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

Abstract

Time series measurements of the ground level 14CO concentration were made at Olympic Peninsula, Washington (48°N), and Point Barrow, Alaska (71°N), between 1991 and 1997. Measurements of the meridional gradients of the 14CO concentration at sea level were made during five oceanographic cruises in the Pacific Ocean between 55°N and 65°S during 1991–1995. These measurements were combined with earlier time series measurements of atmospheric 14CO at 41°S and 77°S [Brenninkmeijer, 1993] and at 13°N [Mak and Southon, 1998] and meridional transects of 14CO at 6–8 km [Mak et al., 1994]. These 14CO 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 14CO. Additionally, the interannual trend in tropospheric methyl chloroform concentration and the stratospheric time history of bomb 14CO2 were simulated by the model. The results of this analysis indicate that an average tropospheric OH concentration of ∼l0×l05 radicals cm−3 explains both the 14CO and methyl chloroform trends. The model‐predicted 14CO concentrations, however, are sensitive to the rate of stratosphere‐troposphere exchange and horizontal mixing in the troposphere. Model predictions of tropospheric 14CO at high latitudes improved when the stratosphere‐troposphere exchange rate was slowed, based on the results of the stratospheric bomb 14CO2 model simulation. Substantial improvement in the model 14CO simulations occurred with increased horizontal diffusion rates in the troposphere and lower cosmogenic 14CO production rates. Significantly lower 14CO concentrations (∼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 14CO.