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Bioquimica, Un

Atmospheric Environment 41 (2007) 8796–8810

www.elsevier.com/locate/atmosenv

Primary product distribution from the Cl-atom initiatedatmospheric degradation of furan: Environmental implications

Florentina Villanueva, Ian Barnes1, Esperanza Monedero, Sagrario Salgado,M

%a Victoria Gomez2, Pilar Martin�

Departamento de Quımica Fısica, Facultad de Quımicas, Universidad de Castilla-La Mancha, Campus Universitario s/n,

13071, Ciudad-Real, Spain

Received 30 April 2007; received in revised form 17 July 2007; accepted 24 July 2007

Abstract

Furan is an aromatic hydrocarbon present in both urban and rural atmospheres, which is emitted mainly by biomass

burning and the combustion of fossil fuel. Reaction of furan and Cl atoms may be important in areas where chlorine atom

concentrations are potentially high such as marine and coastal regions or continental atmospheres where industrial activity

emits molecular chlorine or photo-labile Cl-containing compounds. To assess the importance of this reaction and to

investigate whether any unique chlorine-containing product is formed the products of the reaction of Cl atoms with furan

have been determined under atmospheric conditions. For the study two different sampling/detection methods have been

used: (1) Solid-Phase MicroExtraction, with subsequent analysis by thermal desorption, and gas chromatography with

mass spectrometry or flame ionization detection (SPME/GC-MS/FID), and 2-‘‘in situ’’ with long path fourier transform

infrared spectroscopy (FTIR). The yields of primary reaction products in the absence of NO were: 5-chloro-2(5H)-

furanone (64.5710.7)%, E-butenedial (1173)%, 5-hydroxy-2(5H)-furanone (p2.4%) and Z-butenedial (1.670.5)%.

Other products generated by secondary reactions were 2(3H)-furanone (2.871.9)%, HCl (21.173%) and CO. Maleic

anhydride was detected with a yield of about 2%, however, this yield may be a combination of both primary and secondary

reactions. All errors are 72s. The observed products confirm that addition of Cl atoms to the double bond of furan is the

dominant reaction pathway.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Furan; Chlorine atoms; Mechanism; Chlorofuranone; Reaction products; Quantitative determination

e front matter r 2007 Elsevier Ltd. All rights reserved

mosenv.2007.07.053

ing author. Tel.: +34 926295300;

5318.

ess: mariapilar.martin@uclm.es (P. Martin).

achbereich C – Physikalische Chemie, Bergische

ppertal, Gauss Strasse 20, D-42097 Wuppertal,

epartamento de Quımica Inorganica, Organica y

iversidad de Castilla-La Mancha, Spain

1. Introduction

The products from atmospheric reactions ofaromatic hydrocarbons are extremely complexbecause of the large number of photooxidationreaction pathways, which are possible for thesemolecules. A detailed understanding of the productdistributions arising from the photooxidation ofaromatic hydrocarbons is important because aro-matic compounds constitute a large fraction of the

.

ARTICLE IN PRESSF. Villanueva et al. / Atmospheric Environment 41 (2007) 8796–8810 8797

hydrocarbons emitted into urban atmospheres andthey are predicted to contribute significantly to theproduction of ozone in these areas (Calvert et al.,2002). Accurate formulation of air quality modelsrequires a fundamental understanding of the atmo-spheric fate of aromatic compounds.

Furan, also known as furane and furfuran, is anaromatic heterocyclic organic compound similar tobenzene but more reactive than benzene withrespect to electrophilic substitution reactions. Basedon ample evidence of malignant tumor formation atmultiple tissue sites in multiple species of experi-mental animals (IARC, 1995), furan is anticipatedto be a human carcinogen. Furan is produced whenwood, especially pine-wood, is distilled. Furan mayalso be released in the effluent from oil refining, coalmining, coal gasification and biomass burning(Andreae and Merlet, 2001; Ciccioli et al., 2001;Graedel et al., 1986; HSDB, 2001; IARC, 1995;Knudsen et al., 1993). Pyrolysis studies of biomass,cellulose, lignins, and similar materials, reportfuran, as one of the major degradation products(Antal et al., 1983, 1985; Graham et al., 1984;Milne, 1981; Shafizadeh, 1982; Soyer et al., 1988).Yields of furan (and derivatives) depend very muchon the severity of the pyrolysis, but typically theliquid pyrolysate from the pyrolysis of poplar woodhas been reported to contain about 10mg offuran g�1 wood (Soyer et al., 1988). This datum isimportant for the interest in the possibility of usingbiomass (Spliethoff and Hein, 1998) and wastewood (Adams and Harding, 1998) for NOx reduc-tion in large-scale combustion processes. Furan hasalso been observed from the OH radical-initiatedreaction of 1,3-butadiene (Ohta, 1984; Sprengnetheret al., 2002; Tuazon et al., 1999) and in thecyclization of unsaturated 1,4-hydroxyaldehydes,with subsequent loss of water (Baker et al., 2005).

A relatively complete understanding of the atmo-spheric transformations of furan and some of itsderivatives is necessary in order to integrate itstransformations into air quality models for toxicspecies and to improve regulatory quantitative riskdecision-making analysis.

Progress has been made in recent years inunderstanding the fate of furans and its alkylatedderivatives in the atmosphere. Studies have beenmainly focussed on the determination of kinetic andproduct data for the reactions of OH and NO3 withthese compounds (Atkinson et al., 1985, 1988;Atkinson and Arey, 2003; Berndt et al., 1997;Bierbach et al., 1992, 1995; Kind et al., 1996; Lee

and Tang, 1982). Although reactions with OH andNO3 radicals, and O3 are generally considered to bethe major transformation processes of gaseousorganic compounds in the troposphere, recentstudies show that Cl-atom reactions may also besignificant in the marine boundary layer, in coastalregions and in continental areas with high chlorineemissions (Ariya et al., 1999; Galan et al., 2002;Jobson et al., 1994; Keene, 1995; Keene et al., 1996;Knipping and Dabdub, 2003; Oum et al., 1998;Ramacher et al., 1997). The reason for this is thatmany organic compounds have rate constants forreaction with Cl atoms that are up to two orders ofmagnitude higher than those for the analogousreaction with OH radicals (Atkinson, 1997; Atkin-son and Arey, 2003; Finlayson-Pitts and Pitts, 2000;IUPAC, 2005).

Although there is evidence for the presence ofchlorine atom precursors in the lower troposphere,there is a paucity of data on such species, withrespect to both their spatial and temporal distribu-tion. The lack of widespread and readily availabletechniques for measuring them has limited thenumber of field studies directed toward elucidatingthe role of chlorine chemistry in the marineboundary layer. An alternate approach for investi-gating chlorine atom production in the troposphereis the identification and measurement of uniquechlorine-containing products, which would nototherwise be in the atmosphere except for Clreaction with organics. If unique chlorine-contain-ing products can be identified from the reaction ofCl with a particular organic, they could serve as‘‘markers’’ for chlorine atom chemistry in thetroposphere (Wang and Finlayson-Pitts, 2001).

Although there have been several studies on thereactions of Cl atoms with various classes of volatileorganic compounds, only a few studies on thereactions of Cl atoms with aromatic hydrocarbonshave been performed to date (Atkinson andAschmann, 1985; Fantechi et al., 1998; Finlayson-Pitts and Pitts, 2000; IUPAC, 2005; Market andPagsberg, 1993; Noziere et al., 1994; Shi andBernhard, 1997; Sokolov et al., 1998; Wallingtonet al., 1988, Wang et al., 2005). However, no studieshave been reported for furans, therefore, our workgroup has embarked on a course of research tocharacterize the reactivity of furan and its deriva-tives with chlorine atoms under atmospheric condi-tions. In a previous study from this laboratory, rateconstants for the reactions of Cl with furan, 2-methylfuran, 3-methylfuran, 2,5-dimethylfuran and

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2-ethylfuran were measured (Cabanas et al., 2005).The next step necessary to elucidate the atmosphericdegradation mechanism of these compounds is adetailed speciation of the reaction products andtheir yields.

Consequently, the current goals of the research inthis laboratory are to elucidate, through laboratorystudies, the mechanisms of the reactions of Cl atomswith furan and its derivatives in sufficient detail sothat they can be implemented in atmospheric CTmodels, and to search for unique chlorine-contain-ing compounds, which could serve as ‘‘markers’’ ofchlorine atom chemistry in the troposphere. Pre-sented here are the results from a product study onthe Cl-atom initiated photooxidation of furan.

2. Experimental

The experiments were performed in two differentreaction chambers equipped with two differentsampling and monitoring systems: (1) Solid-PhaseMicroExtraction with analysis by gas chromatogra-phy—mass spectrometry and flame ionizationdetector (SPME GC-MS/FID), (2) in-situ withanalysis by Fourier transform infrared spectro-scopy (FTIR). In both cases the experiments werecarried out at 29872K and 740Torr total pressureof air.

2.1. Analysis by gas chromatography

The experimental system is described in detail inCabanas et al. (2005). The experiments wereconducted in a �200L Teflon bag. This bag wasplaced inside a rectangular cage with four fluor-escent lamps (Philips TUV G13 36W) mounted onthe walls. In preliminary experiments, it was foundthat Cl2 could not be used as a chlorine atom sourcebecause Cl2 reacts with furan in the dark yieldingthe same chlorinated product as in the reaction offuran with chlorine atoms. Therefore, photolysis at254 nm of either thionyl chloride (SOCl2) ortrichloroacetyl chloride (CCl3COCl) was used togenerate atomic chlorine in irradiation steps of8–10 s for SOCl2 and 60–120 s for CCl3COCl with atotal photolysis time of 20–50 s and 120–600 s,respectively, which resulted in a furan conversionof 37–92%. A sample was collected and analyzed inthe GC after each photolysis step. Dark reactionsbetween these sources of chlorine atoms and furanwere not observed. The initial reactant concentra-tions for qualitative experiments were as follows:

furan (1.6–32 ppmV), SOCl2 (3–74 ppmV), andCCl3COCl (6–35 ppmV). The products formed fromthe reaction of Cl atoms with furan were investi-gated using combined gas chromatography-massspectrometry (GC-MS) and electron capture detec-tion (GC-ECD) for product identification and GC-FID for quantification. SPME was the samplingmethod used to collect products for GC-MS/FID/ECD analysis. The SPME fiber sampleswere collected for 30min by exposing a 50/30 mmDVB/CAR/PDMS fiber to the reaction mixturein the Teflon bag. The fibers were thermallydesorbed for 15min in the heated 270 1C GCinjection port in a Shimadzu GC-17A, MS-QP5050A system onto a 30m TRB-1701 fused silicacapillary column. To obtain molecular weightinformation on the different reaction products, theMS was operated in the chemical ionization (CI)mode with isobutane or methane as the reagentionization gas.

In the quantification experiments the initialreactant concentrations were: furan (0.9–4 ppmV),SOCl2 (2–5 ppmV). Irradiations were performed for10 s with a total photolysis time of 60–70 s resultingin a furan conversion of 60–90%. In these experi-ments, SPME was also used as the samplingmethod, however, an adsorption time of only15min was used for the fiber sample collectionand FID (Hewlett Packard 5890, Series II) was usedfor both detection and quantification of products.Quantification in the capillary GC by FID is moreaccurate than other techniques. The particularadvantage of GC analysis is that the quantitativeresponse of the FID is approximately the same forisomers. The unit peak area (Ai) is given byAi ¼ fi�Ci, where (fi) is the response factor andCi is the concentration, therefore, the concentrationis obtained by dividing the area by the responsefactor.

The GC-FID response factors (fi) of eachcompound were determined by introducing differentmeasured amounts of authentic standards (whichcovered the range of experimental conditions) intothe Teflon chamber and conducting several replicateanalyses with the coated SPME fiber. For com-pounds, which were not commercially available orthe synthesis yield of pure compound was notsufficient to do the calibration, only an estimationof yields was possible, using the ECN (effectivecarbon number) method (Scalon and Willis, 1985).Also the sampling efficiency on the fiber waschecked.

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2.2. Analysis by FTIR

To complement the results from the experimentsusing SPME GC-MS/FID experiments using an ‘‘insitu’’ sampling method were also used. Experimentswere performed in a cylindrical quartz-glass reactorof 1080L with in situ FTIR analysis; the chamber isdescribed in Barnes et al. (1994). The photolysis ofClCOCOCl and Cl2 using 10 low-pressure mercurylamps (UV lamps, lmax ¼ 254 nm) and 10 super-actinic fluorescence lamps (Philips TL 05/40W,320 nmolo480 nm, lmax ¼ 360 nm) was used togenerate Cl atoms, respectively. Dark reactionbetween furan and Cl2 was found to be negligiblein the quartz-glass reactor, therefore, use of this Clsource avoided potential photolysis at 254 nm of theproducts generated in the reaction.

The initial concentrations of reactants in this setof experiments were: furan (0.4–0.8 ppmV) andClCOCOCl and Cl2 (1–3 ppmV). The reactantswere injected directly into the reaction chamberusing calibrated gas-tight syringes via an inlet portlocated on one of the chamber end flanges and thechamber was pressurised to 1 atm total pressurewith air. Irradiations were performed for a totalphotolysis time of 17–25min resulting in a furanconversion of 72–100%. The concentration-timebehaviour of furan and the products were mon-itored in situ using a multi-reflection White mirrorsystem (5.6m base length, 498m total path length)mounted in the reactor and coupled to a FTIRspectrometer (thermo Nicolet Nexus) equipped witha KBr beam splitter and MCT detector. IR spectrawere recorded in the 700–3700 cm�1 spectral rangewith 1 cm�1 resolution by co-adding 64 interfero-grams over 60 s. In typical experiments 25 spectrawere recorded over a period of 25min with light. Aspectral subtraction procedure was used to derivethe concentrations of furan and products as afunction of the reaction time. Identification andquantification of furan and products was madeusing calibrated reference spectra. The calibratedspectra were either produced by introducing aknown concentration of an authentic sample ofthe compound into the reactor or were taken froman existing IR spectral library bank.

2.3. Chemicals

The chemicals used and their stated purities wereas follows: furan (+99% Aldrich), Cl2 (499.8%Praxair), N2 (99.999% Praxair), synthetic air

(99.999% Praxair), NO (99% Praxair), 2(5H)-furanone (98% Aldrich), SOCl2 (+99% Aldrich),CCl3COCl (99% Aldrich), ClCOCOCl (98% Al-drich) and maleic anhydride (99% Aldrich).

3. Results and discussion

3.1. Analysis by gas chromatography

Fig. 1A shows some typical chromatogramsobtained before and after 10 and 20 s photolysis timeof a mixture of SOCl2 and furan in 1 atm air at roomtemperature in the absence of NOx. In this figure aseries of peaks appear that grow with the photolysistime and can be assigned to reaction products. PeaksA and C are compounds from the Cl-atom sourceand from the fiber, respectively, peak B is due tofuran, peak D is assigned to E-butenedial based onthe retention time of a synthesized sample (Fredericoet al., 2003, Liu, 1999) and peak E is tentativelyassigned to Z-butenedial, based on the mass spectrumwhich is similar to the mass spectrum of E-butenedialand by comparison with the mass spectrum of Z-butenedial reported by Berndt et al. (1997). Peak F isassigned to maleic anhydride based on the retentiontime of a standard sample of the compound and itsmass spectrum. Peak G is tentatively assigned to2(3H)-furanone, this compound is not commerciallyavailable but it is likely that it has the same retentiontime and mass spectrum as 2(5H)-furanone. Theexperimental mass spectra are shown in Fig. 1B.2(3H)-furanone was also observed as a reactionproduct of the reaction of furan with NO3 (Berndtet al., 1997) and furan with OH (Bierbach et al.,1995). Peak H with a retention time 17.9min is themost intense and has been identified as a chlorinatedorganic compound with molecular formulaC4H3O2Cl based on the following evidence: (a) theCI mass spectrum shows that the molecular ion is118m/z, (b) the fragmentation pattern from theelectron impact mass spectrum are consistent withthis compound, and (c) the gas chromatogramobtained with electron capture detection (ECD),which is especially sensitive to chlorinated com-pounds, shows only one very intense peak atE17.9min. The structure of this compound wasinitially assigned to the lactone compound, 5-chloro-2(5H)-furanone based on data obtained in the FTIRstudies, that are presented in the next section, andalso tropospheric reactivity principles (Finlayson-Pitts and Pitts, 2000.). Since 5-chloro-2(5H)-furanoneis not commercially available, it was synthesized as

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Fig. 1. (A) Gas chromatograms from reaction mixtures of SOCl2 and furan in 1 atm air at room temperature after t ¼ 0, 10 and 20 s

photolysis period. (B) (a–d) Experimental electron impact mass spectra (EI-MS) of peaks D, E, F, G.

F. Villanueva et al. / Atmospheric Environment 41 (2007) 8796–88108800

described in Kumar and Pandey (2000) and Doerrand Willette (1973). Injection of a sample ofsynthesized 5-chloro-2(5H)-furanone resulted in apeak with the same retention time and mass spectrumas peak H (Fig. 1A) and thus confirmed that5-chloro-2(5H)-furanone is a reaction product.

Fig. 2A shows the gas chromatograms andFig. 2B the mass spectra of both a sample from

the reaction mixture (a1, b1) and an authenticsample of 5-chloro-2(5H)-furanone (a2, b2). In thechromatogram (a2) in Fig. 2A a peak appears at12.8min that corresponds to maleic anhydride,which is an impurity formed in the synthesis of5-chloro-2(5H)-furanone.

Finally, peak I at 27.0min (Fig. 1A) is assigned toan organic compound with molecular formula

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Fig. 2. (A) Gas chromatograms of a sample of a SOCl2/furan/air reaction mixture after 20 s photolysis (a1) and an authentic sample of 5-

chloro-2(5H)-furanone (a2). (B) EI mass spectrum of peak 17.9min (b1) and EI mass spectrum of an authentic sample of 5-chloro-2(5H)-

furanone (b2).

F. Villanueva et al. / Atmospheric Environment 41 (2007) 8796–8810 8801

C4H4O3. The compound has been tentativelyidentified as 5-hydroxy-2(5H)-furanone based onits mass spectrum and the synthesis of a similarcompound (methyl-hydroxy-furanone) from 3-methylfuran.

The concentrations of the reaction products havebeen calculated using the response factors deter-mined with the procedure outlined in the Experi-mental Section. The response factors determined forfuran, 2(5H)-furanone and maleic anhydride were(1.3870.16)� 105, (13.671.5)� 105 and (23.671.7)� 105 ppm�1, respectively. The response factorfor 2(5H)-furanone was used to estimate theconcentrations of 2(3H)-furanone, butenedial and5-chloro-2(5H)-furanone. Tests with thiophene andchlorothiophene, showed that the presence of thechlorine atom in the molecule hardly affected theresponse factor in the detector FID. For these testsdifferent concentrations of thiophene and chlor-

othiophene were put into the chamber and sampledusing the fiber (SPME) to establish the responsefactor of FID. The sampling efficiency of thebutenedials on the fiber was also checked to besimilar to sampling efficiency of 2(5H)-furanonebased on the agreement between the yields obtainedin both experimental systems, GC-MS and FTIR.

Yields for the reaction products have beenobtained from the slopes of plots of the concentrationof the reaction product formed versus the amount ofconsumed furan. Corrections for wall loss of themajor products were made; the corrections were,however, negligibly small. Corrections for possibleloss of the products due to reaction with Cl were notpossible as kinetic data for the reactions are notavailable. The yields obtained for the reactionproducts are listed in Table 1. These yields are thearithmetic average of five experiments. The resultsshow that butenedial and 5-chloro-2(5H)-furanone

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Table 1

Yields of the products observed in the Cl atom oxidation of furan in 1 atm air at room temperature from the SPME-GC-FID and FTIR

analyses

tr (min) M (CI) Compound Structure Molar yield (%)

SPME-GC/FID FTIR

9.2 84 E-butenedial

C = O

H

H

H

O= C

H

C = C

1172 1172a

9.4 – Z-butenedial H

CH

O

HC

O

HC=C

1.670.4 –

12.8 98 Maleic anhydride

OOO

2.470.3 871b

15.8 84 2(3H)-furanone

OO

371 –

17.9 118 5-chloro-2(5H)-furanone

OO Cl

5877 7779a

27 100 5-hydroxy-2-(5H)-furanone

OOHO

Detected –

– CO Not detected –

– HCl Not detected 2173c

The errors quoted in table are a combination of the 2s statistical errors from the regression analysis and the errors from the spectral

subtraction procedure in the case of FTIR or of the calibration curve in the case of SPME-GC-FID.aYields are calculated using UV and VIS lamps.bYield is calculated using only UV lamps.cYield is calculated using only VIS lamps.

F. Villanueva et al. / Atmospheric Environment 41 (2007) 8796–88108802

are major products of the reaction of Cl with furan.Low molar yields of maleic anhydride and 2(3H)-furanone are observed. The FTIR analysis describedbelow suggests that these compounds are mainlysecondary products, although there are some indica-tions that maleic anhydride may also be a minorprimary product formed together with 5-hydroxy-2(5H)-furanone (see below).

3.2. Analysis by FTIR

Fig. 3A shows FTIR spectra recorded fordifferent stages of the reaction of furan with Cl:

(A) is a spectrum of furan+Cl2 in air beforephotolysis, (B) is a spectrum of furan and pro-ducts after 15min of photolysis (absorptionsfrom CO2 has been zeroed due to saturation ofthe band and those from H2O have been sub-tracted), and (C) is the residual product spectrumafter subtraction of furan. In spectrum C, absorp-tion bands due to HCl and CO are readilyidentifiable, other tangible features include a weakabsorption band around 1724 cm�1 typical ofcarbonyl compounds, several absorption bands inthe fingerprint region and one very intense bandcentred at 1829 cm�1.

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Fig. 3. (A) (a) FTIR spectrum of furan before photolysis, (b) spectrum of furan and products after 15min of photolysis, and (c) residual

spectrum from (b) after subtraction of furan, H2O with the CO2 absorption zeroed. (B) Comparison of a residual product spectrum from a

SOCl2/furan/air reaction after subtraction of the reactants and all identified products (lower trace) with an authentic spectrum of 5-chloro-

2(5H)-furanone (upper trace).

F. Villanueva et al. / Atmospheric Environment 41 (2007) 8796–8810 8803

The band at 1724 cm�1 is assigned to E-butene-dial based on a comparison with a referencespectrum recorded by Bierbach et al. (1995).This compound is, unfortunately, not commer-cially available. The same reference spectrum ofE-butenedial was used in the spectral analyses of theproduct spectra. Fig. 3B shows the residual productspectrum obtained, after subtraction of HCl, CO,E-butenedial and maleic anhydride, in the range1900–700 cm�1. As described in the previous sectionthe GC-MS results show that a reaction productwith molecular weight corresponding to C4H3O2Clis formed. The lactone 2(5H)-furanone is known tohave an intense carbonyl absorption band at1800 cm�1 (Aldrich Library of FT-IR Spectra,

1997), the absorption band at 1829 cm�1 is in linewith the formation of a chlorinated furanone; thepresence of the Cl atom, which will inductivelywithdraw electron density from the ring, wouldbe expected to shift the carbonyl absorption to ahigher wavenumber position. The possibility thatthe compound was 4-oxo-2-butenoyl chloride(ClC(O)–CHQCH–CHO) has been discarded sincethe infrared spectrum is devoid of any absorptionbands in the region 2695–2900 cm�1 which would becharacteristic for the stretching vibration of C–H inthe –CHO group. The displaced spectrum shown inFig. 3B is a spectrum of an authentic sample of5-chloro-2(5H)-furanone. There is very good agree-ment between the majority of the spectral features in

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the residual product spectrum and those of theauthentic sample of 5-chloro-2(5H) furanone, whichstrongly supports that the residual spectrum IRfeatures are mainly attributable to 5-chloro-2(5H)furanone. A theoretical infrared spectrum generatedby ‘‘ab initio’’ calculations using GAUSSIAN 03(Frisch et al., 2004) also supports the assignment.Therefore, both the results from the FTIR analysisand the data obtained in the GC-MS (EI/CI)experiments confirm that 5-chloro-2(5H) furanoneis a major product of the reaction of Cl atoms withfuran.

The reaction products maleic anhydride, HCl,and CO can be formed from secondary reactions ofbutenedial (Bierbach et al., 1994) or reactions of Clwith impurities, or in the case of CO interaction ofCl with substances absorbed on the chamber walls.Tang and Zhu (2005) found that when a UV lampwas used to irradiate a cis- and trans-butenedialmixture in air only (3H)-furanone was formed, thisis contrary to the work of Bierbach et al. (1994) inwhich formation of maleic anhydride was alsoobserved. The gas-phase IR absorption coefficientsfor butenedial and 5-chloro-2(5H)-furanone are notknown and synthesis of highly pure samples of thecompounds has not been possible. As a consequencethe concentrations of these compounds have beenestimated using the known absorption coefficientsfor the structurally related compounds 3-hexene-2,5-dione for butenedial (Bierbach et al., 1995) and2(5H) furanone for 5-chloro-2(5H)-furanone whichwas measured in the Wuppertal laboratory.

Fig. 4. Concentration-time profiles of the products observed by FTIR in

Fig. 4 shows concentration-time profiles for theidentified products in the Cl-atom initiated oxida-tion of furan. The curve contours for maleicanhydride and CO are typical for substances formedin secondary reactions, that for HCl appears to beintermediate. The yields of the primary products5-chloro-2(5H)-furanone and butenedial obtainedfrom the slopes of plots of the amounts of reactionproduct formed (corrected for wall loss) versus theamounts of consumed furan (see Fig. 5) are given inTable 1. A similar plot of the concentration data forHCl (Fig. 5) also gave a linear correlation. Theyields of 5-chloro-2(5H)-furanone and butenedialare the arithmetic averages for 10 experiments usingUV and VIS light. In the case of UV lamps theyields are derived in the early stage of the reaction,when the secondary processes are largely negligible.

In the case of HCl the yield could only becalculated from the experiments using VIS radiationand in the case of maleic anhydride the yield couldonly be calculated from the experiments using UVradiation. That is a proof that maleic anhydride isalso a secondary product from butenedial photo-lysis at l ¼ 254 nm.

Comparing the results from the quantificationmethods, SPME/GC-FID and FTIR, shown inTable 1 it is obvious that 5-chloro-2(5H) furanoneis the major gas-phase reaction product of thereaction of chlorine atoms with furan. Within theoverall uncertainties associated with each quantifi-cation method, the yields of the products are infair quantitative agreement. A total carbon mass

the Cl-atom initiated oxidation of furan at 29872K in 1 atm air.

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Fig. 5. Examples of plots of the main reaction product concentrations against the amount of reacted furan obtained by FTIR analysis.

Only 5-chloro-2(5H)-furanone has been corrected for wall loss.

Cl-C(O)-CH=CH-O-CHO P1

OHC-CH=CH-O-CHO P2

O

Cl

OP3

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balance of 76711% is obtained for the pro-ducts detected in GC-FID experiments and90715% for the products determined by FITR.These values must be taken with caution since theyield could be overestimated. Other types ofproducts, such as aerosols, were not observed inthe reaction system and it is unlikely that anysignificant fraction of the products is being lost tothe aerosol phase. Therefore, it would appear thatthe majority of gas phase products have beenaccounted for.

3.3. Reaction mechanisms

Fig. 6 shows the possible reaction pathwaysfollowing addition of Cl to the double bond infuran. Based on a previous kinetic study of theCl+furan reaction (Cabanas et al., 2005) andtropospheric reactivity principles (Finlayson-Pittsand Pitts, 2000), this is expected to be the majorinitial reaction pathway. The products observed inthe present study support that H-atom abstractionfrom the ring is a minor pathway and that additionto the double bond dominates. There are twopossible positions where chlorine atoms can add tothe ring, C-2 (or C-5) and C-3 (or C-4). Since themolecule is symmetric positions C-2 and C-5 and C-3 and C-4 are equivalent.

Both of the radicals are stabilized by resonance.Attack at the ortho position (C-2 (or C-5)) leads tothree resonance structures while attack at the meta

position leads to only two, therefore addition at theortho position is favored. As shown on the right-hand side of Fig. 6 the following products can beformed via Cl addition at the meta position onfuran:

No evidence was found in the product analysesfor the formation of these compounds indicatingthat this pathway is minor. Similarly, no evidencewas found for the products P4, P5, and P6(OQCQCH–CHO), which can arise from reac-tions of one of the resonance structures formed byCl addition at the ortho position (left-hand side inFig. 6).

The major observed product, 5-chloro-2(5H)-furanone from the reaction of Cl with furan canbe formed as follows: addition of oxygen to thechlorofuran radical resonance structure with anelectron in the five position (Fig. 6, centre), willresult in the formation of the following chlorofuranperoxy radical.

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O Cl(chlorofuran peroxy radical, RO )

+ O2

O ClOO

Cl

O HOO

RO2Cl

O HO

RO, O2

(chloro-alkoxy peroxy radical)

F. Villanueva et al. / Atmospheric Environment 41 (2007) 8796–88108806

In the experimental system the peroxy radicalscan either undergo self-reaction or reaction withother peroxy radicals in the system. There areseveral possible reaction pathways for peroxy/peroxy reactions (Lightfoot et al., 1992), in thisparticular instance the dominant pathway is mostprobably reaction to give two chlorofuran alkoxyradicals and molecular oxygen:

2

RO2 RO, O2

O ClOO O ClO

Cl

O HO

-Cl

O HO(Butenedial: P7)

The chlorofuran alkoxy radical will react with O2

to form the observed 5-chloro-2(5H)-furanone andHO2.

O2 HO2

O ClO O ClO

Cl

O HO

Cl

O O

O2 HO2

The pathway leading to formation of butenedialmost probably involves ring opening of a chlor-ofuran resonance radical structure with an electronin the C-3 position,

(5-chloro-2(5H)-furanone, P9)

O Cl

Cl

O H

followed by addition of oxygen to form a chloro-alkyl peroxy radical (ROOd)

Cl

O H

Cl

O HOO

+ O2

(chloro-alkyl peroxy radical)

As discussed above the chloroalkyl peroxyradicals (ROOd) are expected to mainly self-react

or react with other peroxy radicals in the system toform chloro-alkoxy radicals:

There are a number of pathways open to thechloro-alkoxy radical: (i) elimination of a Cl atom,(ii) elimination of an H-atom or reaction with O2,both reactions will yield the same product, andpossibly (iii) three-center elimination of HCl whichhas been observed for chlorinated alkyl radicals(Bilde et al., 1999). (i) elimination of a chlorine atomwill result in the formation of E/Z butenedial whichis observed experimentally

or (ii) reaction with oxygen to form an acid chlo-ride and HO2 (or direct elimination of an H-atom)will both yield 4-oxo-2-buteneoyl chloride (P8):

No evidence could be found for the formation ofP8 in the product analyses implying that this lastprocess is negligible. Similarly for process (iii)although HCl is observed as a product no evidencewas found for the presence of low molecular weightcarbonyl compounds which would be expected to beformed from the further reactions of the CHO–CHQCH–CO radical resulting from the HClelimination.

Although the data analyses of the reactionproducts have confirmed that addition of Cl atomsto the double bonds of furan is the major pathway,the detection of 5-hydroxy-2(5H)-furanone in theGC analyses shows that H-abstraction is alsooccurring to some extent. It was not possible tocalculate the branching ratio for this pathway. Theformation of this compound could be explained bythe reaction scheme shown in Fig. 7. As mentioned

ARTICLE IN PRESS

Fig. 6. Proposed pathways for the reaction of Cl atoms with furan in 1 atm air at room temperature.

Fig. 7. Possible reaction pathway in the Cl-atom oxidation of furan leading to formation of hydroxyfuranone.

F. Villanueva et al. / Atmospheric Environment 41 (2007) 8796–8810 8807

above there are several possible reaction pathwaysfor RO2+RO2 reactions one of which is formationof an alcohol (ROH) and a carbonyl (RdCHO)coproduct as shown for Cl+furan in Fig. 7. In thisparticular reaction scheme the carbonyl coproductformed is maleic anhydride which is observedexperimentally.

The possibility of formation of hydroxyfuranonevia furan and OH reaction, where this OH in ourreactor is from H2O2 photolysis has been discardedsince the detection of hydroxyfuranone in anexperiment of furan with OH is not observed, inaddition works about OH and furan do not showthis compound (Bierbach et al., 1995).

ARTICLE IN PRESSF. Villanueva et al. / Atmospheric Environment 41 (2007) 8796–88108808

4. Environmental implications and conclusions

The rate constant for the reaction of chlorineatoms with furan obtained in a previous study(Cabanas et al., 2005) has shown degradation offuran with this species to be rapid and an atmo-spheric lifetime with respect to this chemicalreaction of 13.6 h was estimated assuming aconcentration of chlorine atoms which might betypical at dawn in a coastal urban area,[Cl] ¼ 1� 105 atoms cm�3 (Spicer et al., 1998).Similarly, assuming a tropospheric OH concentra-tion of [OH] ¼ 1� 106 radicals cm�3 as a 12-hdaytime average (Spivadovsky et al., 2000), andconsidering the rate coefficients for OH (Atkinson,1994) the lifetime for the atmospheric removal offuran by OH is 6.8 h. Hence in regions such asmarine boundary layers or coastal regions, wherechlorine concentration may be relatively high, theimpact of chlorine chemistry on the fate of furan inthe troposphere may have to be carefully consid-ered, as it could constitute a competitive channel forits atmospheric removal. A similar effect may takeplace in some urban contaminated areas, where highlevels of chlorine may originate from industrialemissions (Galan et al., 2002). The products andmechanistic data obtained in this work indicate thatchlorine-containing products such as 5-chloro-5(2H)-furanone may possibly be used as a tracerfor the Cl-atoms initiated chemistry of furan inthose areas where the atmospheric levels of furanare high. The prerequisite being that 5-chloro-5(2H)-furanone is itself not being emitted.

On the other hand, the yield of 1,4-dicarbonylcompounds as butenedial, shows that this furandegradation path can contribute indirectly tophotochemistry smog pollution, because thesecompounds are very reactive and can act likeimportant sources of free radicals, promote theformation of organic aerosols and serve likecarboxyl acid precursors, hydroperoxides and oxi-dants like O3, PAN. The results of reaction productsfrom this work show that the use of biomassburning as an alternative energy source should becarefully considered, since it is the main source offurans to the atmosphere and could have importantenvironmental consequences. Clearly, further stu-dies, on the reactions of butenedial and maleicanhydride with Cl, are needed to assess theimportance of chlorine atom chemistry with furanin industrial areas or in special situations, forexample, in forest fires. The photolysis of butenedial

will dominate in the atmosphere, however, in abiomass burning situation where the light may notpenetrate Cl might play a role.

Acknowledgements

Florentina Villanueva Garcıa thanks ‘‘Junta deComunidades de Castilla La Mancha’’ for apersonal grant. The authors thank Iustinian Bejanfor his help in Wuppertal. This work was supportedby Project PAI06-0042 granted by the JCCM(‘‘Junta de Comunidades de Castilla La Mancha’’).

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