The impact of CO on secondary organic aerosols formed from the mixture of α-pinene and n-dodecane

Abstract. Atmospheric simulation chambers are powerful tools for investigating atmospheric processes and form the basis for model parameterisations. Ensuring the atmospheric relevance of experimental conditions is crucial for understanding and predicting the impacts of secondary organic aerosols (SOA) on air quality and climate. However, chamber studies are often conducted under simplified conditions, which may limit their applicability to real-world scenarios. Here, we investigated the impact of CO on the mass yields and chemical composition of SOA particles formed from a biogenic volatile organic compound (VOC, α-pinene), an anthropogenic intermediate-volatility organic compound (IVOC, n-dodecane), and their mixture in the presence of nitrogen oxides (NO_x = NO₂ + NO) in the Manchester Aerosol Chamber (MAC). This photochemical system better represents polluted atmospheric conditions. The results show that the influence of CO differed between single- and mixed-precursor systems. In the single-precursor systems, CO significantly suppressed SOA particle mass yields, whereas no such suppression was observed in the mixture. Moreover, compared with the single-precursor systems, CO exerted a diminished impact on the organic peroxy (RO₂) radical reaction pathways in the mixture, with the extent of this change differing between α-pinene and n-dodecane. These findings demonstrate that variations in reaction conditions can lead to different responses in SOA particle properties between the single- and mixed-precursor systems, highlighting the importance of conducting laboratory experiments under atmospherically relevant conditions.