Isoprene reaction with ozone

Trees and shrubs contain a group of fragrant compounds called terpenes. The simplest terpene is isoprene. All other terpenes are built around carbon skeletons constructed from one or more isoprene units. Plants emit terpenes into the atmosphere, as anyone who has walked in a pine or eucalyptus forest will have noticed. The possible effect of terpenes on the concentration of ozone in the troposphere has been the subject of much debate and has led to careful measurements of rates of reaction with ozone. 

Pedersen and Sehested (2002) showed that the aqueous-phase reaction of isoprene with ozone was insignificant for the processing of isoprene in the atmosphere. They estimated the overall and individual lifetimes of isoprene due to reactions with ozone and the hydroxyl radical, at 25 "C and typical in-cloud conditions. The results (Table 3) indicate that clouds generally should not contribute much to the processing of isoprene in the atmosphere. Only in the aqueous phase, were the lifetimes of isoprene due to reactions with ozone and with OH radicals comparable. Similar conclusions were drawn for methyl vinyl ketone, while for methacrolein the clouds could reduce the overall atmospheric lifetime by 50 %. 

These reactions proceed by initial ozone attack on the C = C bond of the olefin. An intermediate ozonide is formed, which rapidly decomposes to a carbonyl and a biradical. The biradical can be stabilized, or it can decompose. Paulson et al. (1991b) found the products methacrolein, methyl vinyl ketone, and propene, in yields of 68%, 25%, and 7%, respectively. Based on the presence of epoxides in the ozone/isoprene system, Paulson et al. concluded that 0(3P) was being formed. Calculations indicated that 0.45 0(3P) radicals were formed for every ozone/isoprene reaction. However, Atkinson et al. (1993) recently showed that the epoxides were formed directly from the reaction with ozone rather than the reaction with 0(3P). The epoxides formed were l,2-epoxy-2-methy 1-3-butene and l,2-epoxy-3-methyl-3-butene, in yields of 0.028 and 0.011, respectively. There was also definite evidence for the formation of OH radicals in the ozone system, thus causing difficulties in product analyses. Each ozone/isoprene reaction yielded 0.68 OH radicals (Paulson et al., 1991b). 

The MCM mechanism for isoprene also includes reactions of NO3 and of ozone. With NO3, a single site of addition is included (the 1-position, associated with formation of the peroxy radical in the 4-position). Four channels are included for reaction with ozone, which lead to methacrolein (0.3), methyl vinyl ketone (0.2) and the associated Criegee biradical CH2OO, and CH2O and the associated Criegee biradicals derived from methacrolein (0.3) and methyl vinyl ketone (0.2), where the numbers in brackets refer to the channel yields. 

O3 + terpene products Rate =. [03] [terpene] We expect the reaction rate to depend on two concentrations rather than one, but we can isolate one concentration variable by making the initial concentration of one reactant much smaller than the initial concentration of the other. Data collected under these conditions can then be analyzed using Equations and, which relate concentration to time. For example, an experiment could be performed on the reaction of ozone with isoprene with the following initial concentrations ... 

Data for the reaction of isoprene with ozone under conditions when the concentration of isoprene is isolated. The experimental behavior is first order under these conditions. 

The most widely used gas-phase chemiluminescence reagent is ozone. Analytically useful chemiluminescence signals are obtained in the reactions of ozone with NO, SO, and olefins such as ethylene and isoprene, but many other compounds also chemiluminesce with ozone. Ozone is conveniently generated online at mixing ratios of =1-5% by electrical discharge of air or 02 at atmospheric pressure [14]. 

As Barr et al. (2003) pointed out, the importance of such emissions is determined mainly by their impact on the three processes taking place in the atmosphere. The first consists in that such NMHCs as isoprene form in the course of carboxylization in plants and contribute much thereby to the formation of biospheric carbon cycle. The second process is connected with NMHCs exhibiting high chemical activity with respect to such main oxidants as hydroxyl radicals (OH), ozone (03), and nitrate radicals (N03). Reactions with the participation of such components result in the formation of radicals of alkylperoxides (R02), which favor efficient transformation of nitrogen monoxide (NO) into nitrogen dioxide (N02), which favors an increase of ozone concentration in the ABL. Finally, NMHC oxidation leads to the formation of such carbonyl compounds as formaldehyde (HCHO), which stimulates the processes of 03 formation. Finally, the oxidation of monoterpenes and sesquiterpenes results in the intensive formation of fine carbon aerosol with a particle diameter of <0.4 pm... 

The data presented have important implications in the behavior of tropospheric nonanthropogenic ozone, aerosol, and other trace constituents. Observational and experimental data have been reported by Rip-perton et al. 20) indicating the natural synthesis of ozone in the troposphere. Considering this study, the ubiquitous presence of various terpenes (21), isoprene (22), and oxides of nitrogen (20) suggest that some ozone is synthesized in the lower troposphere by the reaction NO2 + a-pinene + hv. Conversely, the destruction of ozone in the troposphere is partially ascribed to reactions with the terpenes and intermediates of the photochemical mixture. 

Respiratory irritant mixtures can arise from environmental chemical reactions. For example, ozone reacts rapidly with terpenes under environmental ambient conditions to produce aldehydes, ketones, and carboxylic acids. Several studies that have been carried out demonstrated that reaction of ozone with a-pinene, c/-limonene, and isoprene produce low level concentrations (at or below NOEL levels) of oxidation products and that along with residual ozone and terpenes act as respiratory irritants. 1012 Table 17.3 lists the species typically contained in these mixtures along with their K values. As can be seen, the mixtures contain lipophiles (residual terpenes) and hydrophiles (the reaction products). Similar results have also been reported for environmental reaction of terpenes with ozone and nitrogen dioxide. 9 ... 

Chambers were also used in the research on heterogeneous reactions of isoprene and of other atmospheric trace compounds. As already discussed, Czoschke et al. (2003) studied the formation of SOA from products of isoprene oxidation in 500 dm Teflon-bag chambers at UNC. FoUcers et al. (2003a,b,c) studied the partitioning and influence of dicarboxylic acids on aerosol formation in Aerosol Chamber in Julich. Shantz et al. (2003) investigated the growth of aqueous organic particles and cloud condensation nuclei in the CALSPAN chamber, linuma et al. (2004 paper submitted to this book) studied the reaction of a-pinene with ozone on acidic particles in the Leipzig tent-chamber (9 m ). 

In addition to the reaction with OH, ozone reactions play a significant role in daytime loss processes for isoprene and a- and /3-pinene. Room-temperature rate constants for the ozone reactions with these olefins are 1.43 X 10 17 cm3 molecule-1 s-1, 8.5 X 10-18 cm3 molecule-1 s-1, and 1.6 X 10- 7 cm3 molecule-1 s-1, respectively. 

The oxidation of isoprene, CH2=CHC(CH3)=CH2, in the atmosphere has not yet been fully delineated and thus remains speculative. Hanst et al. (1980) have discussed some of several possibilities. The successive addition of OH and 02 to either of the two double bonds of isoprene may be modeled in analogy of that to propene and is expected to give formaldehyde and methacroleine or formaldehyde and methyl vinyl ketone as the products. The reaction of isoprene with ozone is expected to produce dioxirane and... 

Because of their double bonds, isoprene and a-pinene are Ukely to react with ozone (Weschler and Shields, 1999). These reactions may cause a difference between indoor and outdoor atmospheric chemistry, since these compounds have known indoor sources. An important product from these reactions may be the far more reactive OH- radical (Atkinson et al., 1992 Weschler and Shields, 1996). 

The results of LACTOZ have provided an extended kinetic data base for the following classes of reactions reactions of OH with VOCs, reactions of NO3 with VOCs and peroxy radicals, reactions of O3 with alkenes, reactions of peroxy radicals (self reactions, reaction with HO2, other RO2, NO, NO2), reactions of alkoxy radicals (reactions with O2, decomposition, isomerisation), thermal decomposition of peroxynitrates. Photolysis parameters (absorption cross-section, quantum yields) have been refined or obtained for the first time for species which photolyse in the troposphere. Significantly new mechanistic information has also been obtained for the oxidation of aromatic compounds and biogenic compounds (especially isoprene). These different data allow the rates of the processes involved to be modelled, especially the ozone production from the oxidation of hydrocarbons. The data from LACTOZ are summarised in the tables given in this report and have been used in evaluations of chemical data for atmospheric chemistry conducted by international evaluation groups of NASA and lUPAC. 

This summary will indicate that, even for the simplest of the molecules, isoprene, there remain uncertainties about the degradation pathways. One problem concerns poor carbon balance in almost all of the studies of attack by OH, NO3 and O3 Another concerns the effect of humidity on product distributions. Yet a further question hangs over the significance of the ozonolysis reactions as a source of OH radicals. Almost nothing is known about the mechanisms and specific pathways of reactions of the terpenes, and there are substantial experimental obstacles to investigation of these systems. Much further work is clearly warranted, in order to determine whether ozone is only lost in its reaction with biogenic VOCs or whether the reactions might constitute a source of atmospheric ozone when NOx is present. 

Photooxidation of isoprene in the atmosphere was thought to yield only lighter and more volatile products such as formaldehyde, methacrolein, and methyl vinyl ketone. Evidence that the formation of hygroscopic polar products such as 115 can give rise to aerosols by gas-to-particle formation processes has opened a new panorama for the role of isoprene in the atmosphere. Under simulated atmospheric conditions, photooxidation of isoprene by ozone was shown to be relatively slow and to occur mainly by reaction with OH radicals [65]. Furthermore, when the OH -initiated... 

Apart from pre-vulcanized latex where the rubber molecules have been chemically crosslinked by sulfur, the chemical nature of the rubber molecules of the other latices described above remain chemically intact during and after the process. There are several other latexes available on the market in which the rubber molecules of the latex have been chemically modified. The chemical reactivity of the rubber molecules arises from the olefinic structure of the cis-1,4-isoprene unit within the molecule, which can undergo rapid reactions with, for example, halogens, ozone and hydrogen chloride. Some of these will be described in this section. They are prepared to serve niche applications. 

Orzechowska, G.E. and Paulson, S.E., Photochemical sources of organic acids. 1. Reaction of ozone with isoprene, propene, and 2-butenes under dry and humid conditions using SPME, /. Phys. Chem., 109, 5358,2005. 

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