Journal of Atmospheric Chemistry 32: 375–395, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
375
Iodine Chemistry and its Role in Halogen
Activation and Ozone Loss in the
Marine Boundary Layer: A Model Study
RAINER VOGT1, ROLF SANDER2, ROLAND VON GLASOW2 and
PAUL J. CRUTZEN2
1 Ford Forschungszentrum Aachen GmbH, Süsterfeldstr. 200, 52072 Aachen, Germany
2 Max-Planck Institut für Chemie, Postfach 3060, 55020 Mainz, Germany
(Received: 8 July 1998; in final form: 28 October 1998)
Abstract. A detailed set of reactions treating the gas and aqueous phase chemistry of the most
important iodine species in the marine boundary layer (MBL) has been added to a box model
which describes Br and Cl chemistry in the MBL. While Br and Cl originate from seasalt, the I
compounds are largely derived photochemically from several biogenic alkyl iodides, in particular
CH2 I2 , CH2 ClI, C2 H5 I, C3 H7 I, or CH3 I which are released from the sea. Their photodissociation
produces some inorganic iodine gases which can rapidly react in the gas and aqueous phase with
other halogen compounds. Scavenging of the iodine species HI, HOI, INO2 , and IONO2 by aerosol
particles is not a permanent sink as assumed in previous modeling studies. Aqueous-phase chemical
reactions can produce the compounds IBr, ICl, and I2 , which will be released back into the gas phase
due to their low solubility. Our study, although highly theoretical, suggests that almost all particulate
iodine is in the chemical form of IO−
3 . Other aqueous-phase species are only temporary reservoirs
and can be re-activated to yield gas phase iodine. Assuming release rates of the organic iodine
compounds which yield atmospheric concentrations similar to some measurements, we calculate
significant concentrations of reactive halogen gases. The addition of iodine chemistry to our reaction
scheme has the effect of accelerating photochemical Br and Cl release from the seasalt. This causes
an enhancement in ozone destruction rates in the MBL over that arising from the well established
reactions O(1 D)+H2 O −→ 2OH, HO2 +O3 −→ OH+2O2 , and OH+O3 −→ HO2 +O2 . The given
reaction scheme accounts for the formation of particulate iodine which is preferably accumulated in
the smaller sulfate aerosol particles.
Key words: aerosol, iodine chemistry, halogen chemistry, marine boundary layer, modeling, ozone
loss, sea salt.
1. Introduction
Methyl iodide, the most abundant organic I compound in the marine boundary
layer (MBL), which is present at the pmol mol−1 (= 10−12 mol mol−1 = pptv)
level (Cicerone, 1981) is released from the ocean surface into the atmosphere at
an estimated rate of 1–2 Tg year−1 . Besides methyl iodide, a number of other
organic iodine compounds have been detected in the ocean water, and also in the
376
RAINER VOGT ET AL.
MBL (Rasmussen et al., 1982; Singh et al., 1983; Class and Ballschmiter, 1988;
Carlier et al., 1991; Reifenhäuser and Heumann, 1992; Schall and Heumann, 1993;
Carpenter et al., 1998). Iodocarbons, such as CH2 I2 , CH2 ClI, C2 H5 I, or C3 H7 I are
formed by various types of macroalgae and phytoplankton, most likely because of
their antimicrobial activity (Fenical, 1981). Due to their low solubility, the ocean
water can become supersaturated in these compounds, causing a flux from the
ocean to the atmosphere (Singh et al., 1983). Photochemical transformation of
organic iodine can be the source of the observed enrichment of the aerosol and gas
phase inorganic I compared to sea salt (e.g., Moyers and Duce, 1972; Cicerone,
1981).
The chemistry and photochemistry of iodine has been investigated in several
modeling studies (Chameides et al., 1980; Jenkin et al., 1985; Chatfield and Crutzen,
1990; Jenkin, 1992; Solomon et al., 1994a; Davis et al., 1996). Davis et al. (1996)
used a one-dimensional tropospheric chemistry model which considered photolysis
of CH3 I as the sole iodine source. The model treated cycling of iodine in the gas
phase and removal of inorganic iodine compounds by wet and dry deposition.
Depending on the methyl iodide concentration the potential impact of iodine on
tropospheric ozone and other photooxidants was calculated (Davis et al., 1996).
In the present study we developed a detailed iodine reaction scheme which we
have added to a photochemical box model of the MBL which so far had treated
Br and Cl chemistry (Sander and Crutzen, 1996; Vogt et al., 1996). Besides CH3 I,
other alkyl iodides (CH2 I2 , CH2 ClI, and C3 H7 I) were considered to be released
from the sea into the MBL. Rapid photolysis of these compounds (Roehl et al.,
1997) leads to the production of reactive inorganic iodine species. We find that
very intense chemical interactions between iodine and chlorine and bromine are
possible. Uptake of iodine by the marine aerosol and subsequent aqueous phase
reactions enhance halogen activation in the marine aerosol, leading to additional
ozone destruction.
2. Modeling of Iodine Chemistry in the Marine Boundary Layer
The box model MOCCA (Model Of Chemistry Considering Aerosols) previously
used to study halogen chemistry in the polluted and the remote MBL has been extended with an explicit iodine chemistry module. The MOCCA model has already
been described in great detail (Sander and Crutzen, 1996; Vogt et al., 1996). The
model treats gas phase and aqueous phase reactions in the deliquesced sea-salt and
the ammonium sulfate aerosol. Also, exchange between the aerosol and the gas
phase is considered. In total, including I-chemistry the model now considers 101
reactions in the gas phase, 37 photolysis reactions, as well as 42 phase transfer and
102 aqueous phase reactions in the sea-salt and sulfate aerosol. Temperature and
relative humidity were set equal to T = 293 K and φv = 76%. Photolysis rate
constants were calculated for equinox spring at 45◦ N latitude using the method of
Brühl and Crutzen (1989). The ozone mixing ratio was set equal to 20 nmol mol−1
IODINE CHEMISTRY AND ITS ROLE IN HALOGEN ACTIVATION
377
Figure 1. A simplified scheme of iodine cycling in the MBL. Organic iodine compounds are
shown in rectangular boxes; temporary iodine reservoir species in the gas and in the aerosol
phase are shown in gray octagons.
(nmol mol−1 = 10−9 mol mol−1 = ppbv) and kept constant during the simulation.
At the assumed relative humidity, sea-salt aerosol concentration was 3 × 10−11
m3 per m3 of air (i.e., 9.4 µg NaCl per m3 of air) and that of the sulfate aerosol
1 × 10−12 m3 per m3 of air consisting mainly of NH4 HSO4 , (NH4 )2 SO4 and H2 O
3
(i.e., 0.9 µg SO2−
4 per m of air; for details and references see Sander and Crutzen
(1996) and Vogt et al. (1996)).
The detailed iodine chemical reaction scheme is given in Tables I–IV. The major
iodine reaction cycles are illustrated in Figure 1. In our base model run we consider
oceanic emission rates of CH3 I, i-C3 H7 I, CH2 ClI, and CH2 I2 at 0.6×107 cm−2 s−1 ,
2.0×107 cm−2 s−1 , and 3.0×107 cm−2 s−1 , respectively. These values were chosen
such that the molar mixing ratio of total organic iodine was 3–4 pmol mol−1 (Rahn
et al., 1976; Yoshida and Muramatsu, 1995) with mixing ratios of the individual
compounds in agreement with field observations (Class and Ballschmiter, 1988;
Rasmussen et al., 1982; Singh et al., 1983; Schall and Heumann, 1993; Carpenter
et al., 1998).
During daytime, iodine atoms are released through the photolysis of the alkyl iodides. Unlike chlorine and bromine atoms which are scavenged by various organic
compounds, I atoms do not abstract hydrogen from hydrocarbons. The predominant fate of I atoms is reaction with ozone (G801). Most of the IO radicals are
378
RAINER VOGT ET AL.
Table I. Gas phase rate constants k at T = 298 K
No.
Reaction (of order n)
n
k
(cm−3 )1−n s −1
1Ox
Reference
G801
G802
G803
G804
G805
I + O3 −→ IO + O2
IO + NO −→ I + NO2
HO2 + I −→ HI + O2
OH + HI −→ I + H2 O
IO + HO2 −→ HOI + O2
2
2
2
2
2
1.2 × 10−12
2.0 × 10−11
3.8 × 10−13
7.0 × 10−11
8.4 × 10−11
0
0
0
0
–1
DeMore et al. (1997)
DeMore et al. (1997)
DeMore et al. (1997)
See note
DeMore et al. (1997)
G806
G807
G808
G809
G810
G811
G812
G813
G814
G815
G816
IO + NO2 −→ INO3
INO3 −→ IO + NO2
IO + IO −→ I2 O2
I + NO3 −→ IO + NO2
IO + ClO −→ I + Cl + O2
IO + BrO −→ I + Br + O2
IO + CH3 SCH3 −→ I + CH3 SOCH3
IO + CH3 OO −→ I + HCHO + HO2
IO + IO −→ 2 I + O2
IO + ClO −→ ICl + O2
I + BrO −→ IO + Br
2
1
2
2
2
2
2
2
2
2
2
3.5 × 10−12
5.0 × 10−3
5.2 × 10−11
4.5 × 10−10
1.3 × 10−11
6.9 × 10−11
1.2 × 10−14
See note
2.8 × 10−11
0
1.2 × 10−11
–2
+2
0
0
–2
–2
–1
–1
–2
–2
0
DeMore et al. (1997)
Jenkin et al. (1985)
DeMore et al. (1997)
Chambers et al. (1992)
DeMore et al. (1997)
DeMore et al. (1997)
DeMore et al. (1997)
G817
G818
G819
G820
I + NO2 −→ INO2
INO2 −→ I + NO2
iC3 H7 I + OH −→ CH3 OO + I
I2 O2 −→ 2 IO
2
1
2
1
5.1 × 10−12
2.4
1.2 × 10−12
See note
–1
+1
0
0
M
M
DeMore et al. (1997)
See note
DeMore et al. (1997)
DeMore et al. (1997)
See note
See note
1Ox describes the ozone destruction capability of a reaction (see text for details); G804: measured by J. Crowley
(pers. comm., 1998); G806, G817: The air pressure (1013 hPa) has been included into these three-body reactions
to obtain a pseudo second-order rate constant; G808, G814: The overall rate constant k(IO + IO) = 8.0 × 10−11
cm3 s−1 is recommended by DeMore et al. (1997) and is somewhat lower than recent data of Harwood et al.
(1997) who found k(IO + IO) = 9.9 × 10−11 cm3 s−1 . The I2 O2 yield of 65% is assumed following Huie et
al. (1995) and is in agreement with Harwood et al. (1997). Since I2 O2 is said to be the dominant channel, we
assumed a 65% yield; G810, G815: We assumed 100% formation of I and Cl atoms and neglected other product
channels. For product branching see Bedjanian et al. (1997); G813: Products assumed in analogy to reactions
of ClO and BrO. In a sensitivity study we have assumed k(IO + CH3 OO) = 2.3 × 10−11 cm3 s−1 which is
based on comparison with the analogous ratio of the reaction rates CH3 OO + BrO, HO2 + BrO and HO2 +
IO; G818: Calculated from G817 and equilibrium constant in Jenkin et al. (1985); G819: Rate constant from
J. Crowley (pers. comm., 1998). We assumed that hydrogen abstraction leads to formation of a peroxy radical
and that the iodine atom is liberated in secondary reactions; G820: Thermolysis of I2 O2 at a rate of k = 31 s−1
(same as for Cl2 O2 ) assumed in a sensitivity study.
rapidly photodissociated to I and O atoms (J22) which regenerate the same amount
of ozone consumed in the reaction (G801). Unlike this ‘null cycle’ the reaction
of IO with HO2 , IO, BrO, or ClO leads to catalytic ozone loss. At low IO levels
the reaction with HO2 (G805) is an important reaction pathway which leads to
formation of hypoiodous acid, HOI, a temporary iodine reservoir. Other temporary
iodine reservoir species are HI, IONO2, I2 O2 , and INO2 which may be formed in
reactions G803, G806, G808, and G818. All of these compounds are unstable; they
decompose thermally, or photolytically, or through reaction with OH.
379
IODINE CHEMISTRY AND ITS ROLE IN HALOGEN ACTIVATION
Table II. Gas-phase photolysis rates J¯ (12-hour mean values)
No.
Reaction
J¯
s−1
1Ox
Reference for spectrum
J22
J23
J24
J25
J26
J27
J29
J30
J31
J32
J33
J34
IO −→ I + O3
HOI −→ I + OH
INO3 −→ I + NO3
CH3 I −→ I + CH3 OO
I2 −→ 2 I
I2 O2 −→ 2 I
ICl −→ I + Cl
IBr −→ I + Br
INO2 −→ I + NO2
iC3 H7 I −→ I + CH3 OO
CH2 ClI −→ I + Cl + 2 HO2 + CO
CH2 I2 −→ 2 I + 2 HO2 + CO
1.6 × 10−1
5.6 × 10−3
1.9 × 10−3
2.2 × 10−6
1.2 × 10−1
5.9 × 10−3
1.6 × 10−2
5.0 × 10−2
1.9 × 10−3
7.0 × 10−6
6.0 × 10−5
3.6 × 10−3
0
0
+2
0
0
–2
0
0
+1
0
0
0
Laszlo et al. (1995)
Bauer et al. (1998)
See note
Roehl et al. (1997)
Tellinghuisen (1973)
See note
Seery and Britton (1964)
Seery and Britton (1964)
See note
Roehl et al. (1997)
Roehl et al. (1997)
Roehl et al. (1997)
The 12-hour mean values were obtained by integrating the photolysis rates over the whole day and
then dividing by 12 h. 1Ox describes the ozone destruction capability of a reaction (see text for
details); J24: Same spectrum as for BrNO3 assumed but with a 50 nm redshift; J27: assumed to be
9 times faster than photolysis of Cl2 O2 (Davis et al., 1996); J31: Same as J24 assumed; J32: It is
assumed that photolysis of iC3 H7 I produces C3 H7 and I. The propyl radical reacts with O2 to a
peroxy radical which in the model is treated as CH3 OO.
Table III. Henry constants kH
and accommodation coefficients α at T = 298 K
kH
M/atm
Reference
α
∞
4.5 × 102
4.5 × 102
See note
See note
Chatfield and Crutzen (1990)
0.055
0.5
0.055
H2 O
∞
See note
0.1
H2 O
∞
3.0
1.1 × 102
2.4 × 101
See note
Palmer et al. (1985)
Wagman et al. (1982) (see note)
Wagman et al. (1982) (see note)
0.1
0.01
0.01
0.01
H2 O
∞
See note
0.1
Substance (+ hydrolysis)
H2 O
HI −→ H+ + I−
IO
HOI
INO2 −→ HOI + HNO2
INO3 −→ HOI + HNO3
I2
ICl
IBr
I2 O2 −→ HOI + H+ + IO−
2
Hydrolysis reactions are assumed to be irreversible, i.e., the Henry constants for HI, INO2 ,
INO3 , and I2 O2 are set to infinity; IO: Same values as for HOI assumed; HOI: Lower limit,
as given by Chatfield and Crutzen (1990); ICl, IBr: Calculated from thermodynamical data;
All accommodation coefficients had to be estimated.
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RAINER VOGT ET AL.
Table IV. Aqueous-phase rate constants k at T = 298 K
No.
Reaction (of order n)
n
k
M1−n s−1
Reference
A801
A802
A803
A804
A805
A806
A807
A809
A810
HOI + I− + H+ −→ I2
HOI + Cl− + H+ −→ ICl
ICl −→ HOI + Cl− + H+
HOI + Br− + H+ −→ IBr
IBr −→ HOI + H+ + Br−
HOCl + I− + H+ −→ ICl
HOBr + I− −→ IBr + OH−
−
IO−
2 + H2 O2 −→ IO3
+
IO + IO −→ HOI + IO−
2 +H
3
3
1
3
1
3
2
2
2
4.4 × 1012
2.9 × 1010
2.4 × 106
3.3 × 1012
8.0 × 105
3.5 × 1011
5.0 × 109
6.0 × 101
1.5 × 109
Eigen and Kustin (1962)
Wang et al. (1989)
Wang et al. (1989)
Troy et al. (1991)
Troy et al. (1991)
Nagy et al. (1988)
Troy and Margerum (1991)
Furrow (1987)
Buxton et al. (1986)
A811
A812
A813
A814
A815
A816
A817
A818
I− + O3 −→ HOI + O2
−
+
Cl2 + HOI −→ IO−
2 + 2 Cl + 3 H
−
−
HOI + HOCl −→ IO2 + Cl + 2 H+
−
+
HOI + HOBr −→ IO−
2 + Br + 2 H
−
−
−
+
IO2 + HOCl −→ IO3 + Cl + H
−
−
+
IO−
2 + HOBr −→ IO3 + Br + H
−
− + H+
IO−
+
HOI
−→
IO
+
I
2
3
−
−
+
I2 + HSO−
3 −→ 2 I + HSO4 + 2 H
2
2
2
2
2
2
2
2
4.2 × 109
1.0 × 106
5.0 × 105
1.0 × 106
1.5 × 103
1.0 × 106
6.0 × 102
1.0 × 106
Magi et al. (1997)
Lengyel et al. (1996)
Citri and Epstein (1988)
Chinake and Simoyi (1996)
Lengyel et al. (1996)
Chinake and Simoyi (1996)
Chinake and Simoyi (1996)
Olsen and Epstein (1991)
H+
A802: Calculated from equilibrium and back reaction; A804: Calculated from equilibrium and back
reaction; A810, A811: Products assumed.
We have also included scavenging reactions into the sea-salt and sulfate aerosol.
Promoted by higher acidity HOI very rapidly reacts with Cl− , or Br− in the sea-salt
aerosol. In the sulfate aerosol the same reactions can occur once HCl and HBr have
been scavenged from the gas phase (Vogt et al., 1996).
HOI + Cl− + H+ ←→ ICl + H2 O
(A802, A803)
HOI + Br− + H+ ←→ IBr + H2 O
(A804, A805)
We have estimated the reaction rates of the forward reactions k(A802) = 2.9×1010
M−2 s−1 and k(A804) = 3.3 × 1012 M−2 s−1 from the corresponding equilibrium
constants and the fast hydrolysis reactions, (A803) and (A805), which have been
measured in the laboratory (Wang et al., 1989; Troy et al., 1991). ICl and IBr are
only slightly soluble and escape to the gas phase.
In previous iodine modeling studies (Chatfield and Crutzen, 1990; Jenkin, 1992;
Davis et al., 1996) scavenging of HOI, IONO2 , INO2 , and HI were considered
as loss processes of gaseous iodine. However, we believe that it is reasonable to
assume that IONO2 and INO2 will hydrolyse in the aqueous aerosol and form
HOI which has the same iodine oxidation state, +1. As shown above HOI very
rapidly reacts with other halide ions by which ICl, IBr, or I2 are liberated from
the aerosol. Because of the relatively large halide ion concentrations other HOI
reactions including the bimolecular HOI self reaction (Truesdale, 1998) can be
IODINE CHEMISTRY AND ITS ROLE IN HALOGEN ACTIVATION
381
Table V. Overview of sensitivity studies and their differences to the base run
A
B
C
D
E
F
G
Base run
No iodine
Iodine emissions increased by a factor of 3
Iodine emissions consist only of CH3 I
Thermolysis of I2 O2 included
Thermolysis of I2 O2 plus high solubility of IO
IO + CH3 OO included
neglected. HI which is a very strong acid is scavenged from the gas phase even
at very low aerosol pH. In the aqueous phase iodide is rapidly oxidized by HOCl,
HOBr, or HOI (reactions A806, A807, and A801, respectively). Therefore none of
the above aerosol scavenging reactions represent a sink for gaseous iodine as has
been assumed before.
We suggest that scavenging of I2 O2 by the aqueous aerosol surface results in a
hydrolysis reaction:
+
I2 O2 + H2 O −→ HOI + IO−
2 +H
(H15)
IO−
2 can be further oxidized by H2 O2 to iodate (Furrow, 1987) which is photochemically and chemically stable and may accumulate in the sea-salt and sulfate
aerosol.
−
IO−
2 + H2 O2 −→ IO3 + H2 O
k(A809) = 60 M−1 s−1 .
(A809)
Another reaction path which may lead to formation of iodate could be initiated by
the scavenging of IO radicals and their self reaction in the aqueous phase (Buxton
et al., 1986):
+
IO + IO(+H2 O) −→ HOI + IO−
2 +H
k(A810) = 1.5 × 109 M−1 s−1 .(A810)
The relative importance of the IO self reaction in the gas phase and in the aqueous
phase, or the scavenging of I2 O2 depends on several unknown parameters, such as
the I2 O2 thermal stability, the I2 O2 photolysis rate constant, and the IO solubility.
Some sensitivity studies under various assumptions have been performed and are
presented in chapter 3.4.
3. Results
To analyse the partitioning of the iodine species and the contribution of iodine to
halogen activation and ozone destruction we have performed several sensitivity
studies which are summarized in Table V.
382
RAINER VOGT ET AL.
Figure 2. Iodine compounds on day 3 of the base run (A): (a) Individual mixing ratios of the
organic iodine compounds and the sum of all organic I species, Iorg , (b) Gas-phase species
I, IO, HOI, I2 O2 , ICl, and IBr, and (c) the sum of all inorganic I species, Iinorg (i.e., HOI +
IO + I + ICl + IBr + HI + 2 I2 + 2 I2 O2 + IONO2 + INO2 ), all organic I species, Iorg (i.e.,
CH3 I + 2CH2 I2 + CH2 ClI + C3 H7 I). Also shown is the total amount of iodine compounds
in sulfate aerosol particles, Ipart (sulfate), and in sea-salt particles Ipart (seasalt); (mcl/cc =
molecules cm−3 ; pmol mol−1 = 10−12 mol mol−1 = pptv).
IODINE CHEMISTRY AND ITS ROLE IN HALOGEN ACTIVATION
3.1.
383
PARTITIONING OF IODINE SPECIES IN THE BASE CASE
In Figure 2 the partitioning of the different iodine species is shown for day 3 of
the model run (A). The mixing ratio of total organic iodine which is the sum of
CH3 I, i-C3 H7 I, CH2 ClI, and 2 CH2 I2 reaches approximately 3 pmol mol−1 during
daytime and a maximum of 4 pmol mol−1 during nighttime. The diurnal variation
is caused by the rapid photolysis of CH2 I2 and CH2 ClI during the daylight hours.
In our base case simulation CH2 I2 and CH2 ClI reach early morning maxima of
approximately 0.5 pmol mol−1 . This is in the order of field observations in the
Arctic of Schall and Heumann (1993), and in agreement with air samples from the
coastlines of Teneriffe (Class and Ballschmiter, 1988), or Ireland (Carpenter et al.,
1998). However, because the photolabile organics were measured during daytime
it is quite clear that there must have been very strong local sources and most likely
a vertical concentration gradient in the MBL. Due to rapid photolysis the modeled
daytime CH2 I2 mixing ratio is very low.
In agreement with the more recent iodine modeling studies (Jenkin, 1992; Davis
et al., 1996) we find that the most abundant inorganic iodine gas is hypoiodous acid,
HOI. The HOI mixing ratio reaches a daytime maximum of about 2 pmol mol−1 .
During nighttime scavenging of HOI dominates and the mixing ratio drops to about
0.5 pmol mol−1 . The products of HOI scavenging reactions, ICl and IBr, reach
early morning maxima of 1.0 and 0.4 pmol mol−1 , respectively. During daytime
the maximum mixing ratio of IO is about 0.8 pmol mol−1 .
The sum of all gaseous inorganic iodine species, i.e., HOI, IO, I, ICl, IBr,
HI, 2 × I2 , 2 × I2 O2 , IONO2, and INO2 increases during daytime and reaches a
maximum of 3.8 pmol mol−1 . At night the sum of the inorganic iodine is about 2
pmol mol−1 . This calculated diurnal variation is comparable to that observed by
Rancher and Kritz (1980) in the tropical MBL near the African coast, if we assume
that the LiOH coated filters used in the field study reflect the same inorganic iodine
compounds as defined above. The absolute value of the sum of the iodine species
is very dependent on the emission rates of the alkyl iodides and the deposition
velocities of the inorganic iodine gases.
Besides HOI deposition to the ocean surface which we have set equal to 1 cm
s−1 the most efficient sink of atmospheric iodine may be the formation of particulate iodine and its removal by dry and wet deposition. In our calculation particulate
iodine entirely consists of IO−
3 which due to its chemical interness is the only iodine
species which may be accumulated at significant concentrations in the sea-salt, or
sulfate aerosol. The absolute amount of the particulate iodine (1–3 pmol mol−1 )
very much depends on aerosol removal processes and iodate formation processes
which are not well known and which have been strongly simplified in the model.
3.2.
HALOGEN ACTIVATION BY IODINE CHEMISTRY
An autocatalytic halogen activation mechanism from sea-salt aerosol has been
proposed recently (Vogt et al., 1996). Briefly, HOBr which has been formed in
384
RAINER VOGT ET AL.
an initiation step is scavenged by sea-salt aerosol. In the presence of acidity it
rapidly reacts with Cl− to BrCl which can further react via Br2 Cl− to form and Br2
and Cl− , if sufficient Br− is present in the aerosol droplet. BrCl and Br2 are only
slightly soluble and escape to the gas phase where they undergo rapid photolysis.
It was shown that bromine and chlorine atoms can have a significant impact on
the ozone budget and the oxidation of hydrocarbons. In addition, HOBr and HOCl
are capable of S(IV) oxidation in the aerosol, so that sulfate formation may be
dominated by these reactions (Vogt et al., 1996).
The dynamics of the autocatalytic halogen liberation from sea-salt aerosol depends on initiation reactions, such as bromide oxidation by HSO−
5 or OH radicals.
In this study we find that iodine which is released from the photodissociation of the
alkyl iodides activates the other halogens, bromine and chlorine. The key activation
reactions are scavenging of hypoiodous acid by the aerosol and the subsequent
formation of ICl and IBr which escape to the gas phase:
HOI + Cl− + H+ −→ ICl + H2 O
(A802)
HOI + Br− + H+ −→ IBr + H2 O .
(A804)
Br and Cl atoms formed upon photolysis react with ozone, and subsequently BrO
and ClO are converted to HOBr and HOCl. Both compounds can be scavenged
by the sea-salt aerosol initiating the autocatalytic bromine and chlorine release as
described above.
The magnitude of this additional halogen activation is illustrated in Figure 3.
The model started with no gas phase halogens present at all. For example, on the
second model day in the absence of iodine compounds a chlorine atom concentration of approximately 1.8 × 103 cm−3 is reached. However, if iodine is included in
the model the chlorine atom concentration reaches 3.3×103 atoms cm−3 . Similarly,
the mixing ratios of the other chlorine and bromine compounds are also increased.
After three days of cycling the difference has almost disappeared, because now the
halogen release is dominated by the HOBr scavenging reaction and the availability
of bromide in the aerosol.
Also shown in Figure 3 is the effect of iodine chemistry on the OH, HO2 and
NOx mixing ratios. In agreement with studies of Jenkin (1992) and Davis et al.
(1996) the [OH]/[HO2 ] ratio is increased because of the HO2 + IO reaction (G805)
and photolysis of HOI (J23). In the presence of iodine the NOx concentration is
lower because of the scavenging of BrONO2 which is enhanced by the higher
reactive bromine concentration.
To investigate the amount of iodine which is necessary for the accelerated liberation of halogens we have performed sensitivity studies (C and D) in which
the emission strength of the organic iodine compounds was varied. In Figure 4
is shown that if we assume that methyl iodide ([CH3 I] = 2 pmol mol−1 ) is the
only iodine source present, the sum of all gaseous inorganic iodine compounds
(Iinorg = 0.3 pmol mol−1 ) is much smaller than in the base run. There is only a
IODINE CHEMISTRY AND ITS ROLE IN HALOGEN ACTIVATION
385
Figure 3. Gas-phase mixing ratios in the base run (A; solid) and in a run without iodine
(B; dotted); NOx (= NO + NO2 ); (mcl/cc = molecules cm−3 ; pmol mol−1 = 10−12 mol
mol−1 = pptv.
386
RAINER VOGT ET AL.
Figure 4. Halogen compounds in the base run (A; solid) and in sensitivity runs with 3×
increased iodine (C; dotted) and CH3 I only (D; dashed). Ipart denotes the sum of all iodine species in sulfate and sea-salt particles, respectively; (mcl/cc = molecules cm−3 ; pmol
mol−1 = 10−12 mol mol−1 = pptv; Iorg and Iinorg as defined in Figure 2).
IODINE CHEMISTRY AND ITS ROLE IN HALOGEN ACTIVATION
387
small increase (10%) of Br, or Cl atoms and ClO, or BrO radicals as compared
to a run without iodine. The concentration maxima during the second model day
are [HOI] = 0.5 × 107 molecules cm−3 , [IO] = 0.2 × 107 molecules cm−3 ,
[BrO] = 1.6 × 107 molecules cm−3 and [Cl] = 2.0 × 103 atoms cm−3 .
If we assume three times increased iodine emissions which yields IO concentrations in agreement with very recent field observations at Mace Head (with IO
maxima of up to 1.8×108 molecules cm−3 ; Alicke et al., 1998) the calculated maxima during the second model day are [HOI] = 4.0 pmol mol−1 , [IO] = 4.6 × 107
molecules cm−3 , [BrO] = 2.8 × 107 molecules cm−3 and [Cl] = 4.5 × 103
atoms cm−3 . This run shows that, compared to the base case, reactive chlorine
and bromine mixing ratios are increased during the first two model days.
3.3.
CATALYTIC OZONE DESTRUCTION BY HALOGEN CHEMISTRY
In the MBL catalytic ozone destruction by halogen chemistry is possible via a
number of reaction cycles. At low iodine concentrations cycle (I) first proposed by
Chameides and Davis (1980) involving formation and photolytic decomposition of
HOI is most important.
Cycle I
I + O3
−→ IO + O2
IO + HO2 −→ HOI + O2
HOI + hν −→ OH + I
(G801)
(G805)
(J23)
O3 + HO2 −→ OH + 2 O2
At higher iodine concentrations the reaction cycle which involves the IO self reaction becomes more important (Chameides and Davis, 1980; Chatfield and Crutzen,
1990; Jenkin, 1992):
Cycle II
2 (I + O3 )
−→ 2 (IO + O2 )
(G801)
IO + IO
−→ 2 I + O2
(G814)
IO + IO + M −→ I2 O2 + M
I2 O2 + hν
−→ 2 I + O2
2 O3
−→ 3 O2
(G808)
(J27)
To parameterize the IO self reaction we followed the current recommendation of
DeMore et al. (1997) (see Table I for more details). Very recent data from the
research groups of R. A. Cox (Bloss et al., 1998) and J. P. Burrows (Spietz et al.,
1998) indicate that there is a significant channel of the IO self reaction leading to
OIO as a reaction product. Photolysis of OIO most likely would yield IO and I, and
388
RAINER VOGT ET AL.
therefore would not effect the ozone budget. Due to the lack of experimental data
we have neglected this channel.
Depending on the BrO concentration a reaction cycle involving the IO + BrO
cross reaction may become important (Solomon et al., 1994b).
Cycle III
I + O3
−→ IO + O2
(G801)
Br + O3 −→ BrO + O2
(G602)
IO + BrO −→ I + Br + O2
(G811)
2 O3
−→ 3 O2
Other possible reaction products of G811 are OIO and/or OBrO which would not
effect the ozone budget. Therefore, the calculated ozone loss in Cycle III represents
an upper limit. The corresponding ClO + IO cycle is also included in the model.
However, it is much less important because of the lower ClO mixing ratio and
smaller reaction coefficient.
To analyze the chemical reactions leading to ozone production and destruction
more easily, we define the odd oxygen family Ox in the gas phase: Ox = O3 +
O(3 P) + O(1 D) + NO2 + 2 NO3 + 3 N2 O5 + HNO4 + ClO + 2 Cl2 O2 + BrO + IO +
2 I2 O2 . The definition was chosen in such a way that production or destruction of
one of its constituents eventually also leads to production or destruction of ozone
through various catalytic reaction cycles. Reactions that recycle species within the
Ox family (even though they might be much faster) do not affect the ozone budget.
Changes in the concentrations of Ox via chemical reactions are shown in the tables
in the column 1Ox .
In Figure 5 the most important Ox destruction rates are shown for model day 3.
The Ox destruction by the IO + HO2 reaction (Cycle I) adds up to 0.44 nmol mol−1
day−1 which is 45% of the sum of the Ox removed by the HOx reactions, O(1 D) +
H2 O, O3 + HO2 , and O3 + OH. In the other iodine cycles 0.13 nmol mol−1 day−1
Ox is destroyed by IO + BrO (Cycle III), 0.04 nmol mol−1 day−1 Ox by IO +
IO −→ 2 I (Cycle II), and 0.07 nmol mol−1 Ox day−1 by I2 O2 photolysis (J27,
Cycle II). In addition, the sum of the BrO + HO2 and BrO + CH3 O2 destruction
rates is about 22% of the total Ox removal by O(1 D) + H2 O, O3 + HO2 , and O3 +
OH. The total destruction rate by all halogen reaction cycles, therefore, adds up to
0.9 nmol mol−1 day−1 , close to the O3 removal by the HOx reactions.
To illustrate the impact of iodine on the Ox destruction rates during the initiation phase of day 2 we have compiled the integrated Ox destruction rates for
sensitivity runs (A)–(D) in Table VI. By comparison of the Ox destruction rates
of the BrO + HO2 and BrO + CH3 O2 reactions for the different iodine scenarios
it becomes evident how the bromine catalysed Ox destruction is enhanced by the
iodine initiation chemistry as described in chapter 3.2. The absolute amount of Ox
destroyed by iodine chemistry very much depends on the total oceanic iodine flux.
389
IODINE CHEMISTRY AND ITS ROLE IN HALOGEN ACTIVATION
Figure 5. Ozone destruction rates on day 3 of the base run (A; 1 mol/mol/day = 2.5 × 1019
molecules cm−3 day−1 ).
Table VI. Ox destruction rates during day 2 (unit: pmol mol−1 day−1 )
Reaction
No iodine (B)
CH3 I only (D)
Base run (A)
3 × RI (C)
H2 O + O(1 D) −→ 2 OH
O3 + OH −→ HO2 + O2
O3 + HO2 −→ OH + 2 O2
BrO + HO2 −→ HOBr + O2
BrO + CH3 OO −→ products
IO + HO2 −→ HOI + O2
IO + BrO −→ I + Br + O2
IO + IO −→ 2 I + O2
I2 O2 + hν −→ 2 I + O2
623
88
310
74
16
0
0
0
0
623
89
309
78
17
31
4
0.17
0.3
623
96
289
107
32
395
69
27
47
623
105
267
120
51
833
185
131
233
390
RAINER VOGT ET AL.
The magnitude of the Ox destruction rates are comparable to results from Davis
et al. (1996) who calculated ozone destruction using a one-dimensional model.
However, comparison of the absolute numbers is difficult because Davis et al.
(1996) adjusted the total Ix at 10 km altitude and the iodine content at 0–1 km
is only given for their low iodine case.
In sensitivity study (G) we have assumed an additional reaction of IO with
methyl peroxy radicals.
IO + CH3 O2 −→ I + HCHO + HO2 .
(G813)
Since the rate constant of reaction (G813) has not been determined we have estimated the rate coefficient by comparison with the corresponding BrO reaction (see
Table I). From the data recommended by DeMore et al. (1997) and the rate coefficient of the CH3 O2 + BrO reaction (Aranda et al., 1997) we estimate k(G813) =
2.3 × 10−11 cm3 molecule−1 s−1 . In this case reaction (G813) may add about 0.15
nmol mol−1 day−1 of O3 removal, resulting in equal contributions to O3 destruction
by HOx and halogen chemistry.
3.4.
FORMATION OF PARTICULATE IODINE
In our base model scavenging of I2 O2 and the subsequent oxidation of the hydrolysis products eventually leads to the formation of iodate which accumulates in the
aerosol particles. The enrichment of iodate – rather than iodide – is in agreement
with field data of Wimschneider and Heumann (1995) who reported that particulate
iodine in marine air sampled over the Antarctic ocean mainly consists of iodate.
Murphy et al. (1997) reported data from single particle mass spectrometer measurements performed at Cape Grim during the ACE-1 experiment. They observed
a correlation of the MS signal of I− with a signal that is characteristic for organic
compounds. This could be indicative of an initial iodine enrichment during the seasalt aerosol generation process by I bound to organics in a surface active film on
the ocean surface (Moyers and Duce, 1972). However, many quantitative issues,
such as the dependence of the single particle mass spectrometer sensitivity on the
particle diameter, or the instrumental sensitivity of iodide versus iodate, still have
to be resolved with this fairly new exciting technique.
In our model most of the iodate accumulates in the sulfate aerosol because of
the much smaller droplet radii at a comparable surface area of the sulfate aerosol
(Figure 6). In our base case simulation we do not consider thermal decomposition
of the IO dimer. It should be noted that the I2 O2 molecule has not been identified in
experimental studies and it might not exist at temperatures typical for the MBL. In
sensitivity studies (E) and (F) we have included the thermal decomposition of I2 O2
at the same rate as the decomposition of Cl2 O2 (k(G820) = 31 s−1 ). As shown in
Figure 6 (dotted lines) the amount of total inorganic iodine is increased, however
the I2 O2 mixing ratio is very small. As a result there is no particulate iodine formed.
However, if we assume an IO solubility of kH (IO) = 4.5 × 104 M atm−1 , which
IODINE CHEMISTRY AND ITS ROLE IN HALOGEN ACTIVATION
391
Figure 6. Iodine compounds in the base run (A; solid) and in sensitivity runs with thermal
decay of I2 O2 (E; dotted) and with thermal decay of I2 O2 plus increased solubility of IO
(F; dashed). Ipart (sulfate) and Ipart (seasalt) denotes the sum of all iodine species in sulfate
and sea-salt particles, respectively; Iorg and Iinorg as defined in Figure 2; mcl/cc = molecules
cm−3 ; pmol mol−1 = 10−12 mol mol−1 = pptv).
is 100× larger than assumed in the base model, particulate iodine is formed by the
self reaction of IO in the aqueous aerosol:
+
IO + IO (+H2 O) −→ HOI + IO−
2 +H
k(A810) = 1.5 × 109 M−1 s−1 (A810)
−
IO−
2 + H2 O2 −→ IO3 + H2 O
k(A809) = 60 M−1 s−1
(A809)
Like in the base model iodate accumulates preferably in the smaller sulfate particles
(Figure 6, dashed lines).
Another possible reaction pathway forming iodate could result from the scavenging of OIO which may be a product of IO self reaction (Bloss et al., 1998; Spietz
et al., 1998), or the IO + BrO reaction. However, because there is no experimental
data of the OIO hydrolysis and subsequent aqueous phase oxidation available we
did not investigate this possibility.
392
RAINER VOGT ET AL.
4. Discussion and Conclusions
We have developed a detailed iodine, gas and aqueous phase, chemistry reaction
scheme which was included in the box model MOCCA. Rapid photolysis reactions
of several alkyl iodides which have been detected in the MBL are also considered in
the model. Based on field observations the concentrations of the organic, inorganic,
and particulate iodine species are calculated. In the aerosol droplets HOI reacts
with Br− , or Cl− , and IBr and ICl are released to the gas phase. Depending on
the iodine concentration present in the MBL autocatalytic bromine and chlorine
release from sea-salt aerosol can be initiated. The magnitude of catalytic ozone
destruction depends on the amount of reactive iodine present. In our base model
the additional ozone loss by iodine chemistry is calculated to be about 0.6 nmol
mol−1 day−1 which, although significant, is rather too small to be detected in a
field campaign.
Ozone destruction by all halogen reactions is calculated to be similar to that of
the sum of the O(1 D) + H2 O, O3 + HO2 , and O3 + OH reactions. Although many
assumptions were made in this study, the results indicate that iodine, and more
generally halogen chemistry may play a role in the O3 budget in the MBL.
We also present a chemical mechanism leading to particulate iodine which is
formed in the course of I2 O2 , or IO scavenging and subsequent IO−
2 oxidation by
H2 O2 to IO−
.
Iodate
is
preferably
accumulated
in
the
fine
aerosol
mode which is
3
in agreement with a recent study of aerosol samples from the Antarctic ocean.
This study is highly theoretical and our conclusions, therefore, strongly need
testing with observational data. Foremost of these are:
1. Further identification of the marine biological processes leading to the production of organic iodine gases and their releases to the atmosphere,
2. quantification of the regional and global emission rates,
3. measurements of organic and reactive inorganic halogens in the MBL as well
as chemical analyses of the aerosol,
4. determination of the uptake rates of inorganic halogen species on the sea surface; in this study deposition velocities have been set equal to values between
0.2 cm s−1 and 1.0 cm s−1 ; if they would be smaller (Rancher and Kritz, 1980),
higher concentrations of the reactive halogen gases would have accumulated
in the MBL, resulting in larger ozone destruction rates, or vice versa,
5. improved determinations of the rate constants of the reactions between reactive halogen compounds, especially in the aqueous phase.
Model improvements are also needed, especially regarding: (a) realistic treatment
of the size distributions of the sea-salt and sulfate aerosol and its effect on the MBL
chemistry, and (b) more realistic simulation of the meteorological processes in the
MBL and exchange processes at the sea surface and with the free troposphere.
IODINE CHEMISTRY AND ITS ROLE IN HALOGEN ACTIVATION
393
Acknowledgements
We would like to thank C. Brühl for calculating photolysis rate constants and
C. Roehl for providing UV spectra of several iodine compounds prior to publication. This work was partially supported by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 233. P. Crutzen acknowledges a Grant
provided by the Ford Motor Company.
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