1. Radioactive decay
The phenomenon which accounts for the decrease in fallout exposure rates over time is called Radioactive decay.
Fallen particles accumulate on the ground, on the roofs of buildings, trees, ladders, and all other areas exposed to nature. As fallout accumulates, the unsheltered exposure rate will increase until most of the particles have fallen.
At this point, exposure rates will begin to decline over time due to radioactive decay. The decrease is rapid at first, then slower and slower. The exposure rates decrease with respect to the rate of radioactive decay.