## 5 Conclusions
While the timing of dust outbreaks is rather well simulated, one identified shortcoming of CHIMERE is the difficulty to simulate the background concentrations and the magnitude of dust concentrations far from emission source areas. Additional improvements must be performed to better simulate the lowest concentration values far from emission areas which seems overestimated. Processes such as wet deposition will be revised in the next version of CHIMERE to improve the model results. The implementation of a more complete aqueous chemistry module is also one of the next challenges of the developments for the CHIMERE community [147]. The evolution of the size distribution has to be better analyzed in the next studies. The nitrate coating of coarse mineral particles could also influence their removal [148; 149]; this process must be implemented in the next version of the model. In addition, the capacity of the model to transport thin dust layers has to be improved, indeed the chemistry transport models tend to vertically diffuse the dust within the boundary layer and the free troposphere, more layers has to be implemented in CHIMERE in accordance with the available layers in the meteorological model driver. This issue has been recently studied [150], the long-range transport of pollutants is improved by an increase of vertical resolution; 100 m layers in the free troposphere is expect to favor a more realistic transport of thin dust layers, even if CHIMERE displayed a rather good ability to simulate the vertical profile of dust concentrations [151]. An improvement of the vertical resolution within the boundary layer seems less important [152], except close to the ground where the roughness plays an important role on the local dispersion of emitted pollutants. As identified in recent studies [153], the interactions of dust with meteorology through the radiative budget have also a strong influence on the emissions and diffusion within the boundary layer justifying the current work on a full-coupled version of CHIMERE with a mesoscale meteorological model [3]. These interactions must be considered between dust, clouds and chemical processes, which could influence the chemical regimes of trace elements in cloud droplets [154]. At the urban scale, the resuspension processes of natural dust or issued from anthropogenic activities (industries, constructions, road traffic, etc. )must be revisited in CHIMERE, particularly to better simulate the PM\({}_{10}\) concentrations in urban areas influenced by dry climatic conditions such as Beijing in early spring before the rainy season. **Supplementary Materials:** The following Supplementary Materials are available online at www.mdpi.com/2073-4433/8/12/251/s1, Table S1. Error statistics (Correlation, Root Mean Square Error-RMSE, Bias) for CHIMERE at several AERONET stations over the Northern Hemisphere for March 2014; Table S2. Characteristics, locations of EBAS-EMEP sites used in this study; Figure S1. Cross sections of CHIMERE dust concentrations (right) for several orbits zoomed over the highest values area versus the corresponding CALIPSO qualitative data (left) showing in yellow the dust load and in orange the polluted dust, for the CALIPSO trajectories NAM1-6 as reported in Figure 2 in the publication. The dotted line represents the height of the boundary layer, the dashed lines represent the fraction of fine dust in percent; Figure S2. Cross sections of CHIMERE dust concentrations (right) for several orbits zoomed over the highest values area versus the corresponding CALIPSO qualitative data (left) showing in yellow the dust load and in orange the polluted dust, for the CALIPSO trajectories EURI-5 as reported in Figure 2 in the publication. The dotted line represents the height of the boundary layer, the dashed lines represent the fraction of fine dust in percent; Figure S3. Cross sections of CHIMERE dust concentrations (right) for several orbits zoomed over the highest values area versus the corresponding CALIPSO qualitative data (left) showing in the publication.