Highly oxidized products from the atmospheric reaction of hydroxyl radicals with toluene

Simple aromatics, largely toluene (C₇H₈) and benzene (C₆H₆), are emitted into Earth´s atmosphere by different sources. Their degradation initiated by the gas-phase reaction with hydroxyl radicals (OH) forms mainly bicyclic peroxy radicals (BCP) besides phenol-type compounds. The formation of highly oxidized products with at least 6 O atoms (HOMs) starting from BCP is currently controversial. Here, the experimental observation of HOMs and other products is reported for near-atmospheric conditions focusing on OH + toluene. Autoxidation processes starting from BCP generate C₇H₉O₇ and C₇H₉O₉ radicals with molar yields of ${0.25}_{-0.13}^{+0.25}\%$ and ${0.12}_{-0.06}^{+0.12}\%$ , respectively. Labeling experiments revealed that the ring H atoms are involved in autoxidation. The atmospherically relevant reaction of C₇H₉O₇ and C₇H₉O₉ with NO almost exclusively forms organic nitrates as HOM products, C₇H₉O_6-8NO₂, with a total molar yield ≤ 0.1%. Other HOMs, such as C₇H₈O₈ and C₇H₈O₁₀, represent only small trace products. Results from OH + benzene confirmed the findings from the toluene system pointing to a minor relevance of both aromatics for HOM generation.