<i>Cis</i> -pinonic acid diffusion in several atmospheric aerosol-like environments: A molecular dynamics study

Using molecular dynamics simulations, we examine the diffusive behavior of a typical secondary organic aerosol component, cis-pinonic acid (CPA), in binary and tertiary mixtures containing water and ammonium sulfate. The goal is to estimate the diffusion coefficient of CPA in bulk organic-rich environments representative of atmospheric particle interiors at room temperature. Simulations were performed at various temperatures and water mass fractions (3.7–15% w/w) to quantify the effects of relative humidity and inorganic salt levels on CPA mobility. Even low water fractions (5% w/w) increase CPA diffusivity by about a factor of 2 compared to its self-diffusivity. At 10% w/w, mobility increases further, whereas at 15% w/w, no additional increase is observed, attributed to phase separation of water from CPA. Adding ammonium sulfate leads to ion-rich aqueous regions that reduce water content in organic-rich regions and inhibit CPA diffusion. The diffusivity follows Arrhenius behavior regarding temperature, allowing extrapolation to 300 K. However, these values are upper-limit estimates because of potential deviations from Arrhenius behavior at lower temperatures. At higher water fractions in binary mixtures, the CPA diffusivity can be up to 10 times that in pure CPA. This upper limit is about 3.7 × 10⁻⁹ cm² s−1 at room temperature. In mixtures containing also ammonium sulfate, diffusivity decreases to values equal to or below the CPA self-diffusion coefficient (i.e., in its pure phase). At relative humidity up to 65% and room temperature CPA remains mobile in the organic-rich areas of aerosol particles, but its mobility is reduced by about 5 times due to the presence of ion-dominated areas. The upper-limit estimated diffusion coefficient of CPA at 300 K is 7.7 × 10⁻¹⁰ cm² s⁻¹. The accuracy of these results depends on the accuracy of the force-field used and on that of the extrapolation procedure used to estimate room-temperature values.Copyright © 2026 American Association for Aerosol Research