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We directly correlate the local (20-nm scale) and global electronicproperties of a device containing mono-, bi- and tri-layer epitaxial graphene(EG) domains on 6H-SiC(0001) by simultaneously performing local surfacepotential measurements using Kelvin probe force microscopy and global transportmeasurements. Using well-controlled environmental conditions, where thestarting state of the surface can be reproducibly defined, we investigate thedoping effects of N2, O2, water vapour and NO2 at concentrations representativeof the ambient air. We show that presence of O2, water vapour and NO2 leads top-doping of all EG domains. However, the thicker layers of EG are significantlyless affected by the atmospheric dopants. Furthermore, we demonstrate that thegeneral consensus of O2 and water vapour present in ambient air providingmajority of the p-doping to graphene is a common misconception. Weexperimentally show that even the combined effect of O2, water vapour, and NO2at concentrations higher than typically present in the atmosphere does notfully replicate the state of the EG surface in ambient air. All doping effectscan be reproducibly reversed by vacuum annealing. Thus, for EG gas sensors itis essential to consider naturally occurring environmental effects and properlyseparate them from those coming from targeted species.

Atmospheric doping effects in epitaxial graphene: correlation of local and global electrical measurements