Discussion: Laboratory study of NO2 reaction with dispersed and bulk liquid water. Schwartz, S. E. and Lee, Y.-N. Atmos. Environ. 29, 2557-2559 (1995).

Recently Novakov and colleagues (Bambauer et al., 1994) have reexamined the rate of the reactive uptake of nitrogen dioxide (NO2) by water under laboratory conditions. Two types of gas-liquid contactor were employed, a bubbler, similar to that employed in our earlier work (Lee and Schwartz, 1981a), and a cloud chamber, in order to examine the applicability of kinetic information determined in bulk liquids to a dispersed system such as a cloud. We commend Bambauer et al. for undertaking this study and recognize the difficulty of the experiment, especially in maintaining constant conditions in the cloud chamber for the rather long periods (typically an hour) required to collect sufficient amounts of cloudwater to permit chemical analysis. We nonetheless appreciate the advantage afforded by direct comparison between reaction rate measured in the cloud-chamber reactor with that measured in the bubbler in confirming the applicability of "bulk" rate laws to reactions in dispersed clouds. The expectation of course is that the rates would be equal. In fact the reaction rate was found to be substantially greater in the cloud-chamber reactor, leading Bambauer et al., to conclude that different mechanisms govern the reaction rate in the bulk reactor versus in the dispersed system. Here we suggest that the rate measured in the cloud chamber reactor may have been enhanced artificially because of nonuniform concentration distribution of the reagent gas NO2 together with the second order dependence of the rate on NO2 concentration. Consequently, the conclusion by Bambauer et al. that the rate expression determined using a bulk reactor is inapplicable to evaluation of the rate of this reaction in clouds in the ambient atmosphere may not be warranted.

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