Clara-Magdalena Saak

Heterogeneous Photochemistry at Atmospheric Aqueous Surfaces

Accurate climate models are absolutely essential in order to design climate change policy, as well as to model the changes that will occur in the environment. A central part of these models is understanding the radiative forcing of difference atmospheric components, in particular those from anthropogenic sources. Greenhouse gases such as CO2 have a pronounced warming effect (positive radiative forcing), whereas aerosols have a net cooling effect (negative radiative forcing) due to their ability to act as cloud condensation nuclei and to reflect incoming solar radiation. In order to determine the efficiency of these micron sized particles to nucleate cloud condensation, one needs to accurately model their hygroscopicity, i.e. how easily they take up water, over time. A particle’s hygroscopicity strongly depends on the composition of the aerosol particle, which can vary depending not only on the environment the aerosol droplet is in, but also on the degree of particle ageing. Ageing mainly describes the successive oxidation of compounds contained within the aerosol. In order to describe how the composition of the particle changes over time, which in turn affects the hygroscopicity of the particle, one needs to be able to accurately describe the ageing process. Since the behaviour of aerosols in the atmosphere is highly complex and depends on many factors their effective radiative forcing is currently the largest source of uncertainty in predictive climate models. A deeper understanding of the influence of aerosols on atmospheric processes and chemistry is therefore required to aid in the development of more reliable climate predictions.

In aerosols, the surface is a particularly important reaction site, since many reactive species such as larger organic molecules and even smaller halide ions are surface enriched. The surface also allows for the heterogeneous reaction of these solvated molecules with highly oxidising compounds in the gas phase, such as ozone. It has even been suggested that the efficiency of some reactions may be enhanced at the aqueous surface. Therefore, the aerosol surface may be regarded as a unique chemical environment with its own reactive landscape, distinct from that of the bulk.

Characterising the molecular properties and reactions at an interface is a challenging task, as one needs a probe that is sensitive to the molecular boundary layer only and excludes signal from the bulk. Sum frequency generation (SFG) spectroscopy is one of the few tools that fulfil these criteria, allowing us to build a detailed understanding of the surface structure. Using time-resolved SFG we can also follow photochemical reactions at the aqueous interface, characterise reactive intermediates and study effects such as surface enhanced reactivity.

Providing a fundamental, molecular level understanding of the aqueous surface and its chemistry is essential to improve the physical science basis of the climate models necessary to accurately judge the impact of potential climate change interventions, which are becoming increasingly important.