Multi-generational oxidation mechanism of M-xylene: Unexpected implications for secondary organic aerosol formation

Secondary organic aerosol (SOA) affects air quality, human health and climate. The multi-generation reactions of volatile organic compounds (VOCs) have become key process for explaining the SOA formation. M-xylene, as one of the most important VOCs in urban atmosphere, has got extensive study. However, the reaction mechanism of the first-generation products hydroperoxides (M-ROOH) and organonitrates (M-RONO2) remain unknown. In this work, the hydroxyl radicals (·OH) initiated reactions of M-ROOH and M-RONO2 were investigated by employing quantum chemical methods and kinetics modeling. The results show that new ring-retaining epoxides, a range of dialdehydes and less volatile hydroperoxides and organonitrates are formed, contributing to SOA formation. By adopting the unveiled new mechanism into the SOSAA box modeling, we found that the yield of SOA from m-xylene oxidation is up to 38%, which is comparable to the experimental values (36%). This work advances current understanding of multi-generational chemistry and elucidates the role of aromatic hydrocarbons oxidation in the formation of SOA.