On the Impact of Time-Resolved Boundary Conditions on the Simulation of Surface Ozone and PM10

The grid-spacing of chemistry-transport models (CTM) is always limited by computational resources and ranges from 100-200 km of global scale models, to 25-50 km of continental scale models, to 1-10 km of regional and local scale models. We push to higher resolution in hope of better reproducing small scale processes that affect our ability to assess the environmental and health impacts of emissions. Running simulations at a resolution less than 50 km is often feasible only using a limited area model, which uses a domain ascribed to the region of interest. However, even the air quality of a single city is in principle affected by all the emission sources at global level: we thus account for such long-range transport of pollutants and oxidants specifying the chemical state of the atmosphere outside the domain through boundary conditions (BC). BC concentrations are usually taken from typical profiles or from larger-scale simulations with a procedure called “nesting”. In this chapter, we focus on the latter technique, exploring in particular the effect of different BC timeresolutions (monthly to hourly) on the simulation of ozone and particulate matter on a nested domain at European scale. For the sake of completeness, we point out here that even very high-resolution models cannot explicitly simulate processes at all possible spatialtemporal scales and thus a certain degree of parameterization is always required. For further insights on the “subgrid” issues we refer the reader to the literature (e.g. Galmarini et al., 2008; Qian et al., 2010; Denby et al., 2011; Paoli et al., 2011).