Warmer temperatures were expected to continue on Tuesday, but a thick haze blankets the Seoul metropolitan area.
On Wednesday, it is forecast to rain or snow across the nation.
The National Institute of Environmental Research said that the concentration of fine particles in the Seoul metropolitan area will rise to 81-120㎍/㎥ on Tuesday, and warned the elderly and those with respiratory diseases against staying outside for long.
In Seoul, the morning low hovered around -2 degrees Celsius, and the day's high will reach 7 degrees, the Korea Meteorological Administration forecast. That is similar to the average in early March.
It will begin to rain or snow in southwestern parts early Wednesday morning, expanding to the entire country through the day.
Most parts of the nation will clear up by nightfall, but then the mercury will plummet and a cold snap will sweep the country, the KMA said.