The greenhouse should have a port for co2 injection.

Real industrial greenhouses do it to boost their plant's yield. It also reduces the plant's water usage.
https://extension.okstate.edu/fact-sheets/greenhouse-carbon-dioxide-supplementation.html

They use very, very little CO2. It's not relevant on industrial scales. Like, a few cubic meters for an entire array of greenhouses.

Add the ability to pipe co2 into green houses to boost crop yield this would one reduce air pollution and two be in line with how many greenhouses work

Suggest adding CO2 input for Greenhouse II only, to make it qualitatively distinct from Greenhouse I. The CO2 should increase crop yield just like fertilizer.
Aside from that, greenhouses must be superior to open-air farms in terms of water consumption - that is one of the feature of the greenhouses in reality, which seems to be working opposite in game, where greenhoused crops require MORE water instead (?!).

in the netherlands i think it's shells refinery that recycles about 1/3 if it's CO2 with greenhouses

@reeper, van confirm that, but not the exact amount. But yes, the shell refinery is pumping pure Co2 to the greenhouses in the area called 'westland'