Two simple and computationally efficient models for simulating stratospheric ozone in three-dimensional global transport models are presented. The first, linearized ozone (or Linoz), is a first-order Taylor expansion of stratospheric chemical rates in which the ozone tendency has been linearized about the local ozone mixing ratio, temperature, and the overhead column ozone density. The second, synthetic ozone (or Synoz), is a passive, ozone-like tracer released into the stratosphere at a rate equivalent to that of the cross-tropopause ozone flux which, based on measurements and tracer-tracer correlations, we have calculated to be 475±120 Tg/yr. Linoz and Synoz have been evaluated in the UC Irvine chemical transport model (CTM) with three different archived meteorological fields: the Goddard Institute for Space Studies (GISS) general circulation model (GCM) version II′, the GISS GCM version II, and merged forecast data from the European Centre forecast model (EC/Oslo). Linoz produced realistic annual, cross-tropopause fluxes of 421 Tg/yr for the GISS II′ winds and 458 Tg/yr for the EC/Oslo winds; the GISS II winds produced an unrealistic flux of 790 Tg/yr. Linoz and Synoz profiles in the vicinity of the tropopause using the GISS II′ and EC/Oslo winds were found to be in good agreement with observations. We conclude that either approach may be adequate for a CTM focusing on tropospheric chemistry but that Linoz can also be used for calculating ozone fields interactively with the stratospheric circulation in a GCM. A future version of Linoz will allow for evolving background concentrations of key source gases, such as CH₄ and N₂O, and thus be applicable for long-term climate simulations.

Nitrous oxide (N₂O), an important greenhouse gas, has been increasing since 1980 at a rate of about +3% per decade. Recently, a notably greater rate of increase of about +5% per decade since 1980 was reported for measurements of stratospheric nitrogen dioxide (NO₂) over Lauder, New Zealand. Since N₂O is the dominant source of odd-nitrogen compounds in the stratosphere, including NO₂, this presents an obvious conundrum. Analysis here shows that these apparently conflicting trends are generally consistent when viewed in a global-change framework, specifically, when concurrent trends in stratospheric ozone and halogens are included. Using a combination of photochemical and three-dimensional chemistry-transport models, we predict a 1980–2000 trend in the NO₂, as measured over Lauder, New Zealand, of +4.3%/decade when these concurrent trends are considered. Of this, only +2.4%/decade is attributed directly to the increase in N₂O; the remainder includes +2.5%/decade due to the ozone change and −0.6%/decade to the increased halogens' impact on odd-nitrogen partitioning. The slant column densities of NO₂, as measured from the zenith scattered sunlight during twilight, are found to (1) overestimate the trend by +0.4%/decade as compared to the true vertical column densities and (2) display a diurnally varying trend with a maximum during the night and large gradients through sunrise and sunset in good agreement with measurement. Nonetheless, measurements such as these are essential for identifying global change and provide a lesson in understanding it: careful simulation of the time, location, and geometry of measurements must be combined with concurrent trends in related chemical species and climate parameters.