What may be some possible contributors to the higher carbon dioxide levels found during the fall and winter months vs. the summer months?

One is that, there are no leaves on the trees in his mom's or they are dying because they have stored up energy for winter and fall so they're not taking any more oxygen so we are putting out CO2 that's not getting absorbed by plants. (hope this helps but that's the only reason I could think of)

The answer is observations and interpreting.