Influence of particle viscosity on mass transfer and heterogeneous ozonolysis kinetics in aqueous–sucrose–maleic acid aerosol

Abstract

Mass transfer between the gas and condensed phases in aerosols can be limited by slow bulk diffusion within viscous particles. During the heterogeneous and multiphase reactions of viscous organic aerosol particles, it is necessary to consider the interplay of numerous mass transfer processes and how they are impacted by viscosity, including the partitioning kinetics of semi-volatile organic reactants, water and oxidants. To constrain kinetic models of the heterogeneous chemistry, measurements must provide information on as many observables as possible. Here, the ozonolysis of maleic acid (MA) in ternary aerosol particles containing water and sucrose is used as a model system. By varying the mass ratio of sucrose to MA and by performing reactions over a wide range of relative humidity, direct measurements show that the viscosity of the particle can be varied over 7 orders of magnitude. Measurements of the volatilisation kinetics of MA show that this range in viscosity leads to a suppression in the effective vapour pressure of MA of 3-4 orders of magnitude. The inferred values of the diffusion coefficient of MA in the particle phase closely mirror the expected change in diffusion coefficient from the Stokes-Einstein equation and the change in viscosity. The kinetics of ozonolysis show a similar dependence on particle viscosity that can be further investigated using the kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB). Two scenarios, one constraining the diffusion coefficients for MA to those expected based on the Stokes-Einstein equation and the other including the diffusion coefficients as a fit parameter, yield similarly adequate representations of the ozonolysis kinetics, as inferred from the experimental decay in the signature of the vinylic C-H stretching vibration of MA. However, these two scenarios provide very different parameterisations of the compositional dependence of the diffusion coefficients of ozone within the condensed phase, yielding qualitatively different time-dependent internal concentration profiles. We suggest that this highlights the importance of providing additional experimental observables (e.g. particle size, heterogeneity in composition) if measurements and models are to be universally reconciled.

Effect of inorganic-to-organic mass ratio on the heterogeneous OH reaction rates of erythritol: implications for atmospheric chemical stability of 2-methyltetrols

“…The physical state of particles can play a key factor in determining the composition, morphology and properties (e.g., viscosity) of the particles, which in turn influence the heterogeneous reactivity (Renbaum and Smith, 2009;Slade and Knopf, 2014;Fan et al, 2015;Marshall et al, 2018;Karadima et al, 2019). Marsh et al 2017have measured the hygroscopicity of erythritol particles and found that erythritol particles are spherical droplets over their experimental RH range (60 %-100 %).…”

“…At low RH, erythritol particles are known to be viscous (Song et al, 2016;Grayson et al, 2017;Chu et al, 2018). With high particle viscosity, the diffusion of erythritol molecules from the bulk phase to the particle surface for oxidation slows down, and the overall heterogeneous reaction rate can be controlled by the diffusion (Chim et al, 2018;Marshall et al, 2018). To investigate whether particle viscosity affects the heterogeneous OH reactivity of erythritol particles at 85 % RH in this work, we calculated the characteristic timescale for diffusive mixing time (τ D ) of erythritol within the particle and that for gas-phase OH-erythritol particle collisional timescale (τ coll ) (Chim et al, 2018).…”

“…To date, the heterogeneous kinetics and chemistry of many pure organic compounds or organic mixtures have been investigated (Zhang et al, 2015;Enami et al, 2016;Socorro et al, 2017;Marshall et al, 2018;Zhao et al, 2019). There remains large uncertainty in how inorganic salts alter the heterogeneous kinetics and chemistry of organic compounds (McNeill et al, 2007(McNeill et al, , 2008Dennis-Smither et al, 2012).…”

“…To date, the heterogeneous kinetics and chemistry of many pure organic compounds or organic mixtures have been investigated (Zhang et al, 2015;Enami et al, 2016;Socorro et al, 2017;Marshall et al, 2018;Zhao et al, 2019). There remains large uncertainty in how inorganic salts alter the heterogeneous kinetics and chemistry of organic compounds (McNeill et al, 2007(McNeill et al, , 2008Dennis-Smither et al, 2012). A few recent laboratory studies have revealed that the presence of dissolved inorganic ions (e.g., ammonium sulfate, AS) can reduce the heterogeneous OH reactivity of organic compounds in aqueous organic-inorganic particles (e.g., methanesulfonic acid and 3-methylglutaric acid) but does not significantly alter the reaction mechanisms (Mungall et al, 2017;Kwong et al, 2018b;Lam et al, 2019b).…”

Heterogeneous oxidation of amorphous organic aerosol surrogates by O<sub>3</sub>, NO<sub>3</sub>, and OH at typical tropospheric temperatures

“…In general, the solid condensed phase showed significantly lower γ O 3 values compared to the ozonolysis of the liquid phase. Recent experimental studies focused on how relative humidity (RH) influences the reactive uptake of gas oxidants via its impact on the phase state of the organic species (Berkemeier et al, 2016;Steimer et al, 2015;Shiraiwa et al, 2011;Slade and Knopf, 2014;Li et al, 2018;Davies and Wilson, 2015;Hu et al, 2016;Marshall et al, 2018). The heterogeneous oxidation of a typical component of biomass burning aerosol (BBA) particles, levoglucosan (LEV) by OH yielded γ values in the range from 0.008 to 1 with implications for the lifetime of LEV ranging from weeks at dry conditions (Slade and Knopf, 2013;Kessler et al, 2010;Slade and Knopf, 2014) to a couple of days when LEV is in a more https://doi.org/10.5194/acp-2020-82 Preprint.…”