Turbulent Fluxes and Air Pollution Concentrations During Wintertime Cold Air Pool Events

Meteorological conditions and orographic features cause elevated pollutant concentrations in multiple cities in the western U.S. (e.g., 24-hr particulate matter concentrations ~90 ug m­^-3­). The complex mountainous terrain in these regions along with favorable meteorology leads to the formation of cold air pools (CAP), where cold dense air settles on the valley floor. During wintertime, persistent multi-day CAPs can form when background winds are weak, or stagnant, due to slow moving high-pressure systems. The atmospheric boundary layer is strongly stable during CAP events, leading to a decrease in planetary boundary layer height and inhibited turbulent mixing, both of which contribute to elevated air pollution concentrations. Recent research has shown that numerical weather prediction (NWP) models can capture the synoptic weather patterns associated with CAP formation and destruction. However, the models do not capture the small-scale turbulent mixing or heat fluxes near the earth’s surface, therefore impacting simulation results related to the land-atmosphere exchange of heat, moisture, and momentum. Additionally, this small-scale mixing process is responsible for the turbulent mixing of pollutants in the atmospheric boundary layer, and research suggests that decreased turbulent mixing contributes to the increase in pollutant concentrations during the CAP periods. In NWP models, the surface-atmosphere exchange is typically over-estimated during CAP events, which leads to under-estimated pollutant concentrations. The focus of this talk will be on recent observations of turbulent fluxes and surface energy balance closure, quantified for seven land use types in the Salt Lake Valley, Utah. The turbulence observations and surface energy balance closure are used to investigate the relationship between decreased turbulent mixing and pollutant accumulation during wintertime persistent CAPs. Multiple gas and aerosol phase pollutant concentrations will be investigated, specifically to determine the temporal relationship between turbulent mixing, gas phase precursor emissions, and secondary aerosol formation. Results from the Weather Research and Forecasting model will also be presented to compare simulated surface fluxes with observations, where results indicate that the model results over-estimate the surface fluxes during CAP events.