Smog initially caused by pollutants from China

The Seoul city government said the unprecedented London-like smog that descended on Seoul from Sunday to Thursday was initially caused by the inflow of pollutants from China.

“Our conclusion was that the smog was initially caused by the inflow of pollutants from China around Sunday, then worsened on Monday and continued to worsen from Tuesday to Thursday,” the city government said in its statement on Friday. “The recent levels were largely brought on by the inflow of international pollutants and worsened by domestic causes.”

The smog persisted for several days and worsened due to rising levels of nitrate, which the city government attributes to vehicle emissions and residential boilers.

“The pattern we saw in the past few days in Seoul was very rare,” said Hwang Bo-yeon, head of the Climate and Environment Headquarters of the Seoul Metropolitan Government. “We saw nitrate in the air expand almost 10 times within days. Nitrate is formed when emissions from cars and boilers meet ammonia in the air.”

“The nitrate in the air grew 10 times, from 2.2 micrograms per cubic meter on Jan. 12 to 22.6 micrograms on Jan. 16,” said Jung Kweon, general director of the Research Institute of Public Health and Environment of the Seoul city government. “When you see the level of PM2.5 fine dust particles in the air, you can tell it coincides with the rising level of nitrate. So the two are directly related.”

The highest hourly average of PM2.5 per cubic meter of air in Seoul from Jan. 14 to 18 was 105 micrograms, according to the institute. The lowest was 35 micrograms.

“The smog was made worse because of these secondary reactions of pollutants in the air,” Hwang said. “And the yellow dust may not have so much to do with this recent phenomenon in particular, because it carries mostly PM10 particles, which don’t react with the air to create secondary pollutants. The cause of this smog was nitrate, which mostly comes from emissions from cars and boilers, because Seoul doesn’t have coal plants in the city.”

PM2.5 fine dust particles are less than 2.5 micrometers in diameter, about 30 times smaller than the width of a human hair, and can travel through the respiratory tract and reach the bloodstream. The World Health Organization describes fine dust as carcinogenic and recommends exposure of no more than a daily average of 25 micrograms.

PM10 fine dust particles are less than 10 micrometers in diameter.

In a study in 2016, the Seoul Institute said that 25 percent of fine dust particles in the air was caused by vehicle emissions. Seoul’s recent policy of providing free public transportation during severe air pollution to encourage more people to take buses and subways instead of driving their own cars has come under public scrutiny. The policy costs some 4.8 billion won ($4.5 million) a day.

“The Seoul city government knew that pushing through the free public transportation policy on its own, without cooperation from Incheon and Gyeonggi, may not produce the desired effect immediately,” Hwang said. “But the local governments and the Ministry of Environment are holding discussions to coordinate their fine-dust policies.”

The local governments of Seoul, Incheon and Gyeonggi, and the Environment Ministry held a meeting Friday in central Seoul to discuss possibilities of extending their ban on odd- and even-number license plates of vehicles of public officials during severe air pollution to ordinary citizens.

“We realize there are loopholes in our fine-dust policies,” said the ministry’s climate and environment policy official Kim Jong-ryul. “We will discuss how to better fight air pollution.”

BY ESTHER CHUNG [chung.juhee@joongang.co.kr]