Atmospheric Environment 35 (2001) 1171}1182
Tropospheric concentrations of the chlorinated solvents,
tetrachloroethene and trichloroethene, measured in the remote
northern hemisphere
C.H. Dimmer *, A. McCulloch, P.G. Simmonds , G. Nickless ,
M.R. Bassford, D. Smythe-Wright
School of Chemistry, University of Bristol, Bristol BS8 1TS, UK
ICI Chemicals & Polymers Ltd., P.O. Box 13, The Heath, Runcorn, Cheshire WA7 4QF, UK
Department of Physics, University of Toronto, 60, St. George St., Toronto, Canada M5S 1A5
Southampton Ocenography Centre, Empress Dock, Southampton SO14 3ZH, UK
Received 29 September 1999; received in revised form 18 August 2000; accepted 23 August 2000
Abstract
A fully automated twin ECD gas chromatograph system with sample enriching adsorption}desorption primary stage
was deployed on two "eld campaigns } Ny-As lesund, Svalbard, Arctic Norway (July}September 1997), and the RRS
Discovery CHAOS cruise of the northeast Atlantic (April}May 1998). Concentrations of an extensive set of halocarbons
were detected at hourly intervals at pptv levels. We present here the results obtained for the chlorinated solvents,
tetrachloroethene (PCE) and trichloroethene (TCE). Average baseline PCE and TCE concentrations of 1.77 and 0.12
pptv, respectively, were recorded in Ny-As lesund. During pollution incidences, concentrations rose to 5.61 (PCE) and
3.18 pptv (TCE). The cruise data showed average concentrations ranging from 4.26 (PCE) and 1.66 pptv (TCE) for air
masses originating over the North Atlantic and Arctic open oceans, to maxima of 15.59 (PCE) and 17.51 pptv (TCE) for
polluted air masses from Northern Europe. The data sets emphasise the di$culties in de"ning remote sites for
background tropospheric halocarbon measurements, as Ny-As lesund research station proved to be a source of tetrachloroethene. The data also suggest possible oceanic emissions of trichloroethene in the sub-tropical ocean. 2001
Elsevier Science Ltd. All rights reserved.
Keywords: Tetrachloroethene; Trichloroethene; Arctic troposphere; Emissions; Svalbard; Atlantic
1. Introduction
Tetrachloroethene (perchloroethene, PCE) and trichloroethene (TCE) are used as industrial solvents and
degreasers and are excellent markers of anthropogenically polluted air. Knowledge concerning global
atmospheric concentrations of TCE is limited owing
to the poor global distribution of reported measurements (Wiedmann et al., 1994), although global seasonal
* Corresponding author.
E-mail address: [email protected] (C.H. Dimmer).
variations in the distribution of PCE have been reported
(Wang et al., 1995). Industrial emission estimates
(McCulloch and Midgley, 1996) suggest that source regions are broadly similar to chlorinated solvents such as
CH Cl and other halocarbons like the HCFCs. There
has been a suggestion that certain types of macroalgae
present in the sub-tropical oceans may be capable of
synthesising chlorinated alkenes such as TCE and PCE
(Abrahamsson et al., 1995a, b). However, Scarratt and
Moore (1999) detected no production of TCE and PCE
from any cultures of the red microalga, Porphyridium
purpureum, used in the work of Abrahamsson et al. under
either low or high irradiance. Tropospheric measurements made by Quack and Suess (1999) on a western
1352-2310/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved.
PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 4 2 7 - 1
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C.H. Dimmer et al. / Atmospheric Environment 35 (2001) 1171}1182
Paci"c cruise in September 1994, suggested a natural
source of TCE in the area of the Indonesian Archipelago,
as did TCE and PCE ocean water supersaturations measured on a cruise in 1981, between 403N and 303S (Khalil
and Rasmussen 1998). However, atmospheric measurements made onboard the Polarstern research vessel
which travelled across the Atlantic Ocean between 453N
and 303S in August/September 1989 found that the concentrations of TCE and PCE were highest between 40
and 453N (Koppmann et al., 1993). There was a distinct
inter-hemispheric gradient, implying that TCE and PCE
are not well mixed in the global atmosphere and that the
dominant source regions are located within the northern
hemisphere.
PCE and TCE concentrations in the Arctic troposphere were measured at Alert, Canada from January
1992 to June 1994 (Yokouchi et al., 1996). A signi"cant
seasonal variation was observed, with higher concentrations in winter and spring than in summer, suggesting
greater reactive loss in summer due to more abundant
hydroxyl and other radicals. Concentrations were lowest
for both compounds from May to September inclusive.
The winter/summer concentration ratio was as much as
60 for TCE.
During June 1992, Blake et al. (1996) recorded PCE
concentrations in the marine boundary layer and the free
troposphere on a series of #ights in the North Atlantic,
east of the Azores, from 30 to 413N and 20 to 263W.
Marine boundary layer concentrations ranging from
a minimum of 7 pptv in clean marine air masses to
59 pptv in aged polluted European air masses were
observed.
Measurement of PCE and TCE concentrations in the
ocean water column during cruises in the western Atlantic in 1997, and as part of the GASEX 1998 (investigating
air}sea gas exchange) experiment, north of the Azores,
showed supersaturations, indicating that the ocean was
acting as a source of TCE and PCE to the atmosphere
(Moore, 1999). TCE and PCE concentrations in seawater were reported to increase with depth down to
500 m, unlike CH I and isoprene, where high sub-surface
concentrations decline rapidly with increasing depth. The
PCE depth pro"le was less extreme than the TCE pro"le.
Moore postulated that depending on their lifetimes in
seawater, the seasonal concentration variations for PCE
and TCE could drive a wintertime #ux of the gases from
the atmosphere to the ocean, and a reverse #ux during
the summer. The winter, high-latitude cold ocean in
contact with high winter atmospheric concentrations,
particularly of TCE, could result in high gas dissolution
in the oceans. These highly concentrated water masses
could then be distributed by ocean circulation. In the
summer at low latitudes, where atmospheric concentrations are low due to high OH attack, a positive ocean to
atmosphere #ux could result, explaining the observations
of sub-tropical production of TCE. The e!ects of any
biological production or consumption of PCE and TCE
would therefore be superimposed on the physically
driven #uxes.
The primary atmospheric loss mechanism for both
TCE and PCE is through oxidation by the OH radical.
The rate constants for oxidation by OH are di!erent
for TCE and PCE (2.49;10\ and 1.24;10\
molecule cm\ s\, respectively), and consequently the
two halocarbons have di!erent atmospheric residence
times which has important implications for long-range
atmospheric transport. Assuming a globally averaged
value of [OH]"9.7$0.6;10 radicals cm\ (Prinn et
al., 1995), atmospheric lifetimes of 4.8$0.3 and 97$6 d
have been calculated for TCE and PCE, respectively
(Bassford, 1998).
An area of environmental concern has been the toxicity of PCE, and its ability to produce decomposition
products such as phosgene, trichloroacetyl chloride
(TCAC), trichloroacetic acid (TCA) and CCl . PCE,
TCE, TCA and TCAC have been investigated as possible
contributors to forest decline in central Europe, where
they have been implicated in the destruction of photosynthetic pigments in the presence of UV radiation and
other herbicidal e!ects (Frank and Frank, 1986).
Measureable concentrations of PCE and TCE and their
degradation products have been detected in forest foliage
(Diezel et al., 1988; Frank and Frank, 1989; PluK macher
and SchroK der, 1993; Brown et al., 1999). The lipophilic
nature of these compounds, suggests that they may be
enriched in the epicuticular wax and cellular membranes.
In this paper we discuss a two month continuous data
set for tetrachloroethene and trichloroethene, measured
in the Arctic troposphere at Ny-As lesund, Svalbard during July}September 1997. We also present tropospheric
PCE and TCE measurements made on the RRS Discovery CHAOS cruise of the NE Atlantic during April
and May 1998.
2. Instrumentation and sampling methodology
The instrumentation was deployed at the Zeppelin
station, Ny-As lesund, from 19 July to 14 September 1997.
The Zeppelin station is located on a mountain ridge
south of Ny-As lesund (78355N, 11356E) at a height of
475 m above sea level, minimising any e!ects of emissions
from the local settlement and infrastructure. From 21
April to 31 May 1998 the instrumentation was also
installed on the RRS Discovery for the CHAOS cruise of
the NE Atlantic covering a transcept from 203N 203W to
Iceland (63.43N 203W), followed by a study in the
Rockall Trough (Fig. 1). The instrumentation utilised
consisted of a gas chromatograph (GC) (Model 6890,
Hewlett Packard Ltd.) equipped with two electron capture detectors (ECD) and modi"ed to direct the e%uent
from the "rst normal ECD into the second oxygen doped
C.H. Dimmer et al. / Atmospheric Environment 35 (2001) 1171}1182
Fig. 1. CHAOS Cruise track, from 21 April to 31 May 1998.
detector (Bassford et al., 1998). PCE and TCE concentrations were only calculated from the "rst detector results,
while the second ECD was used for accurate determination of less responsive halocarbons such as CH Cl,
CH Cl and CH Br, detailed elsewhere (Dimmer, 1999).
Chromatographic separation was achieved using
a gas}liquid WCOT CP Sil-5 methyl silicone column
(0.32;50 m, 5 lm "lm thickness, Chrompack International BV), with ultrapure helium as the carrier gas at
a #ow rate of 1.9 ml min\. The temperature programme
used consisted of 8 min at 303C followed by a temperature ramp to 1553C at a rate of 53C min\. The "nal
temperature was then held for 12 min giving a total
analysis time of 45 min, allowing 15 min for cool down in
the hourly sampling cycle.
An automated front end, adsorption}desorption system (ADS) was utilised based on the design described by
Simmonds et al. (1995) to perform routine analysis of air
and standard samples (200 ml) in a continuous 3 h cycle
(two air analyses followed by a standard analysis). The
bracketing of air analyses enabled quanti"cation of the
atmospheric measurements and allowed for any drift in
sensitivity. The working standard containing halocarbons at near ambient concentrations was obtained from
the ultraclean dilution of a gravimetrically prepared
($1% accuracy) 16 component calibration standard in
N O (Linde Gases, UK). To assess system precision,
each standard analysis was compared with the bracketing standards, therefore correcting for any drift in
detector sensitivity. The percentage accuracy of standard}standard ratio consistency was within $1% for
both compounds. Over the two month Ny-As lesund
sampling period a total of 1287 analyses were carried
out, with 858 air analyses and the remainder, standard
1173
analyses. Over the 5.5 week cruise monitoring period,
a total of 978 analyses were carried out with 563 air
analyses. On both "eld campaigns the initial 3}4 d worth
of analyses were excluded from subsequent data analysis
to allow for instrument stabilisation time.
The "nal calibration standard was compared with
absolute calibration standards maintained by the
Scripp's Institute of Oceanography (SIO) and the National Oceanic and Atmospheric Administration
(NOAA) CMDL laboratory, USA, as part of an ongoing
series of intercalibration exercises. Final concentrations
were determined by calculating absolute values using
CFC-12 as a bootstrap to the SIO/NOAA scale, and
assuming the Linde standard component ratios were
correct. Canned real air samples were also intercompared
with the University of East Anglia, the NOAA CMDL
laboratory, and the University of California, Irvine.
Values for PCE were in agreement to 4.6, 12.5 and 9% for
the three laboratories, respectively. TCE concentrations
were only intercompared with the University of California and values were found to be 24}36% greater using
the GC-twin-ECD system, suggesting possible calibration discrepancies (overestimates). Accurate detection
limits for PCE and TCE were approximately 0.01 pptv.
During the Ny-As lesund campaign air was pumped
into the ADS through an 1/8 o.d. stainless steel tube
connected into the main Norwegian Institute for Air
Research (NILU) air inlet in the Zeppelin station. Air was
continuously pumped through this neutral anodised aluminium tube (100 mm diameter) at a rate of approximately 2 m s\ from an inlet approximately 4 m above the
roof of the Zeppelin station. The sample lines were
purged for 3 min (at 200 ml min\ for air and
50 ml min\ for standard) prior to acquisition of a 200 ml
sample on the microtrap, when the sampling #ow was
reduced to 30 ml min\. When sampling was not taking
place, the microtrap was continuously purged with
a stream of ultrapure helium.
Onboard the research ship Discovery, air was sampled
through 70 m o.d. copper tubing attached to the fore
mast at a height of 16.5 m above the load waterline (sea
surface), and 11 m above the ship's deck, through which
air was pumped for 10 min at 200 ml min\ before
a 200 ml sample was taken. Prior to using the equipment
remotely from the laboratory, the sample tubes were
tested using zero air to ascertain potential contamination
sources, and these were found to be insigni"cant.
Throughout the cruise, the ability to detect rapid changes
in concentration level coincident with changing air mass
origin, suggested that sample line #ushing was adequate,
and carry over from pollution episodes within the long
lengths of sample line was not signi"cant.
The air or standard sample passed through a simple ice
trap, which e$ciently removed bulk water present in the
air sample prior to "nal drying with a Na"on drier
counter purged with dry nitrogen gas. Use of a potassium
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C.H. Dimmer et al. / Atmospheric Environment 35 (2001) 1171}1182
carbonate or magnesium perchlorate drying agent trap
was avoided due to possible contamination and/or
halocarbon removal by the drying agent.
Atmospheric halocarbon levels were analysed using
meteorological data and 3-D 5 d isentropic back trajectories obtained at a frequency of one per day during the
Ny-As lesund monitoring period (Deutscher Wetterdienst
} Global Model), and every 12 h for the cruise data set
(UK Met. O$ce Trajectory Model). Trajectories were
visually analysed to qualitatively identify the source regions of air masses reaching Ny-As lesund, and hence to
link concentrations recorded to geographical source regions, and height of air parcel tracks. Using the trajectories, the Ny-As lesund data were allocated to six source
sectors, Eastern Europe, Western Europe, Greenland
Sea/Iceland, Canada/USA, Arctic Ocean and Northern
Greenland/Northern Alaska (Fig. 2). If a trajectory crossed over several regions, the data were assigned to the
region that will have the strongest in#uence. For
example, the Canada/USA trajectories always pass over
the Greenland Sea/Iceland areas, but are allocated to
the former source region. The data for 5 d trajectories
originating from the Greenland Sea, Arctic Ocean and
N. Greenland/N. Alaska were averaged to provide baseline concentration data. The cruise data were allocated
to seven trajectory source regions: African coast;
Northern central Europe including the UK and Ireland;
SW Europe covering Spain and Portugal; Canada/USA;
Iceland/Greenland coast; Scandinavia coast/Svalbard
and the open N. Atlantic ocean. Wind speed data were
utilised to identify any local contamination from the
Ny-As lesund settlement or the ship. Pollution incidences
identi"ed during periods of calm weather, were attributed to local sources and rejected from subsequent
analysis.
3. Observations: Ny-As lesund, Svalbard
3.1. Tetrachloroethene
During the two month monitoring period only two
major incidences of polluted air reaching Ny-As lesund
were recorded (Fig. 3). The "rst incident, from 30 August
to 2 September 1997, was due to wind trajectories originating over central Russia and Eastern Europe. The second event, from 5 to 9 September 1997, was due to air
from Western Europe, particularly Scandinavia and
Scotland. Earlier in the monitoring period smaller pollution incidences can also be identi"ed in the time series,
showing polluted air from Scotland and Eastern Canada
and the USA. The pollution incident from Canada and
the USA (7 August 1997) is more signi"cant in the PCE
trace. This can be explained by the much shorter lifetime
for TCE of only 4.8 d, and hence more rapid decay of
elevated concentrations.
On 15 August 1997 the Zeppelin station cable car cable
was reoiled, causing levels of PCE to rise from baseline
levels to give a peak of 846 pptv (lubricant analysis was
performed by the Norwegian Institute for Air Research
(NILU) and the presence of PCE was con"rmed). As
a consequence of the introduction of this local PCE
source, low wind speeds were marked by sharp rises in
the PCE concentration recorded for the remainder of the
Fig. 2. Map showing example trajectories from each air mass sector used in the analysis of the Ny-As lesund data.
C.H. Dimmer et al. / Atmospheric Environment 35 (2001) 1171}1182
1175
Fig. 3. The time series of PCE and TCE concentrations (pptv) during the Ny-As lesund campaign.
Fig. 4. The time series of PCE and CH Cl concentrations (pptv) from 19 August to 15 September 1997 during the Ny-As lesund
campaign.
monitoring period and the average baseline level of PCE
rose by 0.55 pptv. This rise in baseline was due to the
close proximity of the sampling inlet to the cable (5 m),
rather than system contamination, since elevated PCE
was not present as a concomitant artefact on standard
runs. Using the CFC-11, CH Cl and wind speed data,
the sharp local contamination peaks were removed by
visual analysis (Figs. 4 and 5). The CH Cl data only
shows peaks in concentration coincident with large-scale
transport of pollution from the urbanised northern hemisphere, with no evidence of local sources. CFC-11, on the
other hand, does not rise in concentration in response to
European pollution incidents, as a consequence of the
stringent phase out limitations imposed within Europe.
However, like PCE it shows sharp spikes, on a time scale
of hours, coincident with low wind speed that indicate
a local Ny-As lesund source, most probably insulation
foam used in the station construction. (It should be noted
that the Zeppelin station building was replaced in 1999,
and should no longer be a source of CFC-11.)
The tendency when using this visual analysis "ltration
method is to exclude more data than would be the case
with mathematical methodology, that statistically "lters
out pollution incidents. Therefore, any bias should be
1176
C.H. Dimmer et al. / Atmospheric Environment 35 (2001) 1171}1182
Fig. 5. The time series of PCE and CFC-11 concentrations (pptv) and wind speed (ms\) from 20 August to 7 September 1997 during the
Ny-As lesund campaign.
towards having a lower absolute population of background determinations and a purer data set. The presence of these local sources of both PCE and CFC-11
certainly undermines the importance of Ny-As lesund as
a remote location away from anthropogenic sources, as
the accuracy of the baseline measurements is compromised (a 0.55 pptv rise due to local PCE contamination).
We hope that the detailing of the contamination episode
will raise awareness of potentially signi"cant local sources due to the construction and maintenance of other
remote baseline stations. Canister measurements would
be particularly susceptible to bias from baseline contamination, as the lack of continuity and real-time monitoring
may not identify the impact of local sources.
It must be emphasised that the local PCE contamination was a single event. The paper is based on two "eld
campaigns and only one month of the Spitzbergen data
was a!ected. When background concentrations are
quoted for Ny-As lesund, these are for the "rst month of
the campaign before the contamination incident. The
major European peaks after the local contamination
incident lasted at least 2 d in length, and were during
periods of high wind speed, when local in#uence would
have been almost negligible.
Table 1 shows the mean, standard deviation, maximum and minimum concentration values for each trajectory source sector. The background concentrations of
PCE recorded at Ny-As lesund (1.77$0.07 pptv) are
lower than the northern hemisphere measurements
of Wiedmann et al. (1994) (21$5 pptv, 90}303N;
7$3 pptv; 30}03N, 1982}1989), and Alaskan measurements of Wang et al. (1995) (7.6$0.5 pptv for September
1989 and 21.5$1.8 pptv for March 1990). However,
both groups made their measurements during the 1980 s
when PCE emissions were signi"cantly higher, and the
reported concentrations of Wiedmann et al. (1994) represent data averaged throughout all seasons, and thus
will be biased by winter maxima. Yokouchi et al. (1996)
observed average summer PCE levels at Alert, Canada
during 1992}1993 of 4.4 pptv, with minimum values of
2.8 pptv. Our measurements thus support the continued
decline of PCE emissions as suggested by the emissions
estimates of McCulloch and Midgley (1996).
During the European pollution incidences, PCE maximum concentrations in Ny-As lesund rose to concentrations similar to those recorded at other sites in the
northern hemisphere (5.61 pptv for Eastern European air
masses), suggesting that the low concentrations observed
during background periods are indicative of the dominance of clean air masses reaching Ny-As lesund, and the
distance of the station from major pollution sources.
Indeed, trajectories allocated to clean baseline (nonEuropean) air masses made up 75% of trajectories reaching Ny-As lesund during the 2 month monitoring period.
Baseline PCE concentrations recorded using the same
instrumentation during a previous "eld campaign (28
June to 8 August 1996) at Mace Head, Ireland averaged
2.0 pptv. These baseline levels were associated with maritime westerly air masses. During periods when southwesterly air originating from the sub-tropical Atlantic
Ocean reached Mace Head, concentrations as low as
1.2 pptv were recorded. However, during European/UK
pollution events concentrations reached 23.6 pptv
(Bassford, 1998).
C.H. Dimmer et al. / Atmospheric Environment 35 (2001) 1171}1182
1177
Table 1
Statistics for trajectory sorted Ny-As .lesund data, showing the mean, standard deviation, maximum and minimum PCE and TCE concentrations for the complete data set and trajectory sorted data before and after the introduction of the local source. The count indicates the number
of data points allocated to each trajectory. The locally polluted data have been "ltered out. Concentrations are measured in pptv
PCE
Complete "eld campaign
Before contamination
TCE
Complete "eld campaign
Before contamination
All
Greenland
sea
Arctic
ocean
Canada/
USA
Average
Max
Min
SD
Average
Max
Min
SD
2.28
5.61
1.62
0.62
1.84
2.4
1.62
0.11
2.04
3.11
1.7
0.26
1.84
1.99
1.7
0.07
2.26
4.49
1.69
0.43
1.79
1.87
1.69
0.04
1.81
2.4
1.69
0.16
1.81
2.4
1.69
0.16
2.03
2.73
1.62
0.28
1.76
1.95
1.62
0.1
Average
Max
Min
SD
Average
Max
Min
SD
0.41
3.18
0.04
0.51
0.16
0.91
0.04
0.12
0.25
0.9l
0.05
0.17
0.13
0.91
0.05
0.09
0.23
0.62
0.07
0.07
0.09
0.12
0.07
0.01
0.09
0.25
0.04
0.05
0.09
0.25
0.04
0.05
0.21
0.3
0.05
0.06
0.15
0.27
0.05
0.04
3.2. Trichloroethene
The average baseline concentration of TCE was only
0.12$0.03 pptv at Ny-As lesund, rising to a maximum of
3.18 pptv during the E. European/Russian pollution incidences. However, during the Ny-As lesund campaign
TCE was at the lowest point of its annual concentration
cycle. Yokouchi et al. (1996) measured similar summer
TCE levels at Alert during 1992}1993 of 0.1 pptv, with
annual maximum and minimum of 8.0 and 0.01 pptv,
respectively. Measurements made from 1979 to 1986 at
Hokkaido, Japan by Makide et al. (1987) showed highest
mixing ratios of about 20 pptv in January and lowest
values of about 3 pptv in summer, while Koppmann et al.
(1993) measured TCE mixing ratios of 3$1 pptv in the
northern hemisphere. Again, the measurements reported
here support the declining trend of trichloroethene emissions (McCulloch and Midgley, 1996).
4. CHAOS cruise
4.1. Tetrachloroethene
Trajectory sorting of the cruise data gave a range of
average PCE concentrations from 4.26$0.84 pptv for
air masses with 5 d back trajectory origins over N. Atlan-
N. Greenland/
N. Alaska
W. Europe
E. Europe
Baseline
2.56
5.22
1.78
0.71
1.93
2.25
1.78
0.1
3.69
5.61
2.33
0.88
2.15
3.61
1.66
0.35
1.77
1.9l
1.66
0.07
0.91
2.59
0.07
0.68
0.29
0.83
0.07
0.18
1.41
3.18
0.33
0.82
0.22
0.46
0.06
0.07
0.12
0.2
0.06
0.03
tic open ocean and Arctic open ocean, to a maximum
value of 15.59 pptv for polluted air masses from Northern
Europe, showing the closer proximity to and prevalence
of polluting air masses (Table 2). Periods of low wind
speed were again marked by sharp rises in concentration
indicating that the ship was a contaminating source of
PCE (Fig. 6).
4.2. Trichloroethene
During the CHAOS cruise, TCE concentrations measured ranged from 1.58$0.45 pptv for remote Atlantic
and Arctic open ocean levels, to 6.24$3.13 pptv average
values for Northern European polluted air masses
(Table 2). For this compound, the ship appears not to be
a signi"cant local source of contamination, as during
PCE contamination spikes, TCE levels only rise to
a maximum of 6.27 pptv, signi"cantly less than the maximum levels recorded in European air masses of
17.51 pptv. Air masses originating from Southern Europe
(Portugal and W. Spain) are characterised by low levels
of TCE, explicable by the high exposure to photochemical activity during transport from this source region, and
the long transit time to the ship. Trajectories allocated to
the African coast and Canary Islands sector have high
TCE values (maximum of 17.12 pptv), and will be discussed further.
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C.H. Dimmer et al. / Atmospheric Environment 35 (2001) 1171}1182
2.94
6.27
1.34
2.06
13
2.42
1.83
20.56
0.83
2.99
55
10.22
1.37
2.42
0.82
0.30
105
19.52
1.65
2.65
0.86
0.49
60
11.15
1.79
5.00
0.78
0.60
163
30.30
1.35
1.51
1.18
0.16
4
0.74
6.24
17.51
1.41
3.13
103
19.14
% trajectory
allocation
2.80
20.56
0.42
2.90
538
Average
Max
Min
SD
Count
TCE
6.04
17.12
1.30
4.68
35
6.51
36.69
184.47
5.44
51.32
4.09
5.69
2.44
0.70
4.53
12.58
3.16
1.18
5.16
19.91
3.62
2.34
4.16
6.51
2.90
0.64
5.57
6.57
4.69
0.77
9.66
15.59
4.60
2.65
4.87
6.51
3.06
0.81
6.35
184.47
2.44
10.11
Average
Max
Min
SD
PCE
SW Europe,
Spain/Portugal
N. Central
Europe,
UK, Ireland
African
coast
Total
Table 2
Statistics for trajectory sorted cruise data. Concentrations are measured in pptv
Open Ocean
central N. Atlantic
E. Canada/
USA coast
Iceland/
Greenland coast
Scandinavian
coast/Svalbard
Ship-board
contamination
5. Biogenic trichloroethene production?
Previous comparison (McCulloch and Midgley, 1996)
of the 1988}1992 calculated atmospheric concentrations
for PCE and TCE from the emissions estimates, with
reported observations such as Koppmann et al. (1993),
raised discrepancies, particularly for TCE concentrations. The calculated TCE concentrations were very
much less than the reported observations of Koppmann
et al. (1993), suggesting either the #uxes of TCE were too
low by a factor of 10, or the rate constant for the reaction
with OH radicals was too fast by a similar order of
magnitude. However, McCulloch and Midgley (1996)
reported that their rate constant was consistent with the
series of halocarbon rate constants provided by Atkinson
(1992), and was unlikely to have such a large error. This
discrepancy between observations and emission estimates could be resolved if the reported natural sources of
TCE and PCE (Abrahamsson et al., 1995a, b; Moore,
1999) were con"rmed.
There is one possible indication of algal production of
TCE in the air mass arriving at Ny-As lesund on 13 August
1997 (Fig. 3). This air mass has experienced surface contact with the coastal waters o! east Greenland, and
appears elevated in both TCE and PCE, with proportionally greater elevation for TCE. Abrahamsson et al.
(1995a) observed that the formation rate of TCE by
macroalgae is far greater than that for PCE. However,
this is an isolated incident, and no strong inferences can
be drawn from it.
Abrahamsson et al. (1995a) also reported greater TCE
production by tropical coastal algae, than temperate
species. The cruise data were examined to investigate
whether they provided any evidence of biogenic production in the sub-tropics. The data suggest that there are
two dominating source regions of high TCE } the African
Coast and N. Central Europe (Table 4), where average
concentrations are 6.04 and 6.24 pptv, respectively, compared to a mean concentration of 1.60 pptv for all other
air parcel source regions. N. Central Europe is likely to
be an industrial source, while the African coast source is
almost certainly not. The TCE/PCE concentration ratios
also give a notable di!erence between air masses of
African coastal origin and those of N. European origin
(Table 3). The considerably higher TCE/PCE ratio for
the African coast sector suggests the presence of more
substantial TCE sources than PCE sources to this data
set. An African or Canary Island anthropogenic source of
TCE without an associated PCE source would be a very
unusual possibility, especially due to the shorter TCE
lifetime. The Canary Islands have a high population
density, and there may be some use of TCE in cleaning
applications, with some low level release also monitored
from the principal land "ll site (TCE } 0.9 tonnes/yr, PCE
!2.5 tonnes/yr) (Lima et al., 1999). However, from 27 to
29 April 1998, the period of elevated TCE concentrations,
C.H. Dimmer et al. / Atmospheric Environment 35 (2001) 1171}1182
1179
Fig. 6. The time series of PCE and TCE concentrations (pptv) and ship's latitude during the CHAOS cruise.
Table 3
Table illustrating range of TCE/PCE concentration ratios for
African coast and North European continental air masses for
the CHAOS cruise data
TCE/PCE ratio
African coast
N. Europe continental
Mean
Maximum
Minimum
1.13
2.71
0.53
0.56
1.28
0.12
the Canary Islands were between about 400 and 920 km
from the ship, so considerable air mass dilution and
reaction would be expected during transport over this
distance. McCulloch et al. (1999) reported extremely limited anthropogenic TCE and PCE sources in West Africa, with generally much greater usage of PCE than TCE,
for example Morocco emitted approximately 108 Mg of
TCE and 243 Mg of PCE in 1990, compared to 84 Mg of
TCE and 172 Mg of PCE for Ivory Coast, and 15,147 Mg
of TCE and 18,033 Mg of PCE for the UK. It seems
extremely unlikely that after several days transport, TCE
levels should still be signi"cantly elevated above baseline,
without an associated PCE concentration increase. The
same argument can be used to eliminate the case for
a coastal macroalgae source of TCE o! the coast of
Africa, since again levels should be signi"cantly depleted
by the arrival of the air parcel at the sampling ship. It
therefore seems likely that the elevated concentrations in
the data allocated to the African coast source sector are
in fact due to a local source in the immediate proximity of
the ship, either microalgae present in the open ocean as
suggested by Abrahamsson et al. (1995a) or release from
supersaturated water, originating from high latitudes
during the winter months, as postulated by Moore
(1999).
6. A south}north concentration gradient and the e4ect
of light intensity on TCE/PCE ratios
TCE baseline concentrations on the CHAOS cruise
showed a considerable decline in concentration as
the ship moved northwards (Fig. 6). On 6 May 1998,
background levels of 1.21$0.47 pptv were observed
(35.503N, 20.823W to 37.003N, 20.003W), 22% more than
concentrations (0.99$0.17 pptv) measured 203 further
north (56.013N, 9.173W to 55.363N, 15.633W), over the
period 27}28 May 1998. CH Cl , on the other hand,
showed an increase in mean concentration with increasing latitude from the sub-tropics (Fig. 7) (approximately
8 pptv 103 latitude\), as was also observed by
Koppmann et al. (1993) (7 pptv 103 latitude\), which is
compatible with a predominantly industrial source. This
south}north positive concentration gradient being the
opposite to the TCE observations, further supports
a sub-tropical oceanic source of TCE. Koppmann et al.
(1993) observed a slight increase in TCE from the intertropical convergence zone to 453N. However, their cruise
track passed closer to the European land mass, and
therefore may have been subjected to greater in#uence
from anthropogenic sources of TCE. Their cruise also
occurred during August and September, with background TCE concentrations at the lowest point of the
annual cycle, after possible spring microalgae blooms
1180
C.H. Dimmer et al. / Atmospheric Environment 35 (2001) 1171}1182
Fig. 7. The time series of CH Cl concentrations (pptv) during the CHAOS cruise.
and with the OH concentration falling as the cruise
progressed.
The lifetime of TCE is governed mainly by its reaction
with OH and in order to assess the extent of a possible
biogenic or other ocean source contribution to the
south}north concentration gradient, it was informative
to determine the extent of OH concentration variation
along the cruise track. As a general case, it was assumed
that the OH concentration at any point on the cruise
track was dependent on the light intensity at that point
(Prinn, 1994). Standard insolation on the Earth's surface,
averaged over 24 h is 29.4;10 J m\, consistent with an
average tropospheric OH concentration of 9.7;10 radicals cm\ (Prinn et al., 1995). During the CHAOS
cruise, the average insolation would have been approximately constant at between 36 and 38;10 J m\
(Peixoto and Oort, 1992), the reduction in solar zenith
angle due to the northward track of the ship being
compensated by increasing day length. This suggests
that, during the course of the cruise, the e!ective OH
concentration should have remained almost constant,
and thus TCE concentrations monitored would have
been little in#uenced by time or position of sampling.
Considering the Ny-As lesund data, the TCE lifetime
will be signi"cantly longer by September due to the
decrease in solar radiation at Ny-As lesund. The longer
TCE lifetime supports the substantial TCE polluted air
mass peaks during September compared to the start of
August (Fig. 3). When the TCE/PCE ratio is calculated
for a Western European polluted air mass at the start of
August (max } 0.43), it is considerably lower than the
same ratio determined for concentrations measured in
September (max } 0.74) (Table 4). The theoretical TCE/
Table 4
Table illustrating the TCE/PCE ratio determined for Western
Europe in trajectory sorted data for two 3 d periods during the
Ny-As lesund campaign
TCE/PCE
1/8/97 7:26 to 3/8/97 17:46
Maximum
Minimum
Mean
0.43
0.05
0.18
5/9/97 4:49 to 8/9/97 19:17
Maximum
Minimum
Mean
0.74
0.30
0.48
PCE ratios are shown in Table 5 where, with equal
emissions of TCE and PCE and a standard air mass
transport time from Western Europe of approximately
3 d, the TCE/PCE ratio would be 0.48 under average light
intensities of 36;10 J m\, but 0.82 under lower light
intensity levels of 10 ;10 J m\. The good agreement
between measured and predicted TCE/PCE ratios suggests that anthropogenic emissions are the greatest contributor to elevated TCE and PCE levels in Ny-As lesund.
Although these results suggest evidence for sub-tropical oceanic production of TCE, we are aware that without simultaneous ocean water measurements of TCE and
PCE concentrations, marine production of TCE can only
be speculated. Incorporation of TCE measurements in
oceanic depth pro"les on future cruises would be highly
informative. As a result of the TCE and PCE ocean/
air recycling system proposed by Moore (1999), depth
C.H. Dimmer et al. / Atmospheric Environment 35 (2001) 1171}1182
1181
Table 5
Table illustrating the ratio of TCE to PCE in varying aged air masses arriving at Ny-As lesund and on Discovery under varying light
intensities The ratio is assumed to be 1.00 at trajectory start on day 0
Insolation (;10 J m\) Day number TCE/PCE factor
Standard
29.40
0.00
1.00
2.00
3.00
4.00
5.00
1.00
0.82
0.67
0.55
0.45
0.37
pro"les need to be investigated in both winter high-latitude oceans, southern hemisphere oceans, and summer
low-latitude oceans. It is possible that cold, deep water
oceans could provide an environment for high levels of
storage of TCE and PCE, which has important implications on the global budgets of these two trace gases.
7. Conclusions
The deployment of the dual channel ECD instrument
at the Zeppelin station, Ny-As lesund and on the
Discovery cruise of the NE Atlantic has enabled nearreal-time continuous tropospheric monitoring of the
chlorinated solvents, tetrachloroethene and trichloroethene. The work represents one of the few reported comeasurements of TCE and PCE and as such helps to
verify comparative source strengths. PCE and TCE show
low background tropospheric concentrations measured
at Ny-As lesund, explained by TCE and PCE seasonality,
with lowest concentrations in the summer due to high
OH attack; the remoteness of Ny-As lesund from anthropogenic sources of PCE and TCE; and the large distance
from any possible oceanic sources of both halocarbons.
The CHAOS cruise data correspond more closely with
previously published northern hemisphere concentration
observations. However, the observed baseline TCE data
exceed the tropospheric concentrations suggested by
emissions estimates such as those in McCulloch and
Midgley (1996) by a factor of approximately six. The
cruise data suggest the possibility of oceanic TCE sources
in the tropics, either in the form of an open ocean microalgae source raising baseline concentrations, or #ux of
TCE from supersaturated water, originating from high
latitudes during the winter months. Ocean water depth
pro"les for TCE and PCE would be highly informative on
future open ocean and coastal cruises. To further increase
our understanding of the global budgets of PCE and TCE,
anthropogenic emission sources and estimates should be
validated independently. An interesting approach would
CHAOS cruise
37.50
1.00
0.78
0.60
0.47
0.36
0.28
Ny-As lesund (range)
36.00
10.00
1.00
0.78
0.62
0.48
0.38
0.30
1.00
0.93
0.87
0.82
0.76
0.71
be to monitor concentrations of PCE and TCE within
anthropogenic plumes at a range of global locations close
to urban conurbations and industrial plants.
Acknowledgements
We acknowledge Sverre Solberg at NILU for the provision of the meteorological data for the Zeppelin station,
and Simon Josey of SOC for the CHAOS cruise MET
data. We are grateful to Barbara Fay of the Deutscher
Wetterdienst for the provision of 5 d back trajectories for
the Ny-As lesund data set and Matt Smith of the University of Bristol Computing Service for his help in the
trajectory data visualisation. We thank Derrick Ryall of
the UK MET O$ce for the provision of the trajectories
for the CHAOS cruise. We particularly wish to thank the
Norwegian Polar Institute sta! based in Ny-As lesund
during summer 1997 for their logistical support and
assistance. We also thank the members of the Bristol
Atmospheric Monitoring Group for their technical assistance, during the preparation for the "eld campaigns.
Thanks are due to the scienti"c party of RRS Discovery
cruise D233 (CHAOS) which was funded by Southampton Oceanography Centre and the Natural Environment Research Council. Financial assistance for the
Ny-As lesund "eld campaign was provided by the NyAs lesund Large Scale Facility Fund. The instrumentation
was funded by an NERC IFMA grant and developed by
M.R. Bassford in partial ful"llment of his Ph.D. degree.
C.H.D. was in receipt of an NERC Ph.D. studentship
during preparation of this work.
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