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. Mass accommodation is an essential process for gas–particle partitioning oforganic compounds in secondary organic aerosols (SOA). The massaccommodation coefficient is commonly described as the probability of a gasmolecule colliding with the surface to enter the particle phase. It is oftenapplied, however, without specifying if and how deep a molecule has topenetrate beneath the surface to be regarded as being incorporated into thecondensed phase (adsorption vs. absorption). While this aspect is usuallynot critical for liquid particles with rapid surface–bulk exchange, it canbe important for viscous semi-solid or glassy solid particles to distinguishand resolve the kinetics of accommodation at the surface, transfer acrossthe gas–particle interface, and further transport into the particle bulk. For this purpose, we introduce a novel parameter: an effective massaccommodation coefficient αeff that depends on penetrationdepth and is a function of surface accommodation coefficient, volatility,bulk diffusivity, and particle-phase reaction rate coefficient. Applicationof αeff in the traditional Fuchs–Sutugin approximation ofmass-transport kinetics at the gas–particle interface yields SOApartitioning results that are consistent with a detailed kinetic multilayermodel (kinetic multilayer model of gas–particle interactions in aerosols and clouds, KM-GAP; Shiraiwa et al., 2012) and two-film model solutions (Modelfor Simulating Aerosol Interactions and Chemistry, MOSAIC;Zaveri et al., 2014) but deviate substantially from earlier modelingapproaches not considering the influence of penetration depth and relatedparameters. For highly viscous or semi-solid particles, we show that the effective massaccommodation coefficient remains similar to the surface accommodationcoefficient in the case of low-volatility compounds, whereas it can decrease byseveral orders of magnitude in the case of semi-volatile compounds. Such effectscan explain apparent inconsistencies between earlier studies deriving massaccommodation coefficients from experimental data or from molecular dynamicssimulations. Our findings challenge the approach of traditional SOA models using theFuchs–Sutugin approximation of mass transfer kinetics with a fixed massaccommodation coefficient, regardless of particle phase state and penetrationdepth. The effective mass accommodation coefficient introduced in this studyprovides an efficient new way of accounting for the influence of volatility,diffusivity, and particle-phase reactions on SOA partitioning in processmodels as well as in regional and global air quality models. While kineticlimitations may not be critical for partitioning into liquid SOA particlesin the planetary boundary layer (PBL), the effects are likely important foramorphous semi-solid or glassy SOA in the free and upper troposphere (FT–UT)as well as in the PBL at low relative humidity and low temperature.

atmospheric chemistry and physics

Mass accommodation and gas–particle partitioning in secondary organic aerosols: dependence on diffusivity, volatility, particle-phase reactions, and penetration depth