We report observations and calculations of peroxynitric acid, HO2NO2, in the stratosphere and upper troposphere. The simulations show that photolysis of HO2NO2 via excitation of purely vibrational modes at wavelengths longward of 760 nm (the near IR) can dominate loss of this species. Consideration of this photolytic pathway reduces calculated HO2NO2, resolving a large discrepancy between standard model calculations and observations of HO2NO2 at high-latitude spring. The lower calculated abundance of HO2NO2 reduces the efficiency of the OH + HO2NO2 sink of HOx. Consideration of this process leads to large increases in calculated HOx (20 to 60%) for high-latitude spring and better agreement with observed stratospheric abundances of HOx. Near IR photolysis of HO2NO2 alters the coupling between NOx and HOx in stratospheric and upper tropospheric photochemical models.