Smog Chamber Study on the Role of NO x in SOA and O 3 Formation from Aromatic Hydrocarbons.

China is facing dual pressures to reduce both PM 2.5 and O 3 pollution, the crucial precursors of which are NO x and VOCs. In our study, the role of NO x in both secondary organic aerosol (SOA, the important constituent of PM 2.5 ) and O 3 formation was examined in our 30 m 3 indoor smog chamber. As revealed in the present study, the NO x level can obviously affect the OH concentration and volatility distribution of gas-phase oxidation products and thus O 3 and SOA formation. Reducing the NO x concentration to the NO x -sensitive regime can inhibit O 3 formation (by 42%), resulting in the reduction of oxidation capacity, which suppresses the SOA formation (by 45%) by inhibiting the formation of O- and N-containing gas-phase oxidation products with low volatility. The contribution of these oxidation products to the formation of SOA was also estimated, and the results could substantially support the trend of SOA yield with NO x at different VOC levels. The atmospheric implications of NO x in the coordinated control of PM 2.5 and O 3 are also discussed.