Effects of NOₓ on secondary organic aerosol yields and composition from a biogenic–anthropogenic mixture

Abstract. Nitrogen oxides (NO_x = NO₂ + NO) play a crucial role in secondary organic aerosol (SOA) formation. The effects of NO_x on SOA formation from single precursors have been extensively studied. However, in the real atmosphere, biogenic and anthropogenic precursors often coexist, and it remains unclear whether the effects of NO_x in such mixtures can be directly extrapolated from those observed in single-precursor systems. In this study, we investigated the effects of NO_x on SOA particle mass yields and chemical composition from α-pinene, n-dodecane, and their mixture under high- and moderate-NO_x conditions in the Manchester Aerosol Chamber. The results show that SOA particle mass yields were higher under high-NO_x conditions across all systems. Enhanced oxidant levels and alkoxy radical (RO) isomerisation appear to more than compensate for the negative impacts associated with the formation of more volatile products via the reaction of organic peroxy radicals (RO₂) with NO. However, compared with the single-precursor systems, the increase in SOA particle mass yields in the mixed-precursor system was less pronounced. In the mixed-precursor system, enhancement of RO₂ + NO termination pathways was stronger, and the contribution of α-pinene-derived alkoxy-peroxy pathway may have been comparatively weaker. These changes would be expected to favour the formation of more volatile products and thus suppress SOA formation. Collectively, these observations provide evidence that the effects of NO_x in the mixed-precursor systems cannot be interpreted as a simple combination of behaviours observed in individual precursor systems.