Mass accommodation and gas–particle partitioning in secondary organic aerosols: dependence on diffusivity, volatility, particle-phase reactions, and penetration depth
<p>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 ofte...
Main Authors: | , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2021-02-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://acp.copernicus.org/articles/21/1565/2021/acp-21-1565-2021.pdf |
Summary: | <p>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.</p>
<p>For this purpose, we introduce a novel parameter: an effective mass
accommodation coefficient <span class="inline-formula"><i>α</i><sub>eff</sub></span> that depends on penetration
depth and is a function of surface accommodation coefficient, volatility,
bulk diffusivity, and particle-phase reaction rate coefficient. Application
of <span class="inline-formula"><i>α</i><sub>eff</sub></span> 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.</p>
<p>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.</p>
<p>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.</p> |
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ISSN: | 1680-7316 1680-7324 |