Identification and Yields of 1, 4-Hydroxynitrates Formed from the

Aug 21, 2014 - Yields based on analyses of particles increased with increasing carbon number from 0.00 for C8 to an average plateau value of 0.130 ± ...
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Identification and Yields of 1,4-Hydroxynitrates Formed from the Reactions of C8−C16 n‑Alkanes with OH Radicals in the Presence of NOx Geoffrey K. Yeh†,‡ and Paul J. Ziemann*,†,§ †

Air Pollution Research Center and ‡Department of Chemistry, University of California, Riverside, California 92521, United States ABSTRACT: A series of C8−C16 n-alkanes were reacted with OH radicals in the presence of NOx in an environmental chamber and particulate 1,4-hydroxynitrate reaction products were collected by filtration, extracted, and analyzed by highperformance liquid chromatography with UV absorption and electron ionization mass spectrometry (HPLC/UV/MS). Observed mass spectral patterns can be explained by using proposed ion fragmentation mechanisms, permitting the identification of each hydroxynitrate isomer. Reversed-phase retention of these compounds was dictated by the length of the longer of two alkyl chains attached to the 1,4-hydroxynitrate subunit. 1,4-Hydroxynitrates were quantified in particles using an authentic analytical standard for calibration, and the results were combined with gas chromatography measurements of the n-alkanes to determine the molar yields. Yields based on analyses of particles increased with increasing carbon number from 0.00 for C8 to an average plateau value of 0.130 ± 0.008 for C14−C16, due primarily to corresponding increases in gas-to-particle partitioning. The value at the plateau, where essentially all 1,4-hydroxynitrates were in particles, was equal to the average total yield of C14−C16 1,4-hydroxynitrates. The average branching ratio for the formation of C14−C16 1,4hydroxynitrates from the reaction of NO with the corresponding 1,4-hydroxyperoxy radicals was 0.184 ± 0.011. This value is ∼20% higher than the plateau value of 0.15 for reactions of secondary 1,2-hydroxyperoxy radicals and ∼40% lower than the plateau value of 0.29 for reactions of secondary alkyl peroxy radicals, both of which were reported previously. The branching ratios determined here were used with values reported previously to calculate the yields of C7−C18 alkyl nitrates, 1,4hydroxynitrates, and 1,4-hydroxycarbonyls, the three products formed from the reactions of these n-alkanes.

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INTRODUCTION Alkanes are emitted to the atmosphere primarily by combustion of fossil fuels and biomass and constitute approximately 40− 50% of anthropogenic nonmethane organic compound (NMOC) emissions.1−3 In the atmosphere they react primarily with OH radicals at rates that result in typical lifetimes of a few days or less.4 In polluted air containing high concentrations of NOx, these reactions lead to the formation of first-generation products consisting of alkyl nitrates, 1,4-hydroxynitrates, 1,4hydroxycarbonyls, and carbonyls, as well as higher generation multifunctional products.4−7 The formation of organic nitrates affects radical cycling and NOx and O3 concentrations by sequestering NOx,8 and when the reaction products have sufficiently low volatility they can contribute to secondary organic aerosol (SOA). For example, recent studies have indicated that alkane emissions contain a larger fraction of higher molecular weight alkanes than previously believed and that the oxidation of these compounds may be a major source of SOA.9,10 In spite of the importance of alkane oxidation in atmospheric chemistry, however, comprehensive reaction mechanisms that are necessary to accurately model the effects of these compounds on the formation of NOx, O3, and SOA,7,11 and thus air quality and climate,12−18 are still not available. The mechanism of the reaction of n-alkanes with OH radicals in air in the presence of NOx to form first-generation products is shown in Scheme 1.19−22 The reaction is initiated by abstraction of an H atom to form an alkyl radical and H2O. The © 2014 American Chemical Society

alkyl radical reacts solely with O2 to form an alkyl peroxy radical, which reacts with NO to form either an alkyl nitrate or an alkoxy radical and NO2. The alkoxy radical then isomerizes via a 1,5 H atom shift to a 1,4-hydroxyalkyl radical, which reacts with O2 to form a 1,4-hydroxyperoxy radical. Although alkoxy radicals can also react with O2 or decompose, for n-alkanes larger than C6 (such as in this study) those reactions are too slow to compete with isomerization. The 1,4-hydroxyperoxy radical reacts with NO to form either a 1,4-hydroxynitrate or NO2 and a 1,4-hydroxyalkoxy radical, which reverse isomerizes and reacts with O2 to form a 1,4-hydroxycarbonyl and HO2. The quantities α1 and 1 − α1, and α2 and 1 − α2 in Scheme 1 are reaction branching ratios that are equal to the fraction of alkyl peroxy radicals or 1,4-hydroxyperoxy radicals that react by each of the two alternative pathways shown. The molar yields of alkyl nitrates formed from reactions of nalkanes have been investigated over the range of carbon numbers from C2−C14 and observed to increase with increasing carbon number to a plateau value of ∼0.3 at ∼C12−C14 at room temperature,19,23 consistent with model predictions.19,24 The yields of 1,4-hydroxynitrates are less well-known. Arey et al.19 measured yields of 1,4-hydroxynitrates formed from reactions of C5−C8 n-alkanes, obtaining values of 0.047 and 0.054 for C7 Received: June 12, 2014 Revised: August 20, 2014 Published: August 21, 2014 8797

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results from two studies of OH radical-initiated reactions of 1alkenes,26,29 which indicate that the branching ratios for the formation of 1,2-hydroxynitrates from reactions of 1,2hydroxyperoxy radicals with NO are approximately one-half those of alkyl peroxy radicals (due to the presence of a vicinal hydroxyl group), another recent study30 indicates that the branching ratios are similar. In light of the considerable uncertainties associated with the methods used thus far to measure yields of 1,4-hydroxynitrates, and the large differences in reported 1,2-hydroxynitrate yields, additional studies are warranted to better understand the effects of hydroxyl groups and molecular structure on the formation of organic nitrates. In the present study, C8−C16 n-alkanes were reacted with OH radicals in the presence of NOx in an environmental chamber and particulate 1,4-hydroxynitrates were collected on filters and analyzed by high-performance liquid chromatography with UV and MS detection (HPLC/UV/MS), as we have done previously for other hydroxynitrates.29,31 This approach is well suited to determining yields at the plateau, because the vapor pressures of 1,4-hydroxynitrates larger than C13 are sufficiently low that for the aerosol mass concentrations (seed particles + SOA) present in these experiments 1,4-hydroxynitrates are expected to partition essentially entirely to the particle phase within a few seconds of formation and then remain there, with negligible loss of gaseous C14−C16 1,4hydroxynitrates to the walls.32 The amounts of C14−C16 1,4hydroxynitrates measured in particles can therefore be used to quantify the total yields of 1,4-hydroxynitrates. These values must be corrected for losses of particles to the walls, but this correction is much simpler and more accurate than the corrections we have recently shown are required for gas−wall partitioning of more volatile compounds such as alkyl nitrates.23 The accuracy of quantitation was also maintained by the use of an authentic analytical standard of 1,4-hydroxynitrates that were purified from filter extracts and used for the preparation of a calibration standard. Structural confirmation of 1,4-hydroxynitrate products was obtained using our thermal desorption particle beam mass spectrometer (TDPBMS) coupled to the HPLC downstream of the UV detector, and an electron ionization fragmentation mechanism was developed that can be used to explain the observed mass spectra and identify each of the 1,4-hydroxynitrate isomers. Because standards of these compounds are not commercially available, this information can be useful for interpreting mass spectra obtained in laboratory and field studies using other instruments that employ electron ionization, such as the Aerodyne Aerosol Mass Spectrometer (AMS)33 and gas chromatography−mass spectrometers (GCMS).

Scheme 1. Mechanism of Formation of First-Generation Products from the Reaction of n-Alkanes with OH Radicals in the Presence of NOx

and C8 1,4-hydroxynitrates whose yields are not affected by formation of carbonyls from alkoxy radical decomposition or reaction with O2. In that study they used direct air sampling atmospheric pressure chemical ionization mass spectrometry with NO2− as the reagent ion and employed the 2-nitrooxy-3butanol formed in the reaction of cis-2-butene as an internal standard for quantification. The yield of 2-nitrooxy-3-butanol was assumed to be 0.0355, the average of values measured by Muthuramu et al.25 and O’Brien et al.,26 and they also assumed that the 1,2-hydroxynitrate standard and 1,4-hydroxynitrates have identical mass spectrometric responses. They estimated an overall measurement uncertainty of a factor of ∼2 for the yields, which is in addition to the uncertainty in the assumed 2nitrooxy-3-butanol yield. We estimated a yield of ∼0.16 for the formation of 1,4-hydroxynitrates from the reaction of npentadecane using the measured SOA yield and the fraction of 1,4-hydroxynitrates in SOA determined using temperatureprogrammed thermal desorption particle beam mass spectrometry (TPTD).27 When the same TPTD approach is used with data presented in Jordan et al.28 for reactions of a series of nalkanes, the average yield is ∼0.09 ± 0.02 for C14−C17 1,4hydroxynitrates. The variability in the yields measured by this approach is mainly due to uncertainties in fitting 1,4hydroxynitrate desorption profiles in complex SOA mixtures and possible overlapping contributions from other products. All previously measured 1,4-hydroxynitrate yields, as well as the corresponding branching ratios for the formation of 1,4hydroxynitrates, are much smaller than those of alkyl nitrates with similar carbon numbers. Although this is consistent with

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EXPERIMENTAL METHODS Chemicals. Acetonitrile, ethyl acetate, and water were HPLC grade and were purchased from Fisher Scientific. nOctane (99+%), n-decane (99+%), n-dodecane (99+%), ntridecane (99+%), n-pentadecane (99+%), n-hexadecane (99+ %), and dioctyl sebacate (97%) were purchased from SigmaAldrich. n-Tetradecane (99+%) was purchased from Alfa Aesar. Methyl nitrite was synthesized according to the procedure of Taylor et al.34 and kept on a glass vacuum rack in liquid nitrogen until used. Environmental Chamber Experiments. Experiments were conducted in a 8.2 ± 0.4 m3 Teflon FEP environmental chamber filled with clean, dry air ( C6 can be used with the mechanism shown in Scheme 1 to calculate the yields of not only alkyl nitrates (yield = α1) and 1,4hydroxynitrates (yield = [1 − α1] × α2) but also 1,4hydroxycarbonyls (1 − alkyl nitrate yield − 1,4-hydroxynitrate yield = [1 − α1] × [1 − α2]). Yields of C7−C18 alkyl nitrates, 1,4-hydroxynitrates, and 1,4-hydroxycarbonyls calculated using this approach are given in Table 2. For this range of carbon numbers the yields differ relatively little from their respective plateau values of 0.292, 0.130, and 0.578. For example, for the reaction of n-octane the calculated yields of alkyl nitrates, 1,4hydroxynitrates, and 1,4-hydroxycarbonyls are 0.242, 0.116, and 0.642. These values agree quite well with the most recent yields

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CONCLUSIONS In this study, a homologous series of C8−C16 n-alkanes were reacted with OH radicals in the presence of NOx in an environmental chamber and the yields of 1,4-hydroxynitrate products present in particles were measured. Yields increased with increasing carbon number to a plateau at about C14−C16 due to enhanced partitioning into particles, with the average value at the plateau of 0.130 ± 0.008 being equal to the total yield (gas + particle) because at these high carbon numbers essentially all 1,4-hydroxynitrates were in particles. The average branching ratio for the formation of C14−C16 1,4-hydroxynitrates from the reaction of NO with the corresponding 1,4hydroxyperoxy radicals was determined to be 0.184 ± 0.011, which is ∼20% higher than the value of 0.15 determined previously for secondary 1,2-hydroxyperoxy radicals formed from reactions of internal alkenes, 1-alkenes, and 2-methyl-1alkenes.29,31 These values are ∼60% and ∼50%, respectively, of those measured for alkyl peroxy radicals with the same carbon number.23 Quantum chemical calculations conducted on 1,2hydroxyperoxy radicals indicate that the lower branching ratios are due to intramolecular hydrogen bonding between the hydroxyl and peroxy group that weakens the O−O bond in the intermediate complex formed with NO (illustrated in Scheme 1) and thus increases the rate of dissociation to the 1,2hydroxyalkoxy radical and NO2 relative to the rate of formation of the 1,2-hydroxynitrate.26 Hydrogen bonding in the 1,2- and 1,4-hydroxyperoxy radicals occurs through five-member and seven-member rings, respectively, both of which are energetically favorable configurations in intramolecular reactions and are thus likely to have similar effects on hydroxynitrate formation. The results presented here are inconsistent with the results of Teng et al.,30 who observed that the branching ratios for the formation of 1,2-hydroxynitrates from the reactions of NO with C2−C8 1,2-hydroxyperoxy radicals were nearly the same as those measured by others for alkyl peroxy radicals with the same carbon number. The methods used in that study included chemical ionization mass spectrometry, gas chromatography, and thermal dissociation laser-induced fluorescence. In our opinion the approach has no obvious weaknesses, although it would be useful to determine whether calibration using an authentic standard gives the same results. With regard to the approach used here, there are a number of potential opportunities for errors that could lead to underestimates of yields. These include purification and gravimetric analysis of the authentic 1,4-hydroxynitrate standard, quantitation of alkane concentrations that are affected by gas−wall partitioning, particle filter collection, and 1,4-hydroxynitrate extraction efficiency, 1,4-hydroxynitrate losses due to gas-phase reactions with OH radicals or particle-phase reactions that form strongly bound oligomers that are not extracted or do not dissociate during extraction or HPLC analysis, and gas and particle wall losses during filter sampling. For reasons discussed in previous sections we believe that these potential problems could increase the measured plateau yields and branching ratios reported here by up to ∼15%, but the effects could also be negligible.

Table 2. Calculated Yields of C7−C16 Alkyl Nitrates, 1,4Hydroxynitrates, and 1,4-Hydroxycarbonyls carbon no.

alkyl nitrate yield

1,4-hydroxynitrate yield

1,4-hydroxycarbonyl yield

7 8 9 10 11 12 13 14 15 16

0.210 0.242 0.262 0.273 0.280 0.285 0.288 0.290 0.292 0.294

0.105 0.116 0.122 0.125 0.127 0.128 0.129 0.130 0.130 0.131

0.685 0.642 0.617 0.602 0.593 0.587 0.583 0.580 0.577 0.576 8804

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AUTHOR INFORMATION

Corresponding Author

*P. J. Ziemann. Telephone: 303-492-9654, Fax: 303-492-1149. E-mail: [email protected]. Present Address

§ Department of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This material is based on work supported by the National Science Foundation under Grant AGS-1219508. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation (NSF). The authors thank Roger Atkinson and Janet Arey for helpful discussions.

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