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

Abstract

Abstract. Mass accommodation is an essential process for gas–particle partitioning of\norganic compounds in secondary organic aerosols (SOA). The mass\naccommodation coefficient is commonly described as the probability of a gas\nmolecule colliding with the surface to enter the particle phase. It is often\napplied, however, without specifying if and how deep a molecule has to\npenetrate beneath the surface to be regarded as being incorporated into the\ncondensed phase (adsorption vs. absorption). While this aspect is usually\nnot critical for liquid particles with rapid surface–bulk exchange, it can\nbe important for viscous semi-solid or glassy solid particles to distinguish\nand resolve the kinetics of accommodation at the surface, transfer across\nthe gas–particle interface, and further transport into the particle bulk. For this purpose, we introduce a novel parameter: an effective mass\naccommodation coefficient αeff that depends on penetration\ndepth and is a function of surface accommodation coefficient, volatility,\nbulk diffusivity, and particle-phase reaction rate coefficient. Application\nof αeff in the traditional Fuchs–Sutugin approximation of\nmass-transport kinetics at the gas–particle interface yields SOA\npartitioning results that are consistent with a detailed kinetic multilayer\nmodel (kinetic multilayer model of gas–particle interactions in aerosols and clouds, KM-GAP; Shiraiwa et al., 2012) and two-film model solutions (Model\nfor Simulating Aerosol Interactions and Chemistry, MOSAIC;\nZaveri et al., 2014) but deviate substantially from earlier modeling\napproaches not considering the influence of penetration depth and related\nparameters. For highly viscous or semi-solid particles, we show that the effective mass\naccommodation coefficient remains similar to the surface accommodation\ncoefficient in the case of low-volatility compounds, whereas it can decrease by\nseveral orders of magnitude in the case of semi-volatile compounds. Such effects\ncan explain apparent inconsistencies between earlier studies deriving mass\naccommodation coefficients from experimental data or from molecular dynamics\nsimulations. Our findings challenge the approach of traditional SOA models using the\nFuchs–Sutugin approximation of mass transfer kinetics with a fixed mass\naccommodation coefficient, regardless of particle phase state and penetration\ndepth. The effective mass accommodation coefficient introduced in this study\nprovides an efficient new way of accounting for the influence of volatility,\ndiffusivity, and particle-phase reactions on SOA partitioning in process\nmodels as well as in regional and global air quality models. While kinetic\nlimitations may not be critical for partitioning into liquid SOA particles\nin the planetary boundary layer (PBL), the effects are likely important for\namorphous semi-solid or glassy SOA in the free and upper troposphere (FT–UT)\nas well as in the PBL at low relative humidity and low temperature.\n