Differences in Secondary Organic Aerosol Formation from α-Pinene Photooxidation in a Chamber with Purified Air and Ambient Air as Matrices: Preliminary Results

α-Pinene is a biogenic volatile organic compound (BVOC) that significantly contributes to secondary organic aerosols (SOA) in the atmosphere due to its high emission rate, reactivity, and SOA yield. However, the SOA yield measured in chamber studies from α-pinene photooxidation is limited in a purified air matrix. Assessing SOA formation from α-pinene photooxidation in real urban ambient air based on studies conducted in purified air matrices may be subject to uncertainties. In this study, α-pinene photooxidation and SOA yield were investigated in a smog chamber in the presence of NO and SO2 under purified air and ambient air matrices. With the accumulation of ozone (O3) during the photooxidation, an increasing part of α-pinene was consumed by O3 and finally nearly half of the α-pinene was oxidized by O3, facilitating the production of highly oxidized organic molecules and thereby SOA formation. Although the ambient air we introduced as matrix air was largely clean, with initial organic aerosol mass concentrations of ~1.5 μg m−3, the α-pinene SOA yield in the ambient air matrix was 42.3 ± 5.3%, still higher than that of 32.4 ± 0.4% in the purified air matrix. The chemical characterization of SOA by the high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) revealed that CxHy accounted for 53.7 ± 1.1% of the total signal in the ambient air matrix experiments, higher than 48.1 ± 0.3% in the purified air, while CxHyO and CxHyO>1 together constituted 45.0 ± 0.9% in the ambient air matrix, lower than 50.1 ± 1.0% in the purified air. The O:C ratio in the ambient air matrix experiments was 0.41 ± 0.01, lower than 0.46 ± 0.01 in the purified air. The higher SOA yield of α-pinene in the ambient air matrix compared to that in the purified air matrix was partly due to the presence of initial aerosols in the ambient air, which facilitated the low volatile organic compounds produced from photochemical oxidation to enter the aerosol phase through gas-particle partitioning. The in-situ aerosol acidity calculated by the ISORROPIA-II model in the ambient air matrix experiments was approximately six times higher than that in purified air, and the higher SOA yield in the ambient air matrix experiments might also be attributed to acid-catalyzed SOA formation.