At the Swiss Light Source (SLS), a new photochemical reaction cell was developed for the X-ray microscope at the PolLux beamline. This allowed the researchers to mimic sunlight mediated chemical reactions in airborne particles we normally inhale. Utilizing the new reaction cell, the X-ray microscope was used to image the interior of particles for the chemistry that produced a high concentration of persistent carbon centered radicals (CCR) and reactive oxygen species (ROS), which are harmful compounds when inhaled and can cause damage in the respiratory tract. Two main factors were 1) a very high particle viscosity that effectively locks the CCRs in a glass-like state and 2) oxygen deficiency, or anoxia, to prevent smaller ROS to be formed with a shorter lifetime that easily diffuse out of the particle before inhalation. When relative humidity in air is <60%, particles can become highly viscous or even glass-like, which drastically reduces the mobility of all molecules. Although sunlight induced radical formation is likely to be unhindered, high viscosity would instead inhibit molecular diffusion and block oxygen from accessing the particle interior. This leads to preservation of large amounts of radicals. Amazingly, this may apply to all organic light absorbing atmospheric compounds making radical abundance and persistence an unforeseen issue until now.
Particles composed of citric acid and iron were investigated as a model for iron containing organic particles. About 1 in 20 airborne particles contain iron in urban areas at a significant concentration as identified by previous studies. The oxidation state of iron was mapped across individual particles using X-ray spectromicroscopy to reveal where photochemical reactions, oxidation and molecular diffusion took place inside. Oxidation and formation of ROS took place rapidly, but surprisingly, only near the particle surfaces, i.e. an oxidized reaction front extending only hundreds of nanometers was directly observed. This was entirely due to the rapid depletion of oxygen in the particle due to slow molecular transport and fast reaction cycling. In addition to X-ray microscopy, the researchers used an electrodynamic balance (collaboration with ETHZ) and a coated wall flow tube reactor to study these radical forming particles and constrain the overall reactive cycle and the production and release of radicals to air.
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Image: A chemical scheme and X-ray image showing particles oxidized only near their surface. Light in iron-organic particles start a cycle of oxidizing reactions (purple text) forming carbon centered radicals (yellow text) and reactive oxygen species (red text). We directly imaged oxidation happening only near the particle surfaces indicated by the brighter colour in micrometer and submicrometer viscous particles in the right image.