Department of Earth System Science

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.