Measurements and modeling of contemporary radiocarbon in the stratosphere

\ Measurements of the C-14 content of carbon dioxide in air collected by high-altitude balloon flights in 2003-2005 reveal the contemporary radiocarbon distribution in the northern midlatitude stratosphere, four decades after the Limited Test Ban Treaty restricted atmospheric testing of nuclear weapons. Comparisons with results from a 3-D chemical-transport model show that the (CO2)-C-14 distribution is now largely governed by the altitude/latitude dependence of the natural cosmogenic production rate, stratospheric transport, and propagation into the stratosphere of the decreasing radiocarbon trend in tropospheric CO2 due to fossil fuel combustion. From the observed correlation of (CO2)-C-14 with N2O mixing ratios, an annual global mean net flux of (CO2)-C-14 to the troposphere of 1.6(0.4)x10(17)molCO(2)yr(-1) and a global production rate of 2.2(0.6)x10(26) atoms(14)Cyr(-1) are empirically derived. The results also indicate that contemporary (CO2)-C-14 observations provide highly sensitive diagnostics for stratospheric transport and residence times in models.