On the impact of urban surface exchange parameterisations on air quality simulations: the Athens case

Most of the standard mesoscale models represent the dynamic and thermodynamic surface exchanges in urban areas with the same technique used for rural areas (based on Monin–Obukhov similarity theory and a surface energy budget). However it has been shown that this technique is not able to fully capture the structure of the turbulent layer above a city. Aim of this study is to evaluate the importance for meteorological and air quality simulations, of properly capture the dynamic and thermodynamic surface exchanges in urban areas. Two sets of simulations were performed over the city of Athens (Greece): a first using a mesoscale model with a detailed urban surface exchange parameterisation (able to reproduce the surface exchanges better than the traditional method), and a second with the traditional approach. Meteorological outputs are passed to a Eulerian photochemical model (the photochemical model is run offline). Comparison with measurements shows better agreement for the simulation with the detailed parameterisation. The differences between the simulations concern, mainly, wind speed (maximum difference of 0.5–1 m s−1), night-time temperatures (2–3°C), turbulence intensity (2 m2 s−2) and heat fluxes (0.15 K m s−1) over the urban area, urban nocturnal land breeze intensity, timing and extension of sea breeze. These differences modify the pollutant distribution (e.g. for ozone maximum differences are of the order of 30 ppb). Differences between the simulations are also found in AOT60 values (inside and outside the city) and in O3 chemical regimes.