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. Typical tropospheric temperatures render possible phasestates of amorphous organic aerosol (OA) particles of solid, semisolid, andliquid. This will affect the multiphase oxidation kinetics involving theorganic condensed-phase and gaseous oxidants and radicals. To quantify thiseffect, we determined the reactive uptake coefficients (γ) ofO3, NO3, and OH by substrate films composed of single and binary OAsurrogate species under dry conditions for temperatures from 213 to 313 K. Atemperature-controlled coated-wall flow reactor coupled to a chemicalionization mass spectrometer was applied to determine γ withconsideration of gas diffusion transport limitation and gas flow entranceeffects, which can impact heterogeneous reaction kinetics. The phase stateof the organic substrates was probed via the poke-flow technique, allowingthe estimation of the substrates' glass transition temperatures. γvalues for O3 and OH uptake to a canola oil substrate, NO3 uptaketo a levoglucosan and a levoglucosan / xylitol substrate, and OH uptake to aglucose and glucose / 1,2,6-hexanetriol substrate have been determined as afunction of temperature. We observed the greatest changes in γ withtemperature for substrates that experienced the largest changes in viscosityas a result of a solid-to-liquid phase transition. Organic substrates thatmaintain a semisolid or solid phase state and as such a relatively higherviscosity do not display large variations in heterogeneous reactivity. From213 to 293 K, γ values of O3 with canola oil, of NO3with a levoglucosan / xylitol mixture, and of OH with aglucose / 1,2,6-hexanetriol mixture and canola oil, increase by about a factorof 34, 3, 2, and 5, respectively, due to a solid-to-liquid phase transitionof the substrate. These results demonstrate that the surface and bulklifetime of the OA surrogate species can significantly increase due to theslowed heterogeneous kinetics when OA species are solid or highly viscous inthe middle and upper troposphere. This experimental study will further ourunderstanding of the chemical evolution of OA particles with subsequentimportant consequences for source apportionment, air quality, and climate.

atmospheric chemistry and physics

Heterogeneous oxidation of amorphous organic aerosol surrogates by O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;, NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;, and OH at typical tropospheric temperatures