Significantly Accelerated Hydroxyl Radical Generation by Fe(III)-Oxalate Photochemistry in Aerosol Droplets

Fe(III)-oxalate complexes are ubiquitous in atmospheric environments, which can release reactive oxygen species (ROS) such as H2O2, O center dot 2-, and OH center dot under light irradiation. Although Fe(III)-oxalate photochemistry has been investigated extensively, the understanding of its involvement in authentic atmospheric environments such as aerosol droplets is far from enough, since the current available knowledge has mainly been obtained in bulk-phase studies. Here, we find that the production of OH center dot by Fe(III)-oxalate in aerosol microdroplets is about 10-fold greater than that of its bulk-phase counterpart. In addition, in the presence of Fe(III)-oxalate complexes, the rate of photo-oxidation from SO2 to sulfate in microdroplets was about 19-fold faster than that in the bulk phase. The availability of efficient reactants and mass transfer due to droplet effects made dominant contributions to the accelerated OH center dot and SO42- formation. This work highlights the necessary consideration of droplet effects in atmospheric laboratory studies and model simulations.

Automated summary

Fe(III)-oxalate complexes are commonly found in atmospheric environments, releasing reactive oxygen species under light irradiation. However, their role in aerosol droplets has not been well understood. This study reveals that Fe(III)-oxalate produces significantly more OH radicals and enhances the photo-oxidation of SO2 in aerosol microdroplets compared to bulk-phase conditions. Droplet effects, including efficient reactants and mass transfer, contribute to the accelerated formation of OH radicals and sulfate. These findings emphasize the importance of considering droplet effects in atmospheric laboratory studies and model simulations.