## 2 Experiments
### Modeling Tools
#### 2.1.2 Model Set-Up  
The model grid covers the Northern Hemisphere using a polar stereographic projection with 1440 \(\times\) 1440 points. With this projection and this number of points, the horizontal model resolution varies from 18 \(\times\) 18 km\({}^{2}\) at the Equator to 9 \(\times\) 9 km\({}^{2}\) at the Pole. Global chemical fields from a six-year reanalysis with the LMDz4-INCA3 model [58; 59] are used as boundary conditions for the model top and lateral borders. For this simulation, the lateral boundary conditions (under the Equator) will have a low impact since the selected domain covers the whole Northern Hemisphere with few interactions with Southern Hemisphere. The vertical discretization has 16 layers extending from approximately 997 hPa to 200 hPa. This number of layers is a compromise between accuracy and computing capability, certainly this is not sufficient to well simulate the long-range transport of pollutants, and this has to be analyzed in follow-up works. The period of March 2014 is simulated with 15 days spin-up in February 2014. The model configuration is summarized here, but the reader is referred to the reference CHIMERE publication [46] for details on the corresponding model components and references as well as non-user-specific model characteristics. The gas-phase chemical mechanism is MELCHIOR2, which consists of a simplified version (40 species and 120 reactions) of the full chemical mechanism based on the concept of chemical operators. Modeled particulate matter includes primary particulate matter, and secondary inorganic (nitrate, sulfate, ammonium based on the ISORROPIA thermodynamic equilibrium model) and organic aerosol (whose formation is represented according to a single-step oxidation of the relevant anthropogenic and biogenic precursors and gas-particle partitioning of the condensable oxidation products). For the computation of thermodynamic equilibrium, the relative humidity is capped with a maximum value of 97% to avoid a too large growth of particle size. Biogenic emissions are computed with MEGAN version 2.04, sea-salt and mineral dust emissions are also considered, emission parameterization for the latter species is discussed later. The particle size ranges from 10 nm to 40 \(\upmu\)m over 10 bins according these ranges from bin No. 1 to bin No. 10 which are, respectively, 10.00-22.01 nm, 2.20-48.43 nm, 48.43-106.7 nm, 106.7-234.7 nm, 234.7-516.2 nm, 516.2-1.14 \(\upmu\)m, 1.14-2.50 \(\upmu\)m, 2.5-5.0 \(\upmu\)m, 5.0-10.0 \(\upmu\)m, and 10-40 \(\upmu\)m. In this paper, SIA, SOA, DUST, SALT, and TPPM, respectively, refer to Secondary Inorganic Aerosol (sum of nitrate, sulfate and ammonium), Secondary Organic Aerosols (anthropogenic and biogenic in origins), natural mineral dust, sea salt and Total Primary Particle Matter (total primary anthropogenic carbonaceous and non-carbonaceous species). The attribute \"\(-\)10\", \"\(-\)25\" or \"coarse\" are affected, respectively, for the PM\({}_{10}\), and PM\({}_{2.5}\) and the coarse (PM\({}_{10}\)-PM\({}_{2.5}\)) fractions. \"Total dust\" (sum over all bins including the bin above 10 \(\upmu\)m) is a terminology sometimes used in this study; the \"coarse\" fraction can include the last bin in some analyses. Horizontal transport is solved with the second-order Van Leer scheme. Subgrid scale convective fluxes are considered. The present set-up also benefits from an improved representation of turbulent mixing in urban areas that yields lower horizontal wind and vertical mixing to better capture the difference between the urban canopy (where the first CHIMERE model level lies) and the top of the urban sublayer of which the lowermost meteorological model level is representative [55]. As an offline CTM, CHIMERE requires prescribed meteorological fields which were provided here by ECMWF with the Integrated Forecasting System model at 0.125\({}^{\circ}\) resolution with data assimilation consistent with the CHIMERE horizontal resolution. Emissions for the main pollutants are issued from the EDGAR-HTAP initiative [60; 61; 62].