A new look at the Effect of NOx on Biogenic SOA Yields

Secondary organic aerosol is a large fraction of the total aerosol mass and is thought to be responsible for growth of many particles from the Aitken to accumulation mode. However, gaps still exist in the scientific understanding of the basic chemical and physical properties of SOA, its formation mechanisms, volatility, and atmospheric lifecycle. To address these knowledge gaps, we recently conducted a collaborative study (SOAFFEE) involving seven institutions, hosted at PNNL in a continuous-flow environmental chamber, in which oxidation of several biogenic hydrocarbons and their oxidation products were investigated as a function of seed particle concentration, NOx concentration, oxidant identity, relative humidity, and aging time. We will discuss results from this study, focusing on a subset of these experiments investigating the role of precursor VOC, oxidant, and NO concentration on the yield and chemical composition of SOA from biogenic VOCs. When investigating α-pinene and isoprene photooxidation, we found that addition of progressively larger amounts of NO to the system had a complex, but relatively muted effect on SOA yield. When α-pinene was oxidized by ozone, addition of NO to the system also had a negligible effect on SOA yield, but substantially changed the condensed phase chemical composition, generating a significant mass of organic nitrates. Photooxidation of Δ-carene generated SOA in higher yield than α-pinene under similar conditions, despite their similar molecular structure. A general anti-correlation was observed between SOA yield and SOA volatility with more volatile SOA systems displaying lower yield. These results indicate that the role of monoterpene structure and NOx in determining SOA yield is more complex than typically represented in models.