Carbon dioxide levels tend to peak in May,
no sh!t Sherlock, that’s because that’s when more trees have their leaves out and doing this thing called photosynthesis. It’s really a cool thing - trees breath in CO2 and expel O2, been happening since the beginning of time.
Full foliage also doesn’t appear till June in many places. Now the photosynthesis has a chance to catch up with the CO2.