I have a HEPA filter air cleaner with 16 pounds of activated carbon. It's suppose to last up to years, but my experience is that after a few months (if not weeks) of operation under a light load like a bedroom, the air starts to exude a sweetish smell that is sometimes like honey and sometimes like citrus fruit. This has been explained by the vendor as possibly due to the fact that the activated carbon is derived from coconut hust, and I've verified from Wikipedia that this is typical.
I noticed repeatedly over the years, however, that as the temperature drops a few degrees (e.g., from 22 to 19 Celsius), the honey smell seems like an unpleasant sour smell. It only seems that way during downward transitions in temperature. Once the temperature is at its low, it starts to smell like fruit again.
What mechanisms in activated carbon could explain this?
2023-02-09 afternote: I wonder if the cause could be related to humidity. Small swings in temperature indoors are correlated with high swings outdoors, which correlate with relative humidity.