Stratospheric N2O-NOy system: Testing uncertainties in a three-dimensionalframework

Nitrous oxide (N2O) is an important greenhouse gas and the major source of stratospheric reactive nitrogen (NOy), an active participant in the stratos pheric chemistry controlling ozone depletion. Tropospheric N2O abundances a re increasing at nearly 0.3% yr(-1) and this increase is expected to contin ue in the near future as are direct stratospheric NO, perturbations, for ex ample, from aircraft. In order to test and gain confidence in three-dimensi onal (3-D) model simulations of the stratospheric N2O-NOy system, a simplif ied photochemistry for N2O and NOy is developed for use in chemistry transp ort 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 NOy of po tential errors in model transport, in column and local ozone, and in strato spheric temperatures. For example, with the three different 3-D wind fields used here, modeled N2O lifetimes vary from 173 to 115 years, and the unrea listically long lifetimes produce clear errors in equatorial N2O profiles. The impact of Antarctic denitrification and an in situ atmospheric N2O sour ce are also evaluated. The modeled N2O and NOy distributions are obviously sensitive to model transport, particularly the strength of tropical upwelli ng in the stratosphere. Midlatitude, lower-stratospheric NOy/N2O correlatio ns, including seasonal amplitudes, are well reproduced by the standard mode l when denitrification is included. These correlations are sensitive to cha nges in stratospheric chemistry but relatively insensitive to model transpo rt. The lower stratospheric NOy/N2O correlation slope gives the correct net NOy production of about 0.5 Tg N yr(-1) (i.e., the cross-tropopause flux a s in the Plumb-Ko relation) only when N2O values from 250 to 310 ppb are us ed. As a consequence., the Synoz calibration of the flux Of O-3 from the st ratosphere to the troposphere needs to be corrected to 550 +/- 140 Tg O-3 y r(--1).