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Global chemical transport models (CTMs) are used extensively to study airpollution and transport at a global scale. These models are limited bycoarse horizontal resolutions that do not allow for a detailed representation ofsmall-scale nonlinear processes over the pollutant source regions. Here wecouple the global GEOS-Chem CTM and its three high-resolution nested modelsto simulate the tropospheric carbon monoxide (CO) over the Pacific Oceanduring five High-performance Instrumented Airborne Platform forEnvironmental Research (HIAPER) Pole-to-Pole Observations (HIPPO) campaigns between 2009and 2011. We develop a two-way coupler, the PeKing University CouPLer (PKUCPL), allowing for the exchange andinteraction of chemical constituents between the global model (at2.5° long. × 2° lat.) and the three nestedmodels (at 0.667° long. × 0.5° lat.)covering Asia, North America, and Europe. The coupler obtainsnested model results to modify the global model simulation within therespective nested domains, and simultaneously acquires global model resultsto provide  lateral boundary conditions (LBCs) for the nested models.  Compared to the global model alone, the two-way coupled simulation resultsin enhanced CO concentrations in the nested domains. Sensitivity testssuggest the enhancement to be a result of improved representation of thespatial distributions of CO, nitrogen oxides, and non-methane volatileorganic compounds, the meteorological dependence of natural emissions, andother resolution-dependent processes. The relatively long lifetime of COallows for the enhancement to be accumulated and carried across the globe.We found that the two-way coupled simulation increased the globaltropospheric mean CO concentrations in 2009 by 10.4%, with a greaterenhancement at 13.3% in the Northern Hemisphere. Coincidently, the globaltropospheric mean hydroxyl radical (OH) was reduced by 4.2%, resulting ina 4.2% enhancement in the methyl chloroform lifetime (MCF; via reactionwith tropospheric OH). The resulting CO and OH contents and MCF lifetime arecloser to observation-based estimates.  Both the global and the two-way coupled models capture the generalspatiotemporal patterns of HIPPO CO over the Pacific. The two-way coupledsimulation is much closer to HIPPO CO, with a mean bias of 1.1 ppb (1.4%)below 9 km compared to the bias at −7.2 ppb (−9.2%) for the global modelalone. The improvement is most apparent over the North Pacific. Our testsimulations show that the global model alone could resemble the two-waycoupled simulation (especially below 4 km) by increasing its global COemissions by 15% for HIPPO-1 and HIPPO-3, by 25% for HIPPO-2 andHIPPO-4, and by 35% for HIPPO-5. This has important implications forusing the global model alone to constrain CO emissions. Thus, the two-waycoupled simulation is a significantly improved model tool for studying theglobal impacts of air pollutants from major anthropogenic source regions

Tropospheric carbon monoxide over the Pacific during HIPPO: two-way coupled simulation of GEOS-Chem and its multiple nested models