Distinct Roles of NO ₂ versus NO and Synergisms with SO ₂ in Secondary Organic Aerosol Formation from β-Myrcene Photooxidation

Elucidating the impacts of gaseous pollutants on secondary organic aerosol (SOA) formation mechanisms is critical for developing effective PM2.5 mitigation strategies. However, the combined effects of NOx (NOx = NO₂ + NO) and SO₂ on SOA compositions under complex photochemical conditions remain mechanistically unresolved. Here, we investigated the individual roles of NO₂/NO and their combined effects with SO₂ on β-myrcene photooxidation. Our results showed that NO₂ enhanced the SOA yield and the O:C ratio through oxidant amplification, whereas NO suppressed nucleation and oxidation states of products but facilitated particle size growth via organic nitrate partitioning. SO₂ exhibited significant synergistic effects with both NO₂ and NO, facilitating particle formation and growth processes. The newly built vacuum ultraviolet free electron laser photoionization aerosol mass spectrometer enabled the observation of a series of new compounds (i.e., organic peroxides, organic nitrate, and organosulfates) and identified new mechanisms of SOA formation. Notably, a novel compound with a molecular weight of 287 was characterized to be a nitrogen- and sulfur-containing species. These findings highlight the critical roles of pollutants in particle formation in environmental processes and provide key scientific support for regional air quality management and climate change mitigation.