Journal cover Journal topic
Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
Geosci. Model Dev., 10, 3025-3057, 2017
https://doi.org/10.5194/gmd-10-3025-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Model evaluation paper
17 Aug 2017
The TOMCAT global chemical transport model v1.6: description of chemical mechanism and model evaluation
Sarah A. Monks1,2,3, Stephen R. Arnold1, Michael J. Hollaway1, Richard J. Pope1,4, Chris Wilson1,4, Wuhu Feng1,5, Kathryn M. Emmerson6, Brian J. Kerridge7, Barry L. Latter7, Georgina M. Miles7, Richard Siddans7, and Martyn P. Chipperfield1 1Institute for Climate and Atmospheric Science, University of Leeds, Leeds, UK
2Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA
3Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
4National Centre for Earth Observation, University of Leeds, Leeds, UK
5National Centre for Atmospheric Science, University of Leeds, Leeds, UK
6CSIRO Oceans and Atmosphere Flagship, Aspendale, Australia
7Remote Sensing Group, STFC Rutherford Appleton Laboratory, Harwell Oxford, UK
Abstract. This paper documents the tropospheric chemical mechanism scheme used in the TOMCAT 3-D chemical transport model. The current scheme includes a more detailed representation of hydrocarbon chemistry than previously included in the model, with the inclusion of the emission and oxidation of ethene, propene, butane, toluene and monoterpenes. The model is evaluated against a range of surface, balloon, aircraft and satellite measurements. The model is generally able to capture the main spatial and seasonal features of high and low concentrations of carbon monoxide (CO), ozone (O3), volatile organic compounds (VOCs) and reactive nitrogen. However, model biases are found in some species, some of which are common to chemistry models and some that are specific to TOMCAT and warrant further investigation. The most notable of these biases are (1) a negative bias in Northern Hemisphere (NH) winter and spring CO and a positive bias in Southern Hemisphere (SH) CO throughout the year, (2) a positive bias in NH O3 in summer and a negative bias at high latitudes during SH winter and (3) a negative bias in NH winter C2 and C3 alkanes and alkenes. TOMCAT global mean tropospheric hydroxyl radical (OH) concentrations are higher than estimates inferred from observations of methyl chloroform but similar to, or lower than, multi-model mean concentrations reported in recent model intercomparison studies. TOMCAT shows peak OH concentrations in the tropical lower troposphere, unlike other models which show peak concentrations in the tropical upper troposphere. This is likely to affect the lifetime and transport of important trace gases and warrants further investigation.

Citation: Monks, S. A., Arnold, S. R., Hollaway, M. J., Pope, R. J., Wilson, C., Feng, W., Emmerson, K. M., Kerridge, B. J., Latter, B. L., Miles, G. M., Siddans, R., and Chipperfield, M. P.: The TOMCAT global chemical transport model v1.6: description of chemical mechanism and model evaluation, Geosci. Model Dev., 10, 3025-3057, https://doi.org/10.5194/gmd-10-3025-2017, 2017.
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The TOMCAT chemical transport model has been updated with the chemical degradation of ethene, propene, toluene, butane and monoterpenes. The tropospheric chemical mechanism is documented and the model is evaluated against surface, balloon, aircraft and satellite data. The model is generally able to capture the main spatial and seasonal features of carbon monoxide, ozone, volatile organic compounds and reactive nitrogen. However, some model biases are found that require further investigation.
The TOMCAT chemical transport model has been updated with the chemical degradation of ethene,...
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