We directly correlate the local (20-nm scale) and global electronic properties of a device containing mono-, bi-and tri-layer epitaxial graphene (EG) domains on 6H-SiC(0001) by simultaneously performing local surface potential measurements using Kelvin probe force microscopy and global transport measurements. Using well-controlled environmental conditions, where the starting state of the surface can be reproducibly defined, we investigate the doping effects of N2, O2, water vapour and NO2 at concentrations representative of the ambient air. We show that presence of O2, water vapour and NO2 leads to p-doping of all EG domains. However, the thicker layers of EG are significantly less affected by the atmospheric dopants. Furthermore, we demonstrate that the general consensus of O2 and water vapour present in ambient air providing majority of the p-doping to graphene is a common misconception. We experimentally show that even the combined effect of O2, water vapour, and NO2 at concentrations higher than typically present in the atmosphere does not fully replicate the state of the EG surface in ambient air. All doping effects can be reproducibly reversed by vacuum annealing. Thus, for EG gas sensors it is essential to consider naturally occurring environmental effects and properly separate them from those coming from targeted species.

A correction for the undesirable effects of direct and indirect cross-interference from water vapour on ammonia (NH 3) measurements was developed using an optical laser sensor based on cavity ring-down spectroscopy (CRDS). This correction relied on new measurements of the collisional broadening due to water vapour of two NH 3 spectral lines in the near infra-red (6548.6 and 6548.8 cm −1), and on the development of novel stable Primary Standard Gas Mixtures (PSMs) of ammonia prepared by gravimetry in passivated gas cylinders at 100 μmol mol −1. The PSMs were diluted dynamically to provide calibration mixtures of dry and humidified ammonia atmospheres of known composition in the nmol mol −1 range, and were employed as part of establishing a metrological traceability chain to improve the reliability and accuracy of ambient ammonia measurements. The successful implementation of this correction will allow the extension of this rapid on-line spectroscopic technique to exposure chamber validation tests under controlled conditions and ambient monitoring in the field.

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21Keywords 22 ammonia in ambient air, traceability, reference gas standards, optical transfer standard, validation and testing 23 infrastructure 24 Abstract 25The environmental impacts of ammonia (NH 3 ) in ambient air have become more evident in the recent decades, 26 leading to intensifying research in this field. A number of novel analytical techniques and monitoring 27 instruments have been developed, and the quality and availability of reference gas mixtures used for the 28 calibration of measuring instruments has also increased significantly. However, recent inter-comparison 29 measurements show significant discrepancies, indicating that the majority of the newly developed devices and 30 reference materials require further thorough validation. There is a clear need for more intensive metrological 31 research focusing on quality assurance, intercomparability and validations. MetNH3 (Metrology for ammonia in 32 ambient air) is a three-year project within the framework of the European Metrology Research Programme 33 (EMRP), which aims to bring metrological traceability to ambient ammonia measurements in the 0.5 -34 500 nmol/mol amount fraction range. This is addressed by working in three areas: 1) improving accuracy and 35 2 stability of static and dynamic reference gas mixtures, 2) developing an optical transfer standard and 3) 36 establishing the link between high-accuracy metrological standards and field measurements. In this article we 37 describe the concept, aims and first results of the project. 38