Representation of Multiphase OH Oxidation of Amorphous Organic Aerosol for Tropospheric Conditions

Abstract

Organic aerosol (OA) is ubiquitous in the atmosphere and, during transport, can experience chemical transformation with consequences for air quality and climate. Prediction of the chemical evolution of OA depends on its reactivity with atmospheric oxidants such as the OH radical. OA particles undergo amorphous phase transitions from liquid to solid (glassy) states in response to temperature changes, which, in turn, will impact its reactivity toward OH oxidation. To improve the predictability of OA reactivity toward OH oxidation, the reactive uptake coefficients (γ) of OH radicals reacting with triacontane and squalane serving as amorphous OA surrogates were measured at temperatures from 213−293 K. γ increases strongest with temperature when the organic species is in the liquid phase, compared to when being in the semisolid or solid phase. The resistor model is applied, accounting for the amorphous phase state changes using the organic species' glass transition temperature and fragility, to evaluate the physicochemical parameters of the temperature dependent OH uptake process. This allows for the derivation of a semiempirical formula, applicable to models, to predict the degree of oxidation and chemical lifetime of the condensed-phase organic species for typical tropospheric temperature and humidity when OA particle viscosity is known.

The number fraction of iron-containing particles affects OH, HO<sub>2</sub> and H<sub>2</sub>O<sub>2</sub> budgets in the atmospheric aqueous phase

“…On organic surfaces, γ(OH) values are generally higher than on inorganic surfaces which demonstrates the high reactivity of OH with organics in the condensed phase and/or on organic surfaces. In several lab studies, squalane was used as a proxy for long-chain alkane; the reactive uptake coefficients have been determined in a range of ∼0.2 < γ(OH) < 0.5 (e.g., Che Li and Knopf (2021)). A similar value range was also found for a wide variety of other organics ( (Bertram et al, 2001)).…”

A Kinetic Model for Predicting Trace Gas Uptake and Reaction

“…This is consistent, for example, with OH surface reactions on organic aerosols, which find that γ is independent of size. 24,87 To derive a closed form expression for the time dependence of [Y(total)] we start with the following expression,…”

“…Kinetic resistance models have been developed and refined by many authors. 2,[14][15][16][17][18][19][20][21][22][23][24][25][26] These formalisms approximate uptake as set of decoupled and normalized fluxes expressed as resistances, which can be added in series or in parallel in analogy to electrical circuits. Resistor models provide simple expressions for estimating uptake coefficients and in some cases the associated decay kinetics of the trace gas or solute.…”

Ozone depletion in the Arctic and Antarctic stratosphere induced by wildfire smoke

“…At conditions of the UTLS, it can be assumed that the organic species in the biomass burning particles are in a solid (glassy) state (Koop et al, 2011;Shiraiwa et al, 2017;Knopf et al, 2018). The solid-phase state is the likely explanation for the long chemical lifetime against multiphase oxidation (Arangio et al, 2015;Knopf et al, 2011;Li et al, 2020;Li and Knopf, 2021). Furthermore, heterogeneous oxidation of glassy organic aerosol particles likely increases their ability to take up water (Slade et al, 2017) and thus contributes to PSC formation and heterogeneous ozone-depleting reactions such as the hydrolysis of N 2 O 5 (Solomon et al, 2022).…”