## 2 Status and Risks of Particulate Matter in Korea  
### Characteristics of Particulate Matter Generation in Seoul  
To identify the characteristics of PM generation in Seoul, we investigated the annual average concentration of PM along with its highest levels of concentration. As shown in Figure 1, the average concentrations of PM\({}_{10}\) and PM\({}_{2.5}\) in Seoul were calculated using Seoul's atmospheric environment information from the past 10 years. From 2009 to 2018, the average annual concentration of PM\({}_{10}\) in Seoul was 53.8 \(\upmu\)g/m\({}^{3}\) and the average annual concentration of PM\({}_{2.5}\) was 27.2 \(\upmu\)g/m\({}^{3}\)[3]. The average annual concentration of PM\({}_{10}\) decreased from 76 \(\upmu\)g/m\({}^{3}\) in 2002 to 41 \(\upmu\)g/m\({}^{3}\) in 2012 but this decline has since slowed. Since the government enacted the Special Act on the Improvement of the Air Quality in the Seoul Metropolitan Area and established and performed its Basic Plan for the Management of Air Quality in the Seoul Metropolitan Area in 2003, air pollution as well as PM concentrations have been reduced [7]. However, no further improvement has occurred since 2012.  
The results of a survey of \"bad\" and \"very bad\" days exceeding a PM\({}_{2.5}\) of 35 \(\mu\)g/m\({}^{3}\) for an average of 24 h showed that 44 days in 2015, 73 days in 2016, 64 days in 2017 and 61 days annually on average were recorded as \"bad\" and \"very bad\". This means that the number of days when the concentration of PM was significant is very large. This can be seen by comparing the number of PM warnings and days of alarm in Seoul. Figure 2 shows the number of PM and ultra-PM warning days using Seoul's atmospheric environment information. If there is a PM\({}_{10}\) of 150 \(\mu\)g/m\({}^{3}\) or a PM\({}_{2.5}\) of 75 \(\mu\)g/m\({}^{3}\) for more than 2 h, a warning is issued. If there is a PM\({}_{10}\) of 300 \(\mu\)g/m\({}^{3}\) or a PM\({}_{2.5}\) of 150 \(\mu\)g/m\({}^{3}\) for more than 2 h, an alarm is issued. In 2018, there were 17 PM warning days, 1 PM alarm day and 7 ultra-PM warning days. Seoul residents were exposed to extremely high dust concentrations on these days.  
This result is closely related to the monsoon season experienced in the geographical location of Korea. Korea's winter atmospheric circulation is affected by the northwestern winds associated with the winter monsoon in East Asia and the location of the barometer over the Korean peninsula. Inflow from China and Mongolia in winter greatly affects the concentration of PM in Korea [13]. Figure 3 shows the changes in the concentrations of PM and ultra-PM from March 2018 to February 2019 [3]. It shows that the levels of PM are high in winter and spring and low in summer. Specifically, they are lowest in September and highest in January. The atmospheric environmental standards for domestic PM are below 50 \(\mu\)g/m\({}^{3}\) on average for 24 h and below 25 \(\mu\)g/m\({}^{3}\) on average per year. Korea's levels exceed these standard values in all seasons except summer.  
Figure 1: Annual mean particulate matter (PM) concentration in Seoul (2009–2018).  
Figure 2: High PM concentration days: (**a**) days exceeding a PM\({}_{2.5}\) of 35 \(\mu\)g/m\({}^{3}\) on average for 24 h; (**b**) PM and ultra-PM warning and alarm days (Source: Seoul Atmospheric Environment Information).  
The air quality in Korea deteriorates considerably as the number of days with seasonally high concentrations of PM increase [14].