Abstract
Abstract. Mass accommodation is an essential process for gas–particle partitioning of organic compounds in secondary organic aerosols (SOA). The mass accommodation coefficient is commonly described as the probability of a gas molecule colliding with the surface to enter the particle phase. It is often applied, however, without specifying if and how deep a molecule has to penetrate beneath the surface to be regarded as being incorporated into the condensed phase (adsorption vs. absorption). While this aspect is usually not critical for liquid particles with rapid surface–bulk exchange, it can be important for viscous semi-solid or glassy solid particles to distinguish and resolve the kinetics of accommodation at the surface, transfer across the gas–particle interface, and further transport into the particle bulk. For this purpose, we introduce a novel parameter: an effective mass accommodation coefficient αeff that depends on penetration depth and is a function of surface accommodation coefficient, volatility, bulk diffusivity, and particle-phase reaction rate coefficient. Application of αeff in the traditional Fuchs–Sutugin approximation of mass-transport kinetics at the gas–particle interface yields SOA partitioning results that are consistent with a detailed kinetic multilayer model (kinetic multilayer model of gas–particle interactions in aerosols and clouds, KM-GAP; Shiraiwa et al., 2012) and two-film model solutions (Model for Simulating Aerosol Interactions and Chemistry, MOSAIC; Zaveri et al., 2014) but deviate substantially from earlier modeling approaches not considering the influence of penetration depth and related parameters. For highly viscous or semi-solid particles, we show that the effective mass accommodation coefficient remains similar to the surface accommodation coefficient in the case of low-volatility compounds, whereas it can decrease by several orders of magnitude in the case of semi-volatile compounds. Such effects can explain apparent inconsistencies between earlier studies deriving mass accommodation coefficients from experimental data or from molecular dynamics simulations. Our findings challenge the approach of traditional SOA models using the Fuchs–Sutugin approximation of mass transfer kinetics with a fixed mass accommodation coefficient, regardless of particle phase state and penetration depth. The effective mass accommodation coefficient introduced in this study provides an efficient new way of accounting for the influence of volatility, diffusivity, and particle-phase reactions on SOA partitioning in process models as well as in regional and global air quality models. While kinetic limitations may not be critical for partitioning into liquid SOA particles in the planetary boundary layer (PBL), the effects are likely important for amorphous 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.
Highlights
Secondary organic aerosols (SOA) are major constituents of atmospheric particulate matter, affecting air quality, climate, and public health (Jimenez et al, 2009; Kanakidou et al, 2005; Pöschl and Shiraiwa, 2015; Shrivastava et al, 2017a)
To investigate and demonstrate the relevance of the kinetics of mass accommodation and the applicability of αeff, we simulate the temporal evolution of partitioning and equilibration of semi-volatile organic compounds (SVOC) with C0 = 100 μg m−3 and Dg = 0.1 cm2 s−1 interacting with non-volatile seed particles, with a number concentration of 5000 cm−3, an initial diameter of 200 nm, and a surface accommodation coefficient αs = α(0) = 1
Traditional SOA modeling approaches often use the Fuchs– Sutugin approximation of mass-transport kinetics at the gas– particle interface in combination with mass accommodation coefficients that are not appropriately defined, leading to inconsistent results and conclusions. To overcome such deficiencies and difficulties, we have introduced an effective mass accommodation coefficient αeff that depends on penetration depth and is a function of surface accommodation coefficient, volatility, bulk diffusivity, and particle-phase reaction rate coefficient
Summary
Secondary organic aerosols (SOA) are major constituents of atmospheric particulate matter, affecting air quality, climate, and public health (Jimenez et al, 2009; Kanakidou et al, 2005; Pöschl and Shiraiwa, 2015; Shrivastava et al, 2017a). In most current air quality, atmospheric chemistry, and climate models, the limiting step of SOA formation is assumed to be gas-phase oxidation of VOC to form semi-volatile and lowvolatility products. Most model studies use the Fuchs–Sutugin approximation of mass-transport kinetics at the gas–particle interface with a fixed mass accommodation coefficient that does not vary with particle phase state nor with the volatility and diffusivity of the investigated organic compounds. We investigate the influence of volatility, diffusivity, and particle phase state on the mass accommodation and gas–particle partitioning of organic compounds in SOA by detailed and simplified kinetic modeling approaches, comparing the Fuchs–Sutugin approximation to a detailed kinetic multilayer model (kinetic multilayer model of gas–particle interactions in aerosols and clouds, KM-GAP; Shiraiwa et al, 2012) as well as approximate and transient two-film model solutions (Model for Simulating Aerosol Interactions and Chemistry, MOSAIC; Zaveri et al, 2014)
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