Exploring the influence of particle phase in the ozonolysis of oleic and elaidic acid

Aerosol particles in the atmosphere undergo heterogeneous transformations due to interactions with various gas-phase oxidants such as ozone. While it is known that these reactions are significantly affected by the phase state of the particle, a direct comparison between the reaction kinetics and product distributions for the same reactive process occurring in the different phases remains elusive. This study uses single particle levitation and flow-tube methods to measure and compare the ozonolysis of particles containing oleic acid and its trans isomer elaidic acid in liquid, supercooled liquid, and solid states. We measure their evolving size, optical properties, phase, and chemical composition during reaction. Both primary reactions and secondary chemistry were explored, along with the influence of particle phase state on reaction kinetics and product formation. Notably, we directly compare the reaction kinetics of supercooled liquid elaidic acid particles with liquid oleic acid particles, revealing similar uptake coefficients indicative of similar inherent reactivity of the C = C moiety. We go on to compare the kinetics of solid elaidic acid particles, formed due to solidification at room temperature and freezing at low temperature. We found a significant slowing in reaction kinetics that may be attributed to the phase of the particle and the influence of slowed molecular diffusion, supported by qualitative agreement with a multilayer kinetic model (KM-SUB). We further explore differences in the product distributions between particles exhibiting different phase states. These results provide important insights into how the chemical aging of ambient aerosol particles may be influenced by their physicochemical characteristics.