Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey, UK.
Phys Chem Chem Phys. 2009 Sep 21;11(35):7699-707. doi: 10.1039/b906517b.
The oxidation of organic films on cloud condensation nuclei has the potential to affect climate and precipitation events. In this work we present a study of the oxidation of a monolayer of deuterated oleic acid (cis-9-octadecenoic acid) at the air-water interface by ozone to determine if oxidation removes the organic film or replaces it with a product film. A range of different aqueous sub-phases were studied. The surface excess of deuterated material was followed by neutron reflection whilst the surface pressure was followed using a Wilhelmy plate. The neutron reflection data reveal that approximately half the organic material remains at the air-water interface following the oxidation of oleic acid by ozone, thus cleavage of the double bond by ozone creates one surface active species and one species that partitions to the bulk (or gas) phase. The most probable products, produced with a yield of approximately (87 +/- 14)%, are nonanoic acid, which remains at the interface, and azelaic acid (nonanedioic acid), which dissolves into the bulk solution. We also report a surface bimolecular rate constant for the reaction between ozone and oleic acid of (7.3 +/- 0.9) x 10(-11) cm2 molecule s(-1). The rate constant and product yield are not affected by the solution sub-phase. An uptake coefficient of ozone on the oleic acid monolayer of approximately 4 x 10(-6) is estimated from our results. A simple Kohler analysis demonstrates that the oxidation of oleic acid by ozone on an atmospheric aerosol will lower the critical supersaturation needed for cloud droplet formation. We calculate an atmospheric chemical lifetime of oleic acid of 1.3 hours, significantly longer than laboratory studies on pure oleic acid particles suggest, but more consistent with field studies reporting oleic acid present in aged atmospheric aerosol.
有机薄膜在云凝结核上的氧化有可能影响气候和降水事件。在这项工作中，我们研究了臭氧对空气-水界面上单层氘代油酸（顺-9-十八烯酸）的氧化，以确定氧化是否会去除有机薄膜或将其替换为产物薄膜。研究了一系列不同的水亚相。通过中子反射法跟踪氘代材料的表面过剩，同时使用Wilhelmy 板跟踪表面压力。中子反射数据表明，在臭氧氧化油酸后，大约有一半的有机物质仍留在空气-水界面上，因此臭氧对双键的裂解产生了一种表面活性物质和一种分配到体相（或气相）的物质。最可能的产物，产率约为（87 +/- 14）%，是非酸，它留在界面上，和壬二酸（壬二酸），它溶解在体相溶液中。我们还报告了臭氧与油酸之间反应的表面双分子速率常数为（7.3 +/- 0.9）x 10(-11) cm2 分子 s(-1)。速率常数和产物产率不受溶液亚相的影响。根据我们的结果，估算出臭氧在油酸单层上的吸收系数约为 4 x 10(-6)。简单的 Kohler 分析表明，臭氧对大气气溶胶中油酸的氧化将降低云滴形成所需的临界过饱和度。我们计算出油酸在大气中的化学寿命为 1.3 小时，明显长于纯油酸颗粒的实验室研究，但与报告大气中存在老化气溶胶的油酸的现场研究更一致。
