Heterogeneous Adsorption of Volatile Organic Compounds to Aerosol Particle Surfaces Probed with <i>In Situ</i> Surface Vibrational Sum Frequency Scattering

Vibrational sum frequency scattering (VSFS) has shown great utility in monitoring organics at sub-micrometer droplet surfaces, but the adsorbates were present upon generation. Herein, we present the first direct observation of heterogeneous gas-phase adsorption to droplet surfaces using <i>in situ</i> VSFS spectroscopy. This marks a significant development in the VSFS technique by allowing direct observation of heterogeneous adsorption to droplet surfaces <i>in situ</i> under ambient conditions, not relying on bulk removal or uptake. Using a flow tube system, we investigated formic acid vapor adsorption under different interaction times, concentrations, and physical environments. VSFS was used to compare adsorption to droplet surfaces from the gas phase and the underlying bulk at different interaction times. We then quantified the adsorption free energies for the different adsorption processes and found no significant difference between adsorption from the aqueous particle phase and that from the gas phase to the particle surface, with similar results at depressed relative humidity.