Measurements of Diffusion Coefficients of Organic Dyes in Proxies of Atmospheric Particles and Comparison with Predictions Using the Stokes-Einstein Equation

Because of their high impact on the environment, particles consisting of secondary organic material (SOM) have moved into the centre of attention in atmospheric research. These SOM particles are formed in the atmosphere and can influence the Earth's climate directly and indirectly by absorption and scattering of solar radiation and acting as ice and cloud nuclei. Furthermore, it has been found that SOM particles can affect the human health and reduce air quality e.g. visibility significantly.

To better understand the effect of SOM particles on the Earth's climate, several characteristics of SOM particles of different compositions have been studied. Diffusion rates are essential for the investigation of processes such as particle growth and the rate of heterogeneous reactions within SOM particles. In the past, diffusion rates of SOM particles have been estimated by converting viscosity values into diffusion coefficients using the Stokes-Einstein relation.

Here, we present values of diffusion coefficients, which were measured directly using the fluorescence microscopy method FRAP (fluorescence recovery after photobleaching), of organic dyes in organic-water proxies of atmospheric particles. Fluorescent dyes of varying sizes and molecular weights (320-1.50x10⁵ g/mol) were used and diffusion coefficients were measured across a wide range of relative humidities (40-93% RH). These measurements add to the limited amount of data on the diffusion rate constants of organics within organic-water solutions.