FACE (Free Air Carbon Enrichment) is a technique used to increase CO₂ concentrations in open plots (as compared to enclosures like green houses). Several projects all over the world use this technique. 

Where are all the FACE experiments in the world ?http://cdiac.esd.ornl.gov/programs/FACE/whereisface.html

Aspen FACE experiment
http://aspenface.mtu.edu/

Bangor FACE experiment in Wales
http://www.bangorface.org.uk/

Brookhaven National Laboratory FACE experiment
http://www.face.bnl.gov/

Duke Forest FACE experiment
http://c-h2oecology.env.duke.edu/Duke-FACE/description.cfm

EUROFACE FACE experiment in Italy
http://www.unitus.it/euroface/

Jasper Ridge global change experiment
http://globalecology.stanford.edu/DGE/Dukes/JRGCE

Swiss FACE experiment
http://www.gl.ipw.agrl.ethz.ch/research/projects/closed/face

TasFACE experiment on Tasmania
http://www.utas.edu.au/docs/plant_science/Ecophys/TasFACE.html