Modeling Cl₂ formation from aqueous NaCl particles: Evidence for interfacial reactions and importance of Cl₂ decomposition in alkaline solution

Abstract

[1] A series of experiments have demonstrated that a significant yield of chlorine gas is produced when mixtures of ozone and sodium chloride particles above their deliquescence point are irradiated at 254 nm. In order to obtain expressions for future modeling studies, a comprehensive model is used to analyze the system and to determine its sensitivity. This work reexamines and expands previous studies [Knipping et al., 2000]. The enhanced model, described in detail herein, reaffirms that current known physical and chemical processes fail to reproduce the observed Cl 2 formation in the experiments. A methodological analysis, proposed as a framework for similar studies, of the physicochemical system supports the accountability of an overall mechanism initiated by the formation of a relatively stable complex of the hydroxyl radical and chloride ions at the gas-liquid interface for the observed chlorine generation. Different potential fates of the OH...Cl .À surface intermediate are discussed. A rate expression and kinetic parameters are presented for the overall reaction of the interfacial mechanism. In addition, sensitivity studies underscore the importance of accurately modeling chlorine decomposition processes in alkaline solution-in particular, the reactions of chlorine with hydroxide, carbonate, and basic hydrogen peroxide. Recommended aqueous-phase rate constants for these reactions are drawn from a literature evaluation illustrating the limited availability and lack of agreement of related kinetic data.

Inland Concentrations of Cl2 and ClNO2 in Southeast Texas Suggest Chlorine Chemistry Significantly Contributes to Atmospheric Reactivity

“…Other work suggests that the reaction of hydroxyl radicals (OH • ) at the particle surface more accurately depicts the chemistry active during heterogeneous Cl2 production [30,33,34], where the formation of a surface complex between particulate Cl − and gas phase OH • was suggested as the rate limiting step, as is shown in Reactions (R3) and (R4).…”

“…Such a Cl2 source could potentially lead to the enhanced oxidation of CH4 and other VOCs [23,25,58], and therefore enhanced formation of secondary organic aerosol. Additionally, enhanced O3 production can result from the presence of a Cl • source [1,33]. The implications of this are discussed further in Section 3.4.…”

Chlorine chemistry in urban atmospheres: a review

“…Rates are given in parts per billion per second at 298 K and rate constants for calculations are from Manion et al [115] Cl and [5] Aschmann and Atkinson, [15] Finlayson-Pitts et al, [109] Stutz et al, [110] Gill and Hites, [111] Atkinson et al [112] and Aschmann et al [113] A mechanism involving the formation of a surface complex between particulate chloride and OH (OH?Cl À ) (Reactions 20, 21) has been proposed to explain the processes involved in Reaction 14. [41,42] OH ðgÞ þ Cl…”

Reactive chlorine in the marine boundary layer in the outflow of polluted continental air: A model study

“…As possible explanation, Sadanaga et al [2001] suggested heterogeneous reaction of ozone with sea salt in the presence of ferric ions. Another possibility for the production of Cl 2 would be the reaction between OH and Cl À on aerosol surfaces [Knipping et al, 2000;Knipping and Dabdub, 2002], in addition to the autocatalytic mechanism involving oxidation of Cl À by HOCl/HOBr in acidified sea salt aerosol [e.g., Sander and Crutzen, 1996;Vogt et al, 1996]. Using a similar method as Spicer et al [1998], Finley and Saltzman [2006] measured Cl 2 in coastal urban air in Irvine, California, reporting mixing ratios up to 20 pmol mol À1 with a 2-month average of 3.5 pmol mol À1 .…”