The three-dimensional global distribution of OH over a year is calculated as a function of temperature, ultraviolet irradiance, and densities of H₂O, CO, O₃, CH₄, and NO_t, (defined as NO + NO₂ + NO₃ + 2N₂O₅ + HNO₂ + HNO₄). The concentration of OH is computed within a chemical tracer model (CTM) with an accuracy comparable to that of a detailed photochemical model. Distributions of CO, NO_t, O₃, CH₄, and the density of O₃ column were specified on the basis of observations. Meteorological fields were derived from the general circulation model developed at the Goddard Institute for Space Studies. The numerical method for parametrization of chemistry is described inSpivakovsky et al. (this issue). The CTM is used to simulate the global distribution of CH₃CCl₃. The computed distribution of OH implies a lifetime of 5.5 years for CH₃CCl₃ (obtained by relating the global burden of CH₃CCl₃ to the global loss, integrated using simulated three-dimensional distributions). Analysis of the long-term trend in CH₃CCl₃ as defined by observations suggests a lifetime of 6.2 years (consistent with Prinn et al. (1987)), indicating that model levels of OH may be too high by about 13%. This estimate for the lifetime depends on industry data for global emissions and on the absolute calibration of observations. It is argued that seasonal variations of CH₃CCl₃ provide an independent test for computed OH fields that is insensitive to the uncertainties in the budget of CH₃CCl₃. The annual cycle of CH₃CCl₃ from about 25°S to the South Pole is dominated by seasonal changes in OH. Observed seasonal variations of CH₃CCl₃indicate that the OH field south of 20°S±4° should be scaled by 0.75±0.25 from computed values, consistent with the result based on long-term trends. Reactions involving non-methane hydrocarbons were not included in the current model. These reactions could account for lower concentrations of OH than computed. Seasonal variations of CH₃CCl₃ in the tropics and in the northern mid-latitudes are dominated by effects of transport. If use of CH₃CCl₃ is phased out (as envisioned by the Montreal protocol), the dynamically driven seasonal variations of CH₃CCl₃ will decrease dramatically, whereas the chemically driven variations will remain proportional to the concentration of CH₃CCl₃; then the annual cycle of CH₃CCl₃ in northern mid-latitudes will provide a measure of OH as does at present the annual cycle in southern mid-latitudes. The influence of chemistry on the latitudinal distribution of CH₃CCl₃ is small and at present does not provide a constraint for the globally averaged OH or for the latitudinal distribution of OH. However, if emissions of CH₃CCl₃ were to cease, the tropical depression in the concentration of CH₃CCl₃ caused by high levels of OH in the tropics may provide an additional means to test OH models.

The equations which determine partitioning of Cl_x in steady state have multiple (three) solutions under conditions which might arise in the high-latitude winter stratosphere. Two of these solutions are stable, one is unstable, to infinitesimal perturbations. The relative stability of solutions is examined by subjecting the system to finite perturbations. The more stable solution is found to eliminate the less stable when semi-infinite volumes of the two solutions are placed in contact. The high-ClO, low-NO₂ solution is more stable under most conditions. Transitions from less to more stable states are slow in winter but may occur more rapidly when the seasonal variation of insolation is taken into account.

It is shown that the equations describing chemical partitioning among Cl_x (HCl, Cl, ClO, ClNO₃), NO_t (NO, NO₂, NO₃, N₂O₅, HNO₂, ClNO₃) and HO_x (OH, HO₂) may admit multiple solutions. These solutions apply to the high latitude winter stratosphere where abrupt spatial variations may be expected for NO₂, ClO and ClNO₃.

A global climatological distribution of tropospheric OH is computed using observed distributions of O₃, H₂O, NO_t (NO₂ +NO + 2N₂O₅ + NO₃ + HNO₂ +HNO₄), CO, hydrocarbons, temperature, and cloud optical depth. Global annual mean OH is 1.16×10⁶ molecules cm⁻³(integrated with respect to mass of air up to 100 hPa within ±32° latitude and up to 200 hPa outside that region). Mean hemispheric concentrations of OH are nearly equal. While global mean OH increased by 33% compared to that from Spivakovsky et al. [1990], mean loss frequencies of CH₃CCl₃ and CH₄ increased by only 23% because a lower fraction of total OH resides in the lower troposphere in the present distribution. The value for temperature used for determining lifetimes of hydrochlorofluorocarbons (HCFCs) by scaling rate constants [Prather and Spivakovsky, 1990] is revised from 277 K to 272 K. The present distribution of OH is consistent within a few percent with the current budgets of CH₃CCl₃ and HCFC-22. For CH₃CCl₃, it results in a lifetime of 4.6 years, including stratospheric and ocean sinks with atmospheric lifetimes of 43 and 80 years, respectively. For HCFC-22, the lifetime is 11.4 years, allowing for the stratospheric sink with an atmospheric lifetime of 229 years. Corrections suggested by observed levels of CH₂Cl₂ (annual means) depend strongly on the rate of interhemispheric mixing in the model. An increase in OH in the Northern Hemisphere by 20% combined with a decrease in the southern tropics by 25% is suggested if this rate is at its upper limit consistent with observations of CFCs and ⁸⁵Kr. For the lower limit, observations of CH₂Cl₂ imply an increase in OH in the Northern Hemisphere by 35% combined with a decrease in OH in the southern tropics by 60%. However, such large corrections are inconsistent with observations for ¹⁴CO in the tropics and for the interhemispheric gradient of CH₃CCl₃. Industrial sources of CH₂Cl₂ are sufficient for balancing its budget. The available tests do not establish significant errors in OH except for a possible underestimate in winter in the northern and southern tropics by 15–20% and 10–15%, respectively, and an overestimate in southern extratropics by ∼25%. Observations of seasonal variations of CH₃CCl₃, CH₂Cl₂,¹⁴CO, and C₂H₆ offer no evidence for higher levels of OH in the southern than in the northern extratropics. It is expected that in the next few years the latitudinal distribution and annual cycle of CH₃CCl₃ will be determined primarily by its loss frequency, allowing for additional constraints for OH on scales smaller than global.