Fis2002c

Fis2002c
Chemosphere 48 (2002) 981–992
www.elsevier.com/locate/chemosphere
Levels and pattern of volatile organic nitrates and
halocarbons in the air at Neumayer Station (70S), Antarctic
Ralf Fischer
a
a,1
, Rolf Weller b, Hans-Werner Jacobi
Karlheinz Ballschmiter a,*
b,2
,
Department of Analytical and Environmental Chemistry, University of Ulm, Albert-Einstein-Allee 11, D-89081 Ulm, Germany
b
Alfred-Wegener-Institute for Polar and Marine Research, Am Handelshafen 12 D-27570 Bremerhaven, Germany
Received 13 February 2001; received in revised form 14 February 2002; accepted 28 February 2002
Abstract
Levels and patterns of C1–C4/C9 organic nitrates were measured for the first time in Antarctica. The sampling was
done by adsorptive enrichment on Tenax TA followed by thermodesorption cold-trap high resolution capillary gas
chromatography with electron capture detection. 2–70 l air on-column have been analyzed this way. C1–C9 alkyl
mononitrates, C2–C4 alkyl dinitrates, C2–C4 hydroxy alkyl nitrates, and halocarbons could be identified in air samples
collected near the German Neumayer Research Station, Antarctica, in February 1999. Volatile biogenic and anthropogenic
halocarbons were used to assess the origin of the air parcels analyzed. The average concentration measured
P
for C2–C6 alkyl nitrates was in the range of 9:2 1:8 ppt(v), while the sum of the mixing ratios of six C2–C4 hydroxy
alkyl nitrates was in the range of 1:1 0:2 ppt(v). Moreover, C2–C4 alkyl dinitrates were found at levels near the
detection limit of 0.1–0.5 ppt(v). The concentrations of organic nitrates found in Antarctic air represent ultimate
baseline levels due to chemical and physical loss processes during long-range transport in the air. The South Atlantic
and the Antarctic Ocean as a general secondary source for organic nitrates in terms of an air/sea exchange equilibrium
has to be evaluated yet, but it seems logical. Our results confirm the common assumption that there are no biogenic
marine sources of C2–C9 organonitrates. We have found a level of >80 ppt(v) for methyl nitrate. This level if it can be
confirmed in a systematic survey requires a strong biogenic source of methyl nitrate in the Antarctic Ocean.
2002 Elsevier Science Ltd. All rights reserved.
Keywords: Alkyl mononitrates; Methyl nitrate; Hydroxy alkyl nitrates; Alkyl dinitrates; Air; Antarctica; Thermal desorption; Capillary
gas chromatography; Baseline levels; Long-range transport
1. Introduction
*
Corresponding author. Tel.: +49-731-502-2750; fax: +49731-502-2763.
E-mail address: karlheinz.ballschmiter@chemie.uni-ulm.de
(K. Ballschmiter).
1
Present address: Dr. Th. B€
ohme KG, Isardamm 79-83, D82538 Geretsried.
2
Present address: Department of Hydrology and Water
Research, 1133 E. North Campus Dr., University of Arizona,
Tucson, AZ 85721-0011, USA.
The troposphere has to be considered in general as an
oxidative medium with the tendency for species to be
converted to a more oxidized state. The results of the
light induced photochemistry and oxidation chemistry
of hydrocarbons in air depend on the levels of the reaction of starting radicals such as OH, or NO3 , and on
the levels of trace gases like odd-nitrogen NOy , (particularly NOx (NO þ NO2 ) as main part), ozone, and organosulfur compounds (Atlas et al., 1992a,b; Carroll
et al., 1992; Barrie et al., 1994; Kondo et al., 1997; Platt
0045-6535/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 5 - 6 5 3 5 ( 0 2 ) 0 0 1 1 0 - 8
982
R. Fischer et al. / Chemosphere 48 (2002) 981–992
and Le Bras, 1997). As a result of combustion emissions,
levels of NOx greatly enhanced over those in the background troposphere can be found in urban and densely
inhabited continental areas. NOx is the key in the organonitrogen chemistry of the urban troposphere. When
NO and NO2 are present in sunlight, ozone formation
occurs as a result of photolysis of NO2 at wavelength
k < 424 nm.
• Formation of alkyl radicals (initial step):
RH þ OH ! R þ H2 O
RH þ
NO3
ð5aÞ
! R þ HNO3
ð5bÞ
R0 RHC–O þ D ! R þ R0 HC@O
0
0
R ðC@OÞR þ hm ! R þ R C@O
ð5cÞ
ð5dÞ
• Formation of peroxyalkyl radicals:
NO2 þ hm ! NO þ Oð3 PÞ
ð1Þ
Oð3 PÞ þ O2 þ M ! O3 þ Mz
ð2Þ
On the other hand, ozone reacts with NO to regenerate
NO2 .
O3 þ NO ! NO2 þ O2
ð3Þ
Therefore the steady-state ozone concentration is approximated by Eq. (4)
½O3 ¼
J1 ½NO2 k3 ½NO
ð4Þ
Expression (4) is named the photostationary state relation. The ozone concentration is in a first approximation
proportional to the NO2 =NO ratio, which means high
NO2 levels lead to high ozone concentrations. Conversion of NO to NO2 by HO2 or peroxyalkyl radicals (7a)
will lead to increases in ozone and deviation from Eq.
(4).
Members of the NOy pool are also involved in these
processes by increasing or reducing NOx levels in the air.
Alkyl nitrates as parts of the NOy pool are therefore of
potential interest.
Alkyl nitrates besides being compounds of the atmospheric chemistry are used as propellants, and explosives (K€
ohler and Meyer, 1995). Together with the
alkyl nitrites they are a group of potent pharmaceuticals,
e.g. isosorbid dinitrate is a powerful vasodilator (Ahlner
et al., 1991). None of the technical and pharmaceutical
uses leads however to a general environmental contamination. Local contaminations by explosives may occur.
The source of alkyl nitrates in the troposphere is the
conversion of hydrocarbons (Fraser et al., 1997). OH/O2
(5a) or NO3 /O2 (5b) attack on an aliphatic carbon–
hydrogen or an olefinic C@C bond forms peroxyalkyl
radicals RO2 (6). They are also formed by a thermolysis/
O2 reaction (5c) of long chain alkoxy radicals and by the
photolysis (5d) of carbonyl compounds followed by the
addition of O2 . The reaction of peroxyalkyl radicals with
NO leads to nitrogen dioxide and alkoxy radicals––
finally stabilizing as carbonyl compounds (7a) and
(7b)––or in a side reaction to alkyl nitrates (reactions
(7c)) (Atkinson et al., 1982; Finlayson-Pitts and Pitts,
1986; Atkinson, 1990; Seinfeld and Pandis, 1998).
R þ O2 ðþ MÞ ! RO2 ðþ Mz Þ
ð6Þ
• Formation of stable products:
RO2 þ NO ! RO þ NO2
RO þ O2 ! R1 R2 CO þ HO2
ð7aÞ
ð7bÞ
RO2 þ NOðþ MÞ ! ðRO2 NOÞðþ Mz Þ ! RONO2
ð7cÞ
This reaction scheme may repeat itself with alkyl
nitrates leading finally to carbonyl alkyl nitrates or to
non-vicinal alkyl dinitrates.
The reaction of alkenes with either OH during daytime or with NO3 during nighttime in the presence of NOx
leads to the formation of multifunctional organic nitrates
like hydroxy alkyl nitrates or alkyl dinitrates (O’Brien
et al., 1995; O’Brien et al., 1997; Kastler and Ballschmiter, 1998; Kastler and Ballschmiter, 1999). Moreover,
organic nitrates formed by the reaction of isoprene have
recently been reported (Werner et al., 1999). A detailed
summary of the reaction schemes leading to alkyl nitrates is given by Roberts (Roberts, 1990).
The yield of alkyl nitrates in the branching of the
reaction of a peroxyalkyl radical with NO (7c) increases
from <0.014 for ethane to 0.33 for octane. Thus a wide
range of homologues and isomeric alkyl nitrates is expected in the atmosphere. The decreasing concentrations
of long-chain alkanes are partly offset by the increased
yields of alkyl nitrate formation (Atkinson et al., 1982;
Schneider et al., 1998a).
While for alkyl mononitrates 6 C5 the dominant loss
process is photolysis (8) (Clemitshaw et al., 1997), alkyl
nitrates with more than five C-atoms and multifunctional alkyl nitrates are mainly degraded by OH radicals
(9) (Talukdar et al., 1997).
• Atmospheric chemistry of alkyl nitrates
RONO2 þ hm ! RO þ NO2
ð8Þ
RONO2 þ OH
! multifunctional nitrates; polar products
ð9Þ
In continental air alkyl nitrates contribute 2% to the
NOy budget, increasing up to 15% for marine air (Atlas
et al., 1992a,b). They are a sink of the NOy pool par-
R. Fischer et al. / Chemosphere 48 (2002) 981–992
ticularly during polar winters. Thus, in arctic air masses
the alkyl nitrates can enrich up to 20% of the total NOy
budget (Bottenheim et al., 1993). Photodegradation of
alkyl nitrates results in the formation of NOx . They can
act as an important NOx source in the polar regions with
highest release rates of NOx during polar sunrise.
Since the first measurements of alkyl nitrates in the
marine atmosphere 1988 by Atlas (Atlas, 1988), this
class of compounds found increasing interest. Several
studies took place in the USA, Canada and Germany.
Moreover field campaigns took place in the Pacific air
(equatorial and Hawaiian region) and the Atlantic air on
board RV ‘‘Meteor’’ (Schneider and Ballschmiter, 1999)
and RV ‘‘Polarstern’’ (Fischer et al., 2000). In spite of
increasing activities in this field of atmospheric chemistry the number of studies in polar regions is limited.
Concentrations in the low ppt(v) range were found
for the North Pacific air by Atlas (Atlas et al., 1992a,b;
Atlas et al., 1993). P
Roberts measured concentrations of
14 8:3 ppt(v) for C1–C4 alkyl nitrates at Chebogue
Point, Nova Scotia (Roberts et al., 1998). de Kock
found mean concentrations of 17:5 8:4 ppt(v) for C3–
C5 alkyl nitrates at the South African southeast coast
(de Kock and Anderson, 1994). More
P recently Schneider
reported levels of 3–8 ppt(v) for C3–C5 alkyl nitrates
for the South Atlantic air (Schneider, 1998a,b; Schneider
and Ballschmiter, 1999). Fischer
reported a mean conP
centration of 1.7 ppt(v) for C4 þ C5 alkyl
P nitrates for
the North Atlantic air and 1.3 ppt(v) for C4 þ C5 for
the South Atlantic air (Fischer, 1999; Fischer et al.,
2000). Fischer gives a global overview on the levels of
short chain alkyl nitrates at different continental and
marine sampling sites (Fischer and Ballschmiter, 2001).
O’Brien reported 1995 for the first time the occurrence of four alkyl hydroxy nitrates and one alkyl dinitrate (O’Brien et al., 1995; O’Brien et al., 1997).
Recently additional hydroxy alkyl nitrates and 30 alkyl
dinitrates were identified in urban air (Kastler and
Ballschmiter, 1998; Kastler and Ballschmiter, 1999; Fischer et al., 2000).
We believe to be the first to present in this work levels
and patterns of higher organic nitrates in the lower
troposphere of the Antarctic. Furthermore we compare
the occurrence of alkyl nitrates with the levels of halocarbons as atmospheric markers. A comparison with
values obtained in South Atlantic air places the Antarctic data in a greater spatial context.
2. Short hand nomenclature of organic nitrates
Schneider and Ballschmiter recently introduced a
shorthand nomenclature of alkyl mononitrates that
correlates the structure of a specific alkyl nitrate to the
hydrocarbon precursor (Schneider and Ballschmiter,
983
1996). In our extended shorthand nomenclature the
expression organic nitrates represents the whole family
of mono- and multifunctional alkyl nitrates (Table
1) keeping the basic settings given by Schneider and
Ballschmiter (Fischer et al., 2000).
The longest unbranched alkyl chain is taken as the
skeleton of the molecule; e.g. C7 means in this case that
the longest unbranched carbon chain has seven Catoms. Furthermore we define for unbranched alkyl
nitrates that the nitrooxy group possesses the highest
priority and is numbered first.
For branched alkyl nitrates the alkyl side chains
possess now the highest priority above all other groups
in the molecule, and therefore the positions of alkyl side
chains are numbered first. M is used for methyl, and E is
used for ethyl side chains, respectively.
This convention is particularly important and helpful if isomeric hydrocarbon skeletons have to be distinguished. For a better understanding we explain the
abbreviations introducing some examples for alkyl mononitrates, alkyl dinitrates, hydroxy alkyl nitrates, keto
alkyl nitrates, and alicyclic alkyl nitrates.
Alkyl mononitrates: 2,4M5C7 indicates a heptyl chain
with two methyl groups at the 2 and 4 position and the
nitrooxy group at the 5 position, specifically a 2,4-dimethyl-5-nitrooxyheptane.
Alkyl dinitrates: 2,3M1,4C5 would be a alkyl dinitrate with a carbon skeleton of five atoms, with two
methyl groups at the 2 and 3 position, and two nitrooxy
groups at position 1 and 4, specifically a 2,3-dimethyl1,4-dinitrooxy pentane.
Hydroxy alkyl nitrates: OH is used for the hydroxy
group. 3OH1C4 would be 3-hydroxy-1-nitrooxy butane.
Keto alkyl nitrate: The letter O labels the carbonyl
group, e.g. 2O1C5 is a 2-keto-1-nitrooxy pentane.
Alicyclic alkyl nitrates: The letters c and t are used for
cis and trans positions, respectively. Cy indicates an
alicyclic skeleton. c1,2CyC6 indicates a cis-1,2-dinitrooxy cyclohexane.
Aryl alkyl nitrates: Aryl alkyl nitrates are named
considering the phenyl- or naphthyl group as a substituent of the alkyl chain (Woidich et al., 1999).
3. Position of the Neumayer Research Station in the
Antarctic
The Neumayer Station (70390 S, 8150 W) was established in 1992 on the Ekstr€
om Shelf Ice as a research
observatory for geophysical, meteorological and air
chemistry measurements. Fig. 1 shows the geographical
position of the research station. The snow-covered station is located on shelf ice that is 200 m thick and almost
flat. The shelf ice margin where supply ships (e.g. RV
‘‘Polarstern’’) dock is 10 km away. The isolated location
984
R. Fischer et al. / Chemosphere 48 (2002) 981–992
Table 1
IUPAC and short style nomenclature of the alkyl mononitrates and short-chain alkyl hydroxy- and dinitrates identified in the Antarctic air samples
No.
IUPAC
Abbreviation
Cn
No.
Alkyl mononitrates
1
2
3
4
5
6
7
8
IUPAC
Abbreviation
Cn
Alkyl dinitrates
Nitrooxy-methane
Nitrooxy-ethane
1-Nitrooxy-propane
2-Nitrooxy-propane
1-Nitrooxy-butane
2-Nitrooxy-butane
2-Methyl-1-nitrooxy-butane
1-Nitrooxy-pentane
2-Nitrooxy-pentane
C1
C2
1C3
2C3
1C4
2C4
2M1C3
1C5
2C5
3-Nitrooxy-pentane
2-Methyl-3-nitrooxy-butane
1-Nitrooxy-hexane
2-Nitrooxy-hexane
3-Nitrooxy-hexane
1-Nitrooxy-heptane
2-Nitrooxy-heptane
3-Nitrooxy-heptane
4-Nitrooxy-heptane
1-Nitrooxy-octane
3C5
2M3C4
1C6
2C6
3C6
1C7
2C7
3C7
4C7
1C8
2
3
4
5
6
3
4
5
6
1,2-Dinitrooxy-ethane
1,2-Dinitrooxy-propane
1,3-Dinitrooxy-propane
1,3-Dinitrooxy-butane
2,3-Dinitrooxy-butane
2-Methyl-1,2-dinitrooxy-propane
2,3-Dinitrooxy-pentane
2-Methyl-3,4-dinitrooxy-pentane
Hydroxy alkyl nitrates
1,2C2
1,2C3
1,3C3
1,3C4
2,3C4
2M1,2C3
2,3C5
2M3,4C5
1-Hydroxy-2-nitrooxy-propane
2-Hydroxy-1-nitrooxy-propane
3-Hydroxy-2-nitrooxy-butane
4-Hydroxy-2-nitrooxy-butane
2-Hydroxy-1-nitrooxy-pentane
1-Hydroxy-2-nitrooxy-pentane
2-Hydroxy-3-nitrooxy-hexane
4-Hydroxy-3-nitrooxy-hexane
2-Methyl-4-hydroxy-5-nitrooxy-pentane
2-Methyl-5-hydroxy-4-nitrooxy-pentane
1OH2C3
2OH1C3
3OH2C4
4OH2C4
2OH1C5
1OH2C5
2OH3C6
4OH3C6
2M4OH5C5
2M5OH4C5
4. Experimental
4.1. Air sampling procedure
Fig. 1. Geographical position of Neumayer Station; on the
Ekstr€
om Shelf ice in the Atka Bay, Northeast Weddell Sea
(703900 S, 81500 W).
of the station makes it a valuable reference site for
measurements under conditions of extreme air purity. In
addition, the proximity to the ice margin permits the
detection of substances released from the ocean to the
atmosphere. Table 2 summarizes the meteorological
parameters related to the Neumayer Station during the
sampling period.
The air sampling was done by pulling air through
glass tubes (160 mm length, 3 mm, i.d.) filled with 80–
100 mg of Tenax TA, 60–80 mesh (Chrompack, Middelburg, Netherlands) at a flow rate of 80–100 ml/min
with a sampling pump (SKC model 222-3, Eighty Four,
Pennsylvania, United States). Two sampling tubes connected in series have been used to control the sampling
efficiency. Volumes of 2–68 l air were sampled. The
sampling tubes were flame-sealed in larger glass tubes
for transport and were kept refrigerated until analysis.
To detect the whole range of alkyl nitrates parallel
sampling of low volumes of 2 l for C2 up to C5 alkyl
nitrates and high volumes of 50–70 l as well for alkyl
nitrates C > 5 is recommended. In polar regions the
breakthrough loss due to sampling temperatures is
negligible.
4.2. Analytical procedure: GC separation, detection and
quantitation
The high resolution gas chromatography with electron capture detection (HRGC/ECD) chromatographic
separation was performed on a Chrompack Model 9001
gas chromatograph equipped with a thermal desorption
cold trap (TCT) device (Chrompack, Middelburg,
R. Fischer et al. / Chemosphere 48 (2002) 981–992
985
Table 2
Meteorological parameters related to the Antarctic air samples
Sample
Date
Sample volume
(l)
Air temperature
(C)
Wind direction
Wind velocity
(m s1 )
Humidity (%)
NM
NM
NM
NM
13.02.1999
22.02.1999
22.02.1999
26.02.1999
2.0
30.2
68.0
10.8
7.5
5.5
10.0
6.3
130SE
75ENE
160ESE
90E
4
5
3
12
75
73
87
96
1
2
3
4
Netherlands). We used a DB-1701 capillary (58 m 0:312 mm i:d: 1:0 lm film thickness, J&W Scientific,
Folsom, United States). A detailed description of the
analytical conditions is given in (Fischer et al., 2000).
Quantitation was done by external calibration with solutions containing known amounts of alkyl nitrates and
halocarbons. The limit of detection based on a 30 l air
sample was about 0.3 ng/m3 or 0.05 ppt(v) for alkyl
nitrates and 0.2 ng/m3 or 0.02 ppt(v) for the three halocarbons trichloroethane, tetrachloroethene and bromoform. The overall error for the alkyl nitrates and
halocarbons was estimated to be less than 20% (Fischer, 1999).
5. Results
The measured concentrations of organic nitrates and
halocarbons of this work are summarized in Table 3.
The results presented in this work are unique for organic
nitrates. Table 4 lists all literature data and compares
them with the results of this work. Highest in abundance
are the short chain alkyl mononitrates C2, 1C3, 2C3 and
2C4. This observation is consistent with the literature
data for Arctic regions (Table 4). The alkyl nitrate levels
for the Arctic regions are one order of magnitude higher
than the values found in the Antarctic, indicating that
the sampling sites in the North are closer to the highly
industrialized regions of the North American continent.
In terms of the vicinity to non-point sources the Arctic
and Antarctic regions have to be considered differently;
the effects of a limited photodegradation in the respective polar nights will be similar.
Fig. 2a and b depict TCT–HRGC–ECD chromatograms of air samples collected at Neumayer Station,
Antarctica. To our knowledge these are the first chromatograms showing alkyl nitrates in the air of a South
Polar region.
The biogenic marker dibromomethane and tribromomethane (bromoform) show relative to alkyl nitrates
higher mixing ratios indicating marine emissions sources
(the sampling location is only 10 km away from the
coastal line). Anthropogenic markers like carbontetrachloride and 1,1,1-trichloroethane with long kOH
Table 3
Mixing ratios for the alkyl mononitrates, hydroxy alkyl nitrates, alkyl dinitrates, and bromo- and chloro-halocarbons in the Antarctic
air in ppt(v) (n ¼ 2–4)
Mononitrates
Mean (ppt(v))
Hydroxy nitrates
Mean (ppt(v))
Halocarbons
Mean (ppt(v))
C1(n ¼ 1)
C2
1C3
2C3
1C4
84
4.6
1.1
0.7
0.03
2OH1C4
2OH1C3
RR-3OH2C4a
1OH2C3/RS-3OH2C4a
<LOD
0.03
0.3
0.8
Tetrachloromethane
1,1,1-Trichloroethane
Trichlorethene
Tetrachlorethene
Hexachloroethane
95
75
<LOD
0.3
0.06
2C4/2M1C3
1C5
2C5
3C5
0.5
0.7
<LOD
0.03
1OH2C4
3OH1C4
P
OH
0.02
0.01
1.1
Dibromomethane
Bromodichloromethane
Dibromochloromethane
Bromoform
1,2-Dibromoethane
0.8
0.05
0.02
0.3
0.1
Dinitrates
1C6
2C6
3C6
2C7
4C7
0.5
1.0
0.08
0.18
0.02
1,2C3/1,2C2
2M1,2C3
0.5
0.1
P
C4 þ C5
P
C3–C6
1.3
4.6
Limit of detection ðLODÞ ¼ 0:01 ppt(v).
a
RR/SS-3OH2C4; RS/SR-3OH2C4 co-elution with 1OH2C3.
986
R. Fischer et al. / Chemosphere 48 (2002) 981–992
Table 4
Comparison of mixing ratios of alkyl nitrates in the Antarctic air with Arctic and Alaska reference data in ppt(v)
Antarctic (this work)
b
1C3
2C3
1C4
2C4/2M1C3
2M3C4
1C5
2C5
3C5
2C6
3C6
2C7
3C7
P
C2–C6
P
C3–C7
1.11
0.49
0.03
0.48
<LOD
0.71
<LOD
0.03
0.08
0.95
0.18
(0.01)
9.2
–
Arctic, Canadaa
Alert, Canada
b
Polar night
Polar day
3.33
12.44
1.7
18.41
4.84
1.01
5.44
4.31
2.46
4.27
1.45
1.86
–
–
3.14
13.08
1.18
13.73
2.32
0.53
2.47
2.31
0.98
1.65
0.56
0.68
–
–
3.96
12.5
2.05
13.98
2.65
1.03
4.18
3.02
1.50
2.55
–
–
34c
144d
Limit of detection ðLODÞ ¼ 0:01 ppt(v).
a
Canadian Arctic (69N–83N) ‘‘Polar Sunrise Experiment’’ April 1992 (Leaitch et al., 1994).
b
Alert, Northwest Territories of Canada, ‘‘Polar Sunrise Experiment’’ January–April 1992, Polar night January–March; day period
March–April (Muthuramu et al., 1994).
c
Poker Flat Research Rang, Alaska (64N/147W) winter/spring 1993 (Beine et al., 1996).
d
Canadian Arctic March–April 1988 (Bottenheim et al., 1993).
lifetimes (s ¼ month to years) were also found, but
markers with short kOH lifetime like trichloroethene
(s ¼ 7 days) are below the limit of detection (<0.01
ppt(v)).
The patterns of the Antarctic air samples are quite
similar to the patterns found in the South Atlantic Trade
Wind region. Fig. 2c depicts for comparison a chromatogram of this marine region, taken during the ALBATROSS campaign on board of RV ‘‘Polarstern’’
(ANT XIV/1) at 10.4S/25.5W (Fischer et al., 2000).
The Antarctic and the Southern Trades apparently
represent baseline levels of clean marine air in terms of
organic nitrates.
a mean value of 0.3 ppt(v). During the Albatross–
Campaign we found similar mixing ratios in the South
Atlantic Ocean and levels by a factor 3 higher for the
North Atlantic (Fischer et al., 2000). The good correlation of the measured halocarbon ratios with literature
data backs the accuracy of our analytical procedure.
6. Discussion
To interpret the data of the South Polar region in a
wider spatial context we enclose the data of the Albatross–Campaign, which also covered the South Atlantic
(Fischer et al., 2000).
5.1. Levels of anthropogenic halocarbons in Antarctic air
6.1. Levels of butyl nitrates in marine and Antarctic air
The mixing ratios for the long-lived halocarbons
(Table 3) are in good agreement with earlier measurements. We found 95 ppt(v) for tetrachloromethane
(s ¼ 42a) while Derwent measured 101 ppt(v) in Ireland
(Derwent et al., 1998). This indicates a more or less even
global distribution in the troposphere. For 1,1,1-trichlorethane we observed 75 ppt(v) (s ¼ 4:8a); estimations
by Derwent for 1999 results in ratios of 70–80 ppt(v)
(Derwent et al., 1998). The situation seems to be similar
as observed for tetrachloromethane. The mean value for
hexachloroethane is in the range of 0.06 ppt(v) and
somewhat lower than 0.24 ppt(v) observed by Atlas in
1993 for the Pacific (Atlas et al., 1993). For medium
lived C
2 halocarbons like tetrachlorethene we observed
Fig. 3 is a plot of the mixing ratios of the 2C4, the
secondary, and the 1C4, the primary butyl nitrate along
the Atlantic Ocean (Albatross–Campaign) and at Neumayer Station, Antarctic. The main emission sources
are in the industrialized areas of the Northern hemisphere. The accumulation in the Northern hemisphere is
enhanced in fall and wintertime by a reduced photodegradation. The concentration for 2-butyl nitrate (2C4) in
the coastal region is 3.5–5 ppt(v). An average value of
1 0:2 ppt(v) is observed for 1-butyl nitrate (1C4) in the
air of the North Atlantic. A 2C4/1C4 ratio close to 4
indicates a recent continental input or marine local input
by islands.
R. Fischer et al. / Chemosphere 48 (2002) 981–992
987
Fig. 2. (a) TCT–HRGC(DB-1701)/ECD chromatogram of an Antarctic air sample, taken at Neumayer Station, sampling and oncolumn volume (a) 2.0 l (sample NM 1), (b) 68 l (sample NM 3). (c) TCT–HRGC(DB-1701)/ECD chromatogram of a South Atlantic
air sample of the trade wind region 10.4S/25.5W, sampling and on-column volume 105 l.
988
R. Fischer et al. / Chemosphere 48 (2002) 981–992
masses. It is the 2-butyl-, 2-pentyl nitrate/tetrachloroethene quotient (traffic/industry indicator) and the ratio
of isomeric pentyl nitrates (traffic indicator).
6.3. The 2-butyl-, 2-pentyl nitrate/tetrachlorethene ratio
as an air mass marker
Fig. 3. Distribution of butyl nitrates from 67N to 70S over
the Atlantic Ocean mainly across 30W longitude (Albatross–
Campaign on board RV Polarstern ANT XIV/1, 1996) and at
the Neumayer Station, 1999.
In the South Atlantic trade wind region the values for
2C4 decrease to 0:2 0:05 ppt(v) leading to a 2C4/1C4
ratio to about 1 and even below 1. The levels of 2C4 rise
again to 0:4 0:08 ppt(v) in the west wind belt of the
South Atlantic and remain at 0:4 0:1 ppt(v) for air
collected at the Neumayer Station. The levels of 1C4
decrease from 1:0 0:2 ppt(v) in the Northern hemisphere passing the intertropical convergence zone (ITCZ)
to the south to levels of 0:4 0:08 ppt(v). A further
decrease to levels down to the limit of detection of 0.05
ppt(v) is observed for the air at the Neumayer Station.
The 2C4/1C4 ratio covers a span between 0.2 and 7.0
reflecting in one part the dominance of 1C4 in ‘‘old’’ air
masses and also a very low level of possible local input in
the Antarctic. In the latter case only the dominant 2C4
would be detected.
6.2. Characterization of the age of air masses
An air mass is called ‘‘young’’ if pollution deriving
from densely inhabited regions with a corresponding
level of traffic and industrial emissions is observed.
Several attempts have been made to characterize air
mass ages. Atherton used a pure deductive kinetic model
(Atherton, 1989). Other models consider differences in
photochemical degradation velocities using pairs of
molecules or groups of molecules like carbon monoxide
and alkyl nitrates (Roberts et al., 1996), PAN and alkyl
nitrates (Buhr et al., 1990), hydrocarbons and alkyl nitrates (Roberts et al., 1998), NOy and NOx (Trainer
et al., 1991), two isomeric alkyl nitrates (Flocke et al.,
1998; Schneider and Ballschmiter, 1999) or consider the
presence of benzyl nitrate (Schneider et al., 1998a;
Schneider and Ballschmiter, 1999). Here we present two
different approaches to characterize the ‘‘age’’ of air
Here, we present a method based on the differences in
the degradation rates of short chain alkyl nitrates and
semi-stable halocarbons (e.g. tetrachloroethene). Short
chain alkyl nitrates are formed in all regions with traffic
that are releasing significant NO and short chain hydrocarbon emissions. Butyl- and pentyl nitrates are degraded by photolysis and OH on their long-range
transport finally reaching remote areas like the Antarctic. The anthropogenic halocarbons emerge basically
from industrialized continental regions and then undergo photochemistry on their long-range transport.
They are degraded mainly by OH radicals and only to a
minor extent by photolysis (Class and Ballschmiter,
1986).
The kOH lifetime of tetrachloroethene is in the range
of 4 month. Tetrachloroethene shows a three-step decrease in the global mixing ratios in the lower troposphere (Wiedmann et al., 1994). A first decrease is
observed when going from continental air to marine air
of the Northern hemisphere. This is a mixture of a decrease by degradation and by mixing of the source air
with less polluted air of the Northern Hadley cell of the
general circulation. The second decrease is observed in
the Northern tropics and a third one when passing the
ITCZ reaching the air of South East Trades of the
Southern hemisphere. This clearly indicates the separation of the air masses of both hemispheres in the lower
troposphere by the ITCZ. This has been observed for
other anthropogenic volatile chlorocarbons before (Class
and Ballschmiter, 1986).
The butyl nitrates do not give this clear difference in
atmospheric levels but decrease more or less continuously in going to remote regions. The increased levels
around the Azores indicate the importance of quasilocal inputs in the marine atmosphere due to traffic on
these islands. Fig. 5a depicts the ratios of the mixing
ratios of the secondary alkyl nitrates 2-butyl- and 2pentyl nitrate relative to tetrachloroethene (C2 Cl4 ) in the
air of the Atlantic Ocean and in the air of Neumayer
Station. Three clusters are formed. Marine air influenced
by air masses coming from the South American continent is characterized by a surplus of 2-butyl and 2-pentyl
nitrates relative to tetrachloroethene. Unpolluted marine air has low mixing ratios for 2-pentyl nitrate itself
and the ratio 2C5/C2 Cl4 is <1 indicating a strong degradation for 2-pentyl nitrate. The third cluster represents the global baseline level. It is characterized by the
fact that both the ratios for 2C4/C2 Cl4 and 2C5/C2 Cl4
R. Fischer et al. / Chemosphere 48 (2002) 981–992
are well below 1. This third cluster represents the
photochemically aged pollution depleted air masses.
6.4. The ratio of isomeric pentyl nitrates as an air mass
marker
A further method to decide about the origin or age of
an air parcel is to compare the levels of the isomeric
pentyl nitrates. In the case of pentyl nitrates significantly
increased mixing ratios for primary and secondary
pentyl nitrates are found with a surplus of secondary
pentyl nitrates (3C5 > 2C5 > 1C5). Very low concentrations of secondary pentyl nitrates, and in contrast
noticeably higher concentrations of 1-pentyl nitrates
characterize aged air masses (e.g. collected in remote
marine or Antarctic regions). The former sequence of
the levels in air 3C5 > 2C5 > 1C5 turns now to
1C5 3C5 > 2C5. The mixing ratio for 2C5 in the air
of the Antarctic Neumayer Station is below the limit of
detection (Table 3). The missing of 2C5 and barely detectable traces of 3C5 while 1C5 can be measured confirm that highly degraded air masses were collected.
6.5. Methyl nitrate in Antarctic air
The level found for methyl nitrate found in the
Antarctic is unexpectedly high and requires a specific
discussion. The branching ratio for the atmospheric reaction Eqs. (7a)–(7c) is normally set close to zero for C1.
Degradation of peroxyacetylnitrate (PAN) by the loss
of CO2 or the esterfication of methanol with nitric acid
on aerosols (Senum et al., 1986; Orlando et al., 1992;
Fan et al., 1994) can result in higher yields of metyl
nitrate in polluted continental air.
The reaction of methyl nitrate with OH is the major
loss process. The reaction rate constant was first measured by Gaffney et al. (Gaffney et al., 1986). It has recently be redetermined (Kakesu et al., 1997).
Methyl nitrate is very volatile and substantial
breakthrough losses will occur if not specific precautions
(low sample volume, high amount of adsorbent, low
temperature) are set to sample this very volatile alkyl
nitrate by adsorption techniques. A correct quantitation
of methyl nitrate is therefore difficult and requires optimized sampling conditions. We have found a level of
>80 ppt(v) for methyl nitrate in the low volume sample.
Co-elution with other compounds in the gas chromatogram of our sample can be excluded by identification of
the C1 peak by HRGC/MSD. Recently Jones measured
methyl nitrate from 27 to 46 ppt(v) and ethyl nitrate
from 5 to 13 ppt(v) sampled at Neumayer Station 1997
(Jones et al., 1999).
Walega et al. found for methyl nitrate 4 ppt(v) at
Mauna Loa, Hawaii (Walega et al., 1992) and Flocke
989
et al. measured 8 ppt(v) in the air over the North Atlantic (Flocke et al., 1998). Atlas supposed a marine
emission source for methyl nitrate (Atlas et al., 1997). In
the marine boundary layer (MBL) maximum concentrations were found as high as 50 ppt(v) near Christmas
Island (1–3N) and concentrations of 20–35 ppt(v) near
Western Samoa (13S) during ACE 1 (Blake et al.,
1999). The authors conclude that methyl nitrate has a
significant equatorial marine source. The most logic biogenic source would be the methylation of nitrate ions.
We may observe a similar reaction for the nitrate ion
leading to methyl nitrate as it is found for the methylation of chloride (Urhahn and Ballschmiter, 1998).
Methionine methyl sulfonium chloride (MMSL) e.g.
gave an intense release of monohalomethanes when
mixed in a buffer with potassium halides.
6.6. Levels of hydroxy alkyl nitrates in marine and
Antarctic air
Fig. 4 is a plot of the mixing ratios of hydroxy alkyl
nitrates along the Atlantic Ocean (Albatross–Campaign)
and at Neumayer Station, Antarctic. All of the observed
compounds are vicinal hydroxy alkyl nitrates. Hence,
their formation pathway via OH radical addition to
olefins is most likely. We observe a NH/SH gradient of
about factor 3 with 4:3 0:8 ppt(v) for the sum of 4
hydroxy alkyl nitrates in the air of the Northern hemisphere and 1:6 0:3 ppt(v) in the South Atlantic air.
The mixing ratio for hydroxy alkyl nitrates in the air of
the Neumayer Station is with 1:2 0:2 ppt(v) somewhat
lower than in the South Atlantic air. In accordance with
previous discussions the formation of secondary alkyl
radicals is preferred to the formation of primary ones
(Kastler, 1999). Therefore, if 1-alkenes are degraded by
OH radicals, the yield of 1-hydroxy-2-nitrooxy-alkanes
must be higher than the yield for 2-hydroxy-1-nitrooxy
alkanes. This is consistent with our observations.
Fig. 4. Distribution of hydroxy propyl- and butyl nitrates from
67N to 70S over the Atlantic Ocean and at Neumayer Station.
990
R. Fischer et al. / Chemosphere 48 (2002) 981–992
6.7. Principal compound analysis (PCA) of organic
nitrates in marine and Antarctic air
The principal compound analysis is a statistical
model to compare the pattern of air samples between
different regions. The theoretical feedback of the method
is given in (Smith, 1991). The n-dimensional expression
is reduced to a two dimensional expression. The distance
between two points is a measure of the similarity of air
samples. Fig. 5b depicts the PCA of all marine and
Antarctic air samples. 15 locations and the ratios of 14
alkyl mononitrates are the basis for the PCA given here.
A grouping of air masses is also obtained by plotting
two ratios of selected air constituents. This approach has
been used to characterize air masses by their complex
pattern of polychlorinated biphenyls (PCBs) (Schreitm€
uller et al., 1994). We have plotted here the ratios
of the mixing ratio of 2-butyl nitrate/tetrachloroethene
versus 2-pentyl nitrate/tetrachlorethene. The results are
given in Fig. 5a. Three clusters of air masses can be
differentiated. Cluster I contains all sampling locations
representing global baseline levels. Cluster II summarizes marine air samples with slightly polluted air, indi-
cating the influence of islands. The last cluster contains
polluted air coming in from the South American continent. The PCA as an independent method confirms the
characterization of marine and polar air samples as
discussed above.
7. Conclusion
In this paper we presented for the first time a data set
of higher organic nitrates in the air of the German South
Polar Neumayer Station. We are in a unique position to
discuss and compare the levels and patterns of alkyl
nitrates and halocarbons in the air of the Atlantic Ocean
and in the air of the South Polar region. The organonitrate concentrations in the marine and Antarctic
samples reached only 1–10% of the continental samples.
The mixing ratio in the South East Trade Wind region
and in the air of Neumayer Station are similar and
represents the global baseline level for the organonitrates. This is a strong indication for the long-range
transport of these molecules. No new alkyl nitrate formation is expected due to the lack of NOx sources in the
marine and the Antarctic atmosphere. Methyl nitrate
seems to be the exception from this rule. A biogenic
source for this compound appears to be likely as it has
been discussed before.
Halocarbons can be used as marker molecules to
distinguish between inhabited and biogenic sources. The
ratio of tetrachloroethene to secondary alkyl nitrates
can be used as a tool to decide about the origin and age
of an air parcel analyzed. The ratio of the isomeric
pentyl nitrates can also be used to characterize air
masses.
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