Changes in gas-to-aerosol-phase partitioning ratio of semi-volatile products affect secondary organic aerosol formation from α-pinene photooxidation

α-Pinene is one of the most important precursors of secondary organic aerosols (SOA). The formation of α-pinene derived SOA is strongly affected by NOx. However, the effects of NOx on α-pinene derived SOA formation, especially the enhancing effect of NOx on SOA yield, are still not comprehensively understood. A series of α-pinene photooxidation experiments were performed at different NOx concentrations through an atmospheric chamber in this study. The yields of α-pinene SOA initially increased with rising NOx concentrations but subsequently decreased at higher levels. The maximum SOA yields were 8.0 % and 26.2 % in 115 ppb and 250 ppb α-pinene experiments, respectively. It is found that the fitted curves of SOA mass concentration (M₀) versus SOA yield shift downward with increasing NOx, which means the volatility of the oxidation products gradually increases. However, the higher SOA yields observed with the increasing M₀ during each photooxidation process, which were attributed to the enhanced gas-to-aerosol-phase partitioning ratio. The relationship of SOA yields with M₀ for different NOx experiments shows that, under low-NOx conditions, the elevation in M₀ which was driven by enhanced VOC consumption would still promote SOA yield with increasing NOx concentrations, despite the position of the Odum curve shift downward. That is to say, the change of M₀ leading to the variation gas-to-aerosol-phase partitioning ratio should be taken into account in the facilitation of NOx on SOA yield. The relation of nitrogen-containing organic compound (NOCs) concentrations with NOx was also quantified in this study. The rapid increase in NOCs formation under low NOx conditions is another factor contributing to the increase of SOA yields. This study greatly enhances our understanding of the mechanisms by which NOx promotes SOA yields, and provides crucial information for improving the accurate simulation of SOA formation.