Chemical oxidation in the atmosphere removes pollutants and greenhouse gases but generates undesirable products such as secondary organic aerosol. Radicals are key intermediates in oxidation, but how they interact with aerosols is still not well understood. Here we use a laser to measure the loss of radicals onto oxidised aerosols generated in a smog chamber. The loss of radicals was controlled by the thickness or viscosity of the aerosols, confirmed by using sugar aerosols of known thickness.