Near IR photolysis of HO_2NO_2: Implications for HO_x

We report observations and calculations of peroxynitric acid, HO_2NO_2, in the stratosphere and upper troposphere. The simulations show that photolysis of HO_2NO_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_2NO_2, resolving a large discrepancy between standard model calculations and observations of HO_2NO_2 at high-latitude spring. The lower calculated abundance of HO_2NO_2 reduces the efficiency of the OH + HO_2NO_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_2NO_2 alters the coupling between NO_x and HO_x in stratospheric and upper tropospheric photochemical models