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Atmospheric aerosol particles with a high viscosity may become inhomogeneously mixed during chemical processing. Models have predicted gradients in condensed phase reactant concentration throughout particles as the result of diffusion and chemical reaction limitations, termed chemical gradients. However, these have never been directly observed for atmospherically relevant particle diameters. We investigated the reaction between ozone and aerosol particles composed of xanthan gum and FeCl 2 and observed the in situ chemical reaction that oxidized Fe 2+ to Fe 3+ using X-ray spectromicroscopy. Iron oxidation state of particles as small as 0.2 μm in diameter were imaged over time with a spatial resolution of tens of nanometers. We found that the loss off Fe 2+ accelerated with increasing ozone concentration and relative humidity, RH. Concentric 2-D column integrated profiles of the Fe 2+ fraction, α , out of the total iron were derived and demonstrated that particle surfaces became oxidized while particle cores remained unreacted at RH = 0–20%. At higher RH, chemical gradients evolved over time, extended deeper from the particle surface, and Fe 2+ became more homogeneously distributed. We used the kinetic multi-layer model for aerosol surface and bulk chemistry (KM-SUB) to simulate ozone reaction constrained with our observations and inferred key parameters as a function of RH including Henry's Law constant for ozone, H O3 , and diffusion coefficients for ozone and iron, D O3 and D Fe , respectively. We found that H O3 is higher in our xanthan gum/FeCl 2 particles than for water and increases when RH decreased from about 80% to dry conditions. This coincided with a decrease in both D O3 and D Fe . In order to reproduce observed chemical gradients, our model predicted that ozone could not be present further than a few nanometers from a particle surface indicating near surface reactions were driving changes in iron oxidation state. However, the observed chemical gradients in α observed over hundreds of nanometers must have been the result of iron transport from the particle interior to the surface where ozone oxidation occurred. In the context of our results, we examine the applicability of the reacto-diffusive framework and discuss diffusion limitations for other reactive gas-aerosol systems of atmospheric importance.