NH<sub>3</sub> Weakens the Enhancing Effect of SO<sub>2</sub> on Biogenic Secondary Organic Aerosol Formation
Lin Du, Li Xu, Kun Li, C. George, Maofa Ge
Abstract
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 SO 2, NH 3, and O 3 and biogenic β-caryophyllene was investigated. It is shown that although SO 2 considerably enhanced SOA formation, this enhancing effect was weakened by NH 3 when SO 2 and NH 3 coexisted. NH 3 -induced neutralization of particle acidity generated by SO 2 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 NH 3 -induced changes in SOA composition and aerosol acidity. Specifically, the lower relative abundances of several main products generated in the presence of SO 2 and NH 3 than those formed in the presence of only SO 2 were consistent with their suppressed formation by lower seed acidity. The suppression of oligomer formation by NH 3 provided more evidence for the weakening of acid-catalyzed processes caused by acidity neutralization. Accordingly, the current study demonstrates that NH 3 as a less effectively regulated alkaline gas resulting from an unbalanced reduction of different pollutants must be considered with caution when evaluating effects of SO 2 on SOA formation via anthropogenic–biogenic interactions.