seen for all inorganic species with a maximum in the surface
where the concentration of sea-salt aerosols is highest. Over the subtropical
area, little difference is seen between WRF-NODEBROM and WRF-DEBROM.
Regional average vertical partitioning of inorganic bromine
(Bry) for the three different simulations
– WRF-NOHET (a, d), WRF-NODEBROM (b, e) and
WRF-DEBROM (c, f) – during January and February 2012.
Panels (a, b, c) are over the subtropical area and
panels (d, e, f) over the tropical. Units are in
pptv.
Figure compares model results sampled along 16 flight tracks
with the observations for IO separating tropical from subtropical flights for
the five simulations: WRF-NOHET, WRF-NODEBROM, WRF-GAMMADV2, WRF-DIFF and
WRF-DEBROM. No clear impact is seen with the inclusion of the heterogeneous
chemistry. At the surface, simulations with heterogeneous chemistry
(WRF-DEBROM, WRF-GAMMADV2, WRF-DIFF and WRF-NODEBROM) have slightly lower IO
concentrations than the simulation without heterogeneous chemistry
(WRF-NOHET). The main reason for that reduction is the sink for the iodine
oxides (I2Ox, where X=2,3,4) included in the
heterogeneous chemistry. Over the tropical region, the model overestimated
surface IO. This overestimation might be explained by the large modelled
inorganic iodine oceanic fluxes in this area. The biggest uncertainty in the
inorganic iodine emissions parameterization is the calculation of the iodide
concentration in the seawater. Over the subtropics, IO enhancements observed
below 4 km are not captured by the model. Some studies suggest that there is
abiotic CH3I production when dust contacts seawater containing iodide
. Implementing this chemistry into the
model is out of the scope of this paper and further investigation is needed
to explain whether the production of CH3I enhances the IO
concentration or if there are other missing IO precursors.
presented an analysis of observations of several
gas-phase iodine species made during a field campaign in the eastern Pacific
marine boundary layer and suggested that the presence of elevated
CH3I does not have a big impact on the IOx
concentrations due to CH3I in the MBL having a long lifetime (∼2 days at the Equator). An overestimation of modelled IO in the UT needs
further investigation. This overestimation is similar to other modelling
studies . Changing the heterogeneous rate constants
(difference between the WRF-DEBROM, WRF-GAMMADV2 and WRF-DIFF runs) has very
little impact on IO.
Mean vertical profile of IO (pptv) over the subtropics (a)
and tropics (b). An average over 16 flights of the TORERO campaign
(red line) is compared to the five different WRF-Chem simulations: WRF-NOHET
(blue line), WRF-NODEBROM (green line), WRF-DEBROM (black line), WRF-GAMMADV2
(pink line) and WRF-DIFF (yellow line). Orange and grey horizontal bars
indicate the 25th–75th quartile interval for the observations of the TORERO
campaign and WRF-DEBROM simulation, respectively. Values are considered in
0.5 km bins and the number of aircraft measurement points for each altitude
is given on the right side of each plot.
Figure shows the vertical profile distribution for inorganic
iodine (Iy) for the three simulations – WRF-NOHET
(Fig. a, d), WRF-NODEBROM (Fig. b, e) and
WRF-DEBROM (Fig. c, f) – over the subtropics
(Fig. a–c) and tropics (Fig. d–f).
Iy is higher in the MBL where it is emitted, especially in
the tropical region, with HOI being the dominant species. Concentrations
start to decrease above the MBL due to the removal of soluble species by the
wet deposition. Unlike Bry, we do not see a big impact on
the vertical profile of Iy partitioning with the inclusion
of the heterogeneous chemistry. The only differences are the
Iy decreases in the surface with the inclusion of the
heterogeneous chemistry, due to the removal of the iodine oxides, and the
production of more dihalogens in the MBL, especially when debromination is
included. Heterogenous iodine reactions (Reactions R11–R19) compete with the
photolysis. Iodine species are more readily photolysed, so less is taken up
into the aerosol and the impact of heterogeneous chemistry is lower.
Regional average vertical partitioning of inorganic iodine
(Iy) for the three different simulations –
WRF-NOHET (a, d), WRF-NODEBROM (b, e) and
WRF-DEBROM (c, f) – during January and February 2012.
Panels (a, b, c) are over the subtropical area and
panels (d, e, f) over the tropical. Units are in
pptv.
Impact on VOCs
Several VOC oceanic fluxes have been included in the model (see
Sect. ) as well as the oxidation of VOCs by halogens. In order
to see the impact of halogen reactions with the VOCs, average loss rates of
all organic compounds due to the Cl and Br families are calculated as percent of
the total tropospheric losses over the ocean for the WRF-DEBROM simulation.
Bromine accounts for 9.2 % of the oxidation of CH3CHO, 1.4 %
of CH2O, 0.8 % of C2H4 and 4.1 % of C3H6.
Chlorine accounts for 0.6 % of the oxidation of CH3CHO, 0.3 %
of CH2O, 7.7 % of CH3OH, 0.8 % of CH3OOH,
0.6 % of CH3O2, 35.5 % of C2H6 and 10.5 % of
C3H8.
A subset of nine flights from the TORERO campaign over the tropics is compared
with the WRF-DEBROM, WRF-NOBRVOCS, WRF-NOBRALKE and WRF-NOBRALD simulations
for BrO (pptv) in Fig. . Comparisons between WRF-DEBROM and
WRF-NOBRVOCS simulations show a clear difference (1–4 pptv) throughout the
whole troposphere. VOCs play an important role in the MBL regulating the
reactive halogens. Without the bromine reactions with the VOCs, BrO
concentrations are higher than observed in the MBL. In the middle and upper
troposphere, where VOCs emitted from the ocean and forests are transported by
convection, the model underestimates the amounts of BrO when these reactions
are considered. The results obtained indicate that BrO is highly sensitive to
the conversion of reactive bromine into more stable species by these
reactions. The partitioning of the products of these reactions
(HBr/Br), and thus the conversion of reactive bromine to more stable
species, is highly uncertain (see Sect. ) and the results
suggest that it might be too effective in these upper layers of the model.
(a) Mean vertical profile of BrO (pptv) over the
tropics. A subset of nine flights from the TORERO campaign (red line)
are compared to the four different WRF-Chem simulations: WRF-NOBRALKE (blue
line), WRF-NOBRALD (green line), WRF-NOBRVOCS (blue line) and WRF-DEBROM
(black line). (b) Regional average vertical partitioning of
inorganic bromine (Bry) for the WRF-NOBRVOCS run over the
tropical area during January and February 2012. (c, d) The
WRF-DEBROM (black line) simulation is compared with acetaldehyde and
formaldehyde TORERO observations for the same nine flights (red line). Orange
and grey horizontal bars indicate the 25th–75th quartile interval for the
observations of the TORERO campaign and WRF-DEBROM simulation, respectively.
Values are considered in 0.5 km bin and the number of points for each
altitude is given on the right side of each plot. Units are in
pptv.
In order to understand which families of VOCs have a higher impact on the BrO
concentrations, the oxidation of alkenes and aldehydes by Br has been
studied separately in the WRF-NOBRALKE and WRF-NOBRALD simulations. Differences
between WRF-DEBROM and WRF-NOBRALD are seen in the whole troposphere with
higher differences up to 2 pptv in the MBL, where the concentrations of both
bromine and aldehydes are high. The concentrations of the aldehydes are
underestimated by the model, especially for CH3CHO, meaning that
BrO-modelled concentrations would be even lower if the modelled concentrations of
the aldehydes were reconciled with the observations. The model also seriously
underestimates the observed glyoxal mixing ratios. The modelled values are
typically ∼1 pptv, whilst the observed values are around 30–40 pptv
in the MBL, decreasing to around 5–10 pptv in the upper troposphere
. This illustrates that there are
large gaps in our understanding of OVOCs in the remote marine atmosphere.
Small differences are observed between WRF-DEBROM and WRF-NOBRALKE. However,
differences up to 2 pptv between WRF-NOBRVOCS and WRF-NOBRALD are clearly
seen especially in the MBL.
These findings suggest that when aldehyde oxidation by Br is included,
reactive Br is reduced considerably, thus limiting the amount of alkene
oxidation by Br (difference between WRF-DEBROM and WRF-NOBRALKE). However,
when the oxidation of aldehydes is included, there is sufficient
Bry present for the oxidation of alkenes by Br to have an
impact on the BrO (difference between WRF-NOBRALD and WRF-NOBRVOCS).
Figure also shows the vertical profile distribution for
inorganic bromine (Bry in pptv) for the WRF-NOBRVOCS run
over the tropics (Fig. b). When reactions of bromine with VOCs are
not included, the amount of Bry increases considerably
(difference between WRF-DEBROM and WRF-NOBRVOCS from Figs.
and ), reaching values of 14 pptv in the MBL over the
tropics. Moreover, when this chemistry is included, the partitioning of
Bry shifts to more stable bromine species such as HBr.
Figure shows the vertical profile distribution for inorganic
chlorine (Cly in pptv) for the two simulations –
WRF-NOHALVOCS (Fig. a, d) and WRF-DEBROM (Fig. b,
e) – over the subtropics (Fig. a–c) and tropics
(Fig. d–f). Regional average vertical partitioning of reactive
chlorine species (Cl*) is also shown (Fig. c, f) where
Cl* is defined as Cly gases other than HCl. When the
VOCs react with Cl (WRF-DEBROM), almost all the inorganic Cl is in the form
of HCl (see Fig. ). When these reactions are not considered
(WRF-NOHALVOCS), Cly increases and there is a shift in the
partitioning to more reactive chlorine increases, in particular HOCl, but
also ClO and the dihalogens.
Regional average vertical partitioning of inorganic chlorine
(Cly) for the two different simulations –
WRF-NOHALVOCS (a, d) and WRF-DEBROM (b, e) – during January
and February 2012. Regional average vertical partitioning of reactive
chlorine species (Cl*) is also shown (c, f). Cl* is
defined as Cly gases other than HCl.
Panels (a, b, c) are over the subtropical area and
panels (d, e, f) over the tropical. Units are in
pptv.
From this, we concluded that VOCs play an important role in the reactive
bromine and chlorine concentrations. Therefore, marine emissions of VOCs as
well as halogen reactions with VOCs need to be included in models. However,
large uncertainties still exist in some of these reactions (see
Sect. ).
Impact on O3 and Ox
Figure a presents a comparison of modelled O3
from seven simulations (WRF-DEBROM, WRF-GAMMADV2, WRF-DIFF, WRF-NODEBROM,
WRF-NOHET, WRF-NOHALVOCS and WRF-NOHAL) sampled along 13 flight tracks with
the observed O3 (ppbv). O3 is overestimated when halogens are
not included (WRF-NOHAL) except in the upper troposphere. When halogens are
included, the model (WRF-DEBROM) is in line with the observations, capturing
the O3 gradient and variability of data throughout the troposphere.
The average difference between WRF-DEBROM and WRF-NOHAL simulations
throughout the troposphere is 7 ppbv. In the MBL, high concentrations of
halogens due to ocean emissions destroy O3 and contribute to a
negative bias up to 8 ppbv for WRF-DEBROM run. In the middle troposphere,
the model results (WRF-DEBROM) improve with the inclusion of halogens, where
the average underestimation is reduced from 4.0 to 2.4 ppbv. In the upper
troposphere, where the differences between the simulations (WRF-DEBROM
and WRF-NOHAL) are mainly driven by the boundary conditions used for each
simulation, both simulations underestimate the ozone concentrations. The
heterogeneous halogen chemistry has an impact on O3 concentrations
where a difference of up to 3 ppbv of O3 is seen between the
simulations with and without heterogeneous chemistry (WRF-DEBROM run WRF-NOHET
run, respectively) mainly in the MBL. Dividing gamma by 2 (WRF-GAMMADV2) and
considering the diffusion limitation (WRF-DIFF) reduces this difference to
around 2 ppbv. The modelled O3 is highly sensitive to the inclusion
of the reactions of the halogens with the VOCs (WRF-NOHALVOCS) where
O3 concentrations are much lower (between 12 and 7 ppbv) than in the
WRF-DEBROM run.
(a) Mean vertical profile of O3 (ppbv) over the
domain area using 13 flights from the TORERO campaign (red line) compared to
the seven different WRF-Chem simulations: WRF-NOHAL (purple line), WRF-NOHET
(blue line), WRF-NODEBROM (light green line), WRF-DEBROM (black line),
WRF-GAMMADV2 (pink line), WRF-DIFF (yellow line) and WRF-NOHALVOCS (dark
green line). Orange and grey horizontal bars indicate the 25th–75th quartile
interval for the observations of the TORERO campaign and WRF-DEBROM
simulation, respectively. Values are considered in 0.5 km bins and the
number of aircraft measurement points for each altitude is given on the right
side of each plot. (b, c) Mean O3 difference between the
simulation with no halogen chemistry (WRF-NOHAL) and with halogen chemistry
(WRF-DEBROM) for January and February 2012. Surface mean bias (ppbv) is shown
in panel (b) and surface relative mean bias (%) in
panel (c). Relative mean bias (%) is calculated as
(WRF-NOHAL - WRF-DEBROM) / WRF-NOHAL ×100.
Figure b–c show the regional effects of
halogen chemistry on simulated O3 concentrations at the surface.
Surface mean bias (ppbv) and relative mean bias (%) between the simulation
with no halogen chemistry (WRF-NOHAL) and with halogen chemistry (WRF-DEBROM)
for the simulation period are presented. We find that the regional O3
concentrations are reduced by 2–18 ppbv, corresponding to
25 %–70 %, with the inclusion of the halogens. Over the tropics,
there is a substantial decrease of O3 (>8 ppbv, >40 %). As
we see in Figs. and , there are high iodine and
bromocarbon emissions and especially large amounts of bromine produced from
debromination over this area. These destroy ozone and contribute to higher
difference in O3 concentrations in this area.
The odd oxygen Ox is defined as
Ox=O(3P)+O(1D)+O3+NO2+2×NO3+HNO3+HO2NO2+3×N2O5+PAN+MPAN+ONIT+ONITR+ISOPNO3+PBZNIT+MBONO3O2+XO+HOX+XNO2+2×XNO3+2×OIO+2×I2O2+3×I2O3+4×I2O4+2×OClO,
where X is Cl, Br and I; PAN is peroxyacetyl nitrate, MPAN is methacryloyl
peroxynitrate; ONIT is organic nitrate; ONITR is lumped isoprene nitrate;
ISOPNO3 is peroxy radical from NO3+ISOP; PBZNIT is
peroxybenzoyl nitrate; MBONO3O2 is peroxy radical from NO3+ 2-methyl-3-buten-2-ol.
The Ox loss resulting from reactions with each of the
ozone-depleting families (Ox , HOx,
NOy, VOCs, Br, Cl and I) is calculated. Note that to
calculate the Ox loss due to the Ox
depleting family we only consider reactions involving O(3P),
O(1D) and O3. The average tropospheric vertical profile of
Ox loss grouped by ozone-depleting families for the
WRF-DEBROM simulation is given in Fig. . Figure
summarizes the relative contribution of each halogen family averaged at
different altitude intervals for the WRF-DEBROM, WRF-GAMMADV2, WRF-DIFF,
WRF-NODEBROM, WRF-NOHET and WRF-NOHALVOCS simulations.
Regional average percentage contribution of each ozone-depleting
family to the total tropospheric vertical odd oxygen loss
(Ox) for the WRF-DEBROM simulation.
Integrated odd oxygen loss rates for each O3 depleting
halogen family within the troposphere at different altitude levels: MBL
(surface–900 hPa), FT (900–350 hPa), UT (350 hPa–tropopause) and
troposphere (surface–tropopause) for the WRF-DEBROM, WRF-GAMMADV2, WRF-DIFF,
WRF-NODEBROM, WRF-NOHET and WRF-NOHALVOCS simulations.
The regional average Ox percentage loss due to the halogens
in our model domain is 34 %, 18 % and 40 % in the MBL
(p>900 hPa), free troposphere (FT) (350<p<900 hPa) and
UT (350hPa<p<trop), respectively, for the WRF-DEBROM
simulation. The MBL Ox loss is in good agreement with
, who reported 33 % and
reported 31 %. The tropospheric Ox loss due to the
BrOx, IOx and ClOx cycles
is 14 %, 16 % and 1 % throughout the troposphere, respectively,
for the WRF-DEBROM simulation. The very fast catalytic reactions of iodine
species make the iodine loss higher than for bromine and chlorine, especially
in the MBL for all simulations that include halogen–VOC reactions
(19 %–23 %). With the inclusion of the sea-salt debromination,
Ox loss due to the bromine is 14 % in the MBL. In the
upper troposphere, iodine contributes 18 %–23 % and bromine
14 %–19 % to the total Ox loss. The impact of
halogen chemistry on the tropospheric Ox loss is 31 %
for the WRF-DEBROM simulation. This value is comparable with other studies
that reported 28 % over the tropics and 21.4 %
at the global scale . Moreover, our results are in
agreement with , who used a box model and concluded that
bromine and iodine are responsible for 34 % of the column-integrated loss
of tropospheric O3. The tropospheric Ox loss due to
the iodine is higher than the box model study of , which
concluded that the fraction of iodine-induced ozone loss generally is around
10 %. When comparing different simulations with the WRF-DEBROM run, the
biggest difference is seen with the WRF-NOHALVOCS simulation, where around
60 % of Ox is removed by halogens. BrO is much higher
when the VOC reactions are not included (see Fig. ), which
explains why the amount of Ox loss by BrOx
reactions is much larger (20.5 %). Moreover, the big change though is for
the ClOx, which increases from <1 % to 26 %. Cl
is very important in the oxidation of the alkanes. When this chemistry is not
included the concentrations of Cly increases and there is an
impact on the partitioning increasing reactive species (see
Fig. ); hence, the ClOx cycles play an
important role in Ox loss. It should be noted that very
little is known about the abundance and distribution of Cly,
so this is a large uncertainty. Therefore, a large uncertainty in the impact
of halogen cycling on the O3 budget is the reactions of halogens with
VOCs. In the model runs performed, excluding these reactions doubled the
percentage contribution of halogens to Ox loss (i.e.
increase it from 31 % to 60 %) in the troposphere. Heterogeneous
chemistry (including debromination) has the effect of increasing the
Ox loss by halogen cycling from 25 % to 31 % for the
whole troposphere (i.e. comparison between WRF-NOHET and WRF-DEBROM runs).
For the UT, the equivalent values are 37 % to 40 %, for the FT
13 % to 18 % and for the MBL 23 % to 34 %. Hence,
heterogeneous chemistry increases the percentage of the Ox
loss that is attributable to the halogens by about 6 % for the
troposphere, ranging from 3 % to 11 % depending on the region of the
troposphere. Dividing gamma by 2 (WRF-GAMMADV2) and considering the diffusion
limitation (WRF-DIFF) reduces the Ox loss in the troposphere
by the halogens to 3 % and 2 %, respectively. Note that the gas-phase
halogen chemistry makes a bigger contribution of around 25 % (WRF-NOHET
run) to the Ox loss for the troposphere ranging from
13 % to 37 % depending on the region of the troposphere. Therefore,
the overall impact of the halogen chemistry on Ox loss
appears not to be very sensitive to the treatment of the heterogeneous
chemistry.
Conclusions
We have presented a regional 3-D tropospheric model that
includes halogen chemistry (bromine, iodine and chlorine). A comprehensive
description has been provided for the halogen gas-phase chemistry, the
heterogeneous recycling reactions in sea-salt aerosol and other particles,
reactions of reactive halogens with volatile organic compounds (VOCs) and the
oceanic emissions of halocarbons, inorganic iodine and several VOCs. It is
the first time that a comprehensive halogen chemistry mechanism has been
added into the online WRF-Chem model. Our results provide useful insight
regarding the potential importance of reactive halogens in the tropical
marine atmosphere and the many uncertainties that remain. Field data from the
TORERO campaign (January–February 2012) have been used in the model
evaluation.
Two different approaches to compute marine emissions, online and prescribed,
for the VSLHs are discussed here. There is an improvement using online fluxes,
WRF-DEBROM, in comparison with prescribed fluxes, WRF-ZIS, especially for
CH2Br2 and CHBr3 atmospheric concentrations, where the
overestimation seen for the model in comparison with ship measurements is
decreased for specific periods. During the whole period, an underestimation
is seen for both simulations for CHBr3. This underestimation is
similar to other modelling studies, which indicates the oceanic fluxes for
CHBr3 in this region are not well determined. Results indicate that
the input data (especially wind speed and water concentrations) used in this
study to calculate marine fluxes underestimate halocarbon concentrations.
Large underestimation of CHBr3 and CH3I concentrations
throughout the troposphere is seen when compared to the aircraft
observations.
Five sensitivity studies are compared in order to
understand the impact of the heterogeneous chemistry for bromine and iodine
species. Results show that the inclusion of heterogeneous chemistry on marine
aerosol has a considerable impact on the Bry partitioning,
increasing reactive species like BrO. An increase of Bry is
seen in the tropical MBL when debromination processes are included, due to
the presence of relatively acidic particles.
The oxidation of alkenes and aldehydes by bromine has been studied in three
different sensitivity runs. These runs suggest that reactions of bromine with
OVOCs have a big impact on the BrO concentrations. The reactions between Br
and aldehydes were found to be particularly important, despite the model
underestimating the amount of aldehydes observed in the atmosphere.
The model shows an overall good agreement with the observed IO vertical
profile. Higher modelled concentrations in the surface are seen over the
tropics, indicating that inorganic iodine emissions might be too high in this
area. The model is not able to capture the IO enhancements sometimes seen
below 4 km over the subtropical area. Unlike Bry, the
Iy partitioning is found to be relatively insensitive to
inclusion of the heterogeneous chemistry.
The model captures the O3 vertical profile in the free troposphere.
The simulation with halogens (WRF-DEBROM) underestimates the observed
O3 values in the MBL, where the oceanic emissions of the halogenated
species are higher. Over the tropics, the regional surface O3
concentrations are reduced between 2 and 18 ppbv with the inclusion of the
halogens. When heterogeneous chemistry is included, O3 concentrations
are reduced by up to 3 ppbv in the MBL. The biggest difference (7–12 ppbv) in
O3 values is seen when reactions between Br and Cl and VOCs are not
considered (WRF-NOHALVOCS run).
In our simulations, halogens constitute 25 %–60 % of the overall
tropospheric Ox loss. This range of values is comparable
with other studies. Uncertainties in the heterogeneous chemistry accounted
for only a small proportion of this range (25 % to 31 % of the
Ox loss). When reactions between Br and Cl with VOCs are not
considered (WRF-NOHALVOCS), Ox loss by
BrOx, ClOx and IOx cycles
is high (60 %), which accounts for the upper limit of the overall range.
The model results are clearly very sensitive to the VOCs and this is a large
uncertainty given that their emissions over these remote areas are poorly
known.
Our model results suggest that including halogen chemistry has a large affect
on O3 (7 ppbv) and contributes typically about 25 %–30 % of
Ox loss. Including heterogeneous halogen chemistry has a big
impact on the Bry partitioning but not on the
Iy partitioning. However, it does not have a large impact on
the O3 concentrations or the percentage of Ox loss
via halogen chemistry. Therefore, although the uncertainties in the
heterogeneous chemistry are large, the Ox appears to be
relatively insensitive to these uncertainties. However, the modelled
O3 and Ox losses are very sensitive to the reactions
between the halogens and the VOCs. Excluding these reactions leads to greater
amounts of the reactive halogen species (Figs.
and ), less O3 (Fig. ) and greater
Ox loss from halogens (60 %) (Fig. ), in
particular from ClOx. Very little is known about the
abundance and distribution of Cly, so this is a large
uncertainty. There are also large uncertainties in the degree to which Br is
recycled or converted to the more stable product (HBr) in the reactions
following Br reactions with the alkenes. Moreover, there is considerable
uncertainty in the emissions and distributions of the VOCs in the remote
marine atmosphere.
More data are required at the process level from laboratory studies along with
field observations of, for example, more Bry,
Iy and Cly species, to better constrain the
modelled representation of these processes and to verify if halogens really
do have such a large impact on Ox in the tropical
troposphere. This is important given that the oxidizing capacity of this
region of the atmosphere has a large impact on the lifetime of many
pollutants including methane, a key greenhouse gas.
Code availability
The WRF-Chem model code is available from
http://www2.mmm.ucar.edu/wrf/users/download/get_sources.html (last
access: 1 February 2019), with the specific code used in this study available
from the authors upon request (alba.badia.moragas@gmail.com).
Data availability
The TORERO data are available from the TORERO data archive:
https://www.eol.ucar.edu/field_projects/torero, last access: 1 February
2019. The TORERO data set is open for use by the public, subject to the data
policy: https://www.eol.ucar.edu/content/torero-data-policy, last
access: 1 February 2019.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-3161-2019-supplement.
Author contributions
AB carried out all the model simulations
and data analysis, and led the interpretation of the results and prepared the
manuscript with contributions from all co-authors. CER contributed to the
interpretation of the results and provided extensive comments on manuscript.
ARB and AS made several comments and suggestions. RV, TKK, ECA, RSH, LJC and
SJA conducted and provided the TORERO measurements. TS provided input data to
run the model. RvG provided the initial motivation to this study, designed
the research and secured the funding.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
This work is funded by the National Environmental Research Council (NERC)
grant NE/L005271/1. The authors wish to thank the TORERO team, especially
Barbara Dix and Theodore Konstantinos. TORERO was supported by the NSF under
award AGS-1104104 (PI: Rainer Volkamer). The involvement of the NSF-sponsored
Lower Atmospheric Observing Facilities, managed and operated by the National
Center for Atmospheric Research (NCAR) Earth Observing Laboratory (EOL), is
acknowledged. Rainer Volkamer acknowledges funding from NSF award
AGS-1620530. Lucy J. Carpenter acknowledges support from NERC (award
NE/J00619X/1). The authors also thank Carlos Cuevas, Douglas Lowe, Gordon McFiggans,
Kenjiro Toyota, Peter Braüer, Luke Surl, Deanna Donohoue and Roberto
Sommariva for their constructive suggestions and feedback during this study.
Finally, this work is specially dedicated to the friendship and memory of
Roland von Glasow.
Edited by: Aurélien Dommergue
Reviewed by: two anonymous referees
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