Changes in Aerosol/Gas-Phase Distribution Ratio of Semi-Volatile Products Affect Secondary Organic Aerosol Formation with NOx from α-Pinene Photooxidation

Abstract. Atmospheric 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, we still do not comprehensively understand the effects of NO_x on α-pinene derived SOA formation. Therefore, we conducted α-pinene photooxidation experiments in an atmospheric chamber at different NO_x concentrations. The yields of α-pinene SOA increased with NO_x concentration under low-NO_x conditions, but were suppressed under high-NOx conditions. The maximum SOA yields were 8.0 % and 26.2 % in the low- and high-volatility organic compound (VOC) experiments, respectively. We found the increased SOA yields under low-NOx conditions were related to increased consumption of α-pinene. The products of α-pinene photooxidation were mainly semi-volatile, and the change in the aerosol/gas-phase distribution ratio as the formation of α-pinene photooxidation products increased was identified as the main reason for the enhanced SOA yields with increasing NOx. The sensitivity of the SOA yield to changes in NOx and VOCs under different experimental conditions was also analyzed. This study also quantified the nitrogen-containing organic compound (NOC) concentrations. The mass fraction of NOCs in SOA increased monotonically with NOx in the α-pinene photooxidation process, and the maximum NOC mass fraction made up as much as two-fifths of the α-pinene SOA.