Pergamon

Atmospheric Environment Vol. 30, Nos 10/11, pp. 1621-1629, 1996
Copyright © 1996 Elsevier Science Ltd
Printed in Great Britain. All fights reserved
1352-2310/96 $15.00 + 0.00

1352-2310(95) 00425-4

SULFUR HEXAFLUORIDE--A
POWERFUL
ATMOSPHERIC TRACERNEWMANFRED MAISS,* L. PAUL STEELE,l" ROGER J. FRANCEY/F
PAUL jl. FRASER,I'~ RAY L. LANGENFELDS,I" NElL B. A. TRIVETT§and INGEBORG LEVIN**Institut fiir Umweltphysik, University of Heidelberg, INF 366, 69120 Heidelberg, Germany; i'CSIRO
Division of Atmospheric Research, Private Bag No. 1, Mordialloc, Victoria 3195, Australia; ~CRC for
Southern Hemisphere Meteorology, Monash University, Clayton, Victoria 3168, Australia; and §Atmospheric Environment Service, 4905 Dufferin Street, Downsview/Ontario M3H 5T4, Canada(First received 20 March 1995 and in final form 25 September 1995)
Abstract--Long-term observations of the atmospheric trace gas sulfur hexafluoride (SFr) at four background monitoring stations, Neumayer, Antarctica (1986--1994),Cape Grim, Tasmania (1978-1994), Izafia,
Canary Islands (1991-1994) and Alert, Canada (1993-1994) are presented. These data sets are supplemented
by two meridional profiles collected over the Atlantic Ocean (1990 and 1993) and occasional observations
at the regional site Fraserdale, Canada (1994). The analytical system and the method of SF6 calibration are
described. Compared with data from Neumayer and Izafia reported earlier, measurements are updated forall sites until the end of 1994 and the precision has improved by more than a factor of 2. With the Cape
Grim archived air samples, the atmospheric SF6 chronology is extended by 8 more years back to 1978. For
the period from January 1978 to December 1994 the data confirm a stable and unbroken quadratic rise intropospheric SF6 from 0.50 to 3.11 ppt in the southern hemisphere and for July 1991 to December 1994
from 2.69 to 3A4 ppt in the northern hemisphere. The global mean tropospheric increase rate in late 1994
was 0.225 ppt yr- t (6.9% yr- t). The long term trend and interhemispheric gradients are due to industrial
production and emission, rising approximately linearly with time and located predominantly (94%) in thenorthern hemisphere. The interhemispheric exchange time (1.7 + 0.2 yr) derived from SF6 ground level
observations when using a two-box model of the atmosphere is considerably larger if compared to theexchange time derived from two- and three-dimensional models (1.1 yr). The chemical and biologicalinertness of SF s up to stratospheric conditions results in an atmospheric lifetime of more than 800 years
and makes S F 6 a powerful tool for modelling transport processes in the atmosphere. Moreover, thetropospheric SF 6 chronology is a very valuable input function for mixing studies in linked compartments
like the stratosphere, the hydrosphere and the cryosphere.
Key word indeg:: Sulfur hexafluoride, atmospheric trace gas, interhemispheric exchange, GC-ECD.

1. INTRODUCTION
Sulfur hexafluoride is an extremely stable atmospheric
trace gas produced entirely anthropogenically. Its
unique physico-cherrlical properties make this gas
ideally suited for m a r y specialized industrial applications. Most of the SFs produced worldwide ( ~ 80%)is used in and released from electrical equipment,
predominantly Gas I:asulated Switchgear (GIS). This
primary use started :in the 1960s. Historical release
estimates are available for the years 1953-1974 risingfrom 14 to 1200 Mg SF6 yr-1 (Ko et al., 1993, andreferences therein). For the beginning of the 1980s
a world-wide production rate of 2500- 4500 Mg SF 6
yr-1 is reported (Encyclopedia of Chemical Technology, 1980) rising approximately linearly since 1970 toan estimated value of 5000-8000 Mg SF6 y r - t around1990 (SOLVAY, SF~ producer, personal communication, 1993). These numbers on SF6 production

are very rough estimates. Beyond this, the amount of
SF6 that is banked inside electrical equipment is highly uncertain. However, observing the global increase
of SF6 in the atmosphere provides a reliable accumulation record from which release rates can be calculated. This approach has been used by Maiss andLevin (1994) to model an atmospheric emission history, namely the history of SF6 production assuming
different residence times associated with its commercial applications. These results agreed reasonably wellwith the production estimates.
The accumulation of sulfur hexafluoride in the atmosphere is nearly perfectly described by its release
rate due to its long atmospheric life-time of probably
3200 yr (Ravishankara et al., 1993). A minimum estimate of 800 yr is given by Morris et al. (1995). The
oceanic sink to date is negligible due to the low watersolubility of SF6 (Ostwald solubility coefficient
1%, Wanninkhof et al., 1991) and a relatively slow

1621

1622

M. MAISS et al.

oceanic deep-water formation rate. Since the very firstatmospheric measurements in 1970 (Lovelock, 1971),
the SF 6 concentration has increased by two orders ofmagnitude from 0.03 ppt (parts per trillion by mole) toa global mean value of 3 ppt at the end of 1993. This
dramatic increase recently brought SF6 into the climatic impact discussion since, on a per molecule basis,the greenhouse effect of SF6 is likely to be one of thehighest of any atmospheric trace gas (Rinsland et al.,1990). In fact, the Global Warming Potential for SF 6
relative to CO2 on a 100 year time horizon has been
estimated to be about 20,000 (Ko et al., 1993; Stordal
et al., 1993). Only the present low SF 6 concentrations( ~ 10 -8 of CO2; -~ 10 -2 of CFC) preclude any actual climatic relevance.
Here we describe the Heidelberg low-level SF6
analysis technique by electron capture gas chromatography including the calibration procedure which wasused to achieve the complete SF6 data set presented
here. The main purpose of the paper is to present newworld-wide high-precision SF6 data records now
covering the past 18 yr by including samples from theunique air archive from Cape Grim, Tasmania (Fraser
et al., 1991). In a similar way, as described by Maiss
and Levin (1994) we use a simple two-box model ofthe atmosphere to explain the interhemispheric difference and to derive a long-term trend function for the
tropospheric SF6 chronology in both hemispheres.
The potential to apply SF6 as a transport tracer in theatmosphere and in linked compartments like the hydrosphere or the cryosphere is also discussed briefly.

2. ANALYTICALSYSTEM
Analysis of sulfur hexafluoride is performed by gas
chromatography using an electron capture detector(Shimadzu GC-8AIE). The system, shown schematically in Fig. 1, is a compact transportable device,primarily designed for on board SF6 analysis of water
samples during a tracer release experiment in Lake
Constance, Germany (Maiss et al., 1994a, b). Here webriefly summarize the major features with emphasison routine air sample analysis for the present work.
The analytical system can be roughly divided into
five components: (1) the six-port selection valve V1 toload different types of samples. It allows either the gas
content of the stripper reservoir to escape during
water sample injection through the vent of V3, orallows one of the two sample loops selected by valve
V3 to be flushed with nitrogen (positions D and F of
V1) or loaded with standard or a gas sample; (2) the
extraction unit for water samples, including the stripper reservoir, the Mg(CIO4)2 drier, and the ten-portvalve V2. During gas sample analysis, this part is used
as a through pass and a switch for flushing the content
of the in-line sample loop onto the trap; (3) the injection system for gas samples, being loaded throughvalve V1 and consisting of two sample loops selectedby valve V3. Four volume calibrated loops are avail-able: 0.1040 _ 0.0014, 1.006 ___0.002, 4.817 ± 0.002
and 15.024 + 0.002 cm 3. In principle, air samples with
sub-ambient pressure can be analyzed as well byconnecting a vacuum system at the vent of valve V3;(4) the trapping system, namely the trap, heater/cooler
and functions of valves V2 and V4; (5) the chromatographic separation and detection system, including
pre- and main-column, valve V4, and the electron
capture detector (ECD). Since components are integrated into one unit, most of the valves are involved in
more than one function.
Valve switching, heating and cooling of the trap arecontrolled by a portable PC recording the ECD-signal and providing instantaneous peak integration. In
this way, the reproducibility is improved, since only
the sample injection remains manual. Standard andblank measurements are automatic. Gas samples takeabout 10 min analysis time. Ultra-high purity nitrogen further cleaned by an activated charcoal filter isused as carrier gas. This purified nitrogen is tested tobe free of SF 6 ( < 10- 5 ppt) and is used also for blankmeasurements. The four valves V1-V4 (Valco E-2)
can be positioned electronically and have very small
dead volume stainless steel rotary heads. The content
of one or more sample loops is collected in the trap
filled with Porapak-Q © and cooled down to - 77°C
by a dry-ice/isopropanol bath. The cryogenic preconcentration allows large volume samples and thus
high-precision analyses of low mixing ratio samples.
Electromechanical repositioning of the isolated trap
between two copper blocks held at + 100°C completely releases the trapped SF6. Chromatographic
separation starts by the injection of the heated trap
content onto the precolumn (30 cm) which is backflushed when SF6, being the fastest component, hasreached the main column (300 cm). This enhances
sample throughput. Both columns are made of
2.4 mm ID stainless steel tube, packed with 5 A molecular sieve (80-100 mesh, Alltech), located in the GC
oven and kept at 65°C. With a carrier gas flow rate of
20 ml min-1, the SF6 peak reaches the ECD kept at
330°C 2 min after trap injection.
In the present work, air sample analyses were carried out with the 15.024 cm 3 loop (position 2 at V3),
while for standard gas analyses the 1.006 cm 3 loop
(position 1 at V3) was always used. All SF6 measurements are first related to our 93.7 ppt secondary
standard. F o r testing the linearity of the system, different numbers of standard gas aliquots from the 1.006
cm 3 and the 0.104 cm 3 sample loop (at positions 1 and
2 of V3) were collected in the trap and analyzed. The
response of the system to SF6 is linear (R z > 0.9999)
from the minimum detectable level of 0.015 fmol up
to 10 fmol with a typical sensitivity of 22 mV s fmol- x(1 fmol = 10 -15 mol). During routine analysis thesensitivity of the whole analytical system is measured
by two standard gas analyses every 8 air samples. The
analytical precision of a single sample is dominated by
the peak area uncertainty of replicate blank measurements of + 0 . 1 2 m V s leading to an accuracy ofSulfur hexafluoride--a powerful new atmospheric tracer

ALTERNATIVE L

FLUSH/ OPEN
STRIPPERGAS~ISOLATESAMPLE J

WATER
SAMPUNGSYRINGE

1623

D C

O

~

SF6-STANDARD
ISOLATESTRIPPERFLOWCARRIERFLOWBACKFLUSHFLOW

@

3-WA~
STOP
COCV

SELECTION VALVE

VENT
COLUMN.JW~ PRE'COLUMN<-- FORWARD-> BACKFLUSH

I 'VENT@@STRIPPER& LOOPISOLATED(STRIPPER& LOOPIN LINE)

LOOP1 IN UNE& LOOP2 LOAD
(LOOP2 IN LINE& LOOP1 LOAD)®
TRAPLOAD& PRE-COLUMN:BACKFLUSH+ ISOLATE(TRAPINJECT& PRE-COLUMN:FLOWFORWARD+ IN LINE}

Fig. 1. SF6 analysis system: legends describe the different functions of the valves V1-V4 for the shown
position and the reverse position in parentheses.

___1.3%. Higher precision was achieved when working, in addition, with five air standards at the concentration range of 1.6-3.2 ppt. These background air
standards are routi~aely analyzed together with thesamples of unknown concentration. The results forthe air standards on an individual day are used ina second evaluation :step to adjust the peak area of theblank in an optimum way through least squares fitting. This procedure leads to an improvement of theblank determinatior: on individual days and thereby
to an overall analytical precision of about ___0.5%.
A large number of the air samples stored in aluminium or stainless steel tanks over different time periods (up to 6 yr) was analyzed repeatedly over the last
3 yr. No drift in concentration has been observed inthese samples.

3. CALIBRATION M E T H O DAll SF6 measurements are related to our 93.7 ppt
SF6 secondary gas standard prepared in the Heidelberg laboratory from a commercially available gravimetric high pressure 0.1 ppm (parts per million) SF 6 in
N2 primary standard with a certified accuracy of+ 1% (Messer-Griesheim, Germany). A gravimetrically controlled single step static pressure dilution
with ultrapure N2 was used, giving an absolute nom-

inal accuracy of our secondary gas standard of+__1.1%. Even though this method is simple andreliable it is not very common. Therefore we will
describe the preparation procedure here in detail.
Firstly, the volume of a normal 2 E aluminium tank
(Luxfer) fitted with a stainless steel valve and designated to take the final gas standard, is precisely calibrated by weighing it, repeatedly evacuating, and fillingwith N 2 at ambient pressure and temperature. Gas
density and mass difference yield the tank volume VT.
Material and size of the tank were chosen to staywithin the usual range of high precision scales (4000 g)even if filled with 200 atm of N2. The scale precision( ___0.01 g) is sufficient to calibrate a volume of about
2000 cm 3 with an accuracy better than 0.5%. We
obtained Vx = 2059 + 10 cm 3. To contain an aliquotof our primary standard (concentration Co = 0.1 ppm( + 1%)) we used a stainless steel cylinder of calibrated volume (Nupro), Vc = 50cm 3 (-q-1%), fittedwith a stainless steel valve. This cylinder was evacuated, weighed, and filled up to about 6 atm with our
primary standard. Due to the lower weight of thiscylinder the resulting increase in weight, Amc= 0.366 g, is measured with a precision of ___0.001 g.
The filled cylinder was then connected to the evacuated 2 E aluminium tank and its content expanded
into the tank. After pressure equilibration, both containers were closed and disconnected. A 50 ¢ tank ofM. MAISS et al.

1624

ultrapure, SF6-free N2 (200 atm) was then attached tothe aluminium tank to bring it close to equilibrationpressure (,~ 190 atm). The aluminium tank wasweighed and the mass difference relative to the evacuated weight was determined (Amx = 379.91 + 0.01 g).
Even considering an unprecisely known dead volumeof the fittings used for coupling the 50 cm 3 cylinder
to the aluminium tank (VD ~ 7.5 + 1 cm 3 ), thefinal standard concentration Cs=Co.{Vx/(Vr+Vc + VD)}'{Amc)/AmT} = 93.7 ppt is determinedwith a nominal uncertainty of _ 1.1% (Gaussian
addition of errors) being mainly due to the + 1%
uncertainty of the primary standard. The assumptionof ideal gas behavior is used to calculate the tankvolume VT at a pressure of ~ 1 atm where the nonideal correction for N2 ( + 0.03%) can be neglected.
Our calibration method needs no accurate measurement of high pressures. Only simple volume andweight determinations are to be made. A dilution
factor of ~ 10 - s can be achieved by one single step.
There is no internationally accepted gas standard
for SF6 in the ppt range available, and no intercalibration with other laboratories has been undertaken so
far. However, a comparison based on atmosphericbackground concentrations derived recently fromoceanic background observations by Law et al. (1994)
indicates absolute differences between the two laboratories of less than ___5%. Our standard was prepared gravimetrically and is, thus, defined in a mole
fraction scale. Consequently, and therewith following
conventional practice (R. Weiss, personal communication) all concentrations given in the present paper
are mole ratios.

4. S A M P L I N G SITES

Long-term observations of SF6 at four backgroundair monitoring stations, Neumayer (Antarctica), Cape
Grim (Tasmania), Izafia (Canary Islands), and Alert(Canada) are the basis of the present SF 6 study. In
addition to these time series, two shipboard meridional profiles of background air over the Atlantic
Ocean were analyzed as well as occasional samples
from the regional site Fraserdale (Canada). Except for
the Cape Grim baseline station, the samples are highvolume air samples originally collected for methane
isotope analyses in Heidelberg (Levin et al., 1992). The
six sampling sites in the northern and southern hemisphere are characterized as follows:

Neumayer (Antarctica, 71°S, 8°W, 42 m asl). At theGerman Antarctic Neumayer station, situated on the
EkstriSm ice shelf close to the ice edge, monthly, and inlater years up to weekly, spot samples of about 1 m a of
air were directly compressed into 10 E high pressuretanks using a diving compressor (Bauer, Miinchen).
Before 1991 stainless steel tanks were used as sampling containers, in the later years, in most cases, weused aluminium tanks. Air sampling is controlled bywind direction to guarantee background conditions.
With increasing time resolution, 225 samples from
October 1986 till December 1994 have been measured. This data set provides the most comprehensive
quasi-continuous time series of SF6 measurements.
Cape Grim (Tasmania, 41°S, 145°E, 94 m asl). Since
1978 air has been archived at the Cape Grim Baseline
Air Pollution Station in pressurized tanks. A detailed
description of the sampling technique at this stationon the northwestern tip of Tasmania, Australia, isgiven by Fraser et al. (1991) and Weeks et al. (1992).
The Cape Grim air archive has been shown to accurately represent the Cape Grim in situ record for
CFC-11 and CFC-12 over the period 1978-1993(Cunnold et al., 1994). Samples of the Cape Grim
air archive were transferred to previously evacuatedelectropolisbed stainless steel flasks of 1.6 and 0.9 f
volume. Typical pressure in subsamples was 200500 kPa absolute, mostly achieved by direct transfer,
but assisted cryogenically if the parent archive tank
had a lower pressure. The integrity of the subsample
was assessed by comparing measured values in parent
and subsample for a full range of species measured in
at CSIRO, including CO2, CH4, CO, H2, N20, andseveral CFCs. Typically, agreement was better than
2% in all cases. In the Heidelberg laboratory, SF6
analyses were performed on 44 subsamples over thetime range of 1978-1994. This time series represents
the longest atmospheric SF6 chronology available todate.
Iza~a (Tenerife/Canary Islands, 28°N, 16°W,
2367 m asl). At this high altitude maritime background observatory in the North Atlantic trade windsystem operated by the Institnto National de Meteorologica, Madrid, Spain, air is most of the time
representative of the free mid-troposphere in this latitudinal belt (Schmitt et al., 1988). Air has been collected continuously over two weeks in high volume
bags made of polyethylene coated weldable aluminium foil. These integrated samples of about 1-1.5 m 3
STP were subsequently transferred to 10 ~ aluminium
tanks using a diving compressor (Bauer, Mfinchen).
85 Izafia air samples have been analyzed for SF6 since
July 1991.
Fraserdale (Canada, 50°N, 82°W, 200 m asl). Weekly integrated samples collected in 1994 have been
analyzed from the regional station Fraserdale, runby the Atmospheric Environment Service (AES),
Toronto, Canada. The air sampling technique is thesame as used at Izafia. The Fraserdale station is
located in the Hudson Bay Lowlands far away fromanthropogenic sources. Moreover, trajectory analyses
show that the station, for most of the time, is influenced by northerly air masses (AES, unpublishedresults). The station should, therefore, represent continental background conditions in mid-latitudes of thenorthern hemisphere.
Alert (Canada, 82°N, 63°W, 187 m asl). 36 samples
from the High Arctic Research Laboratory, Alert, also
operated by the AES, have been analyzed. The station

Sulfur hexafluoride--a powerful new atmospheric tracer
is located at the northeastern strip of Ellesmere Islandand is thus far removed from major industrial regions.
This site is well suitc;d to monitor the background airof the high northern latitudinal belt (Trivett andWorthy, 1989). SF6 measurements on two-week integrated samples collected in a similar way as at Izafia
and Fraserdale started in April 1993 and are available
until December 199.4 except for a six week gap from2 May to 14 June 1994.
Latitudinal profiles. High volume surface air spot
samples were collected during two cruises of the German RS Polarstern on the Atlantic Ocean. In November 1990 samples were taken from 30°N to 68°S on
a cruise from Germ~my to Antarctica, and in November 1993 we collected samples from 40°N to 56°S on
a cruise from Germany to South Africa.

5. ATMOSPHERIC SFe HISTORY

Figure 2 summarizes all SF6 background observations achieved in this investigation for the northern(NH) and the southern (SH) hemisphere. Comparedto the preliminary ,data from Neumayer and Izafia
reported by Maiss and Levin (1994), the measurements presented here have more than double theprecision, and are updated until December 1994.
Additionally, archived air samples from Neumayer
station have been analyzed together with the new
samples from Alert, Fraserdale and from Cape Grimarchived air. With the latter, the atmospheric chronology of SF6 is extended by 8 yr back to 1978.
Time-series of sulfur hexafluoride in both hemispheres show very little scatter and a steady long-term
increase. The concentration in the NH is persistently
higher when compa:red to the SH due to a relatively
strong increase rate resulting from predominantly N Hsources, and an interhemispheric exchange time of theorder of 1-2 yr. Concentrations observed at Alert
normally exceed those observed at Izafia, on averageby 0.07 ppt in 1994. Maximum "background" concentrations are obserw:d in mid-northern latitudes, atFraserdale, exceeding concentrations observed at
Izafia, on average by 0.14 ppt in 1994. These maximum concentrations can be explained by the fact thatthe main sources of SF6 are located in northern midlatitudes. F o r comparable sampling periods in theSouthern Hemisphere (SH), the Cape Grim and
Neumayer data are identical within their inherent
variations. This impressively shows that, with respect
to SF6, mid-to-high latitudes of the SH seem to bewell mixed. Also before 1986, the Cape Grim data
reflect a nearly perfect back-extrapolation of thelong-term trend established by the Neumayer data.
This means a stable and unbroken trend in the rise of
atmospheric SF6 since 1978. For the period from May
1991 to February 1992, the Neumayer record showsa concentration bump in combination with data variations larger than in the remaining part of the SH
record. An error during sample analysis can be ex-~30:10/11-6

1625

cluded. However, since the period exactly coincides
with the use of new tanks (Spectra-Seal, BOC), we arestill investigating if this concentration bump comesfrom a small contamination in the sample containers.
These debatable data are shown in Fig. 2 but havebeen excluded in the long-term trend fitting given
below.
Both shipboard latitudinal profiles, shown by
the inset of Fig. 2, are consistent with the timeseries. An identical concentration scale for the inset
allows for direct comparison of the latitudinal distribution with the time series data. Southern hemispheric means for the troposphere of 2.34 _ 0.03 ppt
for November 1990 and 2.91 + 0.02 ppt for November 1993 are calculated by weighting the cruise databy the area of the equivalent latitude band. For theNH, means of 2.57 ppt (1990) and 3.15 ppt (1993) were
derived with the same method. These N H means arelikely to be minimum values since missing cruise data
for mid-to-high latitudes result in underestimated N H
means. Therefore, based on the cruise data for thelatitude of Izafia, we have used the above mentioned
mean concentration shifts for the sites Fraserdale( + 0.14 ppt) and Alert ( + 0.07 ppt) to create two supplementary "data points" at 50 ° and 82°N. These
values are shown as well in the inset of Fig. 2. With
these additional estimates for 50 ° and 82°N we havecalculated NH means again and obtain 2.67 ppt(1990) and 3.21 ppt (1993) as probably the more realistic values.
Maiss and Levin (1994) have already shown
that, based on Neumayer observations, an extrapolation of the global increase of SF 6 back to 1970 matches other historical measurements in the 1970s andearly 1980s reasonably well. Further, using a simple
two-hemispheric box model, the long-term trend of
SF6 observed in the troposphere is best fitted bya quadratic increase. Linear or exponential trend
curves would conflict with earlier observations andare not able to describe the onset of marked SF6
emissions in the 1960s when the predominant industrial application in GIS systems started. Interpretation of the SF6 concentrations obtained in both thisstudy and earlier work, requires a model for atmospheric mixing as well as for the global source distribution of SF 6. For the latter we will make the following estimates:
The dominant use of SF6 (80%) is in heavy electrical equipment, such as gas insulated switchgear (GIS)
and substations (Niemeyer and Chu, 1992). The remaining 20% is mainly used in metal industries forblanketing or degassing the molten reactive metals
aluminium and magnesium (Stordal et al., 1993). In
the case of GIS, the occasional release of SF6 into theatmosphere, either through small leakage from equipment or discharge during equipment repair or maintenance, should be distributed down to a country
scale and directly correlated with electric power production. In 1987, the NH produced 94%, 88%
and 100% of total electric power, aluminium andM. MAISS et al.

1626
4.0

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SIN (Latitude)
0

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~-7-7-7-~-5~_7"~-'7"7-7-; NOV.'93
3.0 -I

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Latitudinal Profiles....c,.~ _o_.e..e_.o. -o_. _ .l-rl====================== NOV.'90
2.5 -

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2.0

~

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1.5

1.0

0.5

78

79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

Year

Fig. 2. S F 6 observations in the atmosphere: southern hemispheric records for Neumayer station (©) andCape Grim (rq) are based on spot samples, northern hemispheric time series for Izafia ( + ) Alert (e) andFraserdale (A) are obtained from one- and two-week integrated samples. Not shown are three outliers from
Fraserdale and one outlier from Izafia being probably contaminated by polluted air mass plumes. The
analytical uncertainty of individual data points is always below symbol size. Assuming 94% of S F 6
emissions occurring in the northern hemisphere, results of a quadratic trend model fitting are given as
dashed curve for the southern hemisphere and as solid and dotted curve for the northern hemisphere, using
an interhemisphere exchange time of 1.7 and 1.1 yr, respectively. Inset: (<>) Latitudinal profiles of SF6
ground level concentrations over the Atlantic Ocean. Hemisphere means are indicated by dashed-dottedlines. A second higher mean in the northern hemisphere is obtained if indicated estimates for Alert (e) andFraserdale (A) are considered additionally. Open squares joined by a dashed-dotted line allow to link thelatitudinal profiles information to the time-series of the main figure.

magnesium production, respectively. For a varietyof smaller, more sophisticated SF 6 uses, a 100%
N H contribution seems realistic. If the 80% heavy
electrical use of SF6 is associated with 94% of
the total power production in the NH, and the remaining 20% of SF6 uses are associated with the
88-100% use in the NH, between 93% and 95% of
the total SF6 emissions occur in the NH. Prather et al.
(1987) estimated a figure of 95% for the N H power
production in 1978, and we assume that a similar
percentage of northern hemispheric SF 6 emissions(94 ___1%) is applicable for the period back to thattime.
For a simple analytical long-term representation of
our tropospheric data, a two-box mass balance model
can be formulated assuming an atmospheric lifetimeof SF 6 much longer than its atmospheric occurrence,
and an emission rate E(t) of SF 6 ( 1 0 - 1 2 m o l y r -1)
increasing linearly with time:dCNn (t)/dt = y" E(t)" 2/neff -- ACNs(t) ze~(1)

dcsn(t)/dt = (1 - y)'E(t)" 2/neff + ACNs(t)/zex(2)

where CNn(t) and CsH (t) are the mean surface concentrations in both hemispheres; AcNs = crq, (t) -- CsH(t) is
the mean interhemispheric difference; noff is the massof the effective atmospheric reservoir in moles (to get
the global inventory from surface concentrations); zex
is the interhemispheric exchange time and 7 (94%) is
the ratio of N H emissions to total emissions. With
time t in years AD (e.g. t = 1992.0.¢~ 1 January 1992)
and with CNn(to)= Csrr(to) = 0 the solution of equations (1) and (2) can be approximated fort > (to + 2 yr) by the following quadratic time dependency for the SH:
Csn(t) = u ' ( t -- to)2

[ppt].

(3)

The values of u and to were derived by fitting equation(3) to the Neumayer and Cape G r i m data, and foundto be 0.003865 and 1966.635, respectively. Figure 2
displays this result by a dashed curve providing an
excellent representation of the observed SF 6 record in
the SH. Further, in terms of the revised parametersu and to if compared to the earlier estimate of Maiss
and Levin (1994), the model solution for the emission
rate and the interhemispheric concentration differenceSulfur hexafluoride--a powerful new atmospheric tracer
is given by:
E(t) = 2u n~fe{t - to + z~x" O' -- 0.5)}

r i 0-12 mol y r - 1]

(4)

ACNs(t) = 2UZ¢x{(27 -- 1)(t -- to + z,~'(y -- 1))}[ppt].

(5)

A value for ne, is only needed for emission calculations through (4). From own unpublished vertical
SF 6 profiles we estimated that during the last 15 years
neff is at least 95% of the total atmospheric dry mass(natm= 1.77 x 1020 tool). Neglecting this slight difference of < 5% (n~ee ~ natm) and using derived values
for u, ~, to and %~ = 1.7 yr, equation (4) can be simplified to a emission r~tte estimate in Gg y r - 1 as follows:
E(t) = 0 . 2 - { t - 1966}. For example 5.7 G g y r -~ areobtained for 1994 fitting well into emission scenarios
based on earlier data (Maiss and Levin, 1994, andreferences therein).
Using SH fit values for u and to and adding (5) to (3)gives the model solution CNH(t) for the NH. The only
unknown %x is obtained by least squares fitting of
CNH(t) to the Izafia ,data set. An interhemispheric exchange time z¢~ = 1.7 ___0.2 yr yields the best fit for
7 = 94% and is shown in Fig. 2 by a solid curve.
A variation of 7 by :[%0 alters the value for %~ only by+ 0.04 yr.
With equation (5) solved for %x, interhemisphericexchange times cart be calculated from the n o r t h south concentration difference, AcNs , derived fromhemispheric means of measured latitudinal profiles aswell. Depending on which of the two different estimates for the Northern Hemisperic mean (Fig. 2) thecomputation of AcNs is based on, a range for %~ from
1.4 to 2.0 yr and from 1.3 to 1.6 yr is obtained for
November 1990 and 1993, respectively. The minimum
values for %x are obtained if only measurements up to
40°N are considered. Within the obtained uncertainty
ranges, the results are consistent with the long-termmean %~ = 1.7 ___0.2 yr derived from the continuous
data. On the other hand, the wide ranges also show
how sensitive estimates of zo~ are on the way the NH
means have been determined.

1627

thereby dropping our former assumption that SF6 is
exclusively emitted in the NH, is responsible for one
half of the apparent increase of %x. The second half isdue to a revision of the fitting parameters u and to to
the considerably extended SH database. Further, thetime overlap of the SH and the NH records is now
four years, which ensures 1.7 __+0.2 yr being a more
representative determination of zoo.
We implied as first order approximation that observations from Izafia and Neumayer (or Cape Grim) arerepresentative means for the NH and the SH surface
concentrations, respectively. Hemispheric means calculated from our latitudinal profiles justify this firstorder approximation. Further, two-dimensionalmodel estimates for SF 6 by Levin and Hesshaimer(1995) confirm that the concentration difference between Izafia and Neumayer (or Cape Grim) nearly
exactly represents the mean concentration difference
between the two hemispheres at ground level.
Weiss et al. (1983) and Jacob et al. (1987) haveestimated comparative zox values of 1.7 and 1.6 yr,
respectively, using hemispheric surface means of aSKr
in a two-box model. With a 3D general circulation
model, Jacob et al. (1987) have computed from thesame adKr database a distinctly lower mean zex of
1.1 yr. They pointed out that the obvious difference inZox(1.1 yr vs 1.6 yr) is due to the overestimation of thereal hemispheric difference when using ground level
observations in a simple two-box model of the atmosphere.
The difference between hemispheric surface means
is obviously well described in a two-box model by aninterhemispheric exchange time of 1.7 __.0.2 yr forboth transport tracers, SdKr and SF6. This is verified
also by the time-lag ( ~ zox) between representative
records for surface means in each hemisphere. As
shown in Fig. 2 by a dotted curve, a value zox = 1.1 yrdoes not fit the SF 6 surface difference between bothhemispheres. This is an important finding, since, in thepast, many workers (e.g. Butler et al., 1992; Elkins
et al., 1993) have used the apparently "better" 3Dmodel result for zex (1.1 yr) from Jacob et al. (1987)
being inappropriate for a simple two-box model if
working with surface data.

6. DISCUSSION
7. C O N C L U D I N G REMARKS

Agreement between the interhemispheric exchange
time as derived from long-term observations andthose calculated from November 1990 and November
1993 cruise results is not compelling as each measured
latitudinal distribution is affected by the actual
position of the Inner Tropical Convergence Zone
(ITCZ) showing seasonal and interannual variations.
Interhemispheric exchange times of 1.3-2.0 yr with abest long-term mean of 1.7 yr obtained in the present
work are significanlly larger than the value of 1.1 yrderived from continuous SF 6 observations by Maiss
and Levin (1994) in a previous study. This has two
reasons: introducing the emission ratio ~, = 94%,

All atmospheric SF6 data presented in this study
confirm a stable and unbroken long-term increase ofthis anthropogenic trace gas which is described reasonably well by a quadratic trend model fitting the past
14 yr of observations. This smooth and steadily increasing atmospheric SF6 concentration curve, unaffected by chemical and biological processes, anddistinct for both hemispheres is a powerful tool formodel investigations of transport processes in theatmosphere. With a global mean increase rate of
0.225 ppt yr -1 (6.9% yr -1) in late 1994, SF6 showsa persistently increasing trend, in contrast to CFC-11

1628

M. MAISS et al.

and CFC-12 now rising comparatively slowly as a result of production restrictions (Elkins et al., 1993).
Compared to the established transport tracer SSKr,
SF6 is a promising alternative since it is easier tomeasure and both, temporally and geographicallymore regular in emissions. The high analytical precision now achieved in combination with an extended
continuous observational network, including verticalprofiles extending far up into the stratosphere, willallow the study of longitudinal, meridional and seasonal transport patterns within each hemisphere.
These signals require a general three-dimensionaltransport model of the atmosphere for interpretation.
Beyond the promising application of SF6 as a calibration tracer to validate the transport part of complex
atmospheric circulation models, the long-term observations provide an extremely valuable input function
for mixing studies in linked compartments like thestratosphere, the hydrosphere and the cryosphere.
Such potential applications using troposphericbackground levels of SF 6 are, for example, (1) troposphere/stratosphere mixing studies comparable tothose performed by Schmidt and Khedim (1991) using
CO2; (2) groundwater investigations (Dalmolin, 1993);(3) oceanic mixing studies from surface water to thedeep ocean as proposed by Law et al. (1994); and(4) investigations of the air ventilation rate in firn (seeSchwander et al. (1993) for the principal approach).
A c o m m o n feature in all these applications is thepossibility of air sample dating with the clear tropospheric SF6 chronology applicable for the time span of
the last 20 yr. Data of the present work are available
on request under the E-mail address of Ingeborg
Levin: LV@uphys 1.uphys.uni-heidelberg.de.
Acknowledgements--We wish to thank the staff of the
Neumayer, Cape Grim, Izafia, Fraserdale and the Alertobservatories for carefully collecting the air samples,
G. Schmedding for SF6 measurements, and V. Hesshaimer
for motivating discussions on modelling questions. K. O.
Miinnich is gratefully acknowledged, especially for his expertise in standard preparation. We would like to thank
R. Weiss for his clear words concerning the ppt-by-molescale definition and M. Prather and one anonymous reviewer for their helpful comments on the manuscript.

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