Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK.
Phys Chem Chem Phys. 2012 Jun 14;14(22):8023-31. doi: 10.1039/c2cp24094g. Epub 2012 Apr 25.
Atmospheric aerosol particles are important in many atmospheric processes such as: light scattering, light absorption, and cloud formation. Oxidation reactions continuously change the chemical composition of aerosol particles, especially the organic mass component, which is often the dominant fraction. These ageing processes are poorly understood but are known to significantly affect the cloud formation potential of aerosol particles. In this study we investigate the effect of humidity and ozone on the chemical composition of two model organic aerosol systems: oleic acid and arachidonic acid. These two acids are also compared to maleic acid an aerosol system we have previously studied using the same techniques. The role of relative humidity in the oxidation scheme of the three carboxylic acids is very compound specific. Relative humidity was observed to have a major influence on the oxidation scheme of maleic acid and arachidonic acid, whereas no dependence was observed for the oxidation of oleic acid. In both, maleic acid and arachidonic acid, an evaporation of volatile oxidation products could only be observed when the particle was exposed to high relative humidities. The particle phase has a strong effect on the particle processing and the effect of water on the oxidation processes. Oleic acid is liquid under all conditions at room temperature (dry or elevated humidity, pure or oxidized particle). Thus ozone can easily diffuse into the bulk of the particle irrespective of the oxidation conditions. In addition, water does not influence the oxidation reactions of oleic acid particles, which is partly explained by the structure of oxidation intermediates. The low water solubility of oleic acid and its ozonolysis products limits the effect of water. This is very different for maleic and arachidonic acid, which change their phase from liquid to solid upon oxidation or upon changes in humidity. In a solid particle the reactions of ozone and water with the organic particle are restricted to the particle surface and hence different regimes of reactivity are dictated by particle phase. The potential relevance of these three model systems to mimic ambient atmospheric processes is discussed.
大气气溶胶粒子在许多大气过程中很重要，如光散射、光吸收和云形成。氧化反应不断改变气溶胶粒子的化学成分，特别是有机质量成分，通常是主要成分。这些老化过程了解甚少，但已知会显著影响气溶胶粒子的云形成潜力。在这项研究中，我们研究了湿度和臭氧对两种模型有机气溶胶系统的化学组成的影响：油酸和花生四烯酸。这两种酸也与马来酸进行了比较，马来酸是我们之前使用相同技术研究过的气溶胶系统。相对湿度在三种羧酸的氧化方案中的作用非常复杂。相对湿度对马来酸和花生四烯酸的氧化方案有很大影响，而油酸的氧化则没有观察到这种依赖性。在马来酸和花生四烯酸中，只有当粒子暴露在高相对湿度下时，才能观察到挥发性氧化产物的蒸发。颗粒相强烈影响颗粒处理，水对氧化过程的影响。油酸在室温下（干燥或升高的湿度、纯或氧化的颗粒）在所有条件下都是液态。因此，臭氧可以很容易地扩散到颗粒的主体中，而不管氧化条件如何。此外，水不会影响油酸颗粒的氧化反应，这部分可以用氧化中间体的结构来解释。油酸及其臭氧化产物的低水溶性限制了水的影响。这与马来酸和花生四烯酸非常不同，它们在氧化或湿度变化时从液态变为固态。在固态颗粒中，臭氧和水与有机颗粒的反应仅限于颗粒表面，因此颗粒相决定了不同的反应性范围。讨论了这三种模型系统模拟环境大气过程的潜在相关性。
