## 3 Discussion Results at the Hemispheric Scale
### Overview of PM Concentrations Simulated by CHIMERE over the Northern Hemisphere in March 2014
Better agreements are found over two islands in front of China coasts with for instance in Cheju (33.52\({}^{\circ}\) N-126.48\({}^{\circ}\) E) about 27 \(\upmu\)g m\({}^{-3}\) for the measurements and 20 \(\upmu\)g m\({}^{-3}\) predicted by CHIMERE. The lowest values (<1 \(\upmu\)g m\({}^{-3}\)) of surface dust concentrations are simulated over the Equatorial Pacific, this is certainly due to low dust emissions downwind to the prevailing air mass fluxes from Central and South America. From Central to North Pacific the mean surface concentrations lie in the range 2-10 \(\upmu\)g m\({}^{-3}\) that are usual for a March period which generally exhibits the highest dust concentrations over the Pacific Ocean [91], these orders of magnitude are fully in line with the SEAREX experiments findings [93]. In Alaska, the CHIMERE model exhibits dust concentrations in the fine mode of particles 5 to 10 times overestimated at IMPROVE Stations (Denali 63.72\({}^{\circ}\) N-148.97\({}^{\circ}\) W, Gates of the Arctic 66.90\({}^{\circ}\) N-151.52\({}^{\circ}\) W and Trapper Creek 62.31\({}^{\circ}\) N-150.32\({}^{\circ}\) W) with observational data usually in the range 0.01-0.4 \(\upmu\)g m\({}^{-3}\) while CHIMERE simulates concentrations up to 1 to 5 \(\upmu\)g m\({}^{-3}\), however the end of March 2014 increase of concentrations is well reproduced by the model. This overestimation of background concentrations over remote areas is shared by several global models participating to the AeroCom phase I project [94], and also identified in the Conformal Cubic Atmospheric Model (CCAM) [95] where the dust lifetime is longer than the ones calculated in previous modeling exercises. Even if the objective of the paper is not to perform an exhaustive evaluation of the model for criteria pollutants, as it has been made in past studies (see previous sections for adequate references), Table 1 presents an overview of the model performances for PM\({}_{2.5}\) over Beijing, Europe and the USA. The performances will be more detailed in the next sections to analyze the ability of the model to simulate the identified dust events. For Beijing, the daily correlation is very good and the bias very low for this period. For Europe, as is generally the case in this period, an underestimation of PM\({}_{2.5}\) concentrations is observed but the correlation is rather good, particularly the spatial correlation. This underestimation is due to underpredicted ammonium nitrate concentrations in early spring pollution events and a lack of wood burning emissions (from the residential sector) in emission inventories [54]. For the USA, a negative bias is also observed with a lower correlation mainly driven by a low spatial correlation. In addition, Table S1 in Supplementary Materials provides an overview of the evaluation at AERONET sites; at these sites, CHIMERE generally overestimates the AOD and the time correlation is poor, except at low latitude stations where more hourly data are available thanks to clear sky conditions. However, the spatial correlation is better.