PNNL

Abstract

Secondary organic aerosol (SOA) plays a crucial role in influencing 20 both climate and air 21 quality, and therefore characterizing SOA formation and evolution pathways has been a major 22 research goal over the last decades. To reach this goal, especially relating to reactions occurring in 23 the particle phase, real-time comprehensive molecular-level characterization of SOA composition is 24 required, yet rarely achieved. Here, utilizing state-of-the-art high-resolution mass spectrometry 25 techniques, we deduce important new reaction pathways for the formation and subsequent 26 transformation of SOA in the boreal atmosphere. Our detailed molecular characterization of particle27 phase organics helped identify various compound classes from the oxidation of monoterpenes, 28 including previously reported highly oxygenated molecules (HOMs). Our results suggest that many 29 compounds, and HOMs in particular, undergo further reactions in the particle phase, accelerated by 30 acidic aerosols. Coincident formation of products with higher degree of unsaturation and lower O:C 31 ratios was observed and attributed to high-molecular weight oligomers. Targeted smog chamber 32 experiments confirmed the ambient observations and revealed that monoterpene oxidation products, 33 such as aldehydes, undergo acid-catalyzed heterogeneous reactions yielding oligomers. While the 34 aerosol acidity strongly affected the chemical composition and properties of a-pinene–derived SOA, 35 aerosol liquid water alone had nearly negligible impact. Our findings provide a comprehensive picture 36 of particle-phase processing governing monoterpene-derived SOA composition and highlight the 37 underestimated role of aerosol acidity in SOA evolution and oligomer formation. 38