Ozone, atmosphere isoprene reaction

Paulson, S. E Flagan, R. C., and Seinfeld, J. H., Atmospheric photooxidation of isoprene. Part 2. The ozone-isoprene reaction. Int. J. Chem. Kinet. 24, 103 (1991b). 

As exemplified by the stmctural formulas of a-pinene, P-pinene, A -carene, isoprene, and limonene, shown in Figure 16.1, terpenes contain alkenyl (olefinic) bonds, in some cases two or more per molecule. Because of these and other structural features, terpenes are among the most reactive compounds in the atmosphere. The reaction of terpenes with hydroxyl radical is very rapid, and terpenes also react with other oxidizing agents in the atmosphere, particularly ozone, O3. Turpentine, a mixture of terpenes, has been widely used in paint because it reacts with atmospheric oxygen to form a peroxide, then a hard resin. It is likely that compounds such as a-pinene and isoprene undergo similar reactions in the atmos-... 

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. 

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... 

Figure 1. Gas-phase reactions of isoprene with atmospheric radicals and ozone (for simplicity, only one isomer of each product is shown).

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 %. 

Rudzinski (2004) compared the rates of the aqueous-phase reaction of isoprene with sulphate radicals against the rates of the gas- and the aqueous-phase reactions of isoprene with OH radicals, NO3 radicals and ozone. The rates were evaluated for 25 C, typical atmospheric concentrations of reactants (Herrmann et at, 2000), and a LWC of 10 and 10. The partitioning of reactants between phases was described using Henry s Law. The results, shown in part in Table 4, indicated that the aqueous-phase reaction of isoprene with sulphate radicals was competitive against other reactions only in the aqueous phase and at very high values of liquid water content (LWC =10 ). 

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 ). 

OH and NOx do not appear in the net reaction as they are regenerated in other reactions. Notably, atmospheric oxidation of RH (at sufficient NOx) results in the production of ozone and a carbonyl that can undergo further photochemistry. If RH is the slowly reacting methane (typical lifetime, about 8 y), the carbonyl product is formaldehyde (HCHO). If RH is the more reactive hydrocarbon isoprene (CH2=C(CH3)CH=CH2 typical lifetime, 1-2 h) the net reaction produces two carbonyl products formaldehyde and a 4-carbon carbonyl, either methacrolein (MAC CH2=C(CH3)CH0), or methylvinyl ketone (MVK CH2=CHC(0)CH3) depending on the site of OH attack. 

Biogenic hydrocarbons are principally emitted by trees. Isoprene and a- and /3-pinene are the most abundant biogenic hydrocarbons which are emitted (Dimitriades, 1981) (see Fig. 7). The rate constants of the reactions of the biogenics with OH, ozone, and nitrate radicals have been well investigated (Atkinson, 1991 Atkinson et ai, 1990). However, limited data are available concerning the products of biogenic reactions in the atmosphere. 

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). 

Isoprene and other terpenes are now known to undergo reactions that contribute to the development of pollutants, such as ozone and oxides of nitrogen in the atmosphere. 

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... 

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