I don't know why this pollution spike occurred.
The above sign also asserts that 100 is the "safety limit", though the USA and EU say that anything above 50 is harmful to human health.
I frequently look at these real-time pollution signs in Korea. It may be that 139 is the highest PM-10 level I've ever noticed, except of course for "yellow dust" season, the pollution spikes of which I wrote about way back in post-12 ["The Sky Betook an Awful Shade"].
To compare with home, the Washington DC region's PM-10 air-pollution yearly average is 18 micrograms per cubic meter, according to the WHO. Seoul rarely sees levels that low, maybe only after vigorous rainstorms clear all the junk out of the air.