Tropospheric O 3 from photolysis of O 2

Photolytic dissociation of molecular oxygen (O2) at wavelengths about 205 nm produces ozone (O3) in the upper tropical troposphere. In tropospheric chemistry models that ignore this process, the O3 abundance above 14 km in the tropics (a.k.a. Tropopause Transition Layer) is underestimated by 5 to 20 ppb. Even for models including O2 photolysis, uncertainty in the O2 cross sections yields similar uncertainty in TTL O3. The related impact on global atmospheric chemistry is small, i.e., ±0.2% in CO and CH4 budgets, but the change in the O3 column, ±1.6 DU in the tropics, may be important in calculating heating rates and climate forcing