## 4 Discussion on Regional Dust Events  
### Dust Outbreaks over North America  
In arid and semi-arid climates of North America (Arizona, New Mexico, California, Utah, Colorado and Texas), dust storms are commonly observed, particularly during the March-May period [109; 110]. Several dust episodes can be observed over the southern USA in March 2014, some  
Figure 9: **(Top)** MISR (on board TERRA satellite instrument) monthly mean AOD 555 nm for March 2014 with cloud screening; and (**Bottom**) mean AOD 555 nm simulated by CHIMERE without cloud screening. of them have been discussed and commented in media and press release of scientific institutions only. Over North America, on 11-12, 16, and 18-19 March, episodes were reported in media [111] and institutional Internet resources [112; 113]. The 18 March episode was particularly commented: a low-pressure system brought strong winds--gusting to 85 km h\({}^{-1}\)--to the Southern Plains. The winds picked up exposed soil from the parched landscape, resulting in a large dust storm that covered parts of Colorado, Kansas, Oklahoma, New Mexico, and Texas. The storm was the second in the week to sweep across the region with similar wind patterns. The dust originated from New Mexico. The plume stretched across about 175 km, but was dissipated a couple of hours later. The large images reported in the NASA report [113] shows dust across a wider area, including a second large dust plume in southern Colorado, Kansas, and Oklahoma. Some stations from the USEPA air quality network have been selected to analyze the March 2014 dust events close to soil dust emission areas, mostly in the southwest of US (Figure 10). PM\({}_{2.5}\) observations have been selected to be consistent with the IMPROVE data mainly focused in fine fraction of particles. The three episodes can be identified in most of selected stations and are clearly correlated to the presence of dust in the model outputs. The correlation coefficient based on hourly values is in the range 0.3 to 0.6 at these stations and the timing of the soil dust episodes is rather good (a correlation based on hourly value is very hard for such a model), however the intensity of these events is underestimated at the station. On average, the bias is rather low, the background observed and simulated concentrations are closed to 10 \(\mathrm{\SIUnitSymbolMicro g}\) m\({}^{-3}\). During the dust outbreaks, PM\({}_{2.5}\) concentrations can reach and exceed 100 \(\mathrm{\SIUnitSymbolMicro g}\) m\({}^{-3}\). The evolution of soil dust concentrations in the PM\({}_{2.5}\) matrix of particles is displayed in Figure 11 at IMPROVE stations and confirms the ability of the model to rather well locate the pattern of the plume events with sometimes a clear overestimation of dust concentrations during the dust events. However, on average, for IMPROVE stations a negative bias is observed for March 2014, \(-23\%\), and a correlation of 0.39. From 16 March, the dust plume is advected toward Mexico over the Baja California region. Manned SYNOP observations indicate the presence of dust from 16 to 21 March particularly in Baja California (Ciudad Constitucion: 25.00\({}^{\circ}\) N-111.67\({}^{\circ}\) W) where the presence of dust is reported several times on 16 March. All stations in Chihuahua (city located in the very north of Mexico close to the USA border) displays high PM concentrations on 13 March with PM\({}_{2.5}\) hourly values largely exceeding 100 \(\mathrm{\SIUnitSymbolMicro g}\) m\({}^{-3}\) at the Sur station (urban station located 28.63\({}^{\circ}\) N-106.04\({}^{\circ}\) W) and two other peaks on 17 and 21 March; these three events are well detected by CHIMERE despite an underestimation (Figure S12 in Supplementary Materials) certainly due to local dust or resuspension of road traffic dust. Mexican data can be accessed at [[http://sinaica.inecc.gob.mx/](http://sinaica.inecc.gob.mx/)]([http://sinaica.inecc.gob.mx/](http://sinaica.inecc.gob.mx/)) that is the Air quality information system in Mexico.