Automating the search for new reaction pathways in atmospheric oxidation chemistry

Abstract:

Reactive organic carbon is emitted into the troposphere from a variety of biogenic and anthropogenic sources. Most of these species undergo radical initiated oxidation in a highly complex, multigenerational process involving a variety of organic radical intermediates. These reactions are critical in the secondary production of key atmospheric species including secondary organic aerosol (SOA). Understanding the routes by which volatile organic compounds are oxidized represents a key challenge in atmospheric chemistry. In recent years, several "exotic" organic radical transformations not typically found in standard atmospheric oxidation mechanisms (e.g. Peroxy radical isomerization, epoxide formation) have been shown to be important in the formation of low-volatility organic compounds and SOA. These discoveries suggest that there may be other non-traditional mechanisms that play a role in oxidative aging and aerosol formation, and point to the need for a systematic way to look for such pathways. The Reaction Mechanism Generator (RMG) is an open-source program developed for and typically applied to combustion systems. RMG uses a combination of an extensive database of known chemical information and a functional group based methodology for kinetic and thermodynamic parameter estimation to automatically develop reaction networks with very little a priori information. Here, we use RMG to systematize and automate the search for new mechanisms in atmospheric organic oxidation chemistry. RMG is used to generate reaction networks for the OH-intiated oxidation of a set of approximately 200 monofunctionally and bifunctionally substituted pentanes in the gas phase at atmospheric temperatures and pressures. In addition to the canonical reactions expected for organic radicals in the atmosphere, a number of "exotic" reactions are identified. These include the recently explored but historically underappreciated reaction mechanisms described above, as well as examples of novel radical chemistry which may impact SOA formation. The results demonstrate the utility of RMG as a tool for atmospheric reaction discovery, and provide several interesting targets for further study via quantum mechanical calculations and experiments.