Decrease in the growth rates of atmospheric chlorofluorocarbons 11 and
12
J. W. Elkins*, T. M. Thompson*, T. H. Swanson*+, J. H. Butler*, B. D. Hall*#,
S. O. Cummings*+, D. A. Fisher &, A. G. Raffo
@
* National 0ceanic and Atmospheric Administration, Climate Monitoring
and Diagnostics Laboratory, Boulder, Colorado 80303, USA.
+ Cooperative Institute for Research in Environmental Sciences,
University of Colorado, Boulder, Colorado 80309, USA.
& E. I. DuPont de Nemours Co. & Inc., Scientific Computing
Division, Central Research & Development, Wilmington, Delaware 19880,
USA.
@ E. I. DuPont de Nemours Co. & Inc., Fluorochemicals Division,
Wilmington, Delaware 19898, USA.
# Current address: Washington State University, Department of
Chemical Engineering, Pullman, Washington 99164, USA.
Abstract
The discovery of the Antarctic ozone hole [1]
in 1985 led to international efforts to reduce emissions of ozone-destroying
chlorofluorocarbons [2]. These efforts culminated
in the Montreal Protocol [3] and its subsequent
amendments, which called for the elimination of CFC production by 1996.
Here we focus on CFC-11 (CCl3F) and CFC-12 (CCl2F2),
which are used for refrigeration, air conditioning and the production of
aerosols and foams [4], and which together make
up about half of the total abundance of stratospheric organic chlorine [5].
We report a significant recent decrease in the atmospheric growth rates
of these two species, based on measurements spanning the past 15 years and
latitudes ranging from 83N to 90S. This is consistent with CFC-producers'
own estimates of reduced emissions [6, 7].
If the atmospheric growth rates of these two species continue to slow in
line with predicted changes in industrial emissions, global atmospheric
mixing ratios will reach a maximum before the turn of the century, and then
begin to decline.
We report here the recent change in growth rates of tropospheric CFC-11
and -12, based on 15 years of data from flask samples and in situ instruments
located at the monitoring stations of the National Oceanic and Atmospheric
Administration's (NOAA) Climate Monitoring and Diagnostics Laboratory (CMDL).
These stations are located at Pt Barrow, Alaska, 71.3N, 156.6W, elevation
8 m; Mauna Loa, Hawaii, 19.5N, 155.6W, 3,397m; Cape Matatula, American Samoa,
14.3S, 170.6W, 77 m, and South Pole, Antarctica, 90S, 2,841 m. Cooperative
sampling sites were also employed, located at; Niwot Ridge, Colorado, 40.1N,
105.6W, 3,472 m; Alert, Canada, 82.5N, 62.3W, 210 m, and Cape Grim Baseline
Air Pollution Station. Australia, 40.7S, 144.8E, 94 m. Starting in 1977,
paired samples were collected each week in electropolished, stainless steel
flasks (with volumes of either 300 or 850 ml) at the four NOAA/CMDL stations
and Niwot Ridge. After 1980, flask pairs at South Pole were collected once
a week only during the austral summer when the station was open. Collection
of flask pairs began at Alert in 1988, and at Cape Grim in 1991. All flasks
were shipped back to Boulder for analysis by electron capture gas chromatography
(EC-GC) [8, 9]. In 1987
our flask programme was complemented by automated in situ measurements [9,
10] (also performed by EC-GC) at the four NOAA/CMDL
baseline observatories and Niwot Ridge. Measurements are reported as dry
gas mole fractions or mixing ratios relative to the NOAA/CMDL 1991 gravimetric
calibration scale [10].
Figure 1: legend... and thumbnail of image.
Figure 1 shows monthly means of the mixing ratios
for atmospheric CFC-11 and -12 measured from more than 4,980 flask samples
collected at NOAA/CMDL and cooperative stations between January 1977 and
March 1993. The most obvious feature of the data is the levelling off of
the mixing ratios, particularly CFC-11. Additional support for the slowdown
of the atmospheric growth rates is presented in Fig.
2, which displays data collected (using in situ EC-GC measurements)
at two sealevel sites, Pt Barrow and American Samoa. The seasonal patterns
observed in the CFC data are primarily the result of transport. For Pt Barrow,
the peak mixing ratios observed from November to March are the result of
trapped cold air in the polar tropospheric vortex which restricts ventilation
with cleaner air from the low latitudes".
Figure 2: legend... and thumbnail of image.
In contrast. the CFC maximum at American Samoa occurred during the period
(November to March) of highest frequency of northwesterly flow which transports
air masses enriched with CFCs from the northern hemispherere. It is important
to note that the amplitude of the seasonal cycle for both CFCs has also
decreased with time and that this decrease is consistent with reduced emissions.
The levelling off of mixing ratios after 1989 is a direct result of the
reduced production of CFC-11 and -12 reported recently [7]
by the CFC producers. DuPont scientists estimated the emissions [13]
of both CFCs produced in the past, and those expected in the future, including
the 1996 phaseout deadline. The procedures employed were published by Gamlen
et al. [4] and are based on production surveys,
estimates for production in non-surveyed countries, and market analyses
for use after 1990. Using these new data for worldwide emissions and an
approach similar to our one-box model [14], we
calculated the annual mean mixing ratios of the CFCs for both hemispheres
from 1930 to 2000 with a two-box. finite-increment model that describes
the mean change in the tropospheric mixing ratio for a one-year interval.
The differential equations for the mean annual change in the mixing ratios
are for the northern hemisphere (n):
and for the southern hemisphere (s):
where X is the mean tropospheric mixing ratio in that hemisphere;
gamma is the ratio of the emissions in the northern hemisphere to
the total emissions (~0.95); f is the fraction of total atmospheric
CFC in the troposphere divided by the fraction of the total atmospheric
mass of the troposphere [14], (because f
has a latitudinal dependence, means that were weighted by the cosine of
the latitude were calculated as 1.10+/-0 02 (one standard deviation, s.d.)
for CFC-11 and 1.06+/-0.02 for CFC-12 from measurements taken aboard aircraft
and balloons [15, 18],
and spacecraft [19]); E is the emission rate
(mol yr [-1]); na is the total mass of the atmosphere (moles); tau
is the mean atmospheric lifetime (yr); tauns is the mean interhemispheric
exchange time [20] (1.1 yr), and deltaXns
is the mean interhemispheric difference. The best fits of the observed mixing
ratios in Fig 1 were obtained with mean atmospheric
lifetimes of 55 (+8, -9, 1 s.d.) yr for CFC-11 and 140 (+60, -35) yr for
CFC-12. Thus using projected emissions [13] estimated
by DuPont, the model predicts maximum mixing ratios for the northern hemisphere
as ~290 parts per trillion, p.p.t. of CFC-11 in 1998 and ~555 p.p.t. of
CFC-12 in 1999.
For further analysis of the growth rates for the CFCs, flask data were smoothed
and filtered by the locally weighted least-squares (that is, loess regression
[21]) procedure from the Dataplot graphics and
statistical package [22]. The period of the data
set that was locally weighted was fixed at 24 months, sufficient to examine
the interannual variations of the growth rate. We estimated the growth rates
by differentiating the time series of smoothed and filtered mixing ratios.
These values were then compared to the ratio generated by our two-box model
(Fig. 3). The uncertainty of the growth rates
was estimated by applying the Bootstrap technique [23]
to the residuals of the loess technique.
Figure 3: legend... and thumbnail of image.
One of the most significant features of the observed data shown in Fig.
3 is the dramatic slowdown in growth rates for both CFCs beginning in
late 1989. Previous reports [24, 25,
26] showed that the mean global growth rates
between 1977 and 1984 were linearly increasing at a constant rate; our results
give global rates of 9+/-1 (l.s.d.) p.p.t. yr[-1] for CFC-11 and 17+/-3
p.p.t. yr [-1] for CFC-12. Increased CFC usage during 1985-88 resulted in
the observed growth rates climbing to average maxima for both hemispheres
of 11+/-1 p.p.t.yr[-1] for CFC-11 and 19.5+/-2 p.p.t. yr[-1] for CFC-12.
Global growth rates have decreased significantly since 1988, reaching levels
of 2.7+/-0.7 p.p.t. yr[-1] for CFC-11 and 10.5+/-0.3 p.p.t. yr[-1] for CFC-12
by 1993. Perhaps the most significant fall-off of CFC-11 has occurred at
the most northerly stations, Pt Barrow (Fig. 2)
and Alert, where the rate of increase has essentially reached zero. This
corresponds to the decrease in industrial production [6,
7] in the northern hemisphere where most of the
material is used and therefore emitted. At the same time, growth in the
southern hemisphere has decreased less rapidly because its growth rate is
determined mainly by interhemispheric transport. The dramatic fall-off of
CFC-11 is a result of its curtailed use particularly in the aerosol propellant
and foam blowing applications in 1990. If these slowed growth rates are
common for all compounds under regulation by the Montreal Protocol, the
lower limit for the total cumulative organic chlorine, CCly, introduced
into the stratosphere would be less than 4.0 parts per billion (p.p.b).
(current value [5]= 3.4 +/- 0.2 p.p.b.).
Besides CFC production, other factors such as atmospheric transport may
affect the observed hemispheric growth rates. A statistically significant
divergence of the northern and southern hemispheric growth rates occurred
during the period of the El Nino-Southern Oscillation (ENSO) event of 1986-87.
This ENSO had a much stronger correlation with the growth rates of both
CFCs than did the event of 1982-83. ENSO events, occurring every 4-7 years
[27], are known to affect the interhemispheric
transport of trace gases and have been correlated with lower than average
mixing ratios of CH3CCl3 at Samoa [28]. Suppression
of the westerly winds at 200 millibar (mb) in the upper troposphere which
occurs during ENSOs [27] reduces the interhemispheric
transport [29]. The variations observed in the
hemispheric growth rates for both CFCs correlate well with hemispheric variations
recently noted for methane [30] during 1986-89.
A reason that the ENSO of 1986-87 had such a strong effect on the growth
rates may be the coincidences of a quasi-biennial oscillation event (QBO),
typically occurring every 26 months [29], followed
by the strongest cold event in 15 years, the La Nina [27]
of 1988-89. Also, there is a good correlation between the maximum of easterly
winds of the equatorial stratosphere [31] (QBOs)
and the drop in the southern hemispheric growth rate for both CFCs (Fig.
3). Although there is considerable evidence of tropospheric QBOs [32],
dynamic models also show that strong easterly winds in the stratosphere
can enhance vertical transport upward from the troposphere [33]
and concurrently decrease the north to south tropospheric exchange [29].
As the mixing ratios of the CFCs have always been higher in the northern
than southern hemisphere, any process that inhibits interhemispheric exchange
or enhances tropospheric-stratosphere exchange will slow the growth rates
in the southern hemisphere.
These decreased overall growth rates are encouraging in light of voluntary
national and international efforts to limit emissions of ozone-depleting
substances. Note that a recent report [14] also
observed decreased atmospheric growth rates for the two dominant atmospheric
halons, which are used primarily in fire extinguishers, and pose a significant
threat to stratospheric ozone, although less than that posed by the CFCs
considered here. Furthermore, continuous monitoring of CFCs, together with
accurate tracking of emissions, will help to define the chemical lifetimes
of these species and may provide a more complete understanding of the transport
properties of the atmosphere.
Received 30 June 1992; accepted 7 July 1993.
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Acknowledgements
We acknowledge the contributions of all personnel involved in collecting
flask samples and maintaining the EC-GC at NOAA/CMDL and cooperative stations.
We also acknowledge assistance from R. A. Rasmussen, R. F. Weiss, W. D.
Komhyr, E. G. Dutton, M. McFarland, P. P. Tans, P. J. Fraser, N. B. A. Trivett
and S. A. Montzka. This work was supported in part by the Atmospheric Chemistry
Project of NOAA's Climate and Global Change Program.