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Atmospheric Environment 43 (2009) 4043–4049

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Atmospheric Environment

journal homepage: www.elsevier .com/locate/atmosenv

Influence of temperature on the chemical removal of 3-methylbutanal, trans-2-methyl-2-butenal, and 3-methyl-2-butenal by OH radicals in the troposphere

Elena Jimenez, Beatriz Lanza, Marıa Antinolo, Jose Albaladejo*

Departamento de Quımica Fısica, Facultad de Ciencias Quımicas, Universidad de Castilla-La Mancha, Avda. Camilo Jose Cela, s/n. 13071 Ciudad Real, Spain

a r t i c l e i n f o

Article history:Received 11 February 2009Received in revised form30 April 2009Accepted 5 May 2009

Keywords:AldehydesOH kineticsAbsolute rate coefficientsAtmospheric lifetimesHomogeneous reaction

* Corresponding author.E-mail address: Jose.Albaladejo@uclm.es (J. Albala

1352-2310/$ – see front matter � 2009 Published bydoi:10.1016/j.atmosenv.2009.05.005

a b s t r a c t

Absolute rate coefficients for the gas-phase reactions of OH radical with 3-methylbutanal (k1), trans-2-methyl-2-butenal (k2), and 3-methyl-2-butenal (k3) have been obtained with the pulsed laser photolysis/laser-induced fluorescence technique. Gas-phase concentration of aldehydes was measured by UVabsorption spectroscopy at 185 nm. Experiments were performed over the temperature range of 263–353 Kat total pressures of helium between 46.2 and 100 Torr. No pressure dependence of all ki (i ¼ 1–3) wasobserved at all temperatures. In contrast, a negative temperature dependence of ki (i.e., ki increases whentemperature decreases) was observed in that T range. The resulting Arrhenius expressions (�2s) are:k1(T) ¼ (5.8 � 1.7)�10�12 exp{(499 � 94)/T} cm3 molecule�1 s�1, k2(T)¼(6.9 � 0.9)�10�12 exp{(526 � 42)/T} cm3 molecule�1 s�1, k3(T)¼(5.6 � 1.2)�10�12 exp{(666 � 54)/T} cm3 molecule�1 s�1.The tropospheric lifetimes derived from the above OH-reactivity trend are estimated to be higher for3-methylbutanal than those for the unsaturated aldehydes. A comparison of the tropospheric removal ofthese aldehydes by OH radicals with other homogeneous degradation routes leads to the conclusion thatthis reaction can be the main homogeneous removal pathway. However, photolysis of these aldehydesin the actinic region (l > 290 nm) could play an important role along the troposphere, particularly for3-methyl-2-butenal. This process could compete with the OH reaction for 3-methylbutanal or benegligible for trans-2-methyl-2-butenal in the troposphere.

� 2009 Published by Elsevier Ltd.

1. Introduction

In general, aldehydes are emitted into the lower atmospherefrom vegetation and human activities such as road vehicles andindustries, although they are also formed in situ in the tropospheredue to the atmospheric photooxidation reactions of unsaturatedhydrocarbons and volatile organic compounds (VOCs) (Atkinsonand Arey, 2003; Grosjean et al., 2001; Kirstine et al., 1998; Owenet al., 1997; Wildt et al., 2003). These carbonyl compounds are toxicthemselves and some of their photooxidation products, such asperoxyacylnitrates (PAN), are phytotoxic and strong eye-irritantcompounds. Aldehydes are known to be an important contributionto the formation of ozone and secondary aerosols through hetero-geneous reactions (Jang et al., 2002).

During the last decade an increasing number of oxygenated VOCshave been detected in field measurement campaigns and plantemission studies (e.g., De Gouw et al., 1999; Fall et al., 2001; Kirstineet al., 1998; Owen et al., 1997; Wildt et al., 2003). Among these VOCs,methylbutanals have been identified from anthropogenic (Kean

dejo).

Elsevier Ltd.

et al., 2001) and biogenic sources (De Gouw et al., 1999; Fall et al.,2001). Also methylbutenals, such as 2-methyl-2-butenal and3-methyl-2-butenal, have been proposed as oxidation products ofisoprene initiated by OH radicals in the presence of NO (Yu et al.,1995; Park et al., 2003). Additionally, 3-methyl-2-butenal was alsoidentified in the OH reaction with 3-methyl-1-butene in the pres-ence of NO (Atkinson et al., 1998).

In the troposphere these aldehydes can be removed by OH (duringday-time), NO3 (during night-time), O3 (mainly with unsaturatedcompounds) and halogen atoms (mainly Cl, which can be important inmarine, coastal, and industrial areas) (Atkinson and Arey, 2003; Fin-layson-Pitts and Pitts, 2000). Furthermore, photolysis of the unsatu-rated aldehydes can also become an important removal process forthesecompounds in the actinic region (l>290 nm) (Lanza et al., 2008).

The OH-kinetics of 3-methylbutanal ((CH3)2CHCH2CHO) hasbeen widely studied at 298 K and as a function of pressure(100–760 Torr) (D’Anna et al., 2001; Glasius et al., 1997; Kerr andSheppard, 1981; Semmes et al., 1985). Room temperature ratecoefficient for the reaction of 3-methylbutanal with NO3 wasreported by relative (D’Anna et al., 2001) and absolute (Cabanaset al., 2003) methods. No kinetic measurements on the reaction ofO3 or Cl atoms with 3-methylbutanal were found in the literature.

E. Jimenez et al. / Atmospheric Environment 43 (2009) 4043–40494044

In the case of trans-2-methyl-2-butenal ((E)-CH3CH¼C(CH3)CHO), only its gas-phase kinetics towards O3 has been studied bySato et al. (2004). Although no experimental data are available forthe reaction of OH with trans-2-methyl-2-butenal, Grosjean andWilliams (1992) reported an estimation of its rate coefficient. Roomtemperature rate coefficients for the reaction of OH radicals and O3

with 3-methyl-2-butenal ((CH3)2C¼CHCHO) were reported byTuazon et al. (2005) and Sato et al. (2004), respectively. Again, nokinetic measurements on the reaction of NO3 or Cl atoms with thisunsaturated aldehyde were found in the literature.

Thus, in this work, we report the first kinetic study on thetemperature dependence (T ¼ 263–353 K) of the rate coefficientsof the reaction of OH radicals with 3-methylbutanal (k1), trans-2-methyl-2-butenal (k2), and 3-methyl-2-butenal (k3):

OHDðCH3Þ2CHCH2CHO/Products (1)

OHDðEÞ � CH3CH [ CðCH3ÞCHO/Products (2)

OHDðCH3Þ2C [ CHCHO/Products (3)

As far as we know, k2(298 K) and k3(298 K) constitute the firstmeasurement and the first absolute measurement reported, respec-tively. A discussion based on the different reactivity of these aldehydestowards OH radicals is presented together with a comparison withother degradation routes such as the reaction with NO3 radicals and O3

or photolysis in the actinic region.

2. Experimental and methodology

2.1. Description of the experimental apparatus

The experimental technique used was the Pulsed Laser Photol-ysis and Laser Induced Fluorescence (PLP/LIF). The experimentalset-up has already been described in previous kinetic studies(Albaladejo et al., 2002; Jimenez et al., 2005, 2007). All the exper-iments were performed using a jacketed Pyrex reaction cell of200 cm3. A cooling liquid (ethanol) or a heating liquid (water) wasflowed through the jacket of the reaction cell by means ofa refrigerated/heating circulator (Julabo, model FP 50).

OH precursor was gaseous hydrogen peroxide (H2O2) which wasobtained by bubbling a small flow of carrier gas (He) through anaqueous solution of H2O2. This small gas flow (3–10 sccm, STP cm3

per min) was introduced into the reaction cell, where joints a knownflow (2–10 sccm) of diluted aldehyde (from a bulb with an aldehyde/He mixture) and a carrier gas flow (250–500 sccm). The dilutionfactors in the storage bulbs, f (defined as f¼ paldehyde/{paldehydeþpHe})ranged from 3.44�10�4 to 6.50�10�4. During a kinetic experimentthe flow rates of H2O2 and bath gas were maintained constant. Thus,the aldehyde concentration was changed by varying the flow rateof the aldehyde/He mixture through the cell (see below).

At the centre of the reaction cell, OH radicals were obtained by the248-nm photolysis of H2O2 with a KrF excimer laser (Lambda Physik,model OPTex). The photolysis laser fluence was varied from 0.42 to14.6 mJ pulse�1 cm�2. Taking into account the absorption cross-sections measured by Lanza et al. (2008), no secondary reactions dueto the photolysis products of the aldehydes (<1%) at 248 nm interferein the kinetic experiments at the laser fluences employed. OHradicals were excited by using the frequency-doubled output of a Nd-YAG-pumped dye laser (Continuum, model NY81CS-10 and ND60,respectively) at 282 nm (A2Sv’¼1)X2Pv’’¼0). Off-resonance fluores-cence was filtered by a bandpass filter (Schott, BG3) with a maximumof transmittance of 90% at 350 nm and FWHM ¼ 150 nm. Then, thefluorescence signal was focused onto a photomultiplier tube (ThornEMI, model 9813B) and subsequently analyzed by a computer.

2.2. Measurement of aldehyde concentrations

Aldehyde concentrations were determined by UV absorptionspectroscopy at 185 nm. Firstly, absorption cross-sections (sl¼185 nm)of these aldehydes were obtained at 185 nm by using a Hg/Ar pen-raylamp. A known pressure of different aldehyde/He mixtures (f ¼(3.44–6.50) � 10�4) was introduced into the absorption cell froma blackened storage bulb. The transmitted light intensity wascollected by a solar blind phototube (Hamamatsu, model R5764)coupled to an interference filter (Acton Research Co., 185-N-1D) witha maximum transmittance of 18.9% at 182.8 nm and FWHM ¼26.1 nm. A picoamperimetre (Keithley Instruments Inc., 6485) wasused as a read-out of the detected light by the phototube. Absorptioncross-sections at 185 nm were obtained from the slope of the plot ofthe absorbance in base e (Al) versus partial pressure as describedearlier (Jimenez et al., 2007, 2009). Plots of Al versus the aldehydeconcentration, obtained from the partial pressure in the UV cell, areshown in Fig. 1S (supplementary information). Concentrations of3-methylbutanal in Fig. 1S appeared multiplied by two to avoid theoverlap with Al from 3-methyl-2-butenal. The resulting sl¼185 nm

were (1.02 � 0.04)� 10�17 cm2 for 3-methylbutanal, (2.07 � 0.09)�10�17 cm2 for trans-2-methyl-2-butenal, and (1.06 � 0.01) � 10�17

cm2 for 3-methyl-2-butenal. The uncertainty in sl¼185 nm representstwice the standard deviation from the fit, defining the weightingfactor as the reciprocal of the variance of each measurement.

During the kinetic experiments aldehyde concentrations weredetermined at each flow rate set before and after the reaction cellby using the measured sl¼185 nm and the Beer–Lambert0s law withthe same experimental apparatus used in the determination ofsl¼185 nm. The ranges of the measured aldehyde concentrationswere: (0.16–2.70) � 1014 cm�3 for 3-methylbutanal, (0.30–6.00) � 1014 cm�3 for trans-2-methyl-2-butenal, and (0.12–2.70) � 1014 cm�3 for 3-methyl-2-butenal. Typically, theseconcentrations are lower than those estimated by flow measure-ments. The average difference observed between both methodswas 4% for 3-methylbutanal and 15% for the unsaturated aldehydes.

2.3. Kinetic data analysis

The OH concentration loss is due to its reaction with the alde-hyde, the OH precursor and its diffusion out of the detection zone.Thus, the reaction scheme is:

OHDAldehyde/Products ki ð1—3Þ

OHDH2O2/H2ODHO2 kprec (4)

OH/Diffusional Loss kdiff (5)

Under pseudo-first order conditions ([Aldehyde]>>[OH]0,[Aldehyde]/[OH]0¼90–9000), the temporal profile of the OHconcentration can be described by a single exponential:

½OH�t [ ½OH�0expðLk0tÞ (6)

where [OH]t is the OH concentration at each reaction time t, [OH]0 isthe initial OH concentration (both are proportional to the intensity ofthe fluorescence signal, ILIF), and k0 is the pseudo-first order ratecoefficient. Bimolecular rate coefficients (ki) were obtained at eachtemperature from the slope of the plot of k0 versus different aldehydeconcentrations, according to:

k0[ ki½Aldehyde�Dk0 (7)

k0 [ kprec½H2O2�Dkdiff (8)

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0.0E+00 5.0E+13 1.0E+14 1.5E+14 2.0E+14 2.5E+14 3.0E+14

[3-methylbutanal]/ cm-3

100.0 Torr

55.6 Torr

71.0 Torr

49.5 Torr

80.8 Torr

0

5000

10000

15000

20000

25000

30000

8.0E+14

73.0 Torr

58.0 Torr

50.0 Torr

51.8 Torr

54.2 Torr

k'-k

0/ s

-1k'

-k0/

s-1

0.0E+00 2.0E+14 4.0E+14 6.0E+14

a

b

E. Jimenez et al. / Atmospheric Environment 43 (2009) 4043–4049 4045

where k0 (213–1220 s�1) is the pseudo-first rate coefficient due toprocesses (4) and (5). k0 allowed the estimation of H2O2 concen-trations in the reaction cell, assuming that kdiff was negligible andthe temperature dependence of kprec recommended by the JetPropulsion Laboratory (Sander et al., 2006). Upper limits of H2O2

concentrations were (1.3–7.1) � 1014 cm�3. Taking into account theH2O2 absorption cross-section and the OH quantum yield at 248 nm(Sander et al., 2006) together with [H2O2] and the fluences employedin this work, upper limits for OH concentration were calculatedto range from 2.5 �1010 cm�3 to 2.0� 1012 cm�3 in the reaction cell.Typically, OH concentrations were on the order of 1011 cm�3.

In the presence of aldehyde, k0 (1300–25000 s�1) were obtainedfrom the non-linear analysis of the temporal profile of ILIF at eachaldehyde concentration at a constant temperature and total pres-sure. An example of the linearised equation (6) in terms of ILIF

recorded in the absence and in the presence of aldehyde at 298 K isshown in Fig. 1. The OH decay due to its reaction with 3-methyl-2-butenal, H2O2, and the diffusion loss has been multiplied by a factorof two for distinguishing it from that of trans-2-methyl-2-butenal.The good linearity of these plots (over four OH lifetimes) suggeststhat secondary reactions of OH radicals were negligible at the lowOH concentration used in these experiments.

In Fig. 2 all room temperature kinetic data are plotted for eachaldehyde at different total pressures between 46.2 and 100 Torrof He. As can be seen, no significant pressure dependence ofthe bimolecular rate coefficients was observed in all cases. At alltemperatures, the intercept of these plots was always close to zeroimplying the absence of unwanted secondary chemistry.

2.3.1. ReactantsHe (Praxair, 99.999%) was employed as the bath gas as supplied.

An aqueous solution of H2O2 (Scharlau, 50% w/v) was concentratedby bubbling He bath gas through it for several days prior use. Allaldehydes were from Aldrich with the purities given in brackets:3-methyl-2-butenal (97%), trans-2-methyl-2-butenal (96%), and3-methylbutanal (97%). 3-methyl-2-butenal was introduced intothe glass vessel under inert atmosphere in order to avoid itsoxidation by ambient air. The aldehydes were degassed by freeze/pumped/thaw cycles prior use.

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

0.00 0.50 1.00 1.50

t (ms)

ln(I

LIF

)

k3'(×2) = 10478 s-1

k1' = 3294 s-1

k2' = 4924 s-1

k0= 743 s-1

Fig. 1. Examples of the OH temporal profiles obtained in absence of aldehyde (B) andin the presence of 3-methylbutanal (C, 1.1 � 1014 cm�3), trans-2-methyl-2-butenal(,, 1.5 � 1014 cm�3), and 3-methyl-2-butenal (:, 1.1 � 1014 cm�3) at 298 K.

[trans-2-methyl-2-butenal]/ cm-3

0

2000

4000

6000

8000

10000

12000

0.0E+00 5.0E+13 1.0E+14 1.5E+14 2.0E+14 2.5E+14

[3-methyl-2-butenal]/ cm-3

k'-k

0/ s

-1

60.0 Torr

72.0 Torr

46.2 Torr

56.0 Torr

49.5 Torr

c

Fig. 2. Plots of the pseudo-first order rate coefficient versus the concentration of3-methylbutanal (a) trans-2-methyl-2-butenal (b) and 3-methyl-2-butenal (c) at 298 Kas a function of total pressure.

E. Jimenez et al. / Atmospheric Environment 43 (2009) 4043–40494046

3. Results and discussion

3.1. Room temperature rate coefficients

A summary of the bimolecular rate coefficients for the reaction ofOH radical with 3-methylbutanal, trans-2-methyl-2-butenal, and3-methyl-2-butenal obtained in this work at 298 K is given in Table 1.The error limits given include only the precision of the fit to ourexperimental data (one standard deviation,�s). Our results show nodependence of ki on total pressure between 46.2 and 100 Torr of He.

For 3-methylbutanal, a pressure independence of k1 between46.2 and 760 Torr is shown in Table 1. Our result at room temperatureis in excellent agreement with those reported in the bibliographyobtained by D’Anna et al. (2001), Semmes et al. (1985), and Kerr andSheppard (1981). k1(298 K) obtained by Glasius et al. (1997)disagrees considerably with all the reported values. The reason forthis discrepancy remains unclear. Structure–reactivity relationship(SAR) estimation of k1(298 K) (2.95 � 10�11 cm3 molecule�1 s�1)based on the method proposed by Kwok and Atkinson (1995) is inexcellent agreement with our result. This agreement is also observedfor SAR rate coefficients k2(298 K) (3.89� 10�11cm3 molecule�1 s�1)and k3(298 K) (4.67 � 10�11 cm3 molecule�1 s�1). Regarding 3-methyl-2-butenal, there is a previous relative value of k3(298 K)reported at 740 Torr by Tuazon et al. (2005) (see Table 1), whichshows a deviation outside the error limits from our absolutek3(298 K). The source of such a discrepancy is not clear. The differ-ence in total pressure and the bath gas used would not be the reason,since for other unsaturated aldehydes, such as acrolein and trans-crotonaldehyde (trans-2-butenal), the OH-rate coefficient wasindependent on pT (from 20 Torr to atmospheric pressure) and thebath gas used (air or He) (Magneron et al., 2002).

As far as we know, no product studies have been reported for theOH reaction with 3-methylbutanal and trans-2-methyl-2-butenal.According to the SAR method of Kwok and Atkinson (1995), theH-abstraction from the CHO group is estimated to account for 70%and 43% of the overall rate coefficients k1 and k2, respectively. In theunsaturated aldehyde around 56% of the overall k2(298 K) isexpected to be due to the addition channel. This estimation is inexcellent agreement with the observations of Tuazon et al. (2005)in an end-product study of the reaction of OH radicals with3-methyl-2-butenal. These authors reported that the OH-additionto the double bond and the H-atom abstraction from the CHO groupaccounts for (53 � 4)% and (40 � 6)%, respectively, of the overallk3(298 K).

Table 1Summary of the kinetic studies on the reaction of OH radical with 3-methylbutanal(k1), trans-2-methyl-2-butenal (k2), and 3-methyl-2-butenal (k3) at roomtemperature.

pT/Torr ki�1011/cm3

molecule�1 s�1Techniquea Reference

CompoundReference

3-methylbutanal49.5–100 2.97 � 0.19 PLP-LIF – This work760 2.79 � 0.07 RR-FTIR 1-butene D’Anna et al. (2001)740 4.00 � 0.70 RR-FTIR isoprene Glasius et al. (1997)100 2.58 � 0.40 FP-RF – Semmes et al. (1985)

2.68 � 0.11 RR-GC ethene Kerr andSheppard (1981)

trans-2-methyl-2-butenal50.6–73 4.08 � 0.31 PLP-LIF – This work

3-methyl-2-butenal46.2–72.0 4.77 � 0.53 PLP-LIF – This work740 6.21 � 0.65 RR-FTIR methacrolein Tuazon et al. (2005)

a PLP-LIF, Pulsed laser photolysis with laser induced fluorescence; RR, RelativeRate; FTIR, Fourier transform infrared; FP-RF, Flash Photolysis with ResonanceFluorescence; GC, gas chromatography.

3.2. Comparison of the reactivity of saturated and unsaturatedaldehydes with OH

In Table 2 a comparison of the OH-reactivity with a series ofbranched and linear aldehydes is presented for both saturated andunsaturated aldehydes. As can be seen, alkyl substitution increasesthe OH-rate coefficient both in branched saturated and unsatu-rated aldehydes respect to the corresponding linear aldehyde. Thatincrease is slightly higher for a methyl substitution in a positionrespect to the CHO group (i.e., 2-methylbutanal) than for a methylsubstitution in b position (i.e., 3-methylbutanal). Also, the substi-tution of an ethyl group (in 2-ethylbutanal and 2-ethyl-2-butenal)increases the OH-rate coefficient respect to that of the methyl-substituted aldehyde (2-methylbutanal and trans-2-methyl-2-butenal).

The fact that H-atom abstraction is mainly occurring in the CHOgroup, but also in the neighboring carbon atoms (Atkinson andArey, 2003), is noticeable in the increase of kOH with the length ofthe hydrocarbon chain. The increase in kOH is less pronounced inlonger saturated and unsaturated aldehydes.

The effect of the unsaturation in the OH-reactivity is also seen inTable 2. In all cases (except for acrolein), an increase of the OH-ratecoefficient respect to that of the corresponding saturated aldehydehas been observed. That increase ranges from 11% (in 2-methyl-propanal) to around 80% (in hexanal). A similar effect is observedwhen comparing the OH-reactivity with trans-2-methyl-2-butenal(4.08 � 10�11 cm3 molecule�1 s�1) and the corresponding saturatedaldehyde, 2-methylbutanal (3.28� 10�11 cm3 molecule�1 s�1, D’Annaet al., 2001). That increase of k manifests a large contribution of theaddition channel to the overall OH-rate coefficient.

Therefore, the rate coefficients reported in this work confirm theOH-reactivity trends for the effect of the unsaturation and the alkylsubstitution and its position respect to the CHO group reportedby other authors (i.e., Atkinson and Arey (2003) and referencestherein).

3.3. Effect of temperature on ki (i ¼ 1–3)

A summary of the rate coefficients for reactions (1)–(3) deter-mined as a function of temperature (263–353 K) is given in Table 1S(supplementary information). These values are the weightedaverage of several measurements at each temperature and theerror limits include only the precision of the fit to our experimentaldata (�s).

The averaged rate coefficients listed in Table 1S are presented ina semilog plot versus 1/T in Fig. 3. The resulting Arrheniusexpressions are represented by solid lines in Fig. 3 and the equa-tions that described this behaviour are:

k1(T) ¼ (5.8 � 1.7)�10�12 exp{(499 � 94)/T} cm3 molecule�1 s�1

k2(T)¼ (6.9 � 0.9)�10�12 exp{(526 � 42)/T} cm3 molecule�1 s�1

k3(T) ¼ (5.6 � 1.2)�10�12 exp{(666 � 54)/T} cm3 molecule�1 s�1

The uncertainty stated is �2s. Negative activation energies forall reactions were found to be in concordance with the tempera-ture dependence observed for other saturated and unsaturatedaldehydes (Atkinson and Arey, 2003; Jimenez et al., 2007; Mag-neron et al., 2002). Global activation energies for the reaction ofOH with 3-methylbutanal (�4.2 kJ mol�1), trans-2-methyl-2-butenal (�4.4 kJ mol�1), and 3-methyl-2-butenal (�5.5 kJ mol�1)are similar. Despite the negative temperature dependenceobserved for 3-methylbutanal, the main reaction pathway forsaturated aldehydes with OH radicals seems to be H-atomabstraction from the CHO group (Albaladejo et al., 2002; Atkinsonand Arey, 2003) via complex mechanism with the possibleformation of hydrogen-bonded adducts as suggested by Smith andRavishankara (2002).

Table 2Comparison between the OH-rate coefficients (10�11 cm3 molecule�1 s�1) for a series of (branched and linear) saturated and unsaturated aldehydes at room temperature.

Branched Aldehyde Linear Aldehyde

Saturated Unsaturated Saturated Unsaturated

2-methylpropanal methacrolein propanal acrolein2.60a 2.90d 2.00a 1.99d

2-methylbutanal trans-2-methyl-2-butenal3.28b 4.08c

2-ethylbutanal 2-ethyl-2-butenal, 4.75e butanal trans-crotonaldehyde4.0a 4.75e 2.88f 3.35d

3-methylbutanal 3-methyl-2-butenal2.97c 4.77c

2-methylpentanal 2-methyl-2-pentenal pentanal trans-2-pentenal3.0a 4.76e 2.48f 4.29h

– – hexanal trans-2-hexenal2.60f, 2.78g 4.68g

– – heptanal trans-2-heptenal2.96f 4.40h

a Atkinson and Arey (2003).b D’Anna et al. (2001).c This work.d Magneron et al. (2002).e Kwok and Atkinson (1995).f Albaladejo et al. (2002).g Jimenez et al. (2007).h Davis et al. (2007).

E. Jimenez et al. / Atmospheric Environment 43 (2009) 4043–4049 4047

4. Conclusions and atmospheric implications

These VOCs can be removed from the troposphere by differentpathways, such as homogeneous and heterogeneous reactions,photolysis in the actinic region, and wet and dry deposition. Otherelimination processes rather than homogeneous reactions andphotochemical degradation are expected to be negligible for thesecompounds. For example, although no Henry’s law coefficients(kH,cp) have been reported for 3-methylbutanal, trans-2-methyl-2-butenal, and 3-methyl-2-butenal, by comparison with other alde-hydes kH,cp could range from 4.3 M atm�1 (2-methylpropenal) to60 M atm�1 (trans-2-butenal). At the planetary boundary layer(ca. 1 km) the wet deposition lifetime estimated as described inJimenez et al. (2009) would range from 1 year (using an averageprecipitation rate of 500 mm/year) to 67 years (using an averageprecipitation rate of 145 mm/year). This means that the low

10-11

2

3

4

5

6

7

8

910-10

k i/ c

m3

mol

ecul

e-1 s-1

4.0x10-33.83.63.43.23.02.8

(1/T)/ K-1

3-methylbutanal 3-methyl-2-butenaltrans-2-methyl-2-butenal

Fig. 3. Arrhenius plot for the OH-rate coefficients for 3-methylbutanal (C), trans-2-methyl-2-butenal (,), and 3-methyl-2-butenal (-).

solubility in water of these aldehydes makes negligible this elimi-nation pathway in the troposphere. Thus, only photolysis of thesealdehydes in the actinic region and the homogeneous reactions areconsidered here to estimate their tropospheric lifetime (si) whichcan be defined as:

1si¼ 1

shnþ 1

sOxid¼ Ji þ

XkOxid½Oxid� (9)

where Ji is the photolysis rate coefficient of the aldehyde, kOxid is therate coefficient for the reaction of the aldehydes with an averagetropospheric concentration of a particular oxidant (OH, NO3, and O3),[Oxid]. Averaged (24-h) oxidant concentrations used to estimate sOxid

were 7.0� 1011 molecule cm�3 of O3, 2.5�108 molecule cm�3 of NO3,and 1�106 radical cm�3 of OH (Atkinson, 2000). Taking into accountthe OH-rate coefficients obtained in this work at room temperaturefor reactions (1)–(3), the tropospheric lifetime of these VOCs due tothe OH reaction has been estimated to be 9.4 h for 3-methylbutanal,6.8 h for trans-2-methyl-2-butenal, and 5.8 h for 3-methyl-2-butenal.Considering k298K from Cabanas et al. (2003), sNO3

for 3-methyl-butanal is around several days. Also, sO3

estimated for trans-2-methyl-2-butenal and 3-methyl-2-butenal using kO3

from Sato et al. (2004)are 12.7 and 70.3 h, respectively. Thus, homogeneous reaction withOH is the major sink for these C5 aldehydes.

Regarding the photochemical removal of these aldehydes, Ji inthe actinic region have been recently calculated in our laboratory asa function of altitude (Lanza et al., 2008). The removal of 3-methyl-butanal by photolysis was concluded to occur in less than 10 h.However, similar to trans-2-hexenal, photoisomerization of trans-2-methyl-2-butenal is likely to occur, being the photodissociationdegradation route less important or even negligible. In contrast,photolysis of 3-methyl-2-butenal is expected to be more important.

The relative importance of the OH reaction and photolysis in theelimination of these aldehydes along the troposphere can be seen inFig. 4. The Arrhenius expressions obtained in this work and an averageenvironmental lapse rate (�dT/dz) of 6.5 K km�1 were used to calculatethe bimolecular rate coefficients between 0 and 10 km of altitude(considering a temperature of 298.15 K at 0 km). Loss rates of thesealdehydes due to the photolysis process were calculated as a functionof altitude taken the absorption cross-sections from Lanza et al. (2008).For 3-methylbutanal photolysis quantum yields reported by Zhu et al.

0

2

4

6

8

10

12

1.00E-05 1.00E-04 1.00E-03

3-methylbutanal Loss Rate (s-1)

z (k

m)

0

2

4

6

8

10

12

1.00E-05 1.00E-04 1.00E-03

3-methyl-2-butenal Loss Rate (s-1)

z (k

m)

a

b

Fig. 4. Comparison between the removal rates of 3-methylbutanal (a) and 3-methyl-2-butenal (b) by the reaction with OH (-) and photolysis (,). Total loss rates aresymbolized by full circles (C).

E. Jimenez et al. / Atmospheric Environment 43 (2009) 4043–40494048

(1999) were employed in the calculation of J values. In the case of3-methyl-2-butenal photolysis quantum yields can be expected to belower than unity. As clearly seen in Fig. 4a, the photolysis of 3-methyl-butanal can compete with OH reaction from 0 km to the tropopause. Inthe case of 3-methyl-2-butenal photolysis can be important even if thephotolysis quantum yield is 0.05 (Fig. 4b). Under these circumstances,the photodissociation of 3-methyl-2-butenal would compete with thereaction with OH as a tropospheric sink. Therefore, further studies onthe quantum yields of 3-methyl-2-butenal are needed to better eval-uate the importance of this degradation route.

In the light of our results, we can conclude that OH reaction is themain homogeneous removal process at 0 km for 3-methylbutanaland trans-2-methyl-2-butenal. However, photolysis and OH radicalscan compete by the removal of 3-methylbutanal. In contrast, thephotodissociation channel could be of great importance (even moreimportant than OH-kinetics) for 3-methyl-2-butenal.

Acknowledgements

The authors would like to thank the Spanish Ministerio deEducacion y Ciencia and the Consejerıa de Educacion y Ciencia de laJunta de Comunidades de Castilla-La Mancha for supporting thisresearch. Also, B. Lanza and M. Antinolo wish to thank theseinstitutions for providing them a grant.

Appendix. Supplementary material

Supplementary data associated with this article can be found inthe online version at doi:10.1016/j.atmosenv.2009.05.005

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