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 8.4×10⁵ 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.