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SIR I thankRoberts et al. for their comments on our recent article on PBN (1). The comments by Roberts et al. appear to focus on three issues: nomenclature, coelution of PAN and PiBN, and if PAN and PiBN coelute, the implications for PAN measurements. With respect to nomenclature, I will continue to use in this reply the traditional names and acronyms first introduced in the pioneeringstudies of Hanst and Stephens, e.g., peroxyacetyl nitrate (PAN), peroxyisobutyryl nitrate (PiBN), etc. (2). The two other issues raised by Roberts et al. are discussed below. Implications for PAN MeasurementsIf PAN and PiBN Coelute. I assume that language such as “itcalls to question ... (the vast majority of all measurements)”,“tremendous interest”, “significant advances”, etc. is meant to convey Roberts et al.’s opinion that, should PAN and PiBN coelute, this would have averyimportant impact on past and current measurements of PAN. I disagree with them. In fact, our article (1) includes detailed estimates of urban emission rates for hydrocarbons that are precursors to PAN and to PiBN. These estimates clearly show that, should PAN and PiBN coelute,the positive bias due to PiBN when measuring PAN in ambient air would be small, i.e., 50.1 in most situations. Even this low value is probably an upper limit: similar calculations we made for peroxy-n-butyrylnitrate [PnBN,CH3CH2CH2C(O)OONO21 yielded aPnBN/PANratio of 0.16,while the PnBNlPAN ratios we measured in ambient air averaged 0.08 (31,i.e., only half of the calculated ratio. Thus, the small positive bias from PiBN, i.e., 10% or less, would be of the same magnitude as the accuracy and precision of current methods used to measure PAN in ambient air (3). Coelution of PAN and PIBN. We first reported on the possible coelution of PAN and PiBN in a paper describing measurements of PnBN in ambient air (3). Perhaps Roberts et al. missed this paper, which was published in this Journal about 1 year before our paper on PiBN. We noted at that time that the retention time of PiBN was apparently not consistent with general chromatography considerations. However,we also noted from workwith other PANs, a dozen or so of which have been studied in our laboratory (1,3-6, 10,lI), that (a)PANs with branched-chain alkyl substituents have much shorter retention times than their n-alkylsubstituted isomers and (b) relative retention times of unsaturated and saturated isomers are not always easy to predict. For example, i-C4HgC(0)OON02elutes well before n-C4HgC(0)OON02 (4). The unsaturated isomervinyl-PAN (R = CH2=CH-) elutes after its isomer PPN (R = CzH5) whereas MPAN (R= CH2=C(CH3)-) elutes before PnBN (R = n-C3H7).We have made similar observations for alkyl nitrates. For example, the branched-chain C3 compound isopropyl nitrate “skips”one carbon atom and essentially coelutes with the Cz compound ethyl nitrate (6). These observationssuggestthat caution should be exercisedwhen attempting to predict retention characteristicsof PANs and that simple chromatography considerations may be useful but are no substitute for experiments. Asdescribedindetailinourpaper ( l )alltestsperformed , indicate that PiBN was indeed formed in our experiments. Decomposition in the liquid phase yielded isopropyl nitrate (PAN would yield methyl nitrate). Decomposition with excess NO in the gas phase gave acetone as the major carbonylproduct (PANwould yield formaldehyde). I agree with Roberts et al. that additional characterization tests
0013-936W95/0929-0287$09.00/0
0 1994 American Chemical Society
would be useful. However, using electron impact mass spectrometry, B e r t ” and Roberts failed to obtain conclusive results for both PAN and MPAN (7). Chemical ionization mass spectrometry, successfullyapplied to PAN many years ago (8),would probably yield better results. FT-IR, applied by Tuazon and Atkinson to MPAN (91,may also be useful for additional characterization of PiBN. There is one suggestion of Roberts et al. that I agree should be further investigated,namely, the possibility that PiBN may have completely decomposed under our gas chromatography (GC)conditions. Should this be the case, acetone would still be the major gas-phase decomposition product (as we indeed observed), isopropyl nitrate would still be the major liquid-phase decomposition product (as we indeed observed), and so on. In our experience, many other PANs do not decompose under the GC conditions we used to measure PiBN (3-6, 10, 111, therefore complete decomposition of PiBN would be peculiar indeed but cannot be ruled out. Due to other project commitments, it will be quite some time before we study PiBN again. Thus, I would encourage Roberts et al. to demonstrate that PiBN does not coelutewithPAN but instead decomposesentirelyunder our GC conditions and to document, in the peer-reviewed literature, GC conditions suitable for the separation of PAN and PiBN. Such a contribution would be of value to those of us who are involved in measurements of PAN and other peroxyacyl nitrates in the atmosphere and in laboratory studies of hydrocarbon oxidation mechanisms. Finally, I need to emphasize that the issue of coelution of PAN and PiBN has no bearing on major findings of our study (11, Le., that isobutanal reacts with OH almost exclusively (298%)by H-atom abstractionfrom the carbonyl carbon, that 3-methyl-1-butene reacts with OH predominantly (298%) by addition on the C=C bond, and that reaction with oxygen predominates over unimolecular decomposition for the alkoxy radicals (CH&CH(O) and (CH~)ZCHCHZ(O).
Literature Cied (1) Grosjean, D.; Grosjean, E.; Williams, E. L., 11. Environ. Sci. Technol. 1994, 28, 167-172. (2) Stephens, E. R. Adv. Environ. Sci. Technol. 1969, 1, 119-146. (3) Grosjean, D.; Williams, E. L.; Grosjean, E. Enuiron. Sci. Technol. 1993,27, 326-331. (4) Grosjean, D.; Grosjean, E.; Williams, E. L., 11. Environ. Sci. Technol. 1994,28, 1099-1105.
(5) Grosjean, D.; Grosjean,E.;Williams,E. L.,11. J.Air WasteManage. A~SOC. 1994, 44, 391-396. (6) Grosjean, D.; Williams, E. L., II; Grosjean, E. Enuiron. Sci. Technol. 1993, 27, 830-840. (7) B e r t ” , S. B.;Roberts, J. M. Geophys.Res.Lett. 1991,18,14611464. (8) Pate, C. T.; Sprung, J. L., Pitts, J. N., Jr. Org. Mass Spectrom., 1976, 11, 552-555. (9) Tuazon, E. C.; Atkinson, R. Int. J. Chem. Kinet. 1990,22,591602. (10) Grosjean, D. Paper ENVR 13; Presented at the 205th National ACS Meeting, Denver, CO, Mar 28-Apr 2, 1993. (11) Grosjean, D.; Grosjean, E.;Williams, E. L., 11. Paper ENVR 248; Presented at the 207th National ACS Meeting, San Diego, CA, Mar 13-18, 1994.
Daniel Grosjean DGA, Inc. 4526 Telephone Road, Suite 205 Ventura, California 93003 ES9409568
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