Terpenes reaction with ozone

Figure 13.2 Aerosol formation due to ozone reactions with terpene emitted from a solid air-freshener in a large laboratory chamber. Adapted from (Sarwar et al., 2004).

H. and Nazaroff, W.W. (2008) Secondary organic aerosol from ozone-initiated reactions with terpene-rich household products. Atmospheric Environment, 42, 8234 15. 

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 reactions of terpenes with ozone lead to a complicated array of products, but the rate behavior for ozone reacting with a terpene can be studied by measuring the concentration of the terpene as a function of time ... 

Nolting F, Behnke W, Zetzsch C. 1988. A smog chamber for studies of the reactions of terpenes and alkanes with ozone and OH. Journal of Atmospheric Chemistry 6 47-59. 

Nirmalakhandan, N.N., Speece, R.E. (1988b) QSAR model for predicting Henry s law constant. Environ. Sci. Technol. 22,1349-1357. Nolting, F., Behnke, W., Zetzsch C. (1988) A smog chamber for studies of the reactions of terpenes and alkanes with ozone and OH. J. Atmos Chem. 6, 47-59. 

The gas-phase reactivity of various terpenes has been measured. Stephens and Scott were the first to include two terpenes (pinene and a-phel-landrene) with their study of the relative reactivity of various hydrocar ns. Both monoterpenes showed the high reactivity predicted by their olefinic structure. Conversion of nitric oxide to nitrogen dioxide in e presence of isoprene is at a rate intermediate between those for ethylene and trans-2-butene, and Japar et al, reported rate constants for the a-pinene and terpinolene-ozone reactions. Grimsrud et a/. measured the rate con-... 

Taken as a whole, it is probable that some fraction of ozone uptake and secondary emission in commercial and residential buildings is due to reactions with soaps, cooking oils, human skin oils, terpenes and other products of human inhabitation. This may partially answer the question posed by NazarofF, Gadgil... 

Worth, and Jeffries (211) obtained emissions 2 to 10 times larger than any previous estimate. Rasmussen (199) has concluded that, while the identity of the emissions is well known, a quantitative estimate of the worldwide terpene emission rate is not yet possible. It would appear that the natural emissions are much larger than that estimated for man s activities, 27 x 10 tons yr Ripperton et al. have suggested that the reaction of ozone with terpenes provides an important, if not dominant, sink for both compounds in the troposphere. While large terpene mixing ratios (ppm or more) have been measured locally in isolated areas [Ripperton et al. (211)], no global estimate is available. 

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 ... 

Table 17.3 Chemical Species Contained in Ambient Mixtures Following Environmental Reactions of Terpenes with Ozone and Their K(m Values...

The majority of the processes presented in Table 2 involve combustion (e.g., burning candles) or the heating of a surface (e.g., electric element of a stove). The UFP from air fresheners and cleaners are the result of nucleation due to secondary chemical reactions with ozone. These secondary reactions are most commonly observed with pinene and limonene (and other terpenes) containing compounds found in pine and lemon scented cleaning products (Nazaroff and Weschler 2004). 

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. 

Production of hydrogen peroxide in forest air by reaction of ozone with terpenes. Nature 346 (1990) 256... 

Several forms of aging of SOA vapors have been observed. One clear form is oxidation of multiply unsaturated alkenes. Many terpenes have multiple unsaturations, and in some cases different double bonds have very different rate constants for reaction with ozone. Examples include terpinolene, myrcene, hmo-nene, a-humulene, and p-caryophyllene [149, 150]. In these systems, ozone will react with one double bond in the terpene and produce some SOA. However, after the precursor is completely removed, SOA levels can continue to rise as the first-generation semi-volatile products continue to react with ozone to produce less volatile second-generation products [149]. 

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-... 

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 ... 

Munshi, H.B., Rama Rao, K.V.S., and Iyer, R.M. Rate constants of the reactions of ozone with nitriles, acrylates and terpenes in gas phase, Atmos. Environ., 23(9) 1971-1976, 1989a. 

It is fitting to begin the discussion with one of the most well-studied reactions in indoor air quality science. In 1999, Weschler and Shields (1999) showed that ozone will react with some terpenes at substantial rates in indoor environments. This prompted phenomenological, health, theoretical, kinetic and mechanistic studies to understand the potential and real impact of this chemistry on occupants. 

Referring to Scheme 13.1, we see that other radical products are formed in the reaction of a-pinene (and other terpenes) with ozone. Not discussed here are secondary or tertiary reactions that may occur as the result of reactions between VOCs and these radical products. The complexity increases many-fold as we consider the many different kinds of unsaturated VOCs and the many different... 

Strong upper airway irritants can be found in reaction mixtures of limonene, other terpenes, and ozone. The identified products included aldehydes, ketones, and carboxylic acids. These identified chemicals and some unidentified reaction products have been studied in upper airway irritation studies in mice, with reduction of the respiratory rate as a key end point. 

Secondly, VOCs react with indoor ozone to produce sub-micron-sized particles [ 101,102 ]. Thus, terpenes, which are commonly found in many household consumer products, interact with ozone, which is also widespread in indoor air through outdoor infiltration and the use of office equipment like laser printers and photocopiers, to form particles. Such reactions can markedly increase the number concentrations and mass concentrations of sub-micronsized particles. In addition, styrene and 4-phenylcyclohexene react with ozone to generate appreciable amounts of aldehydes [4,101]. 

Gas-phase products from the reactions of ozone with the mono-terpenes (-)-(i-pinene and (+)-sabinene which include the ketones formed by oxidative fission of the exocyclic C=C bonds as well as ozonides from the addition of ozone to this bond (Griesbaum et al. 1998). 

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