3.1    Improved Treatment of Boundary Layers in Urban Areas for Air    Quality Modeling

Pleim, Jonathan and Robert Gilliam, US Environmental Protection Agency

Recent evaluation studies of the WRF-CMAQ meteorology and air quality modeling system have shown a persistent tendency to overpredict ground-level concentrations of locally emitted pollutants such as CO and NOx in developed areas.  Emissions of these chemical species are predominantly from mobile sources and typically peak during the morning and evening rush hours which correspond to transition times for PBL development.  We have hypothesized that model overpredictions, especially during the evening transition time, are due in part to inadequate representation of urban heat island effects in the Pleim-Xiu land surface model (PX LSM).  Comparisons to tethersonde observations made during the evening and overnight at the University of Houston for several nights in September 2006 show that the lowest 200 m stay neutral until 9 PM LT while the model  has developed a stable potential temperature inversion.  Modification to the PX LSM, such as scaling the surface heat capacity according to the fractional coverage of impervious surfaces from the National Land Cover Data (NLCD) and increasing the roughness length for the 4 developed land-use categories greatly improve the modeling of the nocturnal boundary layer in urban areas and reduce the overprediction of emitted pollutants.