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Title: Timescales of secondary organic aerosols to reach equilibrium at various temperatures and relative humidities
Abstract. Secondary organic aerosols (SOA) account for a substantial fraction of airparticulate matter, and SOA formation is often modeled assuming rapidestablishment of gas–particle equilibrium. Here, we estimate thecharacteristic timescale for SOA to achieve gas–particle equilibrium undera wide range of temperatures and relative humidities using astate-of-the-art kinetic flux model. Equilibration timescales werecalculated by varying particle phase state, size, mass loadings, andvolatility of organic compounds in open and closed systems. Modelsimulations suggest that the equilibration timescale for semi-volatilecompounds is on the order of seconds or minutes for most conditions in theplanetary boundary layer, but it can be longer than 1 h if particlesadopt glassy or amorphous solid states with high glass transitiontemperatures at low relative humidity. In the free troposphere with lowertemperatures, it can be longer than hours or days, even at moderate orrelatively high relative humidities due to kinetic limitations of bulkdiffusion in highly viscous particles. The timescale of partitioning oflow-volatile compounds into highly viscous particles is shorter compared tosemi-volatile compounds in the closed system, as it is largely determined bycondensation sink due to very slow re-evaporation with relatively quickestablishment of local equilibrium between the gas phase and thenear-surface bulk. The dependence of equilibration timescales on bothvolatility and bulk diffusivity provides critical insights intothermodynamic or kinetic treatments of SOA partitioning for accuratepredictions of gas- and particle-phase concentrations of semi-volatilecompounds in regional and global chemical transport models.  more » « less
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Journal Name:
Atmospheric Chemistry and Physics
Page Range / eLocation ID:
5959 to 5971
Medium: X
Sponsoring Org:
National Science Foundation
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