## 4 Discussion on Regional Dust Events
### Dust Outbreaks over China
Figure 25: Average mass distribution of particles simulated by CHIMERE for the period 17 March 12:00–18 March 01:00 (Beijing Time) as studied in Yinchuan [140], China (38.22\({}^{\circ}\) N–106.48\({}^{\circ}\) E). As shown in Figure 26, the ratio PMcoarse/PM10 exceeds 50-60% during the previous identified episode; this threshold is usually mentioned to identify a mineral dust outbreak [16]. On average, the model captures quite well the evolution of PM concentrations over the Beijing area in March 2014 (Figure 27), the correlation is close to 0.8 for most of stations either for PM2.5 and PM10 concentrations with a very low bias. However, the coarse fraction seems underestimated by the model (Figure 26). The dust outbreaks impacting Beijing can be identified by the predicted dust concentrations with a sudden rise of PM concentrations particularly the PMcoarse fraction on 17 March peaking at 400 \(\upmu\)g m\({}^{-3}\) on an hourly basis over Beijing. Under less \"dusty\" conditions at the beginning of March, this coarse fraction is largely below 50 \(\upmu\)g m\({}^{-3}\). During the 17 March episode, the coarse fraction in the observations reaches 70% of PM10 daily compared to a model value of 72% in Yanqing near Beijing. A last dust episode at the end of March occurs in the simulation outputs starting at the end of an anthropogenic episode which took place between 22 and 27 March over Beijing. On 28 March, the CALIPSO data reveals the starting point of this dust episode over the Gobi Desert (Figure S5 in Supplementary Material, CNA5 trajectory) and a slight increase of coarse PM concentrations are observed at the rural station Yanqing. At this station, the coarse fraction reaches 50% on 28 March in the model compared to less than 20% the days before during the anthropogenic pollution episode, this fraction is even larger in the observations reaching 60%. A closer look at the observations in Figure 26 shows that CHIMERE simulates higher coarse fraction ratio for rural stations compared to the urban station while the observations show the opposite with the highest values at the traffic station. This reveals the importance of dust resuspension over the cities. Particles emitted from anthropogenic (road traffic, urban works, other human activities, etc.) or natural processes (windy conditions on flat surfaces) can lead to more dust in urban areas compared to nearby rural zones. These latter processes are not included or activated in CHIMERE for this study. Figure 26: Evolution of the ratio PMcoarse/PM10 in March 2014 for traffic (Dongsihuan 39.95\({}^{\circ}\) N–116.49\({}^{\circ}\) E), urban (Olympic Center 39.98\({}^{\circ}\) N–116.40\({}^{\circ}\) E) and rural (Yanqing 40.45\({}^{\circ}\) N–115.97\({}^{\circ}\) E) stations in the Beijing area from observational data and CHIMERE model outputs (not reported for the traffic station as the simulation resolution is not coherent). The evolution of the dust plumes over Beijing is displayed in Figure 28 which shows the evolution of concentrations along a longitudinal cross section passing over the capital of China. The dust comes from the Mongolian Plateau (1000-1500 m a.s.l. ), the Gobi Desert being the main source of this episode. Figure 27: Hourly time series of PM concentrations and dust concentrations issued from CHIMRE for the PM2.5, PM10 and PM\({}_{\text{coarse}}\) fractions versus observed values for two stations in Beijing: Yanqing (40.45\({}^{\circ}\) N–115.97\({}^{\circ}\) E) as a rural site located in the northeast of Beijing close to the Great Wall, and Olympic Center (39.98\({}^{\circ}\) N–116.40\({}^{\circ}\) E) as a urban site close to the center of Beijing. The orange shade area represents the dust concentrations simulated by CHIMRE for the corresponding PM matrix. The model simulates concentrations largely above 500 \(\upmu\)g m\({}^{-3}\) inside the plume that is located mainly within the boundary layer when hitting Beijing on 17 March.