OH concentrations from a general circulation model coupled with a tropospheric chemistry model

A comprehensive general circulation model has been coupled with a tropospheric chemistry model (TCM) using 'on-line' actinic flux calculation. The three-dimensional global distribution of OH was calculated and is presented in some detail. A 2-year integration generated a volume- and pressure-weighted global, tropospheric, annual mean OH concentration of 8.4 x 105 molecules/cm3. Over 70 gas phase reactions involving 28 chemical species were solved, using a two-step backward differentiation formula (BDF) combined with Gauss-Seidel iteration. The set of chemical equations was solved every model hour. 'On-line' actinic flux calculation allows for photo-radiation feedback between the two model components. Local changes in clouds and radiatively active gas concentrations directly affect the availability of actinic flux which has a direct impact on photochemistry through the photolysis rate constant. The actinic flux was efficiently calculated in each grid cell every model hour by the delta-Eddington radiation scheme of the general circulation model. The spectral resolution of the radiation scheme was 5 nm between 200 and 400 nm, 2 nm between 245 and 350 nm, and 25 nm between 350 and 700 nm. This provided for accurate calculations in the photolytically active spectral regions of O3 and NO2. (C) 2000 Elsevier Science Ltd.; A comprehensive general circulation model has been coupled with a tropospheric chemistry model (TCM) using `on-line' actinic flux calculation. The three-dimensional global distribution of OH was calculated and is presented in some detail. A 2-year integration generated a volume- and pressure-weighted global, tropospheric, annual mean OH concentration of 8.4×105 molecules/cm3. Over 70 gas phase reactions involving 28 chemical species were solved, using a two-step backward differentiation formula (BDF) combined with Gauss-Seidel iteration. The set of chemical equations was solved every model hour. `On-line' actinic flux calculation allows for photo-radiation feedback between the two model components. Local changes in clouds and radiatively active gas concentrations directly affect the availability of actinic flux which has a direct impact on photochemistry through the photolysis rate constant. The actinic flux was efficiently calculated in each grid cell every model hour by the delta-Eddington radiation scheme of the general circulation model. The spectral resolution of the radiation scheme was 5 nm between 200 and 400 nm, 2 nm between 245 and 350 nm, and 25 nm between 350 and 700 nm. This provided for accurate calculations in the photolytically active spectral regions of O3 and NO2.