The Science of the Total Environment 219 Ž1998. 53]81
Dioxins and furans in air and deposition: A review of
levels, behaviour and processes
Rainer Lohmann, Kevin C. JonesU
En¨ ironmental Science Department, Institute of En¨ ironmental and Natural Science, Lancaster Uni¨ ersity,
Lancaster, LA1 4YQ, UK
Received 20 March 1998; accepted 8 June 1998
Abstract
This paper is a comprehensive, critical review of the levels, behaviour and processes affecting polychlorinated
dibenzo-p-dioxins and -furans ŽPCDDrFs. in air and deposition. Aspects of sampling, analysis and quality assurancercontrol are discussed initially, before a review of the PCDDrF concentrations in ambient air is presented. The
general trend in SP4 ] 8 CDDrF Žand STEQ. is: remote sites - 0.5 pgrm3 Ž STEQ - 10 fgrm3 .; rural sites ; 0.5]4
pgrm3 Ž STEQ ; 20]50 fgrm3 .; and urbanrindustrial sites ; 10]100 pgrm3 Ž STEQ ; 100]400 fgrm3 .. The commonly held view that a consistent mixture of PCDDrFs in air exists is evaluated and questioned. Issues of seasonality
and short-term changes in air concentrations are also critically discussed, with respect to the possibility of seasonal
emission sources to air and seasonally dependent loss processes. Data on the gas]particle partitioning of PCDDrFs
in air are reviewed; the limited database to date is believed to provide evidence for an exchangeable transfer of
PCDDrFs between these two phases. The potential importance of photolytic and radical reaction degradation
processes and wetrdry deposition processes in modifying the mixture of PCDDrFs in air is discussed. Some
homologuercongener specific ‘weathering’ of the mixture of PCDDrFs emitted to the atmosphere clearly occurs,
but in general PCDDrFs have ‘long’ atmospheric residence times, rendering them subject to long-range atmospheric
transport. Data are reviewed which relate the mixture of PCDDrFs in air to that in deposition; this leads to the
conclusion that different homologue groups Žwhich are partitioned differently between the gas and particulate phase.
are transferred to the earth’s surface with broadly similar efficiencies. Q 1998 Elsevier Science B.V. All rights
reserved.
Keywords: PCDDrFs; Dioxins; Furans; Air; Atmospheric deposition
U
Corresponding author.
0048-9697r98r$ - see front matter Q 1998 Elsevier Science B.V. All rights reserved.
PII S0048-9697Ž98.00237-X
54
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
1. Introduction
Polychlorinated dibenzo-p-dioxins and -furans
ŽPCDDrFs. are two groups of persistent, semivolatile and toxicologically significant trace organic contaminants. They enter the environment
in ultra-trace amounts from various combustion
sources and as chemical impurities in a range
of manufactured organochlorine products
ŽHutzinger et al., 1985; Rappe, 1992; Hagenmaier
et al., 1994; Ballschmiter and Bacher, 1996.. The
presence of PCDDrFs in the atmosphere and the
processes they undergo there are of particular
interest to environmental organic chemists at the
present time. There are several reasons for this.
Firstly, they are now ubiquitous contaminants and
it is believed that their emission to the atmosphere and their subsequent atmospheric
transport and behaviour have resulted in their
widespread dispersal through the environment.
Secondly, there are ongoing uncertainties over
the relative contributions of different sources of
PCDDrFs to the atmosphere and hence an interest in assessing the relationship between source
inventory estimates, atmospheric levels and loss
processes, including deposition ŽFiedler and
Hutzinger, 1992; Brzuzy and Hites, 1996; Thomas
and Spiro, 1996; Duarte-Davidson et al., 1997;
Alcock et al., in press.. Thirdly, the pathway air]deposition]soilrplants]foodchain transfer]wildliferhuman exposure in terrestrial systems; and the pathway air]deposition]water
bodies]foodchain transfer ]wildliferhuman exposure in aquatic systems are of key importance for
these bioaccumulating chemicals. This paper
therefore focuses on reviewing the levels, behaviour and processes of PCDDrFs in air and
deposition. Data are presented and discussed in
terms of PCDDrF congeners and homologues,
and the toxicity equivalents Ž STEQs..
The paper has a number of specific objectives,
as follows:
1.
To briefly discuss samplingranalysis for
PCDDrFs in air;
2. To summarise data on the typical ambient air
concentrations, TEQs and major TEQ contributors;
3. To discuss the gas]particle partitioning and
particle size distribution of PCDDrFs in the
atmosphere;
4. To consider the loss processes affecting
PCDDrFs in the atmosphere and discuss
their role in influencing the PCDDrF ‘air
pattern’;
5. To evaluate the relative importance of different deposition processes Žwet, dry gaseous
and dry particulate .;
6. To summarise data on the typical fluxes to
derive typical scavenging ratios; and
7. To evaluate the tendency for long-range
transport of PCDDrFs.
However, before these are discussed in detail,
it is appropriate to briefly begin with some comments on the physico-chemical properties of
PCDDrFs which influence their atmospheric and
environmental behaviour and the demands of
sampling and analysis as they pertain to measurements of air concentration and deposition fluxes.
1.1. Physico-chemical properties of PCDD r Fs
There are 75 different PCDDs and 135 different PCDFs. Their physico-chemical properties
differ widely between homologue groups and congeners Žsee Table 1. and are still quite uncertain
Žsee Mackay et al., 1991. due to difficulties in
their determination. However, in general, they
are all poorly water soluble, possess high octanol]water coefficients Ž K o w . and consequently
in environmental systems will partition strongly to
soilsrsediments as opposed to readily entering
the aqueous phase. Their octanol]air partition
coefficients Ž K o a . vary over several orders of
magnitude from the mono-CDDrFs Ž K o a ; 7]8.
to the octa-CDDrF Ž K o a ; 11]12.. This range of
values is important in influencing the gas]particle
partitioning of these semi-volatile organic compounds in the atmosphere under ambient conditions Žsee later..
Only the 2,3,7,8-substituted congeners are toxicologically important and a range of toxicity
equivalent factors ŽTEFs. has been assigned to
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
55
Table 1
Physico-chemical properties for selected PCDDrFsa
1-CDD
2,3,7,8-TCDD
OCDD
2,8-CDF
2,3,4,7,8-PCDF
OCDF
TEFb
pL ŽPa.
S Žmgrm3 .
log Ko w
H ŽPa m3 rmol.
log Ko a
0
1
0.001
0
0.5
0.001
0.075
1.18? 10y4
9.53? 10y7
1.46? 10y2
1.72? 10y5
1.01? 10y7
417
0.0193
0.000074
14.5
0.236
0.00116
4.75
6.80
8.20
5.44
6.5
8.0
6.288
3.337
0.684
6.377
0.505
0.191
7.34
9.67
11.8
8.03
10.2
12.1
a
Subcooled liquid pressure Ž pL ., water solubility Ž S ., octanol]water partition coefficient Ž K o w ., Henry-constant Ž H . and octanol]air
partition coefficient Ž K o a ., from Mackay et al. Ž1991. determined for 258C.
b
I-TEF from Kutz et al. Ž1990..
them ŽKutz et al., 1990.. The TEFs relate the
toxicity of a given 2,3,7,8-substituted PCDDrF to
that of 2,3,7,8-TCDD, the most toxic congener
ŽTEFs 1.. A simple additivity of their effects is
assumed.
2. Comments on the analytical demands for
r Fs
PCDDr
Congener-specific data are required for the
PCDDrFs because the 2,3,7,8-substituted congeners have different TEF-values and each congener behaves somewhat differently in the environment. However, air concentrations of 2,3,7,8TCDD are typically F 1 fgrm3 , i.e. at ultra-trace
levels. Typically the most sensitive high-resolution
gas chromatography]high-resolution mass spectrometers ŽHRGC-HRMS. can have sensitivities
of - 50 fg on column. Detection of ; 1 fgrm3
therefore becomes achievable when ; 500 m3 of
air is sampled and the extract is taken down to
; 15 m l, with 1 m l injected. Sensitive HRGCHRMS is therefore needed to routinely quantify
all the toxicologically relevant congeners. Nevertheless, the problem of interference by other
compounds becomes obvious, as even a mass resolution of 10 000 cannot discriminate between, for
example a TCDD ŽMqU : 321.8936. and a heptachlorinated biphenyl ŽMqU ]Cl 2 : 321.8678.. High
resolution GC separation, monitoring of another
mass channel and an efficient clean-up are therefore needed to help to unambiguously identify
PCDDrFs among co-eluting substances which are
present at higher concentrations. Strict quality
assurance } quality control protocols have been
developed for the quantification of PCDDrFs in
environmental samples.
Ambient air samples are generally taken with
high volume air samplers ŽHi-vols. equipped with
a filter for trapping particles and a solid adsorbent for collecting the vapour phase. Volumes of
500]2000 m3 are commonly taken in a matter of
days to 2 weeks. Bruckmann et al. Ž1993. could
not find any significant data difference related to
the use of different adsorbents Žpolyurethane
foam, PUF, or XAD-2 resin. or to different air
flows Žbetween 11 and 255 lrmin.. Nevertheless,
long sampling duration does enhance the risk of
losing compounds due to breakthrough. There
are differences of opinion in the literature regarding when quantification standards should be
added. Bruckmann et al. Ž1993. concluded that
there is no need to add all the quantification
standards prior to sampling and that addition of
one surrogate prior to sampling should be sufficient. In contrast, Maier et al. Ž1994. and
Ballschmiter and Bacher Ž1996. recommended the
addition of pre-sampling spikes.
Air samples Žsorbent and filter. are usually
Soxhlet-extracted with toluene for 12]24 h. A
broad range of clean-up procedures are currently
in use and typically include a multi-layer silica
column and a basic alumina column for air samples ŽBallschmiter and Bacher, 1996; Kaupp, 1996;
Wallenhorst et al., 1997.. The silica Žacid and
base. removes mainly polar compounds, e.g. unsaturated compounds and lipids. The alumina
column separates PCDDrFs from other persis-
56
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
tent organic pollutants Že.g. PCBs. on the basis of
different polarities. Our laboratory uses a modified clean-up procedure, based on work by Hockel
¨
et al. Ž1994.. The sample is first refluxed with acid
silica and then fractionated on a mini-alumina
column.
Separation of all the key PCDDrFs involves
separate injections on two GC columns. A nonpolar Že.g. DB-5. column is used to calculate the
homologue groups. A polar Že.g. SP-2331. column
is used to determine congener specific concentrations for most of the seventeen 2,3,7,8-substituted
congeners. Co-elutions of non-2,3,7,8-substituted
with 2,3,7,8-substituted congeners occur on every
polar column Ž e.g. 1,2,3,7,8-PeCDF and
1,2,3,7,8,9-HxCDF on SP-2331., so these congeners should be quantified on a non-polar Že.g.
DB5. column to unequivocally assign the correct
STEQs for the air sample ŽRymen, 1994.. In
practice, however, this cross-quantification is
rarely done.
At least two ions of the molecular cluster are
monitored in the selected ion monitoring ŽSIM.
mode. The lock mass, a reference peak from a
constant flow of a compound into the instrument,
is monitored to test for sensitivity suppression by
co-extracted compounds. A calibration verification standard should be run daily prior to real
samples to ensure the required sensitivity. The
concentration of its’ analytes should be chosen to
give a signal-to-noise ratio of not less than 20:1.
The relative standard deviation of response factors and isotope ratios should be less than 15% of
the actual value Žexperimentally determined for
the given instrument ..
3. Comments on acceptance criteria and quality
control
The identity of a given PCDD or PCDF is
confirmed if there is a simultaneous response for
the two channels within a second, if the isotope
ratio is within 15% of its theoretical value and if
the signal-to-noise ratio is above 2:1 for all relevant channels ŽOehme et al., 1995a; Ballschmiter
and Bacher, 1996.. A specific isomer is identified
if all criteria for identification are met and if
there is a simultaneous or within 2 srscan re-
sponse for the analyte and the matching internal
standard. A high resolution mass spectrometer
should be operated at a resolution of G 10 000.
Internal quantification standards are added to
the sample early on in the procedure, either prior
to sampling or prior to extraction. At least one
2,3,7,8-substituted congener is commonly spiked
per homologue group for the determination of
the TEQ-value of a given sample. Following the
EPA method 1613 ŽUS-EPA, 1994. only two congeners Ž1,2,3,7,38,9-HxCDD and OCDF. have to
be cross-quantified; all the others are calculated
relative to their internal Ž 13 C 12 -labelled. standard.
This is the principle of stable isotope dilution,
which is the most appropriate method for the
reliable quantification of PCDDrFs.
Recovery of analytes is likely to be less than
100% and may vary between different samples
and also between different congeners in a single
extract. EPA method 1613 requires samples to be
reanalysed if recoveries of the labelled compounds fall outside the range of roughly 30]140%
Žcongener specific criteria .. Taking into account
the relatively easy handling of ambient air sampling, recoveries can be expected to be between
50 and 115%, otherwise the accuracy of the data
may be compromised ŽAmbidge et al., 1990; Maier
et al., 1994; Oehme et al., 1995a.. Oehme et al.
Ž1995a. believed that an accuracy and precision of
the sampling and measuring method of - 15%
should be achieved. Regarding the tolerance criteria for the daily calibration standard for the
mass spectrometer Žwithin 10% of its theoretical
concentration . this is a demanding target. However, recent method performance data by Maisel
and Hunt Ž1997. gave an average precision of
12% for a 30-day sampling period. A 3-day sampler comparison at Hazelrigg, UK, of five concurrent Hi-vols gave an average precision of 10%
ŽLohmann et al., unpublished..
A good demonstration of the accuracy of ambient air measurements was made by the Environment Agency ŽUmweltbehorde
¨ . in Hamburg,
Germany. In 1989, four laboratories analysed
concurrently sampled ambient air for PCDDrFs,
using their own sampling devices and following
their own analytical procedures. Standard deviations for the mean concentrations of the homo-
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
logue groups and congeners were mostly between
20 and 50% ŽBruckmann et al., 1993.. The accuracy could be improved further if there was a
certified reference material for PCDDrFs in air;
at present none exists for routine laboratory use.
Laboratories therefore either have to use a certified soil or sediment ŽMaier et al., 1994. or omit
this important part of an air QArQC scheme.
4. PCDDr
r Fs in ambient air
4.1. Typical concentrations
Table 2 presents a comprehensive review of
PCDDrF data in ambient air. In general, concentrations of the sum of the tetra- to octa-CDDrFs
homologues in ambient air are between 0.5 and
20 pgrm3. As expected, there is a general gradient, increasing from remote to rural to urbanrindustrial centres. PCDDrF concentrations for the
sum of the tetra- through octa-PCDDrF homologues Žand the STEQ. are typically as follows:
remote - 0.5 pgrm3 Ž STEQ - 10 fgrm3 .; rural
; 0.5]4 pgrm3 Ž STEQ ; 20]50 fgrm3 .; and urbanrindustrial ; 10]100 pgrm 3 Ž STEQ ;
100]400 fgrm3 .. This trend is consistent with
expectations, given that combustion sources and
chemical usage are believed to be the principal
sources of PCDDrFs to the atmosphere.
4.2. Mixture of PCDD r Fs in air
There is interest in the mixture of PCDDrFs
present in air, as this will be related to source
inputs and ‘weathering’ processes. It has often
been reported that there is a quite consistent
homologue pattern of PCDDrFs in air, except
close to important local sources ŽHagenmaier et
al., 1994; Hippelein et al., 1996; Jones and
Duarte-Davidson, 1997.. This has often been described as decreasing concentrations of PCDF
homologues with increasing chlorination level and
increasing concentrations of PCDDs with increasing chlorination level. Based on the thorough
literature review compiled for Table 2, the ‘averaged’ ambient air pattern shown in Fig. 1 was
obtained Žwith ambient air measurements from
Europe, the USA, Japan and Australia.. This
57
includes error bars representing single standard
deviations, and indicating that there is actually a
high variability in the relative abundance of the
different homologues when data reported by different international laboratories is included. This
variability may therefore partly reflect analytical
differences. The relative contribution of OCDD,
which is usually the most abundant homologue, to
the sum of tetra- to octa-CDDrF homologue
groups differs widely from - 10% to ) 60% Žsee
Table 2..
The ratio of PCDDs:PCDFs in ambient air also
varies in the literature, ranging from - 0.5 to
) 2 Žsee Table 2. and with a high variability even
within one country Že.g. Germany, USA.. The
contribution of the different homologue groups to
the PCDDs:PCDFs ratio was also investigated
during compilation of the data for Table 2. For all
the sampling sites, the correlation between the
overall PCDD:PCDF ratio and the individual homologues was calculated. If the homologue groups
existed in similar proportions, PCDDs would be
correlated with the PCDDs:PCDFs ratio with a
value close to 1, while PCDFs would be
close to y1. As shown in Table 3, hexa- to octaCDDs were significantly correlated with the
PCDD:PCDF ratio, whilst the tetra- and pentaCDFs gave a significant correlation. All these
homologues dominate the air PCDDrF mixture
Žsee Fig. 1.. The contribution of the other homologues varied; interestingly, the penta-CDDs were
significantly inversely correlated with the
PCDD:PCDF at the 99% level. In other words,
air masses having a high PCDD burden are likely
to have a very low penta-CDD concentration.
Interestingly, the relative contribution of
OCDD to the SPCDDrF appears to be generally
higher in rural areas and in more recent air
measurements. For example, OCDD contributed
) 50% to the SPCDDrFs in air from rural locations of Mississippi and Niagara in the US ŽSmith
et al., 1990; White and Hardy, 1994.; it was also
ŽSweden.
relatively abundant in air from Rorvik
¨
compared to Stockholm ŽTysklind et al., 1993., in
rural vs. urban sites in Catalunya ŽSpain. ŽAbad
et al., 1997. and air from North-Rhine Westphalia ŽGermany. sampled in 1993 as opposed to
1988 ŽHiester et al., 1997..
Urban
Urban
Rural
Rural
Urban
Remote
Urban
Koln,
¨ Duisburg
Essen, Dortmund
8 towns in NRW
Egge
Bayreuth
Hessen
Mace Head
Milan
Žurban.
Rome
Kracow
Catalunya
Rorvik
¨
Gothenburg
Coast
Stockholm
Germany
Ireland
Italy
Poland
Spain
Sweden
UK
Urban
Flanders
Belgium
Urban
Urban
Industrial
Rural
Remote
Manchester
Cardiff
Bolsoverf
Hazelrigg
East coast
Traffic
Centre
Day
Night
Urban
Remote
Urban
Rural
Rural
Urban
Rural
Industrial
Dayq night
Days
Brixlregg
South Graz
ŽUrban.
Austria
Comment
Location
Country
Ž73]130.
ŽND]62.
2.2
1.3 Ž0.7]3.6.
1.05 Ž0.8]1.4.
1.7
1.4 Ž1.0]1.8.
22a
31
1.2 Ž0.96]1.8.
1.1
1.2
1.8 Ž0.91]2.7.
0.38 Ž0.10]0.82.
1.5
21
18
19
32
7
31
17
26
26
11
1.05 Ž0.72]1.60. 23
0.43 Ž0.24]0.69. 20
8.7
17
1.3 Ž1.0]1.8.
0.32
1.3
1.9 Ž0.3]5.2.
18 Ž6.7]160.
4.2
60
1100
9b
0.35 Ž0.15]0.93.
0.38 Ž0.13]0.92.
3.4 Ž2.8]4.1.
7.7 Ž5.9]9.2.
1.5]2.95b
0.75
1.2
0.61 Ž0.35]1.0.
0.78
1.1 Ž0.80]1.3.
1.6 Ž0.94]2.2.
1.5
0.51 Ž0.39]0.65.
2.3
2.2
PCDDrPCDF
ratio
1.1 Ž0.9]1.2.
13
21
11
27
23
32
32
2
24
24
OCDD
Ž% of S .
0.45 Ž0.37]0.53. 18
12 Ž6.5]17.
6.7 Ž3.2]9.9.
8.6 Ž5.1]15.
2.3
0.81 Ž0.27]1.4.
6.6 Ž5.2]8.8.
3.3
98
20
19
SP4 ] 8 CDDrF
Žpgrm3 .
Table 2
Ambient air levels of PCDDrFs, together with information on other variables
330
11 Ž8]18.
4 Ž2]6.
190
410 ŽND]1800.
22 Ž16]30.
4
19
21 Ž4]60.
250 Ž70]530.
50
950
12 000
85 Ž50]280.
26
29
29
25
17
22
22
33
20
24
28
27
31
18
29
32
32
100 Ž80]150.
50
4 Ž3]4.
48
34
44
40
40
38
32
2,3,4,7,8PCDF
w% of
STEQx
240
90
140e Ž50]160.
110 Ž20]380.
1200 Ž800]1600.
370
390
STEQa
Žfgrm3 .
1997
1997
1991]
1993
1992]
1993
1989]
1990
1988
1987
1995
1995
1990]
1991
1991
Lohmann et al., unpublished
Lohmann et al., unpublished
Jones and Duarte-Davidson,
1997
Duarte-Davidson et al., 1994
Broman et al., 1991
Tysklind et al., 1993
Abad et al., 1997
Grochowalski et al., 1995
Turrio-Baldassarri et al., 1994
Benfenati et al., 1994
Lohmann et al., unpublished
Kaupp, 1996
Konig
¨ et al., 1993
1994
1990
1997
Buck and Kirschmer, 1986
Hiester et al., 1997
Wevers et al., 1993
Christmann et al., 1989
Thanner and Moche, 1995
Reference
1987
1993
1985]
1992
1988
1994
Year
58
R. Lohmann, K.C. Jones r The Science of the Total En®ironment 219 (1998) 53]81
Table 2 Ž Continued.
Location
Comment
Japan
Kobe
Matsuyamac
Urban
Urban
Mississippi
Ohio
Phoenix, AZ
Bloomington
Rural
Urban
Urban
Rural
Wisconsin
Connecticut
North Carolina
New York State
Rural
Urban
Rural
Urban
Niagara
Sidney
Brisbane
US
Australia
Antarctica McMurdo
SP4 ] 8 CDDrF
Žpgrm3 .
STEQa
Žfgrm3 .
OCDD
Ž% of S .
PCDDrPCDF
ratio
9.0 Ž0.1]51.
14.5 Ž4.2]26.7.
39
1.7
0.61
160 Ž80]280.
0.45 Ž0.29]1.0.
6.2 Ž1.2]30.
26.6 Ž9.9]52.
1.8 Ž0.74]12.
47
33
32
4.7 Ž2.9]8.3.
2.9]4.3c
7.0 Ž0.45]21.
1.7
81 Ž16]210.
250 Ž90]450.
25
20
33d
2.9 Ž1.5]4.4.
6.9 Ž0.24]18.9.
1.8 Ž0.72]4.6.
3.9
13
31
14
31
0.83 Ž0.63]1.2.
1.7 Ž0.83]10.
0.71 Ž0.60]0.89.
1.4
58 Ž30]100.
110 Ž8]1900.
25
22
Rural
2.5 Ž0.48]53.
67
4.2
14 ŽND]1300.
Urban
Rural
3.7]15
1.1 Ž0.60]1.2.
22
1.5 Ž0.86]2.9.
16]62
11 Ž4]17.
Remote
0.005
7
0.65
2,3,4,7,8PCDF
w% of
STEQx
31
Year
Reference
1988
1996]
1997
Nakano et al., 1990
Seike et al., 1997
1991
1995
1996
1986]
1989
1989
1987
1991
1986]
1990
1986]
1988
White and Hardy, 1994
Riggs et al., 1996
Hunt et al., 1997
Eitzer and Hites, 1989a
Harless et al., 1990
Hunt and Maisel, 1990
Harless et al., 1992
Smith et al., 1992
Smith et al., 1990
1990
1996
Taucher et al., 1992
Muller,
1997
¨
1992
Lugar et al., 1996
Note. In general, mean values are given, with the range of concentrations in parentheses } values below detection limits taken as half the detection limit.
ND, not detected.
a
NATO-TEF values.
b
Calculated as sum of 2,3,7,8-congeners only; average value from min]max data.
c
Calculated from min]max values.
d
Coelution with 1,2,3,6,9-PCDF.
e
2,3,4,7,8-PCDF not always measured.
f
Values below detection limits taken as detection limit.
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
Country
59
60
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
Fig. 1. General ambient air pattern Žsee text for details..
Table 3
Correlation between individual homologue groups and the
SPCDD:SPCDF ratio a
Homologue
Correlation coefficient
TCDF
PeCDF
HxCDF
HpCDF
OCDF
TCDD
PeCDD
HxCDD
HpCDD
OCDD
SPCDD:SPCDF
y0.59b
y0.66b
y0.18
y0.33
y0.24
y0.34
y0.47b
0.55b
0.76b
0.72b
1
a
Note. The data can be interpreted as follows; a strong negative correlation Že.g. PeCDF vs. SPCDD:SPCDF. means that
the homologue group follows the trend of the SPCDF, whilst
a strong positive correlation Ž e.g. H pC D D vs.
SPCDD:SPCDF. means that the homologue follows the trend
of the SPCDD.
b
Significant at 99%.
In summary, then, this section shows that the
commonly held view that the mixture of
PCDDrFs in air is quite constant may be incorrect, with some differences in the proportion of
different congenersrhomologues and in the
PCDD:PCDF ratio apparent in the literature. This
likely reflects ‘source’ and ‘weathering’ factors
Žsee later..
4.3. TEQs
In almost all data reported, 2,3,4,7,8-PeCDF
makes the single most important contribution to
the STEQ, accounting for between 20% and
) 40% Žsee Table 2.. A compilation of 26 worldwide air measurements ŽEurope, America, Japan
and Australia. gave the profile summarised in Fig.
2, with the error bars representing a single standard deviation. PCDFs typically contributed )
50% of the STEQ. Typically, the tetra- and
penta-CDDrFs account for ) 50% of the STEQ;
however, it should be noted that these congeners
are often reported as close to or less than the
detection limit and many workers then calculate
the STEQ by assuming the actual concentration
is half the detection limit. This may serve to
overestimate the real contribution of these lighter
congeners to the STEQ Žespecially in the case of
2,3,7,8-TCDD..
4.4. Seasonality
There is considerable interest in whether
PCDDrFs exhibit seasonality in air concentrations, because this provides clues as to the link
with sources Žsome combustion sources are
greater in winter, for example, domestic heating.
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
61
Fig. 2. Relative individual contributions to the overall TEQ in ambient air summarised for a comprehensive review of the literature.
and certain atmospheric loss processes Že.g. photolysis., which may also vary seasonally. Hippelein
et al. Ž1996. found a significant seasonal difference between summer and winter PCDDrF concentrations; in their study winter air concentrations were higher by a factor of four to eight than
summer concentrations in rural Germany. Further investigation led them to conclude that domestic heating was responsible for the elevated
winter concentration. Seasonal changes, with winter levels ) summer levels Žboth SPCDDrFs and
STEQ., have also been reported by several other
workers ŽKonig
¨ et al., 1993; Duarte-Davidson et
al., 1994; Sugita et al., 1994; Thanner and Moche,
1994, 1996; Wallenhorst, 1996; Fiedler et al.,
1997a,b., although Jones and Duarte-Davidson
Ž1997. saw no such trend in an urbanised area of
the UK. Seasonality in air concentrations and
mixtures of PCDDrFs is therefore suggested by
many of the studies but not by all Žsee Table 4..
Making a link to sourceŽs. as controlling any
seasonality, however, is complex, for the following
reasons:
1. Many combustion sources which are believed
to make important contributions to the
PCDDrF emissions to the atmosphere, such
as municipal waste solid incinerators ŽMSWI.,
metal smelting and the iron and steel industry
will not be seasonal. Domestic combustion of
coal and wood for space heating is seasonal,
but may be a rather unimportant source of
PCDDrFs to the atmosphere. Calculations of
the UK national source inventory, for exam-
ple, suggest that domestic heating contributes
- 10% of the STEQ released by MSWI
ŽEduljee and Dyke, 1996..
2. Winters may be more prone to temperature
inversion conditions than summers, which can
‘trap’ emissions close to ground level. This
lowering of the height of the mixed atmosphere can give episodic events of elevated
pollutant concentrations which may give a
false picture about source strengths. There
are general differences in boundary layer
heights between summer and winter; in
summer it is assumed to be up to 1500 m
high, whereas in winter it is - 500 m ŽSeinfeld, 1986.. If there was a proportional relation between the Earth’s surface heat flux
and the boundary layer height, the height
would be reduced by a mere 15% in winter
ŽRobinson, 1966., which may only have a small
impact on air concentrations.
3. Several loss processes which may play a role
in removing PCDDrFs from the atmosphere
will also display seasonality. These include:
photolysis; chemical reactivity with oxidising
species; wetrdry deposition fluxes; scavenging
by vegetational surfaces.
Studies on seasonality are therefore clearly interesting and important, but need to be conducted
and interpreted with care.
4.5. Day]night differences
For similar reasons to the seasonality studies
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
62
Table 4
Seasonality of ambient air PCDDrF concentrations
SP4 ] 8 CDDrFs
Žpgrm3 .
Country
Summerr
winter
Augsburg, Germany
S
W
Hessen, Germany
S
W
Karlsruhe and Stuttgart, Germany
S
W
2.2 Ž0.64]5.1.
4.4 Ž1.3]6.6.
16
27
Hornisgrinde,
Germany
S
W
1.6 Ž0.45]2.7.
0.80 Ž0.49]1.1.
Tokyo, Japan
S
W
44
120
Urban Japan
S
W
44
44
Rural USA
S
W
1.2 Ž1.1]1.3.
6.6 Ž4.8]8.9.
OCDD
Ž% of conc..
Year
Reference
1992]93
Hippelein et al., 1996
1990
Konig
¨ et al., 1993
0.75 Ž0.28]2.0.
1.5 Ž0.42]2.4.
1992
Wallenhorst, 1996
15
31
0.45 Ž0.16]0.80.
1.6 Ž1.1]2.0.
1992
Wallenhorst, 1996
10
8
0.69
0.58
1990r1992
Sugita et al., 1994
17
11
1.2 Ž0.38]2.4.
0.86 Ž0.44]1.4.
1992
Kurokawa et al., 1996
3.62 Ž3.4]4.0.
2.4 Ž1.0]3.9.
1996
Fiedler et al., 1997b
29
23
f3
f 10
PCDDrPCDF
ratio
1.4
f 1.2
f 0.5
Ž24]68.
Ž18]87.
0.47 Ž0.29]0.57.
1.28 Ž0.85]1.86.
Note. In general, mean values are given, with the range of concentrations in parentheses } values below detection limits taken as
half the detection limit.
a
NATO-TEF values.
comparisons of ambient air concentrations in the
day and in the night are of interest, because they
could potentially point to the significance or otherwise of certain key processes, namely: photolytic degradation and temperature-mediated
air]surface partitioning ŽKwok et al., 1995; Lee et
al., 1998.. However, such studies are difficult to
undertake definitively, because of the short sampling times required and the fact that ambient air
concentrations can vary over short time-scales in
response to other factors, such as air mass origin
and recent depositional events ŽEitzer and Hites,
1989a; Tysklind et al., 1993. and the diurnal cycling which is often observed in the boundary
layer height ŽStull, 1988; Van Pul et al., 1994..
These limitations should be borne in mind when
considering the two dayrnight studies which have
been published to date, undertaken in the cities
of Milan, Italy ŽBenfenati et al., 1994. and Graz,
Austria ŽThanner and Moche, 1995. Žsee Table 2..
In summary, these reported similarly high day
and night PCDDrF concentrations for Graz during an inversion period ŽThanner and Moche,
1994. whilst Benfenati et al. Ž1994. found in Milan the sum of the tetra- to octa-CDDrFs to be
twice as high in the night than for the day samples. These studies are rather limited and inconclusive as to the diurnal cycling of airborne
PCDDrFs.
4.6. Time trends
There is good evidence that air PCDDrF levels
are declining in urbanrindustrialised centres. This
has been comprehensively reviewed elsewhere
ŽAlcock and Jones, 1996.. In North-Rhine Westphalia, for example, ambient air levels dropped by
more than 60% from 1987 to 1993 ŽHiester et al.,
1997. while Friesel et al. Ž1996. reported a decline of 66% between 1990 and 1995 in Hamburg.
In Hessen, another federal state in Germany,
consecutive measurements between 1990 and 1992
by Liebl et al. Ž1993. suggested decreases of
; 10% per year. These declines are believed to
be largely due to emission abatement actions
taken in the early nineties Že.g. German national
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
emission standard of 0.1 ngrm3 .. These trends
are consistent with those noted for UK urban
centres ŽColeman et al., 1997..
5. Gas–particle partitioning
5.1. Introduction
An interesting research issue with respect to
PCDDrFs in air is the extent to which the
gasrparticle phase distributions are freely exchangeable Že.g. at different temperatures.
andror whether a portion of the PCDDrF compounds is ‘held’ by the particles in a strongly
bound or non-exchangeable form. Combustionderived PAHs have clearly been shown to have a
sizeable non-exchangeable fraction, often associated with the sootrorganic matter fraction of the
aerosol ŽBidleman, 1988; Burford et al., 1993..
This is apparent in plots of the logarithm of the
gas]particle partitioning constant, K p , against the
logarithm of the sub-cooled liquid vapour pressure, pL8. On such plots, PAHs have a higher
particle-bound fraction for similar vapour pressures than PCBs, which are generally assumed to
be more readily available to partition from the
aerosol to the gas phase. Many of the non-polar
organochlorine pesticides and PCBs show rather
different distributions on the log K p vs. log pL8
plots. This issue clearly has practical significance
for PCDDrFs, because it may again provide clues
as to the proportion of combustion-derived vs.
‘other’ sources of PCDDrFs in the atmosphere
and it will affect the deposition processes Že.g. the
relative importance of dry gaseous and dry particulate. which can transfer PCDDrFs to terrestrial
and aquatic foodchains and compartments. It is
therefore clearly important to obtain data on the
gasrparticle partitioning of PCDDrFs, but there
are few such studies to date. We therefore consider in this section how such data may be obtained and some theoretical aspects before reviewing the information that is available. An issue
of practical concern is how to sample accurately
the ambient distribution of semivolatile compounds.
63
5.2. Sampling and sampling artefacts
The gasrparticle distribution of SOCs has traditionally been studied by quantifying the amount
of a given compound associated with the filter
Žglass fibre or Teflon W filter. and the back-up
sorbent Žpolyurethane foam ŽPUF., XAD-2 resin
etc.., nominally ascribing the terms ‘particulate’
and ‘gaseous’ phase, respectively. There are certain sampling artefacts which can potentially influence the ability of the sampling method to
accurately represent the ‘true ambient’ distribution of the SOCs. These artefacts have been
discussed in detail elsewhere ŽPankow and Bidleman, 1992; Falconer and Bidleman, 1998.. In
brief, however, they relate to: volatilisation losses
from particles collected on the filter Ž‘blow-off’.
and adsorption gains to the filter substrate material.
Kaupp and Umlauf Ž1992. compared the sampling efficiency of a conventional GFF with a low
pressure cascade impactor. They found that particle bound fractions of the organochlorines investigated were systematically higher in the impactor-adsorbent combination than in the filteradsorbent sampler. However, these differences
did not exceed a factor of two and were an
average of 36%.
Diffusion denuders have been suggested to
overcome the sampling problems associated with
conventional GFF-adsorbent sampling trains, as
used by Gundel et al. Ž1995.. A denuder traps the
gaseous phase before the particles are collected.
Inherent problems of denuders are that the
gaseous phase is never collected to a 100% efficiency and that volatilisation can occur if the
samplers are operated at high face velocities.
Kamens et al. Ž1995. used a conventional sampling system and a denuder for PAH sampling.
They concluded that the conventional sampling
system gave the better estimates of gas and particulate concentrations.
5.3. Theory
The distribution of semi-volatile organic compounds ŽSOCs. between the gas and particulate
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
64
phases depends upon the available particle
properties, the ambient temperature, the relative
humidity and the compounds themselves. Important particle features are the size distribution,
their concentration in the atmosphere and surface related properties. Distribution of compounds will also depend on the enthalpy of desorption from the surface and the enthalpy of
vaporisation as well as the compounds subcooled
liquid vapour pressure ŽBidleman, 1988; Pankow
and Bidleman, 1992..
An equation which has been successfully used
to describe gas]particle partitioning is:
Kps
FrTSP
A
Ž1.
where K p Žm3rm g. is a temperature-dependent
partitioning constant, TSP Ž m grm3 . is the concentration of total suspended particulate material, and F Žngrm3 . and A Žngrm3 . are the
particulate associated and gaseous concentrations
of the compounds of interest, respectively
ŽYamasaki et al., 1982; Pankow, 1991; Pankow
and Bidleman, 1992.. Plotting log K p against the
logarithm of the subcooled liquid vapour pressure, pL8, gives:
log K p s m r ? log pL8 q br
Ž2.
where m r is the slope and br the y-intercept of
the trendline.
For a given temperature and for a compound
class, plots of log K p should be linear with a
slope m r s y1. As pointed out by Pankow Ž1994.,
a slope of m r near y1 is a necessary, but not
sufficient, condition in proving that gas]particle
partitioning is governed by simple physical adsorption. It was estimated by Pankow Ž1994. that
br-values of y7.3 and y8.9 would specify absorptive partitioning. Two complete datasets on the
gasrparticle partitioning of PCDDrFs have been
reported in the literature. Eitzer and Hites Ž1989b.
studied a site in urban Bloomington, Indianapolis; their data gave values for m r s y0.775
and br s y5.72. Hippelein et al. Ž1996. published
data from a 1-year-long monitoring programme in
Augsburg, Germany. Using their data, we calcu-
lated m r s y0.70 Žy0.62 to y0.78. and br s
y5.5 Žy4.6 to y6.4; regression coefficients r 2 s
0.87]r 2 s 0.95.. The slopes and intercepts of both
regressions are virtually identical. A comparison
of PAH, PCB and PCDDrF partitioning behaviour using these datasets waverage PAH and
PCB data from Falconer and Bidleman Ž1998.
and Simcik et al. Ž1998.x is shown in Fig. 3. The
datasets are from different times and locations,
but the trend is clear. Fig. 3 suggests that for
comparable vapour pressures, PCDDrFs are less
likely to be in the particulate phase than PAHs
and PCBs. More work, preferentially measuring
PCDDrFs and other SOCs simultaneously, is
needed to confirm this unexpected behaviour,
which contradicts the idea of a nonexchangeable fraction for PCDDrFs in combustion-derived particles. The trends in Fig. 3 support work by Kaupp Ž1996., who did not find any
indication of a non-exchangeable PCDDrF fraction, whereas PAHs showed a stronger tendency
to be bound to particles for comparable vapour
pressures.
Finizio et al. Ž1997. suggested using K o a as a
descriptor of gasrparticle partitioning. A similar
ratio of K prK o a for PAHs, PCBs and OC pesticides led them to suggest that the solutes are
actually absorbing into the aerosol particles, as
adsorption processes should vary much more
broadly for such a range of chemicals.
Most field studies investigating the partitioning
behaviour of SOCs obtained slopes m r of around
y0.6 to y0.8 ŽFinizio et al., 1997; Simcik et al.,
1997; Falconer and Bidleman, 1998.. As discussed
by Simcik et al. Ž1997., this is still observed for air
masses having reached an equilibrium. Other factors must therefore influence the slope, such as
differences in enthalpies of evaporation within a
given class of compounds.
5.4. Summary of measured data on the gas]particle
distribution of PCDD r Fs
So far, reported PCDDrF gas]particle distributions have not been corrected for adsorbing
artefacts ŽEitzer and Hites, 1989b; Broman et al.,
1991; Hippelein et al., 1996; Kaupp, 1996..
Unfortunately, gas]particle measurements are
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
65
U
UU
Fig. 3. PAH, PCB and PCDDrF gas-particle partitioning data plotted as log K p vs. log pL8. Simcik et al. Ž1998.; Hippelein et
al. Ž1996.; 8Eitzer and Hites Ž1989a..
not always accompanied by TSP-levels, which
makes a comparison of different datasets difficult
Žsee Table 5.. Particle-bound fractions are comparable for Bloomington, USA, Bayreuth and Asperg, both in Germany, at similar temperatures
for the PCDFs, but tetra- and penta-PCDDs show
a very high variability. This may reflect problems
of working close to detection limits for these
congeners. Higher chlorinated homologues and
lower temperatures give a higher particle bound
fraction Žsee Table 5.. The results for the study by
Hippelein et al. Ž1996. further show that a higher
Table 5
The proportion of PCDDrFs detected in air present in the particulate phase
Homologue
group
PCDDrFs } % particle bound w prŽ g q p .x
Bloomingtona
USA
Stockholmb
Sweden
Augsburgc
Germany
Augsburgc
Germany
Bayreuthd
Germany
Asperge
Germany
F4
F5
F6
F7
F8
D4
D5
D6
D7
D8
Temp
9
37
74
92
95
13
33
78
97
99
Yearly average
29
63
89
98
100
61
70
91
98
100
59
94
99
) 99
) 99
76
93
99
) 99
) 99
08C
14
29
52
80
) 86
20
37
59
) 90
) 98
208C
15
41
70
) 92
) 94
40
56
83
) 94
) 95
118C
14
31
85
100
96
ND
31
97
95
90
98C
ND, not detected.
a
Eitzer and Hites Ž1989a..
b
Broman et al. Ž1991. } calculated distribution values.
c
Average value out of six measurements in a rural area from Hippelein et al. Ž1996..
d
Kaupp Ž1996..
e
Wallenhorst Ž1996..
66
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
particle mass leads to a higher particle bound
fraction.
r Fs in various sizes of
6. Distribution of PCDDr
airborne particles
mode particles are formed by condensation and
coagulation. Coarse particles are generated by
mechanical attrition, soil erosion and by sea spray
ŽWhitby, 1978; Bidleman, 1988..
6.4. Particle size distribution of PCDD r Fs
6.1. Introduction
An important aspect of the behaviour of
PCDDrFs in air is their distribution on aerosols
of different sizes. Size distribution will influence
atmospheric transport } large particles will settle close to their origin, while smaller ones are
prone to long-range transport; both atmospheric
wash-out and dry deposition are influenced by
particle size. The size-distribution of particles,
and most likely of PCDDrFs, changes with time
and distance from a source. In the following
sections, the formation of the particle sizes is
briefly summarised before discussing the few
studies on the particle-size studies distribution of
PCDDrFs. Analogies to PAHs and their distribution on different particles will be used to suggest
likely PCDDrF behaviour.
6.2. Sampling
Sampling of different particle-size fractions is
usually performed with cascade impactors, which
can separate particles with aerodynamic diameter, d ae , - 10 m m ŽPoster et al., 1995.. Larger
particles can be separated by rotary impactors
ŽSheu et al., 1997.. An inherent problem is the
long sampling time required to obtain a sufficient
mass of particles to readily detect the PCDDrFs.
6.3. Formation of different particle sizes
Aerosols are classified into three size groups:
nuclei mode Žor ultrafine. with d ae - 0.1 m m;
accumulation mode Žor mid-sized., 0.1- d ae - 2.0
m m; and coarse size modes, d ae G 2.0 m m. Nuclei
mode particles are formed by gas-to-particle conversions during combustion, when hot vapour
condenses to form primary particles which coagulate. This range contains numerous particles, but
is unimportant in mass. Their lifetime is short
because of rapid coagulation. The accumulation
Kaupp et al. Ž1994. investigated the occurrence
of PCDDrFs in particulate matter using a low
pressure cascade impactor in rural Germany in
the summer. Approximately 90% of the
PCDDrFs were found on particles with d ae - 1.35
m m. The size distribution of these compounds
was unimodal, with a mean diameter between
0.15 and 0.45 m m. The homologue patterns were
very similar on all particle size fractions for a
particular run. PAH-measurements made by
Schnelle et al. Ž1995, 1996. both in the outskirts
of Munich and in rural Germany also showed a
unimodal size distribution, with a mean diameter
around 0.26]0.42 m m. No significant differences
between the particle size distributions measured
by high-volume or low-pressure cascade impactors were observed ŽSchnelle et al., 1995, 1996..
Kurokawa et al. Ž1996. used a similar multistage
sampling device in Japan and found 68]80% of
the PCDDrFs on particles with d ae - 2 m m.
Differences in the congener distribution were
found for different particle sizes: the smallest
particles Ž d ae - 1.1 m m. were dominated by hexato octa-CDDrFs Žespecially hepta-CDD and
hexa-CDF.. On the smallest particles, the percentage of particle-bound PCDDrF was a function of the PCDDrF boiling point. The lighter
PCDDrF congeners were found predominantly
on the larger particles, which therefore reflect
typical gas phase congener patterns ŽKurokawa et
al., 1996., as can be seen in Fig. 4.
PAHs are also predominantly Ž65]90%. associated with fine particles Ždiameter - 2.0 m m.. A
redistribution of the most volatile PAHs can occur close to emission sources with the lower
molecular weight compounds volatilising more
readily from the fine particles and sorbing to the
coarse particles than the heavier PAHs. The relative amounts of PAHs bound to the fine particles
can therefore increase with molecular weight
ŽPistikopoulos et al., 1990; Baek et al., 1991;
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
67
Fig. 4. Correlation of PCDDrFs on particles - 2 m m with corresponding boiling point at three Japanese sites wdata from Kurokawa
et al. Ž1996.x.
Aceves and Grimalt, 1993; Venkatamaran and
Friedhorst, 1994; Venkatamaran et al., 1994;
Poster et al., 1995; Schnelle et al., 1995, 1996;
Allen et al., 1996; Sheu et al., 1997..
In summary, the two studies on the particle size
distribution of PCDDrFs gave rather different
results. The study in rural Germany ŽKaupp et al.,
1994. showed aged particles with a fully equilibrated PCDDrF distribution, giving further hints
that the gas]particle partitioning of PCDDrFs is
fully exchangeable. However, in the Japanese
study ŽKurokawa et al., 1996. there was an increase in the proportion of the lower chlorinated
congeners with particle size. These samples were
taken close to emission sources and may therefore reflect the temporary non-equilibrium of
PCDDrFs primarily on the smallest particles,
with the lower chlorinated congeners volatilising
more readily.
7. Atmospheric behaviour of PCDDr
r Fs
7.1. Introduction
One of the major unknown aspects of PCDDrF
behaviour to date is the extent to which they are
depleted in the atmosphere, either by photolysis
or radical-initiated reactions. This is of obvious
importance for the long-term environmental fate
of PCDDrFs. Clearly if atmospheric degradation
reactions are slow compared to the rates of deposition, then a greater proportion of the emitted
PCDDrFs could reach human and terrestrial
foodchains.
7.2. PCDD r Fs in the ¨ apour phase
The mixture of PCDDrF compounds in the
vapour phase can potentially undergo ‘weathering’ by photolysis and reaction with OH-radicals.
These processes would operate at different rates
for different PCDDrFs, thereby having the
propensity to alter emission patterns. What is still
unclear at present, however, is how significant
these processes are in the environment. Controlled laboratory experiments are difficult to
conduct per se and especially in a way that adequately mimics environmental conditions. Experimental problems include: Ža. difficulties in generating and maintaining gas phase PCDDrF concentrations in experimental chambers; Žb. sorptive loss processes; and Žc. difficulties in sampling
and detection of the compounds and their
products. These problems increase with increasing chlorination Žand hence lower vapour pressures. and have been discussed by Atkinson
Ž1997..
Atmospheric behaviour of PCDDrFs has been
68
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
studied in reaction chambers, where they are
either exposed to natural sunlightrUV lamps or
to radicals. Kwok et al. Ž1994, 1995. found no
observable reaction of dibenzo-p-dioxin or dibenzofuran with ozone and negligible losses with the
NO 3 radical. OH radicals are the dominant atmospheric gas phase reactants for PCDDrFs ŽAtkinson, 1997.. Based on such chamber studies,
lifetimes of the PCDDrFs containing four or
more chlorine atoms are G 3 days and therefore
expected to be sufficiently long in the absence of
precipitation events for long-range transport to
occur. Recent work by Brubaker and Hites Ž1997.
reported half-lives of at least 10 days for TCDDs.
Data from this study and work by Kwok et al.
Ž1995. are summarised in Table 6. Both studies
gave atmospheric half-lives for PCDFs roughly
twice those for PCDDs.
These reported half-lives seem sufficiently long
to allow other reactions to take place. McCrady
and Maggard Ž1993. studied the fate of gaseous
2,3,7,8-TCDD adsorbed to grass; they reported a
photolytic half-life of 1.9 days on the grass surface. Dung and O’Keefe Ž1994. reported half-lives
in the order of a single day for lower chlorinated
PCDFs under natural sunlight in distilled water.
These observations are difficult to reconcile with
those from chamber studies, suggesting uncertainties remain in our knowledge of these important processes.
7.3. PCDD r Fs on particles
Systematic studies on the losses occurring on
particles are also difficult to perform. Atmospheric particles are difficult to study in the
laboratory, so model substrates such as silica gel
or fly ash spiked with PCDDrFs have therefore
been used instead. However, neither matrix possesses the surface properties or size of urban
atmospheric particles.
Koester and Hites Ž1992a. used a rotary photoreactor and a UV lamp to study degradation on
atmospheric particles. PCDFs photodegraded
much more rapidly than PCDDs on silica, with
the photolytic half-lives increasing with the level
of chlorination for PCDDs. However, on fly ash
particles photodegradation was somehow inhibited by both the organic material on the particles
and by the particle’s surface itself. PAH photolysis has also been reported to be less on fly-ash
than silica gel ŽBehymer and Hites, 1988; Baek et
al., 1991.. Pennise and Kamens Ž1996. investigated the behaviour of PCDDrFs on particles of
high- and low-temperature combustion exposed
to natural sunlight in outdoor Teflon film chambers. Greater photochemical reactivity of
particle-bound PCDDrFs and PAHs was
observed on the low-temperature combustion
particles, which have a higher organic matter
content, whereas PCDDrFs on fly-ashes pho-
Table 6
PCDDrF tropospheric lifetimes with respect of OH radical reactions a
Number
of Cl
atoms
0
1
2
3
4
5
6
7
8
a
b
Tropospheric lifetimes and half-life times in days
PCDDs
PCDFs
tlife
t1r2 b
tlife
t1r2b
1.0
3.0
2.0]2.4
2.5]3.3
2.8]7.2 Ž 11.
4.0]8.5 Ž21.
45
89
230
0.7
2.1
1.4]1.7
1.7]2.2
2.0]5.0 Ž 8 .
2.7]5.9 Ž 15 .
31
62
160
3.7
2.9
4.0]5.5
5.5]9.5
7.7]18 Ž 19 .
15]29 Ž 40 .
83
190
580
2.6
2.0
2.8]3.9
3.9]6.7
5.3]13 Ž 13 .
10]20 Ž 28 .
57
130
400
Kwok et al. Ž1995.; values in italics from Brubaker and Hites Ž1997..
ln2
? w OH x s ln2 ? t life .
kOH
t1r 2 s
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
todegrade only slowly ŽTysklind and Rappe, 1991;
Pennise and Kamens, 1996; Sommer et al., 1996..
High-temperature combustion is more complete,
resulting in a smaller liquid layer surrounding the
core. Recent work by Strommen and Kamens
Ž1997. described freshly emitted diesel engine and
wood stove particles as consisting of two layers,
with an outer layer being primarily composed of
liquid-like organic material covering an inner layer
composed of many discrete, solid and impenetrable masses. ‘Ageing’ of particles is then understood as a decrease of the organic matter content
of the inner layer. The rate limiting step in
achieving an equilibrium with the vapour phase is
then the adsorption into and the tortuosity of the
inner layer. PCDDrFs in the inner layer would
presumably be sheltered from photolytic and radical reactions.
Work by Cains et al. Ž1997. suggests that the
elemental content Žnotably Ka and Ca. of a fly
ash can influence dechlorination. While exposing
combustion products to sunlight, Pennise and Kamens Ž1996. found increasing OCDD and OCDF
concentrations; similar observations were made
by Tysklind and Rappe Ž1991.. This is probably
due to the presence of PCP on atmospheric particles; photochemical synthesis of OCDD has been
confirmed in other studies using PCP treated
wood or PCP spiked water ŽLamparski et al.,
1980; Waddell et al., 1995..
7.4. PCDD r Fs in solution
Various workers have studied photochemical
reactions of PCDDrFs in solution experiments.
These often look at very artificial situations, with
compounds in organic solvents to ensure sufficient solubility Že.g. Dung and O’Keefe, 1994;
Wagenaar et al., 1995.. Friesen et al. Ž1990, 1996.
exposed several congeners to sunlight in natural
water as well as in distilled water; photolysis was
relatively enhanced in the natural water, leading
to the hypothesis that natural organic macromolecules may act as sensitisers. The 2,3,7,8-substituted PCDDrFs seem to be more stable in
sunlight than the non-2,3,7,8-substituted congeners ŽFriesen et al., 1990, 1996; Dung and
O’Keefe, 1994..
69
7.5. What is the real importance of photolytic and
OH radicle degradation in the en¨ ironment?
There seems to be evidence for a destruction of
PCDDrFs to a limited extent at the early stages
of their emission from low temperature combustion ŽPennise and Kamens, 1996., but for little
degradation on particles thereafter ŽKoester and
Hites, 1992a..
Once PCDDrFs partition into the environment
they seem to be very stable, with little or no
degradation or other loss from soil ŽMcLachlan et
al., 1996; Schroder
¨ et al., 1997.. Photodegradation
while sampling bulk deposition is } if at all }
only of minor importance. Horstmann et al. Ž1997.
compared daily to monthly deposition samples,
the difference did not exceed 10%.
The real importance of OH radical reactions
for the depletion of PCDDrFs in the atmosphere
has still to be verified. Diurnal measurements for
a given source strength could indicate the real
influence of OH radical losses, as in studies on
PAHs ŽSimcik et al., 1997..
If OH radical reactions are significant, the
mixture of PCDDrFs in the gas phase of a
‘weathered’ air mass may be expected to shift
towards a greater proportion of PCDFs ŽKwok et
al., 1995; Brubaker and Hites, 1997.. This has
been noted in the mixture of PCDDrFs in the
Arctic air after long-range transport ŽSchlabach
et al., 1996.. However, this observation may partially be explained by preferential scavenging of
the higher chlorinated PCDDs Žsee Table 7..
The 2,3,7,8-substituted congeners are generally
more stable in the natural environment than the
non-2,3,7,8-substituted compounds, with the
higher chlorinated congeners more stable than
lower ones; this presumably explains the predominance of OCDD and hepta-CDDrFs in ambient
air relative to emission samples ŽDung and
O’Keefe, 1994; Sivils et al., 1994, 1995; Sommer
et al., 1996..
8. Deposition processes of PCDDr
r Fs
8.1. Introduction
Deposition of PCDDrFs can occur in dry
70
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
gaseous, dry particulate and wet forms. Dry
gaseous deposition is understood as adsorption at
the air]surface interface. Dry particulate deposition results when an airborne particle comes into
contact with a surface and is lost to it. Wet
deposition transports atmospheric compounds to
the surface by precipitation. It can be seen as a
joint action of several mechanisms: scavenging of
particles and gas by droplets Žbelow-cloud removal or ‘wash-out’. and nucleation scavenging
Žin-cloud removal, ‘riming’ in cold clouds.
ŽGraedel and Crutzen, 1992; Van Oss and Duyzer,
1996..
8.2. Deposition samplers and sampling artefacts
Ideally deposition fluxes would be determined
with a sampler which collected the relevant size
spectrum of dry particle deposition and sorbs
gaseous deposition as a real environmental surface Ži.e. soil, vegetation, water.. However, this is
obviously not possible and realistic measurements
are compromised by the choice of sampler design,
together with the need to use sufficiently long
sampling times to collect enough PCDDrF for
detection and the need to minimise losses due to
‘blow-off’rrevolatilisation and environmental
degradation during sampling.
Two kinds of deposition samplers are currently
used to measure deposition fluxes. One method
consists of using a cylindrical glass jar ŽBergerhoff
beaker. placed on a high pole ŽGerman VDI 2119
type.. Blanks can easily be obtained by placing
the jars in a furnace. The ratio of the total inner
surface to the vertical surface is approx. 10:1, thus
resembling grassland and crops. Nevertheless this
kind of sampling device is not suitable for dry
deposition of small particles or gases which, according to Schroder
et al. Ž1997., are of minor
¨
importance for PCDDrFs. Thus the Bergerhoff
model is believed to be appropriate for sampling
the relevant parts of the deposition spectrum.
Inverted Frisbees are another commonly used
type of collector, as described by Hall and Upton
Ž1988.. Their shape does not cause significant
disturbance to the wind flow and their depth
should prevent particles from bouncing out of the
samplers. The blow-out wind speed was de-
termined to be in the range of 5]6 mrs. An
organic film Že.g. Teflon W . can be used to reduce
the problems of blow-out, but may give anomalous
sampling of gas compounds by sorbing them to
the collector surface. Deposition has also been
sampled using flat plates which are often mineral
oil coated to achieve a higher collection efficiency. Tests by Koester and Hites Ž1992b. indicate that flat plates are more efficient collectors
than the inverted Frisbees.
Horstmann and McLachlan have described and
tested a ‘wet onlyrdeposition only’ ŽWODO.
sampler, to try and quantify wet and dry deposition separately. They used a collection funnel for
wet deposition, consisting of 1-m2 stainless steel
which is covered during dry periods by a moveable lid. Conversely, the lid covers the dry deposition tray during rain events with humidity sensors
initiating and terminating the collection of the
wet and the dry deposition events. Thirty minutes
after closing the lid the collecting funnel is solvent rinsed automatically to minimise post-precipitation losses.
Potential sampling artefacts of primary concern
in sampling bulk deposition of PCDDrF are
volatilisation and photodegradation. Horstmann
and McLachlan Ž1997. found similar results for a
30-day sample compared to thirty daily samples at
the same place. They estimated the loss due to
degradation and volatilisation to be at most 30%
for the TCDFs.
Deposition depends on the sampling surrounding and surface, as Horstmann et al. Ž1997. found
atmospheric deposition to a forest to be different
from one to an adjacent clearing. Their results
suggest that the higher deposition fluxes observed
in the forest in summer is related to deposition of
compounds sorbed on wax cuticles of leaves which
can be shed by the plant. It was suggested that
PCDDrFs were liberated from needles in the
canopy during high temperature events. WelschPausch and McLachlan Ž1995. showed an increased deposition to high leaf-area-index plants.
8.3. PCDD r F bulk deposition
The Bergerhoff and Frisbee type collectors
sample ‘bulk’ or wet plus dry deposition. PCDDrF
Cardiff
Indianapolis
Bloomington
Karlshruhe ]
Eggenstein
Hornisgrinde,
rural
Manchester
Karlsruhe
Stuttgart
Summer
Winter
Summer
Winter
Summer
Winter
Summer
Winter
10 130
360
270
1.8
40
69
26
48
33
35
39
42
50
43
16
19
27
29
OCDD
Ž% of S .
0.82
1.9
12.4
1.3 Ž0.39]3.0.
2.9 Ž1.1]12.
1.5 Ž0.51]2.7.
5.4 Ž0.87]19.
2.3 Ž0.26]5.30.
3.9 Ž0.74]7.1.
3.6 Ž0.58]8.5.
2.3 Ž2.3]2.4.
0.58
1.25
1.87
1.6 Ž0.83]2.8.
PCDDrPCDF
ratio
169
16
11
13
30 Ž16]47.
26 Ž9.0]52.
31 Ž6.8]63.
79 Ž7.5]220.
31 Ž23]39.
49 Ž9.5]110.
51 Ž6.2]83.
47 Ž18]87.
173
15 Ž11]18.
570 Ž110]1000.
83 Ž41]220.
10
9
4
26 Ž9.7]83.
STEQa
Žpgrm2
per day.
7
25
26
23
21
27
23
19
35
9
28
11
33
39
2,3,4,7,8CDF
w% of
STEQx
1991
1991
Koester and Hites,
1992b
Duarte-Davidson
et al., 1994
Wallenhorst, 1996
1992
1990]
1993
Wallenhorst, 1996
Wallenhorst, 1996
Wallenhorst, 1996
Friesel et al., 1996
Kurz et al., 1993
Hiester et al., 1993
Liebl et al., 1993
De Fre
´ et al., 1994
Reference
1992
1992
1990
1993
1995
1992
1992
1990
1991
1992
1992
1993
Year
Note. In general, mean values are given, with the range of concentrations in parentheses } values below detection limits taken as half the detection limit.
a
NATO-TEF values.
US
UK
Summer
Winter
2300 Ž1100]4500.
5000 Ž620]14 000.
2700 Ž360]5800.
27 000 Ž630]130 000.
3500 Ž1400]5400.
8700 Ž490]20 000.
6500 Ž550]14 000.
10 000 Ž1300]27 000.
7990
1390
660
Urban
North-Rhine]
Westphalia
Bielefeld
Hamburg
1700 Ž730]5000.
Urban
Hessen
Germany
Background
urban
Flanders
SP4 ] 8 CDDrF
Žpgrm2 per day.
Belgium
Comment
Location
Country
Table 7
Bulk deposition of PCDDrFs
R. Lohmann, K.C. Jones r The Science of the Total En®ironment 219 (1998) 53]81
71
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
72
deposition fluxes reported in the literature range
from ; 100 to ) 10 000 pgrm2 per day Žsee
Table 7.. Higher deposition fluxes have been reported for winter than summer ŽDuarte-Davidson
et al., 1994; Halsall et al., 1997a,b.. Wallenhorst
et al. Ž1997. reported a positive relationship
between total deposition flux Žwet and dry. and
rainfall.
Deposition is in general dominated by the
higher chlorinated congeners, notably OCDD,
which typically accounts for 20]40% of the
SPCDDrF flux.
STEQ deposition fluxes are generally in the
range of 20]100 pgrm2 per day, with 2,3,4,7,8PeCDF consistently contributing between 20 and
35% of the STEQ. Deposition fluxes have declined in Hessen and Hamburg, Germany, in recent years ŽLiebl et al., 1993; Friesel et al., 1996.,
in line with the trends in ambient air.
8.4. Wet deposition of PCDD r Fs
Wet deposition is the sum of vapour dissolution
into rain and cloud droplets and the removal of
atmospheric particles by precipitation. It is commonly thought that equilibrium partitioning occurs between a slightly soluble trace organic compound in the gas phase and a falling rain drop in
the atmosphere ŽLigocki et al., 1985a,b.. If temperature dependent Henry constants Ž H . are
available, the gas scavenging ratio, S g , can be
estimated by:
Sg s
RT
H
Ž3.
where T s ambient temperature and R s
universal gas constant. Particle scavenging, on the
other hand, is an irreversible process. It is not
based on equilibrium considerations and depends
largely on meteorological factors and particle
characteristics.
Rain is formed via nucleation scavenging. A
raindrop Ž; 1 mm. has an atmospheric lifetime in
the order of minutes. Nevertheless, Slinn et al.
Ž1978. suggested that a falling raindrop will reach
equilibrium with its surrounding gas phase after a
fall of approx. 10 m. Scavenging ratios of rain
exceed the calculated a values Žsimple air]water
exchange. by two or three orders of magnitude.
Particle scavenging is used to explain the difference, as a raindrop reaching the ground may
contain as many as 10 000 small particles ŽGraedel and Crutzen, 1992.. Fog is an even better
scavenger of hydrophobic organic compounds. The
micron sized droplets are two to three orders of
magnitude smaller than rain droplets, with an
extremely high surface area and being rich in
both particles and dissolved organic carbon. These
droplets have lifetimes of many hours. Hydrophobic organic compounds are enriched tens
to thousands of times in fog compared to rain
ŽCapel et al., 1991.. Snow is also an extremely
effective scavenger of aerosols. For the same water content, snowflakes have much larger surface
areas and fall much more slowly. As a consequence, they are 10]100 times more efficient in
scavenging particles. This efficiency increases with
particle size ŽGraedel and Crutzen, 1992..
Wet deposition was shown to be the major
pathway responsible for the deposition of the
higher chlorinated PCDDrFs to a bare soil in a
field study in rural Germany ŽSchroder
et al.,
¨
1997.. Wet deposition accounted for 85% of the
total deposition flux for the higher chlorinated
PCDDrFs and more than 50% for the more
volatile congeners.
Deposition estimates to the Great Lakes by
Hoff et al. Ž1996. have also shown wet deposition
to be more important than dry deposition fluxes
for a range of PAHs. Leister and Baker Ž1994.
estimated deposition fluxes into Chesapeake Bay;
wet deposition accounted for approximately half
of the total PAH and PCB deposited.
8.5. Sca¨ enging ratios
Wash-out and scavenging ratios are defined as
the concentration of the pollutant in the hydrometeor divided by the concentration in the surrounding air during the precipitation event. Ideally these concentrations are measured simultaneously, but in general average air concentrations
are used. Scavenging ratios vary largely within
and between different rain, fog and snow events
ŽLeister and Baker, 1994. and, for PCDDrFs,
vary by a factor of ; 10 for a given homologue
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
group ŽKaupp, 1996; Seike et al., 1997.. They
should therefore be used with caution.
Total precipitation scavenging Ž St o t . is the sum
of gas Ž S g . and particle scavenging Ž S p .:
St o t s S g ? Ž1 y F . q S p ? F
Ž4.
where F is the fraction in particulate phase Ž0 F
F F 1. and gas and particle scavengings are the
concentration of the dissolved respective particulate phase in the hydrometeor divided by the
concentrations of the gaseous respective particulate phase in the air.
8.5.1. PCDD r F sca¨ enging
PCDDrFs in rain samples were analysed by
Eitzer and Hites Ž1989a., Koester and Hites
Ž1992b. and Kaupp Ž1996.. They all found PCDFs
to decrease in concentration with increasing level
of chlorination and PCDDs to increase in concentration with increasing level of chlorination. The
pattern in precipitation was therefore a broad
reflection of the typical air patterns discussed in
an earlier section. Scavenging ratios for most
homologue groups were in the order of 1]4 = 10 4 ,
although HpCDD and OCDD were higher, at
6]15 = 10 4 Žsee Table 8.. Particle bound
PCDDrFs decreased in concentration with in-
73
creasing rain intensity, but increased with decreasing temperature. Relative contributions of
particle scavenging were highest for the highest
chlorinated congeners. Total scavenging ratios for
PCDDrFs in fog increase with the level of chlorination ŽCzuczwa et al., 1989. Žsee Table 8..
In summary, scavenging ratios for PCDDrFs in
fog and rain show a trend similar trends to those
reported for PAHs, with fog accumulating higher
molecular weight congeners more efficiently than
rain.
8.6. Dry particulate deposition
For airborne particles, the variation of deposition velocities with diameter can be as great as
two orders of magnitude. Particles larger than 1
m m have too much momentum to follow the
deflected air and forcefully impact on surfaces
towards which they are heading. Much smaller
particles are light enough to behave rather like
gases, diffusing towards surfaces at high speed.
Near the deposition velocity minimum, particles
have diameters of a few tenths of a micrometer,
so both processes are inefficient and particle
lifetimes are long ŽGraedel and Crutzen, 1992..
Koester and Hites Ž1992b. related deposition flux
to temperature. The dry deposition flux was re-
Table 8
Rain and fog overall PCDDrF scavenging ratios
Rain scavenginga
TCDF
PCDF
HxCDF
HpCDF
OCDF
TCDD
PCDD
HxCDD
HpCDD
OCDD
a
Fog scavengingc
U
Air conc.
Žfgrm3 .
Rain conc.
Žpgrl.
St o t
Ž=104 .
%Pb
St o t
Ž=104 .
300
220
130
75
27
1.2
43
160
380
590
5.7
2.8
1.3
2.4
0.6
0.3
0.4
1.6
24
54
1.9
1.3
1.0
3.2
2.2
]
0.9
1.0
6.4
9.1
21
51
77
87
52
]
50
88
92
80
10
12
16
57
113
]
]
]
16
96
Eitzer and Hites Ž1989b..
Percentage of particle scavenging.
c
Fog average concentrations from Czuczwa et al. Ž1989. October 1986 in Dubendorf,
Switzerland. Average air concentrations from
¨
Buck and Kirschmer Ž1986., October 1985 in Koln,
¨ Bochum and Essen, Germany.
b
74
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
ported to increase with decreasing temperature,
as particle deposition dominated the dry deposition process and more PCDDrFs were adsorbed
to particles at cooler temperatures.
In the study by Schroder
et al. Ž1997. on the
¨
relative importance of wet, dry particle and
gaseous deposition of PCDDrFs to a bare soil,
dry particle-bound deposition accounted for 15%
of the total flux of all congeners, more than
two-thirds of which was due to the deposition of
large particles. Dry gaseous deposition was only
of importance for the lower chlorinated homologues, contributing a maximum of 33% for the
TCDFs. As discussed in an earlier section, wet
deposition dominated.
8.7. Air and deposition pattern
It is instructive to compare the pattern of
PCDDrFs in ambient air and deposition sampled
concurrently at the same site because this yields
information on the relative transfer efficiencies of
the compounds. Data from three sampling sites in
Germany reported by Wallenhorst Ž1996. show a
striking similarity between air and deposition Žsee
Fig. 5.; only OCDD made a different contribution
to the SPCDDrF loading, increasing from ;
25% in air to G 40% in deposition. The deposition samples were taken with a Bergerhoff like
sampling device. Jones and Duarte-Davidson
Ž1997. also found a close match to the mixture of
homologues and 2,3,7,8-substituted congeners in
air and bulk deposition sampled in an urban area
with a ‘upturned frisbee’ and grass in a 1-year-long
study. In summary, then, PCDDrFs of different
chlorination levels seem to transfer to collectors
with similar efficiencies, resulting in a consistency
of pattern in air and deposition.
9. Long-range transport of PCDDr
r Fs
9.1. Introduction
Long-range transport is understood as the
movement of compounds to remote regions of the
earth not having local inputs. Oehme Ž1991a.
noted that atmospheric transport is a faster and
more efficient carrier for organochlorines to the
Arctic than the oceans. The higher troposphere is
a particularly efficient long-range transport
medium, typically moving ; 480 kmrday for substances in the gas phase. The adsorbed portion
will follow the transport routes of the aerosols
which, after aggregation, may be deposited by dry
or, more importantly, by wet deposition, as discussed previously. These are generally processes
with a time scale of the order of 2]10 days
ŽBallschmiter, 1991.. Wania and Mackay Ž1996.
Fig. 5. A comparison of the PCDDrF pattern in ambient air and deposition sampled concurrently at German sites by Wallenhorst
Ž1996..
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
classified persistent organic pollutants ŽPOPs. into
four categories based on their K o a , pL8 and temperature of condensation and concluded that only
compounds with certain properties will be susceptible to long-range transport. Obviously the ‘inventory’ of a given compound emitted to the
environment will also be subject to retention and
degradation in source areas and during transport.
In their classification Wania and Mackay Ž1996.
rated the mono- to tetra-CDDrFs as subject to
transport with preferential deposition and accumulation in the mid-latitudes or polar regions Ži.e.
subject to transport to remote areas over times,
following re-emissionrre-cycling of deposited material from the surface .. In contrast, the tetra- to
octa-CDDrFs were considered to be prone to
rapid deposition and retention close to source
regions. This simple classification is complicated,
of course, by the temperature controlled partitioning of the PCDDrFs between gas]particle
phases in air, between the gas phase and ‘condensed’ phases Ži.e. soils, water bodies, vegetation., reaction rates and deposition processes.
In this section we therefore briefly examine the
sparse data on PCDDrFs in remote, polar regions for evidence to support this classification of
their long-range transport potential.
10. Concentrations in polar regions
Long-range atmospheric transport to the Arctic
is only occasionally observed during the summer
season ŽJune]July., whereas the winter weather
situation ŽFebruary]March. allows a periodic
75
transfer of polluted air from the Eurasian continent and North America into the Arctic. Thus
higher concentrations of mobile POPs Že.g. HCB
and a-HCH. have been detected in March 1984
ŽOehme, 1991b; Stern et al., 1997..
Results from the Antarctic show that the background Antarctic air is still free of PCDDrF
compounds at a current detection limit in the
sub-pgrm3 range ŽLugar et al., 1996.. It is assumed that the Antarctic benefits from fewer
industrial sources in the southern hemisphere
ŽOehme et al., 1994.. There are only two ambient
air measurements of PCDDrFs from the Arctic
so far. Schlabach et al. Ž1996. sampled 10 000 m3
˚
air in spring and summer 1995 at Ny-Alesund,
Spitsbergen. PCDDrF concentrations were
between 16 and 28 fgrm3 ; the pattern showed
similar concentrations for the tetra- to octaCDDs, approx. 3 fgrm3. The tetra- to hexa-CDFs
had concentrations in the order of 10]30 fgrm3,
whereas hepta- and octa-CDFs were lower. Recovery problems made the interpretation of the
higher chlorinated congener data difficult. However, if it is assumed that the Spitsbergen air is
developed from a weathered typical profile, it is
interesting to make comparisons to the mixture of
PCDDrFs found in more temperate, industrialised regions Žsee Fig. 1.. This comparison is
made in Fig. 6 and shows that the Arctic pattern
has shifted towards the lower chlorinated PCDDs
while the PCDFs show a similar relative abundance. The Spitsbergen sample will have been
subject to several of the weathering processes
discussed earlier, namely dry and wet deposition,
U
˚
Fig. 6. PCDDrF ambient air concentrations from Ny-Alesund
compared to a general ambient air profile. Data from Schlabach et
UU
al. Ž1996.; see Fig. 1.
76
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
photolysis and chemical reactions. Wet deposition
would tend to selectively remove the heavier congeners Žsee earlier.. The greater abundance of
PCDFs relative to PCDDs points to the reported
shorter tropospheric lifetimes observed in laboratory experiments for PCDD due to OH-radical
reactions being influential.
Interestingly, if air measurements of PCBs and
PCDDrFs at the Lancaster University ŽUK. field
station site are compared to those by Oehme et
al. Ž1995a,b, 1996. at Spitsbergen, there is evidence for ‘fractionation’ or weathering of the
Arctic pattern. As Table 9 shows, the tetra- to
hexa-chlorinated PCDDrFs and lighter Žtri- to
hexa-. PCBs are lower in the Arctic by ; 10, with
the higher chlorinated PCBs and PCDDrFs being of the order of 50- to 100-fold lower in concentration at Spitsbergen Žsee Table 9.. This
observation supports a selective deposition of the
heavier compounds as air masses are subject to
long-range transport northwards.
11. Concluding remarks and outlook
State-of-the-art HRGCrHRMS is necessary to
start to undertake in-depth studies of the fate and
behaviour of PCDDrFs in the atmosphere and
their partitioning and degradation in the natural
environment. Good QArQC procedures must be
followed to make data more accurate, reliable
and comparable.
Air is the main distribution pathway for
PCDDrFs, its PCDDrF composition is therefore
influenced by diff erent em issions and
congenerrhomologue-selective transport and atmospheric loss processes. Several areas of future
research clearly need to be addressed before the
environmental fate and behaviour of these com-
Table 9
PCB and PCDDrF concentrations at Hazelrigg, UK, and at polar regions
Compound air
PCBs Žpgrm3 .
Tri-PCB 28
Tetra-PCB 52
Penta-PCB 101
Penta-PCB 110
Hexa-PCB 138
Hexa-PCB 153
Hepta-PCB 180
PCDDrFs Žfgrm3 .
2378-TCDF
12378-PeCDD
12378-PeCDF
23478-PeCDF
123678-HxCDD
123678-HxCDF
1234678-HpCDD
OCDD
OCDF
Hazelrigg
ŽUK.a
44
19
18
11d
10
18
8
7
5
10
14
8
19
150
400
220
Antarcticb
n.a.
n.a.
2.5 Ž22.
2.2 Ž17.
2.1 Ž46.
2.3 Ž41.
n.a.
]
]
]
]
]
]
]
]
]
Relative
to UK air
f 1r10
f 1r10
f 1r10
f 1r10
c
˚
Ny-Alesund
Relative
to UK
4.3e ]2.4
2.5]2.3
1.3]1.9
n.a.
0.54]1.8
0.61]1.0
0.16]0.23
f 1r10
f 1r10
f 1r10
f 1r10
f 1r10
f 1r20
f 1r50
0.51]0.62
0.2]0.51
1.3]1.6
0.66]1.6
0.26]1.3
1.4]2.4
1.6
4.4
3.8
f 1r10
f 1r10
f 1r10
f 1r10
f 1r10
f 1r10
f 1r100
f 1r100
f 1r100
Note. n.a., not analysed.
a
Average values from 1994 to 1995.
b
Values from 1988 to 1990 ŽLarsson et al., 1992.; n.d. taken as half the detection limit; values in brackets with sample from 16
December 1988.
c
Values from 1993 ŽOehme et al., 1996.; the second values were measured in spring 1992 ŽOehme et al., 1995a..
d
Coelution with PCB 77.
e
Coelution with PCB 16.
R. Lohmann, K.C. Jones r The Science of the Total En¨ ironment 219 (1998) 53]81
plex and challenging compounds is fully understood. These include research to understand and
quantify the key loss processes and deposition;
understand the relationship between sources and
ambient air levelsrmixtures; understand and
model gas]particle partitioning and atmospheric
deposition processes; the extent to which
PCDDrFs undergo long-rangerglobal atmospheric transport and fractionation.
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