As well as gases, the atmosphere also contains solid particles and liquid droplets. This particulate matter usually arises as a result of complex chemical reactions from pollutants. Organic films can form on this particulate matter, altering its properties which can, in turn, affect climate and rainfall.
An international research team including scientists from ISIS Neutron and Muon Source and the Central Laser Facility set out to understand more about this organic layer formed on atmospheric particulates. In particular, they looked at the effect of oxidation caused by interaction with naturally occurring ozone gas. Oxidation is a natural process; however, oxidation with ozone does not completely remove the layer.
In this study, published in Phys. Chem. Chem. Phys., the group investigated the kinetics of the oxidation of oleic acid as a model of this organic layer. An atmospheric particulate is complex as it's made up of many organic and inorganic compounds. To replicate this, they mixed oleic acid with fatty acids, like stearic acid, to mimic atmospheric conditions.
For the study, they made monolayers of oleic acid and oleic/stearic acid mixtures and exposed them to ozone. Using ISIS instrument SURF and FIGARO at the Institut Laue-Langevin (ILL), they could measure the thickness and composition of the built-up oleic acid layers.
The researchers found that the viscosity and ionic strength of oleic acid was independent of the other organic species that were present, such as stearic acid, in the monolayer.
The atmospheric chemistry of aerosol particulate matter is only just now being studied in detail. Studies such as this one allows atmospheric scientists to model the behavior of the organic films and predict how long the oxidation-related process takes.
As we previously mentioned, atmospheric aerosols are coated with pollutants. This coating in turn may limit the transport of water to and from the droplets, which affects the size of the droplets and hence cloud formation. The coating also changes the amount of light that a coated particle may reflect back to space, thus effecting the global climate. Instruments like SURF can be used to advance research on atmospheric aerosols' influence on cloud condensation.
The full paper can be found here.