The chemistry of organic peroxy radical (RO₂) is the core in the oxidation of organic compounds, because RO₂ is formed in the oxidation of essentially every organic compound. RO₂ is a class of highly reactive intermediate. Critically dependent on the environmental conditions, RO₂ can undergo different reactive pathways and generate distinct oxidation products, hence causing different environmental impacts.

I experimentally quantify the unimolecular reaction rates of RO₂, which is a poorly constrained reaction of RO₂. I find that the RO₂ produced from α-pinene and β-pinene photooxidation undergo fast unimolecular reactions (4 and 16 s^-1 for α-pinene and β-pinene ring-opened RO₂, respectively). This fast reaction rate makes the unimolecular reaction the dominant fate of these RO₂ in the atmosphere, even in urban environment, which contradicts with traditional understanding that the major fate of RO₂ is reaction with NO in urban area.

In collaboration with professor Kjaergaard, we performed ab initio calculations and obtained excellent agreement between calculated and measured rates. The agreement builds confidence in applying the multiconformer transition state theory to predict the unimolecular chemistry of RO₂ in other systems.

Related Publications:

• Xu, L.; Møller, K. H.; Crounse, J. D.; Otkjær, R. V.; Kjaergaard, H. G.; Wennberg, P. O. Unimolecular Reactions of Peroxy Radicals Formed in the Oxidation of Α-Pinene and Β-Pinene by Hydroxyl Radicals. The Journal of Physical Chemistry A 2019, 123, 1661-1674 (link).

Aromatic compounds are an important class of anthropogenic emissions. There are growing interests in understanding the influences of aromatic chemistry in urban air quality, chemical evolution of biomass burning, and new-particle formation events. However, the fundamental oxidation mechanism of aromatic compounds in the atmosphere remains obscure.

Our adventure begins with benzene, the simplest aromatic compound with not simple chemistry. We investigate the reaction kinetics and the products of key radicals in benzene oxidation by means of experimental studies and theoretical calculations. Key findings include:

• First, under atmospheric condition, following OH addition to benzene, 50% of carbon funnels into phenol and the rest 50% becomes bicyclic radical.

• Second, the nitrate branching ratio in benzene oxidation is very small (<1%). The implication is that as NO_x emissions are rapidly declining over the North America, the NOx lifetime and O3 formation will be greatly controlled by the formation of organic nitrates. So an accurate nitrate branching ratio is critical.

• Third, the RO2 and HO2 reaction largely produces OH, instead of organic peroxide. Whether the reaction recycles OH or produces peroxide directly affects the atmospheric oxidation capacity in future low-NOx scenario.

• Fourth, MCM mis-assigned 10% of carbon into a product that does not exist. The carbon mis-assigned by MCM is likely associated with the products from the newly identified reactions of the multifunctional alkoxy radical.

Related Publications

• Xu, L.; Møller, K. H.; Crounse, J. D.; Kjaergaard, H. G.; Wennberg, P. O. New Insights into the Radical Chemistry and Product Distribution in the Oh-Initiated Oxidation of Benzene. Environ Sci Technol 2020, 54, 13467-13477 (link). Video presentation.