Although the specific reactions and mechanisms in fine aerosols could not be explicitly determined from this field study, our results provide isotopic evidence that local daily photochemical reduction of divalent Hg is of critical importance to the fate of PM_2.5-Hg in urban atmospheres and that, in addition to variation in sources, photochemical reduction appears to be an important process that affects both the particle mass-specific abundance and isotopic composition of PM_2.5-Hg.