## 4 Discussion on Regional Dust Events
### Dust Outbreaks in Europe  
As mentioned in the Introduction, Europe is commonly influenced by Saharan dust outbreaks, particularly over the Mediterranean basin. France, Belgium, Netherland and Germany were affected mid-March 2014 by an important anthropogenic pollution episode due to a combination of favorable meteorological conditions and emissions from residential heating, industry, road traffic and particularly agricultural ammonia during this intensive spreading period [114]. At the end of March and beginning of April, a second episode was recorded [87]. In the early part of the episode, Saharan dust remained aloft above the UK; the presence of dust at surface level was only simulated during the second part of the episode and over a small geographic area in southern UK. The analysis presented in their study [87] illustrates the capability of air quality models, corroborated with chemically-speciated measurements, to identify the underlying causes of complex PM air pollution episodes. Specifically, the analyses highlight the substantial contribution of secondary inorganic ammonium nitrate PM, with agricultural ammonia emissions in continental Europe presenting a major driver. This event, also observed by other satellites like Meteosat-10 [115], was associated with rainfalls in UK, France and Italy. For March 2014, the time correlation of dust concentrations is quite low for the stations far from the sources but for the Mediterranean station CY0002R (Figure 13 exhibits a good correlation capturing the dust outbreaks from Africa to Asia. The Armenian station AM0001R (Figure 13) is located close to Asian emission areas and is also influenced by Saharan dust outbreaks under southwesterly air masses fluxes. However, an overestimation of predicted concentrations by the model is observed at these stations, in line with the general overestimation far from emission sources. A spectacular dust outbreak over the Atlantic Ocean on 10 March was recorded by MODIS data as shown in Figure 14 and well captured by the model. The dust plume emitted over the Sahara was quickly ventilated in the free troposphere and blocked by high pressure systems developing over the European continental areas which led to the development of the anthropogenic pollution mid-March over the western Europe. On 10 March was the very beginning of this anthropogenic pollution event but this dust outbreak remained over the ocean in the free troposphere and partly in marine boundary  
Figure 13: Daily time series of dust concentrations (in the PM\({}_{10}\) or PM\({}_{2.5}\) matrix) for four sites in Europe in March 2014 (ES0014R 41.40\({}^{\circ}\) N–0.72\({}^{\circ}\) E Els Torms, Spain; AM0001R 44.26\({}^{\circ}\) N–40.38\({}^{\circ}\) E Amberd, Armenia; DK0012R 55.69\({}^{\circ}\) N–12.09\({}^{\circ}\) N–E Risoe, Denmark; CY0002R 35.04\({}^{\circ}\) N–33.06\({}^{\circ}\) E, E Ayia Marina, Cyprus). layer; as shown in Figure S2 (EUR1 and EUR2 trajectories), the model captured well this event and its vertical extent up to 9 km in altitude. The main dust outbreak, which concerns the ground level in Europe, started on 29 March with dust emitted over the Sahara. Calcium measurements (transposed in \"dust\" measurements by multiplying by a factor 8 [84]) confirm that the dust event was mainly confined in the free troposphere as shown by the CALIPSO data displayed in Figure S2 in Supplementary Materials. The comparison with CHIMERE (Figure 13) shows the model tends to transport a fraction of the plume within the boundary layer in several parts of Europe as supported by modeling results presented in Figure S2 in Supplementary Materials. This is confirmed at the Spanish station ES0014R close to Barcelona in the lands where dust concentrations tend to increase (Figure 13).