Atmos. Chem. Phys., 10, 7655–7658, 2010
www.atmos-chem-phys.net/10/7655/2010/
doi:10.5194/acp-10-7655-2010
© Author(s) 2010. CC Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Introducing the bromide/alkalinity ratio for a follow-up discussion
on “Precipitation of salts in freezing seawater and ozone depletion
events: a status report”, by Morin et al., published in
Atmos. Chem. Phys., 8, 7317–7324, 2008
R. Sander1 and S. Morin2,3
1 Air
Chemistry Department, Max-Planck Institute of Chemistry, P.O. Box 3060, 55020 Mainz, Germany
– CNRS, CNRM/GAME, CEN, 1441, rue de la piscine, 38400 St. Martin d’Hères, France
3 CNRS – UJF Grenoble, LGGE, 54 rue Molière, 38400 St Martin d’Hères, France
2 Météo-France
Received: 4 August 2009 – Published in Atmos. Chem. Phys. Discuss.: 2 October 2009
Revised: 29 June 2010 – Accepted: 11 August 2010 – Published: 17 August 2010
Abstract. Sander et al. (2006) proposed that CaCO3 precipitation can be an important factor in triggering tropospheric
ozone depletion events. Recently, Morin et al. (2008b) presented calculations with the FREZCHEM model and concluded that their results and interpretation cast doubt on the
validity of this hypothesis. In this joint publication, we have
re-analyzed the implications of the FREZCHEM results and
show how they can be reconciled with the proposal of Sander
et al. (2006). The chemical predictions of both approaches
are consistent. Although an interpretation solely based on the
alkalinity change in the brine does not support the conclusion
of Sander et al. (2006), we show that the bromide/alkalinity
ratio (which increases during the cooling of the brine) can be
used as an indicator of the potential for triggering bromine
explosions.
1
Introduction
Sander et al. (2006) proposed that carbonate precipitation
in brine could be a trigger for tropospheric ozone depletion
events in polar regions via the following sequence:
1. Cooling of the brine increases its salinity (cryoconcentration).
Correspondence to: R. Sander
([email protected])
2. CaCO3 becomes supersaturated and precipitates.
3. The loss of dissolved carbonate (the major contributor
to the alkalinity AT ) reduces the buffering capacity of
brine on sea ice.
4. Aerosols resulting from the brine can be acidified more
easily.
5. The acid-catalyzed “bromine explosion” produces reactive halogen species (e.g. BrO) in the gas phase.
6. Halogen chemistry leads to tropospheric ozone depletion events.
This mechanism is specific to cold regions (marine boundary layer temperatures sustained below the freezing point
of water), and is therefore considered potentially capable of
explaining the occurrence of tropospheric ozone depletion
events in polar regions. The mechanism does not necessarily have to involve aerosols as suggested in point 4 above.
Indeed, any mechanism inducing physical separation of the
precipitate and the brine prior to acidification would suffice
(see Fig. 1 in Morin et al., 2008b). Sander et al. (2006) supported their proposal with preliminary calculations. However, due to lack of temperature-dependent data, thermodynamic constants at 273 K had to be used. In addition, activity
coefficients were not taken into account. Improved calculations of CaCO3 precipitation were presented by Morin et al.
(2008b) to test this hypothesis. The FREZCHEM model was
Published by Copernicus Publications on behalf of the European Geosciences Union.
Temperature /K
Fig.
1. Precipitation
of calcium
carbonate
during theon
freezing
R.Sander
Sander
and S. Morin:
Follow-up
discussion
Morinofetsea
al.
R.
water. and S. Morin: Follow-up discussion on Morin et al.
27656
100
90
Alkalinity / 10-3 mol kg-1
% precipitation
80
70
60
50
40
No precip.
FREZCHEM calcite
FREZCHEM ikaite
Sander et al.
30
20
10
0
250
255
260
265
Temperature /K
270
275
Fig. 1. Precipitation of calcium carbonate during the freezing of sea
water.
Fig. 1. Precipitation of calcium carbonate during the freezing of sea
water.
Alkalinity / 10-3 mol kg-1
used
20which is able to extrapolate thermodynamic constants
precip. and explicinto the subzero (T <273 K) temperatureNorange,
FREZCHEM calcite
itly calculates activity coefficients in concentrated
solutions
FREZCHEM ikaite
(Marion, 2001). After the public discussion
Sander etof
al. the ACPD
15
Sea water
at 273K availpaper (Morin et al., 2008a), new information
became
able showing that out of the polymorphs of CaCO3 , ikaite
is more likely to precipitate than calcite (Dieckmann et al.,
10 Implementation of ikaite into FREZCHEM led to
2008).
less carbonate precipitation. Morin et al. (2008b) now concluded that their result “strongly contradicts the predictions
5
of Sander
et al. (2006) and casts doubts on the validity of
the hypothesis originally described by these authors”. In this
paper we re-analyze the implications of the FREZCHEM results0and show that they can be reconciled with the proposal
of Sander
(2006). 260
250 et al. 255
265
270
275
Temperature /K
2
Discussion
Fig. 2. Alkalinity AT of the brine during the freezing of sea wa2.1 The
The
effect is
ofAcooling
on −the
of brine 2−
ter.
definition
) + alkalinity
m(HCO−
T = m(OH
3 ) + 2 m(CO3 )
◦
◦
+
◦
+ 2 m(CaCO
(cryoconcentration)
3 ) + 2 m(MgCO3 ) − m(H ), where CaCO3 and
◦
MgCO3 represent dissolved ion pairs.
For a better comparison of the two studies, plots were created that include both the results of Sander et al. (2006) and
2Morin
Discussion
et al. (2008b). Figure 1 shows the precipitation of calcium carbonate during the freezing of sea water. The black
2.1
of that
cooling
on2the
alkalinity
of (2006).
brine (cryline isThe
the effect
same as
in Fig.
of Sander
et al.
The
oconcentration)
red and
the orange lines show the FREZCHEM results for
calcite and ikaite precipitation, respectively. It can be seen
For
better
comparison
of the two
plots
were crethat athe
preliminary
calculations
by studies,
Sander et
al. (2006)
are
ated
that
include
both
the
results
of
Sander
et
al.
(2006)
and
very similar to the FREZCHEM results for calcite precipitaMorin
et al. (2008b).
shows the precipitation
of caltion. However,
when Figure
ikaite is1 considered,
the precipitation
of
cium
carbonate
during
the
freezing
of
sea
water.
The
black
carbonate is substantially lower.
lineThe
is the
same asAthatofinthe
Fig.brine
2 of Sander
et al. (2006).
The
alkalinity
as a function
of temperT
red
and
the
orange
lines
show
the
FREZCHEM
results
of
ature is shown in Fig. 2. This plot is similar to Fig. 5for
calcite and ikaite precipitation, respectively. It can be seen
Morin et al. (2008b) but contains two additional lines: the
Atmos. Chem. Phys., 10, 7655–7658, 2010
20 preliminary calculations by Sander et al. (2006) are
that the
No precip.
very similar to the FREZCHEM results for
calcite precipitaFREZCHEM
calcite
FREZCHEM
ikaite of
tion. However, when ikaite is considered, the
precipitation
Sander et al.
carbonate
15 is substantially lower.
Sea water at 273K
The alkalinity AT of the brine as a function of temperature is shown in Fig. 2. This plot is similar to Fig. 5 of
10et al. (2008b) but contains two additional lines: The
Morin
blue line shows AT when carbonate precipitation is artificially switched off, and the grey line shows the alkalinity of
sea water
at T = 273 K. This figure requires careful in5
terpretation. It shows that although ikaite precipitation removes a substantial fraction of the alkalinity at subzero temperature,
AT of the brine is always above the value for sea0
water at
273
K. 255
This results
two opposing
250
260 from 265
270 effects:
275
1) the alkalinity increases Temperature
because the /K
brine becomes more
concentrated at lower temperatures (cryoconcentration), and
2)Fig.
it decreases
dueAto
carbonate
When
2. Alkalinity
the brine precipitation.
during the freezing
of ikaite
sea waT of
−
−during
Fig.
2. Alkalinity
A
the brine
the freezing
of sea2−
waister.
formed,
the precipitation
does
notm(HCO
completely
compenThe
definition
is TAof
)
+
)
+
2m(CO
)+
T = m(OH
3−
3 2−
−
ter.
The
definition
is
A
=
m(OH
)
+
m(HCO
)
+
2
m(CO
T
◦
◦
◦
+
sate
for the) cryoconcentration
effect.
This
led
Morin
et
3
3al.)◦
2m(CaCO
+
2m(MgCO
)−m(H
),
where
CaCO
and
MgCO
◦
◦
+
◦
3
3
3
+ 2 m(CaCO
), where3in
CaCO
3 ) + 2 m(MgCO
3 ) − m(H
3 and
(2008b)
the conclusion
that ikaite
precipitation
the brine
represent◦todissolved
ion pairs.
MgCO3 represent dissolved ion pairs.
prior to aerosol generation does not deplete the alkalinity of
airborne particles (relative to particles produced from unfractionated
above
freezing),
thereby
challenging
the
blue lineseawater
shows A
carbonate
precipitation
is artifiT when
2 Discussion
conclusions
of Sander
et al.the(2006).
Indeed,
interpreted
this
cially switched
off, and
grey line
shows
the alkalinity
way
it is
difficult
see
how
mechanism
presented
in(crytheinof
water
at Ttoof=
273
K. the
This
requires
careful
2.1sea
The
effect
cooling
on
thefigure
alkalinity
of brine
introduction
can
trigger
bromine
explosions
at
subzero
temterpretation.
It
shows
that
although
ikaite
precipitation
reoconcentration)
peratures.
moves a substantial fraction of the alkalinity at subzero temperature,
AT comparison
of the brineof
is the
always
value
for creseaFor a better
two above
studies,theplots
were
water
at
273
K.
This
results
from
two
opposing
effects:
atedEquilibration
that include both
resultshumidity
of Sander(evapoconcentraet al. (2006) and
2.2
withthe
ambient
1)
the
alkalinity
increases
becomesofmore
Morin
et al. (2008b).
Figurebecause
1 showsthe
thebrine
precipitation
caltion)
cium carbonate
duringtemperatures
the freezing (cryoconcentration),
of sea water. The black
concentrated
at lower
and
line
is
the samedue
as that
in Fig. 2 ofprecipitation.
Sander et al. (2006).
The
2)
it
decreases
to
carbonate
When
ikaite
Up to this point a surface-borne liquid is considered that can
and thethe
orange
lines show
the not
FREZCHEM
results
for
isredformed,
precipitation
does
completely
compenrepresent
e.g. a brine
layer possibly
covered
with frost
flowcalcite
and
ikaite
precipitation,effect.
respectively.
It can
be seen
sate
for
the
cryoconcentration
This
led
Morin
et
ers or brine in salty snow. In the simulations carried out byal.
(2008b)
conclusion
ikaitethat
precipitation
in the brine
Sander
etto
al.the
(2006),
it was that
assumed
bromine explosions
prior
to
aerosol
generation
does
not
deplete
the
alkalinity
occur within brine-derived aerosols. Under such an hypothe-of
airborne
particles
(relative
produced
from unfracsis,
one has
to evaluate
whattoisparticles
the chemical
composition
of
tionated
seawater
above
freezing),
thereby
challenging
the
aerosols derived from the brine. Once the brine has been inconclusions
of
Sander
et
al.
(2006).
Indeed,
interpreted
this
jected into the air and formed aerosol particles, H2 O will
way it is difficult
to see how with
the mechanism
presented
in is
the
evaporate
until equilibration
the ambient
humidity
introduction
can
trigger
bromine
explosions
at
subzero
temreached. Long-term observations by Treffeisen et al. (2007)
peratures.
found
relative humidities between RH = 60 % and RH = 90 %
for near-surface air at Ny-Ålesund. Using a typical value
with
ambient
humidity
+
of2.2RH Equilibration
= 80 %, a NaCl
solution
of m(Na
(aerosol)) ≈ 5
(evapoconcentration)
mol/kg is reached (Tang et al., 1997). According to Koop
et al. (2000), the deliquescence/efflorescence curves of sea
Up to this point a surface-borne liquid is considered that can
salt hardly depend on temperature, suggesting that the warepresent e.g. a brine layer possibly covered with frost flowter activity of sea salt solutions of fixed molality also reers or brine in salty snow. In the simulations carried out by
mains nearly constant with changes in temperature. Thus,
Sander
et al.et(2006),
it wasweassumed
bromine
explosions
like
Sander
al. (2006),
assumethat
a NaCl
molality
of 5
occur
within
brine-derived
aerosols.
Under
such
an
hypothmol/kg in aerosol particles for RH = 80% at all temperatures.
esis, one has to evaluate what is the chemical composition
vaporization
leads the
to an
increase
concentraofThis
aerosols
derived from
brine.
Once of
thethe
brine
has been
tions
of
all
solutes
(evapoconcentration).
The
evaporation
injected into the air and formed aerosol particles, H2 O will
factor fevap describes the molality ratio before and after
evaporate until equilibration with the ambient humidity is
www.atmos-chem-phys.net/10/7655/2010/
1) the al
concentr
2) it dec
is forme
sate for
(2008b)
prior to
airborne
tionated
conclusi
way it is
introduc
perature
2.2 Eq
tio
Up to th
represen
ers or br
Sander e
occur wi
sis, one
aerosols
jected in
evaporat
reached.
found re
for near
of RH =
mol/kg i
et al. (20
salt hard
ter activ
mains n
like San
mol/kg i
This v
tions of
factor f
R. Sander and S. Morin: Follow-up discussion on Morin et al.
3
evaporation:
R. Sander and S. Morin: Follow-up discussion on Morin et al.
7657
+
m(Na
(aerosol))
The black
squares
in Fig. 3 show the brine alkalinity from
fevap =
(1)
+
the FREZCHEM
ikaite model simulation (same as orange
m(Na (brine))
line in Fig. 2). The black circles show the aerosol alkalinThe process
of evapoconcentration
is visualized
in Fig. 3
ity after
evapoconcentration.
It can be seen
that it decreases
for different
temperatures.
Red squares
the at
molalwith
decreasing
temperature. Thus,
aerosolsdenote
generated
low
ity of Na+ incontain
brine (after
cryoconcentration),
and they
red circles
temperature
less alkalinity
even though
were
show the from
molality
in with
the aerosol
(after evapoconcentration).
generated
brine
an alkalinity
higher than that of
Since cryoconcentration
brings
the Na+ molality closer
to
seawater
at 273 K. Below 266
K, evapoconcentration
always
its final
of alkalinities
5 mol/kg, fevap
is smaller
at lower
leads
to value
aerosol
below
15 mmol
kg−1 temperaused by
tures. et al. (2006). Even taking into account the evapocSander
oncentration
of alkalinity
in airborne
theall
situation
Evapoconcentration
increases
the aerosols,
molality of
solutes
modeled
by Sander
et al.
(2006)
be rather
conserequally, thus
the same
factor
fevapseems
can betoapplied
to the
alkavative
of the alkalinity of aerosols (aerosol alkalinlinity in
ATterms
:
ities are rather on the order of 5 mmol kg−1 at temperatures
around 255 K, i.e. one third m(Na
of the+initial
alkalinity used by
(aerosol))
AT (aerosol)
= AT (brine)
×model study).
(2)
Sander
et al. (2006)
in their
+
m(Na (brine))
Introducing the m(Br− )/AT ratio
The black squares in Fig. 3 show
the brine alkalinity from
the
FREZCHEM
ikaite
model
simulation
(same
orange
Rather than computing AT in aerosol formed
fromasbrine
as
line
in
Fig.
2).
The
black
+circles show the aerosol alkalina function of only the Na concentration in the brine, we
ity aftertoevapoconcentration.
It can
seen that
it decreases
propose
also take into account
thebe
bromide
concentration
with
decreasing
temperature.
Thus,
aerosols
generated
at low
in the brine. Thus, equation (2) can be rewritten as:
temperature contain less alkalinity even though they were
generated from brine
with
an alkalinity higher
than that of
m(Br−
(brine))
× m(Na+ (aerosol))
+
m(Na
(brine))
seawater
at
273
K.
Below
266
K,
evapoconcentration
always
AT (aerosol) =
(3)
− (brine))
leads to aerosol alkalinitiesm(Br
below
15 mmol kg−1 used by
AT (brine)
Sander et al. (2006). Even taking into account the evapoconcentration
of in
alkalinity
in airborne
aerosols,
the situation
The first term
the numerator
describes
the seawater
ra+
modeled
et is
al.constant
(2006) seems
to as
be no
rather
consertio
of Br−by
/NaSander
which
as long
significant
vative in terms
of the
of aerosols
(aerosol
alkalinprecipitation
occurs
foralkalinity
either sodium
or bromide
(bromide
−1
on the
orderions
of 5 in
mmol
temperatures
isities
oneare
of rather
the most
soluble
sea kg
wateratduring
freezaround
255not
K, i.e.
one thirdinto
of the
alkalinity
by
ing,
it does
precipitate
any initial
crystalline
formused
within
Sander
et al. (2006)
their model
study).
the
temperature
rangeinconsidered
here
(Morin et al., 2008b)).
2.3
The second term is also constant when a Na+ molality of 5
− )/A ratio
−1
2.3 kgIntroducing
the m(Br
mol
is used. Thus
equation
(3)
T shows that the aerosol
alkalinity is inversely proportional to the ratio m(Br− )/AT .
Rather than computing AT in aerosol formed from brine as
a function of only the Na+ concentration in the brine, we
www.atmos-chem-phys.net/10/7655/2010/
AT aerosol
AT brine
6
30
5
25
4
20
Sander et al. initial
AT level in aerosols
3
15
2
10
1
5
0
250
AT /mmol kg-1
Na+ /mol kg-1
Na+ aerosol
Na+ brine
0
255
260
265
Temperature /K
270
275
Fig. 3. AT (black, dashed) and Na+ (red, solid) molalities in brine
(squares, “cryoconcentrated”) and in aerosols (circles, “evapoconFig. 3. AT (black, dashed) and Na+ (red, solid) molalities
in brine
centrated”) as a function of temperature. A Na+ molality of 5
(squares,
“cryoconcentrated”) and in aerosols (circles, “evapoconmol kg−1 was used. The “evapoconcentration” curve
for
A
T can be
centrated”)
as a function
of temperature.
A Sander
Na+ molality
of 5
directly
compared
to
the
A
value
chosen
by
et
al.
(2006)
T
−1
mol
kg
was
used.
The
“evapoconcentration”
curve
for
A
T can be
to initialize their model runs in the case where carbonate precipitadirectly
compared
to theis A
T value chosen by Sander et al. (2006)
tion is allowed,
which
presented
in blue. Vertical dashed arrows
toillustrate
initialize
model runs in thepathways.
case where carbonate precipitaATtheir
evapoconcentration
tion is allowed, which is presented in blue. Vertical dashed arrows
illustrate AT evapoconcentration pathways.
propose to also take into account the bromide concentration
in the brine. Thus, Eq. (2) can be rewritten as:
m(Br− (brine))
× m(Na+ (aerosol))
+
m(Na (brine))
AT (aerosol) =
(3)
−
10
m(Br (brine))
AT (brine)
No precip.
The first term in the numerator describes
the seawater raFREZCHEM calcite
−
+
tio of Br /Na which is constant as long
as
no significant
FREZCHEM
ikaite
8
et al. (bromide
precipitation
occurs for either sodium orSander
bromide
is one of the most soluble ions in sea water during freezing, it
6 does not precipitate into any crystalline form within
the temperature range considered here, Morin et al., 2008b).
The second term is also constant when a Na+ molality of
5 mol4kg−1 is used. Thus Eq. (3) shows that the aerosol alkalinity is inversely proportional to the ratio m(Br− )/AT .
It 2is clear that the acid-catalyzed bromine explosion can
only occur if the available acidity is larger than AT . However, since bromide and acidity are both needed for bromine
0
explosions,
it is the ratio m(Br− )/AT that determines how
250
255 be liberated
260 from265
270once a 275
much bromine can
the solution
cerTemperature /K
tain amount of alkalinity has been neutralized. It can be considered representative of the potential for triggering bromine
explosions. While cryoconcentration of the alkalinity is reFig.
4. The
of bromideprecipitation,
to alkalinity during
the freezing
of sea
duced
dueratio
to carbonate
the molality
of highly
water.
soluble Br− increases considerably during freezing (Morin
et al., 2008b). Note that this calculation would be similar
using soluble ions other than bromide. Our choice is motivated here by the relevance of the bromide ion to the bromine
explosion mechanism.
Br-/AT molar ratio
+
m(Na
(aerosol))
Long-term
observations by Treffeisen et al. (2007)
(1)
freached.
evap =
+
m(Na
(brine)) between RH=60% and RH=90%
found relative humidities
for near-surface air at Ny-Ålesund. Using a typical value of
The process of evapoconcentration is visualized in Fig. 3
RH=80%, a NaCl solution of m(Na+ (aerosol)) ≈ 5 mol/kg is
for different temperatures. Red squares denote the molalreached (Tang et al., 1997). According to Koop et al. (2000),
ity of Na+ in brine (after cryoconcentration), and red circles
the deliquescence/efflorescence curves of sea salt hardly deshow the molality in the aerosol (after evapoconcentration).
pend on temperature, suggesting that the water activity of sea
Since cryoconcentration brings the Na+ molality closer to
salt solutions of fixed molality also remains nearly constant
its final value of 5 mol/kg, fevap is smaller at lower temperawith changes in temperature. Thus, like Sander et al. (2006),
tures.
we assume a NaCl molality of 5 mol/kg in aerosol particles
Evapoconcentration increases the molality of all solutes
for RH=80% at all temperatures.
equally, thus the same factor fevap can be applied to the alThis vaporization leads to an increase of the concentrakalinity AT :
tions of all solutes (evapoconcentration). The evaporation
+ before and after evapfactor fevap describes the molality
ratio
m(Na
(aerosol))
(2)
Aoration:
T (aerosol) = AT (brine) ×
+
m(Na (brine))
Atmos. Chem. Phys., 10, 7655–7658, 2010
evapoctuation
conseralkalinratures
used by
7658
R. Sander and S. Morin: Follow-up discussion on Morin et al.
10
))
Br-/AT molar ratio
8
rine as
ne, we
ntration
The service charges for this open access publication
have been covered by the Max Planck Society.
6
Edited by: T. Koop
4
References
2
(3)
ater ranificant
romide
freezwithin
008b)).
ty of 5
aerosol
−
)/AT .
Acknowledgements. We would like to thank J. Burrows,
R. von Glasow, L. Kaleschke, T. Koop, G. Marion, D. Voisin, and
E. Wolff for very helpful suggestions and discussions.
No precip.
FREZCHEM calcite
FREZCHEM ikaite
Sander et al.
0
250
255
260
265
Temperature /K
270
275
Fig. 4. The ratio of bromide to alkalinity during the freezing of sea
water.
Fig.
4. The ratio of bromide to alkalinity during the freezing of sea
water.
Figure 4 shows how m(Br− )/AT increases with decreasing temperature for both, calcite and ikaite precipitation. In
the case where ikaite is set to precipitate in the FREZCHEM
run, this increase is lower than for calcite, and also starts
to increase at lower temperatures. Thus the temperature at
which the brine is separated from the precipitate is critical.
Nevertheless, the strong temperature dependence of this ratio makes this a possible explanation of the fact that ozone
depletion events are mainly found in cold regions. This
approach reconciles the results of Sander et al. (2006) and
Morin et al. (2008b). Also, observations are now available
showing that ikaite precipitation occurs in the Arctic (Dieckmann et al., 2010) as well as in the Antarctic (Dieckmann
et al., 2008). It should be noted though, that all results and
discussions presented here assume thermodynamic equilibrium. For future studies, the kinetics of precipitation and
dissolution should also be taken into account, a topic which
is mentioned in Marion et al. (2009).
3
Conclusions
A re-analysis of the FREZCHEM calculations by Morin
et al. (2008b) has shown that, although ikaite precipitation
is less efficiently removing carbonate than calcite precipitation, these results do not contradict the proposal of Sander
et al. (2006), as long as the interpretation of the results is
based on the m(Br− )/AT ratio and not just on the alkalinity
of the medium where bromine activation occurs.
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