Meanings of atmospheric radioactivity in English

He found a high level of radioactivity- atmospheric radioactivity-over the whole area, greater in the north than in the south, as you'd expect.

There's a school of thought among the scientists, a section of them, who consider that this atmospheric radioactivity may be dissipating-decreasing in intensity, fairly quickly.

If Jorgensen is correct, as you go north from the equator the atmospheric radioactivity should be steady for a time and then begin to decrease.

Atmospheric radioactivity was still increasing, but towards the northern end of the flight it was increasing slowly.