SOA particles rank among the least understood atmospheric constituents in the climate system. Key questions concern (1) their size and hence their growth and (2) the relative roles of particle size vs. chemical composition, including the role of surface-localized species, in influencing CCN activity, all of which remain incompletely understood and controversial. These questions are important for making advances in their poorly quantified roles in cloud activation and radiative forcing. Motivated by the notion that the particle surface is the first entity encountered by an approaching gas phase species, we combine vibrational sum frequency generation, organic synthesis, and atmospheric chemistry to show that surface chemistry can help address these questions. As one illustrative example from our recent work, SOA particle growth and CCN activity were shown to involve molecules at the interface between the gas phase and the particle phase. Specifically,
-We reported interfacial tension measurements indicating that trans- -isoprene epoxydiol (IEPOX) may significantly enhance CCN activity of ambient atmospheric particles when compared to other atmospherically relevant epoxydiols and tetraols. Yet, with a few exceptions, chemically specific surface tension effects relevant for CCN activity remain largely uncharacterized. Work to be discussed provides estimates for the depression of supersaturation ratios required for CCN activity by surface-active species.
-We spectroscopically identified trans- -IEPOX at the surface of synthetic isoprene-derived SOA particles, chosen as an atmospheric surrogate to provide mechanistic insight for particle growth. We then quantified that trans- -IEPOX contributes to half of SOA particle growth. Yet, SOA particle growth models and proposed terpene oxidation schemes lack explicit surface reaction mechanisms. The absence of these schemes represents both an intellectual and a practical gap in mechanistic and quantitative descriptions, especially in light of emerging evidence that under some conditions SOA particles are in a low-viscosity rather than liquid state. Work discussed provides mechanistic insight into several relevant gas-to-particle conversion pathways that lead to SOA particle growth.