Impact of Physics Parameterizations on High-Resolution Air Quality Simulations over the Paris Region

The accurate simulation of meteorological conditions, especially within the planetary boundary layer (PBL), is of major importance for air quality modeling. In the present work, we have used the Weather Research and Forecast (WRF) model coupled with the chemistry transport model (CTM) CHIMERE to understand the impact of physics parameterizations on air quality simulation during a short-term pollution episode on the Paris region. A lower first model layer with a 4 m surface layer could better reproduce the transport and diffusion of pollutants in a real urban environment. Three canopy models could better reproduce a 2 m temperature (T2) in the daytime but present a positive bias from 1 to 5 &deg
C during the nighttime
the multi-urban canopy scheme &ldquo
building effect parameterization&rdquo
(BEP) underestimates the 10 m windspeed (W10) around 1.2 m s&minus
1 for the whole episode, indicating the city cluster plays an important role in the diffusion rate in urban areas. For the simulation of pollutant concentrations, large differences were found between three canopy schemes, but with an overall overestimation during the pollution episode, especially for NO2 simulation, the average mean biases of NO2 prediction during the pollution episode were 40.9, 62.2, and 29.7 &micro
g m&minus
3 for the Bulk, urban canopy model (UCM), and BEP schemes, respectively. Meanwhile, the vertical profile of the diffusion coefficients and pollutants indicated an important impact of the canopy model on the vertical diffusion. The PBL scheme sensitivity tests displayed an underestimation of the height of the PBL when compared with observations issued from the Lidar. The YonSei University scheme YSU and Boulac PBL schemes improved the PBL prediction compared with the Mellor&ndash
Yamada&ndash
Janjic (MYJ) scheme. All the sensitivity tests, except the Boulac&ndash
BEP, could not fairly reproduce the PBL height during the pollution episode. The Boulac&ndash
BEP scheme had significantly better performances than the other schemes for the simulation of both the PBL height and pollutants, especially for the NO2 and PM2.5 (particulate matter 2.5 micrometers or less in diameter) simulations. The mean bias of the NO2, PM2.5, and PM10 (particulate matter 10 micrometers or less in diameter) prediction were &minus
5.1, 1.2, and &minus
8.6 &micro
3, respectively, indicating that both the canopy schemes and PBL schemes have a critical effect on air quality prediction in the urban region.