We describe a new and computationally efficient technique for global three‐dimensional modeling of stratospheric chemistry. This technique involves integrating a photochemical package along a large number of independent trajectories to produce a Lagrangian view of the atmosphere. Although Lagrangian chemical modeling with trajectories is an established procedure, this extension of integrating chemistry along a large number of domain‐filling trajectories is a novel technique. This technique is complementary to three‐dimensional Eulerian chemical transport modeling and avoids spurious mixing caused by low resolutions or diffusive transport schemes in these models. We illustrate the technique by studying the chlorine activation in the Arctic winter lower stratosphere. A photochemical model was integrated along large ensembles of calculated trajectories between 20 and 100 mbar for the 1991/1992 winter in order to produce a three‐dimensional chemical picture. Large amounts of chlorine was activated at low altitudes (80 to 100 mbar) as well as altitudes near 50 mbar. This activated air was well contained at all levels, with little indication of mixing into lower latitudes. Model results for early January 1992 were compared to daily Microwave Limb Sounder (MLS) ClO observations at 465 K. The structure and evolution of the activated chlorine was well reproduced, giving faith in the technique, although absolute modeled ClO amounts were smaller than the MLS data. A larger number of domain‐filling isentropic trajectories were also run at 475 K to produce a higher‐resolution picture of vortex evolution in late January 1992. The model successfully reproduced the wave breaking events which characterized this period causing transport of activated air to lower latitudes.
Calculations with a chemical box model along air parcel trajectories between November 1991 to January 1992 show a build up of reactive chlorine in late December and early January, culminating in values of ClOx greater than 2.0 ppbv widespread in the vortex on 9 January 1992. These values are quantitatively comparable to the MLS satellite measurements of ClO. We discuss the chemistry occurring within the vortex, around the vortex edge and outside the vortex.
Absorption cross‐sections for HNO3 have been measured in the wavelength region 220–340 nm, using a dual‐beam diode array spectrometer, with a spectral resolution of 0.3 nm. The results at room temperature were in good agreement with earlier measurements. Absorption over most of the wavelength range showed a distinct temperature dependence, with a decline in cross‐section with decreasing temperature in the range 295–239 K. The results lead to quite large effects on the calculated photodissociation rate of HNO3 in the lower stratosphere, especially at the low temperatures and high solar zenith angles characteristic of the polar winter and spring.
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