Imaging Photoactive and Viscous Organic Aerosol Particles

Abstract:

Heavy aerosol loading threatens human health across the globe and is typically related to photochemical processing and emission of organic, inorganic and trace metal compounds. Aerosol particles dominated by organic solutes may attain a high or ultra-high viscosity (<10¹² Pa s) becoming solid-like in cold and dry air, limiting diffusion of organic and reactive molecules through the particle volume thus slowing chemistry. In contrast, radicals can be produced through the bulk of photoactive particles irrespective of diffusion limitations. We investigated iron oxidation state changes in particles containing citric acid and iron(III) citrate using a new photochemical environmental X-ray spectromicroscopy microreactor, a coated wall flow tube, and an electrodynamic balance. Chemical images of aerosol particles with resolution currently as low as 35 × 35 nm were acquired in the microreactor revealing spatial gradients in the concentration of iron(II) and iron(III) compounds. We have also quantified the release of radicals, and particle mass loss due to decarboxylation subsequent to ligand to metal charge transfer. We have found striking evidence that particles and films quickly became anoxic due to reactions with organic radical species, despite being in an O₂ atmosphere. Atmospheric aerosol anoxia is highly unexpected since the time it takes for small molecules, such as water and possibly oxygen, to diffuse and mix throughout non-reactive viscous particles is thought to be on the order of seconds to minutes at ambient conditions. However, we found that it is the combined effect of reaction and diffusion limitations that compounds such as reactive oxygen species and carbon-centered free radicals can persist and accumulate in aerosol particles. Our results are highly important for the accurate prediction of aerosol phase radical generation and chemical loss of oxygenated organic aerosol dominated by carboxyl functionalities. These findings are important for other photochemically active species commonly found in the atmosphere, implying a ubiquity of persistent radicals in airborne particles.