Effect of Temporal Averaging of Vertical Eddy Diffusivity on the Forecast Quality of Surface Ozone Concentration of the National Air Quality Forecast

Air Quality forecast models are employed to provide numerical guidance for forecasters to issue timely ozone and particulate matter concentration forecast pertinent to human health exposures to the communities they serve. The National Centers for Environmental Prediction (NCEP), National Weather Service Weather Research and Forecasting Non-hydrostatic Mesoscale Model (WRF/NMM) has been coupled off-line with the EPA Community Multi-scale Air Quality (CMAQ) model in 2003 to form the National Air Quality Forecast Capability (NAQFC). Coupling of the turbulent mixing processes of the meteorological driver and CMAQ has been a challenge. WRF/NMM uses the Mellor-Yamada-Janjic (MYJ) turbulence mixing scheme in the boundary layer. The option of directly using MYJ provided vertical eddy diffusivity to calculate turbulent mixing of the atmospheric chemical constituents in CMAQ has been tested. This study investigated the effect of various temporal averaging treatments of vertical eddy diffusivity in redistributing ozone in the boundary layer. Model predicted concentration structures of ozone resulting from these various temporal treatments of vertical eddy diffusivity have been studied and verified using AIRNOW, an EPA surface ozone concentration network, and ozone-sonde data.