The application of a cavity ring-down spectrometer to measurements of ambient ammonia using traceable primary standard gas mixtures

Abstract

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. 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.