Nitrate radical reaction with isoprene

Limited product data are available for the nitrate radical reaction with isoprene and a- and /3-pinene. For the isoprene reaction, Barnes et al. (1990) determined molar yields of total nitrates of 80%, CO of 4%, and formaldehyde of 11%. Methacrolein was also detected, but an accurate product yield could not be determined. Additional work with a- and /3-pinene was not conclusive. Although nitrate features were observed as initial reaction products, these compounds quickly transferred into aerosols, thus preventing identification via the methods employed in the study (FTIR spectroscopy). 

Skov, H J. Hjorth, C. Lohse, N. R. Jensen, and G. Restelli, Products and Mechanisms of the Reactions of the Nitrate Radical (N03) with Isoprene, 1,3-Butadiene and 2,3-Dimethyl-l,3-butadiene in Air, Atmos. Environ., 26A, 2771-2783 (1992). 

H. Skov, J. Hjorth, C. Lohse, N.R. Jensen, G. Restelli Products and mechanism of the reactions of the nitrate radical (NO3) with isoprene, 1,3-butadiene and 2,3-dimethyl-1,3-butadiene in air, Atmos. Environ. 26A (1992) 2771. 

Field studies suggest that the nitrate radical reaction can also be a major contributor to isoprene decay at night, as well as contributing to the formation of organic nitrates in air. For example, Starn et al. (1998b) found that when the product of N02 and 03 (which form N03) was high in a forested region in the southeastern United States, isoprene often decayed rapidly at dusk. This reaction of N03 with isoprene was estimated to be the major sink for N03 under some conditions in this area. 

Isoprene and the terpenes may be attacked by OH, NO3 and O3. The rate coefficients at room temperature for reaction with isoprene are of the order of 10 , 10 and 10 cm molecule" s , respectively. In the case of attack by OH, and in the presence of oxygen, a peroxy-hydroxy radical is formed up to six isomers may be produced. Product studies indicate that the initial addition is to one of the terminal carbon atoms. The peroxy-hydroxy radical may then either react with NO or with other RO2 radicals (including HO2) to form a variety of products, as indicated in Fig. 11. Under conditions where the peroxy-hydroxy radical reacts exclusively with NO, approximately 50 % of the carbon balance is accounted for by three main products methacrolein, methyl vinyl ketone and formaldehyde. Other carbonyl products and hydroxy-nitrates are thought to make up the carbon balance although there is presently no clear indication of the exact identity of these compounds or their yields. 

Skov, H Th. Benter, R. N. Schindler, J. Hjorth, and G. Restelli, Epoxide Formation in the Reactions of the Nitrate Radical with 2,3-Dimethyl-2-butene, cis- and trans-2-Butene, and Isoprene, Atmos. Environ., 28, 1583-1592 (1994). 

In addition to being oxidized by the hydroxyl radical, alkenes may react with the N03 radical as has been described by several investigators (52, 56, 66). Listed in Table I are some of the organic nitrates that have been predicted to be produced via reaction of OH and N03 with isoprene and pro-pene. Analogous compounds would be expected from other simple alkenes and from terpenes such as a- and (3-pinene. Other possible organic nitrates may be produced via the oxidation of aromatic compounds (53, 54) and the oxidation of carbonaceous aerosols (67). Quantitative determination of these species has not been made in the ambient atmosphere. 

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... 

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. 

It is of particular relevance to atmospheric photochemistry that isoprene, a main naturally emitted hydrocarbon, was found to form bifunctional organic nitrates as the predominant products of its reaction with the NO3 radical in air. 

H. Skov, Th. Benter, R.N. Schindler, J. Hjorth, G. Restelli Epoxide formation in the reactions of the nitrate radical with 2,3-dimethyl-2-butene, cis- and rran5-2-butene and isoprene, Atmoj. Environ. 28A (1994) 1583. 

Epoxide formation in the reactions of nitrate radical with 2,3-2-dimethyl-2-butene, cis- and trans-2-butQn and isoprene,... 

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 ... 

For NO3 + dialkenes (butadiene), the identified products were CO (4%), HCHO (12%) acrolein CH =CH-CHO (12%), total nitrates (60%). For the NO3 + isoprene reaction, yields of products were CO (4%), HCHO (11%), methacrolein CH3=CH(CH3)-CHO (uncertain yield), and total nitrates (80%). The aldehydes formed in the reactions of NO3 with the dialkenes can be explained by the thermal decomposition of the related nitrooxy-alkoxy radicals as in reaction (Equation 4.85) for monoalkenes. The formation of small quantities of CO in both the NO3 + dialkenes reaction systems is difficult to explain. It is unclear whether the CO is formed directly in the reaction of NO3 with dialkenes or whether it is a product of secondary reactions of NO3 with acrolein or methacrolein. 

---