Abstract View
Accelerated Chemical Reaction in Ultrafine Droplets: Effect of Droplet Size on Product Formation
YAO ZHANG, Murray Johnston, University of Delaware
Abstract Number: 549
Working Group: Aerosol Chemistry
Abstract
Reaction acceleration in ultrafine (<100 nm diameter) droplets is reported for the first time using droplet assisted ionization (DAI) to monitor product formation. Previous work has shown that DAI is an effective ionization technique to study the kinetics and mechanisms of reactions in droplets since it is able to decouple processes intrinsic to the droplet environment from those unique to the mass spectrometer ion source. The reaction of Girard’s T (GT) reagent with carbonyls, which are abundant functional groups in secondary organic aerosol, is used as a model system. Reaction mechanisms are elucidated based on the droplet size dependence of product formation. Three types of experiments were performed: 1) size selected, dry GT particles reacting with pinonaldehyde vapor; 2) size selected droplets containing GT reacting with pinonaldehyde vapor; 3) Size selected droplets containing both GT and SOA from α-pinene ozonolysis. In the first experiment, the reaction occurs only at the air-particle interface. In the second and third experiments, the reaction occurs at both the interface and in the droplet volume, and the relative importance of the two changes with droplet size. For small droplets, the reaction occurs primarily at the interface. In this case, rapid product formation at the interface, followed by diffusion of the products into the droplet volume allows product formation to build up beyond what is possible by reaction in the droplet volume alone. The reaction occurs on the surface. Products formed at the interface can diffuse to the interior. This process continues until a droplet size-dependent equilibrium state is reached. When the droplet size increases, product formation eventually becomes independent of droplet size and is determined just by reaction in the volume. The accelerated reaction occurs because the free energy of binding of the reactants to the interface compensates for the increase in free energy of droplets compared to the bulk liquid. The mechanism of reaction acceleration was found to be similar no matter it is of a reaction between gas-phase and droplets (experiment 2) or a reaction in droplets (experiment 3). This work gives insight into how accelerated chemistry in droplets might influence the growth of atmospheric aerosol.