Experimental Insights on the NO-Dependent Yields of Aromatic Hydrocarbon Oxidation Products

Aromatic hydrocarbons play a critical role in the formation of urban ozone and secondary organic aerosols, impacting air pollution, climate change, and human health. However, their oxidation mechanisms remain uncertain, particularly under low nitric oxide (NO) conditions (<5 ppb). Using the SAPHIR chamber, this study investigated the yields of key oxidation products and their NO dependence under initial NO concentrations varying from 150 ppt to 20 ppb. For toluene (m-xylene) oxidation, the low-NO experimental yields of benzaldehyde, phenolic products, and products from bicyclic ring-opening, bicyclic ring-retaining, and H-shift reactions of organic peroxy radicals (RO₂) were 8% (5%), 21% (9%), 45% (53%), 6% (8%), and 6% (7%), respectively. The yields from the bicyclic pathway showed the strongest NO sensitivity. A comparison between the MCMv3.3.1 mechanism-simulated yields and experimental results revealed notable discrepancies, with model─measurement differences of +9% and +25% for the bicyclic ring-opening and ring-retaining products in toluene oxidation and +7%, -16%, and +14% for the phenolic, bicyclic ring-opening, and ring-retaining products in m-xylene oxidation. Notably, products from H-shift RO₂ reactions, not included in the model, showed yields comparable to bicyclic closed-ring products, contributing 5-8% to carbon closure in toluene and 6-8% in m-xylene oxidation.