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, C7H9O6-8NO2, 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.