Envlron. Sci. Technol. 1093, 27, 1117-1 124
Preparation, Analysis, and Atmospheric Production of Multifunctional Organic Nitrates Kayambu Muthuramu, Paul B. Shepson,’ and Jason M. O’Brlen
Department of Chemistry and Centre for Atmospheric Chemistry, York University, 4700 Keele Street, North York, Ontario M U 1P3, Canada Ambient measurements of multifunctional organic nitrates have become necessary to understand the complete tropospheric NO, budget. Laboratory studies indicate that multifunctional organic nitrates that are produced from OH and NO3 reaction with alkenes may be important; however, they have yet to be detected in atmospheric samples. We have synthesized several c3-C~alkyl nitrates, a,@-hydroxynitrates,and dinitrates that are produced from oxidation of atmospheric hydrocarbons. In this paper we report on procedures for their synthesis and purification, and on results of structural characterization by ‘HNMR and mass spectrometry. All organic nitrates were found to be stable on charcoal adsorbent (often used for sampling) for periods of weeks. Although good chromatographic separation is possible, we find that there may be serious problems with irreversible adsorption on column or injection port surfaces that may interfere with quantitative measurement. However, when the columns are properly conditioned, reproducible analyses can be conducted. We report the relative ECD response for all compounds for different injection systems. We have determined the yield of 2-nitrooxy-3-hydroxybutaneformation from OH reaction with cis-2-butene (in the presence of NO) to be 0.037 f 0.009. The possible causes, including column conditioning, of the elusive nature of the hydroxynitrates and dinitrates in GC/ECD chromatographic analyses of ambient air samdes are discussed. Introduction In recent years it has been shown that global background ozone concentrations have increased by roughly a factor of 3 over the past century ( I ) and appear to be increasing at a rate of about 1%per year (2). Because of this and the continuing problem of air quality standards being exceeded for ozone, there is a need for a detailed understanding of processes related to tropospheric ozone formation. This includes a complete understanding of the atmospheric processing of odd nitrogen species and the distribution of NO, in the atmosphere. Measurements of NO, in a variety of atmospheric environments have been conducted, along with measurements of the individual components of NO,, i.e., NO, NO,, “03, PAN, and other organic and inorganic nitrites/nitrates. For well-aged air masses (e.g., at Mauna Loa) the sum of the concentrations of the components is often significantly less than the measured NO,. The NO, “shortfall” can be as large as 25% and as high in effective concentration as any individual component. These observations have recently been reviewed by Ridley (3). The shortfall species appears to originate from photochemical processes, and it has been suggested ( 4 )that organic nitrates could contribute to the missing NO, fraction. If this were so, it would have a significant impact on the global distribution of O3 since the organic nitrates are relatively long-lived species (5) and, thus, represent a NO, reservoir. 0013-936X/93/0927-1117$04.00/0
0 1993 American Chemical Society
Alkyl nitrates are known (6) to be produced from OH reaction with alkanes, followed by reaction of the ultimately produced RO, radical with NO:
RO,
+ NO
--
[ROONO]*
-
RO + NO,
(la)
RONO, (1b) The first direct evidence for organic nitrate presence in the remote troposphere was reported by Atlas (7),who made measurements of alkylnitrates over the North Pacific Ocean. Since then, there have been several measurements of low molecular weight alkyl nitrates and their contribution to NO,. Several measurements at low to mid north latitudes showed that the c3-C~alkyl nitrates account for typically =1-2% of NO, (3,8,9);whereas, Bottenheim et al. (10) found that in the Arctic the alkyl nitrates can account for as much as 20% of NO,. The findings of Flocke et al. (9) and those summarized by Ridley (3)do, however, clearly indicate that in well-aged air masses the alkyl nitrates do not represent a large fraction of the missing NO, and that some other species must be responsible. It is also known that organic nitrates can be produced from OH and NO3 reaction with alkenes, followed by reaction of the peroxy radical with NO, NO,, or other peroxy radicals (11, 12). In this case the products are multifunctional organic nitrates, including cr,p-hydroxynitrates and dinitrates. In many environments alkenes are the dominant reactive hydrocarbons, and thus, if they are formed in significant yields,the multifunctional organic nitrates should be more prevalent than the alkyl nitrates produced from alkane oxidation and may account for a significant fraction of NO, (13). However, these compounds have not been detected in ambient air. An interesting example is the study of Flocke et al. (9),who measured organic nitrates in both rural and urban atmospheres by capillary GC using a nitrate-selective detector. After identifying the alkyl nitrates, there were few peaks remaining in the chromatograms. It is thus unclear as to why multifunctional nitrates and particularly hydroxy nitrates are not observed even though laboratory studies indicate that they are produced. The following can be raised as possible explanations: (a) they are not readily chromatographable; (b) they are destroyed (or not recovered) in the sampling process; (c) they are rapidly removed from the atmosphere; or (d)they are not formed in large enough yields to be present in significant concentrations. I t has been recently shown that the hydroxynitrates have relatively large Henry’s law constants (14) and would thus be subject to loss by wet deposition. However, many of the measurements have been made in photochemically active air masses where this would not affect their detection. Because of the polar nature of these compounds it is unclear as to whether they can be determined quantitatively by chromatography, as implied by Kames and Schurath (14). Alkyl nitrates are often sampled using the charcoal trap method of Atlas and Schauffler (151, and it is unknown as to whether the Environ. Sci. Technol., Voi. 27, No. 6, 1993
1117
hydroxynitrates are stable on this substrate. The yield of hydroxynitrate formation from OH reaction with alkenes has been determined only for propene (111,and this yield is significantly smaller than that found for propyl nitrate formation (from propane). Thus, there is significant uncertainty as to their formation yields. To address these questions, we report here on the synthesis of several mono and bifunctional organic nitrates ( c 3 4 6 alkyl nitrates and CZ-C~hydroxynitrates and dinitrates), their purification, spectroscopic characterization, capillary gas chromatographic separation and detection, and a study of their stability on charcoal traps. Although it is believed that y-hydroxy nitrates may be produced either as a result of internal abstraction reactions of alkoxy radicals or by OH addition to dienes, we have focusedhere on the &-hydroxy nitrates (partly due to the relative ease of synthesis). We have also conducted photochemical reaction chamber experiments to measure the yield of 2-nitrooxy-3-hydroxybutane formation from OH reaction with cis-2-butene in the presence of NO. Experimental Section Synthesis, Purification, and Spectroscopic Characterization. Alkyl nitrates: 2-Propyl and isobutyl nitrates from Aldrich were used as received. Other c3-c6 alkyl nitrates used in this study were prepared in the laboratory by reaction of the appropriate alkyl bromide with AgNO3 (16). Pure alkyl nitrates (50-70% in yield) were obtained by fractional distillation under reduced pressure (,-ONO,) 4.502 (dd, 1H; CHZ-ONO,) 1.010 (t, 3 H; CH,) 1.747 (m, 2 H; CH,-CH,-) 2.200 (br, 1H; OH) 3.741 (dd, 1H; CH,OH) 3.839 (dd, 1 H; CH,OH) 5.064 (m, 1H; CKONO,) 1.258 (d, 3 H; CK)-CHOH 2-nitrooxy-3-hydroxybutane 1.348 (d, 3 H; CH&H(ONO,)) 2.525 (br, 1 H; @) 3.854 (m, 1H; CHOH) 4.975 (m, 1 H; C ~ - O N O , ) 4.767 (s, C;), protons) 1,2-dinitrooxyethane 1,2-dinitrooxypropane 1.458 (d, 3 H;),C ! 4.498 (dd, 1 H; CHZ-ONOJ 4.725 (dd, 1 H; CH2-ONO.J 5.399 (m, 1H; CH-ONO,) 1.066 (t, 3 H; C g ) 1,2-dinitrooxybutane 1.802 (m, 2 H; CH,-CH,) 4.492 (dd, 1 H; CI4,-ONO2) 4.755 (dd, 1H; C€I,-ONO,) 5.240 (m, 1H; CH-ONO,) 1.424 (d, 6 H; CH,) 2,3-dinitrooxybutane 5.233 (m, 2 H; CH-ONO,)
12,13
2
1
I
3
4 5,6
7,8,9
10,ll
15
16
compound 2-nitrooxy-1-hydroxyethane
a-Proton resonances (in ppm) of selected alkyl nitrates: isopropyl (5.19), isobutyl (4.24), 2-hexyl (5.07), and 3-hexyl (4.99). Abbreviations used: doublet (d), doublet of doublet (dd), multiplet (m), broad (br), singlet (s), and triplet (t). Data from NMR spectrum of the isomeric mixture.
nitrates may overlap with the smaller hydroxynitrates. Thus, quantitative measurements of the light hydroxy nitrates will require care to ensure that unambiguous species identification is made. The dinitrate peaks appear at somewhat longer retention time relative to the same carbon number hydroxynitrates, and we found that the CZ-C~dinitrates (not shown) overlap with a C4 hydroxynitrate. I t is interesting to note that the hydroxynitrates and dinitrates yield sharp Gaussian peaks even though, as discussed below, they are sometimes subject to serious irreversible adsorption. Calculation of the number of theoretical plates for 1-butyl nitrate and 2-nitrooxy-31120
Environ. Sci. Technol., Vol. 27, No. 6, 1993
L;
1
0
10
5
20
15
25
Retention Time, min. Flgure 1. Capillary GC chromatogram of a mixed standard of alkyl nitrates, hydroxy nitrates, and dinltrates. Experiment A
Column b a k e - o
18
p”9’
i4
02 0.0
CONSECUTIVE INJECTIONS o 1 - P R O P Y L NITRATE n 2-PEVTYL
NITRATE
c
3 - H E X Y L UITRATE 2-NI’ROOXY-3-HYDROXY
Flgure 2. Results of
o
BUTANE
12-0lNlTROOXY 1 2-DINITROOXY
PROPANE BUTANE
GC condltloning studies.
hydroxybutane yielded values of 4.2 X lo4and 2.7 X lo5, respectively, indicating good chromatographic resolution for this column and injection system. Irreversible Adsorption of Multifunctional Nitrates. Initial injections of standards containing multifunctional nitrates indicated very low response and, thus, the possibility of problems with irreversible adsorption for these relatively more polar compounds onto column and/or injection port surfaces. To investigate further, a mixed standard solution containing some alkyl and bifunctional nitrates was injected =30 times in succession (=lh between injections), and the respective area count for each compound was plotted against the injection number. The results of this study are shown in Figure 2 (as experiment A). The concentration of the components corresponds to what would be expected in an ambient air sample of 500-L volume at =1 ppt in individual concentration and extracted into 50 p L of solvent. As shown in the figure, the peak areas for the alkyl nitrates were very reproducible over all injections (1s = 2 5% rel) and showed no significant trend. In contrast, the hydroxynitrates and dinitrates yielded very small peaks for the first several injections, increasing steadily to a plateau after about 20 consecutive injections. Beyond that point, the peak areas were reproducible to f15%. Kames and Schurath (14)
Table 11. Relative ECD Response Factors for Organic Nitrates
GC no. 1" compound
retention time (min)
relative responseb
GC no. 2n relative responseb splitless mode split mode
Alkyl Nitrates (R-ONOz) R= 2-propyl 1-propyl 2-butyl isobutyl 1-butyl 3-pentyl 2-pentyl 1-pentyl 3-hexyl 2-hexyl cyclohexyl
4.98 6.29 8.39 8.51 10.84 12.19 12.55 15.27 15.90 16.67 19.07
1-nitrooxy-2-hydroxypropane 2-nitrooxy-1-hydroxyethane 2-nitrooxy-1-hydroxypropane 2-nitrooxy-3-hydroxybutane 1-nitrooxy-2-hydroxybutane 2-nitrooxy-1-hydroxybutane
19.46 19.88 20.53 20.87 22.58 23.68
1,2-dinitrooxypropane 1,2-dinitrooxyethane 2,3-dinitrooxybutane 1,2-dinitrooxybutane
23.67 23.76 23.96 25.71
1.00 0.83 (0.03) 0.98 (0.05) 0.73 (0.04) 0.78 (0.03) 0.86 (0.03) 1.10 (0.04) 0.78 (0.02) 0.85 (0.03 1.00 (0.03) 0.70 (0.04) Hydroxynitrates 0.85 (0.06) 0.98 (0.10) 1.40 (0.07) 1.81 (0.12) 0.67 (0.03) 1.43 (0.08) Dinitrates 2.74 (0.09) 2.20 (0.11) 4.00 (0.16) 2.60 (0.12)
1.00 0.62 (0.02) 0.73 (0.02) 0.56 (0.01) 0.59 (0.02) 0.62 (0.02) 0.87 (0.03) 0.56 (0.01) 0.61 (0.02) 0.73 (0.02)
1.00 0.67 (0.02) 0.76 (0.02) 0.56 (0.01) 0.56 (0.01) 0.57 (0.02) 0.76 (0.02) 0.47 (0.02) 0.50 (0.01) 0.57 (0.02)
0.73 (0.03) 1.10 (0.13) 1.34 (0.05) 1.51 (0.08) 0.53 (0.02) 0.97 (0.05)
0.57 (0.02) 0.59 (0.06) 0.86 (0.03) 1.01 (0.04)
2.97 (0.07) 2.40 (0.16) 4.01 (0.19) 2.61 (0.20)
1.62 (0.05) 1.02 (0.07) 2.40 (0.07) 0.98 (0.06)
0.62 (0.03)
See Experimental Section for details. From relative calibration curve slopes. Combined standard deviation of the slopes in parentheses.
independently observed a similar GC behavior for l-nitrooxy-2-hydroxyethaneand 1,2-dinitrooxyethane (for gasphase injections) but ascribed the phenomenon to adsorption of the low vapor pressure compounds in their gas syringe. We believe that the phenomenon we observed is entirely due to adsorption onto active sites on the column surface. I t is likely that, for on-column injection, mechanical abrasion of the bonded phase creates active sites with exposed Si-OH groups, where hydrogen bonding of the nitrates can occur. There are also likely active sites present to some extent throughout the column. The adsorption is likely to be much more effective for the hydroxynitrates. As hydroxynitrates are repeatedly injected, the active sites may become occupied, at which point the column is effectively "conditioned", and reproducible analyses can be conducted. We note that injection of a pure solvent blank yields no detectable peaks when the column has been conditioned with hydroxynitrate standards. The above explanation is further supported by the following observation. In a separate experiment (experiment B),we conditioned the GC as described above. Once the peak areas were relatively constant, the column was heated to 250 "C (well above the normal final column temperature of 150 "C) for 1h, and then three consecutive analyses (GC programmed as before) of the same solution were immediately performed. The results are shown after the break, in Figure 2. As shown in the figure, the area counts of the alkyl nitrates showed only a very small increase after column heating, whereas the hydroxynitrate and dinitrate peak areas decreased substantially after column heating. The area counts for the dinitrates decreased by a factor of 5 while that for 2-nitrooxy-3hydroxybutane decreased by a factor of about 100. This treatment is characteristic of what happens to the column when it is not used for several days. At higher temperatures, or given enough time, the compounds occupying
the active sites desorb and elute from the column rendering it "unconditioned". We find, however, that the column can be conditioned quickly by injecting concentrated (=lo-" M) solutions of hydroxynitrates and dinitrates. We have also found that some column conditioning can be achieved through multiple injections of alcohols such as propylene glycol but that they are not as effective as using a concentrated sample of the analyte or another hydroxy nitrate. It is clear, however, that attempts at detection of hydroxynitrates in ambient samples using unconditioned columns are likely to yield very small or no peaks, even though the hydroxynitrates may be present. Determination of Relative ECD Response Factors. It is often the case that retention times for atmospheric pollutants of interest are known from impure samples, but standards are not available to enable quantitative determination. In such cases, the availability of relative detector sensitivities can be valuable. The ECD responses of various organic nitrates were determined from the slopes of calibration curves using mixed standards of the pure compounds, once it had been determined that the column was properly conditioned, as described above. The results are shown in Table I1 as calibration factors relative to isopropyl nitrate, chosen as a readily available standard. The uncertainties presented (fl s) represent the result of combiningthe variancesof the two calibration curve slopes. These data were obtained using both GCs. As can be seen from the table, the relative responses of alkyl nitrates with closely related structures are comparable, and generally secondary nitrates show greater response than primary nitrates. This is reflected in the hydroxynitrates as well. In general, the presence of the hydroxy group yields a slightly greater response factor. Dinitrates yielded significantly higher response factors as expected. However, the values obtained by using two different GCs vary substantially, as shown in the table. We have seen that Environ. Scl. Technoi., Vol. 27, No. 6, 1993
1121
r
30
-
r c I
/
A
05
00
I
I
I
Ll
0
15
20
I 25
I \ J N B E R OF D A K 0 V
1-PQOPYL NITRATE 2 - P E N T Y L NITRATE
ci
3-HEXY. h T R A T E ~-hlTRCOXV-3--YDR3XY3blANE
0
rn
2-3~Nl-ROOXY PROPANE 1 2 - 2 l N l T R 0 3 X Y SUTAkE '
Figure 3. Recoveries of organic nitrates extracted from charcoal tubes.
the peak widths for the hydroxynitrates are much wider (both injection modes) for GC no. 2, which employs a conventional split/splitless injection port with glass inserts. For theee compounds, it is likely that there are significant losses on the injection port surfaces, even though these losses may be reproducible. We have found that, for this GC, injection port and detector heating to high temperatures can significantly change the observed relative responses. Given this fact and the significant difference in relative responses observed for the two GCs,we estimate that the relative responses have an absolute uncertainty (i.e., if used for other GCs) of about A30 % . Thus, although the relative responses should be used with caution for other instruments, the values reported here may be useful for ambient measurements where standards are not readily available. Charcoal Sorbent Stability Studies. To determine if the charcoal sorbent sampling technique is suitable for measurement of the multifunctional nitrates, we measured the amount of each of several compounds extracted from identically prepared tubes over a period of 21 days. The results are presented in Figure 3, as the average (of two tubes) fraction of each compound present relative to that present on day 0. For the duplicate samples, the average deviation from the mean recovery (for each time period) for the alkyl nitrates, dinitrates, and the hydroxynitrate shown was 6.9, 13.1, and 20.0%, respectively. The data for alkyl nitrates (1-propyl, 2-pentyl, and 3-hexyl) show a fairly constant recovery (f10-15%) throughout the duration of the experiment. These results are, however, inconsistent with those found by Atlas and Schauffler (15) in similar experiments where only a =70 % recovery of the alkyl nitrates was obtained after 2 days. For this same decay rate, we would have expected only =4% to remain after 21 days. The source of the discrepancy is unclear. The tubes used for both studies were identical. Although we added the organic nitrates directly onto the charcoal bed as a solution in benzene, the benzene solvent was subsequently removed in a stream of air, as described above. Since Atlas and Schauffler loaded their tubes by injection of a solution of alkyl nitrates in benzene into an airstream before the tubes, the charcoal was loaded in 1122
Environ. Sci. Technol., Vol. 27, No. 6, 1993
effectively similar ways in both cases. The recovery and was relatively GC analysis of 2-nitrooxy-3-hydroxybutane irreproducible from cartridge to cartridge as well as from day to day, but the data do not indicate any evidence for significant decay over the 21-day period. A similar behavior was observed for the dinitrates as well. Since all charcoal tubes were prepared identically, we ascribe the irreproducibility to the GC conditioning problems described above. The stability experiment described here was the first conducted with these compounds, using a column that exhibited a particularly significant surface adsorption effect. It appears that the analytical results for day 0 may have been anomolously low, resulting in recoveries greater than 1. However, the data obtained indicate that the hydroxynitrates and dinitrates do not show any significantly greater decomposition rate on the charcoal, relative to the alkyl nitrates, and are reasonably stable over periods of days. Thus, if ambient samples are extracted within a few days, the analytical uncertainty should not be significantly influenced by decomposition in the tubes. OH/&-2-Butene Experiments. There is currently very little information regarding reliable measurements of the yields of hydroxynitrates from OH reaction with alkenes. To obtain further information on this subject, from we measured the yield of 2-nitrooxy-3-hydroxybutane OH reaction with cis-2-butene. The reaction of OH radicals with cis-2-butene is believed to occur exclusively by addition of OH to the double bond (19),yielding a carbon-centered radical to which 0 2 will rapidly add, as shown in reactions 5 and 6 below. Since the 2 and 3 carbons OH + CH,CH=CHCH, CH,CHCH(OH)CH,
+ 0,
-
CH,CHCH(OH)CH,
-
(5)
CH,CH(OO')CH(OH)CH, (6)
of cis-2-butene are identical, OH reaction with it yields the peroxy radical shown in reaction 6 as the only intermediate. This radical will then react in the presence of NO as shown in reactions l a and lb. Thus it must be true that
- d[C2B]/dt = kl[R02'l[NOl and d[NHBl/dt = klb[RO2'l[NO1 and, therefore, A[ NHB]/-A[C2B] = k,b/k,
where ROz' represents the peroxy radical, NHB is the hydroxynitrate product formed as a result of reactions 6 and lb, and C2B represents cis-2-butene. In Figure 4 we have plotted A[NHBl vs -A[C2Bl (both in units of ppb) for three replicate experiments. The slopes for the individual experiments are 0.0325,0.0370, and 0.0378. The actual yields must, however, be corrected for reaction of the product with OH. This can be done as described by Atkinson et al. (6)using OH rate constants for cis-2-butene and the product hydroxynitrate of 5.6 X cm3. molecule-l.s-l(l9) and 9.3 X lo-', cm3~molecule-1~s-1 [using 2-butyl nitrate as a model for this hydroxy nitrate (20)1, respectively. Although the data were corrected, the worstcase correction amounted to a factor of 1.007. Shown in
V
Experiment 1 Experiment 2 Experiment 3
/i
SLOPE = 0.037 i 0 0 0 2
rl::
/’ 0
200
-A[
400
600
800
1000
cis - 2-but e ne], pp b
Figure 4. Determination of the yield of 2-nitrooxy-3-hydroxybutane from OH reaction with c/s-Pbutene.
Figure 4 is the best-fit linear regression slope, forced through zero. From this slope for the three experiments we find that klb/kl= 0.037 f 0.002, where the uncertainty is the 95 % confidenceinterval of the slope, for cis-2-butene. Since the standards used were purified, we believe that the absolute uncertainty of this value is determined largely by the collectionlextraction efficiency for this compound and the reproducibility of the GC analysis. This should be reflected in the reproducibility of the three experiments. The average of the slopes obtained for individual experiments was 0.036 f 0.007 (95% CL of av). Including an estimate of unidentified systematic error, we estimate an absolute uncertainty for this determination of f25 % ,and thus we report the yield as 0.036 f 0.009. Since the radicals produced would be identical due to free rotation about the CZ-C~ bond, this result also applies to trans-2-butene. Discussion Since the yields of hydroxynitrate production from reaction of NO with 1-hydroxy-2-propyl peroxy and 2-hydroxy-3-butylperoxyradicals are 0.017 (11)and 0.037, respectively, it appears that the yield increases with the size of the organic group, as found for a large number of alkyl peroxy radicals (21). However, the yields for the two radicals mentioned above are smaller than for the respective alkyl peroxy radicals by about a factor of 2. For 2-propyl peroxy and 2-butyl peroxy radicals the ratio klb/ kl is 0.042 and 0.090, respectively (21). There is not sufficient data at this point to determine whether this is a general result. However, the yield for cis-2-butene is sufficiently large to make the hydroxynitrates potentially important components of NO,. Indeed Tuazon and Atkinson (22) estimate the organic nitrate yield from OH reaction with isoprene to be as high as 12% . Evidence for significant yields of organic nitrates has also been obtained for OH reaction with 1-octene (23). The question then remains as to why these compounds have yet to be detected in ambient air. The recent Henry’s law measurements for
hydroxynitrates conducted by Kames and Schurath (14) and the findings reported here allow us to provide some speculation. For well-aged air masses that have been processed by clouds (or that have been subject to wet deposition losses), the hydroxynitrates will be depleted relative to the alkyl nitrates, due to their relatively large Henry’s law constants. There have, however, been measurements of organic nitrates in photochemically active air masses, in particular those conducted by Flocke et al. (9),where there was no evidence for significant concentrations of organic nitrates other than the alkyl nitrates identified. This is particularly interesting for the Flocke et al. study, since they used a detector that was selective for nitrates. However, in light of the high water solubility of these compounds, it may be that they were solubilized in the condensed water in the cryogenic trap used for sample concentration. In addition, our studies indicate that it may be necessary to condition the injection system and column before adequate detection limits can be realized for the hydroxy nitrates. Our experience indicates that for an unconditioned column, injection of low picomole quantities of hydroxynitrates may yield no detectable GC peak. It is thus clear that the availability of pure standards for these compounds will be a limiting factor in their ambient quantitative detection. Conclusions The results reported here and those obtained in previous studies indicate that multifunctional organic nitrates are produced in significant yields from the atmospheric oxidation of alkenes (in the presence of sufficient [NO]). Although they have not yet been detected in ambient air, this is likely due to problems associated with irreversible adsorption on sample lines and on concentration or chromatographic systems. However, these problems can likely be overcomeby judicious choice of sampling methods and proper conditioning of GC systems. Our experiments indicate that charcoal traps can be successfully used for measurements of these compounds in gas-phase samples, at least for our laboratory conditions. Use of the relative ECD response factors reported here may help in making estimated determinations of their ambient concentrations for GC/ECD analyses. Unfortunately, the problems associated with their detection are likely to be more serious for the larger organic nitrates formed in greater yields, e.g., those produced from oxidation of isoprene, due to their lower volatility. However, the larger hydroxynitrates would likely have lower Henry’s law constants, as found by Kames and Schurath (14),and may not be as effectively rained out. I t seems clear that for air masses where alkenes are important, i.e., either those impacted by relatively fresh anthropogenic emissions or those impacted by vegetation, the hydroxynitrates will be important relative to alkyl nitrates and may contribute significantly to NO,. We have recently identified many of these compounds in samples obtained from rural air masses and are in the process of trying to make quantitative measurements. Hopefully such measurements will shed further light on this question. Acknowledgments We thank the Natural Sciences and Engineering Research Council of Canada, Environment Canada, and the Ontario Ministry of the Environment for their support of this project. Envlron. Sci. Technol., Vol. 27, No. 8, lW3
1123
Literature Cited Sandroni, S.; Anfossi, D.; Viarengo S. J.Geophys.Res. 1992, 97, 2535.
Low,P.S.;Davies,T.D.;Kelly,P.M.;Farmer,G. J.Geophys. Res. 1990, 95, 22441. Ridley, B. A. Atmos. Environ. 1991,25A, 1905. Calvert, J. G.; Madronich, S. J. Geophys. Res. 1987, 92, 2211. Roberts, J. Atmos. Eniuon. 1990, 24A, 243. Atkinson, R.; Aschmann, S. M.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. J . Phys. Chem. 1982, 86, 4563. Atlas, E. Nature 1988, 331, 426. Buhr, M. P.; Parrish, D. D.; Norton, R. B.; Fehsenfeld, F. C.; Sievers, R. E. J. Geophys. Res. 1990, 95, 9809. Flocke, F.; Volz-Thomas, A,; Kley, D. Atoms. Environ. 1991, 25A, 1951. Bottenheim, J. W.; Barrie, L. A,; Atlas, E. J.Atmos. Chem., in press. Shepson, P. B.; Edney, E. 0.;Kleindienst, T. E.; Pittman, J. H.; Namie, G. R.; Cupitt, L. T. Environ. Sei. Technol. 1985, 19, 049. Barnes, I.; Bastian, V.; Becker, K. H.; Tong, Z. J. Phys. Chem. 1989,9,419.
1124 Environ. Sci. Technol., Vol. 27, No. 6, 1993
Shepson, P. B.; Anlauf, K. G.; Bottenheim, J. W.; Wiebe, A.; Gao, N.; Muthuramu, K.; Mackay, G. I. Atmos. Enuiron., in press. Kames, J.; Schurath, U. J. Atmos. Chem. 1992, 15, 79. Atlas, E.; Schauffler, S. Environ. Sei. Technol. 1991,25,61. Ferris, A. F.; McLean, K. W.; Marks, I. G.; Emmons, W. D. J. Am. Chem. Soc. 1953, 75,4078. Nichols, P. L., Jr.; Magnusson, A. B.; Ingham, J. D. J.Am. Chem. SOC.1953, 75, 4255. Taylor, W. D.; Allston, D.; Moscato, M. J.; Fazekas, G. D.; Kozlowski, R.; Takacs, G. A. Int. J. Chem. Kinet. 1980,12, 231. Atkinson, R. Chem. Rev. 1986,86, 69. Atkinson, R.; Aschmann, S. M. Int. J. Chem. Kinet. 1989, 21, 1123. Carter, W. P. L.; Atkinson, R. J.Atmos. Chem. 1989,8,165. Tuazon, E. C.; Atkinson, R. Int. J.Chem. Kinet. 1990,22, 1221. Paulson, S. E.; Seinfeld, J. H. Enuiron. Sei. Technol. 1992, 26. 1165.
Received for review August 10, 1992. Revised manuscript received February 4, 1993. Accepted February 8, 1993.
