Cavity Ring-Down Spectrometer (CRDS) Development for Ambient Measurements of Ammonia

Abstract:

Regulating ammonia atmospheric abundances is important for reducing particle emissions of PM2.5 and PM10. A recent study employing three chemical transport models found an underestimation of the formation of ammonium particles, and concluded that the role of NH₃ on particulate matter is larger than originally thought.

Monitoring of NH₃, on a large scale, is often carried out using low-cost diffusive samplers, active sampling with denuders, and a number of optical spectroscopic techniques. Although denuders are currently considered to be an unofficial “reference method”, they suffer from a few limitations, including low accuracy. They also do not provide rapid measurements in real-time, require complex post-exposure analysis by wet chemical techniques, and must be deployed over extended periods of up to one month to achieve the required sensitivity. These devices deliver only average concentration data, and their validation by traceable methods is not presently extensive.

Cavity Ring-Down Spectroscopy (CRDS) is a rapid on-line monitoring technique, which has regularly been employed for trace gas measurements, including that of dry ammonia gas mixtures. One of the goals of this work is, therefore, to extend the technology to enable more accurate ambient measurements of NH₃ to be made in the field where the sampled atmosphere contains a wide range of concentrations and is also humid. We describe the implementation of this by adjustment of the CRDS instrument for the undesirable effects of cross interference due to water vapor (H₂O). Water vapor influences such spectroscopic measurements through the presence of absorption features close to that of ammonia and through differences in matrix broadening effects. We also detail the establishment of metrological traceability of the measurements via new stable NH₃ Primary Standard Gas Mixtures (PSMs), prepared by gravimetry, and subsequent dilution to typical ambient concentrations (parts per billion).