The spacing depends on the canopy and atmospheric conditions. It is important to know where the concentration gradients are the steepest.
For example, in a low agricultural crop canopy, it may be appropriate to use a logarithmic distribution within and near the top of the canopy, and a linear distribution above the canopy: 0.2 m, 0.4 m, 0.8 m, 1.6 m, 3.2 m (logarithmic) and 5 m, 7 m, 9 m (linear).