12252
J. Phys. Chem. A 2002, 106, 12252-12259
Mechanisms for the Reactions of OH with Two Unsaturated Aldehydes: Crotonaldehyde and Acrolein John J. Orlando* and Geoffrey S. Tyndall Atmospheric Chemistry DiVision, National Center for Atmospheric Research, P.O. Box 3000, Boulder, Colorado 80307-3000 ReceiVed: June 28, 2002; In Final Form: October 11, 2002
The mechanisms for the reaction of OH with the unsaturated aldehydes, acrolein and crotonaldehyde, have been determined at 1 atm total pressure in the presence of NOx using an environmental chamber/FTIR spectrometer system. Products observed in the OH-initiated oxidation of acrolein were CO, CO2, CH2O, HOCH2CHO (glycolaldehyde), and HCOOH, while the major products identified in the OH-initiated oxidation of crotonaldehyde were CO, CO2, CH3CHO, and HC(O)CHO (glyoxal). Also observed were two PANtype species, identified as CH2dCH-C(O)O2NO2 (APAN) from acrolein oxidation and CH3-CHdCHC(O)O2NO2 (CPAN) from crotonaldehyde. The near-complete mass balance obtained in these experiments allows for a quantitative assessment of the branching ratios for abstraction and addition in these reactions. It is shown that about 68% (50%) of the OH reaction with acrolein (crotonaldehyde) proceeds via abstraction of the aldehydic H, with the remainder occurring via addition to the double bond. The data allow for a more accurate assessment of the atmospheric source strength of APAN, a species which has now been identified in ambient air. Trends in the reactivity of acrolein and its methylated derivatives, methacrolein and crotonaldehyde, are also discussed; data are shown to be consistent with structure-reactivity considerations.
Introduction Unsaturated aldehydes (acrolein, CH2dCH-CHO, and its methylated derivatives methacrolein, CH2dC(CH3)-CHO, and crotonaldehyde, CH3-CHdCH-CHO) are present in the atmosphere as the result of direct anthropogenic emissions (e.g., from combustion sources), or as the result of the oxidation of dienes. For example, it is well know that methacrolein is a major first-generation product of isoprene degradation,1 while acrolein is generated as a byproduct of butadiene oxidation.2,3 Due to its central importance in the oxidation of isoprene, a great deal is known about the atmospheric fate of methacrolein.4-9 The dominant atmospheric loss for this species is via reaction with OH, with photolysis and reaction with O3 and NO3 playing at most a minor role. With a value for k1 of 2.9 × 10-11 cm3 molecule-1 s-1 at 298 K,6 the atmospheric lifetime during the day for this species is ≈2-3 h. It has been shown that the OH reaction occurs 50-55% via addition of OH to the CdC double bond (R1a and R1b), and 45-50% via abstraction of the aldehydic hydrogen (R1c), leading to the formation of three peroxy radicals in the atmosphere:4,5
OH + CH2dC(CH3)-CHO (+O2) f HOCH2-C(OO)(CH3)-CHO
(R1a)
f OOCH2-C(CH3)(OH)-CHO
(R1b)
f CH2dC(CH3)-C(O)O2
(R1c)
Among the suite of products generated from the ensuing chemistry is methacryloylperoxynitrate (MPAN, an unsaturated PAN analogue), which has been identified in ambient air in many continental regions:10-19
CH2dC(CH3)C(O)O2 + NO2 + M f CH2dC(CH3)C(O)O2NO2 + M (R2) In fact, simultaneous measurements of MPAN (believed to be derived from almost exclusively biogenic sources), PPN (CH3CH2C(O)O2NO2, of anthropogenic origin), PAN (of mixed biogenic/ anthropogenic origin), and O3 have been used to quantify the relative contribution of biogenic and anthropogenic hydrocarbons to ozone production.13,16 However, the recent finding of a rapid loss of MPAN via reaction with OH20 calls into question some of the assumptions made in these analyses. At present, less is known about the mechanism of the reaction of OH with either acrolein or crotonaldehyde:
OH + CH2dCH-CHO f products
(R3)
OH + CH3-CHdCH-CHO f products
(R4)
Rate coefficients for these reactions are well established,21-25 k3 ) 2.0 × 10-11 cm3 molecule-1 s-1 and k4 ) 3.4 × 10-11 cm3 molecule-1 s-1, and some products of the oxidation have been identified and quantified.3,25,26 In particular, during the course of our investigations, Magneron et al.25 published data which suggested that ≈20-25% of (R3) occurs via addition pathways, and that both addition and abstraction pathways are operative in (R4). Nonetheless, a complete accounting of the product yields, and hence an accurate assessment of the branching ratio for OH addition and abstraction channels in (R3) and particularly (R4), has yet to be achieved. Interest in the chemistry of acrolein has been stimulated by the recent observation of acryloylperoxy nitrate (CH2d CHCOO2NO2, APAN, referred to as vinyl PAN in refs 25 and 26) in ambient air.27,28 Tanimoto and Akimoto27 observed this species at both an urban site (suburban Tokyo) and a more
10.1021/jp021530f CCC: $22.00 © 2002 American Chemical Society Published on Web 12/03/2002
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J. Phys. Chem. A, Vol. 106, No. 51, 2002 12253
remote site (Rishiri Island, northern Japan). Roberts et al.28 have identified APAN in ambient air near Houston, TX as part of the recently completed TEXAS 2000 Air Quality Study. Elevated APAN mixing ratios in this region were attributed in part to large emissions of 1,3-butadiene and/or acrolein from the petrochemical industry. Interpretation of these APAN data will require knowledge of both its sources (acrolein oxidation) and its sinks (expected to be reaction with OH and thermal decomposition, by analogy to MPAN20,29). Regarding the source strength, it is clear that an understanding of the relative contribution of addition and abstraction in the reaction of OH with acrolein is required, since only the abstraction channel (R3c) can lead to APAN production:
OH + CH2dCH-CHO (+O2) f HOCH2-CH(OO)-CHO
(R3a)
f OOCH2-CH(OH)-CHO
(R3b)
f CH2dCH-C(O)O2
(R3c)
In this study, experiments are conducted in an environmental chamber/FTIR absorption system to determine the end products of the OH-initiated oxidation of both acrolein and crotonaldehyde. These data are interpreted in terms of the branching ratios for reactions (R3a-c), and for the analogous reaction channels occurring in the reaction of OH with crotonaldehyde:
OH + CH3-CHdCH-CHO (+O2) f CH3-CH(OH)-CH(OO)-CHO
(R4a)
f CH3-CH(OO)-CH(OH)-CHO
(R4b)
f CH3-CHdCH-C(O)O2
(R4c)
Trends in the reactivity of acrolein, methacrolein, and crotonaldehyde are discussed, as are the implications of the results for the atmospheric source of APAN. Experimental Section All experiments described here were conducted in a 47 L stainless steel environmental chamber,20,30 which is interfaced to a Fourier transform infrared spectrometer via a set of Hansttype multipass optics (observational path length 32.6 m). Spectra were obtained at a resolution of 1 cm-1 from the coaddition of 200 scans (acquisition time 3-4 min), and covered the range 800-3900 cm-1. Photolyses were carried out along the length of the chamber, using the output of a Xe-arc lamp filtered to provide radiation in the range 240-400 nm. Products of the OH-initiated oxidation of both acrolein and crotonaldehyde were measured at 700-720 Torr total pressure (O2 partial pressure 150 Torr, balance N2) from the photolysis of mixtures of an OH precursor (methyl or ethyl nitrite), the unsaturated aldehyde, and NO. Ethyl nitrite was used as the OH source for the acrolein experiments, since methyl nitrite photolysis generates formaldehyde, a likely acrolein oxidation product. For studies of crotonaldehyde, most experiments were carried out using methyl nitrite, to avoid acetaldehyde production from ethyl nitrite. A few experiments were conducted using ethyl nitrite, however, to assess the possibility of CH2O production from crotonaldehyde oxidation. Typical initial concentrations were as follows: methyl or ethyl nitrite, (1.5-2.8) × 1015 molecule cm-3; acrolein, (3.5-8) × 1014 molecule cm-3 or crotonaldehyde, (2.4-14) × 1014 molecule cm-3; NO, ≈3.5 × 1014 molecule cm-3. To obtain product yield data, mixtures were
Figure 1. Product concentrations observed from the OH-initiated oxidation of acrolein in 700 Torr of synthetic air, plotted as a function of acrolein consumption. Details of the experimental conditions are given in the text. Open squares, CO2; filled circles, CO; filled triangles, CH2O; open triangles, glycolaldehyde; filled diamonds, APAN; open circles, HCOOH.
photolyzed for four to five periods (each of duration 2-3 min), with an IR spectrum recorded following each irradiation. Some experiments, using Cl-atoms to initiate oxidation, were carried out specifically to determine IR spectra for the unsaturated PAN analogues of MPAN, i.e., APAN from acrolein and CPAN from crotonaldehyde (see results section for details). These experiments were conducted in 1 atm of synthetic air, and involved the photolysis of Cl2 (≈5 × 1015 molecule cm-3) in the presence of either crotonaldehyde or acrolein (≈7 × 1014 molecule cm-3) and NO2 (1.5 × 1014 molecule cm-3). Quantification of reactants and products was done by comparison with reference spectra recorded with our spectrometer system,5,31-33 with the exception of the unsaturated PAN species (CH2dCH-C(O)O2NO2 and CH3-CHdCH-C(O)O2NO2) whose identification and quantification is described in the results section. Quantification was done primarily using the following absorption bands: CO, 2080-2200 cm-1; CO2, 23002380 cm-1; CH2O, near 1745 cm-1; glycolaldehyde, near 1115 cm-1; glyoxal, near 1730 cm-1 and/or 2835 cm-1; CH3CHO, 1320-1490 cm-1 and/or 1745 cm-1. Our absorption cross sections for glycolaldehyde are consistent with previous studies.34,35 Minor corrections (less than 10%) to product yield data were made to account for product loss via reaction with OH. Chemicals used in this study and their sources and purities were as follows: methyl nitrite and ethyl nitrite, synthesized according to the procedure of Taylor et al.;36 O2, UHP, U. S. Welding; N2, boil-off from liquid N2, U. S. Welding; acrolein, 90%, Aldrich; crotonaldehyde, 99%, Aldrich; NO, UHP, Linde; Cl2, HP, Matheson. Gases were used as received. Acrolein and crotonaldehyde were subjected to several freeze-pump-thaw cycles prior to use. Results and Discussion Products of Reaction of OH with Acrolein. Products observed following the OH-initiated oxidation of acrolein were CO2, CO, CH2O, glycolaldehyde (HOCH2CHO), and formic acid; concentrations of these species, corrected for the occurrence of secondary reactions,37 as a function of acrolein consumption, are shown in Figure 1. These product concentrations were observed to be linear with acrolein consumption, indicating that all are likely primary products (with the possible
12254 J. Phys. Chem. A, Vol. 106, No. 51, 2002
Orlando and Tyndall TABLE 1: Peak Absorption Cross Sections (10-18 cm2 molecule-1) for PAN,38 MPAN,4 APAN, and CPAN (This Work, Assumed by Analogy, See Text for Details) 794 901 950 1070 1125 1163 1193 1204 1300 1452 1740 1810 1840
PAN
MPAN
1.14
1.2 1.6
CPAN 0.22 0.65 0.65 0.3
APAN
1.8
1.47 0.18 1.1
1.28
2.9
3.04 0.8
1.1 0.21 3.2 0.6
0.34 1.3 2.8 0.8
1
O2 and NO2, APAN is expected to be the sole product obtained from abstraction, while Cl addition under these conditions would generate chlorinated peroxynitrate species, products which obviously will not be formed following OH attack: Figure 2. Infrared absorption spectra observed following the photolysis of a mixture of Cl2 (5 × 1015 molecule cm-3), acrolein (upper trace, 7 × 1014 molecule cm-3) or crotonaldehyde (lower trace, 7 × 1014 molecule cm-3), NO2 (1.5 × 1014 molecule cm-3), O2 (150 Torr) and N2 (550 Torr). Absorption features due to the parent aldehyde have been subtracted for clarity. Labeled peaks are assigned to APAN, CH2dCH-C(O)OONO2, or CPAN (CH2dCH-C(O)OONO2), see text for details.
exception of formic acid, see below). As will be detailed later, concentrations of CO2, CO, and CH2O are expected to level off somewhat toward the end of an experiment (due to APAN formation), but this effect is not immediately evident in the data. Product yields, obtained from the slopes of the data shown in Figure 1, are as follows: CO2, 70 ( 5%; CO, 62 ( 5%; CH2O, 49 ( 4%; and glycolaldehyde, 32 ( 4%, and formic acid, 1.5 ( 0.7%. The origin of the minor product formic acid is not known, but may be heterogeneous in nature. As its formation at such low levels has no bearing on the data interpretation, it will not be discussed further. Also observed in the product spectra were absorption features at 1075, 1204, 1298, 1745, and 1820 cm-1. The growth of these features was found to be nonlinear with respect to acrolein consumption, accelerating toward the end of an experiment when NO2/NO concentration ratios were at a maximum. Since the intensities of these absorption features grew in unison (i.e., their relative intensities remained constant), it is likely that they are all due to a single species (or at least that they are all obtained from the same reaction). On the basis of the position of the absorption bands (note that PAN and MPAN both have absorption features near 1300, 1740, and 1820 cm-1)4,38 and on the basis of the observed temporal profile, we assign these absorption features to the PAN analogue, acryloylperoxynitrate, CH2dCH-C(O)O2NO2 (APAN). Further confirmation of the identity of the unidentified absorber as APAN was obtained from experiments involving photolysis of Cl2/acrolein/NO2/air mixtures. Following a brief photolysis period (30 s), absorption features at 1075, 1204, 1298, 1745, and 1820 cm-1 again appeared along with other absorption features at 1304 cm-1 and 1725 cm-1 due to HO2NO2,39 see Figure 2 (upper trace). Upon sitting in the dark, the features at 1075, 1204, 1298, 1745, and 1820 cm-1 remained unchanged, while the HO2NO2 features decreased (presumably due to thermal decomposition). As is the case with OH, Cl-atom reaction with acrolein can occur via addition to the double bond, or via abstraction of the aldehydic H atom. In the presence of
Cl + CH2dCH-CHO (+O2,NO2) f ClCH2-CH(OONO2)-CHO
(R5a)
f O2NOOCH2-CHCl-CHO
(R5b)
f CH2dCH-C(O)O2NO2
(R5c)
Thus, it seems reasonable to assume that the absorption features that are common to the Cl- and OH-initiated experiments (1075, 1204, 1298, 1745, and 1820 cm-1) are indeed due to APAN. Furthermore, the observed “dark” behavior is as expected: APAN is stable over the course of an hour or more, while loss of HO2NO2 occurs to a significant extent over this time period. An absolute calibration of the APAN infrared spectrum cannot be done without prior knowledge of the branching ratios for (R5). Instead, an approximate calibration is obtained from an assumption of similar intensities for APAN bands compared to the analogous absorption features in MPAN4 and PAN.38 To accomplish this, an average absorption cross section for the 1300 and 1740 cm-1 bands was established using the MPAN and PAN data, and the APAN spectrum was adjusted to achieve best agreement with these average absorption cross sections. Use of these absorption cross sections (which are given in Table 1) leads to the conclusion that about 20% of the reaction of Cl with acrolein occurs via abstraction, (R5c). This branching ratio is in quantitative agreement with conclusions drawn from a recently published assessment of Cl-atom reactivity with unsaturated oxygenates,40 thus lending support to our APAN calibration. The assumed cross sections can also be used to quantify the APAN yields following reaction of OH with acrolein, as shown in Figure 1. As discussed earlier, these yields vary with the extent of acrolein consumption, reaching a maximum level of 22% at the highest acrolein conversion. Once the contribution of APAN is included, the observed products in the OH-initiated oxidation of acrolein represent on average 97 ( 7% of the reacted carbon, indicating that no major product(s) remain unaccounted for. It now remains to assess these product yields in terms of the branching ratios for (R3a)-(R3c). As pointed out above, reaction of OH with acrolein will result in the generation of three peroxy radicals, HOCH2-CH(OO)-CHO, OOCH2-CH(OH)-CHO, and CH2dCH-C(O)O2. Under the conditions of our experiments, the main fate of these species will be reaction with NO to generate the corresponding oxy radicals:
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J. Phys. Chem. A, Vol. 106, No. 51, 2002 12255
HOCH2-CH(OO)-CHO + NO f HOCH2-CH(O)-CHO + NO2 (R6) OOCH2-CH(OH)-CHO + NO f OCH2-CH(OH)-CHO + NO2 (R7) CH2dCH-C(O)O2 + NO f CH2dCH-C(O)O + NO2 (R8) Organic nitrates are also possible products of these RO2/NO reactions, though yields are expected41 to be small, probably
