Unique products of the reaction of isoprene with atomic chlorine: Potential markers of chlorine atom chemistry

GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO.13, PAGES 1615-1618, JULY 1, 1997 Unique products of the reaction of isoprene with atomic chlorine: Potential markers of chlorine atom chemistry Trent Nordmeyer, Weihong Wang, Mark L. Ragains, and Barbara J. Finlayson-Pitts University of California, Irvine, Department of Chemistry, Irvine, California Chet W. Spicer, Robert A. Plastridge Atmospheric Science and Applied Technology Department, Battelle, Columbus, Ohio Abstract. The contribution of atomic chlorine to the chemistry of marine regions as well as the Arctic at ground level at polar sunrise is the subject of a number of recent studies. However, identifying the specific chlorine atom precursors has proven difficult. One potential approach is the measurement of definitive products of chlorine atom reactions, for example with biogenic hydrocarbons. We report here product studies of the chlorine atom reaction with isoprene using ppm concentrations at one atmosphere air and 298 K in a NOx-free system using atmospheric pressure ionization-mass spectrometry (API-MS) as well as GC-MS. 1-chloro-3-methyl-3-butene-2-one (CMBO) is identified as a unique product of this reaction, and there is evidence of the formation of three additional isomers of CMBO as well. Methyl vinyl ketone (MVK) is formed in small yields (9 + 5 %), consistent with earlier studies of this reaction in which an upper yield of 13% was reported. The stable product expected from allylic hydrogen atom abstraction (measured in earlier kinetic studies to be 15% of the total reaction), 2-methylene-3- butenal, is also tentatively identified using API-MS. Assuming that similar chemistry occurs at the ppb-ppt levels found in the atmosphere, identification of CMBO and/or its isomers in field studies could provide strong evidence of chlorine atom chemistry in low NO x environments where there are also sources of isoprene. identity is not known have also been measured recently at ground level in the Arctic at polar sunrise [Impey et al., 1997]. The sources of these compounds are not known, but may include reactions of NaC1 and NaBr as well as other sea salt components such as MgC12 6H20 [Langer et al., 1997] with various oxides of nitrogen and perhaps ozone, accompanied by a recycling mechanism [Barfie et al., 1988; McConnell et al., 1992; Fan and Jacob, 1992; Finlayson-Pitts, 1993; Graedel and Keene, 1995; LeBras and Platt, 1995; Mozurkewich, 1995; Tang and McConnell, 1996; Sander and Crutzen, 1996; Vogt, Crutzen and Sander, 1996]. Another potential approach to investigating chlorine atom production in the troposphere is to identify and measure any unique chlorine-containing products of its reactions with organics. For example, significant quantities of isoprene are produced by deciduous trees and shrubs in coastal areas [e.g. see Guenther et al., 1995]; in addition, there appears to be a source over the oceans since it has been shown to be generated by phytoplankton in seawater [Bonsang et al., 1992; Moore et al., 1994; Milne et al., 1995; McKay et al., 1996]. Isoprene emitted in coastal areas or in the marine boundary layer may be oxidized by 03, NO3 (at night), OH (during the day) as well as C1 atoms at dawn. If the Cl-isoprene reaction gives unique chlorine- containing products, they could serve as markers for chlorine atom chemistry. The kinetics and mechanism of reaction of chlorine atoms with Introduction The role of sea salt particles in the chemistry of the troposphere has been recognized for a number of years [Cicerone, 1981]. More recently, the potential for generation of photochemically active chlorine-containing products which subsequently photolyze to generate chlorine atoms has been of great interest [Finlayson-Pitts, 1993; Graedel and Keene, 1995; Keene et al., 1996; Behnke et al., 1997]. isoprene have been studied recently [Ragains and Finlayson-Pitts, 1997]. As expected for a di-unsaturated alkene, the reaction is fast, with a rate constant of (4.6 + 0.5) x 10 © cm 3 molecule -• s -• (2 ,) at 298 K and one atmosphere pressure. A small but significant fraction, 15 + 4 % (2 ,), of the overall reaction proceeds by abstraction of an allylic hydrogen, which is expected to lead to the formation of 2-methylene-3-butenal as a stable product in air. The remaining 85% of the reaction must proceed Observations in air to date provide evidence primarily for the via the initial addition of the chlorine atom to one of the double existence of C12 in marine areas. For example, Keene and bonds, followed by reaction of the resulting alkyl radical with 02. coworkers [Keene et al., 1993; Pszenny et al., 1993] measured This suggests the potential for formation of unique chlorine- inorganic chlorine-containing compounds other than HC1 using a containing products characteristic of this reaction. mist chamber. While individual compounds were not identified, We report here laboratory studies identifying 1-chloro-3- they were hypothesized to include C12 and possibly other species methyl-3-butene-2-one (CMBO): such as HOC1. Recently, Spicer and coworkers (1996) specifically identified C12 at concentrations up to 150 ppt for the CH3 O first time in coastal marine areas using atmospheric pressure ionization mass spectrometry (API-MS). One or more C•C photolyzable chlorine and bromine atom precursors whose Copyright 1997 by the American Geophysical Union. Paper number 97GL01547. 0094-8534/97/97GL-01547505.00 CH 2 CH2CI and three of its isomers as unique products of the gas phase chlorine atom reaction with isoprene in air at room temperature. In addition, we have tentatively identified the expected abstraction product, 2-methylene-3-butenal.