Davies James F, Wilson Kevin R
Chemical Sciences Division , Lawrence Berkeley National Laboratory , 1 Cyclotron Road , Berkeley , CA 94720 , USA . Email:
Chem Sci. 2015 Dec 1;6(12):7020-7027. doi: 10.1039/c5sc02326b. Epub 2015 Sep 8.
A key but poorly understood chemical process is how gas phase uptake is governed by the relative mobility of molecules at an interface of an atmospheric aerosol. Citric acid (CA), a model system for oxygenated organic aerosol, is used to examine how changes in viscosity, due to changing water content, govern the reactive uptake of gas phase hydroxyl radicals (OH). By comparing the reaction kinetics measured when probing the outer aerosol surface layers with measurements of the bulk particle composition, the effective OH reaction probability is observed to be a complex and non-linear function of the relative humidity (RH). At RH < 50%, the reactive decay of CA is controlled by the viscosity of the particle, where the depletion of CA and the formation of reaction products occurs over a narrow region near the aerosol interface, on the order of 8 nm at 20% RH. At RH = 50% the reaction zone increases to the particle dimensions ( ∼50 nm) and at RH > 50%, the aerosol becomes aqueous and well-mixed on the timescale of the heterogeneous reaction. These results imply that in the atmosphere, the formation and dissipation of interfacial chemical gradients could be significant in viscous and semisolid aerosol and play important roles altering gas-particle partitioning and aging mechanisms ( bulk interface).
一个关键但却鲜为人知的化学过程是气相吸收如何受大气气溶胶界面处分子相对迁移率的控制。柠檬酸（CA）作为含氧有机气溶胶的模型体系，用于研究由于含水量变化导致的粘度变化如何控制气相羟基自由基（OH）的反应性吸收。通过比较探测气溶胶外层表面时测得的反应动力学与颗粒整体组成的测量结果，发现有效OH反应概率是相对湿度（RH）的复杂非线性函数。在RH < 50%时，CA的反应性衰减受颗粒粘度控制，CA的消耗和反应产物的形成发生在气溶胶界面附近的一个狭窄区域，在20%RH时约为8纳米。在RH = 50%时，反应区增大到颗粒尺寸（约50纳米），而在RH > 50%时，气溶胶在非均相反应的时间尺度上变成水相且充分混合。这些结果表明，在大气中，界面化学梯度的形成和消散在粘性和半固体气溶胶中可能很显著，并在改变气粒分配和老化机制（整体 界面）方面发挥重要作用。
