Evaluating the Effects of Radiative Forcing Feedback in Modelling Urban Ozone Air Quality in Portland, Oregon: Two-Way Coupled MM5-CMAQ Numerical Model Simulations

We summarize an on-line coupled meteorological-emissions-photochemical modelling system that allows feedback from air-quality/chemistry to meteorology via radiative forcing. We focus on the radiative-forcing impacts (direct effects) of ozone. We present an application of the coupled modelling system to the episode of 23-31 July 1998 in Portland, Oregon, U. S. A. Results suggest that the inclusion of radiative-forcing feedback produces small but accountable impacts. For this region and episode, stand-alone radiative transfer simulations, i. e., evaluating the effects of radiative forcing independently of changes in meteorology or emissions, suggest that a change of 1 ppb in ground-level ozone is approximately equivalent to a change of 0. 017 W m^-2 in radiative forcing. In on-line, coupled, three-dimensional simulations, where the meteorological dependencies are accounted for, domain-wide peak ozone concentrations were higher by 2-4 ppb (relative to a simulated peak of 119.4 ppb) when including the effects of radiative-forcing feedback. A scenario of 10% reduction in anthropogenic emissions produced slightly larger decreases in ozone, an additional 1 ppb in local-peak reductions, relative to scenarios without feedback.