Mechanism in the synergistic effects of NH3 and NOx on secondary organic aerosol formation via α-pinene photooxidation

α-Pinene is a key precursor in the formation of secondary organic aerosols (SOA) in the atmosphere. SOA formation via atmospheric photooxidation of α-pinene under varying nitrogen oxides (NO x ) and ammonia (NH 3 ) concentrations was investigated in a laboratory smog chamber, with a focus on SOA mass concentration and chemical composition. The experimental results demonstrated that increasing NO x concentrations from 30 to 80 ppb reduced the α-pinene SOA yield from 15.5% to 9.0%. In contrast, the presence of NH 3 enhanced the SOA yield. The investigation revealed that the suppressive effect of NO x on SOA yield gradually weakened with increasing NH 3 concentration, whereas the promoting effect of lower NH 3 concentrations on SOA yield was more pronounced under lower NO x conditions. The chemical composition of SOA revealed that NH 3 promoted the formation of nitrogen-containing organic compounds (NOCs). Furthermore, NO x increased the abundance of nitrogen-containing functional groups within NOCs. A comparison of the volatile components of SOA indicated that the increase in volatile organic component (VOC) and intermediate-volatility organic component (IVOC) contents with increasing NO x concentration was responsible for the suppression of SOA formation. Furthermore, the enhancement of SOA yield by NH 3 was mainly attributed to the increase in IVOC and semi-volatile organic component (SVOC) contents. Overall, these results demonstrated that NH 3 and NO x exerted distinct effects on α-pinene-derived SOA formation. These findings provide valuable insights into the mechanism of SOA formation under complex pollution conditions involving the coexistence of NO x and NH 3 .