Effects of Temperature and Pressure on Alkyl ... - ACS Publications


The yields of alkyl nitrates formed in the NO, photooxidations of n-pentane and n-heptane have been determined as a function of temperature (284-341 K...
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J. Phys. Chem. lQ83, 87, 2012-2018

2012

Effects of Temperature and Pressure on Alkyl Nitrate Yields In the NO, Photooxidations of +Pentane and n-Heptane Roger Atkinson,' Willlam P. L. Carter, and Arthur M. Wlner Statewide Air Pollution Research Center, Unhwslty of Callfornkr, Riverside, Callfornkr 9252 1 (Received: November 10, 1982)

The yields of alkyl nitrates formed in the NO, photooxidationsof n-pentane and n-heptane have been determined as a function of temperature (284-341 K) and pressure (56-740 torr). Alkyl peroxy radicals were produced by reaction of the n-alkanes with OH radicals generated from the photolysis of methyl nitrite in air or Oz. The alkyl nitrate yields were observed to be markedly temperature and pressure dependent, increasing with increasing pressure and decreasing temperature. These yields reflect the fraction of ROz radicals which react with NO to yield alkyl nitrates, and are thus important inputs into chemical computer models of the atmospheric NO, photooxidations of the n-alkanes. The observed effects of pressure and temperature on the alkyl nitrate yields are discussed in terms of the mechanistic and thermochemical details of the RO, + NO reaction system, and a general empirical formula giving the alkyl nitrate yields for the C3 through c8 n-alkanes as a function of temperature and pressure is derived.

Introduction The reactions of peroxy radicals (RO,) with NO are an important step in the radical chain mechanism which is associated with the formation of photochemical air pollut i ~ n . l - Until ~ recently, based on analogy with the known mechanism for the reaction of methylperoxy radicals with NO,** the reactions of all alkyl peroxy radicals with NO had been assumed to proceed exclusively via the pathway ROz

+ NO

-

RO -t NO2

-

[RO,NO]

-% RONOz

RO2

f NO

-

[ROONO*

(2)

In our most recent study: these alkyl nitrate yields, which under the experimental conditions employed could be equated to the rate constant ratio k z / ( k l + kz),were observed to increase monotonically with the carbon number of the n-alkane at 299 f 2 K and 735 torr total pressure, from 10.014 for ethane to -0.33 for n-octane. The yields appeared to approach a limit of -0.35 for the larger (lC& n-alkanes under those conditions.s On the basis of these results, we proposed that alkyl nitrate formation could be a significant radical and NO, sink in the NO,/air photooxidations of the higher alkanes, which are important constituents of a wide variety of fuels currently in use.9 (1) K. L. Demerjian, J. A. Kerr, and J. G. Calvert, Ado. Enuiron. Sci. Technol., 4,1 (1974). (2)B. J. Finlayson-Pitts and J. N. Pitts, Jr., Adu. Enuiron. Sci. Technol., 7,75 (1977). (3)W. P. L. Carter, A. C. Lloyd, J. L. Sprung, and J. N. Pitts, Jr., Int. J. Chem. Kinet., 11, 45 (1979): (4)C. T.Pate. B. J. Finlayson, and J. N. Pith, Jr., J.Am. Chem. Soc., 96,6554 (1974). (5)R. Simonaitis and J. Heicklen, J.Phys. Chem., 78,2417 (1974). (6)A. R. Ravishankara, F. L. Eisele. N. M. Kreuttar. and P. H. Wine. J. Chem. Phys., 74,2267 (1981). (7) K. R. Darnall, W. P. L. Carter, A. M. Winer, A. C. Lloyd, and J. N. Pitts, Jr., J. Phys. Chem., 80,1948 (1976). (8)R. Atkinson, S. M. Aschmann, W. P. L. Carter, A. M. Winer, and J. N.Pitts, Jr., J. Phys. Chem., 86,4563 (1982). 0022-3654/83/2087-20 128015010

t RON029

-

RO

+

NO2

1. RON02

(1)

which implies that the reaction is 100% efficient as a radical chain carrier. However, studies in these laboratories7i8indicated that in the NO,/air photooxidations of IC3 n-alkanes alkyl nitrates are formed in significant yields via an alternate pathway for the RO, + NO reaction:

R 0 2 + NO

The increase in the relative importance of alkyl nitrate formation via reaction 2 over chain propagation via reaction 1with the number of carbon atoms in the system is attributed3,'p8 to the reaction occurring by the following general mechanism:

where the asterisk denotes vibrational excitation. The dependence of the alkyl nitrate yield on the size of the molecule is due to the fact that the rates of unimolecular decomposition of vibrationally excited species decrease as the number of internal degrees of freedom increase.1° This chemical activation mechanism also predicts that the alkyl nitrate yields should show a significant dependence on the total pressure, but that the temperature dependence should be relatively small. However, prior to the present study the only available data were obtained at -300 K and atmospheric pressure. Therefore, in order to study such temperature and pressure effects, we have determined the alkyl nitrate yields from the NO, photooxidations of n-pentane and n-heptane as a function of both temperature (284-341 K) and total pressure (56-740 torr).

Experimental Section As in the most recent study from these laboratories: ROz radicals were formed in the presence of NO by the photolysis of methyl nitrite/NO/alkane/air (or 0,) mixtures at 2290 nm, with typical initial reactant concentrations being CH30N0 (0.1-1.6) X 1013molecule ~ m - NO ~; (2-3) X 1013molecule ~ m -and ~ ; n-pentane or n-heptane (2-3) X lOI3 molecule ~ m - ~ . Irradiations were carried out in a 5800-L Teflon-coated evacuable, thermostatted, environmental chamber with a 25-kW solar simulator.'l Prior to each irradiation the

--

-

(9)W. P. L. Carter, P. S.Ripley, C. G. Smith, and J. N. Pitts, Jr., Final Report ESL-TR-81-53, Air Force Engineering and Services Center, Tyndall Air Force Base, FL, Nov 1981. (IO) P. J. Robinson and K. A. Holbrook, "Unimolecular Reactions", Wiley, New York, 1972. (11)A. M.Winer, R. A. Graham, G. J. Doyle, P. J. Bekowies, J. M. McAfee, and J. N. Pith, Jr., Adu. Enuiron. Sci. Technol., 10,461 (1980).

0 1983 American Chemical Society

NO, Photooxidations of nPentane and nHeptane

The Journal of Physical Chemistry, Vol. 87, No. 11, 1983 2013

TABLE 111: Pentyl Nitrate Yields from t h e Irradiation of CH,ONO/n-Pentane/NO/Air (or 0,) Mixtures temp,

K 284 k 1

total press., diluent torr gas

pentyl nitrate yielda

h,lklb 0.068 f 0.011

air air air

0.064 c 0.010 0.101 c 0.014 0.150 c 0.016

0.112 c 0.016 0.176 f 0.019

56 57 153 155 352 505 740

0, 0, air 0, air air air

0.029 0.032 0.050 0.054 0.089 0.092 0.125

0.030 0.033 0.053 0.057 0.098 0.101 0.143

327 c 3

151 400 740

air air air

0.037 f 0.004 0.058 f 0.008 0.074 i 0.005

0.038 i 0.004 0.062 c 0.009 0.080 c 0.006

337

740

air

0.071 i 0.004

0.076 i 0.005

i

3

i

0.005

e 0.008

f

0.006 0.003 0.013

f

0.010

i 2

e 0.003

total press., diluent torr gas

temp,

155 356 740

300 c 2

TABLE IV: Heptyl Nitrate Yields from t h e Irradiation of CH,ONO/n-Heptane/NO/Air (or 0,)Mixtures

0.005 0.008 0.007 f 0.004 c 0.015 c 0.011 f 0.004 f i i

2- t 3-pentyl nitrates. Corrected for secondary reactions of t h e alkyl nitrates (see t e x t ) . Error limits are t w o standard deviations of t h e least-squares slopes of plots such Error limits are t w o standard as those shown in Figure 1. deviations.

chamber was evacuated to 5 2 X 10" torr. Methyl nitrite, prepared as described previously,12and NO were introduced into the chamber from a vacuum gas-handling system. The chamber was then filled to the desired pressure with dry p d i e d matrix air"J3 or ultrahigh purity 0,.Known quantities of n-pentane or n-heptane were flushed into the chamber from an 1-L Pyrex bulb by a stream of ultrahigh purity NP. The chamber was maintained at the desired temperature by means of the chamber's heating/cooling system." The n-alkanes were quantitatively analyzed by gas chromatography with flame ionization detection (GC FID) using a 20 f t X 0.125 in. stainless steel (SS) column of 5% DC703/C20 M on 100/120 mesh AW, DMCS Chromosorb G, operated at 333 K, without sample preconcentration. The pentyl nitrates were analyzed by GC FID using a 10 f t X 0.125 in. SS column of 10% Carbowax 600 on C-22 firebrick (100/110 mesh), operated at 348 K, while the heptyl nitrates were analyzed by GC FID using a 5 X 0.125 in. SS column of 5 % Carbowax 600 on C-22 firebrick (100/110 mesh), also operated at 348 K. For the alkyl nitrate analyses 100 cm3of gas sample was preconcentrated in a 1 cm3 SS loop at liquid argon temperature prior to injection onto the column. Gas chromatographic retention times and calibration factors were determined as described previously.8 Gas chromatographic analyses of the n-alkane and of the alkyl nitrates were carried out prior to and during the irradiations. The irradiations were of -60 min duration and gas chromatographic analyses were carried out every 15 min during this time period.

-

-

Results The temperature, pressure, initial reactant concentrations, and the observed amounts of n-alkane consumed and alkyl nitrate formed, as measured at various times during the irradiations, are given in Tables I and I1 for the CH30NO/NO/n-pentane/air (or 0,)and the CH30NO/ NO/n-heptanelair (or 0,)irradiations, respectively. (Tables I and I1 are available as supplementary material (12)R. Atkinson, S.M. Aschmann, A. M. Winer, and J. N. Pitta, Jr., Int. J. Chem. Kinet., 14, 507 (1982). (13)G. J. Doyle, P. J. Bekowies, A. M. Winer, and J. N. Pitta, Jr., Enuiron. Sci. Technol., 11, 45 (1977).

K

heptyl nitrate yield'

k,lk,b

284 i 2

58 159 349 740

0, air air air

0.121 c 0.170 c 0.257 i 0.308 i

0.018 0.016 0.038 0.036

0.138 c 0.021 0.205 f 0.020 0.346 f 0.054 0.445 i 0.057

300 e 2

56 160 353 740

0, air air air

0.105 c 0.008 0.152 i 0.018 0.220 f 0.014 0.287 i. 0.016

0.117 i 0.009 0.179 f 0.022 0.282 k 0.019 0.403 i 0.025

322 i 4

59 60 156 356 740

0, 0, air air air

0.071 c 0.006 0.081 f 0.012 0.105 i 0.014 0.159 c 0.024 0.199 i. 0.024

0.076 0.007 0.088 i 0.013 0.117 f 0.016 0.189 i 0.029 0.248 i 0.031

341 i 4

160 740

air air

0.085 i 0.008 0.153 i 0.010

0.093 f 0.009 0.181 k 0.012

*

+

a 23- t 4-heptyl nitrates. Corrected for secondary reactions of t h e alkyl nitrates (see t e x t ) . Error limits are t w o standard deviations of t h e least-squares slopes of plots such as those shown in Figure 1. Error limits are t w o standard deviations.

-Y w

ta

t 2

4

-J

>-

L

z W

n - I 2

a

- A ["-PENTANE] molecule ~ m ' ~

Flgwe 1. Plots of the combhed 2- and 3-pentyl nibate yields (corrected for secondary reactlons, see text) against the amount of n-pentane consumed In CH,ONO/NO/n-pentandair (or 0,) irradiations at 300 f 2 K at 56, 154 f 1,505, and 740 torr total pressure (0, A,air diluent; 0 , A, O2 dlluent).

for this manuscript; see paragraph at end of text regarding supplementary material.) Also shown in those tables are the alkyl nitrate yields after correction for their consumption by reaction with hydroxyl radicals.14 The method for these corrections was as described previously: and in no case was the correction greater than a factor of 1.39. Typical plots of the total corrected alkyl nitrate yields against the amount of n-alkane reacted are shown in Figure 1for several CH30NO/NO/n-pentane/air (or 0,) irradiations at 300 f 2 K. It can be seen that straight line plots with zero intercepts were obtained. Tables I11 and IV list the least-squares slopes obtained from such plots, which can be identified with the fraction of the n-alkane reacted yielding the observed alkyl nitrates. In all cases the intercepts were within two least-squares standard deviations of zero. It can be seen from these tables and Figure 1that, within the experimental uncertainties, alkyl nitrate formation (14)R.Atkinson, S.M. Aschmann,W. P. L. Carter, and A. M. Winer, Znt. J. Chem. Kinet., 14, 919 (1982).

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The Journal of Physical Chemistry, Vol. 87, No. 11, 1983

occurred without any apparent induction period, and that it increased linearly with the amount of n-alkane consumed. Furthermore, it is evident that the alkyl nitrate yields are both pressure and temperature dependent, decreasing with decreasing pressure and with increasing temperature. These results are discussed in more detail in the following section.

szc r

c 0 6t

2041

~

Discussion As discussed previously: the major reactions occurring in the CH,ONO/NO/n-alkane/air photolysis system can be represented as follows: CH30N0

+ hv

CH30 + O2

-

+ NO OH + RH R+0 2 RO2 + NO R02 + NO RO + NOz

HOz

-

CH30 + NO

HCHO

--*

+ NO2 R + H20

OH

-

+

M

M -*

--*

+ H02 r

233

'OTAL

RON02

+ NO2

RON02

NO RO

0 2 1

-

O H t oxygenated products

NO2

RON02 + OH

-

1 1 1 4sc 630 PRESSURE (torr)

1

1

800

Figure 2. Plots of the n-pentyl nitrate yields as a function of temperature and pressure (0,284 f 1 K; 0,300 f 2 K; A, 237 f 3 K; A,337 f 3 K). The lines are those calculated from eq I and I 1 (see text).

RO2

RO

,

products

In this system, the n-alkane and the alkyl nitrates are consumed essentially solely by reaction with OH radicals (reactions 6 and 10). The only significant sink for the alkyl peroxy radicals formed from the reaction of OH radicals with the n-alkanes is reaction with NO, forming either the corresponding alkyl nitrate (reaction 2) or the alkoxy radical (reaction l), since the reactions of alkyl peroxy radicals with NOz forming alkylperoxy nitrates

RO2 + NO2 s R02N02

(11,-11)

are insignificant due to the rapid back-decomposition of the alkylperoxy nitrates.15-17 The formation of alkyl nitrates can occur either from the reaction of alkyl peroxy radicals with NO (reaction 2) or from the reaction of alkoxy radicals with NO2 (reaction 8). However, under the conditions of our experiments, decomalkoxy radicals will primarily react with 02,3J%-26 (15)E. 0.Edney, J. W. Spence, and P. L. Hanst, J. Air Pollut. Contr. Assoc., 29, 741 (1979). (16)D. G. Hendry and R. A. Kenley, "Atmospheric Chemistry of Peroxvnitrates" in 'Nitrogenous Air Pollutants". D. Grosiean. Ed.. Ann ArboiPress, Ann Arbor, hI,1979,p 137-48. (17)A. Bahta, R. Simonaitis, and J. Heicklen, J. Phys. Chem., 86,1849 (1982). (18)J. R. Barker, S. W. Benson, and D. M. Golden, Znt. J . Chem. Kinet., 9, 31 (1977). (19)A. C. Baldwin, J. R. Barker, D. M. Golden, and D. G. Hendry, J. Phys. Chem., 81,2483 (1977). (20)L. Batt, 'Reactions of Alkoxy Radicals Relevant to Atmospheric Chemistry", in Proceedings of First European Symposium on the Physico-Chemical Behavior of Atmospheric Pollutants, Ispra, Italy, Oct 16-18, 1979,B. Verdino and H. Ott, Ed., Commission of the European Communities, 1980. (21)L. Batt, Znt. J . Chem. Kinet., 11, 977 (1979). (22)L. Batt and G. N. Robinson, Int.J.Chem. Kinet., 11,1045(1979). (23)D. M.Golden, NatZ. Bur. Stand. Spec. hrbl., No. 557, 51-61 (1979). (24)R. A. Cox, R. G. Derwent, S. V. Kearsey, L. Batt, and K. G. Patrick, J . Photochem., 13, 149 (1980). "

I

pose,3J*21~26~27 or i~omerize3,'9,~~,~%-~' to ultimately give rise to products other than alkyl nitrates (shown overall as reaction 9 above). Upper limits for the contribution of reaction 8 to the observed alkyl nitrate yields for these experiments can be estimated from the rate constants for the reactions of alkoxy radicals with NO2 and 02,and the NO2 and O2 concentrations. Alkoxy radicals react with NO2with a rate constant at atmospheric pressure of k8 N 1.5 X lo-'' cm3 molecule-' s-', approximately independent of temperature.26 The rate constants for the reactions of alkoxy radicals with O2 have received little direct attention, but Gutman et al.25recently determined absolute rate constants for the reaction of methoxy radicals with O2 over the temperature range 413-608 K, and of ethoxy radicals with O2at 296 and 353 K. Furthermore, from thermochemical considerations, Gutman et aLZ5estimated rate constants for other selected alkoxy radicals with 02.For secondary alkoxy radicals the estimated rate constants25 are -3 X cm3 molecule-' s-', independent of temperature over the small range studied here. Hence, assuming that all of the initially present NO and CH30N0 ultimately form NO2, the maximum yield of secondary alkyl nitrate formation from the reactions of RO radicals with NO2 can be calculated to be 0.4% at 740 torr of air and 1.7% at 160 torr total pressure of air (the latter being the conditions most favoring nitrate formation from the reaction of RO radicals with NO2). Since all of the initial nitrogenous species are not converted to NO2 during NO,/organic/air irradiations3 and the larger (X,)alkoxy radicals undergo significant decomposition and isomerization reaction^,'^^^,^^-^' it may be concluded that of the observed fractional alkyl nitrate yields 50.01 are due to the reaction of alkoxy radicals with NOz in the (25)D.Gutman, N. Sanders, and J. E. Butler, J. Phys. Chem., 86,66 (1982). (26)R. Atkinson and A. C. Lloyd, J . Phys. Chem. Ref. Data, in press. (27)K. Y.Choo and S. W. Benson, Int. J . Chem. Kinet., 13, 833 (1981). (28)W. P.L. Carter, K. R. Darnall, A. C. Lloyd, A. M. Winer, and J. N. Pitts, Jr., Chem. Phys. Lett., 42, 22 (1976). (29)A. C. Baldwin and D. M. Golden, Chem. Phys. Lett., 60, 108 (1978). (30)R. A. Cox, K. F. Patrick, and S. A. Chant, Enuiron. Sci. Technol., 15,587 (1981). (31)H. Niki, P. D. Maker, C. M. Savage, and L. P. Breitenbach, J . Phys. Chem., 85, 2698 (1981).

NO, Photooxidations of n-Pentane and n-Heptane

r

284?2 h

The Journal of Physical Chemistry, Vol. 87, No. 11, 1983 2015

/

/*

03*t

0.2

=OI6

> a k-

i

c // //=200

4 00

322 K

0.05

9

".0 I10 0 "0

284 K

300 K

600

300' K

20

050

100

200 2

500

1000

2000 2

800

TOTAL PRESSURE ( t o r r )

Flgure 3. Plots of the n-heptyl nitrate yields as a function of temperature and pressure (0,284 f 2 K; 0, 300 f 2 K; A, 322 f 4 K; A, 341 f 4 K). The lines are those calculated from eq I and I1 (see text).

CH30NO/NO/n-alkane/air (or 0,) irradiations carried out in this study. This is minor, or in most cases negligible, for these n-alkanes. The conclusion that alkyl nitrate yields from the RO + NOz reactions are minor is supported by the excellent agreement in the pentyl nitrate yields observed from EC696 (300 K, 153 torr total pressure of air) and EC-694 (300 K, 155 torr total pressure of 0,). Thus the observed alkyl nitrate yields, after corrections for consumption by reaction with hydroxyl radicals,14can be identified with the rate constant ratio k 2 / ( k l + k z ) . The combined yields determined here of the 2- and 3-pentyl nitrates and of the 2-, 3-, and 4-heptyl nitrates of 0.125 f 0.003 and 0.287 f 0.016, respectively, at 300 f 2 K and 740 torr total pressure are in excellent agreement with the corresponding values of 0.117 f 0.013 and 0.293 f 0.042 determined in our previous study at 299 f 2 K and 735 torr total pressure.8 Since these two studies were carried out in reaction chambers of greatly differing volume, 75 L8vs. the present 5800 L, this excellent agreement indicates that the formation of the alkyl nitrates does not involve surface or heterogeneous effects. Furthermore, this agreement between the two sets of data shows that systematic errors are likely to be small, since completely independent n-alkane and alkyl nitrate gas chromatographic calibrations were carried out for each study. Plots of the total corrected alkyl nitrate yields (Le., k , / [ k l + k , ] ) against total pressure are shown in Figures 2 and 3 for the pentyl and heptyl systems, respectively. It can be seen from these figures that even at 740 torr total pressure neither system is at the high-pressure limit. The logarithmic plots of k z / k l against the total pressure, shown in Figure 4 for the pentyl and heptyl systems for the various temperatures studied, are essentially linear over the total pressure range studied here, and show no indication of k 2 / k l approaching a limiting value at high pressures. Thus these data do not rule out the possibility that the alkyl nitrate yields may approach -100% at sufficiently high pressure. However, as may be expected,'O the lower slope for the heptyl system suggests that at any given pressure it is closer to the higher pressure limit than is the pentyl system. The significant increase in alkyl nitrate yields with decreasing temperature is somewhat unexpected, since for simple chemical activation systems the effect of temperature is anticipated to be relatively minor.I0 The data given in Tables I11 and IV and shown in Figure 4 also

TOTAL PRESSURE ( t o r r )

Flgure 4. Logarithmic plots of the rate constant ratio k 2 1 k , for the pentyl (--A- -, --A - -) and heptyi (-O-, -e-) systems against total pressure at the various temperatures studied.

IooF

0051 29

1

31

33

1

35

l000/T(Kl

Flgure 5. Arrhenius plots of In (k21k,) against l O O O l T (K) for the n-pentane and n-heptane systems at 740 torr total pressure.

indicate that the rate of the increase of k z / k l with pressure does not decrease with decreasing temperature. This indicates that the higher yields at the lower temperatures are not due to lower temperature causing the system to be closer to its high-pressure limit at a given pressure but rather that temperature influences the high-pressure k 2 / k l ratio. Although the Arrhenius expression is not strictly meaningful when applied to rate constants (or, in this case, rate constant ratios) which are not at their high-pressure limits, such analyses may give insights as t o the reaction mechanism and energetics involved in this system. The Arrhenius plots of the rate constant ratios k 2 / k 1at 740 torr total pressure are shown in Figure 5 and yield Arrhenius activation energies of ( E , - E,) N 3.2 kcal mol-' for both the n-pentane and the n-heptane systems. The ratios of the preexponential Arrhenius factors at 740 torr total pressure are Al/Az 6 X 10, for the heptyl system and -1.7 X lo3 for the pentyl system, with each being uncertain by approximately a factor of 2 (one standard deviation). Arrhenius plots of k 2 / k l at lower pressures yield essentially the same (within the experimental uncertainties) A1/Az ratios, but give Arrhenius activation energies ( E , - E,) which decrease monotonically with decreasing pressure, from -3.2 kcal mol-' at 740 torr total pressure to -2.8 kcal mol-' at -60 torr for the heptyl system and 2.5 kcal mol-, at 150 torr total pressure for the pentyl system. Hence the ratios of Al/A2 derived probably rep-

-

-

-

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The Journal of f%ysical Chemistry, Vol. 87, No. 11, 1983

resent the high-pressure limiting values, while 3.2 kcal mol-’ must be considered a lower limit for the high-pressure Arrhenius activation energy difference ( E , - E2). The relatively large temperature dependence of the rate constant ratio k 2 / k l is inconsistent with a purely chemical activation mechanism and suggests that the actual mechanism is complex, involving reactions of thermalized ROONO as well as reactions of chemically activated ROONO* and RON02*: RO

RO2

t NO

-

+

NO;!

5 RONOF

ROONO*

5

RON02

IM

-

ROONO

ROONO -% RO

+ NO2

(a)

ROONO -% RON02 (b) where the asterisk denotes vibrational excitation with a range of internal energies up to that initially present from the RO, + NO reaction, and k, and kb are the rate constants for the respective unimolecular decomposition or isomerization of the thermalized ROONO intermediate. Note that this mechanism consists of a hybrid of chemical activation and thermal activation systems, which accounts for the observation that there are both significant temperature and pressure effects on the alkyl nitrate yields. The observation that the temperature dependence becomes somewhat less pronounced at lower pressure is also consistent with this hybrid chemical activation/thermally activated mechanism, since at lower pressures the system is more dominated by the reactions of the higher energy species (Le., more chemically activated) and is then expected to exhibit only a small temperature dependence. It should be noted that our experimental data show that the lifetime of stabilized ROONO must be significantly shorter than the sampling period of 15 min since the formation of alkyl nitrates occurs without any measurable delay on this timescale. Hence the decomposition of any thermalized peroxy nitrite formed must be occurring on a time scale significantly shorter than the experimental sample time intervals (Le.,