We investigated photochemistry of a secondary organic aerosol under three different conditions: in a dilute aqueous solution mimicking cloud droplets, in a solution of concentrated ammonium sulfate mimicking deliquesced aerosol, and in an organic matrix mimicking dry organic aerosol. We find that rate and mechanisms of photochemistry depend sensitively on these conditions, suggesting that the same organic aerosol compounds will degrade at different rates depending on their local environment.