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Tropospheric chemistry and air quality processes were implemented on-line in theGlobal Environmental Multiscale weather prediction model. The integrated model,GEM-AQ, was developed as a platform to investigate chemical weather at scales fromglobal to urban. The current chemical mechanism is comprised of 50 gas-phase species,116 chemical and 19 photolysis reactions, and is complemented by a sectional aerosolmodule with 5 aerosols types. All tracers are advected using the semi-Lagrangian schemenative to GEM. The vertical transport includes parameterized  subgrid-scale turbulenceand large scale deep convection. Dry deposition  is included as a flux boundary conditionin the vertical diffusion equation.  Wet deposition of gas-phase species is treated in asimplified way, and only below-cloud scavenging is considered. The emissions used includeyearly-averaged anthropogenic, and monthly-averaged biogenic, ocean, soil, and biomass burningemission fluxes, as well as NO<sub>x</sub> from lightning. In order to evaluate the ability tosimulate seasonal variations and regional distributions of trace gases such as ozone,nitrogen dioxide and carbon monoxide, the model was run for a period of five years (2001–2005)on a global uniform 1.5°×1.5° horizontal resolution domain and 28 hybridlevels extending up to 10 hPa.  Model results were compared with observations from satellites,aircraft measurement campaigns and balloon sondes. We find that GEM-AQ is able to capture thespatial details of the chemical fields in the middle and lower troposphere. The modelledozone consistently shows good agreement with observations, except over tropical oceans. Thecomparison of carbon monoxide and nitrogen dioxide with satellite measurements emphasizesthe need for more accurate, year-specific emissions fluxes for biomass burning and anthropogenicsources. Other species also compare well with available observations

GEM-AQ, an on-line global multiscale chemical weather modelling system: model description and evaluation of gas phase chemistry processes