## 4 Discussion on Regional Dust Events
### Dust Outbreaks in Europe
The Murcia AERONET station (38\({}^{\circ}\) N-1.17\({}^{\circ}\) W, south of Spain) detects the dust event, the model is rather well in agreement with the observations at this station with a recorded AOD exceeding 1 and a low angstrom coefficient (close to 0) observed on 29 March confirming the coarse nature of particles (Figure 15). The time correlation between observed and modeled values is 0.85 on average for March 2014 on an hourly basis in Murcia. A pattern of high dust concentrations simulated by CHIMERE in the southwest of France and in the center of Spain can be observed in the top panel of Figure 8. Dust concentrations are generally higher in the mountainous regions certainly due to a deceleration of the dust-carrying flow [116] as it approaches the mountains with specific orographic effects around the Pyrenees leading to local peaks within the boundary layer observed in the southwest of France. Southerly air mass fluxes are associated to _foeln_ effects over the Pyrenees, this orographic effect has been observed in China [117] leading to very stable boundary layers in the up valleys enhancing the rise of air pollutant concentrations. However, north of the Pyrenees at about 100 km, this synoptic southerly flux usually generates a southeast wind (\"_Autan_\" wind) which lowers the concentrations and explains the high boundary layer  
Figure 14: **Top panels represent the AOD 555 nm retrieved by MODIS on 10 March as a composite of several orbits from 09:00 to 14:00 UTC (left), CHIMERE AOD 555 nm for the time range (right). Bottom panels represent the ground concentrations of total dust (right) and PM\({}_{10}\) (left) with observations represented by circles. **  
height even at night. Wind gusts up to 70 km h\({}^{-1}\) were recorded on 29 March in Auch, a small town 100 km north of the Pyrenees (43.65\({}^{\circ}\) N-0.58\({}^{\circ}\) E), while the meteorological conditions very close to the mountains were calm with a low boundary layer height. At the EMEP background station Peyrusse Vieille about 100 km north of the Pyrenees (43.62\({}^{\circ}\) N-0.18\({}^{\circ}\) E), an increase of the coarse fraction to 35% of PM\({}_{10}\) on 31 March compared to the days before is observed where usually the coarse fraction is in the range 20-25%. The same phenomenon is observed in Toulouse also close to the Pyrenees with a coarse fraction exceeding 50% for urban background sites on 31 March associated to an increase of PM concentrations (data are freely accessible through the French national forecasting system PREV'AIR, www.prevair.org). These potential orographic effects are captured at this resolution, with a dust plume coming from Sahara and blocked behind the Pyrenees mountain range (Figure 16). Table 2 reports the measurements and models results at the closest station (Billere 43.31\({}^{\circ}\) N-0.39\({}^{\circ}\) E in the suburb of Pau) in the up valley of the Pyrenees and confirms an increase of concentrations with a decrease of the PM\({}_{2.5}\)/PM\({}_{10}\) ratio confirming the presence of mineral dust in this area, although the concentrations are overestimated by the model. Figure 15: Time series of simulated (CHIMERE) and observed AERONET AOD 870 nm for March 2014 at Murcia (southeast of Spain). Figure 16: Vertical cross section of dust concentrations passing over the Pyrenees (PY) and starting from Africa (AF). The dashed lines represent the ratio PM\({}_{25}\) versus Total dust (particle diameter up to 40 \(\mu\)m). Figure 17 shows the end of March 2014 evolution of hourly PM\({}_{10}\) concentrations and mass distributions (expressed in _dM/dlogDp_) at the Pic du Midi station (elevation: 2876 m) located in the Pyrenees mountain range. The observed mass distribution was derived from the number distribution from 0.3 to 10 \(\upmu\)m) using a density of 2.6 g cm\({}^{-3}\) typical for dust [118]. The observations clearly show the sudden increase of coarse concentrations confirming the assumption of a desert dust outbreak.