Ozone removal techniques in the sampling of atmospheric volatile organic trace gases

Pergamon Atmospheric Environment Vol. 31, No. 21, pp. 3635 3651, 1997 © 1997 Elsevier Science Ltd

All rights reserved Printed in Great Britain. PII: S1352-2310(97)00144-1 1352-2310/97 $17.00+0.00

OZONE REMOVAL TECHNIQUES IN THE SAMPLING OF ATMOSPHERIC VOLATILE ORGANIC TRACE GASES

DETLEV HELMIG Department of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental

Sciences University of Colorado, Boulder, CO 80309-0215, U.S.A.

(First received 21 July 1996 and in final form 18 March 1997. Published August 1997)

Abstract--Some of the most detrimental interferences in the atmospheric analysis of volatile organic compounds (VOCs) are reactions between ozone and the analytes occurring during the sampling and enrichment process. This paper provides a review of typical interferences observed and techniques to circumvent this problem by the selective removal of ozone from the air sample. © 1997 Elsevier Science Ltd.

Key word index: Sampling of atmospheric organic compounds, ozone interferences, artifacts, ozone scrubber.

1. INTRODUCTION

More than 150 yr have passed since the first discovery and analytical descriptions of ozone as a gas-phase constituent were made (Burns, 1997). Ozone was one of the first atmospheric gases to receive attention by the monitoring of its atmospheric concentration, with the first records now dating back more than 130 yr (Janach, 1989; Marenco et al., 1994). Despite the interest in ambient ozone and the early knowledge of its physical and chemical properties, in particular its high oxidizing capacity, effects of ozone on analytical procedures of other tra.ce gases were not recognized until very recently. Interference by ozone has only received attention since the late 1970s and the number of published reports describing problems from ozone interferences has steadily increased ever since. Analy- sis of volatile organic compounds (VOCs) appears to be particularly prone to analytical biases from ozone interferences and is the focus of this review.

Common analytical techniques for the sampling of atmospheric VOCs include sample preconcentration steps by cryogenic freezeout, solid adsorbent trapping or derivatization onto cartridges containing a chemical reactant (Klockow, 1987; Rudolph et al., 1990; Westberg and Zimmerman, 1993). A potential problem encountered in these techniques are reactions occurring during the sampling between the concentrated organic gases of interest and other reactive air constituents such as ozone, halogens, the hydroxyl radical, nitrogen oxides, water or hydrogen peroxide. Reactions of this kind may alter the quantities of the trace gases of interest and may also contribute to the formation of artifact products which may mistakenly be interpreted as atmospheric constituents.

In the VOC sampling onto solid adsorbents and onto derivatization cartridges, artifacts may not only be formed by reactions of reactive air constituents with the peviously adsorbed analyte compounds but also from reactions with the solid adsorbent bed or the derivatizing agent, which in many cases have an organic polymer basis, such as poly(oxy-m-terphenyl- 2',5'-ylene) (Tenax) or polyurethane foams (PUF). A similar problem is encountered in derivatization methods, e.g. in the sampling of aldehydes on cartridges coated with 2,4-dinitrophenylhydrazine (DNPH), where either the formed products or the derivatizing agent itself may be depleted during the sampling. The investigation of interferences from reactions during sampling is an important part of method development and improvement strategies for atmospheric monitoring.

The most recognized and significant interference in the analysis of organic trace gases is from reactions with ozone. Only a few studies have focused on inves- tigating these effects. A literature search on a com- puterized data system using a series of selected key words did not result in many quotations. Problems from oxidant interferences are omnipresent, however, they are mostly dealt with in the experimental sections of atmospheric measurement articles and are rarely included in the title or key word listing. Consequently, most of the cited articles resulted from carefully following the research literature over several years and cross-referencing to related articles. This review, therefore, cannot be complete, however it is an at- tempt to provide the most comprehensive review of this matter to date and this article should therefore be of interest and value for those working in the atmospheric analysis field. A review of typical interferences

3635

3636 D. HELMIG

observed in various procedures for the analysis of VOCs (including organic sulfur compounds) is given below. Because of the similarity of intereferences observed in the analysis of hydrogen peroxide (H202), selected references to that literature are also discussed briefly. The second part of the paper reviews developed techniques to circumvent analytical interferences from ozone reactions.

OBSERVED INTERFERENCES

A tabulated summary of the most elaborate descriptions of observed interferences in VOC analysis from reactions with ozone is given in Table 1. Analyti- cal techniques applied, compound classes analyzed, the range of ozone concentration, the kind of interference and the respective references are listed. A narra- tive description sorted by analytical techniques is given in the following.

Cryogenic enrichment

Many techniques used for atmospheric VOC analysis include sampling by cryogenic freezeout. Ozone melting and boiling points at atmospheric pressure are at - 192.1 and - 111.9°C, respectively (Hand- book o f Chemistry and Physics, 1964). During direct cryogenic freezeout of organic gases (for example with liquid argon [boiling point - 186°C] or liquid nitrogen [boiling point - 196°C]) from the atmosphere, ambient ozone is physically concentrated together with the organic trace gases. The ozone concentration thereby may be increased by several orders of magni- tude since nitrogen and oxygen, the main constituents of air do not quantitatively condense under these conditions (Donahue and Prinn, 1993). Reactions with ozone can occur when the concentrate is heated for sample transfer or injection onto a gas chromatographic (GC) system (Goldan, 1990; Montzka et al., 1993; Goldan et al., 1995).

Analytical interferences from ozone on the sampling of dimethylsulfide (DMS, [(CH3)2S]) have been thoroughly investigated (Saltzman and Cooper, 1988; Goldan, 1990; Cooper and Saltzman, 1993; Ferek and Hegg, 1993). Goldan (1990) showed that the presence of ozone in the sample air reduced the recovery of SO2, dimethyldisulfide, CH3SH and DMS during cryogenic enrichment followed by heating to 100°C and GC injection. In these experiments DMS was completely destroyed at atmospheric ozone levels as low as 20 ppb. Johnson and Bates (1993) report more than 90% loss of 70 ppt DMS in a Teflon cryogenic trapping loop by the addition of 40 ppb of ozone. Substantial analyte loss from reaction with ozone was also observed in the trapping and analysis of DMS by deposition on gold wool (Andreae et al., 1985). The actual mechanism of the interference is not well understood. Cooper and Saitzman (1993) hy- pothesized that losses result from the production of reactive radical species from interactions of ozone

with surfaces in the sampling system. It is also ques- tioned that losses of DMS are only from reactions with ozone. Davison and Allen (1994) demonstrated that DMS trapped on Molecular Sieve 5A was par- tially destroyed even after ozone was quantitatively removed from the sampling stream. They suggested that ozone is not solely responsible for the observed DMS sampling losses, but that other potential oxidants such as nitrogen dioxide and hydrogen peroxide as well as reactive radicals including the hydroxyl radical and peroxy radicals may contribute to DMS losses under different sampling regimes. Results from an intercomparison of tropospheric measurements of DMS by six different methods with different ozone scrubbing techniques were given by Gregory et al. (1993).

In an extensive study on ozone effects during the cryogenic preconcentration of light C2-C4 saturated and unsaturated hydrocarbons Koppmann et al. (1995) found 2-10% losses of unsaturated compounds. However, they concluded that for mixing ratios of up to 100 ppb no statistically significant concentration changes occur. Donahue and Prinn (1993) found in laboratory experiments that in the use of their inlet system, synthetically generated air samples containing several ppt of C2-C5 alkenes and roughly 100 ppb of ozone showed no signs of alkene removal during cryotrapping. The authors speculated that this observation could be from ozone losses during sampling or by distillation effects during the cryotrapping which could separate ozone from the alkenes. In other studies it has been shown that unsaturated hydrocarbons, such as isoprene and monoterpenes can be depleted in cryogenic freezeout methods in the presence of ozone. Identified isoprene reaction products include methacrolein and methyl vinyl ketone (Goldan et al., 1995). Greenberg et al. (1992) reported complete loss of ethene and propene for in situ cryogenic enrichment at ozone levels above 30ppb. By collecting ambient air into stainless steel canisters prior to the analysis with cryogenic freezeout techniques, ozone reactions of this kind seem to be significantly reduced and analyte losses are minimal because of the short ozone lifetime (esti- mated to be in the order of minutes) in the stainless steel canisters (Greenberg et al., 1992; Singh and Zimmerman, 1992).

Solid adsorbent sampling

In the sampling of organic trace gases onto solid adsorbents, it was found that unsaturated compounds (such as styrene, cyclohexene and monoterpenes) adsorbed on Tenax TA can undergo reaction with ozone during ambient sampling leading to diminished analyte concentrations (Roberts et al., 1984; Pellizzari and Krost, 1984; Pellizzari et al., 1984; Jiittner, 1988b; Buffer and Wegmann, 1991; Janson and Kristensson, 1991; Hoffmann, 1995; Calogirou et al., 1996) and the formation of oxidated reaction products (Helmig and Arey, 1992; Calogirou et al., 1996). Bunch and

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Pellizzari (1979) found that organic gases adsorbed on Tenax GC can undergo transformations by reactions with mixtures of nitrogen oxides, chlorine and ozone. Cartridges loaded with cyclohexene showed the presence of the oxidation product cyclohexene-2-one after sampling of 16Y of 100ppb and 720ppb ozone enriched air. Experiments by Venema et al. (1983) indicated that aromatic and saturated compounds adsorbed on Tenax are stable even at more than 10 x above ambient ozone levels.

The most thoroughly studied compound class are biogenic terpenoids. Peters et al. (1994) measured substantial losses for ct-pinene (8% loss), fl-pinene (17%), 3-carene (22%) and limonene (56%) when a 1.5 { air sample containing 50 ppb ozone was drawn through Tenax sampling tubes that previously had been loaded with known amounts of the listed monoterpenes. Likewise, Jiittner (1988b) found substantial loss rates for a series of monoterpenes at ambient ozone levels during sampling of forest air on Tenax. The highest losses were observed for limonene (97% loss) and ~-pinene (86%). Loss rates reported by Janson (1993) on Tenax adsorbent were significant but lower. Depletion of up to 50% was observed for limonene when preloaded tubes were exposed to 1 ( air with 60 ppb ozone. Depletion for 3-carene was on the order of 12-20% while ct-pinene, fl-pinene and camphene were recovered at 94-100%. Similar results were reported by Hoffmann (1995) in the sampling onto a dual-bed Tenax/Carbopack B adsorbent tube. At 23 ppb ozone ambient air concentration, loss rates for six monoterpenes were studied and were highest for limonene (approximately 50%) and myrcene (approximately 70%). Hoffmann (1995) also investigated loss rates of sesquiterpenes and found that the major- ity of the nine sesquiterpenes studied showed significant destruction (up to > 95%) during ambient air sampling when ozone was not removed from the sampling stream. Roberts et al. (1983) concluded that depletion of ~-pinene, fl-pinene and limonene was minimal on Tenax GC (< 5%) when sampling vol- umes were kept below 1.0 to 1.5 standard liters.

A thorough report on decomposition of terpenes by ozone (8-150 ppb) during solid adsorbent sampling was recently published by Calogirou et al. (1996) using Tenax. In this study, it was found that the depletion by ozone depends on the chemical structure of the monoterpene. Saturated terpenoids such as 1,8- cineole, camphor and bornyl acetate were unaffected by ozone. Terpenes and terpenoids containing one C - C double bond were only slightly decomposed while compounds containing two or more double bonds showed significant decomposition. Analytical recoveries were enhanced for many compounds when the sampling times were reduced from 10 min to 30 s.

In contrast to the studies discussed above, experiments reported by Steinbrecher et al. (1994) did not show a statistically significant loss of adsorbed monoterpenes on Tenax TA when tubes loaded with ng- amounts of the monoterpenes tricyclene, ~-pinene,

camphene, sabinene and myrcene were flushed with 100 ppb ozone enriched air for 10 min at a flow rate of 150mlmin -1. Kesselmeier et al. (1996) noted that during their laboratory experiments only low degradation of terpenes took place on carbon adsorbents at ozone concentrations of 120 ppb. Similar results were found for Tenax. Consequently, these two groups found that the use of additional ozone scrubbers was not necessary for the technical design used in these measurements. They concluded that "removal of ozone prior to trap heating was found to be essential in preventing VOC decomposition". This conclusion infers the possibility of at least partial adsorption of ozone on the solid adsorbents which then would be released during the thermal desorption step to react with the concentrated monoterpenes. This assump- tion, to date, lacks experimental evidence. Venema et al. (1983) placed an ozone indicator tube in series with a Tenax trap and demonstrated that ozone is efficiently removed by the Tenax trap. However, ozone adsorption on solid adsorbents followed by a later thermal release has not yet been experimentally observed. Also, this process appears quite unlikely because of the labile nature and high reactivity of ozone and therefore an irreversible adsorption or destruction of ozone on solid adsorbents appears to be most likely. From the technical descriptions given by Steinbrecher et al. (1994) and Kesselmeier et al. (1996), it is not clear what differences in the experimental design and procedures resulted in these contradictory findings.

Cooper and Saltzman (1993) observed the conver- sion of DMS to dimethylsulfoxide and dimethylsul- fone in addition to the appearance of elevated levels of organic artifacts when samples were collected on Ten- ax adsorbent without the use of an oxidant scrubber. Artifacts from direct ozone reaction with solid adsorbents themselves were identified in numerous other studies. The most extensively studied adsorbent is Tenax. Frequently identified products from ozone-- Tenax reactions include benzaldehyde, phenol and acetophenone (Mattsson and Petersson, 1982; Roberts et al., 1984; Ciccioli et al., 1984; Walling et al.,

1986; Helmig and Arey, 1992; Cao and Hewitt, 1994; Bowyer and Pleil, 1995). Furthermore, Roberts et al.

(1984) report on the formation of benzene, toluene, C7 to C9 n-alkanes and C6 to C9 n-aldehydes in an ozone - Tenax exposure experiment. Recently, Clausen and Wolkoff (1997) identified a series of semivolatile VOCs in addition to the compounds listed above in ozone-Tenax exposure experiments. The presence of these artifacts can be an interference in qualitative and quantitative VOC analysis. The use of carbon based solid adsorbents, such as Carbotrap and Carbosieve, has recently increased while applications of Tenax are diminishing. The carbon-based adsorbents seem to be more resistant against ozone oxidation (Cao and Hewitt, 1994; Bowyer and Pleil, 1995; Helmig, 1996). Interferences of nitrogen oxides in the sampling with the solid adsorbents Tenax-GC and XAD-2, such as

Ozone removal techniques 3639

the formation of rautagenic reaction products from XAD-2, were reported by Hansen et al. (1981, 1984).

Derivatization techniques

The most extensively studied derivatization technique for atmospheric analysis is the collection of aldehydes and ketones by reaction with DNPH on coated sampling cartridges. Ozone interferences are usually investigated by flushing a cartridge with ozone. Several kinds of interferences caused by either the simultaneous sampling of ozone in the sample air or by the sequential sampling of aldehydes and ozone have been identified: (1) Depletion of the derivatizing agent DNPH, (2)depletion of the carbonyl hydrazones from the previous or simultaneous sampling of carbonyl compounds (negative interference), (3) formation of carbonyl hydrazones as artifacts (positive interference), (4) formation of other DNPH products that may interfere in the subsequent chromatographic separation and detection of the target compounds. These effects have been studied extensively, however results reported in the literature are somewhat contradictory. It appears that the level of ozone interference depends on the kind of carrier material used and on subtle differences in the preparation of cartridges (for example, coating technique, cartridge tubing material, amount and packing of carrier material, vendor), the hydrazone elution technique and the analytical procedures. Only a few selected studies are reviewed here and the interested reader should consult the referenced literature for further and more detailed information.

Ozone interferences on DNPH-coated silica gel cartridges (such as depletion of DNPH and carbonyl- hydrazones [negal:ive interferences]) have been observed by most investigators using this technique (Arnts and Tejada, 1989; Slemr, 1991; Sirju and Shep- son, 1995; Parmer and Ugarova, 1995; Kleindienst et al., 1995). DNPH cartridges made on octadecyl based carrier material (such as Sep-Pak C-18) seem to be somewhat more tolerant towards ozone. Arnts and Tejada (1989) four~d that DNPH/C-18 cartridges did not show ozone interferences in the analysis of formaldehyde with up to 120 ppb of ozone, but the formation of extraneous peaks occurred in their HPLC chromatograms. No interferences were observed by Druzik et al. (1990) and Zhou and Mopper (1990) in their analysis of carbonyl compounds with this material. In contrast, results by Parmer and Ugarova (1995) indicate losses of DNPH and hydrazones on DNPH/C-18. Kootstra and Herbold (1995) showed substantial artifact formation of carbonyl-hydrazones on DNPH/C-18 cartridges at 400 ppb ozone. A positive interference was also found for aldehydes above C3 by Zhou and Mopper (1993). Arlander et al. (1995) observed in their laboratory tests that reaction of ozone with the c~.rtridge material (Waters Sep-Pak C-18 cartridges) yielded C3-C 7 aldehydes in addition to formaldehyde and acetaldehyde. Sirju and Shepson (1995) found losses of DNPH and of formaldehyde

hydrazone on both silica and C-18 coated DNPH cartridges after exposure to 42 ppb of ozone. They also observed losses of the acetaldehyde and acetone hydrazones and the formation of several extraneous peaks in chromatograms of ozone exposed samples.

Methods relying on derivatizing carbonyls in solution rather than on an impregnated solid carrier material appear to be less prone to ozone interferences. Smith et al. (1989) worked with a DNPH impinger system and found depletion of DNPH from ozone reaction as well as the artificial formation of additional hydrazone compounds. Despite these reactions, formaldehyde measurements at low levels of ozone (16 ppb) and high levels of ozone (514 ppb) were consistent. These results are in agreement with the impinger study by Arnts and Tejada (1989) who observed losses of DNPH upon exposure to ozone while formaldehyde-hydrazone levels remained unaffected. A DNPH gas-liquid scrubber method developed by Lee and Zhou (1993) relies on a short air-liquid contact in a coil sampler. Inter- ferences from aqueous-phase reactions of ozone were found to be insignificant in this method. Detailed in- vestigations and discussions of ozone interferences in DNPH derivatization techniques are reported in articles by Vairavamurthy et al. (1992, 1993), Sirju and Shepson (1995), Parmer and Ugarova (1995) and Kleindienst et al. (1995).

In the use of dansylhydrazine (DNSH) impregnated porous glass beads for aldehyde trapping, up to 50% reagent depletion from ozone reaction was observed in the sampling of air containing 150 ppb ozone (Nondek et al., 1992). In a later study DNSH depletion was found to be ,,~ 40% at 300 ppb. This depletion was reduced to ~ 25% when octadecylsilane sorbent material was used instead of silanized glass beads (Rodier et al., 1993). The formation of several artificial carbonyl compounds from ozone reactions was also described by Vairavamurthy et al. (1993) using O-(penta- fluorobenzyi)hydroxylamine (PFBOA) as a derivatizing agent on C-18 cartridges for carbonyl analysis.

Hydrogen peroxide

A wide variety of analytical methods have been developed for the analysis of hydrogen peroxide (H202). Interference of ozone in sampling of H202 is very complex. In their review on the atmospheric chemistry of peroxides, Gunz and Hoffmann (1990) devote ample attention to methods for H202 analysis and to chemical interferences including ozone. Posit- ive interferences from ozone were reported in numerous studies discussed in their review. Extensive studies of the formation of H202 from reactions of ozone in glass inpingers (Heikes, 1984), in diffusion scrubbers and a scrubbing coil used for H202 sampling (de Serves and Ross, 1993) have been reported. 50 ppb ozone resulted in an increase in the HzO2 signal of 5 ppt for a coil and 35 ppt for a diffusion scrubber method (de Serves and Ross, 1993). Tanner et al, (1986) found that in the aqucoua-phase analysis by an enzyme-catalyzed fluorescence technique,

3640 D. HELMIG

concurrently collected ambient ozone increased signal levels to as much as 2-3 times over their response for ambient H202. In a recent paper by Staffelbach et al. (1995) artifact formation of H202 during cryogenic preconcentration was shown to be dependent on the freezeout temperature and on the presence of other VOCs such as ethene and isoprene. For a more comprehensive discussion of ozone interferences in H202 analysis refer to Gunz and Hoffmann (1990).

OZONE REMOVAL TECHNIQUES

Reactions with oxidants can be reduced or eliminated by selectively removing the respective oxidant in the sample flow prior to the concentrating of the organic trace gases of interest. Physical techniques that have been used for ozone removal in atmospheric sampling include e.g. annular denuder techniques (for example: Parmer and Grosjean, 1990; Williams and Grosjean, 1990; Schmidt et al., 1995; Possanzini and Di Palo, 1995), sampling through tubes containing a crystalline reactant retained by glass wool plugs (for example: Montzka et al., 1993, 1995; Goldan et al., 1995; Sirju and Shepson, 1995), impregnated filter or glass wool techniques (Bunch and Pellizzari, 1979; Pellizzari et al., 1984; Greenberg et al., 1994; Bates et al., 1990; Pellizzari and Krost, 1984), metal oxide catalysts (for example: Hoffmann et al., 1993; Hoffmann, 1995; Calogirou et al., 1996) and gas-phase titration of ozone with nitrogen oxide (for example: Tanner et al., 1986; Sirju and Shepson, 1995). In the sampling onto solid adsorbents or reaction cartridges, a direct pretreatment of the adsorbent (Strrmvall and Petersson, 1992) or dansylhydrazine coated derivatization cartridges (Nondek et al., 1992) with antioxidants has also been used. Koppmann et al. (1995) found up to 50% destruction of ambient ozone by pulling the sample air through stainless steel inlet lines kept at 340 K. An extensive study on the room-temperature catalytic decomposition of ozone involving thirty-five different materials was done by Ellis and Tometz (1972). However, this report focused on the destruction rates of ozone on these different materials only and did not consider the reaction of the scrubbers towards other air constituents and potential applications for VOC sampling. Also, the test regimes were quite unusual with test flow rates at 1 ft a min- 1, which is significantly higher than sampling rates used in most VOC analysis techniques.

This review concentrates on chemical reactants and techniques that have either been tested and applied in different technical systems or appear suitable for specific 03 removal in the analysis of atmospheric VOCs, such as unsaturated hydrocarbons, aldehydes and sulfur containing compounds. These methods are discussed in alphabetical order below. A tabulated summary of the most significant techniques and applications is given in Table 2.

Aluminum oxide (A1203): Alumina has been discussed for its ozone decomposition potential in the context of solid rocket motor exhaust (Hanning-Lee et al., 1996). Analytical applications have not been reported and the potential for use in VOC sampling appears minimal because of the strong adsorption strength of this material towards VOCs.

Copper oxide (CuO): A CuO filled cartridge was used for ozone removal in aldehyde/DNPH sampling. No losses of the carbonyl compounds on this material were observed (Vairavamurthy et al., 1993).

Cotton: Cotton wadding has been used as an ozone scrubber in the analysis of sulfur compounds such as hydrogen sulfide [H2S], DMS, and carbonyl sulfide [COS] (Hofmann et al., 1992; Bandy et al., 1993; Andreae et al., 1990, 1993, 1994, 1995; Persson and Leck, 1994; Leck and Persson, 1996). The material used by Persson and Leck was commercial 100% cotton which was chemically clean, bleached, and defatted with a mean fiber length of 16 mm. In the testing of their material Andreae et al. (1993) found quantitative ozone destruction for 4.5 h sampling time in a 80ppb test atmosphere flowing at 3 ~min- through cotton wadding packed into 47-mm diameter Teflon filter holders. Extensive comparisons between cotton scrubbers and Na2CO3/Anakrom scrubbers (see below) were done by Andreae et al. (1990, 1993). These studies conclude that both materials yield reliable analysis of DMS in ambient samples under most conditions. Better performance of the Na2CO3/Anakrom scrubber was observed in dry air/high ozone conditions. Losses for CH3SH on cotton wadding were observed by Hofmann et al. (1992). The overall capacity and efficiency of this scrubber and possible contaminant formation from ozone- cotton reactions were not reported. A combined USP cotton/potassium hydroxide ozone scrubber was used by Blomquist et al. (1993) for DMS analysis. Unsatis- factory results were obtained in the use of polyester cotton (Andreae et al., 1993).

Ferrous sulfate (FeSO4): A not further specified ferrous sulfate trap was used by Watts (1989) for ozone removal in the analysis of the atmospheric sulfur compounds DMS, dimethyl sulfoxide and dimethyl sulfone.

Indigo-carmine: Glass beads and quartz coated with indigo-carmine (common name Acid Blue 74; 5,5'-indigosulfonic acid, di-sodium) were used for ozone removal by Jiittner (1988a, b) in the analysis of monoterpenes in forest air by solid adsorbent sampling. In a later study Jiittner (1988b) noted that this scrubber suffered from water clogging when humid samples were acquired. Better results were obtained by this author using a different commercial (Ante- chnika, Ettlingen, Germany) MnO2 ozone scrubber usually used to obtain ozone-free air in the calibration of ozone analyzers (see below).

Magnesium sulfate (MgSO4): A magnesium sulfate desiccant and ozone trap in combination with a nation dryer and an Ascarite carbon dioxide scrubber was used by Donahue and Prinn (1993) for C1-C5


hydrocarbon measurements during SAGA 3 by a VOC cryotrapping technique. Laboratory experiments performed prior to the cruise showed that the magnesium sulfate trap was able to effectively remove at least 100 ppb of ozone from the sample air stream. Atlas et al. (1993) found in their tests with this material that all efficiency in removing ozone was lost after 1 h.

Manganese dioxide (MnO2): MnO2 scrubbers have been used by several authors mainly in the sampling of biogenic VOCs onto solid adsorbents. The obtained results are somewhat contradictory at this time. While some articles report promising results regarding ozone scrubbing efficiency and VOC recovery rates, other studies revealed problems from VOC losses on this material. It appears that subtle differences in the production and design of the MnO2- coated carrier ma~terial may be responsible for the different findings. A MnO2-coated copper screen was used by Hoffmann et al. (1993) and Hoffmann (1995) upstream of the sampling tubes to prevent losses of biogenic compounds during sampling on Tenax TA/Carbopack B adsorbent cartridges. These nets were taken out of .an ozone analyzer usually used to generate zero air reference gas for the calibration of the instrument. Hoffmann (1995) showed that no losses of monoterpenes on these scrubbers occurred and that they eliminated interferences from ozone in the sampling of monoterpenes and sesquiterpenes under laboratory and ambient conditions. In a later study, Hoffmann 111996) found losses of polar, unsaturated terpene alcohols (e.g. linalol) on their MnO2 scrubber material. Likewise, Jiittner (1988b; 1996) sampled monoterpenes in forest air through MnO2- coated copper scr,~en material taken from a commercial ozone analyzer. Recoveries of several monoterpenes sampled at 120 pgm -3 ozone concentration were significantly higher in the filtered air samples than in non-filtered air. For instance, limonene, ct-pinene and fl-pinene concentrations were approximately a factor of 32, 7 and 2, respectively, higher in the filtered air. Several non-reactive hydrocarbons and halogenated hydrocarbons showed no significant differences, while A3-carene, camphene, camphor and butyl acetate actually exhibited losses on the scrubber and consequently yielded higher results in non-filtered air. The author notes that after 3 months of continuous use the scrubber was still active. A not further specified MnO2 ozone scrubber was used by Janson and Kristensson (1991) to prevent ozone induced monoterpene losses on Tenax. A 30% loss of terpenes was observed on their scrubber during the sampling of ambient air and this scrubber was not found suitable for their work. In contrast MnO2- coated copper nets were found to give the best results in the sampling of terpenes on Tenax by Calogirou et al. (1996). The materials used in this study were again small nets which were cut out from larger nets to be used in commercial ozone analyzers. The optimum number of layers c f these nets to be used was eight. This scrubber was found to enhance recovery of most

terpenes significantly in laboratory experiments and in ambient air sampling. Kleindienst et al. (1995) tested a MnO2 scrubbers for the analysis of formaldehyde and found that in addition to ozone, MnO2 removed formaldehyde quantitatively.

Meta l surfaces: Various metal surfaces were investigated by Kaschtanoff et al. (1936) for their catalytic destruction of ozone. It was found that aluminum, copper, lead and tin had low ozone scrubbing efficiency whereas the metals silver, iron, zinc, gold, nickel (see below), mercury and platinum had high ozone depletion efficiencies. Alloys containing the later metals were a stronger ozone scrubbers than the pure metals. The effect of ozone destruction by certain metal surfaces was later observed by various groups as reflected in the short lifetime of ozone in canisters (see above). One application of the use of nickel tubing for ozone removal is described below.

Nickel tubing: In their study on terpenes and related compounds in ambient air, Riemer et al. (1994) found that by drawing the sample air through 0.32 cm nickel tubing, ozone levels were reduced to less than approximately 20% of the ambient level and allowed the interference-free analysis of biogenic emissions and oxidation products.

Nitrogen oxide (NO) titration: The use of NO for scavenging of ozone as a measure to prevent losses of VOCs was first reported by Holdren et al. (1979) who added NO to stainless steel canisters to achieve a 100 ppb NO concentration in monoterpene analysis. Seila (1981) spiked 10 f teflon bags with 15 ml of a 38 ppm mixture of NO in nitrogen prior to sampling of hydrocarbons. This mixture destroyed any ozone present in the air sampled. The scavenging reaction is

O3 + NO - - , O 2 --[- NO2 (1)

with a reaction rate constant of k (298 K, 1 atm) = 1.8 x 10-14 cm a molecule- i s - 1. Tanner et al. (1986) and Shen et al. (1988) developed a system for H202 analysis, in which ozone concentrations in the sample flow were substantially lowered by this gas-phase titration with nitrogen oxide. A small flow of NO (200 ppm) in nitrogen was admitted to the sample flow to produce a concentration of approximately 6 ppm (Tanner et al., 1986) or 2 ppm (Shen et al., 1988). Ozone lifetime under these conditions is in the order of less than a second. The ozone reduction efficiency is increased by allowing sufficient reaction time and by adjusting to a large NO excess concentration. The reactant mixture is therefore flown through a mixing or reaction chamber. The same technique was also used by Sirju and Shepson (1995) for ozone destruction in the analysis of aldehydes by the DNPH technique. The ambient air sampling flow was mixed with NO resulting in a 2 ppm NO concentration. Quantitative depletion was achieved in a 1 f glass reaction vessel at a residence time of 20 s. Keuken et al. (1988) added NO to a sampling stream for eliminating interferences from ozone in the analysis of NHa, HNO3, HC1, SO2, and H20 2 by a wet annular

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denuder system. While this technique seems suitable for aldehyde analysis with DNPH, Kuster et al. (1986), report unacceptable chromatographic interferences in the use of titration of ozone with NO in a Teflon vessel for the analysis of non-methane hydrocarbons. Interferences from NO and the subsequently formed NO2 in the sample gas are therefore a potential problem which warrants careful investigation. In- terferences of nitrogen dioxide in the determination of aldehydes and ketones by sampling on DNPH were studied by Karst et al. (1993). Two reaction products were identified that potentially may interfere in the liquid chromatographic determination of formaldehyde-hydrazone. Sirju and Shepson (1995) extensively tested the effects of NO and N O / o n aldehyde DNPH sampling. They concluded that NO, at the concentration present in the reaction vessel as well as at above urban maximum concentrations does not interfere in the determination of gas-phase formaldehyde when using DNPH-coated silica gel cartridges. Results for NOz were somewhat inconclusive and difficult to relate to the conditions present in the NO titration reaction system. Hanson et al. (1981, 1984) found that Tenax is prone to formation of artifacts upon exposure to elevated levels of NO~.

Pal ladium~plat inum: Aluminum oxide coated with palladium/platinum has been used as an ozone scrubber in the analysis of total oxidized nitrogen (NOr). 1% platinum on 3.2 mm AI20 3 beads kept at 450°C was used by Hills to reduce ozone levels from approximately 1% to less than 1 ppb (Hills, 1996). A significant fraction of the ozone destruction in this technique is probably from thermal depletion. Because of the stringent reaction conditions and the absorptive properties of aluminum oxide for VOCs it appears unlikely that this method can be used for VOC analysis.

P h e n o x a z i n e (C12H9ON): Phenoxazine reacts selectively with ozone at ambient air concentrations (Lambert et al., 1987). Annular glass denuders treated with a 0.4 g solution of phenoxazine in 20 ml acetone were used by Williams and Grosjean (1990) and found to quantitatively remove ozone. Applications for VOC analysis have not been reported yet.

Polye thy lene: Kuster et al. (1986) used a 1 ( polyethylene vessel with a residence time of approximately 5 rain to reduce ozone to less than 5% in a sample for DMS analysis. Goldan (1990) found that by flowing a sample with 100 ppb ozone through a polyethylene bottle the ozone concentration was reduced to approximately 5-10%. Davison and Allen (1994) used 4 collapsible polyethylene containers, in which 100 ml of a solution of 2gg '-1 sodium hydroxide and 5 g ~ - i potassium iodide was added, for oxidant removal in DMS analysis in polluted atmospheres. Atmospheric samples were stored in these containers for 12 h in the dark. While DMS was quantitatively retrieved from the containers after this procedure, oxidants were depleted to allow interference-free DMS analysis.

Potas s ium carbonate (K2CO3): A trap filled with potassium carbonate was used by Martin et al. (1991)

in the analysis of isoprene and its oxidation products methacrolein and methylvinyl ketone. K2CO3 in copper tubing was originally used as a desiccant by Westberg et al. (1974). A 100% transmission of light hydrocarbons was found. A K2CO3/glycerine coated glass fiber filter used by Goldan (1990) dropped from initially ~ 100% efficiency to ~ 90% ozone removal during a 2 h sampling experiment at 100 ppb ozone concentration and a flow rate of 100 ml min-1.

Potas s ium hydrox ide (KOH): Potassium hydroxide/glycerin coated glass fiber filters used by Goldan (1990) and Bates et al. (1990) were shown to have a high ozone removal capacity and allowed the interference-free sampling of DMS. Goldan (1990) prepared filters by dipping in a 15% aqueous KOH/7% glycerin solution and drying at 120°C. The same kinds of filters were used by Ferek and Hegg (1993) for DMS sampling. A KOH impregnated glass fiber filter was also used by Quinn et al. (1990) to eliminate oxidant interferences in DMS sampling. KOH between layers of USP cotton was used as an oxidant scrubber for DMS measurements by Blomquist et al. (1993). A KOH coated annular glass denuder used by Williams and Grosjean (1990) failed to remove ozone under the conditions applied in their study. A comparison between KOH and sodium carbonate and sodium sulfite ozone traps was given by Kuster et al. (1990).

Potas s ium iodide (KI): A rather frequently used technique for ozone removal relies on the reaction of ozone with potassium iodide (KI). 03 traps utilizing this reaction have been used in numerous studies in the analysis of atmospheric organic trace gases and were shown to effectively remove Oa at ambient levels as well as in smog chamber experiments. Most applications were for the analysis of aldehydes by the DNPH method. Scrubber systems used include KI impregnated glass wool and filters, KI coated denuders and impingers filled with a KI solution (Saltzman and Cooper, 1989; Arnts and Tejada, 1989; Williams and Grosjean, 1990; Parmer and Grosjean, 1990; Zhou and Mopper, 1990; Slemr, 1991; Grosjean et al., 1992; Kit- tler et al., 1992; Greenberg et al., 1994, 1996; Biesenthal et al., 1994; Helmig and Greenberg, 1994; de Andrade et al., 1995; Kootstra and Herbold, 1995; Sirju and Shepson, 1995; Kleindienst et al., 1995; Possanzini and Di Palo, 1995; Helmig et al., 1996; Slemr et al., 1996; Possanzini et al., 1996; Shepson et al., 1996).

Greenberg et al. (1994) made their traps by soaking glass wool in a solution of KI (3 g), methanol (5 ml), water (10 ml) and glycerol (2 ml). The impregnated glass wool was dried in a flow of zero air and traps were prepared by filling the impregnated glass wool in 51 m m x 5 . 3 m m ID stainless steel tubing. Both, Kootstra and Herbold (1995) and Kleindienst et al. (1995) coated the inside of a 1 m x 4.6 mm ID copper tube with KI by purging it with a KI saturated aqueous solution and following dry purge with nitrogen. Slemr (1991) found about 20-100% higher recoveries for formaldehyde and acetaldehyde from DNPH


coated silica gel cartridges when ambient air at 12 to 57 ppb ozone was first pulled through Teflon tubing filled with KI crystals.

Another application field of KI is DMS sampling in marine atmospheres (Saltzman and Cooper, 1989; Cooper and Saltzrnan, 1991, 1993; Kittler et al., 1992; Davison and Allen, 1994; Wylie and de Mora, 1996; Davison et al., 1996). A buffered 2%-KI solution contained in a glass fitted bubbler was used by Saltz- man and Cooper (1989) and Cooper and Saltzman (1991, 1993) for ozone removal in DMS analysis. The bubbler was imme~rsed in an ice water bath to lower the water vapor concentration of the air being sam- pied. This scrubber gave quantitative recovery of standards of DMS added to ambient air at different ozone pollution levels. Kittler et al. (1992) extensively studied KI for use as an oxidant scrubber in the sampling of atmospheric DMS. Their work resulted in the development of a KI indicating oxidant scrubber. Glass fiber filters were soaked in a solution of 5% KI, 5% glycerol and 0.5% Vitex (a starch-based iodine indicator). Studies of the capacities of these filters were done by sampling with two filters placed in series. The KI/glycerol/Vitex filters turn blue/purple upon reaction with oxidants and the appearance of this color on the second filter did indicate oxidant breakthrough of the first filter. The breakthrough volume of KI/glycerol/Vitex filters was determined to 263 f and compared favorably with other chemicals tested for filter impregnation, such as NaOH, KOH, Na2CO3. No losses of DMS on the KI/glycerol/Vitex filters were observed.

In a later study, Davison and Allen (1994) included KI in a series of scrubber materials tested for oxidant removal in their DMS sampling. They found that KI crystal scrubbers were adequate to prevent DMS loss under remote marine air conditions but not suitable for sampling in polluted atmospheres. Instead, Davison and Allen (1994) stored their samples in polyethylene contai~ners and in gas sampling bags, to which a 100 and 21)3 ml solution of sodium hydroxide and KI was added, respectively, for oxidant scrubbing (see above under polyethylene).

Possanzini and Di Palo (1995) prepared ozone scrubbers by coating annular denuders consisting of two coaxial glass tubes (10cm×10mm and 10cm x 13 mm annulus diameter) with a saturated KI solution followed by drying under a nitrogen flow. The collection efficiency of these denuders was > 99.9% at an air flow of 1 •min- 1 and the capacity

was about 250#g ozone (2000ppb h-~). Parmer and Grosjean (1990) and Kleindienst et al. (1995) found that the capacity of their KI-coated denuders and scrubbers depended on the relative humidity level in the sample air. Strongly reduced capacities were observed in dry air while at ambient humidity levels ozo~ae scrubbing capacities were satisfactory. This observation appears plausible because the destruction of ozone by KI is believed to follow a surface reaction including water from atmospheric

moisture:

Oa + 2KI + H20 --~ 0 2 "[- 12 q- 2KOH. (2)

However, a different mechanism suggested by Hoigne et al. (1985) does not include water as follows:

03 + KI ~ 02 + KOI. (3)

At 50% relative humidity levels, Kleindienst et al. (1995) yielded capacities in excess of 25,000 ppb h for their KI denuders and scrubbers. They also recom- mend heating the denuder to 50°C in order to avoid formaldehyde loss.

It has recently been shown that, in the use of KI based ozone traps, reactive iodine products may be formed, such as I2 or I O - (Possanzini et al., 1984; Parmer and Grosjean, 1990; Williams and Grosjean, 1990; Helmig and Greenberg, 1995) which can cause the formation of alkyl iodide artifact compounds in sampling procedures involving solid adsorbent sampling (Helmig and Greenberg, 1995). A possible measure to eliminate the liberation of I2 may be the addition of sodium arsenite to the reacting agent (Possanzini et al., 1984). Besides their use as ozone scrubbers KI coated denuders were also shown to efficiently remove NO2 for particulate nitrate collection (Possanzini et al., 1984).

Rubber: Schmidt et al. (1995) used an annular denuder coated with natural rubber (1,4-polyisoprene) to remove ozone in ambient nitrogen dioxide measurements. Ozone was reduced to below 1-2 ppb for at least 600 ppb *h. This technique has not yet been tested for analysis of organic trace gas species. It appears likely that contamination and interferences from the organic scrubber material may occur.

Sodium carbonate (NazCO3): Sodium carbonate is frequently used as an oxidant scrubber material in the sampling of atmospheric organic sulfur compounds. While a number of studies showed satisfactory properties in this application other studies revealed problems. Andreae et al. (1985, 1993), Berresheim (1987, 1993), Van Valin et al. (1987), Luria et al. (1989), Berresheim et al. (1990, 1991), and Staubes and Georgii (1993) used Na2CO3 (5%) on Anakrom C22 carrier material and Barnard et al. (1982) and Saltzman and Cooper (1988) used Na2CO3 on Chro- mosorb in the analysis of DMS. Saltzman and Cooper (1988) noted that the NazCO3/Chromosorb scrubbers had higher capacities than Na2CO3-coated pre- filters. In a later study Saltzman and Cooper (1989) noted that Na2CO3/Chromosorb failed under elevated ozone concentrations while Na2CO3/Anakrom scrubbers gave successful DMS recovery under these conditions. Recovery experiments by Berresheim (1987) did not show any DMS losses on Na2CO3/Anakrom. A 10% Na2CO3 on Anakrom C22 composition was used by Biirgermeister et al. (1990) and Biirgermeister and Georgii (1991). Goldan (1990) and Kuster et al. (1986) found that their Na2COa/Anakrom traps had only a very limited

3646 D. HELMIG

capacity, respectively were ineffective and consequently were not suitable for interference-free sampling of DMS in their work. The same results and conclusions were reported by Ferek and Hegg (1993). Na2CO3-coated annular glass denuders used by Will- iams and Grosjean (1990) did not yield quantitative ozone removal rates in their experiments. A comparison between sodium carbonate, sodium sulfite and potassium hydroxide ozone traps was presented by Kuster et al. (1990).

Sodium hydroxide (NaOH): Bates and Johnson (1990), Ayers et al. (1991) and Johnson and Bates (1993) used NaOH-impregnated glass fiber filters as an oxidant scrubber in the atmospheric analysis of DMS. Filters were prepared by immersion in a 0.1 M NaOH aqueous solution, followed by drying at 80°C in a nitrogen-purged oven. Davison and Allen (1994) included NaOH as one component in their technique for oxidant removal in DMS analysis (see above under KI). Bandy et al. (1993) used NaOH pellets to remove oxidant interferences in DMS analysis.

Sodium sulfite (Na2SO3): Montzka et al. (1993, 1995) used a tube filled with anhydrous Na2SOa crystals maintained at 50°C for ozone removal in the sampling of isoprene and the atmospheric isoprene oxidation products methacrolein, methylvinylketone and 3-methylfuran. This trap was found to remove over 99% of the ozone in a humid ambient air stream. Montzka et al. (1993) found inconsistent removal efficiencies in the use of Na2SO3 from different sup- pliers and from different batches. Therefore rigorous testing of individual ozone traps was required. A Na2SO3 trap was also used by Goldan et al. (1995). The trap consisted of a 1/4" glass tube filled with I g of Na2SO3 crystals held in place by glass wool plugs and maintained at 100°C to prevent clumping of the Na2SO3. The authors note that this trap was most efficient in the presence of atmospheric water vapor and hence had to be positioned upstream of a water trap. Sodium sulfite traps were compared to the use of sodium carbonate and potassium hydroxide ozone traps by Kuster et al. (1990).

Sodium thiosulfate (Na2S203): Sodium thiosulfate has been used successfully in numerous studies (Bunch and Pellizzari, 1979; Pellizzari et al., 1984; Pellizzari and Krost 1984; Helmig et al., 1997). The reaction between thiosulfate and ozone produces tetrathionate oxygen and water:

2S202- + 03 + 2H + ~ S4062- + 02 "F H20. (4)

The pH dependency of this reaction and possible implication for its application in the scavenging of ozone from the sample air stream is discussed in detail by Lehmpuhl and Birks (1996). Ozone scrubbers are typically prepared by soaking a glass fiber filter in aqueous Na2S203 solutions. Pellizzari and Krost (1984) loaded deuterated test compounds onto Tenax sampling tubes and measured the depletion of these compounds and the formation of deuterated oxida-

tion products in laboratory tests and also during ambient sampling. They found that by first flowing the sample through Na2S203-impregnated filters the formation of deuterated oxidation products was substantially reduced. Furthermore, the formation of ozone-Tenax reaction products diminished. This study also showed that Na2S203-treated glass fiber filters reduce sampling artifacts from reactions with halogens. Helmig et al. (1997, and unpublished results) found complete recoveries for the biogenic hydrocarbons isoprene, ct-pinene, fl-pinene and limonene and for a series of 55 aliphatic and aromatic hydrocarbons sampled through Na2S203-coated glass fiber filters. Filters were prepared by flowing a 10% solution of Na2S203 through commercial Acrodisc glass fiber filters (Gelman Sciences, Ann Arbor, MI) followed by dry purge with nitrogen and had capacities in excess of 1 m 3 air at ambient ozone levels. Str6mvall and Petersson (1992) introduced solutions of Na2S203 and sodium hydrogen carbonate directly into the inlet side of Tenax cartridges and found increased recovery for monoterpenes and reduced artifact formation from the reduction of rearrangement reactions. Lehmpuhl and Birks (1996) added sodium thiosulfate for ozone scavenging to cartridges treated with 2,4,6- trichlorophenylhydrazine used for aldehyde sampling. Sodium thiosulfate has also been used for removal of chlorine and chlorine dioxide in the analysis of chloroform (Eaton et al., 1996). Recovery of chloroform under both dry and moist conditions was not affected by the scrubber.

N,N,N' ,N'- tetramethyl- l ,4-phenylenediamine dihy- drochloride: This compound was used by Nondek et al. (1992) in the sampling of carbonyl compounds on microcartridges containing porous glass particles impregnated with dansylhydrazine (DNSH). The agent was added to the reagent solution at the time of cartridge preparation to serve as an ozone scavenger.

Triethanolamine: Triethanolamine ((HOCH2CH2)aN) in a 1 N KOH solution was used by Williams and Grosjean (1990) for coating of annular glass denuders. In their testing for ozone depletion by theses denuders they found that ozone removal could only be achieved at significant lower flow rates compared to phenoxazine and KI-coated denuders.

SUMMARY

Ozone interferences are being observed in many common analytical techniques for VOC analysis. Be- cause of the significance of the interferences consider- able attention has been given to the analysis of sulfur containing compounds, in particular of DMS, cryogenic preconcentration of unsaturated hydrocarbons, sampling of biogenic emissions on solid adsorbents and sampling of carbonyl compounds on DNPH impregnated carrier materials. Ozone interferences in the later method have been investigated for more than a decade, however it still does not appear


that the interference of ozone and the inhibition of these interferences on DNPH impregnated materials, such as silica gel or Sep-Pak C-18 have been conclusively resolved. It is striking that potential ozone interferences in the sampling of particulate organics on filter materials have not received noticeable attention to date. It is uncertain to what degree these effects may interfere in this sampling regime.

Criteria for the selection of a suitable ozone scrubber and the analytical requirements are numerous. The materials should ideally be: easy to use, inexpen- sive, efficient in the ozone removal rate and have a high scrubbing capacity, long lifetime and eliminate the effects of ozone without interfering with the analysis of the target compounds and without introducing contaminants. Furthermore, it should be universally applicable to allow the analysis of a wide range of compounds. Only very few of the above-described methods have been researched conclusively in all of these aspects. Most ozone removal techniques have only been researched for one particular application. Basically no effort has gone into researching and developing a more universally applicable ozone scrubbing technique.

Another desirable property is an indication mechanism to monitor the state and remaining capacity of the scrubber. Only the indicating oxidant scrubber developed by Kittler et al. (1992) allows to monitor the efficiency of the scrubber by visual indication. Most other techniques require measuring of the ozone concentration downstream of the scrubber for in situ monitoring of the ~crubber efficiency. However, several of the described systems have been proven to have high enough capacities to allow reliable opera- tion over extended sampling periods.

Commonly reported techniques for ozone scrubbers include impregnated filters, impregnated glass wool, coated tubes and coated annular denuders. The capacity of some systems depend on the relative humidity level of the sample air with strongly reduced capacities found for dry air. The most frequently used and best investigated chemical agents are sodium carbonate, sodium sulfite, sodium thiosulfate and potassium iodide. Certain agents, such as sodium carbonate and potassium iodide seem to work well for specific applications such as the analysis of sulfur compounds or for aldehyde and carbonyl analysis. More universal methods for the analysis of a wider range of VOCs are for example sodium sulfite and sodium thiosulfate. A promising technique that only recently has been applied (mainly for monoterpene analysis) are MnO2-coated screens. Even though reports at this time still seem somewhat contradictory and results for formaldehyde were unsatisfying, it appears that this approach should receive more future and thorough investigation for applications of a wider range of VOCs. Advantages of this technique are, for example, its simplicity, the high ozone destruction capacity and chemical inertness towards relative labile VOCs, such as monoterpenes.

Acknowledoements--I wish to thank numerous colleagues for communicating unpublished research and results. I would like to address special thanks to G. Kok, National Center for Atmospheric Research, Boulder, CO; R. Arnts, U.S. Environmental Protection Agency, Research Triangle Park, NC; David Lehmpuhl and Bradley Baker, both Uni- versity of Colorado, Boulder; P. Golden, National Oceanic and Atmospheric Administration and one anonymous reviewer for valuable discussions during the preparation of this review and for helpful comments on the draft manuscript, respectively. I also thank the National Center for Atmospheric Research for partial support during the preparation of this review. The National Center for Atmospheric Research is sponsored by the National Science Foundation.

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Ozone removal techniques in the sampling of atmospheric volatile organic trace gases