"Aerosol catalysis" is shown to be a powerful tool for investigating the catalytic properties of freshly formed nanoparticles in situ and without substrate interference. The method is first outlined conceptually, followed by an illustrative application to the catalytic formation of methane on a nickel nanoaerosol. Reaction order and activation energy were found conform with generally accepted values from supported Ni catalysts. The TOR decreases strongly during the first 10's as the reaction proceeeds toward a steady value. The decrease correlates with a buildup of about 0.3 monolayer equivalents of carbon on the particle surface measured by TGA and a decline in particle photoelectric activity observed via measurement by aerosol photoemission spectroscopy (APES). APES is shown to be capable of detecting the progressive degradation of the freshly formed particle surface due to a heterogeneous surface reaction on a millisecond time scale. Furthermore, it was possible to induce order-of-magnitude changes in TOR via defined changes in particle morphology, induced by aerosol restructuring techniques preceding exposure to the catalytic reaction.