## 3 Discussion Results at the Hemispheric Scale
### Particle Size Distribution  
The period 16-19 March is very active for dust episodes at the global scale; several outbreaks of dust in Asia and USA are observed occurring at the same time (Figure 5). These events will be discussed later in the regional focus of the study. The dust transport in Asia can be observed with pollution plumes advected toward the Pacific Ocean later in March, a transatlantic transport of dust from Africa to the Caribbean is also simulated on 16 March. Over North America, dust from the southwest of USA are transported over the Pacific Ocean. For this date, the bin number providing the maximum PM concentration is also plotted (Figure 6). This maximum is the highest close to the dust emissions area with a peak around bin 9 and 10, i.e., around 10 \(\upmu\)m. This peak is below 2.5 \(\upmu\)m (bin 5 and 6) for areas influenced by anthropogenic sources, and in-between elsewhere. At the bottom of Figure 6, the mean normalized size distributions for dust is displayed and compared to black carbon and sulfate species. This distribution is averaged over the whole hemispheric domain for each 16 model levels from the surface to the top on 18 March 2014. On average, for dust, a single peak around 2 \(\upmu\)m in particle diameter is predicted by the model, the contribution of the last bin decreases for the highest levels consistent with the effect of particles sedimentation, however a slight shift of the accumulation mode toward the coarse particles is also simulated as a consequence of particle growing by condensation of secondary species. The black carbon distribution displays a bi-modal pattern around 0.2 \(\upmu\)m and 3 \(\upmu\)m and consistent to what was observed in Europe during field campaigns on background sites [96]. For sulfates, the model simulates a single distribution peak around 0.8 \(\upmu\)m and also high concentrations around 0.2 \(\upmu\)m; this is consistent with experimental findings [97; 98] as the results of condensation of sulfate on the maximum of available particle surface for mass transfer. For AERONET sites, a main mode is generally observed around 4 \(\upmu\)m and a second peak around 0.2 \(\upmu\)m [99], this second peak being certainly due to anthropogenic sources (since the measurement is performed within the city), the main peak being driven by windblown dust emissions. In CHIMERE, internal mixing is assumed for aerosol particles; therefore, mineral dust participates directly or indirectly to chemical and microphysical processes (coagulation, condensation so far). This assumption is validated by experimental evidences in many studies [100] and allows a full interaction between all constitutive species of particles. Nevertheless, in this version of CHIMERE the formation of coarse nitrate due to the reaction of calcium with nitric acid (issued from the anthropogenic pollution) is not considered and could enhance the interactions between natural and anthropogenic emissions as it has been studied in China [101]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Domain**} & **Obs.** & **Mod.** & **RMSE * ** & **Total Number of** & **Spatial Correlation \(\ddagger\)** \\  & \(\upmu\)g m\({}^{-3}\) & \(\upmu\)g m\({}^{-3}\) & \(\upmu\)g m\({}^{-3}\) & **Cor \(\ddagger\)** & **Daily Data** & **(Data Points Per Month)** \\ \hline Beijing & 89.9 & 91.8 & 30.9 & 0.91 & 1030 & 0.75 (34) \\ Europe & 20.9 & 13.3 & 11.7 & 0.69 & 1037 & 0.86 (34) \\ USA & 13.3 & 8.1 & 8.4 & 0.30 & 12,953 & 0.04 (367) \\ \hline \multicolumn{6}{l}{* Root Mean Square Error; \(\ddagger\) Space-Time correlations based on total number of daily data; \(\ddagger\) Spatial correlation based} \\ \multicolumn{6}{l}{on monthly data for each station given in brackets.} \\ \end{tabular}
\end{table} 
Table 1: Average performance of CHIMERE for PM\({}_{2.5}\) based on daily values in March 2014 over the Beijing area (all urban and rural background stations), USA (all background rural stations) and Europe (EMEP background rural stations).