Atmos. Chem. Phys., 9, 9237–9247, 2009
www.atmos-chem-phys.net/9/9237/2009/
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Modeling the transport of very short-lived substances into the
tropical upper troposphere and lower stratosphere
J. Aschmann1 , B.-M. Sinnhuber1 , E. L. Atlas2 , and S. M. Schauffler3
1 Institute
of Environmental Physics, University of Bremen, Bremen, Germany
School of Marine and Atmospheric Science, University of Miami, USA
3 National Center for Atmospheric Research, Boulder, Colorado, USA
2 Rosenstiel
Received: 14 August 2009 – Published in Atmos. Chem. Phys. Discuss.: 9 September 2009
Revised: 16 November 2009 – Accepted: 17 November 2009 – Published: 7 December 2009
Abstract. The transport of very short-lived substances into
the tropical upper troposphere and lower stratosphere is investigated by a three-dimensional chemical transport model
using archived convective updraft mass fluxes (or detrainment rates) from the European Centre for Medium-Range
Weather Forecast’s ERA-Interim reanalysis. Large-scale vertical velocities are calculated from diabatic heating rates.
With this approach we explicitly model the large scale subsidence in the tropical troposphere with convection taking
place in fast and isolated updraft events. The model calculations agree generally well with observations of bromoform
and methyl iodide from aircraft campaigns and with ozone
and water vapor from sonde and satellite observations. Using a simplified treatment of dehydration and bromine product gas washout we give a range of 1.6 to 3 ppt for the contribution of bromoform to stratospheric bromine, assuming
a uniform mixing ratio in the boundary layer of 1 ppt. We
show that the most effective region for VSLS transport into
the stratosphere is the West Pacific, accounting for about
55% of the bromine from bromoform transported into the
stratosphere under the supposition of a uniformly distributed
source.
1
Introduction
Recent studies have shown the importance of halogenated
very short-lived substances (VSLS) to stratospheric halogen loading and their contribution to ozone depletion (e.g.,
Correspondence to: J. Aschmann
([email protected])
WMO, 2007). In particular brominated VSLS are found to
contribute significantly to stratospheric ozone depletion (e.g.,
Sinnhuber et al., 2009). However, measurements of VSLS
are scarce and the estimations of abundance and source
strength still remain uncertain.
In this paper we focus mainly on brominated VSLS, particularly bromoform (CHBr3 ). According to the estimations
published in the WMO (2007) report it is the most abundant
VSL bromocarbon. The major part of this compound is emitted naturally from the ocean by maritime lifeforms such as
macroalgae, ice algae and phytoplankton groups (Quack and
Wallace, 2003). Average bromoform mixing ratios in the marine boundary layer (MBL) are estimated to be in the range
of 1 to 2 parts per trillion by volume (ppt) (WMO, 2007)
but Quack and Wallace (2003) noted that local mixing ratios
can be much higher, especially in tropical coastal regions or
in areas which show strong upwelling. The most important
sinks of bromoform are photolysis, which dominates at the
upper troposphere/lower stratosphere (UTLS), and reaction
with the hydroxyl radical at lower altitudes. The average lifetime of bromoform is about 26 days (WMO, 2007).
Various studies were performed to investigate how VSLS
are able to reach the stratosphere even though their lifetimes are relatively short in comparison to typical atmospheric transport times (e.g., Nielsen and Douglass, 2001;
Warwick et al., 2006; Sinnhuber and Folkins, 2006; Kerkweg
et al., 2008; Gettelman et al., 2009). It is generally assumed
that the most important pathway of air entering the stratosphere is in the tropics through the tropical tropopause layer
(TTL) via convection (e.g., Sinnhuber and Folkins, 2006).
Here, the level of zero clear sky radiative heating (at an altitude of about 15.5 km) marks the transition from large-scale
subsidence to large-scale ascent, as diagnosed from diabatic
Published by Copernicus Publications on behalf of the European Geosciences Union.
J.
J. Aschmann
Aschmann et
et al.:
al.: Transport
Transport of
of very
very short-lived
short-lived substances
substances
organic bromine (Bry ) is actually HBr and HOBr, which are
both highly soluble.
There have already been several modeling studies of
VSLS.
400For example, Nielsen and Douglass (2001) supposed
a uniformly distributed bromoform emission over the oceans
of approximately 500 Gg/yr which corresponds to a surface
390
concentration
between 1 and 2 ppt in their three-dimensional
(3-D) model. They concluded that roughly one third of Bry
in the UTLS is due to the destruction of CHBr3 . Warwick et
380 conducted 3-D model runs with various emission
al. (2006)
scenarios for bromoform and compared them with observational data. According to their results a scenario which con370 bromoform emissions of the 500 Gg/yr suggested
centrates
by Nielsen and Douglass (2001) in the tropical oceans is able
to reproduce the observed data thus strengthening the impor360
tance of the tropical oceans as source regions for bromine
VSLS. Sinnhuber and Folkins (2006) used a 1-D model to
give350
estimations about the amount of bromine reaching the
ECMWF detrainment
stratosphere taking into account a convective
rate that was calculated by balancingDessler(2002)
the diabatic CO
heating
Dessler(2002) O3
rate 340
and washout of Bry due to TTL dehydration. They con50
100
strained0the contribution
of bromoform
to 150
stratospheric200
Bry
convective turnover time [days]
to 0.5 to 3 ppt for an assumed ground level mixing ratio of
1 ppt. More recently, Kerkweg et al. (2008), Gettelman et
and Hossaini
et al.averaged
(2009) convective
further investigated
the
al. (2009)
Fig.
03. Comparison
of tropical
turnover times
transport
ofthe
VSLS
withdetrainment
detailed chemistry
convective
derived
from
ECMWF
rate (solid) and
and those
calcuparametrizations
(seefrom
Sect.CO
4).(dashed, circles) and O3 (dashed,
lated
by Dessler (2002)
triangles)
In our measurements.
model experiments we use a 3-D chemical transport
model (CTM) to simulate the transport of bromine VSLS
into the stratosphere. The novelty of our approach is to use
an isentropic model with a parameterization of deep convective transport. In contrast to the fast and localized convective
mass flux we calculate the slow large-scale vertical motion
from diabatic heating rates. With this tool we have studied
the effect of dehydration on VSL tracers and investigated the
relative
importance of different source regions using several
80oN
o
emission
scenarios.
60 N
θ [K]
8
9238
01.Schematic
Schematic
relevant
transport
processes
in the
TTL.
BeFig. 1.
ofof
relevant
transport
processes
in the
TTL.
Beside
sidelarge-scale
the large-scale
vertical
transport
is by
driven
by diabatic
the
vertical
transport
which which
is driven
diabatic
heating
heating
and localized
convective
rates
ωr rates
thereωisr athere
fast is
anda fast
localized
convective
mass fluxmass
ωc . flux
The
ωc . Theof
amount
of airthe
leaving
the convective
by the
amount
air leaving
convective
cloud is cloud
given is
bygiven
the convecconvective
detrainment
. Note
airdetraining
parcels detraintive
detrainment
rate dc .rate
Notedcthat
onlythat
air only
parcels
above
ing level
aboveofthe
level
of zeroheating,
radiative
thebetween
transition
between
the
zero
radiative
theheating,
transition
large-scale
large-scale to
subsidence
to ascent,
large-scale
ascent,
are able
to reach the
subsidence
large-scale
are able
to reach
the stratosphere.
stratosphere.
At 17temperature
km the TTLprofile
temperature
reaches
a minAt
17 km the TTL
reachesprofile
a minimum
(see
also
imum
Fig.traversing
07). Air
parcels
traversing
altitude
Fig.
7).(see
Air also
parcels
this
altitude
are likelythis
to be
subjectare
to
likely to be subject to dehydration.
dehydration.
Heating rate [K/day]
Altitude [km]
heating rates (e.g., Corti et al., 2005) and thus representing
−1
0
1
2
3
a natural border for air parcels (see Fig. 1). In general, air
masses have to be lifted by deep convection above this border to leave the troposphere, which is considered to be the
20
common pathway into the stratosphere. Since air lifted by
convective clouds is able to rise several kilometers within a
18
few hours
this transport mechanism is fast enough for VSLS
cold point
whose life times are rather in the order of weeks.
Another
important process to be considered here is the de16
hydration of the TTL. At an altitude of about 17 km the averlvl. of zeroaradiative
heating(cold point). Air
age temperature profile reaches
minimum
14
masses entering the stratosphere must be dehydrated to mixing ratios that correspond roughly to the saturation mixing
ratio12
at this level. This process of dehydration is not fully
understood and is still subject to current discussions. Studies indicate (e.g., Holton and Gettelman, 2001; Fueglistaler
102005) that during the relatively slow ascent through
et al.,
the TTL it is highly probable that air masses are transported
horizontally
into the coldest areas of the tropical tropopause
8
0 residing
0.02 over
0.04
0.06 Continent
0.08
0.1
0.12
typically
the Maritime
and Western
Detrainment rate [1/day]
Pacific regions. The
so called cold trap hypothesis states
that a major parts of the whole TTL air gets dehydrated in
these particular areas. Along with the dehydration of air this
Fig. 02. Zonally averaged profiles of ECMWF ERA-Interim deimplies
specificand
amount
of heating
solublerate
species
residing
in
trainmentthat
ratea(dashed)
diabatic
(dotted)
between
◦ TTL could
◦
the
eventually
be
washed
out
by
falling
ice.
This
20 N and 20 S. These profiles are temporally averaged from 2000
process
to 2005. is important for modeling bromine compounds since
model calculations indicate that a large fraction of total inAtmos. Chem. Phys., 9, 9237–9247, 2009
40oN
o
20 N
2
o
0Model
description
20oS
o
40 SThree-dimensional chemical transport model
2.1
60oS
o
80 S
The
model used here iso an updated
version
of our
CTMo de180oW 120oW
60 W
0o
60oE
120oE
180 W
scribed by Sinnhuber et al. (2003). It is forced by temperatures and horizontal windfields
from
TT5 VMR
[ppt] ERA-Interim reanalysis
of the European Centre for Medium-Range Weather Fore0.075 0.1 0.125 0.15 0.175 0.2 0.225 0.25 0.275 0.3 0.325
cast (ECMWF).
The spatial resolution of our runs were 2.5◦
◦
lat.×3.75 lon. with a vertical resolution of 29 isentropic levels
330 and
2700
K (about
10Ito
55 km). We used a
Fig.between
04. Snapshot
of TT5
(idealized
CH
3 tracer) distribution at
timestep
of
15
min
for
all
model
runs.
380 K (about 17 km altitude in the tropics) on 1 August 2003
Transport on isentropes is calculated from the meteorological wind fields while the large scale vertical motion is derived from diabatic heating rates. In contrast
to the before-mentioned earlier CTM version which used
www.atmos-chem-phys.net/9/9237/2009/
large-scale subsidence to large-scale ascent, are able to reach the
stratosphere. At 17 km the TTL temperature profile reaches a minimum (see also Fig. 07). Air parcels traversing this altitude are
likely
to be subject
to dehydration.
J. Aschmann
et al.:
Transport of very short-lived substances
−1
Heating rate [K/day]
0
1
y
2
3
20
18
Altitude [km]
cold point
16
lvl. of zero radiative heating
14
12
10
8
0
0.02
0.04
0.06
0.08
Detrainment rate [1/day]
Fig. 03. Comparison of tropical averaged convective turnover times
derived from the ECMWF detrainment rate (solid) and those9239
calculated by Dessler (2002) from CO (dashed, circles) and O3 (dashed,
triangles)(Br
measurements.
bromine
) which is assumed to be the degradation prod-
0.1
0.12
uct of bromoform (ref. to Sect. 2.4). The ozone tracer is calculated by a linearized ozone chemistry which is an updated
version of LINOZ (McLinden et al., 2000); treatment of water vapor is described in detail in Sect. 2.3.
We have conducted three modeling experiments with an
integration time of 6 years (2000 to 2005) each. The first
experiment concentrated on comparison between model calculations
and observational data for specific tracers in order
80oN
o
N our approach (Sect. 3.1). As next step we used our
to 60
test
40oN to study the effect of dehydration on the transport
model
o
N
of 20
short-lived
tracers and their soluble byproducts into the
0o
stratosphere
(Sect.
3.2). Since convection plays a keyrole
o
S
in 20
these
processes,
we
also conducted a sensitivity run with
o
S
the40same
setup
but
with
convection switched off above 380 K
60oS
(for ofurther details on our convective transport please refer
80 S
o
2.2).120
Finally,
we
applied0othe mechanisms
to Sect.
180oW
W
60oW
60oE
120ointroduced
E
180oW
in the second experiment to several emission scenarios to inTT5 VMR of
[ppt]specific source regions
vestigate the relative importance
(Sect. 3.3).
0.075 0.1 0.125 0.15 0.175 0.2 0.225 0.25 0.275 0.3 0.325
2.2
Fig. 02.
2. Zonally
averaged
profiles
of ECMWF
ERA-Interim
detrainFig.
Zonally
averaged
profiles
of ECMWF
ERA-Interim
dement rate (dashed)
and diabatic
heating
rate (dotted)
between
20◦ N
trainment
rate (dashed)
and diabatic
heating
rate (dotted)
between
◦
◦ S. These
and
profiles
are temporally
averaged
from
2000
to
20
N20
and
20◦ S. These
profiles
are temporally
averaged
from
2000
2005.
to 2005.
interactive heating rate calculation from an ozone tracer, the
current version relies on the ECMWF data for tendency of
clear sky long and short wave radiation. They are found
to be more accurate in the tropopause region and are (at
least in principle) consistent with the large-scale wind fields
and convective mass fluxes. However, the ERA-Interim radiative heating rates lack a feedback to interactive ozone
(Fueglistaler et al., 2009). Figure 2 shows a zonally averaged profile of the heating rate in tropical latitudes (20◦ S
to 20◦ N). Fueglistaler et al. (2009) found in the diabatic heat
budget of ERA-Interim a residual diabatic cooling in the lowermost stratosphere over the Maritime Continent, most likely
due to the model’s turbulent mixing parameterization. It remains to be shown if this is a real feature or an artefact and
whether or not this is related to cooling by overshooting deep
convection. This diabatic tendency is not considered in our
CTM calculations. In addition to the relatively slow vertical heating rate-driven advection we have also included a
fast convective transport mechanism in our model that will
be further discussed in Sect. 2.2.
The tracers which are modeled are ozone, water vapor and
two idealized tracers with different chemical lifetimes (5 and
20 days) to resemble typical VSL halogen substances such
as methyl iodide (CH3 I) and bromoform (CHBr3 ) which will
be further referred to as test tracer 5 and 20 (TT5 and TT20),
respectively. Furthermore, there are two tracers for inorganic
www.atmos-chem-phys.net/9/9237/2009/
Convection
Fig. 04. Snapshot of TT5 (idealized CH3 I tracer) distribution at
380 K (about 17 km altitude in the tropics) on 1 August 2003
As a novel feature of our model we have included a parameterization of convective transport. As noted above, isentropic
coordinates have significant conceptual and practical advantages for modeling the large-scale transport in the UTLS.
However, because it is difficult to extend a purely isentropic
model into the lower troposphere and down to Earth’s surface, so far isentropic models either did not include any parameterization of convection at all, or have used a hybrid
coordinate where isentropic surfaces are blended with terrain following surfaces in the lower atmosphere (e.g., Rasch
et al., 1997; Chipperfield, 2006). Instead of using an interactive convective parameterization, we have parameterized
convective transport by using archived convective updraft
mass fluxes from the ERA-Interim reanalysis. This approach
has the additional advantage that the convective mass fluxes
are calculated during the assimilation process in a consistent way. In contrast, when using a convective parameterization inside a CTM, there is always the potential problem that
the driving large-scale temperature and pressure fields from
the meteorological analysis have already been stabilized by a
convective parameterization within the assimilation.
Here we have used the 6-hourly updraft convective mass
fluxes from ERA-Interim, or more specifically the updraft
detrainment rates, which are the vertical divergence of the
updraft convective mass fluxes. An example is shown in
Fig. 2. The detrainment rate dc gives the convective flux of
mass into a grid box per time (unit 1/s). It is derived from
the original ECMWF updraft detrainment rate by integration
over the model grid box divided by the air density inside the
box. Convective injection of tracers is then modeled by specifying a detrainment tracer mixing ratio [X]c that multiplied
Atmos. Chem. Phys., 9, 9237–9247, 2009
J. Aschmann et al.: Transport of very short-lived substances
9240
J. Aschmann et al.: Transport of very short-lived substances
derived from ECMWF dc with calculations made by Dessler
(2002) based on CO and O3 observations show excellent
agreement up to 380 K (Fig. 3). To assess the influence of
detrainment above the coldpoint on our model calculations
we have conducted a sensitivity run where we cut off the
convective transport above 380 K (Sect. 3.2).
400
390
380
θ [K]
2.3
370
360
TL. Bediabatic
ass flux
by the
detrainetween
ach the
a minude are
350
340
0
ECMWF
Dessler(2002) CO
Dessler(2002) O3
50
100
150
convective turnover time [days]
200
Fig. 03.
3. Comparison
Fig.
Comparison of
of tropical
tropical averaged
averaged convective
convective turnover
turnover times
times
derived from
from the
the ECMWF
ECMWF detrainment rate (solid) and those calcuderived
Dessler (2002)
(2002) from
from CO
CO (dashed, circles) and O33 (dashed,
lated by
by Dessler
lated
triangles) measurements.
measurements.
triangles)
3
with the detrainment rate dc and the total grid box mass m
gives the convective tracer flux fX,c into the grid box:
fX,c =dc · m · [X]c
0.12
rim deetween
m 2000
(1)
We assume that the detrainment mixing ratio [X]c is simio
N the tracer mixing ratio in the planetary boundary layer,
lar80to
60oin
N general entrainment of mid tropospheric air during
but
40oN
convective
updrafts may also influence the detrainment mixo
20 ratio.
N
ing
0o
With
this model approach we are able to simulate the slow
20oS
large-scale
transport constrained by diabatic heating rates
o
40 S
with
fast
updrafts
taking place quasi instantaneously as a sub60oS
grid
scale
process.
However, this approach is not strictly
o
80 S
o
mass
180conserving,
W 120oW which
60oWis the0ocase also
60oEin other
120oE isentropic
180oW
CTMs even without convective transport. To force mass conTT5 VMR [ppt]
servation the total mass inside
a grid box after advection and
convection
steps
is
set
to
the
calculated
mass
from
large0.075 0.1 0.125 0.15 0.175 0.2 0.225 0.25
0.275
0.3 the
0.325
scale meteorological analysis. Tracer masses are scaled proportionally to keep the calculated tracer mixing ratios constant.
Fig.
04. Snapshot of TT5 (idealized CH3 I tracer) distribution at
380Note
K (about
in the tropics)
on 1tropics
August above
2003 the
that17dckm
is altitude
still significant
in the
level of zero radiative heating and even above the coldpoint
at 380 K as it can be seen in Fig. 2. With our approach of convective transport this means a considerable amount of air is
detrained at relatively high altitudes and it is unclear, whether
this frequent high reaching transport is a realistic representation. However, comparisons of the convective turnover time
Atmos. Chem. Phys., 9, 9237–9247, 2009
Treatment of water vapor
Water vapor plays an important role in our calculations because we need an adequate representation of atmospheric humidity to model possible loss of soluble Bry due to washout
(Sect. 2.4). Furthermore its distribution in the atmosphere
is another convenient means to validate the model assumptions since there are plenty of measurements of this quantity
that can be considered. The treatment of the H2 O tracer is
based on three simplistic assumptions: (1) For the convective transport, the source term [H2 O]c in Eq. (1) is set to the
saturation mixing ratio of water vapor over ice of the particular model box, following Dessler and Sherwood (2004).
This corresponds to the assumption, that air detraining from a
convective cloud is fully saturated. The saturation mixing ratio is calculated after the formula of Marti and Mauersberger
(1993). (2) Likewise, if the relative humidity of a box exceeds 100%, all surplus water is removed immediately thus
modeling precipitation as instantaneous fallout of ice particles. (3) Finally there is a second water source in the stratosphere implemented by a simple methane oxidation scheme
as described in the ECMWF Integrated Forecast System documentation (IFS Documentation, 2007). The basic supposition is, that the sum of methane and water is constant in the
stratosphere and can be approximated by:
2[CH4 ]+[H2 O]=6.8 ppm
(2)
Therefore the production of water via methane oxidation
can be expressed as:
d [H2 O] (6.8 ppm−[H2 O])
=
dt
τ
(3)
Here τ denotes a pressure dependent chemical lifetime of
methane which is set to 100 days at 50 pa pressure and increases roughly linearly to ≈6000 days at 10000 pa. Above
this mark the lifetime is set to infinity, i.e. the H2 O production due to methane oxidation is zero at lower altitudes.
2.4
Bry production and loss processes
As for VSL bromine substances we focus mainly on bromoform (CHBr3 ) and its role in taking bromine to the stratosphere, since it is one of the most abundant VSL bromine
species. Stratospheric bromine loading due to bromoform
can be divided into processes called source or product gas
injection (WMO, 2007). Either the bromine atoms enter the
stratosphere directly as bromoform or as its inorganic degradation products, summarized here as Bry . The distinction
www.atmos-chem-phys.net/9/9237/2009/
20
J. Aschmann et al.: Transport of very short-lived substances
9241
18
cold point
Altitude [km]
between source and product gases is important, because unlike
16 CHBr3 itself a major part of its degradation products is
highly soluble, in particular HBr and HOBr. Therefore prezero radiative heating
cipitation may removelvl.a ofsignificant
amount of bromine from
14
the atmosphere.
To investigate these processes we use a simplified approach,
however one that is more physically motivated than
12
the use of a fixed washout lifetime used in most of the previous modeling studies. Bromoform is approximated by an
10
idealized
tracer (TT20) which has a constant chemical lifetime of 20 days and a constant detrainment mixing ratio of
[TT20]c =1 ppt. At each timestep, the amount of TT20 is re8
duced
the assumed
represents
0 according
0.02 to0.04
0.06lifetime
0.08which0.1
0.12the
Detrainment
rate [1/day]
loss of CHBr3 caused
by photolysis
and oxidation. For each
unit of TT20 destroyed there are three units of Bry produced
in the same grid box thus ignoring the detailed chemistry of
Fig.bromoform’s
02. Zonally averaged
profiles
of ECMWF
degradation
products,
which isERA-Interim
according todethe
trainment
rate
(dashed)
and
diabatic
heating
rate
(dotted)
between
Hossaini
et
al.
(2009)
a
valid
approximation.
recent
study
of
◦
20◦ N
and 20
S. other
These source
profilesof
areBrtemporally
averaged from 2000
There
is no
y in our model. The only sink
to 2005.
of Bry is supposed to be through washout by falling rain or
ice. To avoid the complexity of cloud and aerosol microphysics we investigate the two extremal cases using two different Bry tracers: one is not affected by washout at all, the
other is removed completely if dehydration occurs in the particular model grid box. This will give an estimation of the
upper and lower limit of stratospheric bromine loading due
to bromoform.
3
Results
3.1
Comparison with observational data
In order to test whether our new approach in modeling convective and large-scale vertical transport produces realistic
results we compared the model output with different observational data. In this particular experiment we used the following tracers: Ozone, water vapor and two idealized VSL
tracers.
3.1.1
Very short-lived source gases
The model tracers TT5 and TT20 are idealized VSL tracers that resemble CH3 I and CHBr3 regarding to their chemical lifetime, which were assumed to be 5 and 20 days, respectively. Due to their short lifetime the distribution of
these tracers in the UTLS is mainly localized at distinct areas where significant convective activity occurs. However,
even for very short-lived tracers horizontal transport can be
very important in the TTL. Figure 4 shows an example of the
distribution of the idealized methyl iodide VSL tracer (TT5).
Profiles of TT5 and TT20 are presented in Fig. 5 together
with observational data of CH3 I and CHBr3 from NASA
WB57 high-altitude aircraft measurements during two Aura
Validation Experiment (AVE) campaigns. Both campaigns,
Pre-AVE and Cr-AVE, were performed around 80–90◦ W at
www.atmos-chem-phys.net/9/9237/2009/
80oN
60oN
40oN
o
20 N
0o
20oS
o
40 S
60oS
o
80 S
180oW
120oW
60oW
0o
60oE
120oE
180oW
TT5 VMR [ppt]
0.075 0.1 0.125 0.15 0.175 0.2 0.225 0.25 0.275 0.3 0.325
Fig.04.
4. Snapshot
Snapshot of
of TT5
TT5 (idealized
(idealized CH
CH33II tracer)
tracer) distribution
Fig.
distributionatat
380KK(about
(about17
17km
km altitude
altitude in
380
in the
the tropics)
tropics) on
on11August
August2003
2003
latitudes between 20◦ N and 3◦ S in January/February 2004
and 2006, respectively. The profiles show the mean, the
range between minimum and maximum values and the standard deviation over all measurements at the particular altitude. Likewise, the model data from January/February 2004
was averaged over the same area allowing a direct comparison to the Pre-AVE campaign. There is in general good
agreement between model calculations of the 5 and 20 day
tracers and the Pre-AVE observations of CH3 I and CHBr3 if
we assume a uniform detrainment mixing ratio of 0.3 ppt and
0.75 ppt, respectively. There is in general also good agreement with the 2006 Cr-AVE observations, although there are
also notable differences. These may either reflect real interannual changes or could (in particular for the CH3 I measurements during Cr-AVE) indicate a small high bias in the
observations due to a possible residual artifact observed in
new canisters that were used during Cr-AVE.
3.1.2
Ozone
Ozone, albeit itself not our primary subject in this paper, is quite suitable to test whether our modeling approach works properly, because there are a lot of observational data available for comparison. In Fig. 6 we compare modeled ozone data with sonde measurements from the
Southern Hemisphere Additional Ozonesondes (SHADOZ)
project (Thompson et al., 2003). For comparison, we picked
measurements from two stations, Watukosek, Java (7.5◦ S,
112.6◦ E) and San Cristóbal Island, Galapagos (0.9◦ S,
89.6◦ W) and the corresponding model grid boxes. We kept
the detrainment mixing ratios [O3 ]c constant for all locations at 20 ppb. Even with this relatively simple treatment
of ozone, the model reproduces generally well the observed
profiles. The difference in convective activity and therefore
the different ozone profiles of the two stations are well reproduced by the model. However, especially the modeled profile over the Java region show too low ozone mixing ratios in
Atmos. Chem. Phys., 9, 9237–9247, 2009
9242
J. Aschmann et al.: Transport of very short-lived substances
10
J. Aschmann et al.: Transport of very short-lived substances
9
35
20
8
Model 0.75 ppt
Pre−AVE WB57
Cr−AVE WB57
a)
18
6
0
16
0.2
0.4
0.6
0.8
CHBr3 VMR [ppt]
1
30
1.2
25
Altitude [km]
20
14
Model 0.3 ppt
Pre−AVE WB57
Cr−AVE WB57
18
12
Altitude [km]
Altitud
12
14
12
15
35
8
10
a)
6
0
30
0.2
10
20
0.4
0.6
0.8
CHBr3 VMR [ppt]
1
1.2
5 −8
10
25
Model 0.3 ppt
Pre−AVE WB57
Cr−AVE WB57
8
18
b)
Altitude [km]
6
016
0.1
0.2
CH I VMR [ppt]
0.3
0.4
3
14
Fig. 05. Comparison between modeled CHBr3 (a) and CH3 I (b)
12
tracers (TT20 and TT5, resp.) and aircraft measurements from PreAVE and Cr-AVE campaign. The observations were performed
at about1080-90◦ W, 20◦ N - 3◦ S during January/February 2004 and
2006, respectively. Both measurements and model data were averaged over this area. Please note that for this comparison we used
8
only model
data from January/February 2004 therefore the modeled
profile fits b)
especially well to the Pre-AVE campaign observations.
The red 6curves mark modeled profiles while for the observations
0
0.1 of the individual
0.2
0.3
0.4 a
symbols denote
the mean
measurements
within
CH
I
VMR
[ppt]
1 km altitude range. Thick and3 thin horizontal lines represent one
standard deviation and the range between minimum and maximum
values of the measurements and the model data, respectively. The
detrainment
mixing ratio
of TT20
was CHBr
assumed
0.75
and
Fig.5.05.
Comparison
between
modeled
CHBr
(a)beand
(b)
3to
3 I tracFig.
Comparison
between
modeled
and
CH
I ppt
(b)
3 (a)
3CH
0.3
ppt
for(TT20
TT5.
tracers
and TT5,
and aircraft
measurements
Preers
(TT20
and TT5,
resp.)resp.)
and aircraft
measurements
fromfrom
Pre-AVE
AVE
and Cr-AVE
campaign.
The observations
were performed
and
Cr-AVE
campaign.
The observations
were performed
at about
◦
◦
◦
◦
◦
◦
at
about
80-90
W,
20
N
3
S
during
January/February
80–90 W, 20 N–3 S during January/February 2004 and2004
2006,and
re2006, respectively.
Both measurements
model
were over
avspectively.
Both measurements
and modeland
data
were data
averaged
eraged
this note
area. that
Please
for this comparison
wemodel
used
this
area.over
Please
for note
this that
comparison
we used only
only
model
data
from
January/February
2004
therefore
the
modeled
data from January/February 2004 therefore the modeled profile fits
profile fitswell
especially
to the campaign
Pre-AVE campaign
observations.
especially
to the well
Pre-AVE
observations.
The red
The red
curves
mark profiles
modeledwhile
profiles
for the observations
curves
mark
modeled
forwhile
the observations
symbols
symbols denote the mean of the individual measurements within a
denote the mean of the individual measurements within a 1 km al1 km altitude range. Thick and thin horizontal lines represent one
titude range. Thick and thin horizontal lines represent one standard
standard deviation and the range between minimum and maximum
deviation and the range between minimum and maximum values of
values of the measurements and the model data, respectively. The
the measurements and the model data, respectively. The detraindetrainment mixing ratio of TT20 was assumed to be 0.75 ppt and
ment
mixing ratio of TT20 was assumed to be 0.75 ppt and 0.3 ppt
0.3 ppt for TT5.
for TT5.
Atmos. Chem. Phys., 9, 9237–9247, 2009
−7
10
−6
−5
10
O VMR
10
3
Altitude [km]
Altitude [km]
16
10
20
Fig.2006.
6. Comparison
Fig.
Comparison between
between modeled
modeled ozone
ozone profiles
profiles (red)
(red) and
and
SHADOZ measurements
measurements (blue).
(blue). Shown
Shown are
are yearly
SHADOZ
yearly averages
averages from
from
2004. The
The solid
solid and
and dashed
dashed lines
lines mark
mark profiles
profiles from
2004.
from Watukosek,
Watukosek,
◦◦ S, 112.6◦◦ E) and San Cristóbal Island, Galapagos (0.9◦◦S,
15
Java
(7.5
Java (7.5 S, 112.6 E) and San Cristóbal Island, Galapagos (0.9 S,
89.6◦◦W),
W),respectively.
respectively.
89.6
10
the TTL. This could indicate an overestimation of convective
transport in our model but may also be a consequence of too
little5 chemical
ozone production.
−8
−7
−6
−5
10
3.1.3
10
Water vapor
O VMR
10
10
3
Our06.modeled
waterbetween
vapor modeled
agrees generally
well(red)
withand
meaFig.
Comparison
ozone profiles
SHADOZ
measurements
Shown
are yearlyinaverages
from In
surements
despite the(blue).
simplistic
treatment
our model.
2004.
The solid and
linesprofile
mark profiles
from humidity
Watukosek,
the troposphere
thedashed
averaged
of relative
with
◦
◦
◦
Java
(7.5
S,
112.6
E)
and
San
Cristóbal
Island,
Galapagos
S,
respect
to ice (RH) is in qualitative agreement with(0.9
observa◦
89.6 W), respectively.
tions (e.g., Folkins et al., 2002), in particular the decrease of
RH with decreasing altitude in the lower TTL (Fig. 7). This
decrease in RH results from the large-scale subsidence and
associated adiabatic warming of air that was initially saturated at convective detrainment. The successful representation of this effect is thus only possible with the separation
of slow large-scale descent and fast isolated convective updrafts in the model. In the stratosphere the modeled water
vapor shows a clear tape recorder signal in the stratosphere
(Fig. 8). Comparison with HALOE measurements (Grooß
et al., 2005) show in general excellent agreement in the dry
phases and also the speed of the tape recorder signal is well
reproduced (Fig. 9). But these comparisons reveal also that
in our model too much water (on average by about 1 ppm) is
transported into the tropics at altitudes above 17 km during
Northern Hemisphere (NH) summer. In our calculations the
www.atmos-chem-phys.net/9/9237/2009/
10
J. Aschmann et al.: Transport of very short-lived substances
Altit
Fig. 07. Zonally averaged profiles of modelled relative humidity of
water vapor with respect to ice (dashed) and temperature (dotted)
between 20◦ N and 20◦ S. These profiles are temporally averaged
J. Aschmann
et al.: Transport of very short-lived substances
from
2002 to 2005.
9243
20
18
190
200
Temperature [K]
210
220
30
230
16
240
28
24
14
2000
26
22
24
Altitude [km]
Altitude [km]
20
18
cold point
22
Fig. 09
recorde
HALOE
20
16
18
lvl. of zero radiative heating
30
14
16
28
12
10
20
30
40
50
Relative humidity [%]
60
2001
2002
Fig.07.
7. Zonally
Fig.
Zonally averaged
averaged profiles
profiles of
of modelled
modelled relative
relative humidity
humidity of
of
water vapor
vapor with
with respect
respect to
to ice (dashed) and temperature (dotted)
water
(dotted)
between 20
20◦◦NN and
and 20
20◦◦S.S. These
These profiles
profiles are temporally averaged
between
averaged
from2002
2002 to
to 2005.
2005.
from
2003
Date
2004
2005
H O VMR [ppm]
70
2
24
Altitude [km]
10
0
14
2000
26
2
2.4
2.8
3.2
3.6
4
4.4
4.8
5.2
5.6
6
22
Fig. 08.
8. Modeled
Fig.
Modeled distribution
distribution of
of tropical
tropical water
water vapor
vapor mixing ratio
(“tape
recorder”)temporally
temporallyaveraged
averagedfrom
from2000
2000 to
to 2005.
2005.
(‘tape
20recorder’)
18
Altitude [km]
30
“moist
head” of the tape recorder is located at about 18 km
which is not supported by the HALOE observations. How28 other measurements do show high water vapor mixing
ever,
ratios at 17 km and above in NH summer above mid-latitudes
(Dessler
and Sherwood, 2004; Hanisco et al., 2007). It is not
26
clear if the discrepancy in water vapor between our model
and24HALOE observations are a result of too much convective
transport above the cold point or whether our assumption that
air detrains with 100% RH overestimates the flux of water
22
into the stratosphere. Moreover, a more realistic treatment of
dehydration (e.g., Gettelman et al., 2002a; Read et al., 2008)
20 lead to an improved water vapor representation.
may
3.218 The effect of dehydration on soluble gases
In 16
the second experiment we introduced a pseudo chemistry scheme for our idealized bromoform tracer as stated in
Sect. 2.4. Since the product gas tracer Bry was modeled to
14
2001
2002plays an
2003
2004 role in
2005
be 2000
soluble, dehydration
important
the quesDate
tion which amount of bromine compounds reaches the stratoH O VMR [ppm]
2
sphere.
7
shows
profiles
of
modeled
relative
humidity
(RH)
Figure
2
2.4
2.8
3.2
3.6
4
4.4
4.8
5.2
5.6
6
and temperature averaged zonally over 20◦ N and 20◦ S.
Mean RH peaks with 60% at around 16 km which is slightly
lower
the important
level
minimum
temperature,
the
Fig.
08.atModeled
distribution
of of
tropical
water vapor
mixing ratio
cold
point,
at
around
17
km.
However,
local
areas
show
much
(‘tape recorder’) temporally averaged from 2000 to 2005.
www.atmos-chem-phys.net/9/9237/2009/
higher values for RH, especially during NH summer over
the Indian Monsoon and West-Pacific regions. Our model
16
suggests
that a significant part of TTL dehydration occurs at
these areas which is on the line with the results of Gettelman
et al.14
(2002a)2001
and Fueglistaler
et2003
al. (2005).
2000
2002
2004
2005
Under these circumstances, Date
our model suggests an additional bromine loading dueH2to
an[ppm]
idealized VSL tracer in
O VMR
the range of 1.6 to 3 ppt as shown in Fig. 10. Here we as2.4
2.8
3.2
3.6 mixing
4
4.4
5.2 source
5.6
6
sume a 2constant
detrainment
ratio4.8of the
gas
TT20 (CHBr3 ) of 1 ppt. Due to the fact that our soluble
Bry tracer is subject to dehydration and is therefore directly
Fig. 09. Distribution of tropical water vapor mixing ratio (‘tape
linked with the water vapor tracer we see also a tape recorder
recorder’) temporally averaged from 2000 to 2005 measured by
signal
in its stratospheric mixing ratio (Fig. 11). ConsiderHALOE (Grooß et al., 2005).
ing the differences of the tape recorder phases and comparing the TT20 and soluble Bry profiles we estimate that about
1.2 to 1.3 ppt of the soluble Bry is formed above the coldpoint (about 17 km or 380 K) and is thus not significantly
affected by dehydration. The remaining 0.3 to 0.4 ppt were
most likely produced at lower altitudes and are consequently
subject to periodical dehydration as seen in the tape recorder
plot. Note that these results are not dependent on significant convection above the coldpoint, since our sensitivity run
with no convective transport above 380 K produces virtually
identical results (Fig. 10). For the estimations we have made
above this means that a significant amount of source gas
reaches the stratosphere via the slow large-scale upward motion, even if we don’t consider deep convection above 380 K.
Atmos. Chem. Phys., 9, 9237–9247, 2009
9244
J. Aschmann et al.: Transport of very short-lived substances
30
26
28
24
26
22
Altitude [km]
t
24
22
20
18
70
Altitude [km]
idity of
(dotted)
veraged
Altitude [km]
24
22
20
16
2000
20
18
16
Fig. 01
rally av
soluble
18
14
16
CHBr3
Bry insoluble
Bry soluble
12
14
2000
2001
2002
2003
Date
2004
2005
10
0
H O VMR [ppm]
2
2
2.4
2.8
3.2
3.6
4
4.4
4.8
5.2
5.6
2
VMR [ppt]
3
4
6
9. Distribution
Fig. 09.
Distribution of
of tropical
tropical water
water vapor
vapor mixing
mixing ratio
ratio (“tape
(‘tape
recorder”) temporally
temporally averaged
averaged from
from 2000 to 2005 measured by
recorder’)
HALOE (Grooß et al., 2005).
3.3
1
Transport efficiency for different source regions
Fig. 010.
10. Averaged
Fig.
Averaged tropical
tropical profiles
profiles of
of idealized
idealized bromoform
bromoform (dots)
(dots)
and insoluble/soluble
insoluble/soluble Br
Bryy tracers (upward/downward facing
and
facing triantriangle,resp.).
resp.). The
Thetwo
two Br
Bry tracers constrain the stratospheric bromine
gle,
bromine
loading to
to 1.6
1.6 to
to 33 ppt given the assumption that the
loading
the source
source gas
gas
CHBr3 has a ground-level mixing ratio of 1 ppt.
CHBr
ppt. The
The dashed
dashed proprofilesare
are derived
derived from
from aa sensitivity
sensitivity calculation where the convective
files
convective
detrainment above
above 380
380 K (about 17 km)
detrainment
km) was
was switched
switched off.
off.
o
80
o
60
o
40
o
20
0
o
20
o
40
o
60
o
6
ng ratio
In a third experiment we applied the approach of CHBr3
degradation into Bry product gas (ref. Sect. 2.4) to different emission scenarios to investigate their relative importance
for stratospheric bromine loading. As shown in Fig. 12 we
introduced nine source areas which roughly correspond to
Earth’s oceans and continents. Each region is associated with
a group of three tracers: one source gas tracer (TT20) and
the upper and lower limit product gas tracers (insoluble and
soluble Bry , resp.). The detrainment mixing ratio parameter [TT20]c for a particular region is 1 ppt inside and zero
outside the corresponding area. Note that this approach only
takes the different transport efficiencies into account. In general there will of course be variations in the source strengths
between the different regions as well, which is not considered
here.
Our model results show clearly that even tracers with a
lifetime of only a few days can travel vast horizontal distances in the TTL. Figure 13 illustrates this fact in showing
an arbitrary snapshot of the Western Pacific source gas tracer
distribution: of course the highest concentrations are located
above the emission region but there are also significant mixing ratios thousands of kilometers away above Central America and the Atlantic. Considering this fact it is interesting
to return again to the Pre-AVE measurements above Central
Atmos. Chem. Phys., 9, 9237–9247, 2009
America conducted in early 2004 (ref. to Sect. 3.1.1). We
already showed in Fig. 5, that our model is able to reproduce
the observed profile of bromoform in this particular region.
We now use our individual source regions approach to investigate their relative contribution to this profile. Figure 14
shows the contribution of the different source regions to the
total amount of TT20 in the before mentioned Pre-AVE area
drawn against altitude. It indicates that the Western Pacific
(WPa) is clearly dominating all other regions, interestingly
also the American (Ame) and Eastern Pacific (EPa) sources,
in whose areas the observation took place. At lower altitudes,
the WPa, EPa and Ame tracers contribute roughly one third
of total CHBr3 each. At the UTLS, however, the influence of
the latter diminish and the WPa tracer makes up about 70%.
As generalization, Fig. 15 shows the relative contribution
of the investigated source regions to TT20 for the whole
tropics (note that the plot looks virtually the same if made
for Bry instead). The picture is similar to the local example presented above: At lower altitudes the relative contribution is roughly equal for most of the source regions whereas
at the UTLS the Western Pacific is clearly identified as the
most important area which contributes about 55% to the total TT20 abundance. Apparently, the Western Pacific and the
Maritime Continent are the only regions where substantial
www.atmos-chem-phys.net/9/9237/2009/
80
1
Fig. 01
model.
Americ
Arabian
tralia pl
40oN
tances
4
ts)
uble
nble
ne 4
as
ove
m (dots)
g trianbromine
rce gas
ed pronvective
11
26
20oN
o
Altitude [km]
0
J. Aschmann et al.: Transport of very short-lived substances
24
20oS
9245
40oS
60oS
30
oo
8080
NS
o
o180 W
60 N
22
120oW
60oW
0o
60oE
120oE
180oW
o
40 N
28
West−Pacific TT20 VMR [ppt]
20oN
o
20
0
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
o
20 S
26
40oS
Fig.
60oS013. Snapshot of the Western Pacific source gas tracer (TT20)
distribution
at 380 K on 4 December 2005.
80oS
Altitude [km]
18
24
180oW
16
200022
2001
2002
2003
Date
2004
0.2
0.4
0.6
0.8
1
1.2
1.6
0o
60oE
120oE
180oW
West−Pacific TT20 VMR [ppt]
0.1
1.4
60oW
2005
Bry (soluble) VMR [ppt]
20
120oW
1.8
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
2
Fig.013.
13. Snapshot
Fig.
Snapshot of
of the
the Western
Western Pacific
Pacific source
source gas
gastracer
tracer(TT20)
(TT20)
distributionatat380
380 K
K on
on 44 December
December 2005.
distribution
2005.
18
Fig. 011. Modeled distribution of tropical Bry mixing ratio tempo16
2000
2001 2000 2002
2005 here to be
rally averaged
from
to 2005. 2003
Since Br2004
y is assumed
Date
soluble the distribution shows also a ‘tape recorder’ shape.
Bry (soluble) VMR [ppt]
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Fig. 015
gions to
from 8-
11. Modeled
Fig. 011.
Modeled distribution
distribution of
of tropical Br
Bryy mixing
mixing ratio temporally averaged from 2000 to 2005. Since Bry is assumed here to be
soluble the distribution shows also a ‘tape recorder’ shape.
o
80 N
o
60 N
o
40 N
Eur
Med
o
20 N
o
0
EPa
Afr
Ame
o
20 S
Fig. 01
gions
from 8
WPa
Atl
Ind
Aus
o
40 S80oN
o
60 S60oN
o
80 S40oN
oo
180
20 W
N
o
0
o
20 S
o
120 W
EPa
o Med
o
60 W
Ame
0
Eur
o
60 E
o
o
120 E
Afr
180 W
WPa
Atl
Ind
Aus
Fig. 14.
014. Relative
Relative contribution
contribution of
of individual
individual source
source regions to
the total amount of TT20 (idealized bromoform tracer) in January/February 2004 above
W, 20
20◦◦NN–3
above 80–90
80-90◦◦ W,
- 3◦◦ S (area of Pre-AVE
campaign, see also Fig. 5).
05).
o
Fig. 12.
40 SMap of the nine different source regions used in our model.
o
The regions
are from left to right: Eastern Pacific (EPa), America
60 S
o
80 SAtlantic (Atl), Mediterranean (Med), Africa plus Arabian
(Ame),
4 Discussion and conclusions
o
o
o
o
o
o
o
W
120 the
W nine
60 W different
0
60 E
120 E
180
W in our
Fig.Peninsula
012. 180Map
of
source
regions
used
(Afr),
Indian
Ocean (Ind),
Eurasia
(Eur),
Australia
plus
model.
The Continent
regions are
from
left toPacific
right:(WPa).
Eastern Pacific (EPa), We have successfully implemented deep convective transport
Maritime
(Aus),
Western
America (Ame), Atlantic (Atl), Mediterranean (Med), Africa plus Fig.
into014.
an isentropic
This allows
us to study
the transport
Relative CTM.
contribution
of individual
source
regions to
Arabian Peninsula (Afr), Indian Ocean (Ind), Eurasia (Eur), Aus- the
of total
VSLSamount
into the
UTLS inbromoform
a physically
motivated
of tropical
TT20 (idealized
tracer)
in Janhigh-reaching
(i.e. above the
levelWestern
of zero radiative
heating)
◦
tralia
plus
Maritime
(Aus),
Pacificused
(WPa).
uary/February
2004large-scale
above 80-90
W, 20◦ N
- 3◦ S (areaby
of diabatic
Pre-AVE
framework with
transport
constrained
Fig.
012.
Map ofContinent
the nine different
source regions
in our
convection occurs. Thus these regions represent the most imcampaign,
see also
05). transport taking place in fast isoheating rates
and Fig.
convective
model. The regions are from left to right: Eastern Pacific (EPa),
portant transport pathway into the stratosphere, in agreement
America (Ame), Atlantic (Atl), Mediterranean (Med), Africa plus
lated updrafts. In this study we focussed on two idealized
with earlier
studies
(e.g.,
Gettelman
et al.,Eurasia
2002b;
Liu Ausand
Arabian
Peninsula
(Afr),
Indian
Ocean (Ind),
(Eur),
tracers whose chemical lifetime roughly corresponds to broZipser,
2005).
tralia
plus
Maritime Continent (Aus), Western Pacific (WPa).
moform and methyl iodide but this approach can be easily
extended to other VSLS as well.
An initial validation of the model with observations of the
VSLS bromoform and methyl iodide, ozone and water vapor
shows in general good agreement (Figs. 5, 6, 8, resp.). There
www.atmos-chem-phys.net/9/9237/2009/
Atmos. Chem. Phys., 9, 9237–9247, 2009
80oW
6
9246
(TT20)
ons to
n Jane-AVE
Fig.
Averaged
relative
contribution
of individual
source
reFig. 015.
15. Averaged
relative
contribution
of individual
source
regions
gions
theamount
total amount
TT20 (idealized
bromoform
tracer)
to the to
total
of TT20of(idealized
bromoform
tracer) from
8–
from
kmtropics.
in the tropics.
20 km8-20
in the
is some evidence from the ozone and water comparisons that
could indicate too much convective transport into the 16 to
18 km altitude region. However, a comparison of the convective turnover time scales in the TTL from our model with
estimates from Dessler (2002), based on O3 and CO observations, shows excellent agreement (Fig. 3), so it may also
be that existing deviations from ozone and water vapor observations are a consequence of our simplified approach in
the treatment of these species. Furthermore, a sensitivity calculation with convective detrainment set to zero above 380 K
altitude shows that our main results are not dependent on the
amount of convective transport to altitudes above the coldpoint (Fig. 10).
Comparisons with recent model studies show also in general good agreement with our results taking into account the
differences in the modeling approaches, source strengths,
lifetimes and other parameters. Estimations of the amount of
bromoform reaching the tropical stratosphere (CHBr3 abundance at 18 km altitude) are ranging from 0.1 ppt (Warwick
et al., 2006) over 0.15 ppt (Hossaini et al., 2009) up to 0.5 ppt
(Gettelman et al., 2009) with our results of 0.1 ppt residing at
the lower bound.
Our main result is an estimation of the impact of dehydration on Bry from VSLS entering the stratosphere. Given
the commonly used assumption of a bromoform mixing ratio of 1 ppt at ground-level, we constrain the stratospheric
bromine loading due to bromoform (TT20) to 1.6 to 3 ppt
(Fig. 10). Note that complete solubility and instantaneous
washout only removes about 50% of the Bry transported into
the stratosphere compared to the insoluble tracer. Considering the lower limit of bromine loading due to the soluble
Bry tracer of 1.6 ppt we estimate that about 1.2 to 1.3 ppt is
Atmos. Chem. Phys., 9, 9237–9247, 2009
J. Aschmann et al.: Transport of very short-lived substances
originated from source gas injection whereas the remaining
0.3 to 0.4 ppt are from product gas injection. Comparing the
estimations for bromine loading due to VSLS with previous
work is difficult because of some significant differences between the conducted studies. Warwick et al. (2006) give an
estimation of 6 to 7 ppt of additional inorganic bromine due
to VSL bromocarbons at about 18 km (for bromoform only
about 3.4 to 4 ppt). Gettelman et al. (2009) give a range of
1.1 to 4.1 ppt (bromoform only: 0.3 to 2.1 ppt) for the same
quantity. Our estimations of additional bromine loading due
to bromoform (1.6 to 3 ppt) are in between.
Finally we applied the bromoform degradation approach
to different emission scenarios. We have identified the regions with most effective transport, e.g. the Western Pacific
with a relative contribution of 55% (Fig. 15) assuming a uniform source. Along with the evidence that horizontal transport plays an utterly important role in the TTL (Fig. 13) we
can confirm that the Western Pacific and the Maritime Continent are the most important pathways into the whole TTL and
finally the stratosphere especially for short-lived substances.
With our current study we have established a modeling
tool to further investigate the transport and transformation
of VSLS into the stratosphere. The comparison with VSLS
measurements indicates that we can successfully reproduce
the source gas input of VSLS into the tropical UTLS. A next
step would be a more sophisticated treatment of Bry uptake
on ice with subsequent sedimentation and the inclusion of
more realistic VSLS emission scenarios.
Acknowledgements. J. A. acknowledges financial support by the
University of Bremen through project VSLS. Parts of this work
were supported by the EU projects SCOUT-O3 and SHIVA.
ERA-Interim data were provided through the ECMWF special
project DECDIO. Measurements of halocarbons were supported
by the NASA Upper Atmosphere Research Program. We thank R.
Lueb for field support, and X. Zhu and L. Pope for analytical work.
Edited by: M. Dameris
References
Chipperfield, M. P.: New Version of the TOMCAT/SLIMCAT OffLine Chemical Transport Model: Intercomparison of Stratospheric Tracer Experiments, Q. J. Roy. Meteor. Soc., 132, 1179–
1203, doi:10.1256/qj.05.51, 2006.
Corti. T., Luo, B. P., Peter, T., Fu, Q., and Vömel, H.: Mean radiative energy balance and vertical mass fluxes in the equatorial
upper troposphere and lower stratosphere, Geophys. Res. Lett.,
32, L06802, doi:10.1029/2004GL021889, 2005.
Dessler, A. E.: The effect of deep, tropical convection on the
tropical tropopause layer, J. Geophys. Res., 107(D3), 4033,
doi:10.1029/2001JD000511, 2002.
Dessler, A. E. and Sherwood, S. C.: Effect of convection on the
summertime extratropical lower stratosphere, J. Geophys. Res.,
109, D23301, doi:10.1029/2004JD005209, 2004.
Folkins, I., Kelly, K. K., and Weinstock, E. M.: A simple explanation for the increase in relative humidity between 11 and 14 km in
www.atmos-chem-phys.net/9/9237/2009/
J. Aschmann et al.: Transport of very short-lived substances
9247
the tropics, J. Geophys. Res., 4736, doi:10.1029/2002JD002185,
2002.
Fueglistaler, S., Bonazzola, M., Haynes, P. H., and Peter, T.: Stratospheric water vapor predicted from the Lagrangian temperature
history of air entering the stratosphere in the tropics, J. Geophys.
Res., 110, D08107, doi:10.1029/2004JD005516, 2005.
Fueglistaler, S., Legras, B., Beljaars, A., Morcrette, J.-J., Simmons,
A., Tompkins, A. M., and Uppala, S.: The diabatic heat budget
of the upper troposphere and lower/mid stratosphere in ECMWF
reanalyses, Q. J. Roy. Meteor. Soc., 135(638), 21–37, 2009.
Gettelman, A., Randel, W. J., Wu, F., and Massie, S. T.: Transport
of water vapor in the tropical tropopause layer, Geophys. Res.
Lett., 29(1), 1009, doi:10.1029/2001GL013818, 2002a.
Gettelman, A., Salby, M. L., and Sassi F.: The distribution and influence of convection in the tropical tropopause region, J. Geophys.
Res., 107(D10), 4080, doi:10.1029/2001JD00104, 2002b.
Gettelman, A., Lauritzen, P. H., Park, M., and Kay, J. E.: Processes
regulating short-lived species in the tropical tropopause layer, J.
Geophys. Res., 114, D13303, doi:10.1029/2009JD011785, 2009.
Grooß, J.-U. and Russell, J. M. III: Technical note: A stratospheric
climatology for O3 , H2 O, CH4 , NOx , HCl and HF derived from
HALOE measurements, Atmos. Chem. Phys., 5, 2797–2807,
2005, http://www.atmos-chem-phys.net/5/2797/2005/.
Hanisco, T. F., Moyer, E. J., Weinstock, E. M., St. Clair, J. M.,
Sayres, D. S., Smith, J. B., Lockwood, R., Anderson, J. G.,
Dessler, A. E., Keutsch, F. N., Spackman, J. R., Read, W. G. and
Bui, T. P.: Observations of deep convective influence on stratospheric water vapor and its isotopic composition, Geophys. Res.
Lett., 34, L04814, doi:10.1029/2006GL027899, 2007.
Holton, J. R. and Gettelman, A.: Horizontal transport and the dehydration of the stratosphere, Geophys. Res. Lett., 28, 2799–2802,
2001.
Hossaini, R., Chipperfield, M. P., Monge-Sanz, B. M., Richards,
N. A. D., Atlas, E. and Blake, D. R.: Bromoform and dibromomethane in the tropics: a 3-D model study of chemistry and
transport, Atmos. Chem. Phys. Discuss., 9, 16811–16851, 2009,
http://www.atmos-chem-phys-discuss.net/9/16811/2009/.
IFS Documentation – Cy31r1, www.ecmwf.int/research/ifsdocs/
CY31r1/PHYSICS/IFSPart4.pdf, last access: 14 August 2009,
2007.
Kerkweg, A., Jöckel, P., Warwick, N., Gebhardt, S., Brenninkmeijer, C. A. M., and Lelieveld, J.: Consistent simulation of bromine
chemistry from the marine boundary layer to the stratosphere
– Part 2: Bromocarbons, Atmos. Chem. Phys., 8, 5919–5939,
2008, http://www.atmos-chem-phys.net/8/5919/2008/.
Liu, C. and Zipser, E. J.: Global distribution of convection penetrating the tropical tropopause, J. Geophys. Res., 110(D23),
D23104, doi:10.1029/2005JD006063, 2005.
Marti, J. and Mauersberger, K.: A survey and new measurements
of ice vapor pressure at temperatures between 170 and 250 K,
Geophys. Res. Lett., 20, 363–366, 1993
McLinden, C. A., Olsen, S. C., Hannegan, B., Wild, O., Prather, M.
J., and Sundet, J.: Stratospheric ozone in 3-D models: A simple chemistry and the cross-tropopause flux, J. Geophys. Res.,
105(D11), 14653–14665, 2000.
Nielsen, J. E. and Douglass, A. R.: A simulation of bromoform’s contribution to stratospheric bromine, J. Geophys. Res.,
106(D8), 8089–8100, 2001.
Quack, B., and Wallace, D. W. R.: Air-sea flux of bromoform: Controls, rates, and implications, Global Biogeochem. Cycles, 17(1),
1023, doi:10.1029/2002GB001890, 2003.
Rasch, P. J., Mahowald, N. M., and Eaton, B. E.: Representations
of transport, convection, and the hydrologic cycle in chemical
transport models: Implications for the modeling of short-lived
and soluble species, J. Geophys. Res., 102(D23), 28127–28138,
1997.
Read, W. G., Schwartz, M. J., Lambert, A., Su, H., Livesey, N.
J., Daffer, W. H., and Boone, C. D.: The roles of convection,
extratropical mixing, and in-situ freeze-drying in the tropical
tropopause layer, Atmos. Chem. Phys., 8, 6051–6067, 2008,
http://www.atmos-chem-phys.net/8/6051/2008/.
Sinnhuber, B.-M., Weber, M., Amankwah, A., and Burrows, J.
P.: Total ozone during the unusual Antarctic winter of 2002,
Geophys. Res. Lett., 30(11), 1580, doi:10.1029/2002GL016798,
2003.
Sinnhuber, B.-M. and Folkins, I.: Estimating the contribution of
bromoform to stratospheric bromine and its relation to dehydration in the tropical tropopause layer, Atmos. Chem. Phys., 6,
4755–4761, 2006,
http://www.atmos-chem-phys.net/6/4755/2006/.
Sinnhuber, B.-M., Sheode, N., Sinnhuber, M., Chipperfield, M. P.,
and Feng, W.: The contribution of anthropogenic bromine emissions to past stratospheric ozone trends: a modelling study, Atmos. Chem. Phys., 9, 2863–2871, 2009,
http://www.atmos-chem-phys.net/9/2863/2009/.
Thompson, A. M., Witte, J. C., McPeters, R. D., Oltmans,
S. J., Schmidlin, F. J., Logan, J. A., Fujiwara, M., Kirchhoff, V. W. J. H., Posny, F., Coetzee, G. J. R., Hoegger,
B., Kawakami, S., Ogawa, T., Johnson, B. J., Vömel, H.
and Labow, G.: Southern Hemisphere Additional Ozonesondes (SHADOZ) 1998–2000 tropical ozone climatology 1. Comparison with Total Ozone Mapping Spectrometer (TOMS) and
ground-based measurements, J. Geophys. Res., 108(D2), 8238,
doi:10.1029/2001JD000967, 2003.
Warwick, N. J., Pyle, J. A., Carver, G. D., Yang, X., Savage,
N. H., O’Connor, F. M., and Cox, R. A.: Global modeling
of biogenic bromocarbons, J. Geophys. Res., 111, D24305,
doi:10.1029/2006JD007264, 2006.
WMO (World Meteorological Organization): Scientific Assessment
of Ozone Depletion: 2006, Global Ozone Research and Monitoring Project-Report No. 50, 572 pp., Geneva, Switzerland, 2007.
www.atmos-chem-phys.net/9/9237/2009/
Atmos. Chem. Phys., 9, 9237–9247, 2009