Atmospheric 14CO:: A tracer of OH concentration and mixing rates

Time series measurements of the ground level (CO)-C-14 concentration were made at Olympic Peninsula, Washington (48 degrees N), and Point Barrow, Alaska (71 degrees N), between 1991 and 1997. Measurements of the meridional gradients of the (CO)-C-14 concentration at sea level were made during five oceanographic cruises in the Pacific Ocean between 55 degrees N and 65 degrees S during 1991-1995. These measurements were combined with earlier time series 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 meridional 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. Additionally, 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 concentrations, however, are sensitive to the rate of stratosphere-troposphere exchange and horizontal mixing in the troposphere. Model predictions of tropospheric (CO)-C-14 at high latitudes improved when the stratosphere-troposphere exchange rate was slowed, based on the results of the stratospheric bomb (CO2)-C-14 model simulation. Substantial improvement in the model (CO)-C-14 simulations occurred with increased horizontal diffusion rates in the troposphere and lower cosmogenic (CO)-C-14 production rates. Significantly lower (CO)-C-14 concentrations (similar to 50%) are observed in the Southern versus Northern Hemisphere. Model simulations indicate that either higher tropospheric horizontal mixing or higher OH concentrations in the Southern Hemisphere can explain the hemispheric asymmetry in (CO)-C-14.