In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained through sunrise and sunset in the lower stratosphere during SPADE are compared to results from a photochemical model constrained by observed concentrations of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N₂O₅ on sulfate aerosols agree with measured concentrations of NO, NO₂, and ClO throughout the day, but fail to account for high concentrations of OH and HO₂ observed near sunrise and sunset. The morning burst of [OH] and [HO₂] coincides with the rise of [NO] from photolysis of NO₂, suggesting a new source of HO_xthat photolyzes in the near UV (350 to 400 nm) spectral region. A model that allows for the heterogeneous production of HNO₂ results in an excellent simulation of the diurnal variations of [OH] and [HO₂].