Toward a better understanding of ferric-oxalate complex photolysis: The role of the aqueous/air interface of droplet.

In this work, the photo reactivity of ferric oxalate (Fe(III)-Ox) complex in atmospheric particles was investigated. Raman spectroscopy was used to explore the mechanism and kinetics of Fe(III)-Ox photolysis occurring at the aqueous/gas interface, inside the droplet and in bulk solution. Ferrous carbonate (FeCO 3) was detected indicating that carbonate ion (CO 3 2−) formed inside the droplets would compete with oxalate ligands for iron complexation. A higher concentration of photoproduct Fe(II)-Ox was observed at the surface and inside of the droplets than in bulk solution. In particular, Fe(III)-Ox on the droplet surface was quickly reduced with light and Fe(II)-Ox concentration gradually decreased with irradiation time. The evolution of Fe(II)-Ox concentration was similar inside the droplet and in bulk solution with a trend of first increasing and then gradually decreasing during irradiation time. Although FeCO 3 would hinder Fenton intermediate reaction, the photolysis rate of Fe(III)-Ox in droplets was almost two orders of magnitude times faster than that observed during bulk experiment. In general, the photolysis mechanism and kinetics of Fe(III)-Ox in aqueous/air interface, inside of droplet and bulk solution were distinct, and the production of oxide species from the atmospheric Fe(III)-Ox droplets was underestimated. The reaction mechanism diagram in the Fe(III)-Ox droplet. [Display omitted] • The photolysis rate of ferric oxalate for a droplet was approximately 100 times that in bulk solution. • Fast and intense photoreduction of ferric oxalate occurs at the aqueous/air interface of droplet. • For the first time, the formation of FeCO 3 in atmospheric droplets was reported. [ABSTRACT FROM AUTHOR]