PII:
Atmospheric Environment Vol. 32, No. 21, pp. 3689—3702, 1998
( 1998 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
S1352–2310(98)00092–2
1352—2310/98 $19.00#0.00
THE IMPACT OF THE MONTREAL PROTOCOL ON
HALOCARBON CONCENTRATIONS IN NORTHERN
HEMISPHERE BASELINE AND EUROPEAN AIR MASSES AT
MACE HEAD, IRELAND OVER A TEN YEAR PERIOD FROM
1987—1996
R. G. DERWENT,*- P. G. SIMMONDS,‡ S. O’DOHERTY‡ and D. B. RYALL-Atmospheric Processes Research, Meteorological Office, Bracknell, Berkshire, RG12 2SY UK; and
‡ School of Chemistry, University of Bristol, Bristol, UK
(First received 30 September 1997 and in final form 22 February 1998. Published August 1998)
Abstract—The international concern following the discovery of Antarctic stratospheric ozone depletion has
prompted unprecedented international action by governments to control the production, sales and usage of
a range of ozone-depleting chemicals. These international treaty obligations include the Montreal Protocol
and its London and Copenhagen Amendments. They address, amongst many halocarbon species, the
chlorofluorocarbons: CFC-11, -12 and -113 and the chlorocarbons: carbon tetrachloride and methyl
chloroform. These chemicals have been routinely monitored at the remote, baseline monitoring station at
Mace Head on the Atlantic Ocean coast of Ireland as part of the GAGE/AGAGE programme. The
available monitoring data for the period 1987—1996 are presented here with a view to confirming the extent
of compliance with the above Protocols on a global and European basis. Daily wind direction sectors
provided by EMEP are used to sort the halocarbon data into northern hemisphere baseline air and
European polluted air masses and trends have been determined for each wind direction sector. Evidence of
the European phase-out of halocarbon usage is clearly apparent in the sorted halocarbon concentrations.
A simple climatological long-range transport and a sophisticated Lagrangian air parcel dispersion model
have been used to interpret the Mace Head halocarbon measurements and to derive estimates of European
emission source strengths for each year. These emission source strengths confirm that the phase-out of
halocarbon manufacture and sales is being followed in Europe. ( 1998 Elsevier Science Ltd. All rights
reserved
Key word index: CFCs, CFC-11, CFC-12, CFC-113, carbon tetrachloride, methyl chloroform, halocarbons, baseline monitoring, long-term trends, European emissions.
1. INTRODUCTION
The global growth in the concentrations of certain
uniquely man-made chlorofluorocarbons and chlorocarbons has been accurately monitored in the ALE
(Rasmussen and Lovelock, 1983), GAGE (Cunnold
et al., 1994) and AGAGE (Cunnold et al., 1997) global
monitoring networks. The remote, baseline station at
Mace Head, Ireland (and its predecessor Adrigole,
Ireland) has been an integral part of these networks
and, in addition to recording the global baseline
halocarbon concentrations (Cunnold et al., 1986), has
also recorded valuable information on their concentrations in European air masses (Simmonds and Derwent, 1991; Simmonds et al., 1993, 1996). This is
because of the unique nature of the Mace Head station, ideally situated on the Atlantic Ocean coast of
Ireland, where it is able to monitor halocarbon con-
centrations in baseline and polluted air masses, depending on the prevailing synoptic weather conditions. It is straightforward to infer European
emissions source strengths from the magnitudes of the
observed pollution events at the Mace Head station
(Prather, 1985, 1988).
Many of the chlorofluorocarbons and chlorocarbons monitored in the ALE/GAGE/AGAGE
networks are of prime concern for their role in the
atmospheric transport of chlorine-containing trace
gases to the stratosphere where they take part in
stratospheric ozone layer depletion. The principal
man-made halocarbons include:
f
f
f
f
f
* Author to whom correspondence should be addressed.
3689
CFC-11, (or CCl F or trichlorofluoromethane),
3
CFC-12, (or CCl F or dichlorofluoromethane),
2 2
CFC-113, (or CCl FCClF or 1,1,2-trichloro-1,2,22
2
trifluoromethane),
CCl , (or carbon tetrachloride),
4
CH CCl , (or methyl chloroform or 1,1,1-trichloro3
3
ethane).
R. G. DERWENT et al.
3690
The international concern following the discovery of
Antarctic stratospheric ozone depletion has prompted
unprecedented international action by governments
to control the production, sales and usage within
developed countries of a range of ozone-depleting
chemicals. These international treaty obligations
(WMO 1988) include the Montreal Protocol (1987)
together with its London (1990) and Copenhagen
Amendments (1992) address, amongst many halocarbon species, the chlorofluorocarbons: CFC-11, -12
and -113 and the chlorocarbons: carbon tetrachloride
and methyl chloroform which have been routinely
monitored as part of the GAGE/AGAGE programme. The obligations in the Montreal Protocol
and its Amendments amount to a complete phase-out
in their manufacture and sale within developed countries by the end of 1995. The ten years of monitoring
data for the above three chloro-fluorocarbons and
two chlorocarbons for the Mace Head monitoring
station cover the period of the phase-out and are
analysed here with a view to confirming the extent of
compliance with the Montreal Protocol within developed countries on a global and European basis and
investigating the difference between halocarbon sales
and emissions. In a companion study, analogous results are presented for the greenhouse gases (Derwent
et al., 1998).
2. METHODOLOGY
Automated two-hourly electron capture-gas chromatography (ECD-GC) measurements of the principal atmospheric halocarbons, and nitrous oxide have been collected
at Mace Head, Ireland since 1987 as part of the GAGE
(Global Atmospheric Gases Experiment) programme. Details of the site location, with maps and a photograph,
together with a description of the meteorological conditions
and climate for Mace Head are given elsewhere (Cvitas and
Kley, 1994). Under the GAGE programme, CFC-11 was
measured on two separate channels with different chromatographic columns and electron capture detectors, which give
systematic differences due to the inherently different nonlinearities between different ECDs (Cunnold et al., 1994).
Beginning in February 1994, a new gas chromatograph,
developed by the Scripps Institution of Oceanography, was
installed alongside the ageing GAGE chromatograph. The
two instruments were operated in parallel for approximately
five months until the end of June 1994 when the older
chromatograph was retired. During this overlap period, the
agreement between the GAGE and AGAGE instruments
was between 0.3—1.0% for all species. The new Scripps instrument is controlled by a Sun workstation with custom
software and incorporates a Hewlett Packard 5890 GC with
twin ECDs, and a Carle FID. Alternate calibration and
ambient air measurements are acquired every 20 min.
Details of the calibration procedures for each of the measured species have been reported elsewhere (Rasmussen and
Lovelock, 1983; Cunnold et al., 1994). In brief, secondary
calibration standards are used in the field for about 4—6
months and then re-calibrated against the primary standards
which are maintained at the CSIRO and Scripps Institutions. As a further check, periodic intercalibration exercises
are conducted with other laboratories making similar global
measurements of these trace gases. The entire ALE/GAGE
data base comprising every calibrated measurement includ-
ing pollution events is accessible at the Carbon Dioxide
Information and Analysis Center (CDIAC) at the US Department of Energy, Oak Ridge National Laboratory,
through Internet (ftp to cdiac.esd.ornl.gov).
An important aspect of our methodology is the ability to
give an air mass attribution to each of the measurments
made. In this study, three methods of air mass attribution are
used: halocarbon sorting and daily air mass sectors. Previous
work has shown that halocarbons are unique tracers of
human activities (Lovelock, 1972) and simultaneous increases in the concentrations of the halocarbons have proved
an effective method for sorting ‘‘pollution events’’ associated
with air masses from the continent of Europe (Cunnold et al.,
1986). Here, simultaneous increases of at least three halocarbons have been used to sort air masses into ‘‘polluted’’ and
‘‘unpolluted’’ categories. ‘‘Polluted’’ conditions usually occur
when air influenced by local sources or by more distant
sources in Europe is advected to Mace Head. ‘‘Unpolluted’’
conditions usually occur when air is advected across the
Atlantic Ocean and is completely uninfluenced by local
pollution sources. The origins of the air masses which arrive
at Mace Head from the continent of Europe have been
identified by using the daily wind sector allocation technique
pioneered by the UN ECE EMEP programme (Schaug et al.,
1987). The 96 h back trajectories are determined at 6-h
intervals throughout the year for the Mace Head station
(53°N, 10°W; EMEP coordinates [12.93, 12.42]). Within
each 24 h period if all four trajectories lie within a specific 45°
sector for 50% of the time, then that day is allocated to that
sector. This procedure produces a sector allocation, 1—8,
representing sectors centred on north (N), north-east (NE)
and so on to north-west (NW). Where the four daily trajectories cannot be allocated to a particular sector then that day is
unclassified and allocated to sector 9 (Lemhaus, 1985). Each
of the daily mean or individual trace gas concentrations were
sorted using this method and the mean concentrations determined for each wind direction sector over the period of
a year. The final method of air mass attribution employed
a sophisticated Lagrangian dispersion model. The U.K. MO
NAME model can, in principle, identify the most likely
source of the halocarbons arriving at Mace Head with a spatial resolution of 1°]1° within the European continent.
3. HALOCARBON CONCENTRATIONS BY AIR MASS
ORIGINS
The individual or daily mean halocarbon concentrations were sorted using the EMEP daily sector
allocation method (Schaug et al., 1987), see Section 2
above for details, and the mean concentrations were
determined for each wind direction sector for each
year of data in the 10 yr period and are presented in
Fig. 1a—e. From a consideration of the location of the
Mace Head station, ‘‘polluted’’ air masses from Europe were assigned using the wind sectors: NE, E, SE,
and S, while clean, ‘‘unpolluted’’ Northern hemisphere
baseline air masses were attributed to the wind sectors: N, NW, W, and SW.
3.1. Changes in the mean concentrations by wind
direction sector
Figure 1a shows the dramatic changes that have
occurred in the mean CFC-11 concentrations in each
of the wind direction sectors over the 10 yr period.
CFC-11 concentrations have steadily risen in the
baseline air masses attributed to the N—SW sectors, at
The impact of Montreal Protocol on halocarbons from 1987—1996
least until 1992 and have fallen slightly subsequently.
The concentrations in European air masses, particularly in the E and SE sectors, have fallen dramatically to the extent that the influence of European
pollution is barely detectable above the northern
hemisphere baseline by the years 1995 and 1996
on an annual mean basis. As will be apparent
later, European ‘‘pollution events’’ are still evident as
excursions above the baseline, although the magnitudes of these excursions have decreased over the
10 yr period.
Mean CFC-12 concentrations have been increasing
in all wind direction sectors continuously throughout
the ten year period at Mace Head. The steady increase
3691
in baseline concentrations attributed to the N—SW
sectors is clearly apparent in Fig. 1b. As with CFC-11,
CFC-12 concentations in European air masses have
declined to such an extent that they are now barely
discernible.
The pictures emerging in Fig. 1c and d for CFC-113
and CCl , respectively, also look similar to that for
4
CFC-11. Both the turn-over in northern hemisphere
baseline concentrations and the reduction in European polluted concentrations are readily apparent.
Indeed, the CCl concentrations in all sectors in 1996
4
are the lowest reported throughout the 10 yr period.
Figure 1e presents a picture of the changes that
have occurred with the wind direction sorted data for
Fig. 1. (continued overleaf ) (a) Average CFC-11 concentrations at Mace Head by wind direction sector for
1987—1996. (b) Average CFC-12 concentrations at Mace Head by wind direction sector for 1987—1996. (c)
Average CFC-113 concentrations at Mace Head by wind direction sector for 1987—1996. (d) Average CCl
4
concentrations at Mace Head by wind direction sector for 1987—1996. (e) Average methyl chloroform
concentrations at Mace Head by wind direction sector for 1987—1996.
3692
R. G. DERWENT et al.
Fig. 1. Continued (caption on p. 3691).
The impact of Montreal Protocol on halocarbons from 1987—1996
3693
Table 1. Northern Hemisphere baseline halocarbon concentrations in ppt at the Mace Head station estimated by daily wind
direction sector allocation and halocarbon sorting over the 10 yr period 1987—1996 and the differences between them
Year
CFC-11
CFC-12
CFC-113
CCl
4
CH CCl
3
3
Wind direction sector allocation method, concentrations in ppt
1987
240.95
440.30
1988
251.25
462.75
1989
257.60
480.80
1990
262.68
490.45
1991
265.73
503.20
1992
267.60
514.13
1993
267.08
520.43
1994
266.90
526.60
1995
265.60
529.90
1996
265.20
535.40
56.40
64.05
69.35
74.18
80.98
84.43
84.83
84.00
84.00
83.50
101.55
106.13
108.95
109.45
106.50
104.13
104.30
103.23
102.30
101.00
133.28
143.05
145.70
152.83
152.45
150.50
138.35
125.30
112.90
92.80
Halocarbon sorting method, concentrations in ppt
1987
240.51
440.75
1988
250.62
462.57
1989
257.64
481.41
1990
263.60
493.99
1991
266.43
504.00
1992
266.07
513.94
1993
267.58
519.48
1994
267.38
526.30
1995
266.51
531.90
1996
265.36
535.35
55.93
63.72
69.33
75.08
80.87
83.59
84.40
83.83
84.10
83.65
100.61
104.84
107.91
106.65
104.50
103.74
102.97
102.25
101.82
101.03
130.33
138.53
142.83
149.79
150.25
147.64
136.43
122.68
108.90
93.13
Concentration differences between the two sorting methods, in ppt
1987
!0.44
0.45
!0.47
1988
!0.63
!0.18
!0.33
1989
0.04
0.61
!0.02
1990
0.93
3.54
0.91
1991
0.70
0.80
!0.10
1992
!1.53
!0.18
!0.83
1993
0.51
!0.95
!0.42
1994
0.48
!0.30
!0.17
1995
0.91
2.00
0.10
1996
0.16
!0.05
0.15
!0.94
!1.28
!1.04
!2.80
!2.00
!0.38
!1.33
!0.97
!0.48
0.03
!2.94
!4.52
!2.88
!3.03
!2.20
!0.38
!1.33
!0.97
!0.48
0.03
a. Northern hemisphere baseline air masses have been allocated to the 45° wind direction sectors centred on the directions:
south-west, west, north-west and northerly.
b. CFC-11 data were taken off the Porasil channel for 1987—1993.
c. Data for 1994 onwards were taken off the AGAGE instrument.
d. Data presented with two significant figures merely to facilitate comparison.
methyl chloroform. Methyl chloroform concentrations in baseline air masses rose to a distinct maximum in 1990 of just under 153 ppt and have declined
dramatically since. The mean concentration for the
year 1996 was about 93 ppt which is lower than that
reported for Adrigole at the start of the ALE programme in 1978 (Prinn et al., 1992). A dramatic decline has also been found in the methyl chloroform
concentrations in polluted European air masses as
with the other halocarbons.
3.2. Changes in baseline concentrations
The mean northern hemisphere baseline concentrations were calculated for each halocarbon and each
year as the mean concentration over the four sectors,
N—SW, and the results are presented in Table 1. For
all the halocarbons, the mean baseline concentrations
derived from the wind sector analyses correspond
exactly with the mean concentrations for ‘‘unpolluted
air’’ based on sorting by simultaneous halocarbon
concentrations. Both methods of sorting appear to
generate baseline northern hemisphere concentrations that are entirely and accurately consistent.
According to Table 1, northern hemisphere baseline concentrations of CFC-11 based on both sorting
methods, reached a maximum concentration of
267.3 ppt in mid-1993 and have subsequently declined
by 2.1 ppt in the three intervening years to mid-1996.
The decrease in the last year between mid-1995 and
mid-1996 has been 0.8$0.2 ppt. Based on a 50$5 yr
lifetime (WMO, 1995), in the absence of sources,
global concentrations should have declined by
5$0.5 ppt yr~1. The decline in the observed baseline
concentrations is about one-sixth of that expected
based on its stratospheric sink alone, pointing to
significant global CFC-11 sources still persisting beyond the date of the Montreal Protocol phase-out of
production and sales in the developed countries. The
R. G. DERWENT et al.
3694
Mace Head data would indicate an approximate global CFC-11 source of about 110$10 thousand
tonnes yr~1 during 1996. This estimate is about onethird of the annual global CFC-11 emissions during
the 1980s (AFEAS, 1996). Atmospheric release of
CFC-11 is expected beyond the phase-out from the
‘‘bank’’ of material already in use.
The baseline concentrations of CFC-12, in contrast
to the case with CFC-11, have continued to increase
and have not turned over. The annual rate of increase
in concentrations has slowed since the 1980s and
levelled off to about 3—6 ppt yr~1. There seems little
prospect that CFC-12 concentrations will stabilise in
the immediate future at the Mace Head station. Based
on a 102 yr lifetime (WMO, 1995), this behaviour is
consistent with a global CFC-12 source of about
200$20 thousand tonnes yr~1 during 1996. This estimate is about one-half of the annual global CFC-12
emissions during the 1980s (AFEAS, 1996).
Baseline concentrations of CFC-113 have followed
a pattern similar to CFC-11 in that they increased to
a maximum concentration of 84.6 ppt in mid-1993
and have declined subsequently. The decline has however been hesitant, showing that some global sources
have continued well beyond the date of the Montreal
Protocol phase-out in the developed countries. Accepting an 85 yr lifetime (WMO, 1995), the concentration decrease during 1996 should have been about
1$0.1 ppt in the absence of global sources. The
observed behaviour at the Mace Head station is consistent with a global CFC-113 source of about 20
thousand tonnes yr~1, about one-tenth of the peak
annual global CFC-113 emission rate during the
1980s (AFEAS, 1996).
Carbon tetrachloride baseline concentrations were
the first of any of the halocarbons to reach their
maximum at the Mace Head station. The peak concentration reached just under 110 ppt during the year
1990. Subsequently, concentrations have shown an
erratic decline which has more recently firmly set in at
just under 1 ppt yr~1. Concentrations are now below
those found at the start of the GAGE monitoring
network in 1987. Accepting a 42 yr lifetime to stratospheric removal (WMO, 1995), CCl concentrations
4
should decline at about 2 ppt yr~1, in the absence of
global sources. The observed behaviour at the Mace
Head station is consistent with a global source of
about 30 thousand tonnes yr~1.
Methyl chloroform baseline concentrations peaked
just after those of carbon tetrachloride at the Mace
Head station during the years 1990—1991. Subsequently, baseline concentrations have shown a
steepening decline, reaching about 18$2 ppt yr~1
between mid-1995 and mid-1996. Using the recent
recalibration of the ALE/GAGE/AGAGE methyl
chloroform monitoring data, a global lifetime of
4.8$0.2 yr has been estimated (Prinn et al., 1995),
giving a decline in global concentrations of about
20.8$0.9 ppt yr~1 in the absence of global sources.
This decline is almost exactly the same as that seen at
the Mace Head station during 1996. The observed
behaviour at the Mace Head station is consistent with
a global methyl chloroform source of about 70 thousand tonnes yr~1, about one-tenth of the peak annual
global emission rate during the 1980s (Midgley and
McCulloch, 1995).
3.3. Changes in European air mass concentrations
The total European contribution was obtained by
summing the mean concentration minus the baseline
for each of the 45° wind direction sectors NE, E, SE
and S. The sum of the concentration differences over
the four sectors in ppt*sectors, therefore represents
the area above the baseline in the plot of concentration excess vs. wind direction. The European concentration contributions in ppt*sectors observed at
Table 2. Excess halocarbon concentrations in ppt*sectors above baseline concentrations in European air
masses at the Mace Head station estimated by daily wind direction sector allocation over the 10 yr period
1987—1996
Year
ppt*sectors
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
CFC-11
CFC-12
CFC-113
CCl
4
CH CCl
3
3
147.4
97.3
67.1
51.8
45.8
18.4
16.6
21.6
8.0
7.5
149.3
95.6
84.6
62.4
57.3
31.7
21.9
30.7
23.7
12.6
43.1
27.2
39.6
40.0
28.1
19.9
13.5
12.8
3.4
3.1
21.0
15.7
20.4
9.8
11.2
7.1
4.7
4.2
1.4
1.9
182.3
101.7
140.3
135.1
95.1
77.6
66.8
101.9
13.3
25.1
a. European air masses have been allocated to the 45° wind direction sectors centred on the directions:
north-east, east, south-east and southerly.
b. Concentration excesses were calculated by subtracting off the Northern Hemisphere baseline concentration and summing over the above four sectors.
c. CFC-11 data were taken off the Porasil channel for 1987—1993, inclusive.
d. Data for 1994—1996, inclusive, were taken off the AGAGE instrument.
The impact of Montreal Protocol on halocarbons from 1987—1996
Mace Head show evidence of a clear decline over the
10 yr period 1987—1996, see Table 2. This we take as
evidence that European emissions of halocarbons
are declining. These results support our previous
observations (Simmonds and Derwent, 1991; Simmonds et al., 1993; Simmonds et al., 1996) up to the
end of 1994 and extend them through to the end of
1996. They confirm that what was seen was not a meteorological nor statistical artefact but the result of a
measured European response to international treaty
obligations concerning halocarbon manufacture and
use which culminated in an almost complete phaseout in atmospheric release by the end of 1996.
Closer scrutiny of Table 2, however, shows that the
entries for 1994 tend to stand out as anomalously high
compared with the entries for the years on either side.
The halocarbon concentration records for the calendar year 1994 show the presence of two particularly
dramatic pollution events during February and October which stand out for their intensity and persistence.
It may well be that these events have led to an apparent overestimation of the magnitude of European
contribution during 1994 by about a factor of two.
This point will be taken up later when these table
entries are used in the estimation of European source
strengths.
4. MODELLING EUROPEAN HALOCARBON EMISSION
SOURCE STRENGTHS
4.1. European sources using a simple climatological
long-range transport model
A simple trajectory model approach using climatological data (Derwent and Nodop, 1986) was used
to calculate the integrated concentrations of each pollutant over the European sectors. The model is described in our previous study (Simmonds et al., 1993)
and provides long-term mean concentrations averaged over all the trajectories which arrive at a chosen
location, in this study, the Mace Head station. The
time dependence of the concentration, c, of a chemic-
3695
ally inert trace gas within the moving air parcel was
obtained by integrating the continuity equation:
dc/dt"E/Ah
where E is the instantaneous trace gas emission rate,
A is the base area of the air parcel taken to be
150 km]150 km for the EMEP grid (Amble, 1981)
and h is the boundary layer depth. Trace gas concentrations were set to zero at the start of the trajectory
or at the boundary of the EMEP grid. The instantaneous trace gas emission rate was calculated from
the current trajectory position in the EMEP grid, the
population density for that 150 km]150 km grid
square (Amble, 1981) and the emission rate of the
trace gas per head of population per year. A distribution of windspeeds and their long-term frequency,
together with an average boundary layer depth of
800 m were selected as representative of the longrange transport situation over north west Europe
(presumably, principally for U.K. conditions) over
each season of the year (Jones, 1981). The emission
rates of each trace gas per head of population per year
were used to scale from population densities to trace
gas emissions in the model calculations, using a European population of 508 millions. A European source
strength of 1 thousand tonnes yr~1 of CFC-11 distributed with population gave a European contribution
of 0.85 ppt*sectors, integrated over the ‘‘polluted’’ 45°
wind direction sectors. The long-range transport
model employed should produce concentrations
which are within a factor of 2—4 of other more sophisticated models (Jones, 1981).
Therefore, from the magnitudes of the pollution
peaks observed at Mace Head and the simple climatological long-range transport model, it is possible to
estimate European source strengths for all of the trace
gases, and these results are summarised in Table 3.
Most dramatic has been the year-by-year decline in
the European emission source strengths of the manmade halocarbons. For example, there has been a
94% reduction in the European emissions of CFC-11,
see Table 3. There have been similar large reductions
Table 3. European emission source strengths in thousand tonnes yr~1 estimated from the daily sector
allocations using a simple, climatological long-range transport model
Year
CFC-11
Thousand tonnes yr~1
1987
170
1988
112
1989
78
1990
60
1991
53
1992
21
1993
19
1994
25
1995
9 (19.5)
1996
9 (13.7)
CFC-12
CFC-113
CCl
4
CH CCl
3
3
152
97
86
64
58
32
22
31
24 (37.4)
13 (23.3)
68
43
63
63
44
32
21
20
5 (14.5)
5 (8.5)
27
21
27
13
15
9
6
6
2 (7.7)
2 (8.5)
204
114
157
151
107
87
75
114
15 (93)
28 (40)
a. Using a climatological long range transport model (Derwent and Nodop, 1987).
b. Based on a European population of 508 million.
c. Figures in parentheses from the sophisticated Lagrangian dispersion model (Ryall et al., 1998).
3696
R. G. DERWENT et al.
in the overall European source strengths for the other
man-made halocarbons viz., CFC-12 (91%), CFC-113
(92%), CCl (92%), and methyl chloroform (86%).
4
Clearly, there are a number of assumptions and
simplifications inherent in the application of such
a simple long-range transport model to this situation.
Windspeeds, wind directions, trace gas emissions and
boundary layer depths have all been set to constant
values appropriate to the long term by averaging over
seasonal variations. Because of the relative infrequency of some wind direction sectors, the method
appears to be appropriate only to annual averaging
periods or longer. As a result bias can be introduced
into the European source strength estimates if the
daily contributions from particular wind direction
sectors are not accumulated evenly throughout the
year. The biases introduced by the inadequate sampling of the seasonal behaviour of each halocarbon
may be significant. Individual annual European
source strength determinations may be uncertain by
as much as $25% due to this cause alone. Furthermore, if some years such as 1994 contain pollution
events which are more persistent and intense than
typical, then the use of climatological model will necessarily lead to the significant overestimation of
emission sources by up to a factor of two. Clearly,
a more sophisticated meteorological approach is
called for if more precise emission estimates are required as a check on the validity of the estimates in
Table 3, hence the application of the Lagrangian
dispersion model.
4.2. Comparison with a sophisticated ¸agrangian
dispersion model
In an alternative and more sophisticated approach,
the UKMO Lagrangian dispersion model ‘NAME’
has been used to describe the detailed nature of the
long-range transport of European halocarbon emissions to Mace Head and to estimate European source
strengths. In contrast to the simple climatological
model the NAME model attempts to describe directly
the transport of pollutants from Europe and to provide model predictions on an hourly timescale. A detailed description of the model and its application to
modelling the long-range transport of halocarbons to
Mace Head can be found in Ryall et al. (1998), so only
a brief description of the model is given here.
The model is of a Lagrangian type, in which emissions are represented by large numbers of parcels
which are advected in three dimensions by the wind,
with mixing due to turbulence represented by random
walk techniques. Meteorological data were taken
from the regional version of the U.K. Met Office
numerical weather prediction model (Cullen, 1993)
at 50 km horizontal resolution and at three hourly
intervals and were interpolated in time and space
as required. A two year simulation was performed
covering the period January 1995 to December 1996.
European sources of CFC-11 were represented within a
model domain of 20°W to 30°E and 35°N to 75°N.
Emissions on a 1°]1° grid were based on the global
fluorocarbon inventory of Prather et al. (1987), scaled
by the release rates in each country and weighted
within each country by population density (McCulloch et al., 1994) as used in the GAGE/AGAGE analyses (Hartley and Prinn, 1993) and the EDGAR
database (Olivier et al., 1996). A cut-off release rate
was defined such that particles are only released from
sources contributing'0.05% of the European total
emission. This resulted in 93% of the European emissions being represented, but reduced the number of
sources required from 600 to 299. The same emission
distribution was assumed for the other halocarbons
CFC-12, CFC-113, carbon tetrachloride and methyl
chloroform.
Boundary layer mean concentrations were calculated every 15 min. (the model timestep used) over
a grid volume defined by the boundary layer depth
and an area 0.5°]0.5°, centred on Mace Head. These
model concentrations were compared with observations of CFC-11 taken at 40 min intervals at Mace
Head. As the model only aims to describe the fluctuations above background levels, background levels
are first subtracted from the observations. For statistical comparisons a 3 hr moving average filter was
applied to both model and observed data to reduce
noise, then the model values were linearly interpolated in time to match exactly the observation times.
European source strengths were determined by scaling the model predictions to obtain a best fit between
model predictions and observations. The scaling factor is determined from the slope of the linear regression between the several thousand pairs of 3 hr model
predictions and observations for 1995 and 1996 and
for each halocarbon.
Figures 2 and 3 compare observations and scaled
model predictions for each of the halocarbons for
1995 and 1996. The overall correlation between
CFC-11 predictions and observations is good, with
the model clearly reproducing the main features of the
observed fluctuations above baseline levels. This implies that the model has captured the important mechanisms involved in the long-range transport of trace
gases to Mace Head in European polluted air masses,
and that the majority of observed increases in CFC-11
above background levels can be explained by European sources. The high correlations also indicate that
the emission distribution used is a fair representation
of the actual source distribution.
The agreement between the model predictions and
obervations of the other halocarbons are also good,
with correlations only slightly lower than those obtained for CFC-11. Given the similar inert nature of
the different halocarbons considered it is unlikely that
any model bias present would differ between the species, so the reduced correlations are likely to be due to
limitations in using the CFC-11 source distribution to
represent emissions for the other halocarbons. Evidence for this can be seen in Figs 2 and 3 where there
are a number of observed peaks not modelled and vice
The impact of Montreal Protocol on halocarbons from 1987—1996
3697
Fig. 2. Comparison of model and observed halocarbon concentrations at Mace Head for 1995 using the
Lagrangian dispersion model.
versa in the plots for CFC-12 and CFC-113, suggesting
a different source distribution compared with CFC-11.
For all the halocarbons, except carbon tetrachloride, the source strengths calculated using the Lagrangian dispersion model for 1996 are significantly lower
than those calculated for 1995, see Table 3. The reductions in source strength between 1995 and 1996 range
from 30% for CFC-11—57% for methyl chloroform.
These results support the view that European sources
of these halocarbons are declining dramatically but
have not yet declined to zero. In contrast, the carbon
tetrachloride source strength has remained fairly
constant throughout 1995 and 1996, suggesting no
significant decline in European emissions. There are
3698
R. G. DERWENT et al.
differences between the European emission source
strengths estimated with the climatological longrange transport model and the Lagrangian dispersion
model and these are highlighted in Table 3. Generally,
the emission estimates are higher with the Lagrangian
dispersion model compared with the simple model.
For CFC-11 and -12, the agreement is within a factor
of two and thus is considered acceptable. For CFC-
113, CCl and methyl chloroform the discrepancies
4
are significantly larger in relative terms. However, the
magnitudes of the European source strengths in 1996
for these particular halocarbons are very small in
historical terms and have become difficult to model
accurately using the climatological dispersion model.
The range implied by the two modelling approaches
for 1996 is therefore taken as our best estimate of the
Fig. 3. (continued opposite) Comparison of model and observed halocarbon concentrations at Mace Head
for 1996 using the Lagrangian dispersion model.
The impact of Montreal Protocol on halocarbons from 1987—1996
3699
Fig. 3. Continued (caption opposite).
European halocarbon emissions required to support
the observations at Mace Head.
The Lagrangian dispersion model can be used to
estimate the mean surface concentrations of CFC-11
across Europe and these are plotted in Fig. 4 for 1995
and 1996. The mean concentrations clearly show the
main source regions, corresponding to the main industrial regions. The differences in the concentration
patterns between 1995 and 1996 are mainly due to
differences in the mean annual meteorology. The an-
nual average CFC-11 concentrations at Mace Head
are 0.99 ppb in 1995 and 0.82 ppb in 1996, which
when normalised by the different annual emission
source strengths give concentrations above the
Northern hemispheric baseline of 0.051 ppt (1995) and
0.06 ppt (1996) for European source strengths of
1 thousand tonnes CFC-11 yr~1. This corresponding
estimate from the climatological long-range transport
model for 1996 is 0.0668 ppt, in good agreement with
the Lagrangian model estimate.
R. G. DERWENT et al.
3700
Fig. 4. Annual mean CFC-11 concentrations in ppt across Europe for 1995 and 1996.
5. DISCUSSION AND CONCLUSIONS
The successes of the international agreements to
control and eventually phase-out the emissions of the
ozone-depleting CFCs and other halocarbons within
the developed countries through the Montreal Protocol and its various amendments are powerfully demonstrated in Fig. 1a—e and the analysis of the Mace
The impact of Montreal Protocol on halocarbons from 1987—1996
Head observations reported here. At the end of 1996,
only CFC-12 does not have a negative trend in its
northern hemisphere baseline concentrations at the
Mace Head station, implying that its atmospheric
burden has not yet peaked, although its rate of growth
has slowed to about 5 ppt yr~1 (&1.0% yr~1). The
northern hemisphere baseline concentrations of all
the other major man-made halocarbons: CFC-11,
113, methyl chloroform and carbon tetrachloride, are
declining dramatically. The European countries have
clearly made a substantial contribution to the global
decline in these man-made halocarbons through significant reductions in emissions over the 10 years of
observations.
Estimates of European halocarbon emissions are
given in McCulloch and Midgley (1997), calculated
from audited sales to refrigeration, foam blowing,
aerosol and other end uses (CEFIC, 1995). Comparisons of these estimated emissions with the European
source strengths from the atmospheric measurements
illustrate generally close agreement between the decrease in industrial emissions and the observed decline in European source strengths derived from the
Mace Head data. For CFC-11, McCulloch and Midgley (1997) report European emissions declining from
132 thousand tonnes yr~1 in 1987 to 44 thousand
tonnes in 1996. The corresponding estimates required
to support the Mace Head observations, see Table 3,
are 170 and 9—14 thousand tonnes yr~1 for 1987 and
1996. There is an indication that the industrial production and use data for CFC-11 may have led to an
underestimation of CFC-11 emissions at their peak
and an underestimation of their decline, following
international action. This would indicate that prompt
CFC-11 emissions have been underestimated and that
the tail of emissions long after use has been overstated
by McCulloch and Midgley (1997).
For CFC-12, McCulloch and Midgley (1997) have
reported European emissions declining from 100 to
8.5 thousand tonnes yr~1 over the period from 1987
to 1996. In this study, Table 3 presents European
emissions declining from 152 to 13—23 thousand
tonnes yr~1 over the same period with the suggestion
that prompt emissions appear to have been overestimated by McCulloch and Midgley (1997). A similar
conclusion also applies for CFC-113, where here we
report European emissions of 5—8.5 thousand
tonnes yr~1 compared to 0.6 thousand tonnes yr~1 in
McCulloch and Midgley (1997). This present study
also shows that European emissions of carbon tetrachloride and methyl chloroform have continued
throughout 1996, albeit at a low level compared with
their historic levels, and that a phase-out of their
emissions has not been achieved in Europe.
In summary our conclusions are that:
f
by careful sorting, it has been possible to distinguish
halocarbon concentrations in northern hemisphere
baseline and European polluted air masses,
f
f
f
f
3701
the trends in global and regional trace gas concentrations appear distinctly different,
the concentrations of all the major man-made
halocarbons, except for CFC-12, have stopped
growing in northern hemisphere baseline air masses
and are now steadily declining,
European halocarbon emissions have declined in
response to international treaty obligations, though
they were still significant throughout 1996 and
a phase-out in atmospheric release has yet to be
achieved,
global sources of CFC-11, CFC-12 and CCl have
4
remained highly significant during 1996.
Acknowledgements—The operation of the Mace Head station was supported as part of the Department of the Environment Global Atmosphere Division under contract
PECD 7/10/154 and data interpretation under contract
EPG 1/1/25. The cooperation of all members of the AGAGE
team in collecting and calibrating the Mace Head measurements is specifically acknowledged. We also thank the Physics Department, University College, Galway for making the
research facilities at Mace Head available. Mr Gerry Spain
and Mr Duncan Brown provided valuable on-site daily
technical assistance. Thanks are due to Mr David Simpson,
Ms Helga Styve and Mr Egil Storen of the Norwegian
Meteorological Institute for providing the EMEP daily sector allocations.
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