Molecular rearrangement of bicyclic peroxy radicals: key route to aerosol from toluene

Abstract

Aromatic compounds contribute significantly to the formation of secondary organic aerosol (SOA) that have implications on health and on climate. Toluene is the most abundant aromatic compound in the atmosphere and is emitted through anthropogenic activities such as incomplete combustion and industrial processes. To form SOA, participating vapors such as toluene need to have sufficiently low volatility, which in practice implies molecules with multiple oxygen containing polar functional groups called highly oxygenated organic molecules (HOMs). The oxidation of toluene by OH can lead first to bicyclic peroxy radicals (BPR), and then to HOM. While the formation of HOM has been shown to be efficient, the underlying molecular level mechanism has been challenging to accurately elucidate because of the sheer number of potential pathways. This leads to a major gap in the understanding of the formation of SOA from toluene in the atmosphere. In this work, we conclusively demonstrate for the first time that the toluene derived BPR is unstable under atmospheric conditions and undergoes spontaneous molecular rearrangement to lead to completely ring broken RO2s at rates competitive with bimolecular reactions with NO even under polluted conditions. Intriguingly, several of the closed shell products deriving from the BPR are likewise unstable and decompose in finite timescales. This is relevant for those aromatic compounds that lead to BPRs that are otherwise stable against rearrangement reactions, providing, e.g., a long-range transport vehicle for NOx similar to peroxyacyl nitrates (PAN). Using quantum chemical calculations and master equation simulations that account for energy non-accommodation, we elucidate the molecular level mechanism of the subsequent autoxidation of the ring broken RO2s that leads rapidly to the HOM O9-RO2. Furthermore, using targeted flow reactor experiments of the OH reaction of toluene and partially deuterated toluene with nitrate chemical ionization mass spectrometry (CIMS) detection, we corroborate the proposed new mechanism. This is the only unambiguous reaction pathway to toluene derived HOM reported to date, and it has implications on the aerosol and ozone forming potential of toluene.