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Small-scale nonlinear chemical and physical processes over pollution sourceregions affect the tropospheric ozone (O3), but these processes are not captured by current global chemical transport models (CTMs) andchemistry–climate models that are limited by coarse horizontal resolutions(100–500 km, typically 200 km). These models tend to contain large (andmostly positive) tropospheric O3 biases in the Northern Hemisphere.Here we use the recently built two-way coupling system of the GEOS-Chem CTM tosimulate the regional and global tropospheric O3 in 2009. The systemcouples the global model (at 2.5° long.  ×  2° lat.) and its three nested models (at 0.667° long.  ×  0.5° lat.) covering Asia, North America and Europe,respectively. Specifically, the nested models take lateral boundaryconditions (LBCs) from the global model, better capture small-scaleprocesses and feed back to modify the global model simulation within thenested domains, with a subsequent effect on their LBCs. Compared to the global model alone, the two-way coupled system bettersimulates the tropospheric O3 both within and outside the nesteddomains, as found by evaluation against a suite of ground (1420 sites fromthe World Data Centre for Greenhouse Gases (WDCGG), the UnitedStates National Oceanic and Atmospheric Administration (NOAA) Earth SystemResearch Laboratory Global Monitoring Division (GMD), the ChemicalCoordination Centre of European Monitoring and Evaluation Programme (EMEP), and the United States Environmental Protection Agency Air Quality System (AQS)), aircraft (the High-performance Instrumented AirbornePlatform for Environmental Research (HIAPER) Pole-to-Pole Observations(HIPPO) and Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC)) and satellitemeasurements (two Ozone Monitoring Instrument (OMI) products). The two-way coupled simulation enhances thecorrelation in day-to-day variation of afternoon mean surface O3 withthe ground measurements from 0.53 to 0.68, and it reduces the mean modelbias from 10.8 to 6.7 ppb. Regionally, the coupled system reduces the biasby 4.6 ppb over Europe, 3.9 ppb over North America and 3.1 ppb over otherregions. The two-way coupling brings O3 vertical profiles much closerto the HIPPO (for remote areas) and MOZAIC (for polluted regions) data,reducing the tropospheric (0–9 km) mean bias by 3–10 ppb at most MOZAICsites and by 5.3 ppb for HIPPO profiles. The two-way coupled simulation alsoreduces the global tropospheric column ozone by 3.0 DU (9.5 %, annualmean), bringing them closer to the OMI data in all seasons. Additionally,the two-way coupled simulation also reduces the global tropospheric meanhydroxyl radical by 5 % with improved estimates of methyl chloroform andmethane lifetimes. Simulation improvements are more significant in theNorthern Hemisphere, and are mainly driven by improved representation ofspatial inhomogeneity in chemistry/emissions. Within the nested domains, the two-way coupled simulation reduces surfaceozone biases relative to typical GEOS-Chem one-way nested simulations, dueto much improved LBCs. The bias reduction is 1–7 times the bias reductionfrom the global to the one-way nested simulation. Improving modelrepresentations of small-scale processes is important for understanding theglobal and regional tropospheric chemistry

Improved simulation of tropospheric ozone by a global-multi-regional two-way coupling model system