## 3 Discussion Results at the Hemispheric Scale  
### Overview of PM Concentrations Simulated by CHIMERE over the Northern Hemisphere in March 2014  
The map of total emissions for March 2014 is displayed in Figure 3. This map is a combination of the various meteorological and soils related processes computed on an hourly basis. The main emission areas are closely linked to the erodibility map (Figure 1), the main desert area can be clearly identified (Algeria, Tunisia, Libya, Gobi Desert in China and Mongolia, Arabian Peninsula and to a lesser extent the US desert area) and particularly the _Bodele_ depression in the south of the Sahara (Chad) which is the most intense source of dust worldwide. In Europe, and especially in Spain, the too high soil moistures in this season makes impossible the emission of agricultural dust. Significant dust emissions around the Caspian Sea are simulated; this specific pattern is discussed in Section 4.4. Figure 4 represents the mean surface concentration maps of PM for March 2014 over the Northern Hemisphere. The main hot spots of PM\({}_{10}\) concentrations are located over the arid and semi-arid regions and also over the most populated areas in Asia. High PM levels are simulated in the southwest of the USA. For the SIA species, the patterns are rather different with high concentrations observed in Europe dominated by ammonium nitrate concentrations and in Asia with mainly due to sulfate. The influence of PM pollution events [87] is very strong and usual for the early spring period. In March, high SOA concentrations are simulated below the 40\({}^{\circ}\) N latitude, the contribution of biogenic species is important in Africa and South America while the contribution of anthropogenic emissions is more important in Asia. The primary emissions are very large over the most populated areas in Asia and Africa (e.g., Nigeria). Sea salt emissions are mostly influenced by wind speed and the resulting concentrations are obviously high over the North Pacific and Atlantic oceans where the low-pressure systems are still active in early spring, the order of magnitude is in agreement with observational data as reported on coastal areas [88; 89]. Figure 4: Mean monthly surface PM concentrations and its composition in the PM\({}_{10}\) matrix in March 2014. Figure 3: Total dust emissions (particles diameter from 10 nm to 40 \(\upmu\)m) in g m\({}^{-2}\) for March 2014 simulated by CHIMERE. At the global scale, the PM pattern is largely influenced by the dust concentrations, two dust plumes issued from the continental areas can be observed from the Southwest US to the South Pacific and from the Sahara to the Caribbean area. A more diluted transport of dust is also observed over the North Pacific Ocean. For mineral dust, the comparison with observational data is difficult, as such data are very scarce in the literature. Globally, the patterns of surface dust concentrations in the bottom panel of Figure 4 are very close to the multiannual structure shown in previous studies based on SYNOP meteorological observations [90], moreover the global increase of dust load in Figures S7-S10 in Supplementary Materials during the course of March 2014 is confirmed by data reported in previous works showing the yearly cycle of dust concentrations with the beginning of the \"dust season\" in February over the Northern Hemisphere [91]. Observational data for a March period based on available data are reported in the literature [92], unfortunately the exact data are not available: observed values are averaged from available data, but a first estimate shows that CHIMERE for March 2014 produces the same order of magnitude for dust concentrations, the main hot spots in Cape Verde (16.70\({}^{\circ}\) N-22.90\({}^{\circ}\) W) and Izana (28.30\({}^{\circ}\) N-16.50\({}^{\circ}\) W) are rather well reproduced by the models with respectively about 100 and 10 \(\upmu\)g m\({}^{-3}\) against observed values generally close to 200 and 30 \(\upmu\)g m\({}^{-3}\).