Secondary aerosol formation and oxidation capacity in photooxidation in the presence of Al2O3 seed particles and SO2

To investigate the sensitivity of secondary aerosol formation and oxidation capacity to NO x in homogeneous and heterogeneous reactions, a series of irradiated toluene/NO x /air and α-pinene/NO x /air experiments were conducted in smog chambers in the absence or presence of Al 2 O 3 seed particles. Various concentrations of NO x and volatile organic compounds (VOCs) were designed to simulate secondary aerosol formation under different scenarios for NO x . Under “VOC-limited” conditions, the increasing NO x concentration suppressed secondary aerosol formation, while the increasing toluene concentration not only contributed to the increase in secondary aerosol formation, but also led to the elevated oxidation degree for the organic aerosol. Sulfate formation was suppressed with the increasing NO x due to a decreased oxidation capacity of the photooxidation system. Secondary organic aerosol (SOA) formation also decreased with the presence of high concentration of NO x , because organoperoxy radicals (RO 2 ) react with NO x instead of with peroxy radicals (RO 2 or HO 2 ), resulting in the formation of volatile organic products. The increasing concentration of NO x enhanced the formation of sulfate, nitrate and SOA under “NO x -limited” conditions, in which the heterogeneous reactions played an important role. In the presence of Al 2 O 3 seed particles, a synergetic promoting effect of mineral dust and NO x on secondary aerosol formation in heterogeneous reactions was observed in the photooxidation. This synergetic effect strengthened the positive relationship between NO x and secondary aerosol formation under “NO x -limited” conditions but weakened or even overturned the negative relationship between NO x and secondary aerosol formation under “VOC-limited” conditions. Sensitivity of secondary aerosol formation to NO x seemed different in homogeneous and heterogeneous reactions, and should be both taken into account in the sensitivity study. The sensitivity of secondary aerosol formation to NO x was further investigated under “winter-like” and NH 3 -rich conditions. No obvious difference for the sensitivity of secondary aerosol formation except nitrate to NO x was observed.