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Outdoor air is often introduced into commercial buildings from ventilation intakes sited on rooftops where vegetation (a green roof) is increasingly present. Little is known about the impact of green roofs on the quality of building outdoor ventilation air supply. In this study, we investigated the potential for green roofs to impact ozone (O-3) levels in ventilation air by parameterizing O-3 dry deposition to vegetation and substrate typical of extensive green roofs in field and laboratory studies. Values obtained constrain a 2-D advection-diffusion model of O-3 transport and reaction at the rooftop scale. The 10th, 50th, and 90th percentiles for O-3 surface resistances measured using flux-gradient methods in field studies were 46 s/m, 155 s/m, and 1700 s/m. Surface resistances measured in laboratory chambers for substrate and green roof samples ranged from 360 s/m to 435 s/m, in the 60th -70th percentile of field measurements. The modeled impact of a green roof on O-3 levels in building outdoor ventilation air intake was a reduction ranging from 0.25 to 1.8 mu g/m(3) for short fetch lengths (1 m) and low vertical mixing to larger fetch lengths (5 m) and stronger vertical mixing, respectively, from ambient O-3 levels of 144 mu g/m(3). Vegetation fetch and vegetation height had the largest impact on modeled O-3 reductions, suggesting large, continuous, intensive green roof designs may enhance O-3 reductions in building ventilation air.