Isoprene oxidation mechanism

The role of isoprene chemistry in controlling atmospheric composition and climate, and the influence of temperature and land use change on isoprene emissions, should not be underestimated. The field observations of OH in isoprene-rich, NO c-poor environments discussed above indicate that isoprene has a considerably smaller effect on OH concentrations than chemistry models predict. This conclusion, based on direct measurements of OH and comparison with model predictions, is also supported by observations of other atmospheric species undertaken in VOC rich NO poor environments. Discrepancies between isoprene concentration measurements and model predictions when constrained to isoprene emission inventories have been reported, as have discrepancies between model predictions of isoprene oxidation product concentrations and those measured. Large model underestimates of OH reactivity and SOA formation under isoprene-rich conditions also point towards significant uncertainties in the OH-initiated isoprene oxidation mechanism. 

Although earlier modelling studies often reduced isoprene emissions to reconcile the modelled isoprene mixing ratio with those observed, a number of observations of canopy scale fluxes have been reported which support the magnitude of isoprene emissions estimated by MEGAN. Perhaps as a consequence of this, the focus in more recent years has turned to the uncertainties associated with the isoprene oxidation mechanism itself rather than the estimated emissions. A number of alternative oxidation schemes have been proposed, some of which are able to reconcile the modelled OH radical concentration with observations (at times), for example the Leuven mechanism. It has been highlighted by recent laboratory studies, however, that the actual amount of OH recycled during isoprene oxidation may be lower than the Leuven mechanism predicts, and currently there does not seem to be one mechanism that can fully satisfy aU field and laboratory based observations to date. 

Stone D, Evans MJ, Edwards PM, Commane R, Ingham T, Rickard AR, Brookes DM, Hopkins J, Leigh RJ, Lewis AC, Monks PS, Oram D, Reeves CE, Stewart D, Heard DE (2011) Isoprene oxidation mechanisms measurements and modelling of OH and HO(2) over a South-East Asian tropical rainforest during the OPS field campaign. Atmos Chem Phys 11 6749-6771... 

Taraborrelli D, Lawrence MG, Butler TM, Sander R, Lelieveld J (2009) Mainz Isoprene Mechanism 2 (MIM2) an isoprene oxidation mechanism for regional and global atmospheric modelling. Atmos Chem Phys 9 2751-2777... 

Poschl, U. von Kuhlmann, R. Poisson, N. Cmtzen, P.J., 2000 Development and Intercomparison of Condensed Isoprene Oxidation Mechanisms for Global Atmospheric Modeling , in Journal of Atmospheric Chemistry, 37 29-52. 

Poschl, U., R. Von Kuhhnann, N. Poisson, and P.J. Crutzen (2000), Development and intercomparison of condensed isoprene oxidation mechanisms for global atmospheric modeling, J. Atmos. Chem., 37, 29—52. 

In a polluted or urban atmosphere, O formation by the CH oxidation mechanism is overshadowed by the oxidation of other VOCs. Seed OH can be produced from reactions 4 and 5, but the photodisassociation of carbonyls and nitrous acid [7782-77-6] HNO2, (formed from the reaction of OH + NO and other reactions) are also important sources of OH ia polluted environments. An imperfect, but useful, measure of the rate of O formation by VOC oxidation is the rate of the initial OH-VOC reaction, shown ia Table 4 relative to the OH-CH rate for some commonly occurring VOCs. Also given are the median VOC concentrations. Shown for comparison are the relative reaction rates for two VOC species that are emitted by vegetation isoprene and a-piuene. In general, internally bonded olefins are the most reactive, followed ia decreasiag order by terminally bonded olefins, multi alkyl aromatics, monoalkyl aromatics, C and higher paraffins, C2—C paraffins, benzene, acetylene, and ethane. 

Measurements of these relatively minor species will not only complete the budget of NO, but will also indicate if our understanding of the hydrocarbon oxidation schemes in the atmosphere is complete. The organic nitrates that completed the NO, budget in the example in Figure 9 arose primarily from the oxidation of the naturally emitted hydrocarbon, isoprene (2-methylbutadiene). To demonstrate the oxidation mechanisms believed to be involved in the production of multifunctional organic nitrates, a partial OH oxidation sequence for isoprene is discussed. The reaction pathways described are modeled closely to those described in reference 52 for propene. The first step in this oxidation is addition of the hydroxyl radical across a double bond. Subsequent addition of 02 results in the formation of a peroxy radical. With the two double bonds present in isoprene, there are four possible isomers, as shown in reactions 2-5 ... 

Oxidation of hydrocarbons has long been considered as a fundamental problem to atmospheric chemists, both from experimental and theoretical points of view, because of the inherent complexity. The reaction kinetics and mechanism of atmospheric hydrocarbons have been the focuses of numerous researches in both experimental and theoretical aspects. Although advances have been made in elucidation of the VOC oxidation mechanisms, large uncertainty and tremendous numbers of unexplored reactions still remain. Several review articles on the atmospheric degeneration of VOCs have been published [4,11-14]. In this review, recent advances in the application of theoretical methods to the atmospheric oxidation of biogenic hydrocarbons are discussed. We will introduce the backgrounds on the quantum chemical calculations and kinetic rate theories, recent progress on theoretical studies of isoprene and a-, y3-pinenes, and studies on other monoter-penes and sesquiterpenes. 

Recently, the explicit oxidation mechanism of isoprene initiated by OH, O3, NO3, and Cl, incorporating the most recent laboratory and theoretical studies, has been evaluated using a box model [69]. The updated mechanism provides explicit reaction steps and detailed intermediates for isoprene oxidation and facilitates more accurate modeling of isoprene photochemistry in the atmosphere. 

Isoprene and terpene oxidation to CO following reaction with OH oxidation mechanism and CO yield not well known. 

This extensive data set showed good agreement with a Master Chemical Mechanism for isoprene oxidation. In addition, one of the predictions of the mechanism is that at low NOx levels, like those seen in Surinam, isoprene hydroperoxides (six isomers, e.g. HOCH2C(OOH)(CH3)CH=CH2) will accumulate. It was noted that correlations between isoprene and other VOCs (different times of day and altitude) were greatest with MlOl ", which could be indicative of isoprene hydroperoxides. This result is an example where PTR-MS analysis can detect previously unmeasured VOCs, although as mentioned above, verification of the identity of unknown positive ions requires complementary methods (e.g. GC-MS). 

In the atmosphere, hydrocarbons are subject to attack by OH radicals and ozone which initiate an oxidation mechanism whereby the materials are first converted to oxygenated compounds and then partly to CO. Hydrocarbon oxidation mechanisms are discussed in Section 6.3. Here we note that not every carbon atom is converted to CO. Accordingly, a yield estimate is required if one wishes to utilize the above data in estimating the production of CO from the oxidation of hydrocarbons. For isoprene the oxidation mechanism has been staked out and one expects a conversion yield of 80% CO, 20% C02. A laboratory study of Hanst et al. (1980) has essentially confirmed these yields. Terpenes, by contrast, pose much large uncertainties, because a substantial portion of the material may be converted to low-volatility products, which condense onto aerosol particles (see Section 7.4.3). The experiments of Hanst et al. (1980) on a-pinene indicated a total yield of CO + C02 of 30% and a CO/C02 ratio of 0.7. Thus, about 20% of carbon in a-pinene was converted to CO. If the conversion efficiencies for other terpenes were similar, one would obtain the following CO... 

Of the atmospheric chemistry of the biogenic hydrocarbons, by far the most is known about isoprene. Whereas rate constants of many other biogenic hydrocarbons with OH, O3, and NO, have been measured, comparatively little is known about the distribution of products. Hatakeyama et al. (1989, 1991) and Arey et al. (1990) have measured some of the products of the a-pinene-O, reaction, including pinonaldehyde, norpinonaldehyde, formaldehyde, CO, and COj. Considerable work remains to be done in elucidating the atmospheric oxidation mechanisms of biogenic hydrocarbons. 

Ruppert, L., Barnes, I, and Becker, K. H. (1995) Tropospheric reactions of isoprene and oxidation products kinetic and mechanistic studies, in Tropospheric Oxidation Mechanisms, edited by K. H. Becker. European Commission, Report EUR 16171 EN, Luxembourg, pp. 91-102. 

Outstanding progress has been made in RO2 chemistry, both in terms of reactivity and reaction mechanisms. The data have been compiled and reviewed [1], and further progress in defining structure-reactivity aspects for more complex VOCs is documented in this Final Report. For example, the mechanism of isoprene oxidation involving six different RO2 radicals, for low NOx and high NOx conditions, has recently been validated against observations. 

Oxidation mechanisms for 2-methyl-propane, 2,3-dimethyl-butane, 2-methyl-butane, cyclohexane and methyl substituted 1-butenes have been fully delineated. The mechanism for n-pentane is understood although not entirely quantified. Progress has been made in deriving the mechanisms for the oxidation of isoprene... 

In this chapter we examine the mechanism for the OH initiated oxidation of isoprene under low NO levels (NO < 50 ppt). At higher NO levels, although it is likely that there are still processes that are missing within atmospheric models (e.g. [3]), isoprene oxidation chemistry is simplified somewhat by the loss of the isoprene-derived peroxy radicals being dominated by reaction with NO. Under low NO c conditions the fate of these peroxy radicals is much less certain. New insights into the isoprene mechanism have been derived using a combinatirHi of ... 

Further evidence for the involvement of isoprene oxidatimi chemistiy in model failures in low to moderate NO regions has also come from laboratoiy and theoretical studies, revealing that the oxidation mechanisms currently adopted in atmospheric models provide inaccurate representations of isoprene-related photochenustiy, with model discrepancies more likely to be apparent under low NOt cOTiditifMis. 

OP3 aircraft measurements have also been used to test our understanding of isoprene oxidation chemistry, using the DSMACC box model to assess the ability of the various proposed oxidation mechanisms to reproduce the observed... 

The oxidation mechanisms of isoprene with OH, O3, and NO3 are addition reaction similar to alkenes and can be thought as an application of reactions described in Sects. 7.2.3, 7.2.4, and 7.2.5. However, since isoprene has asymmetric two double bonds, four reaction pathways have to be considered depending on the addition of active species to either of double bonds and either side of carbons. Many experimental and theoretical studies have been conducted as for the oxidation mechanism of isoprene (Finlayson-Rtts and Pitts 2000 Seinfeld and Pandis 2006), and Fan and Zhang (2004) presented the reaction scheme for each of OH, O3 and NO3 by summarizing those studies. Reaction Schemes 7.4, 7.5 and 7.6 illustrate the oxidation reaction mechanism of isoprene initiated by OH, O3 and NO3, respectively, adapted from Fan and Zhang (2004). 

Fan, J., Zhang, R. Atmospheric oxidation mechanism of isoprene. Environ. Chem. 1, 140-149 (2004)... 

This book covers homogeneous gas-phase kinetics important in the atmosphere, which has been almost established, and provides the solid scientific bases of oxidation of trace gases and oxidant formation. Nevertheless, unresolved problems remain, for example, unsatisfactory reproduction of observed OH/HO2 mixing ratio by model simulation under certain conditions, and oxidation mechanisms involving isoprene, terpenes and other biogenic hydrocarbons, and anthropogenic aromatic hydrocarbons. Therefore, descriptions of these topics are not completed in the book. Heterogeneous reaction chemistry is not covered well except for the chemistry on polar stratospheric clouds (PSCs) and reactive uptake coefficients of selected... 

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