King Martin D, Jones Stephanie H, Lucas Claire O M, Thompson Katherine C, Rennie Adrian R, Ward Andrew D, Marks Amelia A, Fisher Fleur N, Pfrang Christian, Hughes Arwel V, Campbell Richard A
Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey, UK.
Phys Chem Chem Phys. 2020 Dec 23;22(48):28032-28044. doi: 10.1039/d0cp03934a.
Organic films that form on atmospheric particulate matter change the optical and cloud condensation nucleation properties of the particulate matter and consequently have implications for modern climate and climate models. The organic films are subject to attack from gas-phase oxidants present in ambient air. Here we revisit in greater detail the oxidation of a monolayer of oleic acid by gas-phase ozone at the air-water interface as this provides a model system for the oxidation reactions that occur at the air-water interface of aqueous atmospheric aerosol. Experiments were performed on monolayers of oleic acid at the air-liquid interface at atmospherically relevant ozone concentrations to investigate if the viscosity of the sub-phase influences the rate of the reaction and to determine the effect of the presence of a second component within the monolayer, stearic acid, which is generally considered to be non-reactive towards ozone, on the reaction kinetics as determined by neutron reflectometry measurements. Atmospheric aerosol can be extremely viscous. The kinetics of the reaction were found to be independent of the viscosity of the sub-phase below the monolayer over a range of moderate viscosities, , demonstrating no involvement of aqueous sub-phase oxidants in the rate determining step. The kinetics of oxidation of monolayers of pure oleic acid were found to depend on the surface coverage with different behaviour observed above and below a surface coverage of oleic acid of ∼1 × 1018 molecule m-2. Atmospheric aerosol are typically complex mixtures, and the presence of an additional compound in the monolayer that is inert to direct ozone oxidation, stearic acid, did not significantly change the reaction kinetics. It is demonstrated that oleic acid monolayers at the air-water interface do not leave any detectable material at the air-water interface, contradicting the previous work published in this journal which the authors now believe to be erroneous. The combined results presented here indicate that the kinetics, and thus the atmospheric chemical lifetime for unsaturated surface active materials at the air-water interface to loss by reaction with gas-phase ozone, can be considered to be independent of other materials present at either the air-water interface or in the aqueous sub-phase.
在大气颗粒物上形成的有机薄膜会改变颗粒物的光学和云凝结核特性，进而对现代气候和气候模型产生影响。这些有机薄膜会受到环境空气中气相氧化剂的攻击。在此，我们更详细地重新审视了气相臭氧在气 - 水界面处对油酸单层的氧化作用，因为这为大气气溶胶气 - 水界面发生的氧化反应提供了一个模型系统。实验是在大气相关臭氧浓度下，对气 - 液界面处的油酸单层进行的，以研究下层相的粘度是否会影响反应速率，并通过中子反射测量来确定单层中第二种成分硬脂酸（通常认为对臭氧无反应）的存在对反应动力学的影响。大气气溶胶可能具有极高的粘性。发现在一系列中等粘度范围内，单层下方下层相的粘度对反应动力学没有影响，这表明下层相中的水性氧化剂未参与速率决定步骤。发现纯油酸单层的氧化动力学取决于表面覆盖率，在油酸表面覆盖率约为1×10¹⁸分子/平方米以上和以下观察到不同的行为。大气气溶胶通常是复杂的混合物，单层中存在对直接臭氧氧化呈惰性的额外化合物硬脂酸，并未显著改变反应动力学。结果表明，气 - 水界面处的油酸单层在气 - 水界面不会留下任何可检测到的物质，这与该期刊之前发表的工作相矛盾，作者现在认为之前的工作是错误的。此处给出的综合结果表明，气 - 水界面处不饱和表面活性材料与气相臭氧反应损失的动力学以及大气化学寿命，可以被认为与气 - 水界面或水性下层相中存在的其他材料无关。
