25.jaoui,2003b (1)

Journal of Atmospheric Chemistry 46: 29–54, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

29

Gas and Particulate Products Distribution from thePhotooxidation of α-Humulene in the Presence ofNOx, Natural Atmospheric Air and Sunlight

M. JAOUI and R. M. KAMENS

Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill,NC 27599-7431, U.S.A., e-mail: [email protected]

(Received: 7 October 2002; accepted: 18 April 2003)

Abstract. The photooxidation of α-humulene in the presence of NOx , natural sunlight, and ruralbackground air was investigated using a combination of gas chromatography-mass spectrometry(GC-MS) and high performance liquid chromatography (HPLC). Identification and quantificationof gas and particulate reaction products were reported over the course of the reaction. The daytimephotooxidation was carried out in a large outdoor smog chamber (190 m3). A wide range of ringretaining and ring opening products in the gas and particle phase are reported. On average, measuredgas and particle phase products accounted for ∼44% of the reacted α-humulene carbon. Measure-ments show that a number of reaction products with low vapor pressures (e.g. 3-seco-α-humulonealdehyde, 7-seco-α-humulone aldehyde, α-humulal aldehyde, α-humulene 3-oxide or α-humulene7-oxide, α-humulaic/alic acid isomers, and 3-seco-α-14-hydroxyhumulone aldehyde) were found inthe early stage of the reaction and may play an important role in the early formation of secondaryorganic aerosol. A detailed mechanism is proposed to account for most products observed in thisinvestigation.

Key words: sesquiterpene, α-humulene, organic aerosols, photooxidation, NOx .

1. Introduction

Volatile and semi-volatile substances other than CO and CO2 produced by plantsand trees and released into the atmosphere are known as biogenic organic com-pounds (BOCs). The atmospheric chemistry of these BOCs has received particularattention because of the large quantities emitted globally and the high reactiv-ity of these mostly unsaturated biogenic non-methane hydrocarbons ‘NMHCs’(Guenther et al., 1995; Geron et al., 2000; Fuentes et al., 2000).

Although there have been many field and laboratory studies of BOC, most havefocused on isoprene and monoterpenes. Sesquiterpenes (SQT), a class of biogenichydrocarbons made up of three isoprene units (C15H24), are emitted from vegeta-tion, and found in the atmosphere (Andersson et al., 1980; Buttery et al., 1985;Arey et al., 1991; Khalil and Rasmussen, 1992; Turlings and Tumlinson, 1992;Winer et al., 1992; König et al., 1992; Helmig et al., 1994, 1999; Kotzias et al.,

30 M. JAOUI AND R. M. KAMENS

1995; Seufert 1997). Recently, Helmig et al. (1994) reported that about 9% of theNMHCs emitted from vegetation in the United State are SQT. Their biosyntheticproduction pathways and their emission mechanism are not well known (Fuenteset al., 2000). In addition, their overall emission is uncertain due to sampling and/oranalyzing procedures and/or slower detection methods, which are not designedto detect such compounds and does not account for their fast reaction rate withatmospheric oxidants (e.g., O3, OH and NO3 radicals). Thus measured valuesmay not reflect their real composition in the atmosphere. In particular, they of-ten suffer from further oxidation after their emission in the atmosphere, resultingin lower amounts detected. For example, the fast reaction of some SQT (e.g.,β-caryophyllene and α-humulene) with O3 has implications for calculating theiremission rates to the atmosphere. In fact, Fuentes et al. (2000) reported that whenthe emission rates of d-limonene and β-caryophyllene from orange tree brancheswere measured without an ozone scrubber, d-limonene was unaffected, but that ofβ-caryophyllene was dramatically reduced due to reaction with O3. In addition,when the ozone scrubber was used the emission rate of β-caryophyllene was actu-ally a little higher than that of d-limonene. Also, Zini et al. (2001) reported that theamount of β-caryophyllene emitted from Eucalyptus citriodora is higher than theamount of most monoterpenes (e.g., α-pinene, β-pinene, d-limonene). On a reactedmass basis, β-caryophyllene and α-humulene have much higher aerosol formationpotentials than monoterpenes (Griffin et al., 1999). Their contribution, however, tosecondary aerosols is currently unknown, but could be significant, depending ontheir actual emissions. In addition, their role in atmospheric processes remains un-certain due to the lack of the identification and quantification of reaction productsfrom their atmospheric oxidation.

The presence of one, two or three double bonds in SQT (Figure 1) and theirhigh rate constant with O3, NO3 and OH radicals (Shu and Atkinson, 1994, 1995)clearly illustrate the importance of sesquiterpene chemistry in the troposphere.They are of special interest because of their participation in aerosol-formingprocesses (Griffin et al., 1999) and heterogeneous reactions in the atmospheres.The rate constants for gas phase reactions of some SQT with ozone, NO3 andOH radicals have been reported by Shu and Atkinson (1994, 1995). However, todate, there are only three studies on SQT chemistry. In previous papers (Jaoui etal., 2003a; Jaoui and Kamens, 2003b), we reported reaction products and path-ways leading to the gas-particle conversion from β-caryophyllene and α-cedreneozonolysis respectively. Grosjean et al. (1993) investigated the gas phase reac-tion of β-caryophyllene with ozone and its sunlight irradiation in the presence ofNO. Formaldehyde was the only product observed in the ozone-initiated oxida-tion, however, in the sunlight irradiation small amounts of glyoxal and methylglyoxal were also observed. Recently, Calogirou et al. (1997) investigated thegas-phase ozonolysis of β-caryophyllene using a combination of GC-MS andHPLC as detection methods. The authors report the formation of 3,3-dimethyl-

GAS AND PARTICULATE PRODUCTS DISTRIBUTION 31

Figure 1. Structures of some sesquiterpenes emitted to the atmosphere.

γ -methylene-2-(3-oxobutyl)-cyclobutanebutanal, 9-methylene-4,12,12-trimethyl-5-oxabicyclo[8,2,0,0] dodecane, and formaldehyde.

Due to lack of data from sesquiterpene systems, a preliminary study of someimportant sesquiterpene oxidation by ozone or their photooxidation in the pres-ence of NOx and atmospheric air was undertaken in our laboratory (Jaoui et al.,2003a; Jaoui and Kamens, 2003b). To continue this investigation, the current pa-per examines the photooxidation of α-humulene (also known as α-caryophylleneor trans-trans-trans-2,6,6,9-tetramethyl-1,4,8-cycloundecatriene) which has threeinternal double bonds, in the presence of NOx and natural atmospheric air andsunlight. In addition to the examination of secondary organic aerosol (SOA) for-mation, identification, characterization, and quantification of a wide range of gasand particulate reaction products from this system are investigated in this paper.The yields of reaction products in both gas and aerosol phases are reported forthe first time. To account for most observed reaction products, a detailed mecha-nism is proposed. The experiments were carried out at atmospheric pressure air inthe daytime. These investigations should improve our understanding of the effectof biogenic compound on the chemistry of the atmosphere and their effects onSOA formation. It is hoped that this work will increase awareness of the potentialimpacts of SQT chemistry on tropospheric chemistry.

2. Experimental Section

The UNC large outdoor 190 m3 Teflon film chamber was used for the examinationof the photooxidation of α-humulene. Details of basic experimental proceduresand protocols are outlined elsewhere (Kamens et al., 1999; Jaoui and Kamens,2001). Rural background air was used to purge the chamber, and a dehumidifierwas used to partially dry the chamber air for a minimum of 24 h prior to the start ofexperiment. The humidity in the chamber was measured by a dew point hydrometer(EG&G, Waltham model 800, MA), and the temperature was monitored by a tem-perature thermistor positioned inside the chamber in a 1.21 m high-ventilated solarradiation shield. Prior to the start of the experiment, gas and particle backgroundlevels in the chamber were measured simultaneously. NOx (0.57 ppmV) was in-troduced first to the chamber followed by pure α-humulene liquid (0.60 ppmV)that was gently heated in the U-tube with air flowing over the liquid so that it wasvaporized into the chamber (Kamens et al., 1995; Fan et al., 1996). We used arelatively high initial concentration of α-humulene to be able to have enough mass


collected on filters or denuders, and we feel that in this first study, this is necessaryto be able to identify and quantify reaction products. The chamber is equippedwith internal fans to ensure rapid mixing of NOx and α-humulene during theirintroduction to the chamber. The ozone concentration was monitored continuouslyusing a Bendix chemiluminescent ozone meter (model 8002, Ronceverte, WV) andcalibrated via gas phase titration using a U.S. National Institute of Standards andTechnology (NIST) traceable NO tank. NO and NO2 concentrations in the cham-ber were monitored continuously with a thermo-environmental chemiluminescentanalyzer (Model 8440E, San Diego, California). The precision associated with anindividual NOx , and O3 calibration is ±5% (Kamens et al., 1995, 1999; Fan et al.,1996).

Gas and particle samples were collected simultaneously for 20 min at a flowrate of 20 LPM using a sampling train that consisted of two 47 mm Teflon glassfiber filters in series (type T60A20 Pallflex Products Corp., Putnam, CT), whichwas followed by a 5-channel annular denuder coated with XAD-4. Note that, onesample was collected for two hours to allow collection of a maximum amountof reaction products. This sample was used for product identification both withand without derivatization techniques. Filters were re-weighted after sampling todetermine the mass of particulate material collected and transported back to UNCat Chapel Hill in amber glass jars at 0 ◦C for soxhlet extraction. Denuder sampleswere solvent-extracted directly in the field. The entire sampling procedure has beengiven elsewhere (Jaoui and Kamens, 2001).

Product concentrations were measured first by direct injection to a GC-MS operated in EI mode without derivatization (Jaoui and Kamens, 2001).For further identification, carbonyl compounds were derivatized by O-(2,3,4,5,6-pentafluorophenyl) hydroxylamine hydrochloride (PFBHA), and hydroxyl andcarboxylic groups were derivatized by N,O-bis(trimethylsilyl)-trifluoroacetamide(BSTFA), or 10% w/w BF3-methanol (Jaoui and Kamens, 2002a, b). Derivatizedsamples were analyzed using the same GC-MS used for the direct injection. Smallcarbonyl compounds (i.e., formaldehyde, acetaldehyde, and acetone) were sam-pled with a 2,4-dinitrophenylhydrazine (DNPH) aqueous solution by pulling airfrom the chamber at 1.0 LPM through a series of three impingers. Two of theimpingers were pre-filled with 15 ml of acidified DNPH aqueous solution wherethe second was a trap to check for ‘break-through’. Airborne particles as well asgas-phase species passed through the bubblers, hence the method sampled both gasand particle phase carbonyl compounds. The carbonyl compounds in these sampleswere analyzed as their (DNPH) derivatives (Jaoui and Kamens, 2002a, b) using ahigh-performance liquid chromatography and GC-MS.

Most reaction products were quantified using surrogate compounds. In fact, themost significant error source in the present study is the uncertainty associated withsurrogate calibration, which we estimate to be 40% overall (Jaoui and Kamens,2001).


Real-time particle measurements were made using a Scanning Mobility ParticleSizer (SMPS, 3936 TSI). The SMPS was set to measure the size distribution of theparticles in the 13–750 nm range, scanning the complete range in 300 seconds ata sampling flow of 0.3 LPM. Total aerosol number concentrations were measuredwith an associated Condensation Nuclei Counter (CNC, 3025A, TSA). Measure-ment of size distribution allowed for the calculation of the total aerosol volume.Particle measurements started about 40 min before each experiment to characterizethe background aerosol present in the chamber. Filter samples, as described before,were used also to measure aerosol mass concentration.

In addition to the chemicals reported in our previous paper (Jaoui and Kamens,2001), α-humulene (99%), β-caryophyllene, β-caryophyllene oxide (99%), andisocaryophyllene (98%) were purchased from Aldrich Chemical Co.

3. Results and Discussion

This study was dedicated to products identification and examination of SOA for-mation from the photooxidation of a highly unsaturated SQT (α-humulene containsthree endocyclic double bonds: see Figure 1). The direct injection technique previ-ously described (Jaoui and Kamens, 2001) was used to identify reaction productsobserved in this study. This method gives, for most reaction products, a clear spec-trum including the molecular ion (M+ �). To further confirm the identity of theproducts, derivatization techniques were used as a confirmation technique (Jaouiand Kamens, 2001, 2002a, b; Yu et al., 1998, 1999; Smith et al., 1999). A numberof standard biogenic compounds were injected in the GC-MS and their EI massspectra were used as an aid for identification (Jaoui and Kamens, 2001). Briefly,reaction products were identified using the following steps: first we assumed themolecular weight (MW) of reaction products from their EI spectra and we con-firmed their identity and MW by the use of standards, CI spectra, EI and CI spectraof derivatized compounds; second, EI fragmentation patterns of resulting productfunctional groups were compared with the EI fragmentation patterns from of otherterpene standards with the same functional groups, and when combined with addi-tional EI and/or CI spectra of derivatized compounds it was possible to proposedtentative identifications. Note that where no standard is available, the product iden-tification given in Table I must be viewed as very tentative. Direct injection wasuse to semi quantify reaction products from the α-humulene photooxidation.

3.1. IDENTIFICATION

Total ion chromatograms (TIC) of reaction products formed from the photooxida-tion of α-humulene in the presence of NOx are shown in Figures 2 and 3 for gas andparticle phases respectively. The GC-MS analysis of the mixture shows the pres-ence of eight significant products peaks in the gas phase and four products peaks inthe particle phase in the early stage of the reaction. However, a significant number


Table I. α-Humulene and reaction products tentatively identified from the photooxidation ofα-humulene in the presence of air/NOx and natural sunlight and their mass spectra patterns

Systematic Terpene nomenclature Structure Mw m/z (EI)

nomenclature (g mol−1)

trans,trans,trans- α-humulene 93(100)

2,6,6,9-Tetramethyl- 80

1,4,8-cycloundecatriene 204 121

204

3,3,7-trimethyl-11-one- 3-seco-α-humulone 43(100)

dodec-4,7-dien-1- aldehyde 107

aldehyde 236 125

178

4,7,7-trimethyl-11-one- 7-seco-α-humulone 43(100)

dodec-4,8-dien-1- aldehyde 107

aldehyde 236 125

178

2,2,5,9-tetramethyl-10- α-humulal aldehyde 43(100)

formyl-dec-4,8-dien-1- 81

aldehyde 236 93

192

2,6,6,9-tetramethyl-4,5- α-humulene 10-oxide 93(100)

epoxy-1,8- 220 43

cycloundecadiene 69

220


epoxy-1,4- 220 43

cycloundecadiene 80

220


epoxy-4,8- 220 109

cycloundecadiene 138

220

3,3,7-trimethyl-11-one- 3-seco-α-humulaic-3- 43(100)

dodec-4,7-dien-1-oic acid 252 55

acid 43

252

4,7,7-trimethyl-11-one- 3-seco-α-humulaic-7- 43(100)

dodec-4,8-dien-1-oic acid 252 55

acid 43

252


Table I. (Continued)

Systematic Terpene nomenclature Structure Mw m/z (EI)

nomenclature (g mol−1)

3,7,10,10-tetramethyl- α-humulalic 10-acid 43(100)

11-formyl-dec-3,7- 252 55

dien-1-oic acid 43

252

2,2,5,9-tetramethyl-10- α-humulalic 11-acid 43(100)

formyl-dec-4,8-dien-1- 252 55

oic acid 43

252

2,2,5,9-tetramethyl- α-humuladionic acid 43(100)

undec-4,8-dien- 254 84

dicarboxylic acid 107

234

3,3,7-trimethyl-10- 3-seco-α-14- 43(100)

hydroxy-11-onedodec- hydroxyhumulone 268 99

4,7-dien-1-aldehyde aldehyde 82

125

2,2,6-trimethyl-10-one- 3-seco-α- 43(100)

undec-3,6-dien-1- norhumulone 222 99

aldehyde aldehyde 57

207

2,2,5,9-tetramethyl-10- α-norhumulal 149(100)

formyl-dec-4,8-dien-1- aldehyde 222 123

aldehyde 177

222

3,3-dimethyl-7-one-oct- 43(100)

4-ene-1-aldehyde 168 84

98

168

43(100)

∗ ∗ ∗ 210 55

83

110

Acetone

Formaldehyde

Acetaldehyde


Figure 2. Total ion chromatogram (TIC) of gas phase reaction products from the pho-tooxidation α-humulene in the presence on NOx , air and natural sunlight obtained by gaschromatography-mass spectrometry.

of small peaks were clearly observed in the gas and particle phase as the reactiontime goes on reflecting the decomposition of α-humulene to products having lowmolecular weight. The chromatograms in Figures 2 and 3 originated from samplestaken starting at 1 min and 25 min respectively after introducing α-humulene andNOx to the smog chamber. The sampling time for each one was 20 min. Table Isummarizes the reaction products identified in this study, their chemical structures,their molecular weights (MW), and their EI mass spectra patterns. Most reactionproducts are identified for the first time in this study and their systematic nomen-clature (Fox and Powel, 2001) are given in addition to their terpene nomenclature(Larsen et al., 1998) whenever possible.

The molecular weights of the major products observed are easily determined byinterpretation of the EI mass spectra. The chromatogram (Figure 2 or 3) indicatedthe presence of two peaks of parent mass m/z 236 eluted around 13.5 min. Acareful analysis of the reconstructed ion chromatogram of m/z 125 reveals two co-eluted products (Figure 4(B): insert). The two peaks with a molecular weight of 236were positively identified as 3-seco-α-humulone aldehyde and 7-seco-α-humulonealdehyde (Figure 4). This identification is consistent with their similar structure(Table I) and the mass spectra fragmentation patterns observed in this study. Theirmass spectra show ions at m/z 236 (M+ �), 221 (M+ �-CH3), 218 (M+ �-H2O), 203(M+ �-CH3-H2O), 192 (M+ �-CH3-‘-C(O)CH3’) (Figure 4). The first peak (Fig-ure 4(C): insert) was identified as α-humulal aldehyde and shows ions at m/z

236 (M+ �), 221 (M+ �-CH3), 218 (M+ �-H2O), 207 (M+ �-‘-C(O)H’), 178 (M+ �-2x‘-C(O)H’) (Figure 4(C)). A close examination of the ion mass spectra shows thations at m/z 192 have high relative intensity for 3-seco-α-humulone aldehyde (or


Figure 3. Total ion chromatogram (TIC) of particle phase reaction products from the pho-tooxidation of α-humulene in the presence on NOx , air and natural sunlight obtained by gaschromatography-mass spectrometry.

7-seco-α-humulone aldehyde) and low relative intensity for α-humulal aldehyde,unlike the ion at m/z 178, which has a high intensity for α-humulal aldehydeand low relative intensity for 3-seco-α-humulone aldehyde (or 7-seco-α-humulonealdehyde). These findings are consistent with the EI fragmentation patterns ofproducts identified in our previous works (Jaoui and Kamens, 2001, 2003a–e).

Among the products detected are three products having mass m/z 220 andsimilar fragmentation patters as α-pinene oxide, β-pinene oxide, and d-limoneneoxide. Under close analysis of the mass spectra, their fragmentation patterns revealthat these three peaks eluted at 11.86 min (major peak), 11.72 min, and 11.24 minwere tentatively identified as α-humulene 3-oxide, α-humulene 7-oxide, and α-humulene 10-oxide respectively. However, because of their similar fragmentationpatterns, the peaks assigned to α-humulene 3-oxide and α-humulene 7-oxide maybe reversed without having authentic standards available for validation. Figure 4(D)shows, as an example, the mass spectrums for α-humulene 3-oxide or α-humulene7-oxide. An effort was made by using a characteristic ion m/z 93 to determinethe relative ratio of the three components. α-Humulene 3-oxide and α-humulene7-oxide were found at the same ratios, however α-humulene 10-oxide had a 1:10ratio to the sum of α-humulene 7-oxide and α-humulene 3-oxide.

Four products with mass m/z 252 eluted between 13.8 and 15.2 min were de-tected. Their fragmentation patterns, as shown in Figure 4(E), reveals the presenceof carboxylic and keto groups (loss of 15 (CH3), 18 (H2O), and 43 (-C(O)CH3))as observed for β-caryophyllonic acid (Jaoui et al., 2002d) and other keto car-


Figures 4(A)–(C).


Figures 4(D)–(F).


Figure 4(G).

Figure 4. Mass spectra for α-humulene, α-humulene 7-oxide, 3-seco-α-humulone aldehyde,α-humulal aldehyde, α-humulaic/alic acid isomers, 3-seco-α-14-hydroxyhumulone aldehyde,and α-humulal aldehyde.

boxylic compounds (Jaoui and Kamens, 2001, 2002a–c). After closer analyses oftheir mass spectra, these products are identified as 3-seco-α-humulaic-3-acid, 3-seco-α-humulaic-7-acid, α-humulalic 10-acid, and α-humulalic 11-acid (Table I).As before, it was difficult to assign each compound to their correspondent peakswithout authentic standards. Thus in Figures 2 and 3 the name ‘α-humulaic/alicacid isomers’ was assigned to all the four products. α-Humuladionic acid, a diacidsimilar to pinic acid, eluted around 15.7 min with mass m/z 254, was tentativelyidentified and was observed only in small amounts in the particle phase (Figure 3).One compound observed only in the particle phase, with a peak eluted at 18.6 minand a mass m/z 268 (Figure 4(F), Table I) was tentatively identified as 3-seco-α-14-hydroxyhumulone aldehyde based on its mass spectrum fragmentation pattern,and by analogy with 10-hydroxy pinonaldehyde fragmentation patterns (Jaoui andKamens, 2001). 3-Seco-α-norhumulone aldehyde, and α-norhumulal aldehyde,which has a mass m/z 222 were tentatively identified in the gas phase and inlow concentration in the particle phase. Note that some additional products withMW = 222 were observed, and it was difficult to draw a structure for them based onthis work. Possible candidates are hydroxy-keto aldehydes isomers. A number ofpeaks from unidentified products were also present in the chromatograms (Figures2 and 3).

Figure 4 shows mass spectra for α-humulene, α-humulene 7/or 3-oxide, 3-seco-α-humulone aldehyde, α-humulal aldehyde, α-humulaic/alic acid isomers, 3-seco-α-14-hydroxyhumulone aldehyde, and α-norhumulal aldehyde. Note that reactionproducts bearing carboxylic group ‘-COOH’ (e.g., α-humulaic/alic acid isomers)give rise to the common ion fragments M-CH3, M-HO2, M-CO2 as shown for α-humulaic/alic acid isomers in Figure 4(E). All reaction products were tentatively


identified by MS assignment (no brackets) using the direct injection method andthe derivatization techniques discussed above.

The most abundant compounds found in the gas phase early reaction were 3-seco-α-humulone aldehyde, 7-seco-α-humulone aldehyde, α-humulal aldehyde,α-humulene 3-oxide or α-humulene 7-oxide, and α-humulaic/alic acid isomers. Inthe particle phase, the dominant compounds were 3-seco-α-humulone aldehyde,7-seco-α-humulone aldehyde, α-humulal aldehyde, and 3-seco-α-14-hydroxy-humulone aldehyde. Most of reaction products identified in this study wereobserved in particle and gas phase simultaneously (Figures 2 and 3). Severalreaction products may undergo reaction with OH radicals or with ozone (e.g.,3-seco-α-humulone aldehyde and 7-seco-α-humulone aldehyde).

Gas and aerosol products from the reaction were also derivatized using DNPHfor off-line HPLC and GC-MS analysis (Jaoui and Kamens, 2002a, b). Thisanalysis confirmed the formation of formaldehyde, acetone, and acetaldehyde.The identifications were based on retention times from standards and from MSassignment.

3.2. SEMI-QUANTIFICATION OF REACTION PRODUCTS AND TIME SERIES

Dew point temperature and temperature of the outdoor chamber during the ex-periment are shown in Figure 5(A). The time series of O3, NO, NO2, and NOx

are shown in Figure 5(B). The concentration of ozone at the beginning of theexperiment was very low and starts increasing after 15 min of reaction time as theNO was converted to NO2. As soon as α-humulene was injected to the chamber,its concentration decreased as determined from gas phase denuder extracts. Thisdecrease suggests that α-humulene reacts with background ozone and hydroxylradicals (OH), and/or OH radicals formed during decomposition of α-humulene inthe presence of natural sunlight and NOx. Such reaction is consistent with the rela-tive rate of attack OH radicals on α-humulene, which is 29 10−11 cm3 molecules−1

s−1 at 296 ◦C (Shu and Atkinson, 1995). Figure 5(C) shows the amount of aerosol(TSP) measured using a filter-filter-denuder system. The amount measured in thebottom filter (backup filter) represents about 5 to 10% of the total mass measured(top filter + bottom filter). The reaction products observed in the backup filter,(assumed to be sorbed on the filter from the gas phase) could be subtracted from thetop filter to account for the positive artifact associated with this sampling method.However, we observed that products identified in the backup filter included prod-ucts observed only in the particle phase and were similar to those identified in thefront filter. This suggests that a negative artifact (evaporation or off-gassing fromparticles) needs to be taken into account to correct for negative artifacts.

Authentic standards for products identified in this study are not available;hence their quantification was estimated using calibration factors, recoveries andcollection efficiencies of surrogate compounds. Pinonaldehyde was used as sur-rogate for 3-seco-α-humulone aldehyde, 7-seco-α-humulone aldehyde, α-humulal


Figure 5. Outdoor chamber temperature and dew point (A); time series of ozone, NO, NO2,and NOx (B); amount of aerosol (TSP) measured using filter/filter/denuder system in the topand bottom filters (C).


aldehyde, 3-seco-α-14-hydroxyhumulone aldehyde, 3-seco-α-norhumulone alde-hyde, and α-norhumulal aldehyde. Pinonic acid was used as a surrogate for3-seco-α-humulaic-3-acid, 3-seco-α-humulaic-7-acid, α-humulalic 10-acid, andα-humulalic 11-acid. α-humulene was used as surrogate for α-humulene 3-oxide,and α-humulene 7-oxide. Pinic acid was used as a surrogate for α-humuladionicacid. In each case, the extracted ion chromatogram of the most intense ion in themass spectrum was used for quantification. The use of the most intense ion in themass spectrum for quantification purpose is suitable when a complex mixture isto be analyzed (quantified). This becomes more important when the resolutions ofproducts that make up the mixture are not as good as in this study. Note that foreach compound, an ion was chosen to generate the extracted ion chromatogramto be used for quantification purpose when authentic standards were available. Inthe case of reaction products that do not have authentic standards, the ion to beextracted was the one with the higher intensity and is common to the product to bequantified and to its surrogate. Note also that no secondary reactions were takeninto account for the semi-quantifications. Figure 6 shows the time series for semi-quantified reaction products in this study in gas (top) and particle (bottom) phases.As soon as α-humulene and NOx were introduced into the chamber, a number ofproducts were detected in the early stage of reaction with a relatively high con-centration compared to α-pinene photooxidation (Jaoui and Kamens, 2001). Theconcentration of these products starts decreasing gradually due to their reactionwith oxidants present or formed during the photooxidation of the parent hydrocar-bon. This is the first time that experimental time series data have been presentedfor products identified in the gas and particle phases originating from the photoox-idation of α-humulene. These results may be used to predict the contribution ofbiogenic compounds other than monoterpenes to ambient aerosol formation in theregional atmosphere.

3.3. SOA MEASUREMENTS

The aim of aerosol studies is to understand the gas phase carbon balance deficitand the chemical mechanism. Aerosol data include particle size distributions andnumber concentrations. Table II summarizes results obtained in this study. Figure 7shows the typical evolution of the total aerosol number distribution (A), and atypical time profile of the total aerosol number-size distribution (B) observed dur-ing the α-humulene photooxidation. Total background aerosol according to SMPSmeasurements was about 857±60 total particles cm−3 with an overall total volumeof 109 nm3 cm−3. This amount is negligible compared to the total aerosol producedduring the reaction, which typically gives particle concentrations of 6 × 105 parti-cles cm−3, with a volume of 5.5 × 1011 nm3 cm−3. After injection of α-Humulene,the formation of small particles was observed almost instantaneously producing atri-modal size distribution (peak at 12:67 in Figure 7(B)). This finding was not ob-served for the photooxidation of α-pinene and β-pinene (Jaoui and Kamens, 2001,


Figure 6. Time series of most reaction products quantified in this study in gas (top) and par-ticle (bottom) phases. �: 3-seco-α-humulone aldehyde and 7-seco-α-humulene aldehyde; �:α-humulal aldehyde; �: 3-seco-α-14-hydroxyhumulone aldehyde; O: 3-seco-α-norhumulonealdehyde; ∗: α-humuladionic acid; �: α-humulene 10-oxide, α-humulene 3-oxide andα-humulene 7-oxide. —: α-norhumulal aldehyde; ×: α-humulaic/alic acid.

2002a). The particles grew very slowly passing from a tri-modal size distributionto a bi-modal size distribution due to coagulation and heterogeneous nucleationand showing the complexity of the SOA formation in this system. This suggeststhat more than one factor controls the formation of secondary organic aerosol. Theparticle sizes were in basically in the same range from the beginning to the endof the reaction. As expected the number concentration drops with time (Figure 7),partly because of dilution, but mainly because of wall losses. Coagulation alsoplays a role as is shown by an increase in the diameter of the distribution. Allparticles generated in this study were within the measuring range of the SMPSinstrument. Note, that particles less than 13 nm in diameter may form immediatelyin this study due to nucleation, but were not detected by our SMPS system, whichsampled after 2–3 minutes of reaction, and could only detect particles down to13 nm in size.

In the photooxidation experiment, α-humulene was added to the chamber andreacts immediately with OH radicals (Figure 7: top). Bursts of particles were ob-served with a peak number at a diameter of ∼0.026 µm after a few minutes ofreaction. Immediately after adding α-humulene in the chamber, reaction products


Table II. Total particles number (particles cm−3) and theparticles mass (TSP) in mg m−3 obtained in this study overthe reaction time

Time (EDT) Total particles number TSP b

(particles cm−3) a (mg m−3)

10.92 2005 0.0026

12.73 634867 0.5085

13.50 453100 1.4521

14.93 182152 1.2301

15.71 130770 1.3980

a Obtained by SMPS described in the text.b TSP obtained from filters measurements.

Figure 7. Time profile of the total aerosol particle number concentration (top) and time evolu-tion of the number-size distribution measured during the daytime α-humulene photooxidation(bottom).


with low vapor pressures (e.g., 3-seco-α-humulone aldehyde, 7-seco-α-humulonealdehyde, α-humulal aldehyde, and 3-seco-α-14-hydroxyhumulone aldehyde) con-dense on existing background particles or may self nucleate leading to smallparticles in the range of 20 to 80 nm in less than 2 min of reaction (Figure 7:insert). As particle numbers increase very rapidly due to nucleation, the distancebetween the particles decreases rapidly and coagulation occurs. During the nextfew minutes of the reaction, the total mass or volume of the particles increasesquickly, and the mean diameter of the number distribution increased to ∼200 nm.During this period, the particle number concentration was more or less constantand may actually have been decreasing, while the total particle mass or volumewas increasing. The trend strongly suggests that semi-volatile gas phase productswere condensing on existing particles. The total volume of particle increased andreached its maximum a few minutes after the beginning of the experiment, and thenstarted to decrease gradually due to re-partitioning, chamber dilution and wall loss.

Increasing attention has been given to the chemistry of SOA formation frommonoterpenes (Griffin et al., 1999; Yu et al., 1999). However, there are only twostudies (Griffin et al., 1999; Jaoui et al., 2002d) of sesquiterpene chemistry leadingto the formation of SOA. Improved knowledge of this chemistry is important forunderstanding the role and participation of SQT in the formation of SOA and theirpotential impact on the chemistry of the atmosphere. For example, the presenceof low volatility products (e.g., 3-seco-α-humulone aldehyde, 7-seco-α-humulonealdehyde, α-humulal aldehyde, and 3-seco-α-14-hydroxyhumulone aldehyde) inthe early stage of reaction, suggests that these compounds play a crucial role inthe formation of SOA. Vapor pressure is often used to simulate the atmosphericconcentration of any substance and its partitioning between gas and particle phases(Pankow, 1994). The vapor pressure of reaction products identified in this studywere calculated using the method of Mackay (Mackay et al., 1982; Schwarzenbachet al., 1993):

lni PoL = �Si

vap

(Ti

b

)R

[1.8

(1 − Ti

b

T

)+ 0.8 ln

Tib

T

](atm) , (1)

where �Sivap and T i

b are the entropy of vaporization (J mol−1 K−1) and the boilingpoint (K) respectively for a given organic compound (i), R is the gas constant(8.314 J mol−1 K−1), and T is the ambient temperature (K). The molar entropyof vaporization at normal boiling point, �Si

vap, of organic compounds observed inthis study was calculated following a modified Trouton’s rule approach (Zhao etal., 1999a, b). The boiling temperature, Ti

b, of an organic compound was calcu-lated using a predictive method based on the molecular structure of the compound(Cordes and Rarey, 2002). The results show that most oxygenated reaction prod-ucts bearing more than 10 carbons observed in this study have very low vaporpressures (<10−7 mmHg) at 298 K. These low vapor pressures would predisposethese products to exist in the aerosol phase. In addition, Ziemman (2002), Jangand Kamens (2001), and Kamens and Jaoui (2001) have recently suggested that


alcoholic and aldehydic secondary reaction products from aromatics and biogenicsystems may react to form much larger molecules. Unfortunately, current particlesolvent extraction techniques may return these large particle reaction products totheir parent compounds.

4. Gas and Particulate Product Yields

Sampling techniques for particle and gas phases are associated with a number ofinherent errors. The relatively low surface to volume ratio of the 190-m3 chambertends to lead to a small loss of particles by coagulation and subsequent settling tothe chamber walls. However, there are a number of random and systematic errorssuch as incomplete collection of gas and particle compounds, reaction productsand/or aerosol breakthrough to the denuders and/or filters. Thus, the overall prop-agated error associated with the yields presented here is about 40% (Jaoui andKamens, 2001). The dilution rate constant measured in this study kSF6 from SF6

decay in the chamber is 0.8 × 10−6 s−1. The wall loss rate constants for parti-cles, kpmwall, after correcting for the dilution rate constant, was 0.8 × 10−5 s−1.This value was obtained from the aerosol volume decay measured by the SMPSinstrument (Jaoui et al., 2002d). In a previous study (Jaoui and Kamens, 2002c),the wall loss for pinonaldehyde, kwpinald = 0.8 × 10−5 s−1 and for pinonic acid,kwpinacid = 1.0 × 10−5 s−1 were measured in our chamber. The yields of reactionproducts were adjusted for wall losses and dilution. Similar wall rate constantswere used for reaction products observed in this study. For example, rate coeffi-cient kwpinald was used for aldehyde and keto-aldehydes, and kwpinacid for all otherreaction products semi-quantified in this study. The products observed here maycontain one or two double bonds (see Section 5), which can react with OH radicaland/or ozone. The reported yield data have been not corrected for these secondarylosses, because their experimental rate constants are not available in the literature.

The aerosol yield is defined as the ratio of the amount of secondary organicaerosol formed from the oxidation of α-humulene to the amount of α-humulenethat reacted:

y = �M0

�HC, (2)

where �M0 (µg m−3) is the organic aerosol mass formed after the consumptionof �HC (µg m−3) of α-humulene. �M0 was obtained from filters (Table II;Figure 5(C)). Table III shows yields for individual reaction products in the gasand particle phases, as well as the total yields in both phases for α-humulenephotooxidation in the presence of NOx . Products with higher carbon mass yields(>1%) include 3-seco-α-humulone aldehyde + 7-seco-α-humulone aldehyde(11.6%), α-humulal aldehyde (7.9%), α-humulene 3-oxide + α-humulene 7-oxide(3.2%), α-humulaic/alic acid 8.3%), α-norhumulal aldehyde 3.2%), and 3-seco-α-14-hydroxyhumulone aldehyde 3.4%). The total yield of tentatively identified


Table III. Carbon yields (%) of quantified compounds observed in thegas and aerosol phases from the photooxidation of α-humulene in thepresence of natural sunlight

Product Maximum observed

carbon yield, %

Gas Aerosol Total

3-seco-α-humulone aldehyde +

7-seco-α-humulone aldehyde 5.4 6.2 11.6

α-humulal aldehyde 4.1 3.8 7.9

α-humulene 3-oxide +

α-humulene 7-oxide 2.2 1.0 3.2

α-humulaic/alic acid 3.1 5.2 8.3

Compound with MW = 210 3.4 2.8 6.2

α-norhumulal aldehyde 2.2 1.0 3.2

3-seco-α-14-hydroxyhumulone aldehyde ND 3.4 3.4

Total carbon yield, % 20.4 23.4 43.8

ND, not detected.

products from the α-humulene photooxidation accounts for a significant fractionof the secondary organic aerosol. When both the aerosol and gas phase productsare considered, 43.8% of the reacted carbon mass α-humulene is accounted for inthis study (Table III).

5. Mechanisms of Products Formation

Due to its unsaturated structure and the presence of three endocyclic double bonds(two tertiary and one secondary), α-humulene is highly reactive toward the hy-droxyl radical (OH), ozone (O3) and the nitrate radical (NO3). The approach chosenfor α-humulene photooxidation is to use mechanistic information based on reactionpathways, which have been described for lower alkenes, aromatics and monoter-penes (Calvert et al., 2002; Atkinson, 1997). In addition, structural informationand product information obtained in this study were used as a guideline for the pro-posed schematic diagrams reported for α-humulene photooxidation in the presenceof NOx . The presence of three cyclic double bonds, two of them tertiary, results ingreater reactivity toward O3, OH, and NO3 radicals (Calvert et al., 2002, Chapter2). In fact, double bonds with an associated methyl group are favored (Figure 1),and the secondary double bond assumed to be less favored, but non-negligible.Thus, a wide range of reaction products can be observed resulting from the pho-


Scheme 1.

tooxidation of the three internal double bonds in the presence of NOx. The asteriskin Schemes 1–3 stands for an exited state, and the dotted lines stand for multi-steppathways. During daylight the degradation of α-humulene is initiated primarily byOH addition to the double bond ‘C=C’ to form four possible hydroxyalkyl adducts(HML1, HML2, HML3, HML4: Scheme 1), followed by the addition of molecu-lar oxygen to produce peroxy radicals (PHML1, PHML2, PHML3, PHML4). Forsimplicity, Scheme 1 shows OH addition to only one end of the double bond. Theaddition to the other end is assumed to be less likely, but non negligible. Hydrogenatom abstraction can also occur (not presented here).

The peroxy radicals in the presence of NO decompose to form the correspond-ing alkoxy radicals, which decompose via ring opening to form 3-seco-α-humulonealdehyde, 7-seco-α-humulone aldehyde, and α-humulal aldehyde. The oxidation of3-seco-α-humulone aldehyde, 7-seco-α-humulone aldehyde, and α-humulal alde-hyde lead to 3-seco-α-humulaic-3-acid, 3-seco-α-humulaic-7-acid, α-humulalic10-acid, α-humulalic 11-acid, and α-humuladionic acid (Scheme 1: last twocolumns). Due to the complexity of the photooxidation chemistry of α-humulene(e.g., presence of one or two double bonds in reaction products, ozone, OHand NO3 radicals as oxidants), and for simplicity, only OH chemistry is pre-sented in this study. 3-Seco-α-humulone aldehyde, 7-seco-α-humulone aldehyde,and α-humulal aldehyde were observed in the early stage of the reaction, hencethis occurrence strongly suggests that all three bonds were oxidized as soon asα-humulene was injected to the chamber.

A closer look a the chromatograms recorded at the end of the reaction, showsthat the concentration of products observed in the beginning of the reaction de-creased significantly (Figure 6) and other products with low molecular weightwere observed. One explanation for this observation is that products bearing one


Scheme 2.

or two double bonds may undergo further oxidation by ozone, OH and/or NO3

radicals. Also, aldehydes and keto-aldehydes may undergo photolysis or react withOH and NO3 radicals leading to the decrease of their concentration over time. Asan example, the addition of OH radical to 3-seco-α-humulone aldehyde and 7-seco-α-humulone aldehyde is presented in Scheme 2. Scheme 2 shows OH addition toone or more ‘C=C’ double bonds leading to a variety of small molecular weightproducts. Compound with MW = 168 (Table I) is postulated to form via OHreaction with 3-seco-α-humulene aldehyde or 7-seco-α-humulene aldehyde.

Aldehydes observed in this study can undergo OH abstraction of their aldehydichydrogen atoms as illustrated for 7-seco-α-humulone aldehyde in Scheme 3. Theprimarily OH reaction is abstraction of an aldehydic hydrogen atom leading toacylperoxy radical ‘HMLAHDoo’. The reaction of acylperoxy radicals with HO2

or other peroxy radicals ‘RO2’ becomes important when the concentrations of NOand NO2 are very low, such as in remote areas (Finlayson–Pitts and Pitts, 2000).Under these conditions, the acylperoxy radical reacts with HO2 to form 3-seco-α-humulaic 7-acid, and 3-seco-α-perhumulaic 7-acid. The ‘C=C’ double bonds on3-seco-α-humulaic 7-acid can further react with OH radicals and/or ozone to formother products. Acylperoxy radicals can also undergo decomposition to form 7-seco-α-norhumulone aldehyde, which itself reacts with ozone and/or OH radicalsto form other products. 7-Seco-α-humulone aldehyde may also undergo abstractionof a secondary or tertiary H atom (not presented in Scheme 3).

α-Humulene chemistry is similar to that described in our previous papers forthe photooxidation of α-pinene and β-pinene (Kamens and Jaoui, 2001; Jaoui andKamens, 2001, 2002a–c). The ozone generated during the α-humulene photooxida-tion may subsequently react with reaction products still bearing one or two doublebonds. We found that the chemistry of hydrocarbons bearing two or more double


Scheme 3.

bonds is extremely complicated in systems where ozone, and/or OH and/or NO3

radicals are present.The α-Humulene 10-oxide, α-humulene 3-oxide, α-humulene 7-oxide observed

in this study are postulated to form from the reaction of α-humulene with peroxyradicals similar to the α-pinene oxide formation in α-pinene photooxidation (Jaouiand Kamens, 2001). Since, this study was undertaken under a relatively high con-centration of α-humulene and NOx , the formation of α-humulene oxides observedhere may also have originated from α-humulene reaction with the oxygen atom ‘O’(Alvarado et al., 1998).

6. Summary and Conclusion

A substantial number of reaction products in the gas and aerosol phases weretentatively identified and semi-quantified from the photooxidation of α-humulenein the presence of NOx , natural sunlight and air. This investigation showed thatall the three double bonds of α-humulene were oxidized leading to the reactionproducts observed in this work. Also, in this study we presented a time series fora wide range of reaction products identified for the first time in the gas and/or par-ticle phases. To account for most products observed in this investigation, detailedmechanisms were proposed.

Yields for individual products tentatively identified in both gas and/or particlephases have been estimated providing a direct measure of the gas-particle parti-tioning of each product. Identified products in both gas and particle phases are


estimated to account for about 44% of the total reacted mass of α-humulene.Using the response factor of surrogate standards, we were able to estimate theyields of many products, however, an improved carbon balance could be obtainedif authentic standards, or new analytical methods become available.

Acknowledgements

This work was supported by an EPA STAR grant (R-82817601-0) to the Universityof North Carolina at Chapel Hill. The authors would like to thank Sangdon Lee,Chalida U-tapao, Tananya Benjamartarakul, and Kitirote Wantala for helping withthe chamber experiments. Also our thanks go to Nadine Czoschke for editing andher helpful comments. We also wish to acknowledge a Varian Corporation gift toour department of a Saturn II GCITMS.

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