The Combined Effect of NO _<i>x</i> and NH ₃ on Toluene Photooxidation and SOA Formation

Secondary organic aerosols (SOAs) significantly impact air quality and climate, yet their formation mechanisms under complex multipollutant conditions remain insufficiently characterized. This study systematically investigated the synergistic effects of both NOx and NH3 on toluene-derived SOA formation through comprehensive photooxidation chamber experiments. Chemical composition analysis was conducted using a high-resolution Orbitrap mass spectrometer (HR-MS). The experimental results demonstrated that increasing NOx concentrations from 30 to 90 ppb reduced toluene SOA yield from 10.1 to 9.5% primarily owing to the NOx-induced fragmentation of oxidation products as evidenced by elevated proportions of low-molecular-weight components (m/z < 100) in the SOA. The presence of NH3 enhanced the SOA yield, but the facilitative effect of NH3 on the SOA yield gradually weakened as the NOx concentration increased. HR-MS results revealed that NH3 promoted the formation of high-molecular-weight species (m/z > 200) and nitrogen-containing organics (NOCs). The facilitating effect of NH3 on NOC formation strongly correlated with the variation in SOA yield under different NOx conditions, suggesting that NOC formation is a principal mechanism through which NH3 promotes SOA formation. Moreover, four imidazole compounds (IMs) were identified in the presence of NH3, indicating that atmospheric toluene could serve as a precursor for these IMs. The observed nonlinear interactions between NOx and NH3 in SOA formation provide critical mechanistic insights for developing multipollutant control strategies, emphasizing the necessity for coordinated emission regulation of reactive nitrogen species to mitigate SOA pollution.