Environ. Scl. Technol. 1985, 19, 159-163
funded, in part, by the U.S.Environmental Protection Agency under Assistance Agreement R 809474 to L.J.C.L.,it has not been subjected to the Agency's required peer and administrative review and, therefore, does not necessarily reflect the view of
the Agency, and no official endorsement can be inferred. This work was presented in part at the 1984 Pittsburgh Conference on Analytical Chemistry Spectroscopy, Atlantic City, NJ; March 8, 1984, Abstract No. 776.
Kinetics and Atmospheric Implications of the Gas-Phase Reactions of NO3 Radicals with a Series of Monoterpenes and Related Organics at 294 & 2 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 constituent of both clean and polluted nighttime atmospheres, with the naturally emitted monoterpenes myrcene, cis- and trans-ocimene, a- and y-terpinene, and a-phellandrene have been determined in air at atmospheric pressure and 294 f 2 K. On the basis of our recently determined equilibrium constant of 3.4 X cm3molecule-l at 298 K for the reactions NOz + NO e Nz05,the following rate constants (in units of cmB molecule-l s-l) were obtained: myrcene, 1.1 f 0.3; cis- and trans-ocimene, 2.4 f 0.6; a-terpinene, 19.4 f 4.7; y-terpinene, 3.1 f 0.7; a-phellandrene, 9.1 f 2.3. B use of these and our previous data for a-and @-pinene,Axcarene, and d-limonene, NO, radical rate constants are estimated for a further series of monoterpenes and related organics. It is shown that nighttime NO, radical reactions for these and other monoterpenes can be dominant loss processes for these naturally emitted organics and/or for NO, radicals. In addition, rate constants for the reaction of NOz with myrcene and cis- and trans-ocimene were also determined. H
Introduction Long path-length differential optical absorption spectroscopic techniques have recently been used to identify and measure the gaseous NO3 radical in nighttime atmospheres at a variety of locations in the United States and Europe (1-6). Observed NO, radical concentrations have ranged from the detection limit of the technique [ 1 part per trillion (ppt)] up to -350 ppt at a downwind receptor site in the Los Angeles Air Basin (1). These ambient atmospheric data, together with the available kinetic data for NO, radical reactions with a wide variety of organics (7-14), show that the reaction of the NO3 radical with the more reactive alkenes (including the monoterpenes), the hydroxy-substituted aromatics, and dimethyl sulfide can be an important nighttime loss process for NO, radicals and/or these organics ( 8 - 1 2 , 1 4 , 1 5 ) . Indeed, our recent data for the kinetics of the reactions of the NO, radical with the monoterpenes a-and @-pinene,A3-carene, and d-limonene (12) showed that these reactions would lead to large reductions in either the nighttime NO, radical or the monoterpene concentrations, depending on the relative magnitude of the NO, radical formation rates and the monoterpene emission rates ( 1 5 ) . In this work, we have extended our previous measurements of the rate constants for the reaction of NO3radicals with a- and @-pinene,A3-carene,and d-limonene (12) to a further series of monoterpenes. These kinetic measurements, together with our previous data (12), permit predictions to be made of NO, radical rate constants for other monoterpenes and related organics for which experimental data are not available. These experimental or estimated NO, radical rate constants, together with the
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0013-936X/85/0919-0159$01.50/0
corresponding OH radical and O3reaction rate constants, then allow the dominant atmospheric fates and lifetimes of these organics to be assessed under both ambient daytime and nighttime conditions. 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 (10-14). This technique is based upon monitoring the relative decay rates of a series of organics, including at least one organic whose NO, radical reaction rate constant is reliably known, in the presence of NO, radicals. NO3 radicals were generated by the thermal decomposition of Nz05(26) in air:
Nz05 NO2
M
NOz + NO,
(1)
+ NO3 -% N205
(2) Providing that the monoterpenes and the reference organics reacted negligibly with Nz05and NOz (see below), then under the experimental conditions employed the sole chemical loss process for these organics was reaction with NO, radicals: NO3 + terpene products (3)
NO,
--
+ reference organic
products
(4) Additionally, small amounts of dilution occurred from the incremental additions of Nz05to the reactant mixture. During these experiments, the dilution factor D, was typically 0.0025 (i.e., -0.25%) per N205addition. Thus (10-14)
[terpene],, [terpene], [reference organic],, %[In( k4
[reference organic],
) ] - D,
(1)
where [terpene],, and [reference organic],, are the concentrations of the monoterpene and the reference organic, respectively, at time to, [terpene], and [reference organic], are the corresponding concentrations at time t, D, is the dilution factor at time t , and k3 and k4 are the rate constants for reactions 3 and 4, respectively. Hence, plots of [In ([terpene],,/[terpene],) - Dt] vs. [ln([reference organic],,/[reference organic],) - D,] should yield a straight line of slope k 3 / k 4and a zero intercept. With this experimental technique, the initial concentrations of the terpenes and the reference organics were -1 ppm (1 ppm = 2.41 X lo1, molecule cm-3 at 294 K and 735 torr total pressure), and up to five incremental
0 1985 American Chemical Society
Environ. Sci. Technol., Voi. 19, No. 2, 1985
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amounts of N2O5 [ ~(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 NO2 was also included in the reaction mixtures in order to derive the equilibrium between NO3 radicals, NO2 and N205 toward N205. The reference organics 2-methyl-2butene and 2,3-dimethyl-2-butene and the monoterpenes were quantitatively monitored during these experiments by gas chromatography with flame ionization detection (GC-FID), using the columns described previously (12). All rate constant determinations were carried out at 294 f 2 K and atmospheric pressure (- 735 torr) in a -4OOO-L all-Teflon chamber, with the diluent gas being dry purified matrix air (17). As described previously (10-14),N206was prepared by the method of Schott and Davidson (18). Known pressures of N205(as measured by an MKS Baratron capacitance monometer) in 1.0-L Pyrex bulbs were flushed into the chamber for 5 min by a 2 L min-l flow of N2 (199.995% purity level), with simultaneous rapid stirring by a fan rated at 300 L s-'. Initial NO2 concentrations were measured by using a chemiluminescence NO-NO, analyzer. Since several conjugated dialkenes react with NO2 at nonnegligible rates (19),the rate constants for the reactions of myrcene and cis- and trans-ocimene with NO2were also determined as an integral part of this study [those for aand y-terpinene and a-phellandrene had been measured previously (1911. In these experiments, -4 ppm of NO2 was added to a myrcene or cis- and trans-ocimene ( 1 ppm)-air mixture, and the monoterpene concentrations were monitored by GC-FID as a function of time after the addition of NO2, as described in detail previously (19). a-Phellandrene was obtained from Fluka Chemical Corp. with a stated purity level of >99%. Myrcene (technical grade) and a- and y-terpinene (with stated purity levels of 198%) were obtained from the Aldrich Chemical Co. Ocimene (a cis- and trans-mixture) was generously donated by Givaudan. While the cis- and trans-ocimene isomers were resolved by GC-FID, the cis or trans identities of these isomers were not established. However, since these isomers reacted at essentially identical rates (within
