One CO2 monitoring network that already exists and is mature is Aeronet, which has around 30 sites. The network is used to validate the CO2 satellite data we have now, including NASAs OCO-2.
Argh! Correct network for CO2 validation is actually TCCON. Aeronet is a related set of in situ measurements, in some cases collocated with TCCON sites.
https://www.atmos-meas-tech.net/12/169/2019/amt-12-169-2019....