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

Abstract

Abstract. Typical tropospheric temperatures render possible phase states of amorphous organic aerosol (OA) particles of solid, semisolid, and liquid. This will affect the multiphase oxidation kinetics involving the organic condensed-phase and gaseous oxidants and radicals. To quantify this effect, we determined the reactive uptake coefficients (γ) of O3, NO3, and OH by substrate films composed of single and binary OA surrogate species under dry conditions for temperatures from 213 to 313 K. A temperature-controlled coated-wall flow reactor coupled to a chemical ionization mass spectrometer was applied to determine γ with consideration of gas diffusion transport limitation and gas flow entrance effects, which can impact heterogeneous reaction kinetics. The phase state of the organic substrates was probed via the poke-flow technique, allowing the estimation of the substrates' glass transition temperatures. γ values for O3 and OH uptake to a canola oil substrate, NO3 uptake to a levoglucosan and a levoglucosan / xylitol substrate, and OH uptake to a glucose and glucose / 1,2,6-hexanetriol substrate have been determined as a function of temperature. We observed the greatest changes in γ with temperature for substrates that experienced the largest changes in viscosity as a result of a solid-to-liquid phase transition. Organic substrates that maintain a semisolid or solid phase state and as such a relatively higher viscosity do not display large variations in heterogeneous reactivity. From 213 to 293 K, γ values of O3 with canola oil, of NO3 with a levoglucosan / xylitol mixture, and of OH with a glucose / 1,2,6-hexanetriol mixture and canola oil, increase by about a factor of 34, 3, 2, and 5, respectively, due to a solid-to-liquid phase transition of the substrate. These results demonstrate that the surface and bulk lifetime of the OA surrogate species can significantly increase due to the slowed heterogeneous kinetics when OA species are solid or highly viscous in the middle and upper troposphere. This experimental study will further our understanding of the chemical evolution of OA particles with subsequent important consequences for source apportionment, air quality, and climate.

Effects of liquid–liquid phase separation and relative humidity on the heterogeneous OH oxidation of inorganic–organic aerosols: insights from methylglutaric acid and ammonium sulfate particles

“…Laboratory studies have shown that an organic-rich outer shell is always formed for hydrophobic organic compounds in a LLPS scenario, possibly shielding the interior from surface reactions with gas-phase oxidants (McNeill et al, 2007;Li et al, 2020). The organic-rich outer shell (Arangio et al, 2015;Houle et al, 2018) could be viscous, and the het-erogeneous reactivity could be limited by the diffusion of organic species or oxidants across the organic outer shell.…”

“…Organic compounds present at or near the surface of atmospheric aerosols can be efficiently reacted with gas-phase oxidants, such as OH, ozone (O 3 ) and nitrate radicals (George and Abbatt, 2010;Kroll et al, 2015;Chapleski et al, 2016), continuously changing aerosol properties (e.g., water uptake and cloud formation potential) through the alteration of surface and bulk composition (Lambe et al, 2007;Cappa et al, 2011;Harmon et al, 2013;Slade et al, 2015Slade et al, , 2017. The physical state of aerosol particles (and their separate phases, when present) is known to play a role in the heterogeneous reactivity of organic aerosols (Ruehl et al, 2013;Chan et al, 2014;Slade and Knopf, 2014;Li et al, 2020). However, the heterogeneous reactivity of aqueous droplets containing organic compounds and inorganic salts remains unclear.…”

Technical Note: Classical and statistical thermodynamic treatment of adsorption and desorption kinetics and rates

“…Any interaction between gas-phase species and condensed matter, including liquid, semi-solid, and solid phases, commences by adsorption and desorption processes (McNaught and Wilkinson, 2014;Langmuir, 1918Langmuir, , 1916Langmuir, , 1915. Those are of importance in the research areas of catalysis and, in particular, multiphase chemical kinetics or phase transfer kinetics involving environmental surfaces and interfaces (Cussler, 2009;Chorkendorff and Niemantsverdriet, 2007;Finlayson-Pitts and Pitts, 2000;Ravishankara, 1997;Solomon, 1999).…”

“…Especially at low temperatures, large values of τd could counteract slow rates of chemical reaction and diffusion, enhancing the overall gas uptake which may involve reversible, reactive, and catalytic processes on the surface or in the bulk of the particles (Ammann et al, 2013;Crowley et al, 2013;Kolb et al, 2010;Pöschl et al, 2007;Li et al, 2020;Li and Knopf, 2021).…”

Reviewer comment

“…Line 61: The authors might add the recent study by Li et al (2020) on OH uptake by organic matter in various phase states.…”