NH₃ Weakens the Enhancing Effect of SO₂ on Biogenic Secondary Organic Aerosol Formation

Anthropogenic air pollutants can be involved in biogenic secondary organic aerosol (SOA) formation. However, such interactions are currently one of the least understood aspects of atmospheric chemistry. Herein, SOA formation via chemical interactions between anthropogenic SO2, NH3, and O3 and biogenic β-caryophyllene was investigated. It is shown that although SO2 considerably enhanced SOA formation, this enhancing effect was weakened by NH3 when SO2 and NH3 coexisted. NH3-induced neutralization of particle acidity generated by SO2 oxidation may be the primary driving factor of this weakening effect. Molecular-level characterization using high-resolution quadrupole time-of-flight mass spectrometry revealed additional connections between NH3-induced changes in SOA composition and aerosol acidity. Specifically, the lower relative abundances of several main products generated in the presence of SO2 and NH3 than those formed in the presence of only SO2 were consistent with their suppressed formation by lower seed acidity. The suppression of oligomer formation by NH3 provided more evidence for the weakening of acid-catalyzed processes caused by acidity neutralization. Accordingly, the current study demonstrates that NH3 as a less effectively regulated alkaline gas resulting from an unbalanced reduction of different pollutants must be considered with caution when evaluating effects of SO2 on SOA formation via anthropogenic–biogenic interactions.