Nontargeted Tandem Mass Spectrometry Analysis Reveals Diversity and Variability in Aerosol Functional Groups across Multiple Sites, Seasons, and Times of Day

Organic aerosol (OA) is a complex mixture of compounds with diverse elemental and structural features, and its composition affects its health and environmental impacts. A detailed speciation of the functional group distribution in OA is important for constraining atmospheric reaction pathways and products, evaluating chemical mechanisms and models, and understanding OA impacts. We used high-resolution tandem mass spectrometry to perform a nontargeted analysis of OA functional groups from three diverse ambient sites across times of day and seasons. We observed a range of oxygen-, nitrogen-, and/or sulfur-containing functional groups, including oxygenates such as hydroxyls (29-69%) and carboxylic acids (19-59%), that dominated the functional group distribution and that may participate in hydrogen bonding and thus impact the chemical and physical properties of OA (percentages indicate average ion abundance contributions across campaigns). We also observed esters (7-39%) and ethers (13-42%) that suggest the importance of oligomerization. On average, organonitrates represented only 12% of identified nitrogen-containing groups and organosulfates represented 21% of identified sulfur-containing groups, while we observed many other nitrogen- and/or sulfur-containing structures that were important contributors to OA composition (e.g., amines, imines, nitrophenols, and sulfides). Most compounds (81%) were multifunctional and likely multigenerational oxidation products, which typically contained two to five functional groups in total.