A correction for the undesirable effects of direct and indirect cross-interference from water vapour on ammonia (NH₃) measurements was developed using an optical laser sensor based on cavity ring-down spectroscopy. This correction relied on new measurements of the collisional broadening due to water vapour of two NH₃ spectral lines in the near infra-red (6548.6 and 6548.8cm⁻¹), and on the development of novel stable primary standard gas mixtures (PSMs) of ammonia prepared by gravimetry in passivated gas cylinders at 100μmol mol⁻¹. The PSMs were diluted dynamically to provide calibration mixtures of dry and humidified ammonia atmospheres of known composition in the nmol mol⁻¹range 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 online spectroscopic technique to exposure chamber validation tests under controlled conditions and ambient monitoring in the field.