Three-dimensional climatological distribution of tropospheric OH:
update and evaluation.


C. M. Spivakovsky1, J. A. Logan1, S. A. Montzka2, Y. J. Balkanski3, M. Foreman-Fowler4,
D. B. A. Jones1, L.W. Horowitz5, A. C. Fusco1, C. A. M. Brenninkmeijer6,
M. J. Prather7, S. C. Wofsy1, and M.B. McElroy1.
1 Harvard University, Cambridge, MA.
2 NOAA Climate Monitoring and Diagnostics Laboratory, Boulder, CO.
3 Laboratoire des Sciences du Climat, France.
4 University of Colorado, Boulder, CO.
5 National Center for Atmospheric Research, Boulder, CO.
6 Max Plank Institute for Chemistry, Mainz, Germany.
7 University of California, Irvine, CA.

Submitted to the J. Geophys. Res., March, 1999.

ABSTRACT

A global climatological distribution of tropospheric OH is computed using observed distributions of O3, H2O, NOt (NO2 + NO + 2N2O5 + NO3 + HNO2 + HNO4), CO, hydrocarbons and cloud optical depth. The global annual mean concentration of OH in the troposphere is 1.16 × 106 molecules cm-3 (integrated with respect to mass of air). Mean hemispheric concentrations of OH are nearly equal (within 1%). 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 HCFCs by scaling rate constants [Prather and Spivakovsky, 1990] is revised from 277K to 272K. The present distribution of OH is consistent within a few percent with the present budgets of CH3CCl3 and HCFC-22. For CH3CCl3, it results in a lifetime of 4.7 years, including stratospheric and ocean sinks with atmospheric lifetimes of 43 and 78 years, respectively. For HCFC-22, the computed lifetime is 11.4 years, allowing for the stratospheric sink with an atmospheric lifetime of 229 years. Observed levels of CH2Cl2 (annual means) suggest that no correction of hemispheric abundances of OH is necessary if the rate of interhemispheric mixing in the model is increased to the upper limit consistent with observations of CFCs and 85Kr. If this rate is at its lower limit, an increase in OH in the northern hemisphere by 35% combined with a decrease in OH in the southern tropics by 60% is suggested by observations of CH2Cl2. 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 reveal 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 15%-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.

The full text of this paper is available as a PostScript file