Back to the topic at hand...
I think it is psychology rather than science. I suspect that they recommend morning walks because if you wait to do it at night, life happens, we get busy and it doesn't happen at all. Plus, I'm sure that the cool morning air (especially here) is invigorating. I wouldn't know, I walk at night.
As for the quality of the air, I suspect the answer would be regional. For example, in the winter, there are fewer active plants. Temperature inversions may hold pollutants, etc closer to the ground.
Wickpedia states, "In meteorology, an
inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to a
temperature inversion, i.e., an increase in temperature with height, or to the layer within which such an increase occurs.
[1]
An inversion can lead to pollution such as
smog being trapped close to the ground, with possible adverse effects on health."
Prevailing winds may take the pollutants away or bring them in depending on the winds and your location. Perhaps there is a stray meteorologist around who has some input?