The optimal‐interpolation data‐assimilation technique is used to combine TIROS Operational Vertical Sounder total‐ozone columns with the three‐dimensional tracer transport model TM3. An ozone chemistry parametrization and dry deposition of ozone are added to the model to give it the ability to simulate realistic ozone profiles. Starting from a separable form of the forecast covariance matrix, the optimal interpolation equation can be rewritten into a horizontal and vertical analysis step. First, the two‐dimensional ozone column measurements are analyzed using estimates for the horizontal error covariances. In the second step of the assimilation procedure the analysis increment for the column is distributed over the vertical model layers. This step depends only on a normalized vertical weight function which is equal to the vertical covariance. Three different estimates for this weight function are introduced, using either the ozone error covariance, or the ozone time variance, or the actual ozone mass to distribute column corrections in the vertical. A comparison with independent Total Ozone Mapping Spectrometer ozone observations shows a considerable improvement of the total ozone field due to assimilation. The model error growth is small, making it suitable for assimilating sparse measurements. Ozone profiles from the assimilation appear realistic and close to the ones observed by sondes. They are capable to describe dynamical features in the lower stratosphere. However, due to the absence of vertical information, the assimilation of ozone columns has only little impact on the shape of the vertical ozone profile, which is mainly determined by the transport.

A comparison of large‐scale models simulating atmospheric sulfate aerosols (COSAM) was conducted to increase our understanding of global distributions of sulfate aerosols and precursors. Earlier model comparisons focused on wet deposition measurements and sulfate aerosol concentrations in source regions at the surface. They found that different models simulated the observed sulfate surface concentrations mostly within a factor of two, but that the simulated column burdens and vertical profiles were very different amongst different models. In the COSAM exercise, one aspect is the comparison of sulfate aerosol and precursor gases above the surface. Vertical profiles of SO2, SO2−4, oxidants and cloud properties were measured by aircraft during the North Atlantic Regional Experiment (NARE) experiment in August/September 1993 off the coast of Nova Scotia and during the Second Eulerian Model Evaluation Field Study (EMEFSII), in central Ontario in March/April 1990. While no single model stands out as being best or worst, the general tendency is that those models simulating the full oxidant chemistry tend to agree best with observations although differences in transport and treatment of clouds are important as well.

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[1] A new method for the quantitative evaluation of global atmospheric transport and the hydroxyl radical (OH)-based oxidation in three-dimensional (3-D) atmospheric chemistry transport models (CTMs) and general circulation models (GCMs) is developed. The method is based on a cosmogenic 14 CO climatology that has been previously derived from a large number of 14 CO observations. Using 14 CO measurements to constrain model OH distributions and the simulated stratosphere-troposphere exchange (STE) provides a challenging test for 3-D atmospheric models. Here, the evaluation method is applied to the CTMs MATCH and TM3. Whereas MATCH overestimates the STE in both hemispheres, TM3 does reproduce the STE in the Southern Hemisphere (SH) but underestimates it in the Northern Hemisphere (NH). The STE phase in MATCH is 1 month too early, whereas no significant phase shift for TM3 is revealed. These characteristic deficiencies in both models were consistently determined, i.e., with the same boundary conditions (OH distribution and 14 CO source distribution). The robustness of the results is tested by various sensitivity studies, involving the 14 CO source distribution and strength, the tropospheric OH distribution, the stratospheric OH abundance, and the applied numerical advection scheme. Consistency is further checked by comparison of the model simulated vertical 14 CO profiles to 14 CO observations from a number of aircraft campaigns. The 14 CO simulations do not support an interhemispheric asymmetry in the OH abundance with an on average higher concentration in the SH.

A comparison of large-scale models simulating atmospheric sulfate aerosols (COSAM) was conducted to increase our understanding of global distributions of sulfate aerosols and precursors. Earlier model comparisons focused on wet deposition measurements and sulfate aerosol concentrations in source regions at the surface. They found that different models simulated the observed sulfate surface concentrations mostly within a factor of two, but that the simulated column burdens and vertical profiles were very different amongst different models. In the COSAM exercise, one aspect is the comparison of sulfate aerosol and precursor gases above the surface.