PNNL

Abstract

We investigate the relative yields of aqueous secondary organic aerosol (aqSOA) components between photochemical and dark aging and their relationship to water clusters at the air–liquid interface. Our findings show dark aging is a source of aqSOA despite lack of photochemical drivers. Photochemical reactions of glyoxal and hydroxyl radical produce oligomers and cluster ions. Interestingly, different oligomers and cluster ions form intensely in the dark at the interface, contrary to the notion that oligomers are produced only with light irradiation. Furthermore, cluster ions form readily during dark aging, which have a higher water molecule adsorption ability as reflected by more organic water cluster ion formation. The relative yields of water clusters in the form of pronated and hydroxide ions are investigated using the Van Krevelen diagram to explore the underlying formation mechanisms of aqSOA. Large pronated and hydroxide water clusters have reasonably yields during UV aging. In contrast, small pronated and hydroxide water clusters (1=n=17) form after several hours of dark aging. The molecular dynamics simulation shows that cluster ions form stably in UV and dark aging, consistent with in situ observations in this work. Additionally, cluster ions are more sensitive to UV irradiation than protonated water clusters.