GEOS-Chem Chemistry Transport Model. The nest-grid has a horizontal resolution of 0.667° × 0.5° (longitude × latitude), and 47 vertical levels extending from the surface to 0.01 hPa. The model is driven using GEOS-5 meteorology, which is updated every 3–6 hours. Tracer mixing ratios from an off-line global 4° × 5° simulation provide 3-hourly boundary conditions to the grid-edges. Based on a previous model evaluation [Barkley et al., 2011], we use an updated chemical mechanism to simulate O3-NOx-VOC-aerosol photochemistry.