The nitrate radical, NO3, is a fascinating species. As a result of its rapid photolysis, it is typically only present in very small amounts during the daytime, but can build up to significant concentrations at night. NO3 is known to react with alkenes by addition to the double bond, and to abstract weakly-bound hydrogen atoms, such as those in aldehydes. The rate constants for NO3 addition to alkenes increase with the size of the organic molecule, and can become quite rapid with large, biogenic alkenes such as terpenes. Recent field studies in forested areas have suggested that the products of the NO3-terpene reactions can lead to secondary aerosol formation, and act as a loss for NOx in these regions. However, the mechanisms of these reactions have not been thoroughly studied. Following addition of NO3 to the alkene, a series of reactions can lead to the production of alkoxy radicals. The fate of these radicals determines whether the NOx is released back to the atmosphere, or remains in the organic products, which can enhance the rate of secondary aerosol production.
Experiments have been performed to study the mechanisms of reactions of the NO3 radical with a series of linear and branched alkenes in collaboration with Freja Osterstrom (Copenhagen Center for Atmospheric Research, University of Copenhagen, Denmark). These were selected as representative molecules to test our understanding of alkoxy radical behavior, and as a stepping stone to predicting the oxidation products of larger molecules such as terpenes. As an example, some results are shown for trans-2-butene (T2B) in the figure. N2O5 was produced in the ACOM environmental simulation chamber, then the T2B was added. Loss of reactants and formation of products was followed by in-situ FTIR spectroscopy. After 19 minutes, NO was added to the system to scavenge any remaining peroxy radicals. On addition of NO, there is a large increase in the concentration of acetaldehyde, formed from the decomposition of 3-nitrooxy-2-butoxy radicals, accompanied by a smaller increase in the formation of nitrate, from the O2 reaction of the alkoxy radical. The decomposition path is accompanied by release of NO2 from the radical.
CH3CH(ONO2)-CH(O)CH3 → CH3CH(ONO2) + CH3CHO → 2 CH3CHO + NO2
CH3CH(ONO2)-CH(O)CH3 + O2 → CH3CH(ONO2)-C(=O)CH3 + HO2
Comparison of the products from a number of alkenes with predictions from two currently used chemical mechanisms show that the mechanisms generally capture the behavior observed, but the rate constants differ by an order of magnitude. After a full analysis of the data is complete, improvements to the structure-activity relationships underlying the chemical mechanisms will be possible.