Upwind. China is most definitely upwind. Japan is downwind. Winds tend to flow from the West to the East. Has to do with the earths rotation and air density when heated by the sun and relative air pressures. Ever remember wind coming directly from the East. Only seen it a few times. Spinning hurricane remnants and other weird weather anomalies.
That only applies to the Northern Hemisphere. Southern Hemisphere works opposite. East to West there.
Much of it has to do with the upwelling of hot air in the tropics. Some of it has to do with which way the water flows circling a drain.
It is kind of complicated.