I was planting out my beans
The sky betook an awful shade
The queerest one I’d seen
From "The Third Millennium" (a song apparently so obscure that it's not on Youtube)
March and April 2013.
The real-time pollution-board, which stands at a busy local intersection, confirmed it. It showed PM-10 air pollution levels to be through the roof, at 200-some micrograms per cubic meter of air, as I recall. (Normal for Seoul is 60-70. Normal for the Washington, DC area -- my birthplace -- is about 20. The EU declares anything over 50 a health hazard, South-Korea's own arbitrary safety-threshold is 100).
This must have been the vaunted "Yellow Dust", blowing in from Central Asia. The then-new guy at work, C., spoke of "fog". Well, Korea doesn't have fog, I explained. (A foreign editor thinks it's "fog" here. Note on that photo: I'm of the belief that the yellow-dust cannot be adequately captured on camera).
Air Korea is a convenient real-time air quality site. From it, I took the screenshot at left. Each square is one hour of one day.
Green means (by Korean standards) normal air quality. Blue squares are hours when the air was "particularly" (heh) free of particulate-matter. Blues are, I think, associated with rainstorms. Yellow and Orange mean the air is getting unhealthy.
A 72-hour yellow-dust storm is visible on March 7th, 8th, 9th, trailing off before sunrise on the 10th.