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Method for measurement of volatile oxygenated hydrocarbons in ambient air
Pergamon
Atmospheric Environment Vol. 31. No, 20, pp. 3329 3.339, 1997 !? 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain P I I : S1352--2310(97)00155--6 1352 2310/97 5;17.110+ (I,00
M E T H O D F O R M E A S U R E M E N T O F VOLATILE O X Y G E N A T E D H Y D R O C A R B O N S IN A M B I E N T AIR E. LEIBROCK* and J. SLEMR Fraunhofer Institute for Atmospheric Environmental Research, Kreuzeckbahnstrasse 19, D-82467 Garmisch-Partenkirchen, Germany (First received 17 September 1996 and in final form 26 March 1997, Published August 1997) A b s t r a c t - - A n automated gas chromatographic method for the quantitative determination of oxygenated (C2-C5 carbonyls and C1-C2 alcohols) and some non-oxygenated (C5-C8) hydrocarbons in ambient air has been developed. The analytical system consists of a gas chromatograph with a cryogenic sampling trap, a precolumn for the separation of water and other interfering compounds, a cryogenic focusing trap and two analytical columns connected in series. Substances are detected either by flame ionization or by a mass spectrometer. Ozone is removed by a potassium iodide scrubber placed upstream the sampling trap. External gas standards generated by a permeation device are used for calibration. The detection limits range between 0.03 and 0.08 ng (depending on the compound), equivalent to 5 to 56 ppt in 1 { of sampled air. The method was tested by an intercomparison with a different gas chromatographic technique for the determination of NMHC. The system has been applied since 1994 for measurements in ambient air. Data obtained during an intensive campaign in summer 1995 at the field station Wank (1778 m a.s.l.) near Garmisch-Partenkirchen, Germany, are reported and compared with NMHC mixing ratios measured simultaneously in the same air masses. © 1997 Elsevier Science Ltd.
Key word index: Carbonyls, alcohols, tropospheric ozone, two-dimensional gas chromatography, mass spectrometry, GC/MS, field measurements.
I.
INTRODUCTION
Atmospheric carbonyl compounds are both, directly emitted by anthropogenic and biogenic sources and photochemically produced from hydrocarbons (e.g. Atkinson, 1990; Altshuller, 1991a, b). Their photolysis is considered to be a major source of free radicals in moderately and highly polluted atmospheres and can even become the predominant radical source in the polluted planetary boundary layer during winter and early spring (e.g. Kleinman, 1991). Carbonyls react rapidly with OH radicals (Atkinson, 1990) and serve also as precursors of peroxyacyl nitrates and peroxides (e.g. Finlayson-Pitts and Pitts, 1986). Despite their high reactivity, carbonyls can comprise a significant fraction of the total volatile organic compounds in rural air (Schubert et al., 1988; Fehsenfeld et al., 1992; Goldan et al., 1995). Therefore, they play a crucial role in the formation of photooxidants such as ozone (National Research Council, 1991) and their speciation and quantification is necessary in order to understand the production of photooxidants. Measurements of carbonyl compounds and other oxygenated hydrocarbons are difficult due to their
* Author to whom correspondence should be addressed.
high reactivity and polarity. Some techniques reduce the reactivity using derivatization e.g. to oximes (Levine et al., 1981; Le Lacheur et al., 1993), to 2- d i p he n y l a c e t yl- 1,3-in d a n i n on- 1 - h y d r a z o n e s (Swarin and Lipari, 1983) or to 2,4-dinitrophenylhydrazones (Kuntz et al., 1980). Most of the published data on carbonyl compounds have been obtained using the 2,4-dinitrophenylhydrazine (DNPH) method which can be used in a variety of modifications (e.g. Beasley et al., 1980; Neitzert and Seiler, 1981; Grosjean and Fung, 1982). DNPH-coated cartridges, as described e.g. by Tejada (1986), Grosjean et al. (1990), Shepson et al. (1991), Slemr (19911, and Lee et al. (1995) are the most suitable sampling media for monitoring at measurement stations. The D N P H technique, however, requires large sample volumes (up to about 100 •) and reaches thus only a low time resolution. Concurrently, a more sensitive technique - - direct gas chromatographic analysis--has been applied for measurements of oxygenated hydrocarbons (Seizinger and Dimitriades, 1972). It has been used e.g by Jonsson and Berg (19831, Pierotti (19901, Montzka et al. (1993), and Goldan et al. (19951. The common features of these methods are cryogenic collection of air samples with or without adsorbent material and, apart from the latter study, the employment of two-dimensional gas chromatography
3329
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E. LEIBROCK and J. SLEMR
(2D-GC) with a packed precolumn. The ozone interference inherent to the cryo-collection is eliminated using Na2SO3 traps (Montzka et al., 1993; Goldan et al., 1995) and disturbing water from the ambient air is usually separated on a packed precolumn or frozen in a trap prior to cryo-collection (Goldan et al., 1995). Gas chromatographic methods are subject to errors caused by the polarity (leading to adsorption in the analytical system) and the water-solubility of the oxygenates (causing losses in a water trap). Our aim was to develop an automated analytical system which minimizes irreproducible losses of the investigated compounds and reaches detection limits in the ppt range with a sampling time of a few minutes. We used the common configuration of 2D-GC and improved the analytical performance introducing a potassium iodide (KI) ozone scrubber, a capillary precolumn for the separation of water and higher hydrocarbons and we optimized the cryogenic traps. This paper focuses on the description of our system with its elements, their functions and analytical performance. In the second part, measurements in relatively clean ambient air near Garmisch-Partenkirchen, Germany, are reported in order to demonstrate the applicability of the system for automated field measurements. On the basis of these measurements, a preliminary estimation of the relative abundances of oxygenated and non-oxygenated hydrocarbons is made.
2. ANALYTICALSYSTEM 2.1. Instrumentation A schematic diagram of the used gas chromatographic system is shown in Fig. 1. After ozone re-
Gas Sam )le
moval, an air sample is enriched on the cryogenic sampling trap and the sample volume is measured using a mass-flow controller downstream the trap. Concurrently, the precolumn and the analytical columns are provided with helium as carrier gas. After the concentration step, the trap is heated and the evolved gases are transferred through the actuated 6-port valve Vs onto the precolumn, a highly polar, wide bore capillary column. Compared to a packed precolumn, the main advantages of a capillary column are better separation efficiency and higher inertness. Its lower capacity is sufficient for the required sample volume of 1 d. The precolumn separates the fraction of the C1 Cs oxygenates from H 2 0 and the higher nonpolar hydrocarbons. The selected fraction of the precolumn eluate is directed through the actuated 6-port valve V, onto the cryogenic focusing trap, vaporized by heating and passed onto the analytical columns. Eluates are detected either by F I D or MS. The entire pathway of the sample is made of inert material (PFA tubings, deactivated glass, fused silica capillaries). The commercial cryogenic traps delivered with the chromatograph (Varian, Model 3600) had an unnecessarily large inner volume, an uneven temperature distribution due to their geometry, and were not sufficiently inert. We constructed traps which meet the requirements of a quantitative analysis of polar substances. The new concentration traps (Fig. 2) consist of an aluminium bar with parallel drilled holes for the trap tubing, the cooling medium (nitrogen), a heating element, and a temperature sensor (Pt-100). Air samples and cooling medium are introduced into the trap in opposite directions in order to achieve a negative temperature gradient which prevents aerosol formation and thus a possible breakthrough of sample compounds. The tubing of the sampling trap is made of
Carrier Gas
V'sV° Flow
\
Controller Pump ~LJ
/
~
Valve
~L
~/
v.
j/s,,-~
U~f~ ~
.
/f ~,,Q\
x7 " ~ - ~ - - ~ - ~ " ~
. . . . 3Preeolunm/
V~
--
~ Analytical Columns
Sampling Trap
Cryogenic Focussing Trap
Fig. 1. 2D-GC system used to collect and measure oxygenated and non-oxygeanated hydrocarbons in ambient air. Symbols are as follows: FC flow controller; Vs and V, 6-port valves; FID flame ionization detector; MS mass spectrometer.
Measurement of volatile oxygenated hydrocarbons
3331
Aluminium Bar
Flow Pipe for Coolant ( N 2 )
Pt- 100 Temperature Sensor
Fig. 2. Cryogenic sampling trap with (a) 1/4" PFA sampling tube filled with deactivated glass beads, (b) heating element, (c) temperature sensor (Pt-100) and (d) cold nitrogen gas flow pipe. Dimensions: 5.0cm×2.0cmxl.5cm. The cryogenic focusing trap differs only in the dimensions (5.0 cm x 2.0 cm x 1.0 cm) and the tube (a), which is a capillary column.
PFA (1/4" O.D.) and packed with deactivated glass beads which are held in place by fused silica wool plugs on either side. The trap tubing is cooled down to - 100"C and is heated during desorption with a rate of 3.8':Cs-I up to + 80°C. The helium flow of 7.0 ml rain-1 used during desorption is also suitable for the performance of the precolumn. In order to prevent losses due to adsorption during the transport of the concentrated sample to the columns, both 6port valves are maintained at + 70°C. The cryogenic focusing trap (Fig. 1) is smaller than that used for preconcentration (caption of Fig. 2) and the cryogenic focusing occurs at - 160°C on a deactivated metal capillary (Ultimetal CP Sil 8 CB, 0.5 mm I.D., 0.5/~m film thickness). The final release temperature of + 80~C is achieved at a heating rate of 5.8°C s - a. For detection, either flame ionization or a mass spectrometer can be used. The entire analytical system is automatically controlled by a personal computer and can work unattended. The cryogenic traps require~depending on ambient temperature--less than 4 / of liquid nitrogen per analysis. The dewar vessel has to be replenished manually. 2.2. GC separation Cryogenically enriched compounds of each air sample are first separated on the highly polar precolumn (Carbowax CP Wax 52 CB, length 10 m, I.D. 0.53 mm, film thickness 2.0 Fm). The precolumn is kept at 35°C at a helium flow of 7 mlmin-~. After actuating the valve VH (Fig. 1), the fraction of the CI-C5 oxygenates together with some nonmethane hydrocarbons (NMHC) is directed onto the focusing cryotrap and thus separated from subsequently eluting H20. After refocusing, the compounds are separated on the analytical column consisting of CP Sil 5 CB and CP Sil 8 CB capillaries in series (each 50 m long, 0.32 mm I.D., 1.2/~m film thickness). The average helium flow here is 1.7 ml m i n - a. The initial temperature is held at 35°C for 9 min and than raised at three different rates: at 30°Cmin -1 to 75°C, at 5~Cmin -1 to 87°C and finally at 30°Cmin -~ to 150'C where it is held for 5 min. The achieved separ-
ation is shown in a chromatogram of a standard gas mixture (Fig. 3). As can be seen in Fig. 3, propanal and acetone are not completely separated under the chosen gas chromatographic conditions. With MS detection, however, they can be distinguished due to their different mass spectra (Fig. 4). Acetone is determined using its characteristic mass 43 which is subtracted from the sum signal of both compounds (total ion current), and so the propanal signal is obtained. 2.3. Ozone removal Ozone from ambient air, condensed in the cryogenic sampling trap, would react with unsaturated hydrocarbons (e.g. isoprene) during desorption and thus produce oxygenated species. Therefore, we remove ozone by a scrubber installed upstream the cryogenic sampling trap (Fig. 1). The scrubber is made of 10 cm PFA tubing (1/4" O.D.) filled with KI crystals of high purity (p.a. quality, Merck). Its function was tested with humidified standard gas mixtures with different concentrations of ozone (Fig. 5). Without the scrubber, increasing levels of added ozone diminished isoprene and enhanced its oxidation products as shown for methyl vinyl ketone (MVK). The ozone scrubber restored isoprene and MVK to the same levels even at an ozone mixing ratio of 110 ppb. Likewise, no losses of carbonyl compounds due to the KI scrubber occurred, as was also observed earlier by Slemr (1991). 2.4. Calibration system Calibration gas standards are generated by dynamic dilution of gas mixtures obtained from a commercial permeation unit (UPK Kalibriersystem 1000-2). Permeation tubes, made for each compound of interest (caption of Fig. 3), are calibrated gravimetrically and yield permeation rates in the range of 5-60ngmin -~ at 20°C. The permeation rates are regularly checked for stability. The gas stream from the permeation device is introduced into a two stage dynamic dilution system (Fig. 6). Its maximum dilution factor of 1 : 1000 enables to generate gas mixtures
3332
E. LEIBROCK and J. SLEMR
13
16
1.00
0.75
4
i
0.50
12 17
16
0,25
15
10 11
0.00 I
I
I
I
I
I
I
0
2
4
6
8
10
12
Rel. R e t e n t i o n T i m e [rain] Fig. 3. GC-FID chromatogram of a standard gas mixture. Peaks: 1 = acetaldehyde; 2 = methanol; 3 = ethanol; 4 = acrolein; 5 = propanal; 6 = acetone; 7 = isoprene; 8 = methacrolein; 9 = methyl vinyl ketone; I0 = butanal; 11 = methyl ethyl ketone; 12 = n-hexane; 13 = benzene; 14 = pentanal; 15 = nheptane; 16 = toluene; 17 = n-octane.
1.00
0.00 J
I
~
J
I
I
I
I
~
y=
0.45
0.00 0.01
] Propan~
I
with mixing ratios near to detection limits (see below). The ozone scrubber and a humidifier were integrated into the dilution system in order to simulate ambient sampling conditions. Pathways for gas standard mixtures and ambient air samples are made of inert P F A tubings; the ultimately used calibration gas or air sample never passes through a flow controller or a pump. Both, standard gas mixtures and ambient samples, follow the same pathway to the G C inlet so that potential systematic losses would be largely compensated by the calibration. Calibration curves were measured for the range of interest, i.e. up to mixing ratios of about 4 ppb. They are linear and highly correlated (R > 0.999), as shown for some examples in Fig. 7. The low offsets on the ordinate correspond to blank values of the analytical system.
0.00 I I
I
I
I
I
115
120
125
130
135
Rel. Retention Time [s] Fig. 4. Mass spectrometric separation of propanal and acetone by using their different characteristic mass fragments (ambient air sample).
3. ANALYTICAL
PERFORMANCE
The analytical performance of the system was tested with dry and humidified standard gas mixtures and with ambient air.
Measurement of volatile oxygenated hydrocarbons
3333
Isoprene
I
MVK ,!!i!i~ii:'~ii!!i;il
7
:t
:1
H
6
iii~ii!:iii
5
O
4i
I
3i
........
2
t
1
i
0
.........
1
3 Measurement Nr.
4
5
Fig, 5. Influence of ozone on isoprene and methyl vinyl ketone (MVK) levels in a humidified standard gas mixture. 1 = without ozone and scrubber; 2 = with ozone (35 ppb) and without scrubber; 3 = with ozone (110 ppb) and without scrubber; 4 = with ozone (35 ppb) and scrubber; 5 = with ozone (110 ppb) and scrubber.
Permeation ~ Gas
~-"-~ Exhaust
3 -Port 11 V a l v e V~
__~
Flow
J
Humidifier i l ~_q
Exhaust Flow Controller
~
N2
GC ,oo ,
Ozone
......
V 4
4[ Sampling Trap Exhaust < ~ Flow Controller
?
V~'
Valvo Y.
% %
Air Fig. 6. Schematic diagram of the laboratory-made two-stage dynamic dilution system.
3.1. Sampling parameters The system was checked for breakthrough volume and flow as documented in Fig. 8. Using a dry standard gas mixture with mixing ratios in the range of 0.4-8.4 ppb, sample volumes of at least of 6 f could be drawn without breakthrough (Fig. 8(a)). The volume of ambient air sample is, however, restricted by air humidity. The cryotrap is plugged by ice when more than 22 #1 of liquid water are frozen. This water amount corresponds to about 1,2 ~ of air sampled on
a rainy summer day. The sample flow can be enhanced up to 250 m l m i n - t without breakthrough (Fig. 8(b)). Higher values cannot be obtained due to the flow resistance of the whole system. For analyses a sample flow of 200 ml m i n - 1 was chosen. 3.2. Detection limits, blanks, and reproducibility Detection limits for individual compounds measured by F I D were determined at a signal-to-noise ratio of 3:1. They range between 0.03 and 0.08 ng
3334
E. LEIBROCK and J. SLEMR
1200¢ • 1000~
Aecaddehyde,R= 0 , ~ 3
J
• sm~,
x= o.~9
~
• • +
R= 0 . 9 , ~ R= 0 . ~ R=O.99,~
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Q 8000 6000
< " ~ 4000 0~ 2000
0 0
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10
Amount [ng]
1000
b
which corresponds to 5-56 ppt in a 1 • air sample. The non-oxygenated hydrocarbons show detection limits between 5 and 16ppt and the carbonyls between 16 and 31 ppt. The alcohols can be quantified at higher mixing ratios (29-56 ppt) due to wider peak shapes. System blanks are run periodically with zero gas introduced at the beginning of the sample collection system. The blank levels of the individual compounds are below the detection limits. As blanks are included both in calibration and air sample signals, they do not need to be considered. Reproducibility was tested in the entire calibration range with dry and humidified gas standards. Relative standard deviations (5 repetions) were lower than 2% in most cases, only methanol showed 6.2% due to less precise integration of its broader peaks. 3.3. Comparison measurements
C= 1001
o < 10( O
T 1
0,1
10
Amount [ng] Fig. 7. Calibration curves of some Compounds in linear (a) and logarithmic plot ((b) for better resolution of low mixing ratios). 12004
~
.
'-°1 200
0
...... 10
,-20
30 40 Sampling Time [rain]
50
50O
50
100 150 Sampling Flow [ml/min]
200
60
The developed method was compared with a G C / F I D method for analyses of nonmethane hydrocarbons (NMHC) during a field study conducted at the summit of Wank (1778 m a.s.l.) near GarmischPartenkirchen in summer 1995. The method for N M H C differs in some substantial features from the method for oxygenated hydrocarbons: (a) grab air samples are collected in summa polished stainless steel canisters in about 30 s (oxygenates are collected cryogenically for 5 min); (b) N M H C are enriched in the laboratory on an adsorption trap; (c) the calibration of the GC instrument rests on a primary n-butane/benzene gas standard from National Institute of Standards and Technology, Maryland, U.S.A. Some compounds are determined by both methods: isoprene, n-hexane, n-heptane, n-octane, benzene, and toluene. For the comparison, only stable hydrocarbons found above detection limits were chosen, i.e. hexane, benzene and toluene. The results are presented in Fig. 9. The regression lines have negative/ positive offsets which could be explained by overestimated/underestimated blanks of canisters as follow-up checks revealed. The slopes for hexane, benzene, toluene 0.06, 0.97, and 0.91, respectively) are near unity and indicate that the measurements are not subject to systematic errors. The regression coefficients (0.813, 0.884, and 0.905, respectively) show good agreement for ambient air measurements in a low ppt range despite the different averaging times of the methods. For the oxygenated compounds, no direct ambient air intercomparison has been made so far. The data obtained with the presented GC-technique during the Wank field study are however in good agreement with
Fig. 8. Detector signal (peak area) as a function of collection time at a sample flow of 100 mlmin- (a) and sample flow at a collection time of 10 min (b).
Measurement of volatile oxygenated hydrocarbons
3335
45
.~
40
*
35 30 .~
Hexane m - 1.07__.~,Yo = -...~1
,
a
20 15
.
0
5
10
15
/
. ~ f .
20
25
30
35
40
Hexane [ppt], on-fine ..=
250 * 200
o
Benzene ] m = 0.97, Yo= -30
f
J
J
150 100 50 4r..k
0
50
0
100
150
200
250
200
250
Benzene [ppt], on-line e..,
250
,~
~:
*
200
© ,.-.-; 15o e-~ 100 ~D 50 0
Toluene m = 0.91, yo= 10
*
c
~ 0
"A"
*
.
50
~, **
** 100 150 T o l u e n e [ppt], o n - l i n e
Fig. 9. Correlations between n-hexane, benzene, and toluene mixing ratios obtained by the described on-line method (x-axis)and an off-lineNMHC technique (y-axis),resulting from ambient air measurements at the summit of Wank (1778 m a.s.l.).
measurements of acetaldehyde, acetone, and methyl ethyl ketone which were carried out in 1991 with a DNPH-method at the same location (Slemr et al., 1993; see also Table 2).
4. F I E L D
MEASUREMENTS
As mentioned already, the entire automated system was placed at a mountain station Wank during a field campaign in summer 1995 in order to complement measurements of volatile organic compounds. It worked reliably for the whole period. Temporal variations of oxygenated compounds are shown in Fig. 10 and averaged values are presented in Table 1. Acetaldehyde and acetone/propanal concentrations can be compared with levels measured by the DNPH
method at rural sites by other authors (Table 2). The mixing ratios found at Wank in summer 1995 are in good agreement with mixing ratios at other rural areas. Concurrent measurements of oxygenated organic compounds and NMHC during the field study at Wank were used to compare abundances of both groups. For the comparison, 12 oxygenated species (including formaldehyde measured simultaneously by a continuous method described by Kelly et al., 1989) and 35 species of C2-C8 NMHC were taken into account. The sum of the measured oxygenated species, given in ppb of carbon (ppbC), was higher than that of NMHC (15.8 ppbC and 10.7 ppbC, respectively). The ozone forming potentials of both groups were, in first approximation, compared on the basis of their OHreactivities (Middleton et al., 1990) using propylene
3336
E. L E I B R O C K
a n d J. S L E M R
4.0 3.5
---
Acetaldehyde ] Acrolein Propanal + Acetone
3.0 o
2.5 2.0 1.5 1.0 0.5 0.0
~.7.
31.7.
1.8
2.8.
3.8.
4.8.
5.8.
6,8.
7.8.
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.o
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~
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a~ 0.3 0.2 0.1 0.0
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a ~ *
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,~ ~
31.7.
1.8
2.8.
3.8.
4.8.
5.8.
6.8.
7.8.
5.8.
6.8.
7.8.
5.8.
6.8.
7.8.
Isoprene MVK MACR
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.4
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'
30.7.
31.7.
1.8
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4.0 3.5 X~ o 3.0 2.5 2.0 1.5 1.0 ! 0.5 0.0
30.7.
31.7.
1.8
2.8.
Date F i g . 10. T e m p o r a l
variations
of oxygenated
organic compounds mountain Wank.
measured
in J u l y / A u g u s t
1995 o n t h e
Measurement of volatile oxygenated hydrocarbons
3337
Table 1. Levels of volatile organic compounds measured from 19 to 23 July and from 30 July to 8 August 1995 at Wank Average mixing ratios (ppb) at Wank Total 0am-24pm (CET) Compound
Day 8am-8pm (CET)
Night 8pm-8am (CET)
Average + Standard Deviation (6 ++a(c))/ppb
Acetaldehyde Acrolein MACR Butanal Pentanal Acetone + Propanal MEK MVK Methanol Ethanol n-Hexane n-Heptane n-Octane Isoprene Benzene Toluene
0.77 + 0.33 0.60 + 0.41 0.04 + 0.08 0.18 + 0.12 0.11 __+0.12 1.71 _+ 0.41 0.19 + 0.06 0.18 _ 0.08 2.26 _ 0.45 0.24 _ 0.15 0.01 + 0.01
0.72 + 0.36 0.55 + 0.42 0.07 + 0.11 0.19 + 0.11 0.12 + 0.16 1.68 + 0.49 0.18 + 0.06 0.19 + 0.09 2.25 + 0.46 0.25 ___0.15 0.01 + 0.0l < det. lira.
0.84 + 0.26 0.65 + 0.30 0.02 + 0.01 0.17 + 0.12 0.09 + 0.06 1.74 + 0.28 0.20 _+ 0.05 0.16 + 0.05 2.28 + 0.43 0.22 + 0.16 0.01 + 0.01 < det. lim.
Table 2. Summary of carbonyl concentrations at Wank and other rural sites. Mean mixing ratio (ppb) Site Wank, July/August 1995a Southern Germany, June 1984 b Ontario, July/August 1988~ Wank, October 1991d Southern Germany, September 1992e
CH3CHO 0.8 0.5 0.6 0.6 0.7
CH3COCH 3 1.7a 1.6 1.5 2.6
Acetone and propanal (Concentrations of propanal are usually substantially lower than those of acetone so that the sum of their concentrations can roughly be compared with acetone concentrations alone). b Schubert et al. (1988). Shepson et al. (1991). d Slemr et al. (1993). ° Slemr et al. (1996).
equivalents (e.g, N a t i o n a l Research Council, 1991). The resulting total propylene equivalent of the oxygenates was close to t h a t of N M H C (23.0 p p b C a n d 24.9 ppbC, respectively; see Table 3 for the individual species). The c o m p a r i s o n s of the a b u n d a n c e s a n d propylene equivalents of the c o m p o u n d classes underline the necessity to measure both, N M H C a n d oxygenated c o m p o u n d s , for studies of the p h o t o c h e m i c a l f o r m a t i o n of ozone.
5. S U M M A R Y
T h e developed analytical m e t h o d for the determ i n a t i o n of C I - C s oxygenated h y d r o c a r b o n s in clean
a m b i e n t air is based o n two-dimensional gas c h r o m a t o g r a p h y . The cryogenically enriched air samples are pre-separated o n a highly polar wide-bore capillary c o l u m n a n d subsequently analyzed on two capillaries of low polarity connected in series. O z o n e interference is eliminated by a KI filter placed upstream the cryogenic sampling trap. C a l i b r a t i o n gas mixtures are generated using p e r m e a t i o n sources for each c o m p o u n d . Detection limits range between 5 a n d 56 p p t in a n air sample of 1 t~. T h e a u t o m a t e d system worked reliably d u r i n g a field c a m p a i g n o n a m o u n t a i n s u m m i t (Wank, 1778 m asl). C o n c u r r e n t m e a s u r e m e n t s of N M H C a n d oxygenated comp o u n d s d o c u m e n t the high a b u n d a n c e a n d ozone forming potential of oxygenated c o m p o u n d s relative
E. LEIBROCK and J. SLEMR
3338
Table 3. Propylene equivalents of oxygenated hydrocarbons and NMHC (in ppbC) found above detection limits at Wank in the time period 30 July to 7 August 1995 Oxygenated compound Formaldehyde Acetaldehyde Propanal Acrolein MACR Butanal Pentanal Acetone MEK MVK Methanol Ethanol
Sum
Propylene equivalents (ppbC) 2.57 4.20 1.54 5.87 0.91 2.51 2.35 0.18 0.12 2.21 0.33 0.22
23.01
to N M H C and show the necessity to measure both groups.
Acknowledgements--The authors wish to thank Dr Pavel Matuska and Dr Franz Slemr for helpful discussions and the firm Varian (Dr Volker Betz and Dipl.-Ing. Michaela Stahl) for advice and technical support. The development of the analytical system was supported financially by the Bayerisches Staatsministerium fiir Wirtschaft und Verkehr (under grant 3625-VIII/4c).
REFERENCES
Altshuller, A. P. (1991a) Chemical reactions and transport of alkanes and their products in the troposphere. Journal of Atmospheric Chemistry 12, 19-61. Altshuller, A. P. (1991h) Estimating product yields of carbon-containing products from the atmospheric photooxidation of ambient air alkenes. Journal of Atmospheric Chemistry 13, 131-154. Atkinson, R. (1990) Gas-phase tropospheric chemistry of organic compounds: a review. Atmospheric Environment 24A, 1-41. Beasley, R. K., Hoffmann, C. E., Rueppel, M. L. and Worley, J. W. (1980) Sampling of formaldehyde in air with coated solid sorbent and determination by high performance liquid chromatography. Analytical Chemistry 52, 1110-1114. Fehsenfeld, F., Calvert, J., Fall, R., Goldan, P., Guenther, A. B., Hewitt, C. N., Lamb, B., Liu, S., Trainer, M., Westberg, H. and Zimmerman, P. (1992) Emissions of volatile organic compounds from vegetation and the implications for atmospheric chemistry. Global Biogeochemical Cycles 6, 389-430.
Non-oxygenated compound Ethane Ethene Propane /-Butane n-Butane Ethine Cyclopentane i-Pentane n-Pentane Methyl acetylene Pentene Methyl cyclopentane 2, 3-Dimethyl butane 2-Methyl pentane 3-Methyl pentane n-Hexane Isoprene n-Heptane Benzene /-Octane n-Octane Toluene
Propylene equivalents (ppbC) 0.07 4.60 0.12 0.12 0.19 0.10 0.02 0.48 0.10 0.18 0.03 0.16 0.03 0.12 0.08 0.07 17.78 0.01 0.06 0.02 0.01 0.50 24.86
Finlayson-Pitts, B. and Pitts, J. N. (1986) Atmospheric
Chemistry: Fundamentals and Experimental Techniques. Wiley, New York, Goldan, P. D., Kuster, W. C., Fehsenfeld, F. C. and Montzka, S. A. (1995) Hydrocarbon measurements in the southern United States: The Rural Oxidants in the Southern Environment (ROSE) Program 1990. Journal of Geophysical Research 100, 25,945-25,963. Grosjean, D. and Fung, K. (1982) Collection efficiencies of cartridges and microimpingers for sampling of aldehydes in air as 2, 4-dinitrophenylhydrazones. Analytical Chemistry 54, 1221-1224. Grosjean, D., Miguel, A. H., Tavares, T. M. (1990) Urban air pollution in Brazil: acetaldehyde and other carbonyls. Atmospheric Environment 24B, 101 106. Jonsson, A. and Berg, S. (1983) Determination of lowmolecular-weight oxygenated hydrocarbons in ambient air by cryogradient sampling and two-dimensional gas chromatography. Journal of Chromatograph), 279, 307-322. Kelly, T. J., Barnes, R. H., McClenny, W. A. (1989) Development of real time monitors for gaseous formaldehyde. Final Report to U. S. Environmental Protection Agency, EPA/600/3-90/088 Kleinman L. I. (1991) Seasonal dependence of boundary layer peroxide concentration: the low and high NOx regimes. Journal of Geophysical Research 96, 20721-20733. Kuntz, R., Lonneman, W., Namie, G., and Hull, L. A. (1980) Rapid determination of aldehydes in air analyses. Analytical Letters 13(A16), 1409-1415. Lee, Y.-N., Zhou, X. and Hallock, K. (1995) Atmospheric carbonyl compounds at a rural southeastern United state site. Journal of Geophysical Research 100, 25,933-25,944. Le Lacheur, R. M., Sonnenberg, L. B., Singer P. C., Christman, R. F. and Charles, M. J. (1993) Identification of carbonyl compounds in environmental samples. Environmental Science and Technology 27, 2745 2753.
Measurement of volatile oxygenated hydrocarbons Levine, S. P., Harvey, T. M., Waeghe, T. J. and Shapiro, R. H. I1981) O-Alkyloxime derivates for gas chromatographic and gas chromatographic-mass spectrometric determination of aldehydes. Analytical Chemistry 53, 805-809. Middleton, P., Stockwell, W. R. and Carter, W. P. L. (1990) Aggregation and analysis of volatile organic compound emissions for regional modeling. Atmospheric Environment 24A, 1107-1133. Montzka, S. A., Trainer, M., Goldan, P. D,, Kuster, W. C. and Fehsenfeld, F. C. (1993) Isoprene and its oxidation products, methyl vinyl ketone and methacrolein, in the rural troposphere. Journal of Geophysical Research 98, 1101-1111. National Research Council (1991) Rethinking the Ozone Problem in Urban and Regional Air Pollution, p. 232, National Academy Press, Washington D.C. Neitzert. V. and Seiler, W. (1981) Measurement of formaldehyde in clean air, Geophysical Research Letters 8, 79-82. Pierotti, D. (1990) Analysis of trace oxygenated hydrocarbons in the environment. Journal of Atmospheric Chemistry 10, 373-382. Schubert, B., Schmidt, U. and Ehhalt, D, H. (1988) Untersuchungen zum Nachweis und zur Chemie yon Formaldehyd und Acetaldehyd in der unteren Troposphiire. Berichte der Forschungsanlage Jiilich; ISSN 0366-0885, Forschungsanlage Jfilich GmbH, Zentralbibliothek, D-52425 Jiilich. Seizinger, D. E. and Dimitriades, B. (1972) Oxygenates in exhaust from simple hydrocarbon fuels. Journal Air Pollution Control Association 22, 47-51.
3339
Shepson, P. B., Hastie, D. R., Schiff, H. I., Polizzi, M., Bottenheim, J. W., Anlauf, K., Macakay, G. I. and Karecki, D. R. (1991) Atmospheric concentrations and temporal variations of C1-C3 carbonyl compounds at two rural sites in central Ontario. Atmospheric Environment 25A, 2001-2015. Slemr J. (1991) Determination of volatile carbonyl compounds in clean air. Fresenius Journal of Analytical Chemistry 340, 672-677. Slemr J., Schulz S., Seemann S. and Kern M. (1993) Development of techniques for the determination of major carbonyl compounds in clean air. In The Proceedings o[" EUROTRAC Symposium 92, ed. P. Borrell et al., pp, 170-172. SPB Academic Publishing, The Hague, The Netherlands. Slemr, J., Junkermann, W. and Volz-Thomas, A. (1996) Temporal variations in formaldehyde, acetaldehyde and acetone and budget of formaldehyde at a rural site in southern Germany. Atmospheric Environment 30, 3667-3676. Swarin, S. J. and Lipari, F. (1983) Determination of formaldehyde and other aldehydes by high performmance liquid chromatography with fluorescence detection. Journal ~[" Liquid Chromatography 6, 425-444. Tejada, S. B. (1986) Evaluation of silica gel cartridges coated "in situ" with acidified 2, 4-dinitrophenylhydrazine for sampling aldehydes and ketones in air. International Journal of Environmental Analytical Chemistry 26, 167-185.
Atmospheric Environment Vol. 31. No, 20, pp. 3329 3.339, 1997 !? 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain P I I : S1352--2310(97)00155--6 1352 2310/97 5;17.110+ (I,00
M E T H O D F O R M E A S U R E M E N T O F VOLATILE O X Y G E N A T E D H Y D R O C A R B O N S IN A M B I E N T AIR E. LEIBROCK* and J. SLEMR Fraunhofer Institute for Atmospheric Environmental Research, Kreuzeckbahnstrasse 19, D-82467 Garmisch-Partenkirchen, Germany (First received 17 September 1996 and in final form 26 March 1997, Published August 1997) A b s t r a c t - - A n automated gas chromatographic method for the quantitative determination of oxygenated (C2-C5 carbonyls and C1-C2 alcohols) and some non-oxygenated (C5-C8) hydrocarbons in ambient air has been developed. The analytical system consists of a gas chromatograph with a cryogenic sampling trap, a precolumn for the separation of water and other interfering compounds, a cryogenic focusing trap and two analytical columns connected in series. Substances are detected either by flame ionization or by a mass spectrometer. Ozone is removed by a potassium iodide scrubber placed upstream the sampling trap. External gas standards generated by a permeation device are used for calibration. The detection limits range between 0.03 and 0.08 ng (depending on the compound), equivalent to 5 to 56 ppt in 1 { of sampled air. The method was tested by an intercomparison with a different gas chromatographic technique for the determination of NMHC. The system has been applied since 1994 for measurements in ambient air. Data obtained during an intensive campaign in summer 1995 at the field station Wank (1778 m a.s.l.) near Garmisch-Partenkirchen, Germany, are reported and compared with NMHC mixing ratios measured simultaneously in the same air masses. © 1997 Elsevier Science Ltd.
Key word index: Carbonyls, alcohols, tropospheric ozone, two-dimensional gas chromatography, mass spectrometry, GC/MS, field measurements.
I.
INTRODUCTION
Atmospheric carbonyl compounds are both, directly emitted by anthropogenic and biogenic sources and photochemically produced from hydrocarbons (e.g. Atkinson, 1990; Altshuller, 1991a, b). Their photolysis is considered to be a major source of free radicals in moderately and highly polluted atmospheres and can even become the predominant radical source in the polluted planetary boundary layer during winter and early spring (e.g. Kleinman, 1991). Carbonyls react rapidly with OH radicals (Atkinson, 1990) and serve also as precursors of peroxyacyl nitrates and peroxides (e.g. Finlayson-Pitts and Pitts, 1986). Despite their high reactivity, carbonyls can comprise a significant fraction of the total volatile organic compounds in rural air (Schubert et al., 1988; Fehsenfeld et al., 1992; Goldan et al., 1995). Therefore, they play a crucial role in the formation of photooxidants such as ozone (National Research Council, 1991) and their speciation and quantification is necessary in order to understand the production of photooxidants. Measurements of carbonyl compounds and other oxygenated hydrocarbons are difficult due to their
* Author to whom correspondence should be addressed.
high reactivity and polarity. Some techniques reduce the reactivity using derivatization e.g. to oximes (Levine et al., 1981; Le Lacheur et al., 1993), to 2- d i p he n y l a c e t yl- 1,3-in d a n i n on- 1 - h y d r a z o n e s (Swarin and Lipari, 1983) or to 2,4-dinitrophenylhydrazones (Kuntz et al., 1980). Most of the published data on carbonyl compounds have been obtained using the 2,4-dinitrophenylhydrazine (DNPH) method which can be used in a variety of modifications (e.g. Beasley et al., 1980; Neitzert and Seiler, 1981; Grosjean and Fung, 1982). DNPH-coated cartridges, as described e.g. by Tejada (1986), Grosjean et al. (1990), Shepson et al. (1991), Slemr (19911, and Lee et al. (1995) are the most suitable sampling media for monitoring at measurement stations. The D N P H technique, however, requires large sample volumes (up to about 100 •) and reaches thus only a low time resolution. Concurrently, a more sensitive technique - - direct gas chromatographic analysis--has been applied for measurements of oxygenated hydrocarbons (Seizinger and Dimitriades, 1972). It has been used e.g by Jonsson and Berg (19831, Pierotti (19901, Montzka et al. (1993), and Goldan et al. (19951. The common features of these methods are cryogenic collection of air samples with or without adsorbent material and, apart from the latter study, the employment of two-dimensional gas chromatography
3329
3330
E. LEIBROCK and J. SLEMR
(2D-GC) with a packed precolumn. The ozone interference inherent to the cryo-collection is eliminated using Na2SO3 traps (Montzka et al., 1993; Goldan et al., 1995) and disturbing water from the ambient air is usually separated on a packed precolumn or frozen in a trap prior to cryo-collection (Goldan et al., 1995). Gas chromatographic methods are subject to errors caused by the polarity (leading to adsorption in the analytical system) and the water-solubility of the oxygenates (causing losses in a water trap). Our aim was to develop an automated analytical system which minimizes irreproducible losses of the investigated compounds and reaches detection limits in the ppt range with a sampling time of a few minutes. We used the common configuration of 2D-GC and improved the analytical performance introducing a potassium iodide (KI) ozone scrubber, a capillary precolumn for the separation of water and higher hydrocarbons and we optimized the cryogenic traps. This paper focuses on the description of our system with its elements, their functions and analytical performance. In the second part, measurements in relatively clean ambient air near Garmisch-Partenkirchen, Germany, are reported in order to demonstrate the applicability of the system for automated field measurements. On the basis of these measurements, a preliminary estimation of the relative abundances of oxygenated and non-oxygenated hydrocarbons is made.
2. ANALYTICALSYSTEM 2.1. Instrumentation A schematic diagram of the used gas chromatographic system is shown in Fig. 1. After ozone re-
Gas Sam )le
moval, an air sample is enriched on the cryogenic sampling trap and the sample volume is measured using a mass-flow controller downstream the trap. Concurrently, the precolumn and the analytical columns are provided with helium as carrier gas. After the concentration step, the trap is heated and the evolved gases are transferred through the actuated 6-port valve Vs onto the precolumn, a highly polar, wide bore capillary column. Compared to a packed precolumn, the main advantages of a capillary column are better separation efficiency and higher inertness. Its lower capacity is sufficient for the required sample volume of 1 d. The precolumn separates the fraction of the C1 Cs oxygenates from H 2 0 and the higher nonpolar hydrocarbons. The selected fraction of the precolumn eluate is directed through the actuated 6-port valve V, onto the cryogenic focusing trap, vaporized by heating and passed onto the analytical columns. Eluates are detected either by F I D or MS. The entire pathway of the sample is made of inert material (PFA tubings, deactivated glass, fused silica capillaries). The commercial cryogenic traps delivered with the chromatograph (Varian, Model 3600) had an unnecessarily large inner volume, an uneven temperature distribution due to their geometry, and were not sufficiently inert. We constructed traps which meet the requirements of a quantitative analysis of polar substances. The new concentration traps (Fig. 2) consist of an aluminium bar with parallel drilled holes for the trap tubing, the cooling medium (nitrogen), a heating element, and a temperature sensor (Pt-100). Air samples and cooling medium are introduced into the trap in opposite directions in order to achieve a negative temperature gradient which prevents aerosol formation and thus a possible breakthrough of sample compounds. The tubing of the sampling trap is made of
Carrier Gas
V'sV° Flow
\
Controller Pump ~LJ
/
~
Valve
~L
~/
v.
j/s,,-~
U~f~ ~
.
/f ~,,Q\
x7 " ~ - ~ - - ~ - ~ " ~
. . . . 3Preeolunm/
V~
--
~ Analytical Columns
Sampling Trap
Cryogenic Focussing Trap
Fig. 1. 2D-GC system used to collect and measure oxygenated and non-oxygeanated hydrocarbons in ambient air. Symbols are as follows: FC flow controller; Vs and V, 6-port valves; FID flame ionization detector; MS mass spectrometer.
Measurement of volatile oxygenated hydrocarbons
3331
Aluminium Bar
Flow Pipe for Coolant ( N 2 )
Pt- 100 Temperature Sensor
Fig. 2. Cryogenic sampling trap with (a) 1/4" PFA sampling tube filled with deactivated glass beads, (b) heating element, (c) temperature sensor (Pt-100) and (d) cold nitrogen gas flow pipe. Dimensions: 5.0cm×2.0cmxl.5cm. The cryogenic focusing trap differs only in the dimensions (5.0 cm x 2.0 cm x 1.0 cm) and the tube (a), which is a capillary column.
PFA (1/4" O.D.) and packed with deactivated glass beads which are held in place by fused silica wool plugs on either side. The trap tubing is cooled down to - 100"C and is heated during desorption with a rate of 3.8':Cs-I up to + 80°C. The helium flow of 7.0 ml rain-1 used during desorption is also suitable for the performance of the precolumn. In order to prevent losses due to adsorption during the transport of the concentrated sample to the columns, both 6port valves are maintained at + 70°C. The cryogenic focusing trap (Fig. 1) is smaller than that used for preconcentration (caption of Fig. 2) and the cryogenic focusing occurs at - 160°C on a deactivated metal capillary (Ultimetal CP Sil 8 CB, 0.5 mm I.D., 0.5/~m film thickness). The final release temperature of + 80~C is achieved at a heating rate of 5.8°C s - a. For detection, either flame ionization or a mass spectrometer can be used. The entire analytical system is automatically controlled by a personal computer and can work unattended. The cryogenic traps require~depending on ambient temperature--less than 4 / of liquid nitrogen per analysis. The dewar vessel has to be replenished manually. 2.2. GC separation Cryogenically enriched compounds of each air sample are first separated on the highly polar precolumn (Carbowax CP Wax 52 CB, length 10 m, I.D. 0.53 mm, film thickness 2.0 Fm). The precolumn is kept at 35°C at a helium flow of 7 mlmin-~. After actuating the valve VH (Fig. 1), the fraction of the CI-C5 oxygenates together with some nonmethane hydrocarbons (NMHC) is directed onto the focusing cryotrap and thus separated from subsequently eluting H20. After refocusing, the compounds are separated on the analytical column consisting of CP Sil 5 CB and CP Sil 8 CB capillaries in series (each 50 m long, 0.32 mm I.D., 1.2/~m film thickness). The average helium flow here is 1.7 ml m i n - a. The initial temperature is held at 35°C for 9 min and than raised at three different rates: at 30°Cmin -1 to 75°C, at 5~Cmin -1 to 87°C and finally at 30°Cmin -~ to 150'C where it is held for 5 min. The achieved separ-
ation is shown in a chromatogram of a standard gas mixture (Fig. 3). As can be seen in Fig. 3, propanal and acetone are not completely separated under the chosen gas chromatographic conditions. With MS detection, however, they can be distinguished due to their different mass spectra (Fig. 4). Acetone is determined using its characteristic mass 43 which is subtracted from the sum signal of both compounds (total ion current), and so the propanal signal is obtained. 2.3. Ozone removal Ozone from ambient air, condensed in the cryogenic sampling trap, would react with unsaturated hydrocarbons (e.g. isoprene) during desorption and thus produce oxygenated species. Therefore, we remove ozone by a scrubber installed upstream the cryogenic sampling trap (Fig. 1). The scrubber is made of 10 cm PFA tubing (1/4" O.D.) filled with KI crystals of high purity (p.a. quality, Merck). Its function was tested with humidified standard gas mixtures with different concentrations of ozone (Fig. 5). Without the scrubber, increasing levels of added ozone diminished isoprene and enhanced its oxidation products as shown for methyl vinyl ketone (MVK). The ozone scrubber restored isoprene and MVK to the same levels even at an ozone mixing ratio of 110 ppb. Likewise, no losses of carbonyl compounds due to the KI scrubber occurred, as was also observed earlier by Slemr (1991). 2.4. Calibration system Calibration gas standards are generated by dynamic dilution of gas mixtures obtained from a commercial permeation unit (UPK Kalibriersystem 1000-2). Permeation tubes, made for each compound of interest (caption of Fig. 3), are calibrated gravimetrically and yield permeation rates in the range of 5-60ngmin -~ at 20°C. The permeation rates are regularly checked for stability. The gas stream from the permeation device is introduced into a two stage dynamic dilution system (Fig. 6). Its maximum dilution factor of 1 : 1000 enables to generate gas mixtures
3332
E. LEIBROCK and J. SLEMR
13
16
1.00
0.75
4
i
0.50
12 17
16
0,25
15
10 11
0.00 I
I
I
I
I
I
I
0
2
4
6
8
10
12
Rel. R e t e n t i o n T i m e [rain] Fig. 3. GC-FID chromatogram of a standard gas mixture. Peaks: 1 = acetaldehyde; 2 = methanol; 3 = ethanol; 4 = acrolein; 5 = propanal; 6 = acetone; 7 = isoprene; 8 = methacrolein; 9 = methyl vinyl ketone; I0 = butanal; 11 = methyl ethyl ketone; 12 = n-hexane; 13 = benzene; 14 = pentanal; 15 = nheptane; 16 = toluene; 17 = n-octane.
1.00
0.00 J
I
~
J
I
I
I
I
~
y=
0.45
0.00 0.01
] Propan~
I
with mixing ratios near to detection limits (see below). The ozone scrubber and a humidifier were integrated into the dilution system in order to simulate ambient sampling conditions. Pathways for gas standard mixtures and ambient air samples are made of inert P F A tubings; the ultimately used calibration gas or air sample never passes through a flow controller or a pump. Both, standard gas mixtures and ambient samples, follow the same pathway to the G C inlet so that potential systematic losses would be largely compensated by the calibration. Calibration curves were measured for the range of interest, i.e. up to mixing ratios of about 4 ppb. They are linear and highly correlated (R > 0.999), as shown for some examples in Fig. 7. The low offsets on the ordinate correspond to blank values of the analytical system.
0.00 I I
I
I
I
I
115
120
125
130
135
Rel. Retention Time [s] Fig. 4. Mass spectrometric separation of propanal and acetone by using their different characteristic mass fragments (ambient air sample).
3. ANALYTICAL
PERFORMANCE
The analytical performance of the system was tested with dry and humidified standard gas mixtures and with ambient air.
Measurement of volatile oxygenated hydrocarbons
3333
Isoprene
I
MVK ,!!i!i~ii:'~ii!!i;il
7
:t
:1
H
6
iii~ii!:iii
5
O
4i
I
3i
........
2
t
1
i
0
.........
1
3 Measurement Nr.
4
5
Fig, 5. Influence of ozone on isoprene and methyl vinyl ketone (MVK) levels in a humidified standard gas mixture. 1 = without ozone and scrubber; 2 = with ozone (35 ppb) and without scrubber; 3 = with ozone (110 ppb) and without scrubber; 4 = with ozone (35 ppb) and scrubber; 5 = with ozone (110 ppb) and scrubber.
Permeation ~ Gas
~-"-~ Exhaust
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__~
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~
N2
GC ,oo ,
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......
V 4
4[ Sampling Trap Exhaust < ~ Flow Controller
?
V~'
Valvo Y.
% %
Air Fig. 6. Schematic diagram of the laboratory-made two-stage dynamic dilution system.
3.1. Sampling parameters The system was checked for breakthrough volume and flow as documented in Fig. 8. Using a dry standard gas mixture with mixing ratios in the range of 0.4-8.4 ppb, sample volumes of at least of 6 f could be drawn without breakthrough (Fig. 8(a)). The volume of ambient air sample is, however, restricted by air humidity. The cryotrap is plugged by ice when more than 22 #1 of liquid water are frozen. This water amount corresponds to about 1,2 ~ of air sampled on
a rainy summer day. The sample flow can be enhanced up to 250 m l m i n - t without breakthrough (Fig. 8(b)). Higher values cannot be obtained due to the flow resistance of the whole system. For analyses a sample flow of 200 ml m i n - 1 was chosen. 3.2. Detection limits, blanks, and reproducibility Detection limits for individual compounds measured by F I D were determined at a signal-to-noise ratio of 3:1. They range between 0.03 and 0.08 ng
3334
E. LEIBROCK and J. SLEMR
1200¢ • 1000~
Aecaddehyde,R= 0 , ~ 3
J
• sm~,
x= o.~9
~
• • +
R= 0 . 9 , ~ R= 0 . ~ R=O.99,~
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//
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8
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Amount [ng]
1000
b
which corresponds to 5-56 ppt in a 1 • air sample. The non-oxygenated hydrocarbons show detection limits between 5 and 16ppt and the carbonyls between 16 and 31 ppt. The alcohols can be quantified at higher mixing ratios (29-56 ppt) due to wider peak shapes. System blanks are run periodically with zero gas introduced at the beginning of the sample collection system. The blank levels of the individual compounds are below the detection limits. As blanks are included both in calibration and air sample signals, they do not need to be considered. Reproducibility was tested in the entire calibration range with dry and humidified gas standards. Relative standard deviations (5 repetions) were lower than 2% in most cases, only methanol showed 6.2% due to less precise integration of its broader peaks. 3.3. Comparison measurements
C= 1001
o < 10( O
T 1
0,1
10
Amount [ng] Fig. 7. Calibration curves of some Compounds in linear (a) and logarithmic plot ((b) for better resolution of low mixing ratios). 12004
~
.
'-°1 200
0
...... 10
,-20
30 40 Sampling Time [rain]
50
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50
100 150 Sampling Flow [ml/min]
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The developed method was compared with a G C / F I D method for analyses of nonmethane hydrocarbons (NMHC) during a field study conducted at the summit of Wank (1778 m a.s.l.) near GarmischPartenkirchen in summer 1995. The method for N M H C differs in some substantial features from the method for oxygenated hydrocarbons: (a) grab air samples are collected in summa polished stainless steel canisters in about 30 s (oxygenates are collected cryogenically for 5 min); (b) N M H C are enriched in the laboratory on an adsorption trap; (c) the calibration of the GC instrument rests on a primary n-butane/benzene gas standard from National Institute of Standards and Technology, Maryland, U.S.A. Some compounds are determined by both methods: isoprene, n-hexane, n-heptane, n-octane, benzene, and toluene. For the comparison, only stable hydrocarbons found above detection limits were chosen, i.e. hexane, benzene and toluene. The results are presented in Fig. 9. The regression lines have negative/ positive offsets which could be explained by overestimated/underestimated blanks of canisters as follow-up checks revealed. The slopes for hexane, benzene, toluene 0.06, 0.97, and 0.91, respectively) are near unity and indicate that the measurements are not subject to systematic errors. The regression coefficients (0.813, 0.884, and 0.905, respectively) show good agreement for ambient air measurements in a low ppt range despite the different averaging times of the methods. For the oxygenated compounds, no direct ambient air intercomparison has been made so far. The data obtained with the presented GC-technique during the Wank field study are however in good agreement with
Fig. 8. Detector signal (peak area) as a function of collection time at a sample flow of 100 mlmin- (a) and sample flow at a collection time of 10 min (b).
Measurement of volatile oxygenated hydrocarbons
3335
45
.~
40
*
35 30 .~
Hexane m - 1.07__.~,Yo = -...~1
,
a
20 15
.
0
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. ~ f .
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25
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100
150
200
250
200
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Benzene [ppt], on-line e..,
250
,~
~:
*
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*
c
~ 0
"A"
*
.
50
~, **
** 100 150 T o l u e n e [ppt], o n - l i n e
Fig. 9. Correlations between n-hexane, benzene, and toluene mixing ratios obtained by the described on-line method (x-axis)and an off-lineNMHC technique (y-axis),resulting from ambient air measurements at the summit of Wank (1778 m a.s.l.).
measurements of acetaldehyde, acetone, and methyl ethyl ketone which were carried out in 1991 with a DNPH-method at the same location (Slemr et al., 1993; see also Table 2).
4. F I E L D
MEASUREMENTS
As mentioned already, the entire automated system was placed at a mountain station Wank during a field campaign in summer 1995 in order to complement measurements of volatile organic compounds. It worked reliably for the whole period. Temporal variations of oxygenated compounds are shown in Fig. 10 and averaged values are presented in Table 1. Acetaldehyde and acetone/propanal concentrations can be compared with levels measured by the DNPH
method at rural sites by other authors (Table 2). The mixing ratios found at Wank in summer 1995 are in good agreement with mixing ratios at other rural areas. Concurrent measurements of oxygenated organic compounds and NMHC during the field study at Wank were used to compare abundances of both groups. For the comparison, 12 oxygenated species (including formaldehyde measured simultaneously by a continuous method described by Kelly et al., 1989) and 35 species of C2-C8 NMHC were taken into account. The sum of the measured oxygenated species, given in ppb of carbon (ppbC), was higher than that of NMHC (15.8 ppbC and 10.7 ppbC, respectively). The ozone forming potentials of both groups were, in first approximation, compared on the basis of their OHreactivities (Middleton et al., 1990) using propylene
3336
E. L E I B R O C K
a n d J. S L E M R
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5.0 4.5
- -
Methanol 1 Ethanol
4.0 3.5 X~ o 3.0 2.5 2.0 1.5 1.0 ! 0.5 0.0
30.7.
31.7.
1.8
2.8.
Date F i g . 10. T e m p o r a l
variations
of oxygenated
organic compounds mountain Wank.
measured
in J u l y / A u g u s t
1995 o n t h e
Measurement of volatile oxygenated hydrocarbons
3337
Table 1. Levels of volatile organic compounds measured from 19 to 23 July and from 30 July to 8 August 1995 at Wank Average mixing ratios (ppb) at Wank Total 0am-24pm (CET) Compound
Day 8am-8pm (CET)
Night 8pm-8am (CET)
Average + Standard Deviation (6 ++a(c))/ppb
Acetaldehyde Acrolein MACR Butanal Pentanal Acetone + Propanal MEK MVK Methanol Ethanol n-Hexane n-Heptane n-Octane Isoprene Benzene Toluene
0.77 + 0.33 0.60 + 0.41 0.04 + 0.08 0.18 + 0.12 0.11 __+0.12 1.71 _+ 0.41 0.19 + 0.06 0.18 _ 0.08 2.26 _ 0.45 0.24 _ 0.15 0.01 + 0.01
0.72 + 0.36 0.55 + 0.42 0.07 + 0.11 0.19 + 0.11 0.12 + 0.16 1.68 + 0.49 0.18 + 0.06 0.19 + 0.09 2.25 + 0.46 0.25 ___0.15 0.01 + 0.0l < det. lira.
0.84 + 0.26 0.65 + 0.30 0.02 + 0.01 0.17 + 0.12 0.09 + 0.06 1.74 + 0.28 0.20 _+ 0.05 0.16 + 0.05 2.28 + 0.43 0.22 + 0.16 0.01 + 0.01 < det. lim.
Table 2. Summary of carbonyl concentrations at Wank and other rural sites. Mean mixing ratio (ppb) Site Wank, July/August 1995a Southern Germany, June 1984 b Ontario, July/August 1988~ Wank, October 1991d Southern Germany, September 1992e
CH3CHO 0.8 0.5 0.6 0.6 0.7
CH3COCH 3 1.7a 1.6 1.5 2.6
Acetone and propanal (Concentrations of propanal are usually substantially lower than those of acetone so that the sum of their concentrations can roughly be compared with acetone concentrations alone). b Schubert et al. (1988). Shepson et al. (1991). d Slemr et al. (1993). ° Slemr et al. (1996).
equivalents (e.g, N a t i o n a l Research Council, 1991). The resulting total propylene equivalent of the oxygenates was close to t h a t of N M H C (23.0 p p b C a n d 24.9 ppbC, respectively; see Table 3 for the individual species). The c o m p a r i s o n s of the a b u n d a n c e s a n d propylene equivalents of the c o m p o u n d classes underline the necessity to measure both, N M H C a n d oxygenated c o m p o u n d s , for studies of the p h o t o c h e m i c a l f o r m a t i o n of ozone.
5. S U M M A R Y
T h e developed analytical m e t h o d for the determ i n a t i o n of C I - C s oxygenated h y d r o c a r b o n s in clean
a m b i e n t air is based o n two-dimensional gas c h r o m a t o g r a p h y . The cryogenically enriched air samples are pre-separated o n a highly polar wide-bore capillary c o l u m n a n d subsequently analyzed on two capillaries of low polarity connected in series. O z o n e interference is eliminated by a KI filter placed upstream the cryogenic sampling trap. C a l i b r a t i o n gas mixtures are generated using p e r m e a t i o n sources for each c o m p o u n d . Detection limits range between 5 a n d 56 p p t in a n air sample of 1 t~. T h e a u t o m a t e d system worked reliably d u r i n g a field c a m p a i g n o n a m o u n t a i n s u m m i t (Wank, 1778 m asl). C o n c u r r e n t m e a s u r e m e n t s of N M H C a n d oxygenated comp o u n d s d o c u m e n t the high a b u n d a n c e a n d ozone forming potential of oxygenated c o m p o u n d s relative
E. LEIBROCK and J. SLEMR
3338
Table 3. Propylene equivalents of oxygenated hydrocarbons and NMHC (in ppbC) found above detection limits at Wank in the time period 30 July to 7 August 1995 Oxygenated compound Formaldehyde Acetaldehyde Propanal Acrolein MACR Butanal Pentanal Acetone MEK MVK Methanol Ethanol
Sum
Propylene equivalents (ppbC) 2.57 4.20 1.54 5.87 0.91 2.51 2.35 0.18 0.12 2.21 0.33 0.22
23.01
to N M H C and show the necessity to measure both groups.
Acknowledgements--The authors wish to thank Dr Pavel Matuska and Dr Franz Slemr for helpful discussions and the firm Varian (Dr Volker Betz and Dipl.-Ing. Michaela Stahl) for advice and technical support. The development of the analytical system was supported financially by the Bayerisches Staatsministerium fiir Wirtschaft und Verkehr (under grant 3625-VIII/4c).
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