Impact of Particle Phase State on the Competition Between Condensation and Coagulation

KIEUDIEM NGUYEN, Meredith Schervish, Pascale Lakey, James Smith, Manabu Shiraiwa, University of California, Irvine

     Abstract Number: 445
     Working Group: Aerosol Physics

Abstract
The particle size distribution is a critical property for radiative forcing and cloud activation of atmospheric aerosol particles. The evolution of particle size distribution is influenced by condensation and coagulation. While condensation drives the growth of secondary organic aerosols (SOA), coagulation leads to scavenging of small particles to reduce particle number concentrations. It has been shown that amorphous semisolid and glassy solid phase states in SOA can cause kinetic limitations for condensational growth, but their impact 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 simulate the evolution of the particle size distribution by condensation and coagulation for particles with different phase states. Equilibration timescales of SOA partitioning (τ_eq) and coagulation timescales (τ_coag) are calculated for liquid, semisolid, and highly viscous particles in the presence of condensing organic compounds with different volatilities for closed and open systems. Our results show that τ_eq is shorter for condensation of semi-volatile compounds into low viscous particles in a closed system, but it will be prolonged to hours or days for condensation of semi-volatile and intermediate volatile compounds into semisolid and highly viscous particles (D_b ≤ 10^-15 cm² s^-1), which makes coagulation a competitive process. We also illustrate that coagulation is less significant for the growth of SOA in chamber experiments with the presence of relatively low particle number concentration, while coagulation becomes important during nanoparticle growth, which is evidenced by the emergence of a bimodal size distribution in the growth of freshly nucleated particles.