A molecular origin for cloud activation. The latest IPCC Report states that, “aerosols and their interactions with clouds … continue to contribute the largest uncertainty to the total RF (radiative forcing, ed.) estimate”. This situation has not changed when compared to prior IPCC reports. In fact, despite their prominent role in the climate system, the level of scientific understanding regarding aerosol particles has now been rated “very low” for more than a decade. Our research contributes to improving this situation by specifically:

Connecting surface chemistry to particle growth, cloud condensation nuclei (CCN) activity, and cloud activation under conditions of varying, possibly increasing, anthropogenic (sulfate) influence;

Making synthesized compounds available to the scientific community through new collaborations such as those already in place; and

Providing cross-disciplinary training in organic chemistry, laser spectroscopy, and aerosol technology, with a focus on atmospheric science.

Sulfur-containing organic species reported in laboratory- and field-derived SOA have direct relevance to regional outflows from industrial zones in the southeastern United States or growing cities in the tropics into heretofore pristine forest regions, as well as regional and long-range (intercontinental) sulfur influx due to biomass burning, be it in the tropics or the Northern Hemisphere. The mixing of (largely) inorganic sulfur (“sulfate”) from natural ecosystem or anthropogenic sources with terpenes emitted from the large forest ecosystems of our world produces organosulfates (i.e., an R-O-SO₃⁻motif) that we propose should exhibit modified (i.e., enhanced or diminished) surfactant-like properties when compared to the sulfur-free terpene oxidation products that we have studied thus far. We pose this hypothesis because many sulfate esters are detergents. While short, polyhydroxylated sulfate esters, which can be formed from the oxidation products of isoprene, C₅, are unlikely to be highly surface active, C₁₀and C₁₅species may be as surface-active as sodium dodecyl sulfate or other surfactants, which we are testing in this project.

Our work contributes to knowledge about SOA particles, which rank among the least understood atmospheric constituents in the climate system. Relevant for chemistry is the key fact that the particle surface is the first entity encountered by an approaching gas phase species. The important follow-on question is how surface chemistry regulates (1) particle size and growth and (2) the balance between the roles of particle size vs. chemical composition, in influencing CCN activity, all of which remain incompletely understood and debated. Addressing these questions is important for making advances in understanding the poorly quantified role of SOA in cloud activation and radiative forcing, especially under conditions of varying anthropogenic activity. Given this context, then, our studies provide synthesis and measurement approaches for obtaining new physical insights into how surface chemistry drives atmospheric particle growth and cloud activation:

Our synthesis efforts enable (1) the concrete identification of molecular species associated with SOA particles and their surfaces and (2) detailed mechanistic studies of particle growth and the dynamics of the surface processes, while our measurement efforts provide leads for which surface- vs. bulk-localized species most influence particle growth and cloud activation potentials.

Selected Publications: