Dust-Catalyzed Oxidative Polymerization of Catechol and Its Impacts on Ice Nucleation Efficiency and Optical Properties

Dust is the major source of iron in atmospheric aerosols but little is known about its role in catalyzing polymerization reactions of organics in particles. Using Arizona Test Dust (AZTD) and hematite nanoparticles as laboratory standards and proxies for hematite-rich natural dust, respectively, we show that their reactions with catechol in aqueous slurries lead to the formation of black polycatechol. This observation is in contrast to oxalate and sulfate which form surface complexes promoting the dissolution of iron from the dust particles. Results from ultraviolet-visible spectroscopy and microscopy/elemental mapping show that the formation of polycatechol changed the optical properties of the dust particles and surface chemical composition. Results from ice nucleation studies using a droplet freezing technique show that polycatechol did not significantly impact ice nucleation or block ice nucleation sites on AZTD. In contrast, increasing pH decreased the ice nucleation ability of AZTD. These results highlight the complexity of iron's role in aerosol aging processes, brown carbon formation, and ice nucleation.