Line-by-line calculations of thermal infrared radiation

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ARTICLE IN PRESS
Journal of Quantitative Spectroscopy &
Radiative Transfer 97 (2006) 317–331
www.elsevier.com/locate/jqsrt
Line-by-line calculations of thermal infrared radiation
representative for global condition: CFC-12 as an example
Gunnar Myhrea,b,, Frode Stordala,b, Ingvil Gausemelc,
Claus J. Nielsenc, Emanuel Mahieud
a
Department of Geosciences, University of Oslo, Norway
Norwegian Institute for Air Research (NILU), Kjeller, Norway
c
Department of Chemistry, University of Oslo, Norway
d
Institute of Astrophysics and Geophysics, University of Liège, Belgium
b
Received 28 January 2005; accepted 27 April 2005
Abstract
We estimate a current direct radiative forcing due to CFC-12 of 0.18 Wm2 , which is likely to be the peak
radiative forcing for CFC-12. Global measurements of CFC-12 show at present an almost negligible trend
for CFC-12 and measurement in an industrialized region show evidence that the peak concentration is
reached. It is expected that concentration of CFC-12 in industrialized regions begins to decline 1–3 years
before the global concentration.
Our radiative forcing calculations are based on a line-by-line model appropriate for simulation of global
mean radiative forcing, including clouds and stratospheric temperature adjustment. The radiative forcing of
0.33 Wm2 /ppbv is close to earlier published results for this compound. New spectroscopic measurements
for CFC-12 are performed and compared to previously published results.
r 2005 Elsevier Ltd. All rights reserved.
Keywords: Radiative forcing; CFC-12 evolution; Lifetime
Corresponding author. Department of Geosciences, University of Oslo, PO Box 1022, Blindern, 0315 Oslo, Norway.
Tel.: +47 22855801; fax: +47 22855269.
E-mail address: [email protected] (G. Myhre).
0022-4073/$ - see front matter r 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jqsrt.2005.04.015
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1. Introduction
CFC-12 (CF2 Cl2 ) is the component of the atmosphere with the highest abundance among the
constituents of entirely human origin. The current concentration is close to 550 ppbv and is almost
double the CFC-11 concentration, ranking as the second largest concentration of the CFCs.
However, this abundance is much lower than for many of the other greenhouse gases, which are of
both natural and anthropogenic origin. CFC-12 is one of the first CFCs that was developed. From
the 1930s, it was used as a refrigerant [1] and later also as an aerosol propellant. The emissions
and atmospheric concentrations increased strongly from the 1960s.
Among the well-mixed greenhouse gases CFC-12 has the third largest radiative forcing (only
CO2 and CH4 are larger) and it accounts for about half of the radiative forcing from the whole
group of CFCs and CFC replacements [2]. Due to its influence on the stratospheric ozone layer
CFC-12 it has been regulated by the Montreal protocol. However, since CFC-12 has a long
lifetime the concentration will continue to be high during this century [3].
In radiative transfer calculation of CFCs and CFC replacements line-by-line (LBL) calculations
have been used for validation of broad and narrow band models [4–7] and in combination with
narrow band models [8,9]. In this study, we use a LBL model in global estimation of radiative
forcing due to CFC-12. The advantage of a LBL model in calculating global radiative forcing is
that a more consistent and accurate ranking of various CFCs and CFC replacements can be made.
IPCC [2] and WMO [3] have an overview over all radiative forcing and global warming potentials
for CFCs and their replacements. Their estimates have been computed in a wide range of studies.
Jain et al. [10] and Sihra et al. [9] are comprehensive studies that investigate a wide range of CFCs
and CFC replacements.
In addition to describing a method for calculating global and annual mean radiative forcing
from a LBL model, we provide an overview over various absorption cross sections for CFC-12
and investigate the future development of CFC-12 concentration and radiative forcing. We base
our evolution of the CFC-concentrations on observations up to an including 2003. For future
evolution, we use scenarios that were presented in Madronich and Velders [11], which were
incorporated into the IPCC SRES scenarios [12]. In this paper, we are dealing with the direct
radiative forcing of CFC-12 only and not the indirect forcing resulting from ozone destroyed by
CFC-12 in the stratosphere.
2. Cross section data
2.1. Experimental
CFC-12 was obtained from SynQuest Labs., Inc. and had a stated purity of better than 99%.
The sample was condensed and degassed in several freeze-thaw cycles before use. Infrared spectra
of the pure gas and of the gas mixed in 1 atm. of nitrogen at 295 1 K were recorded in the region
500–2000 cm1 with a Bruker IFS 113v FT-IR employing a Ge/KBr beamsplitter and a DTGS
detector. The reported spectra have nominal resolutions of 0.5 and 1 cm1 , and resulted from coadding 500 interferograms and applying the Blackman-Harris 3-Term apodization. A cell of
2:34 0:02 cm length equipped with windows of CsI was employed. The partial pressure of CFC-
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12 in the cell varied from 1.8 to 7.4 mbar and was measured using a MKS Baratron pressure
transducer with a stated absolute accuracy of 0.15%. Spectra were recorded of the pure CFC-12
as well as of CFC-12 mixed with N2 to a total pressure of 1 bar. The cross sections were obtained
from the absorbance spectra assuming that the gas was ideal. The standard deviation of the
derived integrated absorption cross sections was around 1% and the estimated accuracy of the
absolute integrated absorption cross sections is 3%.
2.2. Spectral results
Fig. 1 shows the absorption cross section of CFC-12 resulting from measurements at
0.5 cm1 spectral resolution and 1 bar total pressure. Table 1 summarises the available
integrated absorption cross section data. The present cross section, obtained with a spectral
resolution of 0.5 cm1 , is in good agreement with the high resolution data in the
HITRAN database except for the very narrow Q-branches of the n6 (b1 ) band around
1159 cm1 (the antisymmetric CF2 stretching mode). The integrated absorption cross
sections, however, agrees within the experimental uncertainties. In addition to the very
intense bands in the 1200–1050 and 950–825 cm1 regions two weak bands are observed
around 1242 (n5 þ n8 ) and 670 cm1 (n2 (a1 )—the symmetric CCl2 stretching mode). These
bands are not included in the HITRAN 92-04 data bases. The integrated absorption
cross section of the 1242 cm1 combination band is nearly two orders of magnitude
smaller than those of the n1 (a1 ) and n6 (b1 ) fundamentals in the 1200–1050 cm1 range,
ð6:45 0:19Þ 1019 cm molecule1 , and its contribution to the radiative forcing, see later, is
negligible. The n2 (a1 ) band at 670 cm1 is somewhat more intense, ð1:65 0:08Þ 1018 cm
molecule1 , but is completely covered in the atmospheric spectrum by the CO2 fundamental
at 672 cm1 .
Absorption cross section (base e)
/10-18 cm2 molecule-1
3.5
CFC-12/N2-mixtures
296 K
0.1 MPa
Resolution 0.5 cm-1
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1200
1000
800
600
Wavenumber /cm-1
Fig. 1. Infrared absorption cross section (base e) of CCl2 F2 at 1 bar total pressure and 0.5 cm1 spectral resolution.
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Table 1
Summary of reported CFC-12 integrated absorption cross sections (/ 1017 cm molecule1 ). The listed integration limits
are approximate
T/K
1190–1130/cm1
Varanasi and Ko [13]
300
2:94 0:06
Kagann et al. [14]
296
3:13 0:11
Massie et al. [15]
Nguyen et al. [16]
Varanasi and Chudamani [17]
200, 296
200, 298
300
2:93 0:06
Fisher et al. [18]
McDaniel et al. [19]a
296
293
2:93 0:29
McDaniel et al. [19]
203
3:06 0:31
Clerbaux et al. [20]
Varanasi and Nemtchinov [21]b
HITRAN 2000c
This work
287
200–296
3:08 0:07
295
2:88 0:09
1130–1070/cm1
4:64 0:09
7:58 0:11
4:60 0:16
7:74 0:19
7:1 0:7
7:21 0:22
4:62 0:05
7:55 0:05
7.189
4:2 0:4
7:13 0:5
4:4 0:4
7:46 0:5
4:50 0:08
7:60 0:07
7:63 0:09
4:60 0:13
950–850/cm1
Total
5:32 0:16
12:90 0:20
5:83 0:19
13:57 0:27
6:1 0:6
5:48 0:23
5:81 0:05
13:3 0:9
12:69 0:32
13:36 0:09
5.780
5:5 0:5
12.969
12:6 0:7
5:5 0:5
13:0 0:7
5:88 0:12
5:87 0:07
5:86 0:06
5:98 0:18
13:46 0:16
13:46 0:10
13.484
13:5 0:4
a
HITRAN 1992, 6 (p,T)-data sets at 0.03 cm1 spectral resolution.
HITRAN 1996, 15 (p,T)-data sets at o0:03 cm1 spectral resolution. The values quoted represent the average of the
(p,T)-data.
c
52 (p,T) data sets at o0:03 cm1 spectral resolution. The values quoted represent the average of the (p,T)-data as is
the same in HITRAN-2004.
b
3. Line-by-line model description
It is critically important for radiative forcing calculations of greenhouse gases to be
representative for global conditions that clouds and adjustment of the stratospheric temperature
can be implemented. To take this into account, we have combined the GENLN2 LBL [22]
calculating the optical depth of the gases absorbing in the thermal infrared region and the
DISORT model calculating radiative fluxes ([23], as described in Myhre and Stordal [5] and
Myhre et al. [7] for validation purposes). The GENLN2 LBL uses the water vapor continuum
from Clough et al. [42] and spectroscopic data from HITRAN-1992 [24]. The absorption cross
section data for CFC-12 are taken from several sources (see Section 2). DISORT is a multi-stream
model and we use 16 streams in our calculations.
Myhre and Stordal [5] and Freckleton et al. [25] showed that global radiative forcing
calculations should not be performed using one single vertical global mean profile. For this study
we have generated two vertical profiles; one for tropics the (30N–30S) and one for the extratropics (30N–90N and 30S–90S), as given in Table 2. The cloud data used in the simulations are
from ISCCP [26] and include information about cloud fraction, cloud altitude, and cloud optical
depth. Three cloud layers are used in the calculations and no cloud overlap is assumed. For each
profile four separate LBL calculations are performed, namely for profiles including each of the
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Table 2
Vertical profiles adopted for pressure, temperature, water vapor, and ozone
Level
P (mb)
T (K)
H2 O (mass mixing ratio)
O3 (ppmv)
Tropical profile:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
2.50
5.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
85.00
100.00
125.00
150.00
175.00
200.00
225.00
250.00
300.00
399.71
499.43
696.82
841.06
915.11
977.06
992.38
251.59
241.59
231.59
222.03
216.44
211.59
207.82
204.12
200.99
198.81
196.99
202.37
206.77
213.45
219.23
225.18
230.50
240.42
255.90
267.07
281.50
289.02
292.89
296.62
298.34
4.232E06
4.232E06
4.228E06
4.883E06
5.316E06
5.825E06
6.255E06
6.797E06
7.294E06
7.617E06
7.914E06
1.317E05
2.040E05
4.218E05
7.944E05
1.291E04
2.001E04
3.631E04
8.050E04
1.554E03
4.735E03
8.922E03
1.146E02
1.398E02
1.425E02
6.441E+00
8.963E+00
9.957E+00
7.199E+00
4.436E+00
2.757E+00
1.717E+00
1.029E+00
5.827E01
2.666E01
1.828E01
1.011E01
7.068E02
5.917E02
5.243E02
4.838E02
4.620E02
4.268E02
4.107E02
4.008E02
3.374E02
2.568E02
2.198E02
1.902E02
1.829E02
Extra-tropical profile:
1
2.50
2
5.00
3
10.00
4
20.00
5
30.00
6
40.00
7
50.00
8
60.00
9
70.00
10
85.00
11
100.00
12
125.00
13
150.00
14
175.00
15
200.00
16
225.00
17
250.00
18
299.99
246.30
236.30
226.30
221.63
218.89
216.83
215.22
214.83
214.49
214.45
214.41
215.65
216.66
217.39
218.01
219.42
220.67
225.97
5.756E06
5.755E06
5.701E06
5.797E06
5.894E06
5.997E06
6.105E06
6.119E06
6.138E06
6.197E06
6.258E06
6.694E06
7.187E06
1.133E05
1.799E05
3.162E05
5.314E05
1.184E04
6.302E+00
7.398E+00
7.069E+00
5.752E+00
4.497E+00
3.529E+00
2.763E+00
2.155E+00
1.681E+00
1.185E+00
8.710E01
5.798E01
4.140E01
3.191E01
2.496E01
1.881E01
1.384E01
8.678E02
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Table 2 (continued )
Level
P (mb)
T (K)
H2 O (mass mixing ratio)
O3 (ppmv)
19
20
21
22
23
24
25
398.28
496.57
685.79
824.94
896.17
949.16
969.20
238.77
249.86
264.36
271.15
273.88
275.84
276.68
3.476E04
7.076E04
1.839E03
3.350E03
4.261E03
5.082E03
5.289E03
5.947E02
4.606E02
3.809E02
3.186E02
2.928E02
2.754E02
2.692E02
three cloud layers and for the clear sky. The global mean total cloud cover in the adopted data is
62%. The cloud data are also averaged onto the two vertical profiles. Using these two profiles
gives results within 1% of a global calculation in a 2:5 2:5 resolution using the model and
method as in Myhre and Stordal [5]. The calculations are performed with CO2 , N2 O, and CH4
concentrations following IPCC [2] of 365 ppm, 1745 and 314 ppb, respectively. Vertical profiles of
the various gases absorbing infrared radiation are based on modeled profiles and are the same as
used in Myhre and Stordal [5].
To adjust the temperature in the stratosphere into a new equilibrium, we assume that solar
heating rates are not altered. We use an iterative procedure to adjust the stratospheric
temperatures until the radiative heating rates from the perturbation of the thermal infrared
heating rates is smaller than 1:0 104 K day1 .
4. Results
4.1. Longwave radiative fluxes
Fig. 2 shows the spectral variation in the upward longwave flux at the surface and at the top of
the atmosphere (TOA). The longwave flux at TOA is shown in the case when clouds are included
and for clear sky. The figure shows clearly the strong absorption bands for CO2 (centered at
667 cm1 ) and O3 (centered at 1042 cm1 ). The strong H2 O absorption at wavenumbers below
600 cm1 and above 1200 cm1 and the weak absorption in the ‘atmospheric window’ region
(800–1200 cm1 ) is also evident. The general pattern shown in Fig. 1 is in agreement with Kiehl
and Trenberth [27] and Pinnock et al. [28]. The trapping of longwave radiation from clouds is
generally in the ‘atmospheric window’ region. Fig. 1 shows that clouds clearly can impact the
radiative forcing due to increase in various greenhouse gases differently based on the spectral
absorption feature. Using the LBL model, we calculate the outgoing longwave radiation (OLR) at
the TOA to be 234 Wm2 and the longwave cloud radiative forcing (LWCRF) 19 Wm2 . The
OLR is in good agreement with other estimates [27], whereas the LWCRF is somewhat weaker
than some observationally-based estimates [27], but close to a model estimate using the ISCCP
clouds [29].
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Fig. 2. Global mean upwelling irradiance from the surface and from the top of the atmosphere for clear sky and for a
global representative cloud distribution.
4.2. Radiative forcing due to CFC-12
Fig. 3a shows the spectral distribution of the radiative forcing due to CFC-12 for 1 ppbv and
the cross section shown in Fig. 1. It is displayed in two different spectral regions and spectral
resolutions. The effect of clouds and stratospheric temperature adjustment (STA) on the radiative
forcing is illustrated in Fig. 3b. The figure shows that radiative forcing has a shape which is very
similar to the absorption cross section. However, the effect of STA is seen to be most dominant at
wavenumbers lower than the CFC-12 absorption bands, which is mostly in the CO2 band as a
general increase in the temperature in the lower stratosphere increases the downward emission.
Notice that in a few spectral regions the effect of STA is to reduce the radiative forcing. STA
increases the temperature in the lower stratosphere. However, in the upper stratosphere the
temperature decreases and in some spectral regions this overwhelms the effect of the warming in
the lower stratosphere for the downward irradiance at the tropopause. Clouds reduce the
radiative forcing due to CFC-12 rather spectrally uniformly. This is also what would be expected
based on the absorption cross sections for CFC-12 and shown in Figs. 1 and 2. Table 3 lists
the radiative forcing for various combinations of calculations including STA and clouds.
Clouds reduce the radiative forcing due to CFC-12 by about 30% and STA increases the forcing
by about 10%.
In Table 4 radiative forcing due to CFC-12 is given for various sets of cross section data for
CFC-12. The results show extremely small differences in the forcing between the various sources
of cross section data, except for the calculation with Hitran-1992 data. The differences between
adopting different cross section data are small compared to other studies of CFCs [8,30], but this
is also expected from discussion in Section 2. Fig. 4 shows the spectral difference between the
radiative forcing for different sets of cross section data. The spectral forcing using the
spectroscopical data from this work differs somewhat from the forcing based on the three others
sets of cross section. The spectral forcing is almost identical for cross sections from HITRAN1996 and HITRAN-2000, whereas HITRAN-1992 generally yields smaller forcing than for two
other HITRAN data sets.
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(a)
(b)
Fig. 3. (a) Radiative forcing due to CFC-12 in the spectral region 0–3000 cm1 and spectral resolution 0.02 cm1 where
clouds and stratospheric temperature adjustment are included. (b) Radiative forcing due to CFC-12 in the spectral
region 200–1400 cm1 and results presented with a spectral resolution of 1 cm1 , red line cloud and STA included, blue
line clouds included and STA excluded, green line clear sky and STA excluded.
Table 3
Radiative forcing due to CFC-12 for an increase of 1 ppbv. Results are shown for various experiments where clouds and
stratospheric temperature adjustment (STA) are included or excluded
RF (Wm2 )
STA
STA
STA
STA
and clouds included
excluded, clouds included
included, clouds excluded
and clouds excluded
0.328
0.300
0.440
0.400
For many CFCs and CFC replacements the concentration decreases with altitude and this may
influence the radiative forcing due to these compounds significantly [10,30–32]. CFC-12 is
destroyed at relatively high levels of the atmosphere and thus has a relatively uniform vertical
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Table 4
Radiative forcing due to CFC-12 for an increase of 1 ppbv for different sets of cross section data. Calculations include
clouds and stratospheric temperature adjustment
Cross section data for CFC-12
RF (Wm2 )
This work
HITRAN-1992
HITRAN-1996
HITRAN-2000
0.328
0.307
0.327
0.327
Fig. 4. Difference in spectral radiative forcing due to CFC-12 for various spectroscopic data.
distribution. A control calculation is performed with a constant vertical profile, yielding a 3%
higher radiative forcing.
Our estimate of the radiative forcing due to CFC-12 of 0.33 Wm2 /ppb is slightly higher (3%)
than the value used in WMO [3] and IPCC [2]. The current estimate is within 4% different from
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two model estimates in Myhre et al. [7] and the estimate in Sihra et al. [9]. However, the estimate
of 0.33 Wm2 /ppb is 9% higher than in Jain et al. [10].
We estimate an uncertainty range for CFC-12 from 0.30 to 0.35 Wm2 /ppb.
4.3. Past and future time evolution of the radiative forcing due to CFC-12
4.3.1. Past history
We have established the past evolution of CFC-12 mostly from the observations in the
Advanced Atmospheric Gases Experiment (AGAGE) [36] network and its predecessor
programmes Atmospheric Lifetime Experiment (ALE) and Global Atmospheric Gases Experiment (GAGE). We have taken globally averaged concentrations, available since 1960, from
McCulloch et al. [33], which were based on the work of Prinn et al. [34] (see Figs. 5–7). McCulloch
et al. [33] used the observations to verify their estimates of CFC-12 emissions, by the use of a
simple global box model. They found that the concentrations calculated from emissions were
consistent with the observations at 95% significance level (confidence limits based on uncertainties
in the emissions) for most of the data record. In the 1980s and the early 1990s the observed
concentrations were slightly above the emission-based concentrations. Since the mid 1990s the
observed concentrations have been gradually levelling off.
Since 1990, we have used observations from AGAGE, with global averages derived from a 12
box model [35]. The two AGAGE data series have been combined to one time series by reducing
gradually a small difference in year 1990 between the two series over a ten year period prior to
that year. In addition, we have included observations of total columns of CFC-12 made at
Jungfraujoch [37] from 1986 to 2004, also shown in Fig. 6. The column observations at
Jungfraujoch are, like the in situ surface AGAGE observations at Mace Head, part of the
network of System for Observation of halogenated Greenhouse gases in Europe (SOGE).
Altogether, the observations show a very rapid growth in the period 1960–1990. After 1990, when
the Montreal Protocol came into effect, the observed growth slowed, and recently the CFC-12
Fig. 5. Observed surface in situ concentrations of CFC-s from AGAGE and future scenarios. Scenarios A1, A2, A3
and Ab are shown, assuming 100 (102 for Ab) years lifetime for CFC-12.
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Fig. 6. Observed surface in situ concentrations of CFC-s from AGAGE and column observations in Europe as well as
future scenarios (see text for details). The figure focuses on the period around the peak of CFC-12. The column data
have been scaled to the AGAGE value in 2003 (7:036 1015 molecules/cm2 scaled to 544.4 ppt). The same scenarios as
in Fig. 5 are shown along with a scenario A1 starting from observations in 2003 and developed from a 1-box model
from year 2004 and onward.
Fig. 7. Future scenarios of CFC-concentrations. All scenarios use the emissions from A1 but with a range of lifetimes
for CFC-12 given in Madronich and Velders [11], namely 77, 90, 149 and 185 years in addition to the reference value
100 years (see text for explanation). The AGAGE observations are also shown as in Fig. 5.
abundance peaked in Europe where there is now a weak decline. In Fig. 6, the peak is seen
in the column data from Jungfraujoch, reaching the highest value in 2002. In the AGAGE data
CFC-12 had not reached the peak in 2003, but the trend is extremely slow. The column data
have been scaled to match the AGAGE surface abundance in 2003. Fig. 6 shows that the column
in Europe follows the evolution of the global averaged AGAGE data, but preceding the AGAGE
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data by close to one year. This is consistent with the fact that the major emissions are in
the Northern Hemisphere and that the exchange time between the two hemispheres is about
two years.
4.3.2. Future scenarios
In order to assess the future development of the CFC-12 level, we have adopted three emission
scenarios developed by IPCC [12], as presented by Madronich and Velders [11] (see Fig. 5). These
scenarios, which start with emissions in 1990, were adopted also by IPCC [2]. First, scenario A1 is
the baseline scenario according to the 1997 amendments of the Montreal Protocol. Second, A2 is
an extreme low case, assuming zero emissions from 1999 onward. Third, A3 is a high case,
assuming maximum production allowed under the (1997) Montreal Protocol, i.e. a 15% increase
in production in developed countries for use in developing countries. Fourth, we have included
also a baseline scenario, Ab, presented in Montzka and Fraser [38], including the 1999
amendments to the Montreal protocol.
We have used emissions for A1 and A2 to derive tropospheric abundances using the
simple global box model defined in Madronich and Velders [11], whereas for scenarios A3 and Ab
we have used tabulated abundances (from [43,38], respectively). In our global box model
calculations we have used a lifetime of 100 years for CFC-12 as a reference value, considered the
central value in Prinn and Zander [39] and later Montzka and Fraser [38], and IPCC [2]. The
concentration evolutions A3 and Ab were developed assuming 102 and 100 years lifetime,
respectively. Note that we have started our A1 and A2 calculations in 1990 with observed
AGAGE values (477 ppt) whereas the concentrations in scenarios A3 and Ab are slightly different
(467 and 474 ppt, respectively). The lifetime of CFC-12 is rather uncertain. The range given in
Prinn and Zander [39] is from 77 to 185 years, as derived from surface concentrations. In
comparison, the variation between different models, also given in Prinn and Zander [39], was
90–149 years. We have derived concentration time lines for scenario A1 also for the extreme
lifetimes 77 and 185 years, to span the uncertainty range. We find, in agreement with McCulloch
et al. [33] that the observations were consistent with the concentrations derived from the
emissions, starting in 1990. Considering the large uncertainties in the lifetime, this conclusion
holds for scenarios A1 and Ab, for which the peak concentrations are close to the current
observations assuming a lifetime of 100 years.
The future CFC-12 concentrations according to the three scenarios are also shown in Figs. 5
and 6. For scenario A1, an additional estimate of future concentrations has been made, starting
with the observed 2003 AGAGE value. Naturally, in the A2 scenario the concentrations decline
immediately after the emissions are set to zero in 1999. In A1, A3 and Ab there is still a short
period of growth after 1999, until the concentrations level off, reach the peak and eventually
decline. The given range in lifetimes yields a larger variation in future CFC-12 concentrations
than the four emission scenarios, even including scenario A2 with zero emissions after 1999. For
the A1 scenario the difference between the concentration in 2100 in the high lifetime case (365 ppt)
and the low lifetime case (161 ppt) is about 200 ppt, whereas the scenarios assuming a lifetime
of 100 (or 102) years span over less than 50 ppt (from 199 ppt in A2 to 246 ppt in A3). Even
the somewhat smaller span in lifetimes between models yields a larger span in CFC-12
concentrations in 2100 than the different scenarios (120 from 197 ppt in the short and 317 ppt in
the long lifetime case).
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5. Summary
A LBL model has been developed for calculations of global mean radiative forcing of CFCs
and CFC replacements as recommended in IPCC [2]. LBL models have the advantage of applying
the physical theory for atmospheric radiative transfer without additional major assumptions as in
radiative transfer schemes in global models. The disadvantage of using LBL models is the time
consumption of such codes; in particular this is the case when adjustment of stratospheric
temperature is performed. Stratospheric temperature adjustment and inclusion of clouds are
important in calculations of radiative forcing of CFCs and their replacements.
A significant feature of the combined use of GENLN2 LBL and the DISORT models are that
the OLR is in good agreement with observations. Measurements of OLR can be deduced from
remote sensing both of longwave and shortwave radiation. Note that measurements of LWCRF
are much more uncertain.
Absorption cross sections for CFC-12 show relatively small differences, within about 10%.
Measurements made in this study are in good agreement with recent HITRAN data, in particular
for the integrated absorption cross section. Radiative forcing of CFC-12 is calculated to
0.33 Wm2 /ppb and is rather similar to IPCC [2]. This gives a radiative forcing at its peak
concentration of 0.18 Wm2 . Clouds are likely to be the largest source for uncertainty in radiative
forcing calculation of CFC-12.
Observations of CFC-12 show that there was a rapid growth in the three decades before 1990
when the Montreal Protocol came into effect. Thereafter, the growth slowed, and recently the CFC12 concentration peaked in Europe. With the peak concentration already reached in measurements
in Europe it can be expected that the global mean peak concentration will be reach within 2 years.
The trend in 2003 for the global concentration was extremely small. Due to uncertainties in the
atmospheric lifetime of CFC-12 and future emissions there is a substantial uncertainty also in the
evolution of the CFC-12 abundances during the rest of this century. Assuming the most likely
lifetime of around 100 years for CFC-12 it can be expected, based on the future scenarios, a
radiative forcing for CFC-12 in 2100 similar to the current radiative forcing for CFC-11.
Acknowledgements
Work at NILU was supported by the EU project SOGE (Framework Program 5) and at the
University of Liège primarily by the Belgian Federal Science Policy Office and by the European
Commission, both in Brussels. This work has received support for supercomputing at the
University of Oslo. We acknowledge receiving recent AGAGE data from D. Cunnold.
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