Non-additive Secondary Organic Aerosol Formation Yields from Mixed Biogenic and Anthropogenic Precursors

The mixing of precursors significantly alters secondary organic aerosol (SOA) yield and composition. This study systematically investigates SOA formation from the photooxidation of two polycyclic aromatic hydrocarbons (naphthalene, 2-methylnaphthalene) and two terpenes (isoprene, alpha-pinene), representing anthropogenic and biogenic precursors, as well as their binary mixtures under both low and high NOx conditions in both smog chamber and flow tube reactor. SOA composition is analyzed using a Filter Inlet for Gases and Aerosols coupled to a high-resolution time-of-flight chemical ionization mass spectrometer (FIGAERO-CIMS). Results show that the SOA yield of naphthalene and 2-methylnaphthalene under high NOx is lower than under low NOx, consistent with previous studies. Suppression of SOA formation is observed in mixed precursor systems. This may result from differences in particle volatility between individual and mixed precursor systems, indicating distinct oxidation processes. Additionally, under NOx-free conditions, SOA yields from mixed precursors (e.g., isoprene/naphthalene and α-pinene/naphthalene) are not additive but exhibit a nonlinear dependence on the reactivity ratio—defined as the product of the OH rate constant and consumed concentration of each precursor. A synergistic enhancement of up to 60% is observed at optimal reactivity ratios. Molecular-level analysis reveals unique oxidation products in mixed systems, suggesting novel reaction pathways. The enhanced yield is attributed to an increased condensation sink and potential heterogeneous reactions. A parameterized formula linking yield to reactivity ratio is proposed, which could improve SOA model accuracy. These findings highlight the importance of precursor interactions, quantified via reactivity ratio, for accurately predicting aerosol loading, especially in clean atmospheres. This study provides new insights and a framework for understanding SOA formation from mixed anthropogenic and biogenic precursor systems.