Heterogeneous Reactions of NO2 in Dispersed Aqueous Systems

气液滴相互作用通常不被认为是表面化学反应，而是一系列的传质过程，然后是水相均相反应。用来描述水溶液反应的机理和速率方程是那些从整体溶液测量中获得的，假定这些测量直接适用于云和雾等分散体系。这个项目解决了在实验室云中二氧化氮的水溶液反应中的块状和分散态化学的问题。它的目标是从实验上评估两个用来解释我们之前的结果的工作假说，这两个假说与二氧化氮化学的传统图景不一致。我们的主要发现是：(1)当二氧化氮与实验室云反应时，硝酸盐迅速形成。(2)与不饱和氯化钠气溶胶有关的硝酸盐浓度，特别是与云滴有关的硝酸盐浓度，大大超过清除二氧化氮供应中存在的硝酸杂质所期望的最高水平，即存在气溶胶或云水硝酸盐的净产量。(3)暴露在二氧化氮中的云滴的组成与暴露在同一来源的二氧化氮中的散装水的组成有根本的不同。我们的两个假设中的第一个假设是，观察到的与整体的差异是由于高表面-体积水体系的内在的，可能是表面化学的性质。第二种是，在分散的水体系中，作为云凝结核的次饱和气溶胶、云滴或气溶胶粒子上吸附的微量硝酸，极大地促进了二氧化氮的吸收和向硝酸根离子的转化。

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.