Measurement of the organic nitrate yield from OH reaction with isoprene

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. D19, PAGES 25,563-25,568, OCTOBER 20, 1998 Measurement of the organic nitrate yield from OH reaction with isoprene Xiaohui Chen, David Hulbert, and Paul B. Shepson Departments of Chemistry, and Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana Abstract. This paper describes the results of measurements of the branching ratio (k2 /(k2a +k2b)) for formation of organic nitrates via peroxy radical reaction with NO, following the reaction of the OH radical with isoprene (2-methyl-l,3-butadiene). The experiments were conducted in a 5 m 3 all-teflon photochemical reaction chamber, via the photolysis of isopropyl nitrite in the presence of isoprene and NO. The organic nitrate yield was determined from the measurement of the sum of all organic nitrate isomers, using an organic nitrate selective detector, as a function of isoprene consumed. Organic nitrates were sampled directly from the reaction chamber into a capillary chromatographicolumn, followed by separation and quantitative pyrolytic conversion to NO:, which was detected through luminol chemiluminescence. In this manner, seven isomeric organic nitrates were observed, with a total yield of 4.4%. The structural features of the precursor peroxy radicals that influence the magnitude of the yield is discussed. Emission inventories for isoprene and NO lead to the conclusion that as much as 7% of NO emitted in the eastern United States in the summer months is lost from the atmosphere through the isoprene nitrate channel. 1. Introduction It is now firmly established that the biogenic hydrocarbon, isoprene, can play an important role in tropospheric ozone formation [Trainer et al., 1987; Chameides et al., 1992], even for urban environments [Chameides et al., 1988; Biesenthal et al., 1997]. In addition to its impact on 03 formation, isoprene can also impact the balance of tropospheric ozone through its role in the removal of nitrogen oxides (NOx). Isopren emission can affect this removal by impacting the concentration of the OH radical, and thus the rate of oxidation of NO to HNO3, which rapidly deposits. Isoprene can also directly sequester NOx through the formation of organic nitrates. Organic nitrates are known to be produced through peroxy radical reaction with NO [Atkinson et al., 1982], as shown in reactions (1) and (2). ß OH + VOCs (+02) -- RO2' (1) RO2' + NO --, RO. + NO2 (2a) -- RONO2 (2b) Reaction (2a) is directly responsible for 03 production, while reaction (2b), which is a combination of adduct formation to form an unstable peroxy nitrite followed by rearrangement to produce a stable organic nitrate [Atkinson et al., 1983], results in chain termination and NOx removal. It is known that the organic nitrate yield (k2b/(k2a + k2b)) increases as the size of the organic group increases [Carter and Atkinson, 1989]. Furthermore, it has been shown by Tuazon and Atkinson [1990] that organic nitrates are indeed produced from OH reaction with isoprene, with an estimated yield of 8-13%. A modeling study by Carter and 1Now at Department of Biochemistry, School of Medicine, Case Western Reserve University, Cleveland, Ohio. Copyright 1998 by the American Geophysical Union. Paper number 98JD /98/98JD Atkinson [1996] estimates the yield to be 8.8%. If it is this large, the production of isoprene nitrates could have a significant impact on the fate and processing of atmospheric NOx to oxidized products, not all of which appear to be accounted for [Parrish et al., 1993]. O'Brien et al. [1995; 1997] have calculated that in isoprene impacted environments, the majority of the organic nitrates present in the atmosphere should be those derived from isoprene oxidation, although no measurements have yet been reported. Along with the role of isoprene in accelerating conversion of NOx to HNO3, the organic nitrate route can also return nitrogen to the soil. As discussed by Shepson et al. [1996], the isoprene nitrates are likely to be removed from the atmosphere fairly readily by both wet and dry deposition. In this work, the measurement of the yield of organic nitrates from OH reaction with isoprene, in the presence of NO, is reported. The experiments were conducted using a static-mode photochemical reaction chamber, from which isoprene nitrate measurements were conducted using a novel chromatographic/chemiluminescence detection scheme. We will discuss the significance of the result in terms of the impact of this reaction on the processing of atmospheric NOx. 2. Experiment Isoprene nitrate yields were measured by irradiation of isoprene/no/isopropyl nitrite/air mixtures in an all-teflon 5 m 3 photochemical reaction chamber (PRC). The PRC is a cylindrical chamber consisting of two 1.8 m diameter perfluoroalkoxy (PFA)-coated aluminum end plates, one of which is suspended from the laboratory ceiling, with the other mounted 1 m above the floor. The end plates are 2 m apart and are connected with a 0.08 mm thick FEP Teflon cylindrical bag. As a consequence, all of the reactions were carried out at room temperature and pressure, that is, 25 øc and 743 torr, respectively. The upper end plate has six 1/4 inch TFE bore-through sampling ports and a 25,563

2 25,564 CHEN ET AL.: MEASUREMENT OF THE ISOPRENE NITRATE YIELD centrally mounted TFE-Teflon impeller. The lower end plate also has six 1/4 inch TFE bore-through sampling ports and a centrally located 1/8 inch bore-through TFE sampling port, through which organic nitrate samples are acquired using a 1/8 inch PFA sample line that is inserted 20 cm into the PRC. The PRC is completely enclosed within a light bank structure that residence time between the PRC and the capillary column is less than 2 s. The typical sample volume is 50 cm 3. The column for collection and subsequent separation of the organic nitrates is a RTX m x 0.53 mm diameter column, contained in a Hewlett-Packard 5890 Series 1I gas chromatograph (GC) with liquid nitrogen cryogeni cooling. Grade Low O2 nitrogen is uniformly irradiates the chamber contents with 16 symmetrically used as the carrier gas, at 5.00 ml/min. The carrier gas flow rate mounted GE 48 inch F40BL black lamps. Independent wiring enables four different symmetric radiation intensities. The lamps are not intended to simulate the solar spectrum, but are used as a is independently controlled using a MKS 0-20 sccm flow controller. The column is maintained at -20 øc during sampling, so that all organic nitrates are focused in a sharp band at the head of source of actinic radiation (.m,, = 370nm). In these experiments the column, prior to column temperature programming. The we produced OH from the photolysis of isopropyl nitrite in the presence of NO, as indicated in reactions (3)-(5). oven is then heated at 20 øc/min to a temperature of 120 øc, followed by 2 øc/min to 140 øc, which is maintained for 9 min. CH3CH(ONO)CH3 + hv - CH3CH(O')CH3 + NO (3) The temperature is then increased at 5 øc/min and held at 180 øc CH3CH(O')CH '- CH3C(O)CH3 + HO2 (4) for 10 min. The column exit is connected to an 1/8 inch PFA- HO2 + NO.OH + NO2 (5) Teflon transfer line, maintained at 150 øc in the region between Isopropyl nitrite was synthesized by dropwise addition of the GC oven and the pyrolyzer. Detection is achieved by pyroly- H2804 to a mixture of NaNO2 in isopropyl alcohol, as described sis of the organic nitrates to NO2 in a 64 cm xl.5 mm ID pyrex by Noyes [1933]. tube heated to 400 øc, followed by NO2 detection using a lumi- Samples for organic nitrate measurement were withdrawn nol-based chemiluminescence detector (LND-4D, Unisearch Assfrom the chamber by sampling directly into the analytical colmnn, using the column as the sample loop in a six-port valve, as sociates). This detectoresponds only to those species that are thermally converted to NO2. As described by Hao et al. [1994], shown schematically in Figure 1. This is done to minimize all organic nitrates are detected with identical sensitivity at this losses of the analytes due to contact with surfaces other than the column, as our experience [Muthuramu et al., 1993; J. M. O'Brien et al., "Determination of the hydroxynitrate yields from the reaction of C2-C6 alkenes with OH in the presence of NO" submitted to Journal of Physical Chemistry, 1998, herinafteretemperature. Because of this, calibration can be conducted using any organic nitrate standard sample. Here, isobutyl nitrate was used as an internal standard by adding approximately 20 ppb to the PRC at the start of the experiment. Isoprene concentrations were determined using a Shimadzu ferred to as O'Brien et al., submitted manuscript, 1998] indicates Mini 2 GC, equipped with a heated, manually operated, six-port that hydroxy nitrates are quite adsorptive. As shown in Figure 1, chamber samples are drawn through a Valco six-port valve, that is heated to 120 øc, at an accurately known flow rate (10.0 sccm) using a diaphragm pump and 0-20 sccm MKS mass flow controller that are connected to the analytical column. The total valve, 1 cm 3 sample loop, and flame ionization detector (FID). ^ Gast pump was used to draw samples from the PRC to the isoprene GC through 2.5 m of 1/4 inch PFA-teflon tubing. Isoprene was separated from other chamber contents (essentially only isopropyl nitrite) using a 1/8 inch x 1 m stainless steel column N Inject mode... Sample mode '-I Pyrolyzer I 400 IPerm' I / Olsøro * I" I OC 2 /nteg rator ½ Figure 1. Schematic diagram for sampling and analysis for organic nitrates.

3 CHEN ET AL.' MEASUREMENT OF THE ISOPRENE NITRATE YIELD 25,565 packed with Poropak QS 100/120 with grade 4.8 N2 as the cartier gas. The separation was conducted isothermally at 165 øc. NOx levels were monitored during the experiments using a TECO model 42 NOx monitor. A typical experiment consisted of adding 5 ppm isoprene (Aldrich), 2 ppm NO (Matheson), 20 ppm isopropyl nitrite, and 20 ppb isobutyl nitrate (Aldrich) to the previously clean air flushed PRC (at 295 K), using flowing clean air provided by a Whatman Lab Gas Clean Air Generator. NO was introduced us- ing grade Low O2 nitrogen, instead of air, to minimize conversion to NO2. This was done to preventhe dark reaction of isoprene with NO2, which produces products that interfere with the determination. This led us to conduct a separate dark experiment involving NO2 and isoprene at mixing ratios of 2.0 and 5.0 ppm, respectively, as discussed in the Results section. For isoprene/no/isopropyl nitrite experiments, the chamber contents were irradiated for 10 to 90 s, depending upon the desired consumption of isoprene. The lights are then turned off, and samples are obtained from the dark chamber. This is typically repeated only once or twice for each experiment in order to minimize wall losses of the isoprene nitrates between samples, as well as to minimize the impact of dark NO2-isoprene chemistry. The measurements were always terminated before all of the NO was consumed. The results given here are from seven separate experiments, as well as a limited set of dark NO2-isoprenexperiments. 3. Results 3.1 Reaction Chamber Experiments 40 As discussed above, the organic nitrate chemiluminescence detection system used in this study provides the substantial advantage that all organic nitrates are selectively detected with 30 2O identical sensitivity because the pyrolyzer quantitatively converts all organic nitrates to NO2. Although the detector is known to 10 respond to other oxidizing agents, for example, PAN, 03, and i i i! i H202, these species are all separated or destroyed in the chromatographic process. Thus this system provides highly reliable detection of a wide variety of organic nitrates, as long as they are Retention Time, minutes quantitatively transferred from the PRC to the pyrolyzer. To en- Figure 2. Example results for use of pyrolyzer temperature in sure that this was the case, the chamber contents were rapidly organic nitrate identification. transferred to the analytical column with a very short heated PFA transfer line. Unfortunately, it was not possible to evaluate sampling losses for this system for the specific analytes of interest, as no pure standards were available. As observed and discussed for smaller hydroxy nitrates [Muthurarnu et al., 1993; O'Brien et al., ent ot,ls-hydroxy nitrates. However, when addition occurs at C-I or C-4, an allylic radical is produced, leading to 4,1- or 1,4- hydroxy nitrates, each having both cis- and trans- isomers. As an submitted manuscript, 1998], we found that reproducible results example, we show the mechanism for the OH radical addition at were obtained once the system was "conditioned" with the analyte species. A typical chromatogram from an isopropyl nitrite- C-1 to produce 4-nitrooxy-l-hydroxy-2-methyl-2-butene in scheme 1, below. From examination of the chromatographic data, we can unambiguously identify seven different peaks, at /isoprene/no irradiation is shown in Figure 2 (top). Isoprene nitrates were found to elute from the colunto at retention times ranging from min. Some isopropyl nitrate was also produced from the 2-propoxy radicals generated by the photolysis of isopropyl nitrite. This nitrate had a retention time of =5.5 min and was not included in the integrated total organic nitrate area counts. As shown in the figure, the chromatographic peak shapes are nearly gaussian, suggesting that irreversible adsorption (i.e., loss of the analytes) was not a serious problem. Reaction of OH 2 "ø with isoprene in the presence of O2 and NO should produceight different isomeric peroxy radicals. This results from the fact that OH can add to four different carbon atoms, leading to four differ- loo 9o o O 10 loo 9o 8o 70._m 60 o 50 i i i Pyrolyzer T = 400øC Pyrolyzer T = 275øC

4 25,566 CHEN ET AL.: MEASUREMENT OF THE ISOPRENE NITRATE YIELD retention times of 9.5, 10, 12, 13, 17, 25, and 27 min, with the limited or eliminated the formation of interfering peaks, as well peak at 17 min accounting for 45-65% of the total isoprene ni- as wall losses for the isoprene nitrates, it also minimized the trate formed. Because OH radicals are believed to add preferen- consumption of isoprene due to this reaction. tially at C-1 (yielding methyl vinyl ketone) [Carter and Atkinson, Assuming all OH reactions with isoprene produce peroxy 1996], we believe it likely that this peak represents 4-nitrooxy-1- radicals all of which then react with NO, then -d[isoprene]/dt = hydroxy-2-methyl-2-butene. k2'[ro2']'[no] (where k2 = k2a + k2b) and d[rono2]tototdt = To verify the identification of the observed peaks as organic k2b'[ro2']'[no]. Therefore a plot of [RONO2]total versus nitrates, identical PRC samples were injected at pyrolyzer tem- -A[isoprene] should be linear with a slope of k2b/k2. Such a plot peratures of 400 ø and 275 øc. Hiskey et al. [1991] have studied for all seven experiments is shown in Figure 3, with values for the thermal decomposition kinetics for reaction (6) for several -A[isoprene] ranging from ppb. The slope of the least organic nitrates, and reported an Arrhenius expresssion for the squares regression (forced through zero) yields a ratio k2ffk2 = , where the quoted uncertainty corresponds to the RONO2 (+A) RO' + NO2 (6) 95% confidence level. Since the organic nitrates produced from rate coefficient of k = lx10 %'2ø' rrs4. Using this expression, OH reaction with isoprene are still olefinic, the OH rate constant the lifetime (1/k ) of organic nitrates is estimated to be 1 msec will be relatively large, and thus consumption of the isoprene niat 400 øc and 1 s at 275 øc. For the conditions of the experi- trates during the experiment must be considered. However, usment, the residence time of eluting peaks in the pyrex pyrolyzer ing a value for ko, for the isoprene nitrates of 1.3x tube is 1 s; thus all RONO2 species are quantitatively pyrolzed 10' cm3.molecule4.s', obtained via the estimation method of at 400 øc. However, at 275 øc the reaction is incomplete within Kwok and Atkinson [1995] for the model compound 2-nitrooxy-1-1 s and the peak areas decrease significantly. In this way we can hydroxy-2-methyl-3-butene, we calculate that the correction to unambiguously identify organic nitrates and discriminate against the yield, as described by Atkinson et al. [1982], is never more PAN-type compounds, which would be completely converted to than 2%. It is also possible to correct for losses of the analyte to NO2 at both temperatures. Given the retention times and column the PRC walls with time. However, it was found that this loss temperature program used, less than 1% of the organic nitrates was less than the measurement uncertainty within the first 2 measured would have thermally decomposed in the column. hours of the experiment and was not conducted. In the course of these experiments, it was observed that the In a study of isoprene oxidation products, Tuazon and Atkinunresolved group of peaks with retention times of min son [1990] estimated a yield for total organic nitrates of roughly appeared to be secondary product organic nitrates that increased 8-13%, assuming an IR absorption coefficient equal to that for in the dark. Since it is known that NO2 reacts with alkenes, in- simple alkyl nitrates. Tuazon and Atkinson [1990] indicate that cluding isoprene [Atkinson et al., 1984], a limited series of dark some degree of dark NO2-isoprene reaction had occurred in their NO2 - isoprene experiments was conducted. In these experi- system and corrected the measured isoprene consumption acments it was demonstrated that the group of peaks centered at 20 cordingly. There could also be a contribution to their yield from min was indeed produced from the NO2-isoprene dark reaction, measurement of the NO2-derived organic nitrates. We find that with a yield of approximately 12%. The contribution of this re- if the peaks for the dark reactions are included in our analysis, action to the consumption of isoprene in the isopropyl ni- the calculated yield (k2b/k2) is In a recent modeling trite/isoprene/no experiments was reduced by injection of the study, Carter and Atkinson [1996] simulated environmental reactant NO in a stream of N2, minimizing the conversion of NO chamber data and varied the isoprene nitrate yield, settling on to NO2. To minimize the time available for the NO2-isoprene 8.8% as the best fit. Although for one of their optimizations the dark reactions to occur, all of the experiments were limited to modeled yield was as low as 4.9%, our determined value remains only two or three irradiation/sampling sequences. This not only considerably lower than the previous estimates Experiment y = 0.044x +/ / Date 3/5 0 4/3, 4/16 4/24 _ /-', i i i i A[Isoprene], ppm Figure 3. Isoprene nitrate yield measurement results.

5 CHEN ET AL.: MEASUREMENT OF THE ISOPRENE NITRATE YIELD 25,567 Given the relatively low yield determined here, it is instructive to compare our result to the yields for similar size and structure peroxy radicals. For the secondary peroxy radical produced from isopentane, the 2-methyl-3-butyl peroxy radical, the yield is 12.6% [Carter and Atkinson, 1989]. However, as discussed previously, we expect that OH reaction with isoprene produces a 1,4-hydroxy peroxy radical, that is, primary, instead of a secondary radical, a significant fraction of the time. Carter and Atkinson [1989] recommend a scaling factor of 0.4 for primary radicals; thus a yield of 5% might be expected for the 1,4- (or 4,1-)hydroxy peroxy radicals produced from OH reaction with Ltl isoprenc. Our yield,,r A Ao/_ is reasonable if the majority of the NO. According to the U.S. Environmental Protection Agency's (EPA's) National Air Pollution Emission Trends (available from T. Pierce via FTP at monsoon.rtpnc.epa.gov/pubfoeis2/trend), the total emission of isoprene in the United States, east of the Mississippi River (a relatively high NOx environment), amounts to 8.6xl 0 lø mol of isoprene for the June-August period. If all of this isoprene reacts to produce peroxy radicals that then react with NO, 3.8x109 mol of NO are converted to isoprene nitrates in this period via reaction (2b). From the same EPA database, 5.6x10 ø mol of NO,, are emitted east of the Mississippi River for the same time period. Thus 6.8% of the NOx emitted in the eastern United States in the sumanertime is converted to isoprene niperoxy radicals are primary rather than secondary. However, there is also mounting evidence that there is a significant impact on the branching ratio, k2b/k2, due to the presence of a hydroxy group in the [5-position relative to the peroxy group. For the 2- butyl peroxy radical the yield is 9.0% [Carter and Atkinson, 1989], while the yield for the 3-hydroxy-2-butyl peroxy radical is 3.7% [Muthuramu et al., 1993]. Shepson et al. [1985] report a yield of 1.7% for the 1-hydroxy-2-propyl peroxy radical, while that for the 2-propyl peroxy radical is 4.2% [Carter and AtMnson, 1989]. The hydroxy nitrate yields from OH reaction with 1- butene and 1-hexene have recently been measured (O'Brien et al., submitted manuscript, 1998) and found to be 2.7 and 6.0%, respectively (for the sum of the two a,[3-hydroxy nitrates). When compared to 2-butyl and 2-hexyl radical yields of 9.0 and 19.0%, respectively [Carter and Atkinson, 1989], it appears that the trates. Although this is a rough estimate, it does indicate the likely importance of this mechanism for removal of NOx from the atmosphere in forested regions. These isoprene nitrates can then be removed via wet and dry deposition or be destroyed via reaction with OH. The OH reaction lifetime for these compounds is relatively short at about 21 hours, if a value of 1.3x10' cm 3 molecule ' s ' is used for the OH rate constant for a typical isoprene nitrate [Shepson et al., 1996]. However, the products of OH reaction with the isoprene nitrates are also likely to be lost by wet and dry deposition. For example, OH reaction with 4-nitrooxy-1- hydroxy-2-methyl-2-butene is likely to lead to hydroxyacetone and nitrooxy-acetaldehyde as major products, both of which can be removed by wet and dry deposition, although the latter compound may react with OH to produce a PAN analogue. Thus we presence of a 13-hydroxy group significantly reduces the branch- expect that isoprene nitrate production generally will result in ing ratio. Indeed, as discussed in O'Brien et al. (submitted permanent NOx removal. manuscript, 1998), the 13-hydroxy group significantly weakens the O-O bond in the intermediate peroxynitrite, ROONO, thus 4. Conclusions favoring reaction (2a)(relative to the case for simple alkyl peroxy radicals). In this paper we have reported on the measurement of the yield of organic nitrates resulting from OH reaction with iso- 3.2 Atmospheric Chemistry Impact prene. This work indicates the potential importance of the production of organic nitrates from OH reaction with isoprene, although our reported yield is somewhat lower than anticipated As discussed in the Introduction, reaction (2b) is important in isoprene-impacted regions as an important free radical termination step that also removes NOx from the atmosphere. This can be very important to regions of the troposphere were ozone production is often NOx-limited, such as the eastern United States. based on the literature available to date. It is clear that more information is needed regarding the influence of structural features, for example, the presence of a ls-hydroxy group and whether the peroxy radical is primary versus secondary, on the yields of organic nitrates, so that predictions can be made for Additionally, this reaction can limit the extent of long-range other reactive VOCs. Although our work indicates that isoprene transport of NOx. Given an isoprenemission inventory, the im- nitrates may be a significant NOx sink, other, perhaps higher portance of reaction (2b) as an atmospheric NOx sink can be as- molecular weight biogenic VOCs may be equally or more imsessed. For OH and 03 concentrations of 1.0x106 and 1.0x1012 portant. As an example, it has been recently reported that the molecules/cm 3, respectively, 89% of the isoprene molecules organic nitrate yield from the OH reaction with the terpene otemitted are consumed by OH. Competing with reaction (2) is the reaction of peroxy radicals with other RO2 species, and with HO2. For the eastern rural United States, median daytime (when isoprene is emitted) NO levels are of the order of 0.30 ppb [Parrish et al., 1993], while a typical da) ime value for the total peroxy radical concentration is approximately ppb pinene is 17% [Nozi re et al., 1997]. Thus terpene oxidation may also represent an important NOx sink for terpene-impacted forest regions. Given the effect of the organic nitrate yield on simulated 03 production [Carter and Atkinson, 1996], and the influence of organic nitrate formation in the potential long-range transport of NOx, it is clear that better information about the or- [Cantrell et al., 1995; Arias and Hastie, 1996]. Assuming a ganic nitrate yields for a variety of biogenic VOCs, as well as value for k2 of 8.0x10 '12 cm 3 molecule 'l s 'l [Eberhard and How- their atmospheric fate, is needed to enable reliable modeling of ard, 1997], and 1.0x10 ' cm 3 molecule ' s - for HO2 reaction with ozone production and transport in biogenic VOC-impacted atsimilar size RO2 radicals [Kirchner and Stockwell, 1996], and mospheres. given the estimated average NO and RO2 radical concentrations, we estimate that >90% of all isoprene-derived peroxy radicals in the Eastern United States will react with NO. Thus we may correctly assume that all isoprene will be oxidized by OH and that Acknowledgments. We thank the National Science Foundation for their support of this work (ATM ), and the Jonathan Amy Instrumentation Facility and Ned Gangwet at Purdue for the design and construction of all of the resulting peroxy radicals will then react solely with the photochemical reaction chamber used in this work.

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