Alkane autoxidation and aerosol formation: new insights from combustion engines to the atmosphere

Abstract

<p>Autoxidation is a process whereby organic compounds become oxidized by molecular oxygen (O<sub>2</sub>). It is ubiquitous in various reaction systems, contributing to the spoilage of food and wine, ignition in internal combustion engines, and formation of atmospheric secondary organic aerosol (SOA) from volatile emissions. Autoxidation thus greatly influences both engine operation and efficiency, and, via SOA, climate and air quality. Recent progress in atmospheric chemistry has identified double bonds and oxygen-containing moieties as structural facilitators for efficient autoxidation, and subsequent OA formation. Lacking either of these functionalities, alkanes, the primary molecular class in fuels for combustion engines and an important class of urban trace gases, have been expected to have low susceptibility to undergo autoxidation. In this work, we show that alkanes can indeed undergo efficient autoxidation both under combustion-relevant and atmospheric temperatures, consequently producing more highly oxygenated species than previously expected. By bridging methodologies and knowledge from both combustion and atmospheric chemistry, we mapped the autoxidation potential of a range of alkane structures under various conditions, from the combustion domain to the atmospheric domain. We identified the importance of isomerization steps driven by both peroxy and alkoxy radicals, and show that isomerization and production of low-volatile condensable vapors is efficient even under highly polluted ([NO]>10ppb) conditions. Our findings, currently under review, provide insights into the underlying chemical mechanisms causing highly variable SOA yields from alkanes, which were observed in previous atmospheric studies. The results of this inter-disciplinary effort provide crucial new information on oxidation processes in both combustion engines and the atmosphere, with direct implications for engine efficiency and urban air quality.</p>