S. C. Olsen, C. A. McLinden, and M. J. Prather
Dept. of Earth System Science, University of California, Irvine

Nitrous oxide (N₂O) is an important greenhouse gas and the major source of stratospheric reactive nitrogen (NO_y), an active participant in the stratospheric chemistry controlling ozone depletion. Tropospheric N₂O abundances are increasing at nearly 0.3 %/yr and this increase is expected to continue in the near future as are direct stratospheric NO_y perturbations, e.g., from aircraft. In order to test and gain confidence in 3-D model simulations of the stratospheric N₂O--NO_y system, a simplified photochemistry for N₂O and NO_y is developed for use in chemistry-transport models (CTMs). This chemical model allows for extensive CTM simulations focusing on uncertainties in chemistry and transport. We compare 3-D model simulations with measurements and evaluate the effect on N₂O and NO_y of potential errors in model transport, in column and local ozone, and in stratospheric temperatures. For example, with the three different 3-D wind fields used here, modeled N₂O lifetimes vary from 173 to 115 yr, and the unrealistically long lifetimes produce clear errors in equatorial N₂O profiles. The impact of Antarctic denitrification and an in situ atmospheric N₂O source are also evaluated. The modeled N₂O and NO_y distributions are obviously sensitive to model transport, particularly the strength of tropical upwelling in the stratosphere. Mid-latitude, lower-stratospheric NO_y/N₂O correlations, including seasonal amplitudes, are well reproduced by the standard model when denitrification is included. These correlations are sensitive to changes in stratospheric chemistry but relatively insensitive to model transport. The lower stratospheric NO_y/N₂O correlation slope gives the correct net NO_y production of about 0.5 TgN/yr (i.e., the cross-tropopause flux as in the Plumb-Ko relation) only when N₂O values from 250 to 310 ppb are used. As a consequence, the Synoz calibration of the flux of O₃ from the stratosphere to the troposphere needs to be corrected to 550+/-140 Tg(O₃)/yr.

Manuscript accepted for publication in J. Geophys. Res., July 2001