ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-6881-2011 Artifacts in measuring aerosol uptake kinetics: the roles of time, concentration and adsorption Renbaum L. H. 1 Smith G. D. 1 Department of Chemistry, University of Georgia, Athens, Georgia, USA 18 07 2011 11 14 6881 6893 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/6881/2011/acp-11-6881-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/6881/2011/acp-11-6881-2011.pdf

In laboratory studies of organic aerosol particles reacting with gas-phase oxidants, high concentrations of radicals are often used to study on the timescale of seconds reactions which may be occurring over days or weeks in the troposphere. Implicit in this approach is the assumption that radical concentration and time are interchangeable parameters, though this has not been established. Here, the kinetics of OH- and Cl-initiated oxidation reactions of model single-component liquid (squalane) and supercooled (brassidic acid and 2-octyldodecanoic acid) organic aerosols are studied by varying separately the radical concentration and the reaction time. Two separate flow tubes with residence times of 2 and 66 s are used, and [OH] and [Cl] are varied by adjusting either the laser photolysis fluence or the radical precursor concentration ([O<sub>3</sub>] or [Cl<sub>2</sub>], respectively) used to generate the radicals. It is found that the rates measured by varying the radical concentration and the reaction time are equal only if the precursor concentrations are the same in the two approaches. Further, the rates depend on the concentrations of the precursor species with a Langmuir-type functional form suggesting that O<sub>3</sub> and Cl<sub>2</sub> saturate the surface of the liquid particles. It is believed that the presence of O<sub>3</sub> inhibits the rate of OH reaction, perhaps by reacting with OH radicals or by O<sub>3</sub> or intermediate species blocking surface sites, while Cl<sub>2</sub> enhances the rate of Cl reaction by participating in a radical chain mechanism. These results have important implications for laboratory experiments in which high concentrations of gas-phase oxidants are used to study atmospheric reactions over short timescales and may explain the variability in recent measurements of the reactive uptake of OH on squalane particles in reactor systems used in this and other laboratories.

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