USE OF SATELLITE DATA TO CONSTRAIN THE MODEL-CALCULATED ATMOSPHERIC LIFETIME FOR N2O - IMPLICATIONS FOR OTHER TRACE GASES

The source gases, such as nitrous oxide (N2O) and chlorofluorocarbons (CFCs), are released into the atmosphere at the Earth's surface and are removed mainly by photolysis in the stratosphere. The atmospheric abundance of a source gas is proportional to its emission rate and atmospheric lifetime. The lifetime is, in turn, determined by the local photochemical removal rate of the gas and the efficiency of transport that carries the gas from where it is emitted to the dominant photochemical removal region. The latter is particularly important for source gases for which the removal is restricted to the tropical stratosphere, a region where both the photolysis rates and concentrations are highest. We calculate that approximately 80% of N2O is removed in the stratosphere between 30-degrees-N and 30-degrees-S. Using the data for N2O obtained from the stratospheric and mesospheric sounder (SAMS) instrument on the Nimbus 7 satellite to constrain the model-calculated distributions, we concluded that previous models may have underestimated the magnitude of vertical transport over the tropics and that the calculated lifetimes for N2O and CFC source gases could be 30% shorter than previously reported values. The calculated lifetime for N2O of 110 years would imply a source strength of 13 x 10(6) tons (N) yr-1, compared to a source strength of 9.2 x 10(6) tons (N) per year for a lifetime of 160 years. A shorter lifetime for the CFCs (47 years for CFC-11 and 95 years for CFC-12) would imply a more rapid decrease in the atmospheric chlorine content once the CFC emissions are stopped, making it possible to reach the pre-ozone hole value of 2 ppbv as early as 2045. Accurate determination of the lifetime of CFC-11 is particularly important, since the lifetime is used in the definitions of the ozone depletion potentials (ODP), chlorine loading potentials (CLP), and greenhouse warming potentials (GWP) of the replacement chemicals for the CFCs. A shorter lifetime for CFC-11 would elevate the magnitudes of ODP, CLP, and GWP for these chemicals.