Influence of the PBL scheme on high-resolution photochemical simulations in an urban coastal area over the Western Mediterranean

Abstract

Planetary boundary layer (PBL) and land-surface processes have critical implications for air quality simulations. This contribution analyses the influence of three different surface/PBL schemes from the MM5 mesoscale meteorological model on the predicted photochemical pollutant concentrations using high-resolution EMICAT2000 emission model and Models-3/CMAQ chemical transport model. The Gayno–Seaman (GS), the Medium Range Forecast (MRF), and the Pleim–Chang (PC) PBL schemes are considered. The simulations focus on a complex coastal urban area (Barcelona Geographical Area) located in the Western Mediterranean for a typical summertime situation characterised by the absence of large-scale forcing. The particularities of the area require high spatial (1 km) resolution. Air quality, meteorological and lidar data are used for the evaluation of the results. PBL height inputs have been considered as diagnosed by MM5 and re-diagnosed by the MCIP meteorological pre-processor of Models-3/CMAQ. Several variables playing an essential role in the concentrations of air pollutants, such as PBL height, temperature, and wind speed and direction, are analysed. Important differences are observed in GS scheme depending on the method used in the PBL height estimation. MRF and PC present similar results. Modelled daily maximum pollutants’ concentrations vary in magnitude and location in function of the PBL scheme. GS–MM5 scenario presents the lowest PBL heights, the highest surface temperatures and the weakest winds during daytime, which provokes an enhanced O 3 formation. The higher concentrations of NO x in the GS–MM5 scheme (which are caused mainly by the lower PBL height) provokes a higher depletion of O 3, which yields to the lower concentrations of this pollutant during nighttime in Barcelona downtown. For regulatory purposes, the model tends to underestimate the photochemical formation in the area of study due to an underestimation of VOCs emissions in an VOC-limited area as Barcelona. The lower PBL height shown by GS taken from MM5, as well as the increase in the temperature and the weaker winds over coastal areas (that allow the accumulation of photochemical pollutants) promotes the O 3 concentration and improves the statistical scores of GS–MM5 scenario.