MODEL STUDY OF ATMOSPHERIC TRANSPORT USING CARBON-14 AND STRONTIUM-90AS INERT TRACERS

The observed excess carbon 14 in the atmosphere from 1963 to 1970 prov ides unique, but limited, data up to an altitude of about 35 km for te sting the air motions calculated by 11 multidimensional atmospheric mo dels. Strontium 90 measurements in the atmosphere from 1964 to mid-196 7 provide data that have more latitude coverage than those of carbon 1 4 and are useful for testing combined models of air motions and aeroso l settling. Model calculations for carbon 14 begin at October 1963, 9 months after the conclusion of the nuclear bomb tests; the initial con ditions for the calculations are derived by three methods, each of whi ch agrees fairly well with measured carbon 14 in October 1963 and each of which has widely different values in regions of the stratosphere w here there were no carbon 14 measurements. The model results are compa red to the stratospheric measurements, not as if the observed data wer e absolute standards, but in an effort to obtain new insight about the models and about the atmosphere. The measured carbon 14 vertical prof iles at 31 degrees N are qualitatively different from all of the model s; the measured vertical profiles show a maximum mixing ratio in the a ltitude range of 20 to 25 km from October 1963 through July 1966, but all modeled profiles show mixing ratio maxima that increase in altitud e from 20 km in October 1963 to greater than 40 km by April 1966. Both carbon 14 and strontium 90 data indicate that the models differ subst antially among themselves with respect to stratosphere-troposphere exc hange rate, but the modeled carbon 14 stratospheric residence times in dicate that differences among the models are small with respect to tra nsport rate between the middle stratosphere and the lower stratosphere . Strontium 90 data indicate that aerosol settling is important up to at least 35 km altitude. Relative to the measurements, about three qua rters of the models transport carbon 14 from the lower stratosphere to the troposphere too rapidly, and all models with all three sets of in itial conditions appear to sweep carbon 14 out of the midstratosphere (above 28 km) much more slowly than was observed from 1963 to 1970. At mospheric dynamicists should seriously consider what the carbon 14 mea surements imply with respect to advection in the midstratosphere.