Environ. Sci. Technol. 1984, 18, 370-375
Kinetics of the Gas-Phase Reactions of NO, Radicals with a Series of Dialkenes, Cycloalkenes, and Monoterpenes at 295 f 1 K Roger Atkinson,” Sara M. Aschmann, Arthur M. Winer, and James N. Pltts, Jr. Statewide Air Pollution Research Center, University of California, Riverside, California 9252 1
Rate constants for the gas-phase reactions of the NO, radical, an important reactive constituent of nighttime ambient atmospheres, have been determined for the first time for a series of dialkenes, cycloalkenes, and monoterpenes by using a relative rate technique. By use of a rate constant for the reaction of NO, radicals with trans-2-butene of (2.11 f 0.24) X cm3 molecule-l s-l, the rate constants obtained at 295 f 1K were the following (in cm3 molecule-l s-l units): l,&butadiene, (5.34 f 0.62) X isoprene, (3.23 f 0.38) X cyclohexene, (2.87 f 0.34) X 1,3-cyclohexadiene, (7.2 f 1.7) X 10-l2; 1,3-cyclo1,4-cyclohexadiene, (2.89 f 0.35) X heptadiene, (3.8 f 0.9) X 10-l2;a-pinene, (3.4 f 0.8) X 10-l2; @-pinene,(1.4 f 0.3) X 10-l2; A,-carene, (5.9 f 1.3) X d-limonene, (7.7 f 1.7) X 10-l2. These NO, radical rate constants for the naturally emitted hydrocarbons isoprene and the monoterpenes, in conjunction with measured ambient NO, radical concentrations, show that nighttime reaction with the NO, radical can be an important, if not dominant, loss process for these organics. Similarly, these reactions can also be an important loss process for NO, radicals, and hence for NO,, during nighttime hours. Introduction The nitrate (NO,) radical has been recognized for some time as an important intermediate species in laboratory and environmental chamber NO,-O,-air systems (1-4). The first kinetic data for the reactions of NO, radicals with , organics were obtained by Niki and co-workers ( 5 , 6 ) with rate constants being measured a t room temperature for acetaldehyde (5) and a series of alkenes (5,6) and upper limit rate constants being obtained for benzene and toluene (6).
Within the last 4 years, the NO, radical has been identified and measured by long-path spectroscopic techniques in nighttime atmospheres at a variety of locations in the U.S. and Europe (7-13), at concentrations ranging from the detection limit of the spectroscopictechnique [ 1part per trillion (ppt)] up to -350 ppt at a downwind receptor site in the Los Angeles Air Basin (7). These data, together with the available kinetic data (5, 6, 14), indicated that reaction of this species with the more reactive alkenes and the hydroxy-substituted aromatics could be an important nighttime loss process for both NO, radicals and these organics (6, 14-16). Clearly, kinetic and product data are required for a wide variety of classes and structures of organics to more fully investigate the reactivity of NO3 radicals with organics and to assess the importance of these reactions in the ambient atmosphere. To this end, we are presently conducting a comprehensive study of gas-phase NO, radical reactions and have recently determined rate constants for the reactions of NO, radicals with a series of alkanes (17 ) ,aldehydes (18),alkenes (18),aromatic hydrocarbons (18),and dimethyl sulfide (19). In the present work we have extended this study to the determination of rate constants for the reaction of NO3 radicals with a series of dialkenes (1,3-butadiene and isoprene), cycloalkenes (cyclohexene, 1,3- and 1,4-cyclohexadiene and 1,3-cycloheptadiene),and monoterpenes (a-
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Environ. Sci. Technol., Vol. 18, No. 5, 1984
and @-pinene,A,-carene, and d-limonene) in air a t atmospheric pressure and 295 f 1 K. Isoprene and these monoterpenes, which are emitted into the atmosphere from vegetation (20-22), were included in this study in order to determine the importance of these reactions as loss processes for NO, radicals and these terpenes in the “clean” troposphere. Experimental Section The experimental technique used for the determination of NO, radical reaction rate constants was a relative rate method which has been described in detail previously (17-19), and hence only the relevant details are given here. This technique is based upon monitoring the relative decay rates of the alkenes being studied and at least one organic whose NO, radical reaction rate constant is reliably known in the presence of NO, radicals. NO, radicals were generated by the thermal decomposition of N205in air (23). N206-% NO2 + NO,
NO^ + NO,
A,N
(1)
~ O ~
(2) Providing that the reactant alkenes and the reference organics react negligibly with N206and NO2 (see below), then under the experimental conditions employed the sole chemical loss process of these organics was due to reaction with NO, radicals: NO, + alkene products (3) NO, + reference organic products (4)
--
Additionally, small amounts of dilution occurred from the incremental additions of N205to the reactant mixture. The dilution factor at time t was determined from the volume change within the chamber, being given by D , = In [(chamber volume),/(chamber volume),,], where (chamber volume),, is the initial volume of the chamber at time to and (chamber volume), is the volume of the chamber at time t. During these experiments, the dilution factor D,was typically 0.003 (Le., -0.3%) per N205addition. Thus
In
(r21:)
= k , l tt[oN O , ] d t
+ D,
(I)
and [reference organic],, [reference organic],
= k 4 j f [ N 0 3 ]dt
+ D,
(11)
Eliminating the integrated NO, radical concentration from eq I and I1 leads to
[reference organic],, [reference organic],
)
- D;] (111)
where [alkene],, and [reference organic],, are the concen-
0013-936X/84/0918-0370$01.50/0
0 1984 American Chemical Society
trations of the reactant alkene and the reference organic, respectively, at time to, [alkene], and [reference organiclt are the corresponding concentrations at time t, and k3 and k4 are the rate constants for reactions 3 and 4, respectively. Hence, plots of [In ([alkene],,/[alkene],) - Dtl vs. [In ([reference organic],,/ [reference organic],) - Dt] should yield a plot of slope k 3 / k 4with a zero intercept. For this technique, the initial concentrations of the reactant alkenes and the reference organics were 1ppm (1ppm = 2.41 x 1013molecule cm-3 at 295 K and 735-torr total pressure), and up to six incremental amounts of NzO5 (-0.1-1 ppm per addition) were added to the chamber during an experiment. In order to extend the reaction times beyond the mixing time, 1-4 ppm of NOz was also included in the reaction mixtures to drive the equilibrium between NO3, NOz, and Nz05 toward Nz05. trans-2Butene, 2-methyl-a-butene, 2,3-dimethyl-2-butene, 1,3butadiene, isoprene, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene,and 1,3-cycloheptadiene were quantitatively monitored during these experiments by gas chromatography with flame ionization detection (GC-FID) using a 20 ft X 0.125 in. stainless steel (SS) column packed with 5% DC703/C20M on 100/120 mesh AW, DMCS Chromosorb G, operated at 333 K. The monoterpenes aand @-pinene,A3-carene, and d-limonene, and 2-methyl2-butene and 1,3-cyclohexadienewere monitored by GCFID using a 10 ft X 0.125 in. SS column of 10% Carbowax 600 on (2-22 firebrick, operated at 348 K. All rate constant determinations were carried out at 295 f 1K and atmospheric pressure (-735 torr) in a 6400-L all-Teflon chamber, with the diluent gas being dry purified matrix air (24). N205 was prepared by the method of Schott and Davidson (25). Known pressures of N205in 0.53-L bulbs (as measured by an MKS Baratron capacitance manometer) were flushed into the chamber for 2 min by a 9-Lmin-l flow of ultrahigh purity Nz, with simultaneous rapid stirring by a fan rated at 300 L s-l.
I n
9
ISOPRENE
A
( f,
-
Results By use of this relative rate constant technique, the following sets of alkenes were studied: trans-2-butene, l,&butadiene, and isoprene; trans-2-butene, 2-methyl-2butene, 2,3-dimethyl-2-butene, cyclohexene, and 1,3cycloheptadiene; trans-2-butene, 2-methyl-a-butene, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, and 1,3-cycloheptadiene; trans-2-butene, 2-methyl-a-butene, l,&cyclohexadiene, 1,4-cyclohexadiene, a-pinene, @-pinene, A3-carene, and d-limonene; 2-methyl-2-butene,a-pinene, @-pinene,A3-carene,and d-limonene. trans-2-Butene and 2-methyl-2-butene were used as the reference organics. Experimental data from this (26) and other (27) laboratories show that the conjugated dialkenes (i-e., 1,3-butadiene, isoprene, 1,3-cyclohexadiene, and 1,3-cycloheptadiene among the alkenes studied here) react with NOz at nonnegligible rates under the experimental conditions employed here. In addition, previous work in these laboratories using in situ Fourier transform infrared absorption spectroscopy (18) has shown that in N205NOz-alkene-air mixtures, the NOz concentration remains essentially constant throughout the reaction. Hence, by use of the room temperature rate constants for the reaction of NO2with these conjugated dialkenes recently obtained in this laboratory (26) and the initial NOz concentrations in these reactions, the experimental data obtained for the Nz05-N02-alkene-air mixtures were corrected to account for the loss of the dialkenes due to reaction with NOz. This correction amounted to 4-7% of the total observed decay rate for 1,3-butadiene, 2-4% for isoprene, 2-7% for 1,3cycloheptadiene, and 10-20% for 1,3-cyclohexadiene.
I,3-BUTADIENE
/ I
'6
I
I
I. 2
0.8
0.4
I
I .6
In ([trans-2-BUTENE] t,/[tran~-2-BUTENE])
-Dt
Flgure 1. Plot of eq 111 for 1,3-butadiene and isoprene, with trans2-butene as the reference organic. (The data for 1,3-butadiene and isoprene have been corrected for reaction with NO,; see the text.)
r cii
-
h
c
w
2 W Y
_I
a
u
\ c 7
W
z W Y
_I
a
u W
-c
I
0
0.08
I
I 0.16
1
I 0.24
I
I 0.32
(
In [ ~ ~ ~ ~ S - ~ - B U T E N E ] ~ ~ / [ ~ ~ ~ ~ S - ~-Dt -BUTENE]~) Figure 2. Plot of eq I11 for cyclohexene and 1,4-~yclohexadiene,with trans -2-butene as the reference organic. (The data for 1,4-cyclohexadiene have been displayed vertically by 0.04 unit for clarity.)
Thus, apart from 1,3-cyclohexadiene,for which the correction, though still small, was significant, this correction for NO2 reaction was essentially negligible. For the other alkenes studied in this work (trans-2butene, 2-methyl-2-butene, 2,3-dimethyl-2-butene,cyclohexene, 1,4-cyclohexadiene, and the four monoterpenes), their consumption by reaction with NOz was totally negligible, being
