## 4 Discussion on Regional Dust Events
### Dust Outbreaks over the Caribbean Areas  
The trans-Atlantic transport of dust from Africa to the Caribbean area has been extensively reported in the literature [4; 120]. Dust predictions are generally of acceptable quality for the first several days after dust emission up to 2000 km west of the African continent [121], the removal of dust from the atmosphere is too strong for large transport paths in the global models, and the simulated fine-to-coarse dust ratio (in terms of mass concentration and light extinction) is too high in the models compared to observations. This deviation is already given close to the dust sources and then increases with distance from Africa. No specific scientific publication was published so far on the recorded episodes in March 2014 over the Caribbean area. Figure 18 shows the evolution of dust concentrations along a cross section from Guinea to the French Guiana. This transport occurs in the low troposphere and is confirmed by the latitudinal cross section issued from CALIPSO (Figure 19) and very well reproduced by the model. The increase of the boundary layer height from the African shores toward South America is related to the rise of sea surface temperature along the path (cf. global sea temperature in March 2014 reported Figure S14 in Supplementary Materials accessible at the internet NASA resource: [[https://neo.sci.gsfc.nasa.gov](https://neo.sci.gsfc.nasa.gov)]([https://neo.sci.gsfc.nasa.gov](https://neo.sci.gsfc.nasa.gov))). The high boundary layer height over Africa mixes the dust within the boundary layer with a ventilation up to the free troposphere at about 5 km in altitude in this region. The fine fraction of dust increases from Africa to South America mainly due to sedimentation effects of the coarse particles but the total dust concentrations remain rather high along the pathway. The timing of the arrival on 5 and 6 March over the Caribbean is rather well captured by CHIMERE (Figure 20), however the concentration peak is overestimated. This good behavior of the model is partly due to the ability of the model to simulate the dust emission fluxes over the Sahara, the station in Cinzana close to emission sources exhibits a very good agreement between the model and observational data. The AOD simulated by CHIMERE at Dakar, Cinzana and Capo Verde stations (Figure 21) in March 2014 is very well reproduced compared to the AERONET retrievals with time correlations of 0.76, 0.70 and 0.79 respectively. A second phase of trans-Atlantic transport is observed end of March thanks to the CALIPSO data showing \"dusty marine\" and \"dust\" types of aerosols in the low troposphere also well reproduced by the model (Figure S3 in Supplementary Materials). However, the contribution attributed is low but a clear correlation can be observed between the dust load in CHIMERE and the PMcoarse measurements (Figure 20). Figure 18: Arrival of the main dust episode over South America. Cross section of total dust concentrations fields (Altitude versus Longitude from) from Africa (GU for Guinea 11\({}^{\circ}\) N) to South America (FG for French Guiana 3\({}^{\circ}\) N) from 3 March 0300 UTC to 5 March 0300 UTC. The dotted line represents the height of the boundary layer, the dashed lines represent the fraction of fine dust in percent. Figure 19: Cross section of CHIMERE dust concentrations (**right**) on 5 March 2014 15:00 UTC 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 trajectory CAR1 as reported in Figure 2. The dotted line represents the height of the boundary layer, the dashed lines represent the fraction of fine dust in percent. Figure 20: Hourly time series in March 2014 of simulated and observed PM concentrations in two stations of the Caribbean area operated by the French air quality networks (Cayenne, main city of French Guiana 4.93\({}^{\circ}\) N–52.31\({}^{\circ}\) W, and Fort-de-France in Martinique Island 14.61\({}^{\circ}\) N–61.05\({}^{\circ}\) W).