I started measuring yesterday morning and the CO2 was about 430 ppm but it kept dropping as the day progressed. The good people at CO2 meter.com warned me this would happen but I had no idea it would be this dramatic. The CO2 levels yesterday went from 430 ppm at 8 AM in the morning to 381 at 5:30 PM the height of the Photosynthesis Effect. That is an 11% shift in one day. I have know idea what this means but it was an interesting exercise.