Nitrate- and ammonium ion vs. dicarboxylic acid diffusivity in viscous organic aerosol particles: implications for gas-particle partitioning

Considerable progress has been made in recent years in our understanding of the diffusivity of various species in viscous aqueous organic aerosol. However, little is known about ion diffusivity in such matrices. Here, we use experimental evaporation rates of volatile ammonium nitrate in a levitated, viscous proxy organic aerosol droplet to deduce the diffusivities of the nitrate and ammonium ions. We compare the ion diffusivities with those of semi-volatile maleic and malonic acid in the same proxy organic aerosol droplet. In addition, we measured viscosity of the proxies. Our finding indicates significantly slower diffusion of the ions compared to those of the organic acids, although the viscosity of the mixed solutions is comparable. Overall, the effective diffusivity of the ions seems to follow the Stokes-Einstein relationship, whereas the small organic acids diffuse faster than predicted. These findings have implications for the gas-particle portioning of ammonium nitrate which may be stronger limited by kinetic mass transfer than previously thought.