JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D17, 4341, doi:10.1029/2001JD000942, 2002
Modeling halogen chemistry in
the marine boundary layer
1. Cloud-free MBL
Roland von Glasow1 and Rolf Sander
Atmospheric Chemistry Division, Max-Planck-Institut für Chemie, Mainz, Germany
Andreas Bott
Meteorologisches Institut, Universität Bonn, Bonn, Germany
Paul J. Crutzen2
Atmospheric Chemistry Division, Max-Planck-Institut für Chemie, Mainz, Germany
Received 8 June 2001; revised 26 November 2001; accepted 7 January 2002; published 14 September 2002.
[1] A numerical one-dimensional model of the marine boundary layer (MBL) is
presented. It includes chemical reactions in the gas phase and aerosol particles, focusing
on the reaction cycles of halogen compounds. Results of earlier box model studies were
confirmed. They showed the acid-catalyzed activation of bromine from sea salt aerosol,
and the role of halogen radicals in the destruction of O3. A distinct diurnal variation in
BrO mixing ratios with maxima at sunrise and sunset was found which might be the cause
of the recently published ‘‘sunrise ozone destruction.’’ Maxima of BrO and sea salt acidity
are predicted at the top of the MBL and not close to the sea surface where sea salt spray is
produced. The presence of sulfate aerosol was found to be important for the recycling of
less reactive to photolyzable bromine species. Day/night and seasonal differences in
INDEX TERMS: 0365 Atmospheric Composition and Structure:
halogen chemistry are shown.
Troposphere—composition and chemistry; 0305 Atmospheric Composition and Structure: Aerosols and
particles (0345, 4801); 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and
chemistry; KEYWORDS: sea-salt aerosol, O3 destruction, aerosol chemistry, iodine, bromine, chlorine
Citation: von Glasow, R., R. Sander, A. Bott, and P. J. Crutzen, Modeling halogen chemistry in the marine boundary layer 1. Cloudfree MBL, J. Geophys. Res., 107(D17), 4341, doi:10.1029/2001JD000942, 2002.
1. Introduction
[2] Chemical reactions on and inside atmospheric particles can have a significant influence on the chemistry of
the gas phase (see Jacob [2000] for a review). In the case of
the Antarctic ‘‘ozone hole’’ where rapid O3 destruction by
halogen radicals takes place, interactions between the gas
and the particulate phase (polar stratospheric clouds) are
very important (see e.g., Brasseur et al. [1999] for an
overview). A sudden O3 destruction is also observed in
the Arctic boundary layer during polar sunrise. It is mainly
due to reactions of bromine species that probably originate
from deposits on the snow pack. Reactions on aerosols are
also important here [e.g., Barrie et al., 1988; Bottenheim et
al., 1990; Barrie and Platt, 1997].
[3] In this study we focus on the interactions between gas
and particulate chemistry in the marine boundary layer
1
Now at Scripps Institution of Oceanography, University of California,
San Diego, California, USA.
2
Also at Scripps Institution of Oceanography, University of California,
San Diego, California, USA.
Copyright 2002 by the American Geophysical Union.
0148-0227/02/2001JD000942$09.00
ACH
(MBL). Sea salt aerosol contains the halogens chlorine
and bromine. Provided that an efficient path for the release
of halogens from the aerosol exists, sea salt aerosol is a
potential source for gas phase halogen species in the MBL.
As about 70% of the earth’s surface is covered by oceans,
reactions involving halogens released from sea salt aerosol
are of potential global importance.
[4] Mozurkewich [1995] proposed an acid-catalyzed
mechanism for the release of bromine from Arctic sea salt
aerosol. In several laboratory studies release of Br2 and BrCl
from sea salt solutions could be shown [Abbatt and Waschewsky, 1998; Hirokawa et al., 1998; Disselkamp et al., 1999;
Behnke et al., 1999]. Fickert et al. [1999] could show in the
laboratory that the bromine activation depends on the pH of
the solution. Contrary to HCl, HBr cannot be released
significantly by acid displacement due to its high solubility.
Recently, several authors [Ghosal et al., 2000; Zangmeister et
al., 2001] found that surface segregation of bromide and
chloride ions occurs in salt crystals. This could facilitate the
release of bromine and chlorine from the surfaces. Numerical
simulations by Knipping et al. [2000] and Jungwirth and
Tobias [2001] suggest that in salt solutions the halides are
also present at the surface and Br and I have actually
higher concentrations at the surface than in the bulk.
9-1
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VON GLASOW ET AL.: HALOGEN CHEMISTRY IN THE MBL, 1
[5] Data from kinetic studies have been incorporated into
numerical models showing the activation of reactive bromine in the MBL [Sander and Crutzen, 1996; Vogt et al.,
1996; Keene et al., 1998; Sander et al., 1999; Dickerson et
al., 1999].
[6] Field measurements of sea salt aerosol composition
have been made for a long time, which often show a
decrease of the bromine to sodium ratio of aged sea salt
aerosol compared to sea water [e.g., Duce et al., 1965; Kritz
and Rancher, 1980; Ayers et al., 1999].
[7] Ayers et al. [1999] presented measurements from
Cape Grim, Tasmania, showing bromine deficits of 30%
to 50% on an annual average and maximum monthly mean
values of more than 80%. They also show that Br deficits
were linked to the availability of sulfate acidity in the
aerosol, pointing to the importance of acid catalysis in the
dehalogenation process. The measurements of Rancher and
Kritz [1980] and Kritz and Rancher [1980] show that total
inorganic gas phase bromine and particulate bromine are of
the same order of magnitude.
[8] Individual reactive gas phase halogen species are
difficult to detect because of their relatively low mixing
ratios. Most available measurements of single bromine
species are for BrO using the differential optical absorption
spectroscopy (DOAS) technique. Rather large BrO mixing
ratios of 20–30 pmol/mol were found in the Arctic during
polar sunrise [Hausmann and Platt, 1994; Tuckermann et
al., 1997] which were always associated with dramatic loss
of ozone. Satellite measurements show large plumes of BrO
in the Arctic spring [Richter et al., 1998; Wagner and Platt,
1998]. McElroy et al. [1999] provided evidence for the
presence of BrO in the free troposphere during Arctic polar
sunrise. At the Dead Sea, BrO mixing ratios of more than 90
pmol/mol [Hebestreit et al., 1999] were found, which are
due to the special location of the measurement site downwind of large salt pans. These extreme events show that the
processes involved in bromine related ozone loss do not
necessarily depend on the availability of ice surfaces as was
proposed for the Arctic case. Several groups tried to
measure BrO in the MBL with the DOAS technique. During
these campaigns the detection limit for BrO was between 1
and 10 pmol/mol. Whenever BrO was measured, it was near
the detection limit (personal communication with D. Perner,
B. Allan, and K. Hebestreit). Therefore these measurements
provide us only with upper limits for BrO that are consistent
with our model results (see below). Measurements of total
inorganic bromine, however, show that there is significant
bromine in the gas phase.
[9] Iodine compounds may also be important for the
chemistry of the MBL. Iodocarbons are emitted at the sea
surface and are photolyzed producing chemically reactive
species like I, IO, and OIO. IO and OIO have been measured
in the pmol/mol range [Alicke et al., 1999; Allan et al., 2000;
Hebestreit et al., 2000; Allan et al., 2001]. These studies
highlight the importance of halogen chemistry in the MBL.
[10] In this work we present a modeling study of the
multiphase chemistry in the MBL focusing on the influence
of halogens. For this purpose, we extended a one-dimensional boundary layer model to include chemical reactions
in the gas phase as well as in and on aerosol (sea salt and
sulfate particles). This model (MISTRA-MPIC) is designed
for detailed process studies.
[11] From some measurement campaigns, field data are
available that we use to discuss the model results. Our
findings might also help to design future field campaigns or
laboratory studies.
2. Description of the MBL Model
[12] We used the one-dimensional model MISTRAMPIC, where MPIC stands for Max-Planck-Institut für
Chemie version. The meteorological and microphysical part
of the boundary layer model is described in detail by Bott et
al. [1996] and Bott [1997]. In addition to a description of
the dynamics and thermodynamics, it includes a detailed
microphysical module that calculates particle growth explicitly and treats interactions between radiation and particles.
Chemical reactions in the gas phase are considered in all
model layers, aerosol chemistry only in layers where the
relative humidity is greater than the crystallization humidity
(see section 2.2). Fluxes of heat, moisture, sea salt aerosol
particles, and gases from the ocean are included. Figure 1
shows schematically the most important processes that are
included in the model for the cloud-free MBL. More details
are given by von Glasow [2000].
2.1. Meteorology, Microphysics, and Thermodynamics
[13] The set of prognostic variables comprises the horizontal components of the wind speed u, v, the specific
humidity q, and the potential temperature as listed by Bott
et al. [1996]. For the cloud-free runs that we discuss here,
no subsidence was applied.
[14] The microphysics is treated using a two-dimensional
particle size distribution function f (a, r) with the total
particle radius r and the dry aerosol radius a the particles
would have, when no water were present in the particles.
The two-dimensional particle grid is divided into 70 logarithmically equidistant spaced dry aerosol classes, with the
minimum dry aerosol radius being 0.01 mm and the maximum radius 15 mm. Choosing these values allows to
account for all accumulation mode particles and most of
the coarse particles. Each of the 70 dry aerosol classes is
associated with 70 total particle radius classes, ranging from
the actual dry aerosol radius up to 60 mm. Particle growth is
calculated explicitly for each bin of the 2D particle spectrum
including effects of radiation on the particle growth [see
also Bott et al., 1996].
[15] For the calculation of the radiative fluxes a d-two
stream approach is used [Zdunkowski et al., 1982; Loughlin
et al., 1997]. The radiative fluxes are used for calculating
heating rates and the effect of radiation on particle growth.
The radiation field is calculated with the aerosol particle
data from the microphysical part of the model, so feedbacks
between radiation and particle growth are fully implemented.
[16] Sea salt particles are emitted by bursting bubbles at
the sea surface [e.g., Pruppacher and Klett, 1997]. The
parameterization of Monahan et al. [1986] is used to
estimate the flux F of particles with radius r80 (in mm) at
a relative humidity of 80% per unit area of sea surface (in
particles m2 s1 mm1):
2
dF
3
1:05
¼ 1:373u3:41
101:19expðB Þ
10 r80 1 þ 0:057r80
dr80
ð1Þ
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VON GLASOW ET AL.: HALOGEN CHEMISTRY IN THE MBL, 1
9-3
Figure 1. Schematic depiction of the most important processes included in the one-dimensional
boundary layer model MISTRA-MPIC. The free troposphere is denoted as FT, the marine boundary layer
as MBL. A typical vertical profile of the relative humidity (rh) is shown as the dashed line. Gas phase
chemistry is calculated in all layers.
where B = (0.380 log r80)/0.65 and u10 is the wind speed
at 10 m height in m/s. For each dry aerosol radius bin of the
2D microphysical spectrum the equilibrium radius at the
actual relative humidity is calculated and the appropriate
number of particles according to equation (1) is added in the
lowermost model layer. The Monahan et al. [1986] estimate
of the sea salt aerosol flux based on wind tunnel
experiments is believed to yield good results for small
particles [Andreas, 1998]. Monahan et al. [1986] also
parameterize the emission of particles with radii larger than
r = 10 mm by spume production. However, only the bubble
burst mechanism is included because as pointed out by, e.g.,
Gong et al. [1997] and Andreas [1998] the additional terms
lead to an overestimation of the particle flux compared to
measurements. For higher wind speeds the resulting mass
flux of sea salt particles is less realistic, so for the model
runs with high wind speed the parameterization of Smith et
al. [1993] that is based on measurements off the Scottish
coast is used.
2.2. Chemistry
[17] The multiphase chemistry module comprises chemical reactions in the gas phase as well as in aerosol particles.
Transfer between gas and aqueous phase and surface
reactions on particles are also included. A complete listing
of the reactions is available as electronic supplement1 and
on the web (http://www.rolandvonglasow.de). The reaction
set is an updated version of Sander and Crutzen [1996] (see
1
Supporting Figure A1 and Tables A1 – A6 are available via Web
browser or via Anonymous FTP from ftp://kosmos.agu.org, directory
‘‘apend’’ (Username = ‘‘anonymous’’, Password = ‘‘guest’’); subdirectories
in the ftp site are arranged by paper number. Information on searching and
submitting electronic supplements is found at http://www.agu.org/pubs/
esupp_about.html.
also http://www.mpch-mainz.mpg.de/~sander/mocca) plus
some organic reactions from Lurmann et al. [1986].
[18] The prognostic equation for the concentration of a
gas phase chemical species cg (in mol/mair3) including
turbulent exchange, deposition on the ocean surface, chemical production and destruction, emission and exchange
with the aqueous phases is
@cg
@cg =r
@
Kh r
¼
D þ P Lcg þ E
@z
@t
@z
nkc
X
ca;i
kt;i wl;i cg cc :
kH
i¼1
ð2Þ
Kh is the turbulent exchange coefficient for heat that is
usually used for the treatment of turbulent transport of gases
as well. P and Lcg are chemical production and loss terms,
respectively. The emission E as well as dry deposition D are
effective only in the lowermost model layer. The dry
deposition velocity for gases at the sea surface, vgdry, that is
needed for the determination of D, is calculated using the
resistance model described by Wesely [1989]. The last term
in equation (2) describes the transport from the gas phase
into the aqueous phases according to the formulation by
Schwartz [1986] [see also Sander, 1999], nkc is the number
of the aqueous classes as explained below and kHcc the
dimensionless Henry constant. It is obtained by kHcc = kHRT,
where kH is in mol/(m3 Pa), wl, i is the dimensionless liquid
3
3
/mair
) of bin i. For a single particle, the
water content (maq
mass transfer coefficient kt is defined as
kt ¼
r2
4r
þ
3Dg 3va
1
ð3Þ
withpthe
particle
radius r, the mean molecular speed
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffi
8RT =ð M pÞ (M is the molar mass), the accommodation
v ¼
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VON GLASOW ET AL.: HALOGEN CHEMISTRY IN THE MBL, 1
coefficient a (see Tables A1 – A6 in the electronic
supplement and on the web: http://www.rolandvonglasow.
de), and the gas phase diffusion coefficient Dg. Dg is
approximated as Dg = lv/3 [e.g., Gombosi, 1994, p. 125]
using the mean free path length l.
[19] When aqueous chemistry is treated with size bins,
one has to account for particles with different radii within
each bin. The mean transfer coefficient kt for a particle
population is given by the integral:
4p
kt ¼
3wl
lgr
Z max
1
r2
4r
@N
dlgr;
þ
r3
@lgr
3Dg 3va
ð4Þ
lgrmin
where the size distribution function @N/@lgr is defined in
Table 3.
[20] Aqueous chemistry is calculated in two bins: deliquescent aerosol particles with a dry radius less than 0.5 mm
are included in the ‘‘sulfate aerosol’’ bin, whereas deliquescent particles with a dry aerosol radius greater than 0.5 mm
are in the ‘‘sea salt aerosol’’ bin. Using these values we
ensure that most aerosol mass, which is the important
property for the aerosol chemistry, is included in each class.
Although the composition of the particles changes over time
the terms ‘‘sulfate’’ and ‘‘sea salt’’ aerosol are used to
describe the origin of the particles. In each of these classes
the following prognostic equation is solved for each chemical species ca,i (in mol/mair3), where the index i stands for
the i-th aqueous class:
@ca;i
@
@ca;i =r
ca;i
¼
Kh r
D þ P Lca;i þ E þ kt;i wl;i cg cc
@t
kH
@z
@z
ð5Þ
The individual terms have similar meanings as in equation
dry
, that is
(2). The dry deposition velocity of particles, va,i
needed for the determination of D, is calculated as explained
by Seinfeld and Pandis [1998].
[21] The concentration of H+ ions is calculated like any
other species, i.e., no further assumptions are made. The
charge balance (in other models sometimes used to derive
the pH) is satisfied implicitly.
[22] It is assumed that water is associated with sea salt
aerosol particles above their crystallization point rather than
their deliquescence point because the particles are produced
as droplets at the sea surface. Therefore the hysteresis effect
ensures that, upon shrinking, the particles remain in a
metastable highly concentrated solution state above their
crystallization point, which is at about 45% relative humidity for NaCl [e.g., Tang, 1997; Pruppacher and Klett, 1997].
As long as the relative humidity in the MBL is well above
the crystallization point, sea salt aerosol is not present in
crystalline form. The crystallization humidity for many
mixed aerosol particles containing sulfate or nitrate is below
40% relative humidity [Seinfeld and Pandis, 1998, and
references therein], implying that aerosol particles that
already had been involved in cloud cycles will also be in
an aqueous metastable state. Therefore many soluble aerosol particles will be present in the atmosphere as metastable
aqueous particles below their deliquescence humidity.
Based on this we assume that sulfate aerosols are also
liquified above their crystallization humidity. We assume a
bulk crystallization humidity for the sulfate aerosol particles
and do not include the formation of solid phases explicitly
in the model. In the runs that are presented, this assumption
has no impliciations, because the relative humidity never
drops below the crystallization humidity. It does, however,
drop below the deliquescence humidity.
[23] Aerosols are usually highly concentrated solutions.
Laboratory measurements show that NaCl molalities can be
in excess of 10 mol/kg [Tang, 1997] implying also very
high ionic strengths. Therefore it is necessary to account for
deviations from ideal behaviour, i.e., to include activity
coefficients. We used the Pitzer formalism [Pitzer, 1991] to
calculate the activity coefficients for the actual composition
of each aqueous size bin. The Pitzer model in its original
formulation is applicable to ionic strengths of 6 mol/kg.
According to Luo et al. [1995], the Pitzer model can be
extended to even stronger electrolyte solutions. The parameters used in our model were based on data over a wide
range of particle composition. For example, for (NH4)2SO4
and H2SO4 the model is valid up to 40 mol/kg.
[24] In the atmosphere each aerosol or cloud particle is a
closed ‘‘reaction chamber’’ with its own pH and concentrations of the species. In models the particles have to be
lumped in bins where the individual properties of the
particles vanish. In the model freshly emitted, alkaline sea
salt particles are put into the same chemistry bin as aged
(possibly acidic) particles leading to mean conditions and
reaction paths that can be distinctly different from what is
happening in the atmosphere. Especially in the case of high
wind speeds, when sea salt aerosol production and therefore
also sea salt aerosol loadings are high, the model-calculated
pH is not representative for all particle sizes. This is due to
the fact that the buffer capacity would then be determined
by the large particles and reactions in the smaller, more
acidic particles that involve acidity might be underestimated. In future versions of this model a finer resolution
of the aqueous chemistry bins will be achieved.
[25] Photolysis rates are calculated online using the
method of Landgraf and Crutzen [1998]. For the calculation
of the actinic fluxes a four stream radiation code is used in
addition to the two stream radiation code used for the
determination of the net radiative flux density En because
different spectral resolutions and accuracies are needed for
these different purposes. Based on the findings of Ruggaber
et al. [1997] a factor of 2 is applied to photolysis rates inside
aqueous particles to account for the actinic flux enhancement inside the particles due to multiple scattering.
[26] Constant emission fluxes for the gases DMS and
NH3 from the sea surface are applied with 2 109 molec/
(cm2s) [Quinn et al., 1990] and 4 108 molec/ (cm2s)
[Quinn et al., 1990], scaled to model their measured gas
phase mixing ratios of 19 nmol/mol in clean air masses),
respectively.
2.3. Model Resolution and Integration Scheme
[27] The atmosphere between the sea surface and 2000 m
is divided into 150 layers. The lowest 100 layers have a
constant grid height of 10 m, the layers above 1000 m are
spaced logarithmically. For the ‘‘high boundary layer’’ run
the grid height in the lowest 100 layers was increased to 20 m
and the model domain was extended to 3500 m. The
VON GLASOW ET AL.: HALOGEN CHEMISTRY IN THE MBL, 1
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9-5
Table 1. Overview of the Sensitivity Runsa
Gas Phase
Initialization
(See Table 2)
Name
MBL
Height
SST
Differences From Base Run
base run
remote
700 m
15 C
–
halogen off
remote
700 m
15 C
halogen chemistry switched off
aerosol off
remote
700 m
15 C
low O3 low SO2
remote, but O3 = 12 nmol/mol,
SO2 = 20 pmol/mol
700 m
15 C
halogen and aerosol chemistry
switched off
gas phase initialization
low O3 low SO2
large sulfate surface
continental influence
winter
same as ‘‘low O3 low SO2’’
700 m
15 C
continentally influenced
remote
700 m
750 m
15 C
5 C
spring
remote
700 m
10 C
winter Cape Grim
remote, see text
550 m
9 C
high BL
remote
1300 m
15 C
strong wind
remote
700 m
15 C
carbonate
remote
700 m
15 C
iodine (iod1/iod2)
remote
700 m
15 C
a
sulfate aerosol mode radius
increased by factor 1.5
gas phase initialization
solar declination 20 instead
of 20
solar declination 0 instead
of 20
conditions for ‘‘typical’’ winter
at Cape Grim
inversion at 1300 m instead of
750 m, vertical resolution 20 m
u10 9 m/s (instead of 6 m/s),
sea salt aerosol flux param.
by Smith et al. [1993]
carbonate buffer in sea salt
increased by 50% [Sievering
et al., 1999]
iodine chemistry included
Main Results
BrO vertical profile/diurnal
variation, vertical profile of sea
salt pH, recycling of HBr and HOBr
by sulfate, ‘‘overall’’ effects
of halogen chemistry
overall importance of halogen
chemistry shown
overall importance of aerosol
chemistry shown
low O3 reduces Br activation, initial
mixing ratios of SO2 unimportant
for steady state conditions
recycling of HBr, HOBr by sulfate
aerosol is important
quicker Br activation
higher BrO mixing ratios than in
summer
similar to winter
importance of acidity for release of Br
importance of recycling by sulfate
aerosol, less strong Brx activation
strong increase in S(IV) oxidation
in sea salt particles
changes in importance of S(IV)
oxidation paths
quicker Br activation and greater
O3 loss
SST is the sea surface temperature.
dynamical time step is 10 s and the chemical time step is
chosen to differ between 1 ms for layers with low LWC
associated to the particles or layers including freshly activated particles and 60 s for gas phase only layers. All
chemical reactions in gas and aqueous phases, equilibria
and phase transfer reactions are calculated as one coupled
system using the kinetic preprocessor KPP [Damian-Iordache, 1996] (http://www.cs.mtu.edu/~asandu) which allows
rapid change of the chemical mechanism without major
changes in the source code.
[28] The very stiff chemical differential equation system
is solved with a stable, mass-conserving second order
Rosenbrock method (ROS2 by Verwer et al. [1997]) which
was developed for atmospheric questions. We compared this
solver off-line with the commercial integration system
Facsimile Curtis and Sweetenham [1987] that uses a Gear
solver. Using the complete set of reactions we yielded very
good agreement between the different solvers.
3. Sensitivity Studies
3.1. Overview of the Runs
[29] Here we study only the cloud-free MBL whereas the
cloudy MBL is discussed in a companion paper [von Glasow
et al., 2002]. With the base run it was not intended to mimic
a specific situation that has been observed in a field campaign, it is rather intended to present detailed process studies
of the multiphase chemistry of the MBL under idealized
conditions. The latitude chosen for this run is j = 30N at the
end of July. The boundary layer height is roughly 700 m,
moisture and heat fluxes from the sea surface are adjusted to
yield a stable boundary layer. The relative humidity at the
sea surface is roughly 65%, increasing with height to around
90% below the inversion that caps the MBL. As a result of
the prescribed heat fluxes from the sea surface, the potential
temperature in the well mixed MBL is slowly decreasing
during the runs from = 14.5 C to 13.5 C. The relative
humidity in the different layers stays constant apart from a
minor diurnal variation (±3%). After a spin-up of the
dynamical part of the model for 2 days the complete model
was integrated for 3 days (modelstart is at midnight).
[30] Many different sensitivity studies were performed
and some of them are described here. See Table 1 for an
overview of the runs.
[31] Tables 2 and 3 list the initial gas phase mixing
ratios and parameters for the initial lognormal aerosol size
distribution. Aerosol particles with dry radii less than r =
0.5 mm are assumed to be a mixture of 32% (NH4)2SO4,
64% NH4HSO4 and 4% NH4NO3 [Kim et al., 1995]. The
resulting pH of the sulfate aerosols is between 0.5 and 1
(and even lower for the ‘‘continentally influenced’’ run)
which is in the range estimated by Fridlind and Jacobson
[2000] for sulfate aerosol sampled during the ACE-1
campaign. A run where the initial composition of the
sulfate aerosol was assumed to be pure (NH4)2SO4 showed
a pH between 2.5 shortly after model start and 1 after 3
days due to the uptake of acids from the gas phase.
Consequences for the gas phase were very small, NH3
increased because of reduced uptake by the sulfate aerosol
whereas HCl decreased due to uptake by the sulfate
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VON GLASOW ET AL.: HALOGEN CHEMISTRY IN THE MBL, 1
Table 2. Initial Mixing Ratios of Gas Phase Speciesa
Species
Remote
(MBL)
CO
NO2
HNO3
NH3
SO2
O3
CH4
C2H6
HCHO
H2O2
PAN
HCl
DMS
CH3I
C3H7I
70.0
0.02
0.01
0.08
0.09
20.0
1800.0
0.5
0.3
0.6
0.01
0.04
0.06
0.002
0.001
Remote
(FT)
Cont. Infl.
(MBL)
Cont. Infl.
(FT)
150.0
0.5
0.1
0.2
1.0
50.0
1800.0
5.0
0.3
0.8
0.1
0.04
0.06
0.03
0.05
50.0
0.1
70.0
1.0
a
Units are in nmol/mol. Values are for the two scenarios ‘‘remote’’ and
‘‘continentally influenced.’’ A value for the free troposphere (FT) is given
only when it is different from the MBL value. CH3I and C3H7I are
accounted for only in the ‘‘iodine’’ run.
aerosol. In most acid dependent reactions changes in the
sulfate pH in this very acidic range have no effect. Based
on this we conclude, that for the most relevant situations,
the results do not depend significantly on the initial
chemical composition of the sulfate aerosol.
[32] Particles larger than r = 0.5 mm are assumed to be sea
salt particles. The composition of sea salt aerosol in the
model is simplified by assuming all (unreactive) cations to
be Na+. The anions considered are Cl, Br, HCO3, and for
the iodine runs also I and IO3. Sulfate is not considered as
initial component of sea salt aerosol as it is also assumed to
be chemically unreactive. Therefore all sulfate that is
present in sea salt aerosol in the model runs stems from
uptake from the gas phase and can be labeled ‘‘non-sea salt
sulfate.’’ Upon model initialization ‘‘fresh’’ sea salt with a
pH of about 8—which is roughly the pH of ocean water
[Riley and Skirrow, 1965]—is present everywhere in the
MBL. Uptake of acidic gases like HNO3 from the gas phase
rapidly acidifies the sea salt aerosol particles. Fresh sea salt
aerosol particles emitted from the sea surface also have a pH
around 8.
3.2. Halogen Activation
[33] The model runs start at midnight. Figure 2 shows the
evolution with time of major gas phase species for the base
run, the ‘‘continental influence’’ run and the ‘‘low O3 low
SO2’’ run. Shortly after model start the sea salt aerosol is
acidified due to rapid uptake of acids from the gas phase
with pH values ranging between 6 near the sea surface and
less than 3.5 at the top of the MBL (see section 3.9 for a
discussion of the vertical profile of the sea salt aerosol pH).
Mode i
Ntot, i, 1/cm
RN, i, mm
si
1
2
3
100
120
6
0.027
0.105
0.12
1.778
1.294
2.818
a
The data are after Hoppel and Frick [1990]. The particle
size distribution
2
P3
lgrlgRN ;i Þ
ð
Ntot;i
dN ðrÞ
exp
.
is calculated according to dlgr ¼ i¼1 lgs pffiffiffiffi
2p
2ðlgs Þ2
i
[34] The halogen chemistry is started by reactions that
transform Br into species that can degas from the aerosol
and initiate quick reaction cycles in the gas phase. These
starter reactions are reactions of Br with OH, HSO5 and
NO3.
[35] The reaction cycles responsible in the model for the
degassing of the reactive bromine species (Br2 and BrCl)
involve the following steps as discussed by Sander and
Crutzen [1996] and Vogt et al. [1996].
HOBr
! HOBraq
HOBraq þ Cl þ Hþ
Table 3. Initial Size Distribution of the Aerosola
3
Figure 2. Evolution with time of the main gas phase
species for the base run (solid line), the ‘‘continental
influence’’ run (dotted line) and the ‘‘low O3 low SO2’’ run
(dashed line) (in 50 m). Brx is the sum of all gas phase
bromine species except HBr. The time is given in hours
from model start.
i
! BrClaq þ H2 O
ðI:1Þ
ðI:2Þ
Depending on the concentrations of the species involved in
the aqueous phase equilibria, BrClaq either degasses (when
significant Br depletion has occurred) and is then
photolysed:
BrCl þ hn ! Br þ Cl
l < 560nm
ðI:3Þ
VON GLASOW ET AL.: HALOGEN CHEMISTRY IN THE MBL, 1
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9-7
or takes part in the following autocatalytic cycles (when
sufficient Br is available) leading to the production of Br2:
BrClaq þ Br
Br2 Cl
Br2aq
Br2 þ hn
!
Br2 Cl
ðI:4Þ
! Br2aq þ Cl
ðI:5Þ
! Br2
ðI:6Þ
! 2Br
2ðBr þ O3 Þ !
l < 600nm
2ðBrO þ O2 Þ
2ðBrO þ HO2 Þ ! 2ðHOBr þ O2 Þ
ðI:7Þ
ðI:8Þ
ðI:9Þ
Or, instead of equilibria (I.2), (I.4) and (I.5):
HOBraq þ Br þ Hþ
!
Br2aq þ H2 O
ðI:10Þ
leading in both cases to the following net reaction:
HOBr; Cl
2HO2 þ Hþ þ 2O3 þ Br þ hn !
HOBr þ 4O2 þ H2 O
ðI:11Þ
This autocatalytic bromine activation cycle consumes H+
ions which implies the need for acidification of the sea salt
aerosol as shown by Fickert et al. [1999] in laboratory
experiments.
[36] Keene et al. [1998] studied the influence of the pH
on halogen activation. They showed that significant sea salt
dehalogenation is limited to acidified aerosol but that differences between a pH of 5.5 and 3 are not significant.
[37] Sander et al. [1999] showed that when sufficient
NOx is available, reactions on the surface of aerosols
involving BrNO3 can also lead to the production of Br2
and BrCl without the need for acid catalysis.
[38] These reaction cycles lead to gradual build-up of total
reactive bromine Brx (the sum of all gas phase bromine
species except HBr) in the gas phase with maximum values
between 8 and 10 pmol/mol in the model runs. During
daylight the main Br species are HOBr, HBr and BrO,
during night this is shifted towards Br2 and BrCl which
are photolyzed readily during daylight (see Figure 2). Model
calculated bromide deficits are between 70 and 85% in 50 m
height, rising to more than 90% in higher model layers due to
the lower sea salt aerosol pH in that layers (see section 3.9).
[39] The value of the gas phase mixing ratio of O3 is
important for these cycles because the production of BrO
and therefore also the O3 destruction is catalyzed by O3
(reaction (I.8)). If O3 mixing ratios are small, halogen
activation is slowed down (see run ‘‘low O3 low SO2’’).
[40] The order of magnitude of total inorganic gas phase
bromine (Brt = Brx + HBr) in the lowermost model layers is
in the same range as measurements by Rancher and Kritz
[1980] that were made aboard a ship in 8 m height during a
cruise off the equatorial African coast.
[41] In an additional run we switched off all bromine
related reactions. The development of the gas phase chemistry is very similar to the ‘‘halogen-off’’ run where both
chlorine and bromine chemistry reactions are switched off.
Furthermore, the mixing ratio of Clx is very small. This
Figure 3. Net exchange of Brx and HBr between sea salt
and sulfate aerosol and gas phase (in 215 m). A negative
sign implies loss for the aerosol. The flux of Br atoms is
plotted, i.e. the flux for the molecule Br2 is multiplied with
2 to get the number of Br atoms. The solid line is the net
flux of Br atoms. The time is given in hours from model
start. The nighttime hours are shaded.
shows that in the model significant chlorine activation
depends on interactions with bromine.
3.3. Active Recycling of Brx by the Sulfate Aerosol
[42] The presence of sulfate aerosol particles is important
in the model for the halogen activation cycle. The large
surface area of the sulfate aerosol of 40 to 60 mm2/cm3,
compared to the sea salt surface area of 25 to 45 mm2/cm3, is
the reason that HBr, HOBr and BrNO3 are also scavenged
to a significant amount by the sulfate aerosol and are
involved in reactions on and in sulfate particles. Due to
the low pH of the sulfate aerosol the acid-catalyzed cycle (I)
quickly transforms Br and Cl (from the scavenged HBr
and HCl) and HOBr to BrCl and Br2 which rapidly degas.
As can be seen from Figure 3 the magnitude of recycling of
bromine species is higher than in the sea salt aerosol by
roughly a factor of 5 to 10. Due to this recycling process the
sulfate aerosol is not a significant sink for gas phase
bromine in the model; some bromine, however, will always
be found in the sulfate aerosol.
[43] Particle composition measurements [e.g., Moyers
and Duce, 1972; Duce et al., 1983] often showed an
increase of the bromine to sodium ratio in small particles
compared to sea water. The interpretation of these data is
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VON GLASOW ET AL.: HALOGEN CHEMISTRY IN THE MBL, 1
difficult because for some samples the chemical state of
bromine in the particles is unclear. Furthermore, sodium in
the aerosol samples could be a consequence of the sampling
of an external mixture of sulfate and sea salt particles or of
internally mixed particles that originate from collisioncoalescence or cloud processes (or both). Therefore the
use of the Br/Na+ ratio could be misleading. The enriched
bromine could be in the form of stable bromine-containing
ions or organic molecules. Known reactions [Haag and
Hoigné, 1983] for the production of stable ions like BrO3
in the aqueous phase are, however, too slow to lead to
significant production in the aerosol.
3.4. Nighttime Fluxes of Bromine
[44] During daylight, photolysis is the driving force for
the net transport of bromine from the aqueous to the gas
phase as formation of HOBr is dependent on photolytically
produced HO2. As the photolysis slows down during sunset,
Br2 and BrCl accumulate in the gas phase and the remaining
HOBr is rapidly recycled by the aerosol to Br2 and BrCl.
During night Br2 also degasses as evident from Figure 3.
This is caused by a sudden rise in BrCl mixing ratios after
sunset due to absent photolytical loss. Thus the gas-aqueous
partitioning of these species is perturbed leading to the
reaction sequence:
Figure 4. Diurnal variation of the photolysis frequencies
for O3 and Br2 (note the different units). The difference in
the shape of the diurnal variation is the cause for the noon
minimum in BrO mixing ratios (see text).
Neither cycle (II) nor cycle (III) are a net sink for total gas
phase bromine, they just shift gas phase bromine from Br2
to BrCl which is what is observed both in the model for the
strongly depleted (e.g., ‘‘continental influence run’’) cases
and in the laboratory [Fickert et al., 1999].
[47] At sunrise (high solar zenith angle) the solar spectrum in the lower troposphere is shifted to longer wavelengths. This causes an earlier start of the reaction cycles
that include Br2 and BrCl photolysis compared to cycles
that rely on O3 photolysis (see Figure 4). It also causes a
delay in the photolytically initiated production of OH and
HO2 relative to the production of Br. Since HO2 is the main
sink for BrO (reaction (I.9)) apart from photolysis, this leads
to an increase in BrO mixing ratios (producing the morning
peak). Later during the day HO2 mixing ratios are sufficiently high to reduce BrO mixing ratios leading to the noon
minimum. In the late afternoon a similar mechanism as in
the morning leads to the evening peak of BrO.
[48] For situations with very high BrO mixing ratios like
the ones encountered at the Dead Sea [Hebestreit et al.,
1999] the minimum around noon would not occur because
then HO2 mixing ratios are too small throughout the day to
reduce BrO significantly.
[49] The diurnal variation of BrO is accompanied by an
early morning peak in O3 destruction by XO + YO and XO +
NO2 reactions (X, Y = Br, Cl). Early morning O3 destruction
in our base run is 27% of the maximum O3 destruction at
noon, compared to 4% in the ‘‘halogen off’’ run.
[50] This could explain recently published measurements
of Nagao et al. [1999], who found a ‘‘sunrise ozone
destruction’’ (SOD) in the diurnal variation of O3 based
on measurements over a 3-year period on an island in the
sub-tropical Northwestern Pacific. SOD acts in addition to
the conventional O3 destruction that has its maximum
around noon. They already speculated that bromine spieces
might play a role in SOD. A similar feature was found by
Galbally et al. [2000] in a time series of 13 years of O3
observations at Cape Grim, Tasmania.
3.5. Diurnal Variation of BrO
[46] The model predicts a distinct diurnal variation of
BrO and BrNO3, showing a minimum around noon and
maxima in the morning and evening (see Figure 2). This
feature can be explained by differences in the photolysis
spectra of O3 and Br2 and BrCl. Br2 and BrCl are more
rapidly photolysed in twilight than O3 due to absorption at
longer wavelengths.
3.6. Effect on O3
[51] The net effect of photochemistry in the MBL is to
destroy O3. Dynamics, especially the downmixing of O3rich air from the free troposphere is thought to balance this
destruction [e.g., Monks et al., 2000]. Here we show in
agreement with Vogt et al. [1996] that halogen radicals can
significantly contribute to photochemical O3 loss in the
MBL.
BrCl
BrClaq þ Br
Br2 Cl
Br2aq
!
BrClaq
ðII:1Þ
! Br2 Cl
ðII:2Þ
! Cl þ Br2aq
ðII:3Þ
!
ðII:4Þ
Br2
where the net direction of the equilibria is indicated with the
arrow.
[45] This cycle results in net transfer of Br from the
aqueous to the gas phase:
BrCl þ Br
! Cl þ Br2 :
If the bromide depletion in the sea salt aerosol is close to
complete, sequence (II) is reversed (cycle (II), in
agreement with laboratory experiments of Fickert et al.
[1999]). The Br produced in this reversed reaction cycle
can then take part in a reaction cycle that involves Cl2 and is
similar to cycle (II), leading to a shift of halogen species:
Cl2 þ Br ! BrCl þ Cl
ðIIIÞ
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VON GLASOW ET AL.: HALOGEN CHEMISTRY IN THE MBL, 1
9-9
Table 4. Ox Destruction Ratesa
Net Ox
Destruction,
Net Ox
Destruction by
Halogens,
Run
nmol
mol day
nmol
mol day
base run
halogen off
aerosol off
low O3 low SO2
low O3 low SO2 sulf
cont. influence
winter
spring
high BL (in 130 m)
strong wind
carbonate
iod 1
iod 2
1.9
0.7
0.7
0.6
1.0
3.7
0.8
1.1
2.9
2.0
1.9
2.2
2.7
0.6
0.3
0.5
1.2
0.6
0.5
0.4
0.8
0.6
0.9
1.5
O(1D)+
H2O,
%
O3 +
HO2,
%
O3+
OH,
%
ClO+HO2/
NO2/CH3OO,
%
BrO+
HO2,
%
BrO+
ClO,
%
BrO+
BrO,
%
52.0
70.1
70.0
56.4
49.1
47.4
18.0
39.2
67.0
49.3
52.1
44.4
34.8
12.6
20.8
21.0
14.0
11.1
15.9
10.5
14.5
13.4
12.4
12.7
9.9
7.2
5.3
7.9
7.9
4.7
3.8
8.7
2.9
5.0
7.2
5.5
5.4
4.5
3.6
3.7
12.4
2.7
1.3
6.6
2.5
1.7
3.3
5.4
7.3
4.8
1.7
1.9
3.7
5.0
4.1
11.0
15.0
8.3
20.0
16.0
5.4
14.7
12.3
12.7
9.2
0.9
3.1
2.6
18.0
5.0
0.4
1.8
2.6
2.1
1.6
0.8
2.0
0.7
7.1
2.3
0.3
2.6
1.3
1.7
1.3
4.0
7.0
4.1
10.0
7.1
3.4
7.8
6.5
7.8
6.7
3.2
4.0
0.7
10.2
4.9
0.5
2.6
2.5
2.4
1.9
BrO+ DMS+
CH3OO, BrO,
%
%
a
Net chemical Ox destruction and net chemical Ox destruction involving halogens (first two columns) are given in nmol/(mol day), the contribution of the
different reactions to the total Ox destruction is expressed in %. For each reaction the appropriate Ox has been used as a factor to evaluate the contribution.
The most important Ox destruction paths involving iodine species are IO + HO2: 4.6 (12.4)% and IO + BrO: 3.1 (11.8)% in the ‘‘iod 1’’ (‘‘iod 2’’) run. The
data are given as averages of the second and third day in 65 m height. Only in the ‘‘high BL’’ run for technical reasons a height of 130 m was chosen.
[52] To properly asses the effect of chemical reactions on
O3, the odd oxygen family Ox has been defined (based on
Crutzen and Schmailzl [1983]): Ox = O3 + O + O(1D) + NO2
+ 2 NO3 + 3 N2O5 + HNO4 + ClO + 2 Cl2O2 + 2 OClO + BrO
+ IO + 2 I2O2. Species in this family are readily transfered to
O3 (e.g., NO2 which photolyses forming O which recombines with O2 to O3). Only if a reaction leads to a change in
the mixing ratio of the odd oxygen family (Ox 6¼ 0) an
effective O3 destruction or production takes place.
[53] Chemical Ox production in the MBL is primarily by
reaction of NO with HO2 and CH3OO, both of which form
NO2. In the base run it sums up to 1.9 nmol/(mol day).
[54] Table 4 shows the contribution of the most important
reactions to O3 destruction. The most important Ox loss
reaction is O(1D) + H2O which accounts for 52% of total O3
destruction in the base run followed by O3 + HO2 with 13%.
The main step in bromine-related photochemical O3 destruction is the production of BrO in reaction (I.8) followed by
reactions of BrO with HO2, CH3OO, XO (X = Br, Cl)
radicals, and NO2. In the base run the halogen-related O3
destruction reactions account for roughly 30% of total O3
destruction.
[55] The most important bromine reaction (BrO + HO2)
causes 12% loss. Chlorine related O3 destruction is less
important than the bromine reactions, it accounts for
roughly 4%.
[56] In the ‘‘continental influence’’ run the relative contribution of ClO and BrO changed strongly which is due to
increased release of chlorine (see section 3.8). Furthermore,
there is a strong increase in Ox destruction by HO2 and OH.
[57] The runs ‘‘halogen off’’ and ‘‘aerosol off’’ show the
same net chemical Ox destruction (Table 4). Comparison
with the base run shows a difference in net chemical Ox
destruction of 1.2 nmol/(mol day) or 170%. This is twice as
much as directly attributable to halogen related Ox destruction. The difference is due to indirect effects of halogen
chemistry, namely, the reduction of NOx (see section 3.8)
and thereby the reduction of chemical Ox production.
[58] Very interesting is the high relative contribution of
halogen chemistry to O3 destruction in the winter run. The
reason for this will be discussed in the next section.
3.7. Winter Run
[59] For the winter run, temperature and solar zenith angle
are chosen to model the conditions at the end of January
instead of July as in the base run (see Figures 5 and 6). This
implies that the solar spectrum is shifted to greater wavelengths reducing the photolysis rates for O3 ! O(1D) by
Figure 5. Evolution with time of the main gas phase
species for the base run (solid line) and for the winter run
(dotted line) (in 50 m). The time is given in hours from
model start.
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VON GLASOW ET AL.: HALOGEN CHEMISTRY IN THE MBL, 1
Figure 6. Vertical profile of the main gas phase species for the base run (solid line) and for the winter
run (dotted line) at noon of third day. Note that the height of the MBL is slightly different in the two runs.
66% leading to a decreased formation of OH and HO2
compared to summer conditions. The reduction in the
photolysis rates of Br2 and BrCl are only 7.5 and 15%,
respectively, because they absorb at greater wavelengths.
[60] In the ‘‘winter’’ run net O3 destruction is only 32%
of the destruction in the base (= summer) run (see Table 4)
because of the smaller solar radiation intensity reducing
OH to a third and HO2 to 50% of the summer values. Due
to the reduction in HO2, which is the main sink for BrO
and ClO, the mixing ratios of these radicals increase
compared to the summer run. The absolute rate of halogen-related O3 loss is approximately the same as in the
base run enhancing the relative halogen contribution to
67%. This is mainly due to an increase in the BrO and ClO
mixing ratios, leading to an increased importance of the
XO + YO (X, Y = Br, Cl) reactions, which have a Ox =
2 (see Table 4).
[61] The morning peak in the BrO mixing ratios calculated by the model has increased by approximately 20%,
whereas the noon values increased by more than 90%.
Activation of bromine from the aqueous phase is faster
and total reactive bromine (Brx) increased by 12 to 25%
compared to the summer run at the expense of HBr which
decreased by 1 pmol/mol (roughly 50%). Also Clx (the sum
of all gas phase chlorine species except HCl) increased by
up to 30% (see Figure 5).
[62] Due to lower temperatures compared to the summer
case, the solubility of the gases increases. A reduction in the
temperature from 15 C to 0 C increases the solubility of
SO2 by a factor of 1.8, HNO3 by a factor of 3.8, and HCl by
a factor of 4. Enhanced uptake of acidic gases leads to a
lower sea salt aerosol pH. Caused by the lower pH, bromine
activation is more rapid and bromine depletion even stronger than in the base run.
[63] Comparison with the measurements of Ayers et al.
[1999] shows a difference in the seasonality as they found a
minimum in the Br deficit in winter. During winter the
availability of acids (mainly due to reduced microbial
activity which causes a smaller DMS flux) is reduced at
their measurement site whereas sea salt loadings are
increased due to higher wind speeds in winter. In the model
run the wind speed (which controls the flux of sea salt
aerosol), gas phase acid mixing ratios or the DMS flux were
not changed in order to concentrate on differences in
radiation and temperature only. Therefore these differences
between measurements and model results were to be
expected.
[64] To test the model under the conditions described by
Ayers et al. [1999] another run for winter conditions was
performed (‘‘winter Cape Grim’’), where the geostrophic
wind speed was increased from 7 to 12 m/s yielding a
higher sea salt flux. Furthermore the fluxes of DMS and
NH3 were decreased by a factor of 4 to get typical mixing
ratios of 20 pmol/mol for DMS and less than 5 pmol/mol for
SO2 [Ayers et al., 1997a]. Other chemical initial values and
the meteorology were adjusted to the conditions at Cape
Grim [Ayers et al., 1997b]. Very little bromine activation
was found. The depletion was even less than in the measurements of Ayers et al. [1999]. This possibly points to a source
of acidity that was not considered in the model. Alternatively this can be caused by the fact that in the model only a
mean aerosol pH is calculated, whereas in reality the pH
strongly depends on the particle age, i.e., in the atmosphere
particles of the same size but with different pH coexist. Due
VON GLASOW ET AL.: HALOGEN CHEMISTRY IN THE MBL, 1
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9 - 11
Figure 7. Vertical profile of the sea salt aerosol pH and liquid water content (LWC) for the base run at
noon of third day.
to the reduction of the DMS flux and the resulting decrease
of SO2 mixing ratios, the major source for aqueous phase
acidity was strongly reduced, whereas the aqueous phase
buffer capacity was increased by the increase in sea salt
particle mass (roughly a factor of 3). This lead to sea salt
aerosol pH values greater than 8 (the value for sea water).
When initial gas phase concentrations and fluxes were
chosen for summer conditions at Cape Grim [Ayers et al.,
1997a, 1997b, 1999], strong bromine depletion occurred as
in the other runs that were presented here.
[65] The relative importance of these two opposing
effects, i.e., increase in halogen chemistry relevance caused
by a shift of the radiation spectrum to higher wavelengths
vs. decrease in halogen chemistry relevance due to greater
sea salt fluxes and lower gas phase acidity, will be different
for every site and has to be assessed individually. The
features for the ‘‘spring’’ run (not shown) are the same as
for the ‘‘winter run’’ but the reduction in total net O3
destruction to 58% of the value in the base run is not as
strong as in the ‘‘winter’’ run.
3.8. Influence of NOx
[66] Nitrogen oxides play a very important role in atmospheric chemistry by acting as catalyst in the photochemical
production of O3. When NO2 reacts with OH, nitric acid
(HNO3) is formed which is rapidly taken up by aerosol
(mainly sea salt aerosol), providing acidity to the aerosol. If
halogens are present, NOx (= NO + NO2) mixing ratios are
further reduced by reactions of NO2 with ClO and BrO
yielding XNO3 (X = Cl, Br). XNO3 can either be photolysed,
thermally decomposed or taken up by the aerosol leading to
a loss of reactive nitrogen from the gas phase and to an
increase of NO3 in the particles and further activation of
bromine [Sander et al., 1999]. The scavening of XNO3 by
particles is a significant NOx loss process, so a NOx source
was introduced in the model to avoid unrealistically small
mixing ratios of less than 0.1 pmol/mol. The source strength
is 10 pmol/(mol day) in the MBL which is equivalent to 2 108 molec/(cm2 s) as surface flux. Monks et al. [1998] report
typical mixing ratios of 2 to 6 pmol/mol for NOx for clean
maritime background conditions (Cape Grim) which are
achieved in the model with the NOx source.
[67] This additional NOx could be a photolysis product
from alkylnitrates that are emitted from the ocean (see e.g.,
Blake et al. [1999, and references therein] for latitudinal and
vertical profiles during the ACE-1 campaign). Another
possibility could be the direct emission of NO from the
oceans (formed by photolysis of NO
2 ) as suggested by
Zafiriou et al. [1980] on the basis of NO measurements in
surface waters of the central equatorial Pacific.
[68] Crutzen [1979] pointed to the importance of downward transport of PAN from the free troposphere (where it is
long-lived) and the subsequent thermolysis in the MBL.
This process is included in the model but exchange between
the MBL and the free troposphere is weak as long as no
clouds are present that strongly increase this exchange.
[69] On the other hand the need to include a NOx source
for runs including aerosol/halogen chemistry might point to
the fact, that processes exist that convert aqueous HNO3/
NO
3 back to NOx, that are not included in the model. The
importance of the photolysis of NO
3 in aerosol particles
was also studied [Warneck and Wurzinger, 1988; Zellner et
al., 1990]:
H2 O
NO
3 þ hn ! NO2 þ OH þ OH :
ð3:1Þ
Although this reaction was found to be negligible as a
source of NO2, OHaq concentrations increased significantly
speeding up the activation of bromine from the sea salt
aerosol because OH is involved in one of the starter
reactions (see also section 3.2). The resulting differences in
the gas phase were only apparent on the first model day.
[70] The influence of NOx on halogen chemistry can be
shown when comparing the base with the ‘‘continentally
influenced’’ run. In the continentally influenced run the
initially high mixing ratios of NO2 speed up the bromine
activation and lower the sea salt aerosol pH through the
uptake of HNO3. This causes a very quick depletion of
bromide from the sea salt aerosol favoring the production of
BrCl in the sea salt aerosol instead of Br2 which leads to a
change in the speciation of gas phase Brx (Br2 mixing ratios
are reduced and BrCl enhanced, see discussion above in
section 3.4). Furthermore chlorine (both reactive chlorine,
Clx and HCl) release is strongly enhanced. This results in
increases of about 30– 70% in ClO and the shift in Ox
destruction rates as discussed above in section 3.6.
[71] If NOx mixing ratios are small (base run) the morning peak of BrO is quite large (up to 4.5 pmol/mol in 50 m
height) because destruction of BrO by reaction with HO2 or
NO2 is not efficient. In polluted air, however, there is
enough NOx to make the reaction of BrO with NO2
important. Then a morning and afternoon peak of BrNO3
instead of BrO is predicted by the model. In the ‘‘continentally influenced’’ run this occurs only on the first day,
because later on NOx mixing ratios are too small.
3.9. Variation of Sea Salt Aerosol pH and
BrO With Height
[72] The calculated pH in the sea salt aerosol bin is highest
at the sea surface where alkaline particles are emitted and it
decreases towards the top of the MBL. The vertical decrease
in aerosol pH is associated with increasing relative humidity
and aerosol water content (see Figure 7). This feature is
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VON GLASOW ET AL.: HALOGEN CHEMISTRY IN THE MBL, 1
caused by the high salt loading of the aerosol and exchange
of HCl between the aerosol and gas phases due to changes in
LWC and the aqueous fraction of HCl. A detailed explanation is given by von Glasow and Sander [2001].
[73] An implication of the vertical gradient of the sea salt
particle pH is that acid-catalyzed bromine activation is more
efficient in higher layers of the MBL. The vertical profiles
of both Brx and BrO show a maximum directly below the
temperature inversion that caps the MBL (see Figure 6). In
layers close to the sea surface with lower sea salt aerosol
water content and high sea salt pH this leads to smaller BrO
mixing ratios. The higher mixing ratios of, e.g., BrO at the
top of the MBL might explain difficulties to detect BrO in
surface measurements in the MBL. Based on this finding it
would be desirable to modify measurement instruments for
the detection of halogen radicals such that information on
the vertical distribution in the MBL can be obtained. As a
consequence of the maximum in BrO mixing ratios at the
top of the MBL, O3 destruction rates by bromine radicals
increase with height in the MBL. If significant detrainment
of MBL air into the free troposphere occurs this could lead
to enhanced BrO and Br mixing ratios in this region.
3.10. Iodine Chemistry
[74] The chemistry of iodine in the MBL has been the
subject of some studies in the last decades (see Vogt et al.
[1999] for an overview). In recent years it received even
more attention due to the detection of IO in the MBL [Alicke
et al., 1999; Allan et al., 2000]. Maximum mixing ratios
measured by Alicke et al. [1999] were 6 pmol/mol at Mace
Head, Ireland. Allan et al. [2000] measured IO at Tenerife
and Cape Grim, Tasmania with a mean mixing ratio of
about 1 pmol/mol. These data suggest a rather widespread
presence of IO although it could not be measured on all
days of the measurement campaigns. McFiggans et al.
[2000] used a box model that was constrained with data
from measurements and could show the importance of the
cycling of iodine species on aerosol particles for the Mace
Head and Tenerife measurements. They used a modified
version of the mechanism suggested by Vogt et al. [1999].
[75] In recent field campaigns OIO was also measured in
the MBL at Mace Head [Hebestreit et al., 2000] and
Tasmania [Allan et al., 2001]. OIO was first detected in
the laboratory by Himmelmann et al. [1996]. It is known to
be formed in the self reaction of IO and in the reaction of IO
with BrO [Bedjanian et al., 1998; Misra and Marshall,
1998]. Further reactions of OIO are highly uncertain.
Ingham et al. [2000] studied the photolysis of OIO and
found that OIO is photochemically very stable as already
calculated by Misra and Marshall [1998]. Cox et al. [1999]
and Hoffmann et al. [2001] proposed that OIO could be an
intermediate in the formation of particulate iodate.
[76] Particulate iodine is found in marine aerosol samples 100 to 1000 times enriched compared to sea water but
it is not clear in which chemical state the iodine is present.
Some studies reported it to be IO
3 but in others no IO3
was found (see references and discussion in the work of
McFiggans et al. [2000]. Based on measurements of
aerosol composition, Baker et al. [2000] state that ‘‘iodine
is present in aerosol in varying proportions as soluble
inorganic iodine, soluble organic iodine and insoluble, or
unextractable, iodine.’’
[77] In contrast to gas phase chlorine and bromine that is
mainly released by sea salt particles, iodine compounds
originate from biogenic alkyl iodides released by various
types of macroalgae and phytoplankton in the sea. Photodissociation then produces I radicals that mainly react with
O3 to IO. We assumed the following emission rates for:
CH2I2, CH2ClI, CH3I, and iC3H7I of 3.0 107 cm2 s1,
6.0 107 cm2 s1, 0.6 107 cm2 s1, and 1.0 107
cm2 s1, respectively.
[78] OIO peaks were not measured simultaneously with
IO but rather late in the afternoon and after sunset [Hebestreit et al., 2000; Allan et al., 2001]. Four different stages in
the diurnal evolution can be distinguished: 1) During day
the formation of OIO must not be too much at the expense
of IO, because otherwise IO peaks would become too small.
There should be strong destruction paths to keep OIO at a
reduced level. 2) During dusk the production of OIO wins
over its destruction, pointing to the need of photolytic
processes for the destruction (e.g., reaction with OH, NO).
3) During night there is a slow sink, which might be
reaction with a ‘‘nighttime species’’ like NO3, uptake on
aerosol particles or deposition to the surface. 4) During
dawn no OIO was observed, pointing to differences to the
chemistry at dusk, when OIO increases.
[79] Kinetic data on the formation and reactions of OIO
are extremely scarce, however, so that we can only speculate on the chemistry that is occurring. Apart from the
aforementioned formation reactions for OIO, we assumed
reaction of OIO with NO to yield NO2 and IO. It was
assumed to be 5 times faster than the reaction of OBrO with
NO [Li and Tao, 1999], because the OBrO reaction is also 5
times faster than the OClO reaction [DeMore et al., 1997].
We further included the reaction of OIO with OH. In
contrast to Plane et al. [2001] we assumed two branches
of 50% yield each: HIO3 and HOI + O2. With this it is
avoided that all OIO that reacts with OH ends up as IO3 in
the particles, allowing some recycling of I via the particle
phase. We assumed a rate constant of 2 1010 cm3/
(molec s). The absolute value of the accommodation coefficient a(HIO3) is not important, as a smaller value only
increases the gas phase mixing ratio of the unreactive HIO3.
We chose a(HIO3) = 0.01. Knight and Crowley [2001]
could not detect HIO3 as a product from the reaction IO +
HO2. If, however, HIO3 would be a minor product of this
reaction, than it might compete with OH + OIO due to the
high HO2/OH ratio, which is in the order of 100 – 1000. To
achieve a decrease of OIO during night uptake by the
aqueous phase was included using a(OIO) = 0.01.
[80] It was recently found in the laboratory, that uptake of
HOI on sea salt solution yields not only ICl but also IBr
which degas [Holmes et al., 2001]. ICl is only released
when bromide has been depleted in the aqueous phase. This
points to interhalogen reactions/equilibria in the aqueous
phase, that we implemented in analogy to the respective
equilibria for bromine:
IClaq þ Br
! IClBr
ð3:2Þ
IBraq þ Cl
!
IClBr
ð3:3Þ
As no data are available on the above equilibria we assumed
the ratio to be the same as measured for the BrCl, Br2,
VON GLASOW ET AL.: HALOGEN CHEMISTRY IN THE MBL, 1
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9 - 13
Figure 8. Evolution with time of the main gas phase species for the base run (solid line), for the iod1
run (dotted line), and for the iod2 run (dashed line) (in 50 m). The time is given in hours from model start.
Br2Cl equilibria system [Wang et al., 1994] and varied it.
The gas phase ratio of ICl to IBr depends on the aqueous
phase ratio of Cl/Br and on the ratio of the equilibrium
constants, as already pointed out by Fickert et al. [1999] for
the BrCl, Br2, Br2Cl equilibria system. The remainder of
the iodine reaction mechanism is as described by Vogt et al.
[1999].
[81] We compare a model run where these assumptions
are included (run iod1) with a run where the OIO self
reaction (producing I2O2, k = 5 1011 cm3/(molec s)) was
added to yield higher IO mixing ratios (run iod2) and a base
run without iodine chemistry.
[82] The amount of particulate iodine formed in the
model, 12 (iod1) and 10 (iod2) pmol/mol in sulfate and
3.5 (iod1) and 3 (iod2) pmol/mol in sea salt particles after 3
days, is up to a factor of 10 higher than in measurements.
This might imply that HIO3 formation by the reaction OIO +
OH is still overestimated, because HIO3 is the main precursor for particulate iodine in the model.
[83] As already discussed by Vogt et al. [1999] the
activation of chlorine and bromine chemistry is accelerated
by iodine chemistry due to the reaction of HOI in the
aqueous phase with Cl and Br producing ICl and IBr
which escape to the gas phase, photolyze and produce
halogen radicals that continue the reaction cycles discussed
above. Figure 8 shows that the activation of chlorine and
bromine is faster than in the base run and that the mixing
ratios after 3 days are about 10 and 15% higher for Brx and
ACH
9 - 14
VON GLASOW ET AL.: HALOGEN CHEMISTRY IN THE MBL, 1
Clx. Total inorganic iodine consists mainly of HOI, IO and
OIO during day and of ICl and IBr during night. Maxima of
ICl and IBr mixing ratios are below the inversion at the top
of the MBL, whereas maxima of IO and OIO are found near
the sea surface.
[84] The maximum mixing ratios of IO in the model are
about 0.5 pmol/mol for the run without OIO self reaction
and 1.0 to 1.5 pmol/mol with the self reaction. This and the
above mentioned very high particulate iodine values suggest
that rapid cycling of OIO and possibly also of the reaction
products (HIO3?) are important. Inclusion of iodine chemistry increases chemical net O3 destruction by 15% to 40%
compared to the base run.
[85] In the model, the reaction of IO with BrO is the
major source for OIO. Contrary to field observations, OIO
is already produced at sunrise. The morning peak is even
greater in the model than the afternoon/night peak. It is due
to early morning photolysis in the visible of iodine containing molecules that produce IO. The observed differences
in chemical processes between dusk and dawn are not
captured by the reaction mechanism that we used. The slow
OIO destruction during night is captured by the model, but
in the run including the OIO self reaction significant
amounts of IO are produced during the night from the
thermolysis of I2O2. This is also not observed in the field. It
has to be concluded that the atmospheric chemistry of
iodine and especially that of OIO is not yet understood
and that our speculations based on preliminary data are not
capable of reproducing the features of field measurements.
The inclusion of reactions of OIO with IO, Br, or photolysis
did not change these conclusions. It is obvious that more
kinetic studies are urgently needed.
4. Conclusions
[86] Using a much more elaborate one-dimensional
model of the MBL, results from earlier box model studies
for cloud-free conditions could be confirmed. These are the
activation of bromine from sea salt aerosol by an acidcatalyzed mechanism that follows starter reactions in the sea
salt aerosol. These transform the rather unreactive Br to
reactive substances like HOBraq and Braq. The importance
of catalytic gas phase reaction cycles that involve bromine
for the destruction of O3 was also confirmed. Roughly 30%
of the total chemical O3 destruction in the model was due to
halogen reactions. Furthermore the importance of iodine
chemistry in the MBL for the activation of Br and Cl from
the sea salt aerosol and in the destruction of O3 was
confirmed and speculations on processes involving the
OIO radical which has recently been detected in the MBL
were made.
[87] Additionally it could be shown that under low NOx
situations that prevail in the remote MBL the diurnal
variation of BrO and ClO (= XO) show a maximum in
the early morning and late afternoon and a local minimum
around noon. This diurnal variation was also found in
species that are produced from XO (e.g. XNO3). The reason
for this behaviour is the different wavelength dependence of
the photolytic source of the precursors of BrO and its most
important reaction partner HO2. The early morning BrO
peaks could be the reason for ‘‘sunrise ozone destruction’’
that has been observed in field measurements.
[88] The model further showed a distinct vertical profile
of BrO with maxima below the inversion that caps the
MBL. This is linked to the vertical profile of sea salt aerosol
pH which has its minimum below the inversion where
relative humidity is highest.
[89] In the simulations the presence of sulfate aerosol
particles was important for the recycling of less reactive
bromine species like HBr and HOBr to more reactive
species like Br2 and BrCl. As this cycling is quick the
sulfate aerosol was not found to be an important sink for
bromine in the model.
[90] If the availability of acids is restricted, bromine
activation is reduced in the model. Bromine activation is
also slowed down in low O3 regions, because the reaction of
Br with O3 is an important step in the bromine activation
mechanism. On the other hand bromine activation can be
accelerated in more polluted regions.
[91] As a consequence of the shift in the tropospheric
radiation spectrum at high solar zenith angles the mixing
ratios of BrO and ClO were predicted to be higher in winter
than in summer and the relative importance of halogen
reactions in O3 destruction and DMS oxidation increased
significantly. This is true only for comparable availability of
gas phase acidity and similar sea salt aerosol fluxes. From
the model results large variations in halogen concentrations
are expected for different situations.
[92] The model results showed the existence of vertical
gradients of gas and aqueous phase chemical species. Many
measurements of aerosol composition, etc. are done at the
surface or from ships where different conditions prevail than
in greater heights in the MBL. One should therefore be
careful in extrapolating surface measurements to the complete MBL.
[93] Results from field campaigns with simultaneous
measurements of both gas and particulate phase chemical
composition are becoming more and more available. Comparisons of these results with model data should be made in
the future. The availability of faster computers will make
the introduction of more size bins for the description of
aqueous phase chemistry possible.
[94] Acknowledgments. We thank John Crowley, Bill Keene, Dieter
Perner, Kai Hebestreit, and Rainer Vogt for valuable discussions. The
module for the calculation of the activity coefficients is courtesy of Beiping
Luo, while the photolysis module was provided by Jochen Landgraf.
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A. Bott, Meteorologisches Institut, Universität Bonn, 53121 Bonn,
Germany. ([email protected])
P. J. Crutzen and R. Sander, Atmospheric Chemistry Division, MaxPlanck-Institut für Chemie, P.O. Box 3060, 55020 Mainz, Germany.
([email protected]; [email protected])
R. von Glasow, Scripps Institution of Oceanography, University of
California, San Diego, 9500 Gilman Drive #0221, La Jolla, CA 920930221, USA. ([email protected])