Fig. 1: Imaged photochemical and oxidative cycling of iron(III)-citrate (FeIIICit) inside single aerosol particles.
From: Photolytic radical persistence due to anoxia in viscous aerosol particles
a Sketch of FeIIICit photochemical cycling in a particle oxidized near its surface (green) and anoxic in its bulk (gray). Purple, blue, and red text indicate specific species discernable using scanning transmission X-ray microscopy coupled to near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS), an electrodynamic balance (EDB) and coated wall flow tube (CWFT) methods, respectively. Solid lines are chemical reactions, e.g., ligand-to-metal charge transfer (LMCT) reaction, dashed lines are equilibria conditions, and dash-dotted lines indicate gas evaporation or uptake. Citrate, the iron(II)-citrate complex, and CH2COOH are abbreviated as Cit3-, FeIIHCit, and R, respectively. b A STXM/NEXAFS image of the iron(III) fraction, β, in particles directly showing reoxidation only near the surface of particles. The image was smoothed by increasing the pixel resolution and interpolating. c–e Chemical gradients inside particles as a function of time, t (color coded), and relative humidity, RH, during dark reoxidation at 20 °C after 15 min of UV light exposure. At RH = 40% (c), dark reoxidation occurred slowly while maintaining gradients in β. Reoxidation at RH = 50% (d) and 60% (e) was faster, implying that particles took up more water resulting in greater diffusion coefficients, hence faster molecular transport and oxidation reaction. Gray symbols are the measured initial iron(III) fraction, β0, from multiple images acquired before UV light exposure. Example error bars, Δβ = ±0.07, are shown. Solid colored lines are derived from the photochemical reaction and diffusion (PRAD) model and reproduce our observations. Details on model sensitivity are given in the Supplementary Information (Supplementary Fig. 3).
