Aerosol are ubiquitous in the atmosphere and impact the earth’s radiation budget through scattering of solar radiation and cloud interactions. In this work, we investigate condensed phase mass transport in a model aqueous inorganic-organic aerosol system through single-particle experiments and a multicomponent diffusion model. Diffusion influences many important physical properties in aerosol such as size, phase and cloud nucleation activity. Using a linear-quadrupole electrodynamic balance, water loss measurements were performed on aerosol composed of varying molar ratios of citric acid (CA) and ammonium sulfate (AS). In response to relative humidity steps, characteristic equilibration timescales were measured by monitoring morphology dependent resonances over time. Equilibration times were found to deviate from simple mixing rules wherein for certain molar ratios, timescales can be longer than either limiting binary case. To understand these results, a multicomponent diffusion model was developed using the Maxwell-Stefan (MS) framework. Activities in the mixture were calculated using the aerosol inorganic-organic mixtures functional groups activity coefficient (AIOMFAC) thermodynamic model. Fluxes from the MS equation were incorporated into a spherical model with a moving boundary which was used to solve the mass transport problem numerically. The resulting model accurately reproduced equilibration timescales across the range of AS/CA molar ratios studied experimentally. We conclude that longer equilibration times at certain molar ratios are attributed to thermodynamic nonideality. Furthermore, these results highlight that diffusion in multicomponent systems can behave very differently relative to the often-studied binary systems.

Multicomponent diffusion in a model inorganic-organic aerosol system

ACS Spring Meeting 2021

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