Rate Constants for the Gas-Phase Reactions of a Series of C3–C6

JCK(Wiley) RIGHT BATCH Rate Constants for the Gas- Phase Reactions of a Series of C3 –C6 Aldehydes with OH and NO3 Radicals CHRISTINE PAPAGNI,* JANET AREY,*,** ROGER ATKINSON*,**,† Air Pollution Research Center, University of California, Riverside, CA 92521 Received 28 July 1999; accepted 1 October 1999 ABSTRACT: By using relative rate methods, rate constants for the gas-phase reactions of OH and NO3 radicals with propanal, butanal, pentanal, and hexanal have been measured at 296 Ϯ 2 K and atmospheric pressure of air. By using methyl vinyl ketone as the reference compound, the rate constants obtained for the OH radical reactions (in units of 10Ϫ12 cm3 moleculeϪ1 sϪ1) were propanal, 20.2 Ϯ 1.4; butanal, 24.7 Ϯ 1.5; pentanal, 29.9 Ϯ 1.9; and hexanal, 31.7 Ϯ 1.5. By using methacrolein and 1-butene as the reference compounds, the Ϫ15 3 Ϫ1 Ϫ1 rate constants obtained for the NO3 radical reactions (in units of 10 cm molecule s ) were propanal, 7.1 Ϯ 0.4; butanal, 11.2 Ϯ 1.5; pentanal, 14.1 Ϯ 1.6; and hexanal, 14.9 Ϯ 1.3. The dominant tropospheric loss process for the aldehydes studied here is calculated to be by reaction with the OH radical, with calculated lifetimes of a few hours during daytime. ᭧ 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 79–84, 2000 INTRODUCTION Cn –aldehyde reacts to form mainly the Cn Ϫ 1 –aldehyde [8,9]. Carbonyl compounds are directly emitted into the tro- Rate constants for the gas-phase reactions of the posphere from biogenic and anthropogenic sources OH radical with formaldehyde [8,10]; acetaldehyde Ն [1–7] and are also formed in situ in the troposphere [8,10]; and the C3 aliphatic aldehydes propanal, bu- from the atmospheric reactions of volatile organic tanal, 2-methylpropanal, pentanal, 3-methylbutanal, compounds, including other carbonyl compounds and 2,2-dimethylpropanal [11–16] have been mea- [8,9]. In the troposphere, carbonyl compounds can un- sured at around room temperature. For the reactions dergo photolysis and reactions with OH radicals, NO 3 of the NO3 radical with aliphatic aldehydes, rate con- radicals, and O3 [8,9]. The available literature data in- stants have been measured for formaldehyde [8,10], dicate that aldehydes, RCHO, are generally more re- acetaldehyde [8,10], and a number of C3 –toC6 –al- active than are ketones (due to the weak C9H bond Ն dehydes [17,18]. However, for each of the C3 alde- in the 9CHO group) toward reaction with OH hydes studied to date, rate constants for the NO3 rad- and NO3 radicals [8,9], and in the troposphere a ical reaction are available from only a single study [17,18], and they increase with increasing carbon Correspondence to: R. Atkinson number more rapidly than expected if the reaction in- * Environmental Toxicology Graduate Program volves almost exclusively H-atom abstraction from the ** Also Department of Environmental Sciences 9 † Also Department of Chemistry CHO group [17,19]. In this work, we have mea- short ᭧ 2000 John Wiley & Sons, Inc. CCC 0538-8066/00/020079-06 sured rate constants for the reactions of OH and NO3 standard long JCK(Wiley) LEFT BATCH 80 PAPAGNI ET AL. Ϫ3 ϳ ϫ 14 radicals with propanal, butanal, pentanal, and hexanal molecule cm units) were CH3ONO, 2.4 10 ; at room temperature; no rate constant has been re- NO, ϳ2.4ϫ1014; and aldehydes and reference com- ported to date for the reaction of the OH radical with pound, ϳ2.4ϫ1013 each. Methyl vinyl ketone was hexanal. used as the reference compound because its OH radical reaction rate constant [23–25] is similar to those of Ն the C3 aldehydes measured to date [8] and it could EXPERIMENTAL be analyzed by gas chromatography using the same sampling procedure and gas chromatographic column The experimental methods were similar to those used as the aldehydes studied. Irradiations were carried out previously [20,21]. Experiments were carried out in a at 20% of maximum light intensity for 2–40 min, re- 6500-liter all-Teflon chamber at 296 Ϯ 2 K and 740 sulting in up to 57–76% reaction of the initially Torr total pressure of purified air at ϳ5% relative hu- present aldehydes or methyl vinyl ketone. Because no midity. The chamber is equipped with two parallel additions were made to the chamber during the OH banks of black lamps for irradiation and a Teflon- ϭ radical reactions, Dt 0 for these experiments. coated fan to ensure rapid mixing of reactants during In addition, irradiations of aldehyde (ϳ2.4 ϫ 1013 their introduction into the chamber. Rate constants for molecule cmϪ3) –cyclohexane (8.6 ϫ 1015 molecule Ϫ Ϫ the OH radical and NO3 radical reactions were mea- cm 3)orn-nonane (5.2 ϫ 1015 molecule cm 3)–air sured using relative rate methods in which the relative mixtures were carried out to assess the importance of disappearance rates of the aldehydes and a reference photolysis of the aldehydes during the OH radical re- compound, whose OH or NO3 radical reaction rate action rate constant determinations. Cyclohexane or n- constant is reliably known, were measured in the pres- nonane were present to scavenge any OH radicals ence of OH radicals or NO3 radicals [20,21]. Providing formed during these irradiations, carried out at 20% of that the aldehydes and the reference compound reacted the maximum light intensity for up to 60 min. only with OH radicals or NO radicals, then [20,21] 3 NO3 radicals were generated in the dark by the thermal decomposition of N2O5 [26], and methacrolein or [aldehyde]t ͩͪ0 Ϫ 1-butene was used as the reference compound because ln Dt [aldehyde]t their NO3 radical reaction rate constants [20,24] are of a similar magnitude to those for the ՆC aldehydes k [reference compound]t 3 ϭ 1 ͫͩ0 ͪ Ϫ ͬ ln Dt (i) [17,18]. The initial aldehyde and reference compound k [reference compound] 2tconcentrations were ϳ2.4 ϫ 1013 molecule cmϪ3 each, ϫ 13 Ϫ3 where [aldehyde] and [referenceorganic] are the and (4.8–9.6) 10 molecule cm of NO2 was also tt00 concentrations of the aldehyde and the reference com- included in the reactant mixture [27]. Three additions pound, respectively, at time t ; [aldehyde] and [ref- of N2O5 were made to the chamber during an experi- 0 t ment, with each addition corresponding to an initial erence organic]t are the corresponding concentrations concentration of (1.21–11.0) ϫ 1013 molecule cmϪ3 at time t; Dt is a factor to account for dilution due to any additions to the chamber during the reactions; and of N2O5 in the chamber. For these experiments, the value of Dt was 0.0028 per N2O5 addition to the cham- k1 and k2 are the rate constants for reactions (1) and (2), respectively. ber. The concentrations of the aldehydes and the reference compounds were measured using gas chromatog- OH ϩ !: ͮ aldehyde products (1) raphy with flame ionization detection (GC-FID). For NO3 the analysis of propanal, butanal, pentanal, hexanal, OH methyl vinyl ketone, and methacrolein, 100-cm3 gas ͮ ϩ reference organic !: products (2) NO3 samples were collected from the chamber onto Tenax- TA solid adsorbent with subsequent thermal desorp- Ϫ ϳ Њ Therefore, plots of {ln([aldehyde]t0/[aldehyde]t) Dt} tion at 225 C onto a 30-m DB-1701 megabore col- Ϫ Њ against {ln([reference organic]t0/ [reference organic]t) umn held at 40 C and then temperature programmed Ϫ Њ Њ Ϫ1 Dt} should be straight lines with slope k1/k2 and zero to 240 Cat8C min . For the analysis of 1-butene, intercept. gas samples were collected from the chamber in 100 OH radicals were generated by the photolysis of cm3 all-glass, gas-tight syringes, and transferred via a Ͼ 3 methyl nitrite (CH3ONO) in air at wavelengths 300 1-cm stainless steel loop and gas sampling valve onto nm [22], and NO was added to the reactant mixtures a 30m DB-5 megabore column held at Ϫ25ЊC and then Њ Њ Ϫ1 to suppress the formation of O3 and hence of NO3 temperature programmed to 200 Cat8C min . short radicals [22]. The initial reactant concentrations (in Based on replicate analyses in the dark, the GC-FID standard long JCK(Wiley) RIGHT BATCH RATE CONSTANTS FOR GAS-PHASE REACTIONS OF C3–C6 ALDEHYDES 81 measurement uncertainties for the aldehydes and reference compounds were Յ5% (and generally in the range 1–3%). The NO and initial NO2 concentrations were measured using a Thermo Environmental Instru- ments Model 42 chemiluminescence NO–NO2 –NOx analyzer. The chemicals used, and their stated purities, were propanal (99%), butanal (99%), pentanal (99%), hexanal (99%), methyl vinyl ketone (99%), and methacrolein (95%), Adrich Chemical Co.; NO (Ն99%) and 1-butene (Ն99.0%), Matheson Gas Products; n-nonane, (99.9%), Burdick and Jackson; and cyclohexane (HPLC grade), Fisher Scientific. Methyl nitrite and N2O5 were prepared and stored as described previously [22,26]. NO2 was prepared as needed by reacting NO with an excess of O2. RESULTS Figure 1 Plots of Eq. (i) for the gas-phase reactions of the OH radical with propanal, butanal, pentanal, and hexanal, with methyl vinyl ketone as the reference compound. The OH Radical Rate Constants data for butanal, pentanal, and hexanal have been displaced Photolysis of propanal–butanal–pentanal–n-nonane vertically by 0.2, 0.4, and 0.6 units, respectively, for clarity. (in excess)–air and hexanal–cyclohexane (in excess)–air mixtures showed Ͻ3% losses of any of the aldehydes over 60-min irradiation periods (at the same The data obtained from reacting NO3 –N2O5 –NO2 – light intensity as used in the OH radical reaction rate aldehyde–methacrolein–air mixtures are plotted in constant determination experiments).

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