Atmospheric aerosol particles contain a wide variety of chromophores that, by absorbing solar radiation, trigger physical and chemical transformations and contribute to the heating of the atmosphere. Optical confinement effects within particles can lead to enhancement of the overall light intensity and to structuring of the internal optical fields¹, which influence atmospheric photochemistry and radiative forcing.

Here, we study the influence of optical confinement effects on the photolysis of iron (III) citrate (FeCit) in individual particles. FeCit is a chromophore that is photolyzed by UV light, resulting in the reduction of Fe (III) to Fe (II). Experiments were performed at the PolLux beamline of the Swiss Light Source by means of scanning transmission x-ray microscopy coupled to near edge x-ray absorption fine structure spectroscopy (STXM/NEXAFS). Particles containing FeCit were generated, dried, size selected (640 nm in diameter), and deposited on x-ray transparent silicon nitride windows. Samples were placed in the PolLux end station and they were illuminated in vacuo by a monochromatic UV light source (367.7 nm) located orthogonal to the X-ray beam. Composition maps of individual particles were recorded at the iron L-edge (from 700 to 720 eV) with an image resolution of 25x25 nm. Maps of the iron oxidation state were retrieved using the parameterization described in Moffet et al.² The Fe (III) / Fe (II) maps show inhomogeneous photochemically-generated patterns within individual particles. To analyse the experimental data, we have constructed a 3D particle model that combines light intensity simulations with DDA (discrete dipole approximation) with a photochemical model. Good agreement with the STXM/NEXAFS experimental results was found.

To obtain a better picture of the influence of optical confinement effects in the atmosphere, we simulated the overall amplification of the Earth actinic radiation for a range of particles with typical sizes and compositions of aerosol particles in the atmosphere. On average, the light intensity inside such particles is enhanced by a factor of about 3, which implies an overall non-negligible acceleration of photochemical reactions in atmospheric particles. This acceleration of photochemical reactions might be more important than previously anticipated and should thus be considered in atmospheric aerosol and cloud models.

[1] Johannes W. Cremer, Klemens M. Thaler, Christoph Haisch, Ruth Signorell, Nat. Commun., 2016, 7, 10941 

[2] Ryan C. Moffet, Hiroshi Furutani, Tobias C. Rödel, Tobias R. Henn, Peter O. Sprau, Alexander Laskin, Mitsuo Uematsu, Mary K. Gilles, J. Geophys. Res., 2012, 117, D07204