Effects of Phase State and Phase Separation on Dimethylamine Uptake of Ammonium Sulfate and Ammonium Sulfate-Sucrose Mixed Particles

Unexpectedly high amounts of aminium salts have been detected in ambient aerosol particles, prompting investigations into their role in new particle formation and nanoparticle growth. Amine uptake and particle-phase processes, including the effects of phase state and non-ideal mixing, are poorly understood. In this study, we conducted kinetic multi-layer modeling of dimethylamine (DMA) uptake by crystalline and aqueous ammonium sulfate (AS) and mixed ammonium sulfate-sucrose particles based on measurements at different relative humidity (RH) values. The temporal evolution of particle mass increases and the humidity dependence were successfully reproduced by considering the amine/ammonium exchange reaction and formation of hygroscopic dimethylaminium sulfate. Thermodynamic equilibrium predictions suggest that aqueous sucrose and AS mixtures undergo phase separation at RH < 94%. The kinetic model simulations reveal that DMA uptake is limited by diffusion of DMA and AS through a viscous sucrose-rich shell at lower RH, resulting in strong concentration gradients in the particle bulk. The model predicts that the true uptake coefficients would range from 2.0 × 10-5 to 2.6 × 10-3 for initially solid particles at low RH, while they can be as high as 0.70-0.82 in aqueous particles at high RH. Uptake coefficients increase when RH and associated particle water content increase, while they generally decrease when the molar fraction of sucrose increases at a specific value of RH. Using new measurements of ambient amines from the Holistic Interactions of Shallow Clouds, Aerosols and Land Ecosystems (HI-SCALE) field campaign as a reference, the model is extrapolated to particles with a diameter of 30 nm and amine mixing ratios in the ppt range to emulate atmospheric conditions. At 70% RH or higher with particles in the liquid phase, amine uptake can lead to a mass increase of approximately 20-60%.