Evaluating the effects of radiative forcing feedback in modeling urban ozone air quality in Portland, Oregon: Two-way coupled MM5 – CMAQ simulations

Abstract:

We summarize an on-line coupled meteorological-emissions-photochemical modelling system that allows feedbackfrom 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 radiativeforcing 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-forcingfeedback. 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.