Overseas wildfires were among the causes of Korea’s heavy ultrafine dust episode on Nov. 24 last year, when advisories were issued in multiple central and western regions, according to the Seoul Institute of Health and Environment on Friday. The institute said a detailed chemical analysis of fine dust that day showed smoke from biomass burning abroad had mixed with local emissions and long-range industrial pollution to worsen air quality. Using a high-resolution aerosol mass spectrometer, the institute detected a clear fingerprint of biomass burning organic aerosol, or BBOA, in samples collected during the episode, confirming that overseas wildfires were one of several contributors. An official said there were no major wildfires burning in Korea at the time, while large forest fires were reported in China with winds blowing toward the Korean Peninsula, making it likely that the smoke traveled from there. Seoul’s ultrafine dust concentration that day averaged 76 micrograms per cubic meter, peaking at 92 micrograms per cubic meter. The advisory issued on Nov. 24 lasted for 11 hours and