Atmospheric Chemistry Program Seminar
Tweezing Out the Interfacial Properties that Control the Chemistry of Atmospheric Aerosol Particles
Ryan Sullivan, Associate Professor
Department of Chemistry, Department of Mechanical Engineering, Carnegie Mellon University
Remote only: Please contact Anne.Handschy@colorado.edu for link
Atmospheric aerosol particles have important yet poorly understood effects on air quality, health, atmospheric chemistry, cloud microphysics, and climate change. All these impacts of aerosols are governed by their composition and chemical mixing state – how components are distributed amongst individual particles – and by their morphology or internal structure. Together these two properties determine what lies at each particle’s interface, and this in turn controls how the particle interacts with reactant gases, condensable vapors, radiation, water vapor, and clouds. We advanced the aerosol optical tweezers (AOT) technique to determine the morphology and chemical properties of phase-separated individual droplets that contain complex secondary organic aerosol (SOA). We can now perform AOT experiments on realistic mimics of mixed atmospheric particles to understand their interfacial properties and how these evolve. Our finding of the prevalence of a phase-separated core–shell morphology and the existence of a stable emulsified state of SOA have important new implications for the reactivity and impacts of atmospheric aerosol. Our recent high-accuracy measurements of droplet acidity have enabled novel explorations of the interplay between gas reactive uptake and the resulting changes in pH that can then drive phase separations and alter morphology, which in turn can impede further reactivity.
Biomass burning is a major global source of atmospheric pollutants and much research has focused on the carbonaceous emissions from wildfires. Biomass-burning aerosol is complex and highly heterogeneous, often containing appreciable amounts of inorganic solutes including chloride salts with unknown reactivity and implications for oxidant budgets and atmospheric chemistry. The natural production of a key nighttime reservoir of nitrogen oxides, N2O5(g), in biomass-burning smoke was identified through chamber experiments on authentic biomass-burning aerosol. Our discovery that N2O5(g) often reacts with the chloride salts in smoke aerosol from tall grasses to produce ClNO2(g) instead of HNO3(g) has important implications for the lifetime of nitrogen oxides and the impacts of biomass burning on atmospheric oxidants and photochemical smog production. The surprisingly low reaction probability of N2O5(g) was determined to be due to organic carbon coatings that protect the salt phases from the gaseous reactants but can be removed as the aerosol ages further. Salt deliquescence at high relative humidity can greatly increase the reaction probability as the hydrolysis of N2O5 is driven by chemistry in aqueous phases.