Go to Korea when there are fine dust particles.

Korea is heavily polluted in winter due to Chinese heating.

It is because of winds from China to Korea in winter.

In the summer, the wind from Korea to China blows.

So the best season to travel to Korea is summer.

There is an ozone(O₃ ) warning in summer, but it is a few days.

http://aqicn.org/here/

https://earth.nullschool.net/#current/particulates/surface/level/overlay=pm1/orthographic=-251.21,26.05,976/loc=123.742,18.186

Japan Okinawa has good air.

In contrast to Korea, Okinawa is a low season for summer.

The reason is because of the rainy season.(It is from mid-May to mid-June or the end of June.)

I can go to Korea in summer and Okinawa in winter.

https://www.accuweather.com/ko/jp/okinawa-shi/219498/february-weather/219498

Okinawa is opening its beach from the end of March.