Near IR photolysis of HO 2 NO 2 : Implications for HO x

We report observations and calculations of peroxynitric acid, HO 2 NO 2 , in the stratosphere and upper troposphere. The simulations show that photolysis of HO 2 NO 2 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 HO 2 NO 2 , resolving a large discrepancy between standard model calculations and observations of HO 2 NO 2 at high‐latitude spring. The lower calculated abundance of HO 2 NO 2 reduces the efficiency of the OH + HO 2 NO 2 sink of HO x . Consideration of this process leads to large increases in calculated HO x (20 to 60%) for high‐latitude spring and better agreement with observed stratospheric abundances of HO x . Near IR photolysis of HO 2 NO 2 alters the coupling between NO x and HO x in stratospheric and upper tropospheric photochemical models.