ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-12239-2016Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS-ChemSherwenTomástomas.sherwen@york.ac.ukhttps://orcid.org/0000-0002-3006-3876SchmidtJohan A.https://orcid.org/0000-0002-7297-3851EvansMat J.https://orcid.org/0000-0003-4775-032XCarpenterLucy J.https://orcid.org/0000-0002-6257-3950GroßmannKatjahttps://orcid.org/0000-0002-5154-197XEasthamSebastian D.https://orcid.org/0000-0002-2476-4801JacobDaniel J.DixBarbaraKoenigTheodore K.https://orcid.org/0000-0002-3756-4315SinreichRomanOrtegaIvanhttps://orcid.org/0000-0002-0067-617XVolkamerRainerhttps://orcid.org/0000-0002-0899-1369Saiz-LopezAlfonsohttps://orcid.org/0000-0002-0060-1581Prados-RomanCristinaMahajanAnoop S.https://orcid.org/0000-0002-2909-5432OrdóñezCarlosWolfson Atmospheric Chemistry Laboratories (WACL), Department of Chemistry, University of York, York, YO10 5DD, UKDepartment of Chemistry, University of Copenhagen, Universitetsparken, 2100 Copenhagen O, DenmarkNational Centre for Atmospheric Science (NCAS), University of York, York, YO10 5DD, UKInstitute of Environmental Physics, University of Heidelberg, Heidelberg, GermanySchool of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USADepartment of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215, USACooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309-021, USADepartment of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, CSIC, Madrid, 28006, SpainIndian Institute of Tropical Meteorology, Maharashtra, 411008, IndiaDpto. Física de la Tierra II, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spainnow at: Joint Institute For Regional Earth System Science and Engineering (JIFRESSE), University of California Los Angeles, Los Angeles, CA, 90095, USA now at: Atmospheric Research and Instrumentation Branch, National Institute for Aerospace Technology (INTA), Madrid, SpainTomás Sherwen (tomas.sherwen@york.ac.uk)29September20161618122391227118May201620May201612September201616September2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/12239/2016/acp-16-12239-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/12239/2016/acp-16-12239-2016.pdf
We present a simulation of the global present-day composition of the
troposphere which includes the chemistry of halogens (Cl, Br, I). Building on
previous work within the GEOS-Chem model we include emissions of inorganic
iodine from the oceans, anthropogenic and biogenic sources of halogenated
gases, gas phase chemistry, and a parameterised approach to heterogeneous
halogen chemistry. Consistent with we do not include
sea-salt debromination. Observations of halogen radicals (BrO, IO) are
sparse but the model has some skill in reproducing these. Modelled IO shows
both high and low biases when compared to different datasets, but BrO
concentrations appear to be modelled low. Comparisons to the very
sparse observations dataset of reactive Cl species suggest the model
represents a lower limit of the impacts of these species, likely due to
underestimates in emissions and therefore burdens. Inclusion of Cl, Br, and I
results in a general improvement in simulation of ozone (O3)
concentrations, except in polar regions where the model now underestimates
O3 concentrations. Halogen chemistry reduces the global tropospheric
O3 burden by 18.6 %, with the O3 lifetime reducing from
26 to 22 days. Global mean OH concentrations of
1.28 × 106 molecules cm-3 are 8.2 % lower than in a
simulation without halogens, leading to an increase in the CH4
lifetime (10.8 %) due to OH oxidation from 7.47 to 8.28 years. Oxidation
of CH4 by Cl is small (∼ 2 %) but Cl oxidation of other
VOCs (ethane, acetone, and propane) can be significant
(∼ 15–27 %). Oxidation of VOCs by Br is smaller, representing
3.9 % of the loss of acetaldehyde and 0.9 % of the loss of
formaldehyde.
Introduction
To address problems such as air-quality degradation and climate change, we
need to understand the composition of the troposphere and its oxidative
capacity. A complicated relationship exists between key chemical families and
species such as ozone (O3), HOx (HO2+ OH),
NOx
(NO2+ NO), and organic compounds which include carbon monoxide
(CO), methane (CH4), hydrocarbons, and oxygenated volatile organic
compounds (VOCs) (for example, see ). The most important
tropospheric oxidant is OH, which is itself produced indirectly through
photolysis of O3. Oxidants control the concentrations of key climate
and air-quality gases and aerosols (including O3, methane, sulfate
aerosol, and secondary organic aerosols)
. O3 itself is not directly
emitted, and its tropospheric burden is controlled by its sources through
chemical production from NOx and organic compounds, transport from the
stratosphere, and loss via deposition and chemical reactions
.
Halogens (Cl, Br, I) are known to destroy O3 through catalytic
cycles, such as that shown in Reactions ()–()
. Tropospheric halogens have also been shown to change
OH concentrations and perturb OH to HO2 ratios
towards OH. Halogens perturb the NO to
NO2 ratio and reduce NOx concentrations by hydrolysis of
XNO3. These perturbations also indirectly decrease O3 formation
. Halogens directly oxidise organics species, with Cl
radical reactions proceeding the fastest .
This can cause significant O3 formation through increased RO2
concentrations , notably in regions with elevated
ClNO2. Halogens also play an important role in
determining the chemistry of mercury . The literature on
tropospheric halogens has been the topic of several recent reviews, which
cover the background in more detail .
However, many uncertainties still exist, notably with heterogeneous halogen
chemistry and gas phase iodine chemistry
.
O3+X→XO+O2HO2+XO→HOX+O2HOX+hν→OH+XNet:HO2+O3→2O2+OH
Tropospheric halogen chemistry has been studied in box model studies (see
, and citations within) and more recently in global
models (e.g. ). Modelling has sought to quantify emissions
budgets and evaluate these on a global scale . Global studies have considered impacts of
halogens in the troposphere and reported reductions in the
tropospheric O3 burden by up to ∼ 15 %. However, this field
of research is quickly evolving, with new halogen sources such as inorganic
ocean iodine and ClNO2 produced
from N2O5 hydrolysis on sea salt now appearing to be globally important.
Additional halogen reactions included in this simulation that are
not described in previous work . The full reaction scheme is given in Appendix B
(Tables –). The rate constant is calculated
using a standard Arrhenius expression A⋅exp(-EaRT).
a Reaction from
JPL, only considering the major channel (); product of CH3O reacts to form
CH2O+HO2
(CH3O+O2→CH2O+HO2).
b Only the first channel from JPL was considered. The second channel forms a
criegee (HCl +C2H4O2) and therefore cannot be represented by
reduced GEOS-Chem chemistry scheme. c Reaction defined by JPL and
interpreted as proceeding via hydrogen abstraction; therefore the
acetaldehyde product is assumed. dK(infinity) rate given in
table, K(0) rate =4.00×10-28 with Fc = 0.6 as shown in
Table . e Reaction only proceeds on sea-salt
aerosol, with γ value as described in .
f Reactions which were included in previous work
(), but dihalogen products have been updated, split
between ICl and IBr (see Sect ), and these reactions only proceed on
acidic sea-salt aerosol following . Acidity of aerosol
is calculated as described in . γ values for
uptake of halogen species are given in Table .
Abbreviations for tracers are expanded in Appendix C.
Previous studies of halogen chemistry within the GEOS-Chem
(http://www.geos-chem.org) model have focussed on either bromine or
iodine chemistry. presented a bromine scheme and its
effects on oxidants in the past and present atmosphere.
presented the Unified tropospheric–stratospheric Chemistry eXtension (UCX),
which added a stratospheric bromine and chlorine scheme. This chlorine scheme
was then employed in the troposphere with an updated heterogeneous bromine
and chlorine scheme by . An iodine scheme was employed in
the troposphere to consider present-day impacts of iodine on oxidants
, which used the representation of bromine chemistry from
. Up to this point, the coupling of chlorine, bromine,
and iodine in the GEOS-Chem model and its subsequent impact on the simulated
present-day composition of the atmosphere have not been described.
Here we present such a coupled halogen model built within the GEOS-Chem
framework and consider the present-day tropospheric impacts of halogens. The
model presented here includes recent updates to chlorine , bromine , and iodine
chemistry with further updates and additions described
in Sect. . In Sect. we
describe the modelled distribution of inorganic halogens
(Sects. –) and
compare with observations (Sect. ). We then outline the
impact on oxidants
(Sects. –), organic
compounds (Sect. ), and other species
(Sect. ).
Model description
This work uses the GEOS-Chem chemical transport model
(http://www.geos-chem.org, version 10) run at
4∘× 5∘ spatial resolution. The model is forced by
assimilated meteorological and surface fields from NASA's Global Modelling
and Assimilation Office (GEOS-5). The model chemistry scheme includes Ox,
HOx, NOx, and VOC chemistry as described in . Dynamic
and chemical time steps are 30 and 60 min, respectively. Stratospheric
chemistry is modelled using a linearised mechanism as described by
.
We update the standard model chemistry to give a representation of chlorine,
bromine, and iodine chemistry. We describe this version of the model as
“Cl+Br+I” in this paper. It is based on the iodine chemistry
described in with updates to the bromine and chlorine
scheme described by and . We have made
a range of updates beyond these. Updated or new reactions not included in
, , or are given
in Table with a full description of the halogen
chemistry scheme used given in Appendix B
Tables –.
For the photolysis of I2Ox (x= 2, 3, 4) we have adopted the
absorption cross sections reported by and
and used the I2O2 cross section for
I2O4. A quantum yield of unity was assumed for all I2Ox
species. It is noted that recent work has used an unpublished spectrum for
I2O4 that is much lower than that of I2O3, but this is not expected to have a large effect on
conclusions presented here.
The parameterisation for oceanic iodide concentration was changed from
, as used in , to
because the latter resulted in an improved comparison with observations (see
Sect. 7.5 of ).
Global sources of reactive tropospheric inorganic halogens. Sources
with fixed concentration in the model for Cly (CH3Cl,
CH3Cl2, CHCl3) and Bry (CHBr3) are shown in terms
of chemical release (e.g. +Cl, +OH, +hν) and are in bold. Inclusion
of chlorine and bromine organic species has been reported before in GEOS-Chem
. X2 (I2) and HOX
(HOI) are the inorganic ocean source from ;
XNO2 is the source from the uptake of N2O5 on sea salt
(ClNO2).
* Acid-catalysed sea-salt dihalogen IX (X= Cl, Br) flux is only
stated for Cly and Bry as it does not represent a net Iy source.
The product of acid-catalysed dihalogen release following I+ (HOI,
INO2, INO3) uptake was updated from I2 as in
to yield IBr and ICl following .
Acidity is calculated online through titration of sea-salt aerosol by uptake
of sulfur dioxide (SO2), nitric acid (HNO3), and sulfuric acid
(H2SO4) as described by . Re-release of IX
(X= Cl, Br) is only permitted to proceed if the sea salt is acidic
. Thus aerosol cycling of IX in the model is not a net
source of Iy (and may be a net sink on non-acid aerosol) but alters the
speciation . The ratio between IBr and ICl was set to be
0.15 : 0.85 (IBr : ICl), instead of the 0.5 : 0.5 used previously
. A ratio of 0.5 : 0.5 gives a large
overestimate of bromine monoxide (BrO) with respect to the observations used in
Sect. . We attributed
this reduction to the debromination of sea salt, which we do not consider
here, and the potential for the model to overestimate the BrOx lifetime.
This is discussed further in the next section but future laboratory and field
studies of these heterogenous process are needed to help constrain these
parameters.
Iodine on aerosol is represented in the model with separate tracers based on
the aerosol on which irreversible uptake occurs (see
Table ). We include three iodine aerosol tracers to
represent iodine on accumulation and coarse-mode sea salt and on sulfate
aerosol. The physical properties of the iodine aerosol tracers are assumed to
be the same as their parent aerosol, as previously described for sulfate
and sea-salt aerosol . As in
, no nucleation of iodine species is considered in this
work, with only photolytic and heterogeneous loss being treated.
We have added to the chlorine chemistry scheme described by
to include more tropospheric relevant reactions based on
the JPL 10-6 compilation and IUPAC .
The heterogenous reaction of N2O5 on aerosols was updated to yield
products of ClNO2 and HNO3
on sea salt and 2HNO3 on other aerosol types. Reaction probabilities
are unchanged .
Deposition and photolysis of dihalogen species (ICl, BrCl, IBr) and the
reaction between ClO and iodine monoxide (IO) were also included .
Model results
We run the model for 2 years (1 January 2004 to 1 January 2006), discarding
the first year as a “spin-up” period and using the second year (2005) for
analysis. Non-halogen emissions are described in . A
reference simulation without any halogens (“NOHAL”) was also performed.
Where comparisons with observations are shown, the model is run for the
appropriate year with a 3-month “spin-up” before the observational dates,
unless explicitly stated otherwise. The appropriate month from the 2005
simulation is used as the initialisation for these observational comparisons
to account for interannual variations. The model is sampled at the nearest
timestamp and grid box. The model only calculates chemistry in the
troposphere. To avoid confusion we do not show results above the tropopause
(lapse rate of temperature falls below 2 K km-1).
Average annual halogen surface emission of species and column-integrated fluxes for species that have fixed surface concentrations in the
model (CH3Cl, CH3Cl2, CHCl3, CHBr3) or those
with vertically variable sources (ClNO2 from N2O5 uptake on
sea-salt and IX (X= Cl, Br) production from HOI, lNO2, and
lNO3 uptake). Values are given in kg X m-2 s-1
(X= Cl, Br, I).
Annual global Xy (X= Cl, Br, I) deposition (Xy defined in
Appendix C). Values are given in terms of mass of halogen deposited
(kg X m-2 s-1, X= Cl, Br, I).
Emissions
The emissions fluxes of chlorine, bromine, and iodine species are shown in
Fig. with global totals in
Table . We do not consider the Cl and Br contained
within sea salt as emitted in our simulation, following ,
until a chemical process liberates them into the gas phase. These liberation
processes are the uptake of N2O5 on sea salt and uptake of I+
species on sea salt. We do not include explicit sea-salt debromination for
reasons described in .
The organic iodine (CH3I, CH2I2, CH2ICl,
CH2IBr) emissions are from as described in
. Inorganic iodine emissions (HOI, I2)
are 30 % lower here than reported by
due to use of the
parameterisation for ocean surface iodide rather than that of
. Heterogeneous iodine aerosol chemistry
(Sects. and in
Appendix B4) does not lead to a net release of
iodine, instead just recycling it from less active forms (INO2,
INO3, HOI) into more active forms (ICl / IBr).
Tropospheric distribution of Cly, Bry, and Iy (defined in
Appendix C) concentrations. Upper plots show surface and lower plots show
zonal values. Only boxes that are entirely tropospheric are included in this
plot. The Cly colour bar is capped at 20 pmol mol-1, with a maximum
plotted value of 116 pmol mol-1 at the surface over the North Sea. The
Iy colour bar is capped at 10 pmol mol-1, with a maximum plotted
value of 16.4 pmol mol-1 at the surface over the Red Sea.
Tropospheric distribution of IO, BrO, and Cl concentrations. Upper
plots show surface and lower plots show zonal values. Only boxes that are
entirely tropospheric are included in this plot.
The organic bromine (CH3Br, CHBr3, CH2Br2) emissions
have been reported previously and our
simulation is consistent with this work. A further source of
0.031 Tg Br year-1 (3.5 % of total) is included here from
CH2IBr photolysis. The heterogeneous cycling for Bry (family defined in Appendix C) has been updated here from
, as described in
Sect. /Appendix . An additional
Bry source not considered by is iodine-activated IBr
release from sea salt, which amounts to 0.30 Tg Br year-1 and the
majority (67 %) of this is tropical (22∘ N–22∘ S).
The organic chlorine emission (CH3Cl, CHCl3, CH2Cl2)
for this simulation (Table ) has been described
previously and set using fixed surface concentrations. An
additional source of 0.046 Tg Cl year-1(0.96 % of total) is
present from CH2ICl photolysis . ClNO2
production from the heterogeneous uptake of N2O5 provides a source
of 0.66 Tg Cl year-1 (14 % of total) with the vast majority
(95 %) being in the Northern Hemisphere, with strongest sources in
coastal regions north of 20∘ N. For June we calculate a global
source of 21 Gg Cl month-1, which is substantially less than the
62 Gg Cl month-1 (Sarwar Golam, personal communication, 2016)
calculated in a previous study . The difference in
N2O5 concentrations due to differences in model resolution may
contributes to this. Uptake of HOI, INO2 and INO3 to sea-salt
aerosol leads to the emission of ICl, giving an additional source of
0.76 Tg Cl year-1 (15.7 % of total) mostly (67 %) in tropical
(22∘ N–22∘ S) locations.
Modelled global average vertical Xy (X= Cl, Br, I)
(Xy defined in Appendix C). Units are pmol mol-1 of X (where
X= Cl, Br, I). For Cly the y axis is capped at
20 pmol mol-1 to show speciation. A Cly maximum of
1062 pmol mol-1 is found within the altitudes shown due to additional
HCl contributions increasing with altitude.
Annual mean integrated model tropospheric column for BrO and IO in
molecules cm-2.
Most of the emissions of Br and I species in our simulation occur in the
tropics. It is notable that the chlorine emissions are more widely
distributed (Fig. ). This is a result of longer
lifetimes of chlorine precursor gases, which moves their destruction further
from their emissions, and the ClNO2 source being primarily in the
northern extratropics.
Deposition of halogens
Figure shows the global annual integrated wet and dry
deposition of inorganic Xy (X= Cl, Br, I). Much of the deposition
of the halogens occurs over the oceans (70, 73, and 90 % for Cly,
Bry, and Iy respectively). It is high over regions of significant
tropical precipitation (Intertropical Convergence Zone, Maritime continents,
Indian Ocean) and much lower at the poles, reflecting lower precipitation and
emissions.
We find that the major Cly depositional sink is HCl (94 %), with
HOCl contributing 5.1 % and ClNO3 1.1 %. The Bry sink is
split between HBr, HOBr, and BrNO3 with fractional contributions of
33, 30, and 28 % respectively. The major Iy sink is HOI deposition,
which represents 59 % of the depositional flux. The two next largest
sinks are deposition of INO3 and iodine aerosol (22 and 15 %).
Halogen species concentrations
Figure 3 shows the surface and zonal concentration of annual mean Iy,
Bry, and Cly, with Fig. showing the same
for IO, BrO, and Cl, key halogen compounds in the atmosphere.
Figure shows the global molecule weighted mean
vertical profile of the halogen speciation.
Iodine oxide (IO) surface observations (black) by campaign compared
against the simulation with halogen chemistry (“Cl+Br+I”, red).
Cape Verde measurements are shown against hour of day and others are shown as
a function of latitude. Values are considered in 20∘ bins, with
observations and modelled values at the same location and time (as described
in Sect. ) shown side-by-side around the midpoint of
each bin. The extent of the bins is highlighted with grey dashed lines.
Observations are from Cape Verde (tropical Atlantic; ), TransBrom (western Pacific; ), the Malaspina
circumnavigation , HaloCAST-P (eastern Pacific;
), and TORERO ship (eastern Pacific;
). The number of data points within latitudinal bin is
shown as “n”. The box plot extents give the interquartile range, with the
median shown within the box. The whiskers give the most extreme point within
1.5 times the interquartile range. Locations of observations are shown in
Fig. .
Inorganic iodine concentrations are highest in the tropical marine boundary
layer, consistent with their dominant emission regions. The highest
concentrations are calculated in the coastal tropical regions, where enhanced
O3 concentrations from industrial areas flow over high predicted
oceanic iodide concentrations and lead to increased oceanic inorganic iodine
emissions. Within the vertical there is an average of
∼ 0.5–1 pmol mol-1 of Iy, consistent with previous model
studies . The lowest concentrations of
Iy are seen just above the marine boundary layer, where Iy loss via wet
deposition is most favourable due to partitioning towards water-soluble HOI.
At higher altitudes, lower temperature and high photolysis rates push the
Iy speciation to less-water-soluble compounds (IO, INO3) and hence
the Iy lifetime is longer. IO concentrations
(Fig. ) follow those of Iy, with high
values in the tropical marine boundary layer. IO increases into the upper
troposphere, reflecting a partitioning of Iy in this region towards IO (and
INO3) and away from HOI. The global mean tropospheric lifetimes of Iy
and IOx (IO + I) are 2.2 days and 1.3 min, respectively. IOx loss
proceeds predominately via reaction of IO with HO2 (78 %), with
smaller losses via IO + BrO (7.9 %) and IO +NO2
(7.4 %).
Total reactive bromine is more equally spread through the atmosphere than
iodine. This reflects the longer lifetime of source species with respect to
photolysis, which gives a more significant source higher in the atmosphere.
The highest concentrations are still found in the tropics. Unlike Iy,
Bry increases significantly with altitude, with BrNO3 and HOBr
being the two most dominant species. BrO concentrations
(Fig. ) follow those of inorganic bromine. In
the boundary layer the highest concentrations are found in the tropics. BrO
and IO do not strongly correlate in the tropical marine boundary layer
reflecting their differing sources. BrO concentrations increase towards the
upper troposphere associated with the increase in total Bry. The global
annual-average (molecule weighted) tropospheric BrO mixing ratio in our
simulation is 0.49 pmol mol-1 (Bry= 3.25 pmol mol-1).
When previous implementations are run for
the same year and model version as this work (GEOS-Chem v10), the modelled
BrO concentrations are found to be 11 % higher than
and 33 % higher than . We calculate tropospheric
lifetimes of 18 days for Bry and 8.1 min for BrOx (BrO + Br).
Similarly to IOx, BrOx loss proceeds predominately via reaction of BrO
with HO2 (71 %) and NO2 (18 %).
Total inorganic chlorine has a highly non-uniform distribution at the
surface,
reflecting the ClNO2 source from N2O5 uptake on sea salt. At
the surface ClNO2, HCl, BrCl, and HOCl represent around 25 % of
the total Cly each. Away from the surface the ClNO2 concentrations
drop off rapidly due to the short lifetime of sea salt. HCl concentrations
increase significantly into the middle and upper troposphere and dominates
the Cly distribution. This suggests that stratospheric chlorine freed from
CFCs and organic chlorine strongly contributes to free tropospheric
concentrations of Cly. Cl mixing ratios are very low
(∼ 0.075 fmol mol-1 or ∼ 2000 cm-3) in the marine
boundary layer. Reactive Cl (i.e. Cly excluding HCl) drops from the
surface to around 10 km, where it then increases again towards the
stratosphere. Cl shows a wider distribution than IO and BrO, reflecting the
source wider distribution of Cly. We calculate tropospheric lifetimes of
5 days for Cly and 3.8 h for ClOx (Cl+ClO+ClOO+2Cl2O2). A
global tropospheric mean inorganic chlorine (Cly) concentration of
71 pmol mol-1 in seen in our simulation. ClOx loss proceeds through
reaction of Cl with CH4 (27 %), ClO reaction with
HO2 (21 %), and ClO reaction with NO2 (10 %).
The longer XOx lifetime of ClOx, compared to BrOx and IOx, can
be explained through the importance of the relatively slow dominant loss route
through reaction with CH4.
Vertical comparison of the model (“Cl+Br+I”) and
measured iodine oxide (IO) during TORERO aircraft campaign
. Model and observations are in red and black
respectively. Values are considered in 0.5 km bins, with observations and
modelled values at the same location and time (as described in
Sect. ) shown side-by-side around the midpoint of each
bin. Measurements were taken aboard the NSF/NCAR GV research aircraft by the
University of Colorado airborne multi-axis DOAS instrument (CU AMAX-DOAS) in
the eastern Pacific in January and February 2012
. The box plot extents give the interquartile
range, with the median shown within the box. The whiskers give the most
extreme point within 1.5 times the interquartile range. Locations of
observations are shown in Fig. .
The chemistry of halogens and sea salt is highly uncertain
. Estimates for sea-salt
debromination range from 0.51 Tg year-1 (,
implemented in GEOS-Chem v10 and v9-2) to 2.9 Tg year-1. Other studies have not included sea-salt
debromination as we do not in this work.
found that including debromination of sea-salt aerosol
improved the simulation of the BrO and HOBr observations reported during the
“Combined Airborne Studies in the Tropics” (CAST; )
campaign but resulted in overprediction of the “Tropical Ocean tRoposphere
Exchange of Reactive halogen and Oxygenated VOC” campaign (TORERO;
) BrO observations. Arguably this work
provides a lower estimate of bromine and chlorine sources in the troposphere,
with further work needed to understand the Bry budget.
The difference in lifetimes of inorganic halogen families (Xy) can be
understood from the change in loss routes, which shifts HX to HOX
following the order of group 17 in the periodic table
(Cl → Br → I).
Figure shows column-integrated BrO and IO, which are
the major halogen species for which we have observations (see
Sect. ). Tropospheric ClO concentrations are small (see
Fig. ) and are therefore not shown in
Fig. . Tropical maxima are seen for both BrO and IO,
with BrO concentrations decreasing towards the equator. For IO a localised
maximum is seen in the Arabian Sea. The IO maximum in Antarctica reported
from satellite retrievals is not reproduced in our
model, potentially reflecting the lack of polar-specific processes in the
model.
Comparison with halogen observations
The observational dataset of tropospheric halogen compounds is sparse.
Previous studies that this work is based on have shown comparisons for the
oceanic precursors for chlorine , bromine
, and iodine . The model performance in simulating these compounds has not
changed since these previous publications so we focus here on the available
observations of concentrations of IO, BrO, and some inorganic chlorine
species (ClNO3, HCl, and Cl2).
Seasonal variation of zonal mean tropospheric BrO columns in
different latitudinal bands. Observations from the GOME-2 satellite
instrument in 2007 are compared to GEOS-Chem values at the GOME- 2
local overpass time (09:00–11:00).
Iodine monoxide
A comparison of IO to a suite of recent remote surface observations is shown
in Fig. . The model shows an overall negative bias
of 23 %. This compares with the 90 % positive bias previously
reported in . This reduction in bias to IO observations is due to the use of
the iodide parameterisation over that of
which has reduced the inorganic emission of iodine, along
with the restriction of iodine recycling to acidic aerosol.
Figure shows a comparison between modelled IO with
altitude against observations in the eastern Pacific . In general, the model agreement with observations is good. There
is an average bias of +37 % in the free troposphere
(350 hPa <p< 900 hPa), which increases to +54 % in the
upper troposphere (350 hPa >p> tropopause). As with the surface
measurements, the model bias when comparing to IO observations
in the free and upper troposphere is decreased
from previously reported positive biases of 73 and 96 %, respectively
.
Bromine oxide (BrO) surface observations (black) at Cape Verde
compared against the simulation with halogen
chemistry (“Cl+Br+I”, red). Values are binned by hour of day.
Locations of observations are shown in Fig. .
Comparison between modelled and observed ClNO2.
Concentrations are shown as the maximum and average of the daily maximum
value for the observational and equivalent model time period. The model values are taken for the nearest
time step and location within the analysis year (2005).
Obs. “Cl+Br+I” LocationLat.Long.MaxMeanMaxMeanReferenceCoastalPasadena, CA, US (2010)34.2-118.23.461.480.430.20Southern China, CN (2012)22.2114.32.000.310.600.18Los Angeles, CA, US (2010)34.1-118.21.830.500.430.20Houston, TX, US (2006)30.4-95.41.150.800.190.04London, GB (2012)51.5-0.20.730.230.500.17TX, US (2013)30.4-95.40.140.080.190.04ContinentalHessen, DE (2011)50.28.50.850.200.160.02Boulder, CO, US (2009)40.0-105.30.440.140.000.00Calgary, CA, US (2010)51.1-114.10.240.220.020.01Bromine monoxide
Comparisons of BrO against seasonal satellite tropospheric BrO observations
from GOME-2 are shown in Fig. . As
shown previously the model has some skill
in capturing both the latitudinal and monthly variations in tropospheric BrO
columns. However, it underestimates the column BrO in the lower southern
latitudes (60–90∘ S) and to a smaller degree also in lower
northern latitudes (60–90∘ N), which may reflect the lack of bromine
from polar (blown snow, frost flowers, etc.) sources and sea-salt
debromination processes.
As shown in Fig. , comparisons between the model and
observations of BrO made at Cape Verde show a
negative bias of 22 %. We attribute this to the high local sea-salt
loadings at this site , which is situated in the surf
zone. This may locally increase the BrO concentrations. The model
concentrations of ∼ 1 pmol mol-1 are, however, consistent with
other ship-borne observations made in the region .
Vertical comparison of the model (“Cl+Br+I”) and
measured bromine oxide (BrO) during TORERO aircraft campaign
in the subtropics (left) and tropics (right).
Model and observations are in red and black, respectively. Observations and
modelled values at the same location and time (as described in
Sect. ) are shown side-by-side around the midpoint of
each bin. Measurements were taken aboard the NSF/NCAR GV research aircraft by
the University of Colorado airborne multi-axis DOAS instrument (CU AMAX-DOAS)
in the eastern Pacific in January and February 2012
. Locations of observations are shown in
Fig. .
Figure shows modelled vertical BrO concentrations
against observations in the eastern Pacific .
We find a reasonable agreement within the free troposphere
(350 hPa <p< 900 hPa) in both the tropics and subtropics, with
an average bias of -3.5 and +4.2 %, respectively. A similar
comparison is seen in the upper troposphere
(350 hPa >p> tropopause) with negative biases for the tropics
and subtropics, of 6.3 and 9.7 %, respectively. The decrease in agreement
seen in the TORERO comparison (Fig. ) relative to that
previously presented in is due to reduced BrCl and BrO
production from slower cloud multiphase chemistry (see
Sects. – in
Appendix B). We model higher BrO concentrations in
the tropical marine boundary layer which are above those observed
. Our modelled concentrations are lower than those
reported previously .
Our model does not include sea-salt debromination and yet calculated roughly
the reported concentrations of BrO. Inclusion of sea-salt debromination leads
to excessively high BrO concentration in the model .
Sea-salt debromination is well established; thus the success of the model
despite the lack of inclusion of this process suggests model failure in other
areas. The BrOx lifetime may be too long. The conversion of BrOx to
HBr is dominated by the reaction between Br and organics to produce
HBr. Oceanic sources of VOCs such as acetaldehyde have been proposed
and a significant increase in the
concentration of these species would lead to lower BrOx concentrations.
Alternatively, a reduction in the efficiency of cycling of Bry through
aerosol would also have a similar effect. The aerosol phase chemistry is
complex and the parameterisations used here may be too simple or fail to
capture key processes (e.g. pH, organics). These all require further study in
order to help reconcile models with the rapidly growing body of observation
of both gas and aerosol phase bromine in the atmosphere.
Comparison between global tropospheric Ox budgets of simulations
“Cl+Br+I” (with halogen chemistry) and “NOHAL” (without
halogen chemistry). Recent average model values from ACCENT
are also shown for comparison. For the X1O +X2O halogen crossover
reactions where X1O ≠X2O, we split the O3 loss equally
between the two routes. Values are rounded to the nearest integer value.
Very few constraints on the concentration of tropospheric chlorine species
are available, but an increasing number of ClNO2 observations are
becoming available. Table shows a comparison between
the model an available observations. We find that the model does reasonably
well in coastal regions but does not reproduce observations in continental
regions or regions with very high NOx.
Change in tropospheric O3 on inclusion of halogen chemistry.
Column (left), surface (middle), and zonal (right) changes are shown. Upper
plots show absolute change and lower plots below give change in % terms
((“Cl+Br+I” - “NOHAL”)/“NOHAL” ⋅ 100).
Seasonal cycle of near-surface O3 at a range of Global
Atmospheric Watch (GAW) sites. Observational data shown are 6-year monthly
averages (2006–2012). Model data are for 2005. Data are from GAW, compiled and
processed as described in . Blue and red lines represent
simulations without halogens (“NOHAL”) with halogens
(“Cl+Br+I”), respectively. Grey shaded area gives 5th and 95th
percentiles of the observations. Locations of observations are shown in
Fig. .
reports measurements of HOCl and Cl2 at Cape Verde
for a week in June 2009. For the first 4 days of the campaign, HOCl
concentrations were higher and peaked at ∼ 100 pmol mol-1 with
Cl2 concentrations peaking at ∼ 30 pmol mol-1. For the
later days, HOCl concentrations dropped to around 20 pmol mol-1 and
Cl2 concentrations to ∼ 0–10 pmol mol-1. We calculate
much lower concentrations of Cl2
(∼ 1 × 10-3 pmol mol-1) and slightly lower HOCl
(∼ 10 pmol mol-1). This is similar to findings of
, who also found better comparisons with the later period of
observations. Similar to the comparison with observed ClNO2, our
simulation underestimates HOCl and Cl2.
The model does not include many sources of reactive chlorine. The failure to
reproduce continental ClNO2 is likely due to a lack of representation of
sources such as salt plains, direct emission from power station and swimming
pools, and HCl acid displacement. The inability to reproduce the very high
ClNO2 found in some cities (Pasadena) and industrialised regions (Texas)
may be due to the coarse resolution of the model compared to the spatial
inhomogeneity of these observations. The failure to reproduce the Cape Verde
observations may be due to the very simple aerosol phase chlorine chemistry
included in the model. Overall we suggest that the model provides a lower
limit estimate of the chlorine emissions and therefore burdens within the
troposphere, but constraints of surface concentrations are limited and
vertical profiles are not available. Further laboratory work to better define
aerosol processes and observations will be necessary to investigate the role
of chlorine on tropospheric chemistry.
Comparison between annual modelled O3 profiles and sonde
data (2005). Profiles shown are the annual mean of available observations
from World Ozone and Ultraviolet Radiation Data Centre and
model data for 2005 at given locations. Blue and red lines represent
simulations without halogens (“NOHAL”) with halogens
(“Cl+Br+I”), respectively. Observations (in black) show mean
concentrations with upper and lower quartiles given by whiskers. Locations of
observations are shown in Fig. .
Impact of halogens
We now investigate the impact of the halogen chemistry on the composition of
the troposphere. We start with O3 and OH and then move onto other
components of the troposphere.
Global annual-average tropospheric vertical odd oxygen loss (Ox)
through different reaction routes (Photolysis, HOx, IOx, BrOx, and
ClOx).
Global loss routes (+hν, +Br, +NO3, +Cl, +O3, +OH) of
organic compounds shown as % of total tropospheric losses.
Changes in tropospheric burden of species and families on inclusion
of halogens (“Cl+Br+I”) compared to no halogens (“NOHAL”).
Burdens are considered in elemental terms (e.g Tg S/N/C) and species masses
for OH, HO2, H2O2, and O3. The family denoted by
“VOCs” in this plot is defined as the sum of carbon masses of CO,
formaldehyde, acetaldehyde, ethane, acetone, isoprene, propane, ≥ C4
alkanes, ≥ C3 alkenes, and ≥ C3 ketones. Abbreviations for
tracers are expanded in Appendix C.
Ozone
Figure shows changes in column, surface, and
zonal O3 both in absolute and fractional terms between simulations
with and without halogen emissions (“Cl+Br+I” vs. “NOHAL”).
Globally the mass-weighted, annual-average mixing ratio is reduced by
9.4 nmol mol-1 with the inclusion of halogens and tropospheric burden
decreases by 18.6 % (“Cl+Br+I” - “NOHAL”)/
(“NOHAL” ⋅ 100). A much larger percentage decrease of
30.0 % (8.5 nmol mol-1) is seen over the ocean surface. Large
percentage losses are seen in the oceanic Southern Hemisphere as reported
previously , reflecting the
significant ocean–atmosphere exchange in this regions. The majority
(65 %) of the change in O3 mass due to halogens occurs in the
free troposphere (350 hPa <p< 900 hPa). The location of
O3 concentration decreases is noteworthy as the climate effect of
O3 is highly spatial and vertically variable .
Effects of halogens on tropospheric O3 from preindustrial to
present day are explored elsewhere .
Comparisons of the model and observed surface and sonde O3
concentrations are given in Figs. and
. In the tropics the fidelity of the simulation improves
with the inclusion of halogens, as shown previously by
and . Sonde and surface comparisons north of
∼ 50∘ N and south of ∼ 60∘ S, however, show that
the model now underestimates O3. This is clearly the case for
Neumayer and the South Pole (Fig. ).
The global odd oxygen budget Ox
in the troposphere with (“Cl+Br+I”) and without
(“NOHAL”) halogens is shown in Table . The Ox loss through
chlorine, bromine, and iodine represents 0.8, 8.4, and 12.2 % of the total
Ox loss respectively; thus halogens constitute 21.4 % of the overall
O3 loss. The sum of halogen-driven Ox loss is
1036 Tg Ox year-1, which is similar to the magnitude of loss via
reaction of O3 with HO2 of
∼ 1100 Tg Ox year-1 (21.9 % of total). Halogen
cross-over reactions (BrO + IO, BrO + ClO, IO + ClO) contribute
little to the overall O3 loss. This number compares with
∼ 930 Tg Ox year-1 reported in GEOS-Chem previously by
. found that, between
50∘ S and 50∘ N and over the ocean only, halogens are
responsible for the loss of 640 Tg Ox year-1. We find a higher
value of 827 Ox year-1 with our model.
Halogens represent 39.6 and 33.0 % of Ox loss in the upper troposphere
(350 hPa >p> tropopause) and marine boundary layer
(900 hPa <p), respectively, as shown in Fig. .
The marine boundary layer Ox loss attributable to halogens is comparable
to the 31 % reported by previously, and it is
higher than the 26 % reported solely for iodine . The
inter-reaction of halogen monoxide species is found to less important here
than previous studies (e.g. ), which has been basis in
locations of higher halogen monoxide concentrations. Inclusion of sea salt,
which would increase BrO in the marine boundary layer, would increase the
magnitude of contribution of theses routes to total halogen-driven Ox
loss.
Global annual-average surface and zonal change (%) in HOx,
NOx, and SOx families (as defined in Appendix C) on inclusion of
halogens.
Global annual-average surface and zonal change (%) in ethane
(C2H6), propane (C3H8), ≥ C4 alkanes, and acetone
(CH3C(O)CH3) on inclusion of halogens. For species where all average
changes are negative a continuous colour bar is used (C3H8 and
C2H6) and for species where both negative and positive changes are
present a divergent colour bar is used (≥ C4 alkanes and
CH3C(O)CH3).
Locations of halogen observations against which the model is compared.
IO observations are shown in different colours. ClNO2 observations are shown in gold. BrO observations presented here
were made at the same locations as IO observations. 1 indicates Cape Verde, CV
; 2 is TORERO (aircraft-based;
); 3 is Malaspina
; 4 is TransBrom ;
5 = HaloCAST-P ; 6 is TORERO (ship-based;
); 7 is Texas, US
; 8 is California, US ; 9 is Southern China, CN ; 10 is London, GB
; 11 is Hessen, Germany ;
12 = Colorado, USA ; 13 is Calgary, CA
.
Although the partitioning of the Ox loss processes is significantly
different between the simulations with and without halogens
(Table ), the overall annual Ox loss only increases
by ∼ 0.25 % (4841 vs. 4829 Tg year-1). The Ox production
term decreases by 3.4 %. This decrease is due to a reduction in NOx
concentrations via hydrolysis of XNO3 (X= Cl, Br, I). Our
tropospheric NOx burden decreases by 3.1 % to 167 Gg N (see
Table ) on inclusion of halogens consistent with
previous model studies (;
; ;
). Globally NOx losses through ClNO3
and BrNO3 hydrolysis are approximately equal (1 : 0.88) and
overall proceed at a rate of ∼ 10 % of the NOx loss through the
NO2+ OH pathway. Iodine nitrite and nitrate (INO2,
INO3) hydrolysis is much less significant (∼ 0.2 % of rate
of NO2+ OH). Net Ox is the difference between the production
and loss terms and the change here is much greater, leading to an overall
decrease in net production of tropospheric O3 (POx- LOx)
of 32 % (194 Tg year-1) and a resultant decrease in O3
lifetime of 16 %.
HOx (OH + HO2)
We find that global molecule weighted average HOx (OH +HO2)
concentrations are reduced by 10.2 % with the inclusion of halogens, with
OH decreasing by 8.2 % from 1.40 × 106 to
1.28 × 106 molecules cm-3. Lower O3 concentrations
decreases the primary OH source
(O3→hν 2OH) by 17.4 % and the
secondary OH source (HO2+ NO) by 4.7 %.
The reduction in the sources of OH is buffered by an additional OH source
from the photolysis of HOX (X= Cl, Br, I) which acts to
increase the conversion of HO2 to OH. Previously,
showed an increase of 1.8 % in global OH
concentrations on inclusion of iodine. However, increased Bry and reduced
Iy concentrations in the simulations described here mean that the
increased OH source from HOX photolysis does not compensate fully for the
reduced primary source, resulting in an overall 8.2 % reduction in global
mean OH. This buffering contributes to a change in OH smaller than the
11 % reported previously . As reported previously
, we also find the net effect of halogens on the
OH : HO2 ratio is a small increase (2.3 %).
Organic compounds
The oxidation of VOCs by halogens is included in
this simulation (see Table for reactions). The global
fractional loss due to OH, Cl, Br, O3, NO3, and photolysis (hν) for a
range of organics is shown in Fig. .
Globally, Br oxidation is small in our simulation and contributes 3.9 %
to the loss of acetaldehyde (CH3CHO), 0.8 % of the loss of
formaldehyde (CH2O), 0.63 % of the loss of ⩾ C4
alkenes, and <0.001 % of the loss of other compounds. Recent
work has suggested a significant source of oceanic oxygenated VOCs (oVOCs)
, which we do not include in this simulation.
Furthermore, although our modelled Bry is broadly comparable to some
previous work , it is lower in the marine
boundary layer than in other recent work . The combination of
these two factors suggests that our model provides a lower bounds of impacts
of bromine on VOCs. Significantly higher concentrations of oVOC would
decrease the BrOx concentrations in the model and might then allow an
increased sea-salt source of reactive bromine.
Locations of O3 observations the model is compared against.
Observations made by O3 sonde are shown in brown; surface
observations at GAW sites are shown in gold. Where a site is a location
of both sonde release and surface O3 observation it is shown in brown
(Samoa, Neumayer, Lauder, and Milo).
The oxidation of VOCs by chlorine is more significant. In our simulation
chlorine accounts for 27, 15, and 14 % of the global loss of ethane
(C2H6), propane (C3H8), and acetone (CH3C(O)CH3),
respectively. Loss of other VOCs is globally small. This increased loss due
to Cl is to some extent compensated for by the reduction in the OH
concentrations that we calculate. Thus the overall lifetime of ethane,
propane, and acetone changes from 131, 38, and 85 days in the simulation without
halogens to 113, 37, and 80 in the simulation with halogens. Notably the ethane
lifetime without halogens is 16 % longer. Given that we consider the
chlorine in the model to be a lower limit, ethane oxidation by chlorine may
in reality be more significant than found here.
Methane is a significant climate gas, as it has the second-highest forcing
amongst well-mixed greenhouse gases from preindustrial to present
day . In our simulation without halogens we calculate a
tropospheric chemical lifetime due to OH of 7.47 years. With the inclusion of
halogen chemistry the OH concentration drops, extending the methane lifetime
due to OH to 8.28 years (an increase of 10.8 %). However, in our halogen
simulations, chlorine radicals also oxidise methane (2.0 % of the total
loss), shortening the lifetime to 8.16 years (1.52 %). As noted
previously, the model's chlorine concentrations appear to be underestimated.
estimate a 25 Tg year-1 sink for methane from Cl
(∼ 4 %), significantly higher than our estimate (4 Tg). Overall
the model's CH4 lifetime still appears to be short compared to the
observationally based estimation of 9.1 ± 0.9 from ,
but halogens decrease this bias.
In our simulations, halogens (essentially chlorine) have a significant but
not overwhelming role in the concentrations of hydrocarbons (from
∼ 1 % of methanol loss to ∼ 27 % of ethane loss).
However, as discussed earlier, the low biases seen with the very limited
observational dataset of chlorine compounds would suggest that the impacts
calculated here are probably lower limits.
Other species
With the inclusion of halogens in the troposphere there are a large number of
changes in the composition of the troposphere.
Figure illustrates the fractional global change in
burden by species (for abbreviation see Appendix C). The spatial and zonal
distribution of these changes by species family (HOx, NOx, SOx as
defined in Appendix C)
are shown in Fig. and for a few VOCs
(C3H8, C2H6, acetone, and ⩾ C4 alkanes) in
Fig. . A tabulated form of these changes is given
within Appendix A
(Table )
As discussed in Sects. and
, a clear decrease in oxidants (O3, OH,
HO2, H2O2) is seen. This drives an increase in the
concentrations of some VOCs (4.5 % on a per carbon basis), including CO
(6.1 %) and isoprene (6.2 %). However, as discussed, it also adds an
additional Cl sink term which leads to an overall decrease in some species
(e.g. C2H6, (CH3)2CO, C3H8) particularly in the
northern hemispheric oceanic regions. The SOx burden increases slightly
(0.5 %), which can be attributed to decreases in oxidants.
Summary and conclusions
We have presented a model of tropospheric composition which has attempted to
include the major routes of halogen chemistry impacts. Assessment of the
model performance is limited as observations of halogen species are extremely
sparse. However, given the available observations we conclude that the model
has some useful skill in predicting the concentration of iodine and bromine
species and appears to underestimate the concentrations of chlorine species.
Consistent with previous studies, our model shows significant halogen-driven
changes in the concentrations of oxidants. The tropospheric O3 burden
and global mean OH decreases by 18.6 and 8.2 %, respectively, on
inclusion of halogens. The methane lifetime increases by 10.8 %,
improving agreement with observations.
There are a range of changes in the concentrations of other species. Direct
reaction with Cl atoms leads to enhanced oxidation of hydrocarbons with
ethane showing a significant response. Given that the model appears to provide a
lower limit for atomic Cl concentrations, this suggests a major missing
oxidation pathway for ethane which is currently not considered. NOx
concentrations are reduced by aerosol hydrolysis of the halogen nitrates,
which leads to reduced global O3 production. Our simulation of BrO
appears to be relatively consistent with observations, but we do not
include a sea-salt debromination mechanism. This would suggest that either
the cycling of bromine in our model is generally too fast or that we do not
have sufficiently large BrOx sinks (potentially oVOCs). Both hypotheses
warrant further research.
Significant uncertainties, however, remain in our understanding of halogens
in the troposphere. The gas phase chemistry and photolysis parameters of
iodine compounds are uncertain, together with the
emissions of their organic and inorganic precursors . For
chlorine, bromine, and iodine heterogeneous chemistry, little experimental
data exist and suitable parameterisations for the complex aerosols found in
the atmosphere are unavailable . The uncertainties of this
have been discussed in recent reviews
and considered in previous model studies ,
and they still warrant further exploration.
Understanding fully the impact of halogens on tropospheric composition will
require significant development of new experimental techniques and more field
observations, new laboratory studies, and models which are able to exploit
these developments.
Data availability
The model code used here will be made available to the community through the standard GEOS-Chem repository (http://www.geos-chem.org). Requests for materials should be addressed to Mat Evans
(mat.evans@york.ac.uk).
Tabulated burden changes on inclusion of halogens
Table gives the burdens with and without halogens and
the fractional change.
Tropospheric burden of species and families
with (“Cl+Br+I”) and without halogens (“NOHAL”), and %
change. Burdens are considered in elemental terms (e.g Gg S/N/C) and species
masses for OH, HO2, H2O2, and O3. Families are defined
in Appendix C.
Here is described the full halogen chemistry scheme as presented in previous
works
and with updates as detailed in Sect. and
Table . The complete gas phase photolysis, bimolecular,
and termolecular reactions are described in Tables ,
, and .
Photolysis reactions of halogens included in scheme. Photolysis is
described in (ClNO2, ClNO3, and ClOO),
(I2, HOI, IO, OIO, INO, INO2,
INO3, I2O2, I2O3, I2O4, CH3I,
CH2I2, CH2ICl, and CH2IBr), and
(BrCl, Cl2, ClO, HOCl, ClNO2, ClNO3, ClOO,
Cl2O2, CH3Cl, CH3Cl2, and CHCl3). As stated
in Sect. , we have used the I2O2 cross section
for I2O4.
IDReactionCross-section referenceJ1I2→hν 2IJ2HOI →hν I + OHJ3IO (+O2) →hν I (+O3)J4OIO →hν I +O2J5INO →hν I + NOJ6INO2→hν I +NO2J7INO3→hν I +NO3J8I2O2→hν I + OIOJ9CH3I→hν IJ10CH2I2→hν 2IJ11CH2ICl→hν I + ClJ12CH2IBr→hν I + BrJ13I2O4→hν 2OIOsee captionJ14I2O3→hν OIO + IOJ15CHBr3→hν 3BrJ16Br2→hν 2BrJ17BrO (+O2) →hν Br (+O3)J18HOBr →hν Br + OHJ19BrNO2→hν Br +NO2J20BrNO3→hν Br +NO3J21BrNO3→hν BrO +NO2J22CH2Br2→hν 2BrJ23BrCl →hν Br + ClJ24Cl2→hν 2ClJ25ClO (+O2) →hν Cl (+O3)J26OClO (+O2) →hν ClO (+O3)J27Cl2O2→hν Cl + ClOOJ28ClNO2→hν Cl +NO2J29ClNO3→hν Cl +NO3J30ClNO3→hν ClO +NO2J31HOCl →hν Cl + OHJ32ClOO →hν ClJ33CH3Cl→hν Cl +CH3O2,J34CH3Cl2→hν 2Cl
Bimolecular halogen reactions included in scheme. This includes
reactions from previous updates to descriptions of halogen chemistry in
GEOS-Chem and
those described in Sect. . These are given in the
Arrhenius form with the rate equal to A⋅exp(-EaRT).
Unknown values are represented by a dash and these set to 0 in the model,
reducing the exponent to 1. The bi-molecular reactions with an M above the
arrow represent termolecular reactions where the pressure dependence is not
known or are uni-molecular decomposition reactions. Abbreviations for tracers
are expanded in Appendix C.
a Only first channel from JPL considered. The second channel forms a Criegee (HCl + C2H4O2) and therefore cannot be represented by the reduced
GEOS-Chem chemistry scheme. b Reaction defined by JPL and interpreted as proceeding via hydrogen abstraction; therefore, the acetaldehyde product
is assumed.
Termolecular halogen reactions included in the scheme. This includes
reactions from previous updates to halogen chemistry in GEOS-Chem
and those
detailed in Sect. . The lower pressure limit rate
(k0) is given by A0⋅(300T)x. The high pressure
limit is given by k∞. Fc characterises the fall off curve of the
reaction as described by .
*k∞(T) for Reactions (T7)–(T11) have a form of
k∞(T)=k∞(T300)-m, where m= 3.1, 4.5,
4.5, 3.4, and 3.1 respectively. Abbreviations for tracers are expanded in
Appendix C.
Heterogenous reactions
The halogen multiphase chemistry mechanism is based on the iodine mechanism
(“Br + I”) described in and the coupled (Cl, Br)
mechanism of . The heterogenous reactions in the scheme
are shown in Table and with further detail individual
detail on certain reactions below. The loss rate of a molecule X due to
multiphase processing on aerosol is calculated following .
dnXdt=-rDg+4cγ-1AnX,
where r is the aerosol effective radius, Dg is the gas phase
diffusion coefficient of X, c is the average thermal velocity of X,
γ is the reactive uptake coefficient, A is the aerosol surface area
concentration, and nX is the gas phase concentration of X.
Aerosols
We consider halogen reactions on sulfate aerosols, sea-salt aerosols, and
liquid and ice cloud droplets. The implementation of sulfate type aerosols in
GEOS-Chem is described by and . Sulfate
aerosols are assumed to be acidic with pH = 0.
The GEOS-Chem sea-salt aerosol simulation is as described by
. The transport and deposition of sea-salt bromide follows
that of the parent aerosol. Oxidation of bromide on sea salt produces
volatile forms of bromine that are released to the gas phase. Sea-salt
aerosol is emitted alkaline, but the alkalinity can be titrated in GEOS-Chem
by uptake of HNO3, SO2, and H2SO4.
Sea-salt aerosol with no remaining alkalinity is assumed to have pH = 5.
We assume no halide oxidation on alkaline sea-salt aerosol.
The liquid cloud droplet surface area is modelled using cloud liquid water
content from GEOS-FP and assuming effective cloud
droplet radii of 10 and 6 µm for marine and continental clouds,
respectively. The ice cloud droplet surface area is modelled in a similar
manner assuming effective ice droplet radii of 75 µm. We assume
that ice cloud chemistry is confined to an unfrozen overlayer surrounding the
ice crystal (see for details). Cloud water pH (typically
between 4 and 6) is calculated locally in GEOS-Chem following
.
Halogen multiphase reactions and reactive uptake coefficients
(γ).
IDReactionReactive uptake coefficient (γ)NoteReference1HCl→Cl-(SSA)4.4×10-6exp(2989K/T)Sea salt only2HBr→Br-(SSA)1.3×10-8exp(4290K/T)Sea salt only3HI→I(aerosol)0.14ClNO3→HOCl+HNO30.024Hydrolysis5BrNO3→HOBr+HNO30.02Hydrolysis6INO3→0.85ICl+0.15IBr+HNO30.01Sea salt only7INO2→0.85ICl+0.15IBr+HNO30.02Sea salt only8HOBr+Cl-(aq)→BrClSee Sect. in Appendix B9HOBr+Br-(aq)→Br2See Sect. in Appendix B10HOI→0.85ICl+0.15IBr0.01Sea salt only11ClNO3+Br-(aq)→BrCl+HNO3See Sect. in Appendix B12O3+Br-(aq)→HOBrSee Sect. in Appendix B13I2O2→I(aerosol)0.0214I2O3→I(aerosol)0.0215I2O4→I(aerosol)0.02
Henry's law coefficients and molar heats of formation of iodine
species. Where Henry's law constant equals infinity a very large values is
used within the model (1×1020Matm-1). The
INO2 Henry's law constant is assumed equal to that of BrNO3,
from , by analogy. For I2Ox (X= 2, 3, 4) a
Henry's law constant of infinity is assumed by analogy with INO3.
* Effective Henry's law of HX is calculated for acid
conditions through KH*(T)=KH(T)×(1+Ka[H+]). A pH of 4.5 is assumed for a typical cloud
droplet.
The reactive uptake coefficients depend on the aerosol halide concentration.
For sea-salt aerosol, the bromide concentration is calculated directly from
the bromide content and the aerosol mass. Sea-salt aerosol chloride is
assumed to be in excess (see below). For clouds and sulfate aerosol, the
bromide and chloride concentration is estimated by assuming equilibrium between
gas phase HX and aerosol phase X-.
Reactive uptake coefficientsHOBr + Cl-/ Br-
The reactive uptake coefficient is calculated as
γ=Γ-1+α-1-1,
where the mass accommodation coefficient for HOBr is α=0.6, and
Γ=4HHOBrRTkHOBr+X-[X-][H+]lrf(r,lr)c,
with kHOBr+Cl-=5.9×109M-2s-1 and
kHOBr+Br-=1.6×1010M-2s-1. In the
equation above c is the average thermal velocity of HOBr, and
f(lr,r) is a reacto-diffusive correction factor,
f(lr,r)=cothrlr-lrr,
with r being the radius of the aerosol. For sea-salt aerosol,
HOBr+Cl- is assumed to be limited by mass accommodation, i.e. Γ≫α, due to high concentration of Cl- in sea-salt aerosol.
The reacto-diffusive length scale is
lr=DlkHOBr+X-[X-][H+],
where Dl=1.4×10-5cm2s-1 is the aqueous phase
diffusion coefficient for HOBr. The listed parameters are taken from
, and kHOBr+Br- is from
.
ClNO3+ Br-
The reactive uptake coefficient is calculated as
γ=Γ-1+α-1-1,
where the mass accommodation coefficient for ClNO3 is α=0.108, and
Γ=4WRT[Br-]Dlc,
where c is the average thermal velocity of ClNO3,
Dl=5.0×10-6cm2s-1 is the aqueous phase diffusion
coefficient for ClNO3, and
W=106Msbar-1.
O3+ Br-
The reactive uptake coefficient is calculated as
γ=Γb+Γs,
where Γb is the bulk reaction coefficient,
Γb=4HO3RTkO3+Br-[Br-]lrf(r,lr)c,
with
kO3+Br-=6.8×108exp(-4450K/T)M-1s-1.
In the equation above c is the average thermal velocity of O3, and
f(lr,r) is a reacto-diffusive correction factor,
f(lr,r)=cothrlr-lrr,
with r being the radius of the aerosol. The reacto-diffusive length scale
is
lr=DlkO3+Br-[Br-],
where Dl=8.9×10-6cm2s-1 is the aqueous phase
diffusion coefficient for O3.
The surface reaction coefficient is calculated as
Γs=4ks[Br-(surf)]KLangCNmaxc(1+KLangC[O3(g)]),
where the surface reaction rate constant is
ks=10-16cm2s-1, the equilibrium constant for
O3 is KLangC=10-13cm3, and the maximum
number of available sites is taken as
Nmax=3×1014cm-2. The surface bromide concentration
is estimated as
[Br-(surf)]=min(3.41×1014cm-2M-1[Br-],Nmax).
Abbreviations used in the document. Abbreviated species names used here are defined in the GEOS-Chem manual (http://acmg.seas.harvard.edu/geos/doc/man/appendix_6.html).
AbbreivationExpansionPANperoxyacetyl nitratePPNperoxypropionyl nitrateMPNmethyl peroxy nitratePMNperoxymethacryloyl nitrateMOHmethanolEOHethanolALD2acetaldehydeISOPisopreneALK4≥ C4 alkanesCH3O2methylperoxy radicalA3O2primary RO2 from C3H8B3O2secondary RO2 from C3H8ATO2RO2 from acetoneR4O2RO2 from ≥ C4 alkanesRIO2RO2 from acetoneHOxOH +HO2NOxNO +NO2SOxSO2+SO4+SO4 on sea saltIyI+2I2+HOI+IO+OIO+HI+INO+INO2+INO3+2I2O2+2I2O3+2I2O4BryBr + 2Br2+ HOBr + BrO + HBr +BrNO2+BrNO3+ IBr + BrClClyCl +2Cl2+ HOCl + ClO + HCl +ClNO2+ClNO3+ICl+BrCl+ClOO+OClO+2Cl2O2OxO3+NO2+2NO3+PAN+PMN+PPN+HNO4+3N2O5+HNO3+ MPN +XO + HOX+XNO2+ 2XNO3+2OIO+2I2O2+3I2O3+4I2O4+2Cl2O2+2OClO (where X= Cl, Br, I)PRPE≥ C3 alkenesAcknowledgements
This work was funded by NERC quota studentship NE/K500987/1 with support from
the NERC BACCHUS and CAST projects NE/L01291X/1 and NE/J006165/1.
J. A. Schmidt acknowledges funding through a Carlsberg Foundation
post-doctoral fellowship (CF14-0519).
R. Volkamer acknowledges funding from US National Science Foundation CAREER
award ATM-0847793, AGS-1104104, and AGS-1452317. 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.
T. Sherwen would like to acknowledge constructive comments and input from
GEOS-Chem Support Team at Harvard Univeristy. We also acknowledge
constructive input from Qianjie Chen and Becky Alexander of the University of
Washington. Edited by: R.
Sander Reviewed by: two anonymous referees
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