Atmospheric (CO)-C-14: A tracer of OH concentration and mixing rates

Time series measurements of the ground level (CO)-C-14 concentration were m ade at Olympic Peninsula, Washington (48 degrees N), and Point Barrow, Alas ka (71 degrees N), between 1991 and 1997. Measurements of the meridional gr adients of the (CO)-C-14 concentration at sea level were made during five o ceanographic cruises in the Pacific Ocean between 55 degrees N and 65 degre es S during 1991-1995. These measurements were combined with earlier time s eries measurements of atmospheric (CO)-C-14 at 41 degrees S and 77 degrees S [Brenninkmeijer, 1993] and at 13 degrees N [Mak and Southon, 1998] and me ridional transects of (CO)-C-14 at 6-8 km [Mak et al., 1994]. These (CO)-C- 14 data sets were analyzed using a two-dimensional atmospheric circulation and chemistry model in order to determine the tropospheric OH concentration that could explain the temporal and spatial trends in (CO)-C-14. Additiona lly, the interannual trend in tropospheric methyl chloroform concentration and the stratospheric time history of bomb (CO2)-C-14 were simulated by the model. The results of this analysis indicate that an average tropospheric OH concentration of similar to 10x10(5) radicals cm(-3) explains both the ( CO)-C-14 and methyl chloroform trends. The model-predicted (CO)-C-14 concen trations, however, are sensitive to the rate of stratosphere-troposphere ex change and horizontal mixing in the troposphere. Model predictions of tropo spheric (CO)-C-14 at high latitudes improved when the stratosphere-troposph ere exchange rate was slowed, based on the results of the stratospheric bom b (CO2)-C-14 model simulation. Substantial improvement in the model (CO)-C- 14 simulations occurred with increased horizontal diffusion rates in the tr oposphere and lower cosmogenic (CO)-C-14 production rates. Significantly lo wer (CO)-C-14 concentrations (similar to 50%) are observed in the Southern versus Northern Hemisphere. Model simulations indicate that either higher t ropospheric horizontal mixing or higher OH concentrations in the Southern H emisphere can explain the hemispheric asymmetry in (CO)-C-14.