To protect the citizens’ health, Seoul Metropolitan Government monitors the air quality of the city including the level of fine dust, ozone, and yellow dust in real time and collects basic data to establish plans to improve the air quality. SMG put in place a comprehensive monitoring system which is composed of the City Air Quality Monitoring System, Roadside Air Quality Monitoring System, Acid Deposition Monitoring System, Heavy Metal Monitoring System, and the Photochemical Pollutant Monitoring System.

After introducing 4 air quality monitoring stations in 1973, SMG added 27 City Air Quality monitoring stations, 2 Clean Zone monitoring stations, and 9 Roadside Monitoring stations by 2008 to measure the level of atmospheric concentration. Since Seoul is highly affected by northwestern monsoon unlike many other regions in Korea or cities overseas, the monitoring system was reorganized in 2011 to identify the composition, concentration, and routing of long-range transboundary air pollutants, understand the types of pollutants near national boundaries, and measure and control roadside air pollutants. Every administrative district has its own City Air Quality Monitoring System and three Boundary Monitoring stations were established in Haengju, Gungdong and Segokdong. In addition, the monitoring station at Gwanaksan monitors long-range transboundary air pollutants whereas the stations established in Namsan and Bukhansan measure the air quality of high attitude region in Seoul and the air quality of clean zones, respectively. In addition, 12 monitoring stations were installed in major artillery roads with high traffic levels to manage the rate and change of pollutants emitted by automobiles. Furthermore, monitoring stations were added to the exclusive bus lanes and vehicle-only lanes in 2011, so now there are 15 roadside air quality monitoring stations.
 

City Air Quality Monitoring System
City Air Quality Monitoring System is composed of 25 monitoring stations in Seoul and the main purpose of the system is to improve air quality, collect data on the air pollution level of heavily-populated areas, and check if the area meets the environmental standards. The monitoring stations collect data on various pollutants such as SO2, NO2, CO, O3, PM-10, PM-2.5, and meteorological information including the wind direction, wind speed, temperature, humidity, solar radiation and ultraviolet rays. These data are collected every hour on a continuous basis. Specifically, Pulse U.V Fluorescence Method is used to measure SO2 whereas Non-Dispersive Infrared Method, U.V Photometric Method, and β-Ray Adsorption Method is used to measure carbon monoxide, ozone level, and fine dust, respectively. These data are used when conducting research on the atmospheric condition of Seoul and devising policies to improve air quality.
 
Roadside Air Quality Monitoring System
To measure roadside air quality in area with high traffic and large floating population, 15 Roadside Air Quality monitoring stations were established with 11 installed on city streets, 2 on exclusive bus lanes, 2 vehicle-only lanes. Due to vehicle exhaust and street dust, the level of air pollution on the roadsides tends to be higher compared to other areas in the city. Especially, the concentration of nitrogen dioxide is measured considerably higher than at the City Air Quality monitoring stations. The concentration of carbon monoxide also tends to be high and shows significant difference at different times during the day, which may be the result of nitrogen oxide emitted by automobiles.
 
Heavy Metal Monitoring System
Heavy Metal Monitoring System measures the atmospheric concentration of heavy metals such as mercury, lead, and cadmium to evaluate the environmental impact of such heavy metals. The data collected from the system can also be used as base line data to devise policies on hazardous heavy metals. Mercury is measured around the clock with an automatic measuring instrument installed at monitoring stations in Guro-dong, Bangi-dog, Nowon-gu, and Hannam-dong. Most heavy metals in the atmosphere are emitted from artificial and natural sources and tend to cluster with dust in the atmosphere. Exposure to even a small amount of heavy metal can pose a serious health risk. Samples are collected and measured at the second week of every month, and every day during the yellow dust period using the High Volume Air Sampler. Measurement of 8 types of metals including lead, cadmium, chrome, copper, manganese, steel, nickel and mercury are collected on a regular basis. During the yellow dust period, the amount of aluminum, calcium and magnesium in the atmosphere are also measured. Inductively coupled plasma emission spectroscopy is used as the analysis method for the Heavy Metal Monitoring System
 
Acid Deposition Monitoring System
Acid deposition refers to acidic materials that sink to the surface from the atmosphere. Acid deposition is classified into wet deposition and dry deposition. Wet deposition includes acid rain, snow, and fog whereas dry deposition includes materials in aerosol state that fall or sink to the surface by the forces of gravity, such as particle material PM-2.5, gaseous material NO2, and ionic material SO42- The most common type of wet deposition is acid rain, which refers to rain with a pH level below 5.6.
Acid rain is created from the reaction between sulfur oxide, nitrogen oxide and other materials emitted in the atmosphere and is classified as a long-range transboundary air pollutants because it can affect large areas. Acid rain can corrode buildings and bridges, and cause skin diseases among children or the elderly who are exposed to acid rain for a long time. Furthermore, acid rain damages the ecosystem by preventing plant life from absorbing water, restricting the decomposition of organic matters in the soil, and contaminating the water. Currently, Seoul operates 10 Acid Decomposition monitoring stations to measure the pH level and analyze the acidic/alkali ionic components of acid rain.
 
Photochemical Pollutant Monitoring System
Megacities with high population density and high traffic volume like Seoul tend to record high concentration of ozone in the atmosphere and nitrogen dioxide. Most of the ozone on the earth’s surface is formed by photochemical reaction between nitrogen oxide and volatile organic compounds (VOCs). Thus, to manage the concentration of ozone, controlling nitrogen dioxide and precursor VOCs is very important. Against this backdrop, Seoul installed 9 monitoring stations to measure photochemical pollutants to measure 56 items on an hourly basis. The measurements collected at these monitoring stations are used as basic data to come up effective measures to cope with VOCs that are known to increase ozone concentration and pose serious health risks. The average concentration of Benzene in Seoul is measured at 0.3~0.6ppb, which is lower than the criteria set by the Ministry of Environment (5㎍/m3 or 1.539ppb) and the recommendation of the EU (1.5ppb).
 

By establishing a comprehensive air quality monitoring system, Seoul has secured basic data which can be used to identify the trends in atmospheric conditions and causes of changes in atmospheric concentrations and devise measures to improve air quality. In addition, the city government uploads the collected data in real-time on the website and mobile application to boost administrative transparency and inform the citizens. The data collected at the monitoring stations are used to issue forecasts and warning on fine dust, yellow dust, ozone, and even ultrafine particles, all of which are aimed at ensuring the health and safety of the citizens.