Heterogeneous Reactions of NO2 in Dispersed Aqueous Systems

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Gas-liquid drop interactions are usually not viewed as surface chemical reactions but considered as a sequence of mass transport processes followed by aqueous-phase homogeneous reactions. The mechanisms and rate equations used to describe the aqueous reactions are those obtained from bulk solution measurements that are assumed to assumed to be directly applicable to dispersed systems such as clouds and fogs. This project addresses the question of bulk vs. dispersed state chemistry in aqueous reactions of nitrogen dioxide in laboratory clouds. Its objective is to evaluate experimentally two working hypotheses formulated to explain our previous results, which are inconsistent with the conventional picture of nitrogen dioxide chemistry. Our main findings are: (1) Nitrate forms rapidly when nitrogen dioxide reacts with a laboratory cloud. (2) The nitrate concentrations associated with subsaturated sodium chloride aerosol and especially with cloud drops substantially exceeds the maximum level expected from scavenging the nitric acid impurity present in the nitrogen dioxide supply, i.e. there is a net production of aerosol or cloud water nitrate. (3) The composition of cloud drops exposed to nitrogen dioxide is radically different from bulk water exposed to nitrogen dioxide from the same source. The first of our two hypotheses is that the observed vs. bulk differences are due to an intrinsic, possibly surface chemical, property of high surface-to-volume aqueous systems. The second is that the uptake of nitrogen dioxide and its conversion to nitrate ion in a dispersed aqueous system is strongly enhanced by trace amounts of nitric acid adsorbed on subsaturated aerosols, cloud drops, or aerosol particles that serve as cloud condensation nuclei.