Impact of particle phase state on the competition between condensation and coagulation

Abstract

The evolution of particle size distribution of secondary organic aerosols (SOAs) is influenced by condensation and coagulation. Amorphous semisolid and glassy states in SOAs cause kinetic limitations for condensational growth, but the impact of these phase states on the competition between condensation and coagulation has not been evaluated. In this work, we implement coagulation into the kinetic multilayer model of gas–particle interactions (KM-GAP) to calculate the timescales of SOA partitioning and coagulation for liquid, semisolid, and highly viscous particles for closed and open systems. We find that the phase state may not have a major impact on the coagulation timescale, with particle size playing a more critical role. The equilibration timescale of SOA partitioning is shorter than the coagulation timescale for most conditions in the closed system, while coagulation becomes competitive especially for high particle number concentration and the highly viscous phase state (D_b ≤ 10⁻¹⁵ cm² s⁻¹) due to the prolonged timescale of partitioning. We also illustrate that coagulation is less significant for the growth of seed particles in chamber experiments even for viscous particles, as the condensation sink of low volatility compounds would be larger than the coagulation sink due to their efficient mass accommodation. Coagulation becomes important during nanoparticle growth and the interplay between condensation and coagulation can result in the emergence of a bimodal size distribution with nanoparticles likely adopting a low viscosity phase state. Coagulation is also important for size distribution dynamics in a smoke plume for highly viscous submicron particles as their evaporation and condensation are inhibited with kinetic limitations.