On the light difference between morning and afternoon
I am asking why the light of a clear day is not symmetrically changing centered at noon.


Trying to attribute this to a single cause misses the point. There are multiple causes. Other than lighting itself (i.e., the Sun angle), most aspects of the atmosphere are not symmetric about noon.

The low on a clear day typically occurs right around sunrise (not midnight), while the high typically occurs hours after noon. Assuming a front is not passing through, relative humidity is typically highest in the hours around sunrise, lowest a few hours before sunset, roughly coinciding with the low and high temperatures. Atmospheric stability is high in the early morning on a clear calm day, low in the afternoon and evening. During weekdays, pollution oftentimes is at a minimum around 6 to 8 AM and a peak around 5 to 7 PM.

All of these can contribute to the effects you are seeing, plus other factors I did not list.