This is the average amount of CO2 that is in the atmosphere in a given month. The monthly average highlights the seasonal variation.
The primary unit here is parts per million (ppm), which describes the concentration of atmospheric CO2 per year. The secondary unit here is gigatonnes, which describes the weight of atmospheric CO2 per year. We show this to be able to relate to emissions, which are commonly expressed in gigatonnes.Wikipedia: Parts-per notation
We see a seasonal cycle with the lowest values around September. This is mostly due to the fact that the northern hemisphere has a large temperate land mass where plant growth absorbs CO2 from the atmosphere in spring and summer and plant decay releases CO2 in autumn and winter. The southern hemisphere has the opposite seasonal cycle, but it doesn't have the large land mass to counteract the northern cycle.Wikipedia: Carbon Dioxide in Earth's Atmosphere
NOAA's Global Monitoring Laboratory operates a global network of air sampling sites where they measure CO2 and provide data on monthly averages.
Globally averaged marine surface monthly mean dataCredits: Ed Dlugokencky and Pieter Tans, NOAA/GML (gml.noaa.gov/ccgg/trends/)Update cycle: monthlyDelay: ~ 3 months