## 3 Results and Discussion
### Satellite Images of Yellow Dust Transport
Normally, the transport of the dusts from Beijing city to Gangneung city takes approximately two days by a prevailing northwesterly wind of about 8 m/s to 10 m/s simulated by Weather Research and Forecasting Model (WRF-3.6.1), because the distance between Beijing city and Gangneung city is about 1400 km. Thus, as the two-day earlier PM data of Beijing city correspond to the PM data of Gangneung city (Figure 4), the PM data of Gangneung city from 01:00 LST, 21 March to 00:00 LST, 23 March (during the dust period) can be affected by the PM data of Beijing city from 00:00 LST, 19 March to 01:00 LST, 21 March. Figure 4: Hourly distribution of PM\({}_{10}\), PM\({}_{2.5}\) and PM\({}_{1}\) concentrations from 16 March to 25 March, 2015 in Beijing city, China. As the transport of the dust of Beijing city to Gangneung city takes two days by 8 m/s northwesterly wind simulated by WRF-3.6.1 numerical model, and [PERSON] and [PERSON] [22], two-day-earlier PM data (01:00 LST, 19 March to 00:00 LST, 21) correspond to PM data of Gangneung city for the dust period (01:00 LST, 21 March to 00:00 LST, 23) in Figure 3.  
Figure 3: Hourly distribution of PM\({}_{10}\), PM\({}_{2.5}\) and PM\({}_{1}\) concentrations, before, during and after the dust period from 18 to 27 March, 2015 at Gangneung city, Korea. In Gangneung city, vehicles on roads, boilers for heating and cooking in residential areas are the main sources of pollutant emissions, due to there being no factories emitting a large amount of air pollutants. The PM\({}_{10}\) concentration before the yellow dust period appeared as 28.8333 to 82.1667 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\), which was 10 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\) lower or 20 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\) higher than ones of the previous case studies. When PM\({}_{10}\) increased, the concentrations of PM\({}_{2.5}\) and PM\({}_{1}\) increased at the same time, and vice versa. A very high PM concentration appeared once each in the morning and afternoon, and the concentration was particularly high at 08:00 LST to 09:00 LST (the time to go to work). The maximum concentration was shown at 20:00 LST (the time to leave the office). The high PM concentrations from 08:00 to 09:00 LST were attributed to the emission of a large amount of air pollutants, such as particulate matters and gaseous substances through the combustion of vehicle fuel and a large amount of dust scattered by the movement of the vehicles on the road. Similarly, at 20:00 LST or 22:00 LST (two or three hours after the time to leave work), the PM concentration was also very high, with a maximum concentration. It was due to a large amount of both particulate matters and gaseous emitted from vehicles and scattered dust flied from the road, and added air pollutants emitted from boilers for heating and cooking in the resident area under the relative cool weather in the early evening in the city in March. During the daytime after 09:00 LST, PM concentration was generally low. [PERSON] and [PERSON] [PERSON] and [PERSON] (2016) reported that as the ground surface is heated by the solar radiation, the heated air rises vertically, accompanying the air pollutants emitted from vehicles and fling dusts on the road to the top of the atmospheric boundary layer d to about 1 km height, resulting in the low concentrations of particulate and gaseous. Differently, as the nocturnal surface inversion layer (NSIL) is formed with a very compressed thin layer of about 200 m in height, due to the ground surface cooling (called a strong stable layer). This temperature structure can induce the falling of air toward the ground surface and accumulation of pollutants near the surface, an increase in the pollutant concentration, occurs with a maximum concentration of PM at 22:00 LST, similar to previous cases with a deviated occurrence time [PERSON] and [PERSON] (2016); [PERSON] (2016).