Evaluation of ambient ammonia measurements from a research aircraft using a closed-path QC-TILDAS operated with active continuous passivation
A closed-path quantum-cascade tunable infrared laser direct absorption spectrometer (QC-TILDAS) was outfitted with an inertial inlet for filter-less separation of particles and several custom-designed components including an aircraft inlet, a vibration isolation mounting plate, and a system for optionally adding active continuous passivation for gas-phase measurements of ammonia (NH3) from a research aircraft. The instrument was then deployed on the NSF/NCAR C-130 aircraft during research flights and test flights associated with the Western wildfire Experiment for Cloud chemistry, Aerosol absorption and Nitrogen (WE-CAN) field campaign. The instrument was configured to measure large, rapid gradients in gas-phase NH3, over a range of altitudes, in smoke (e.g., ash and particles), in the boundary layer (e.g., during turbulence and turns), in clouds, and in a hot aircraft cabin (e.g., average aircraft cabin temperatures expected to exceed 30 ∘C during summer deployments). Important design goals were to minimize motion sensitivity, maintain a reasonable detection limit, and minimize NH3 “stickiness” on sampling surfaces to maintain fast time response in flight. The observations indicate that adding a high-frequency vibration to the laser objective in the QC-TILDAS and mounting the QC-TILDAS on a custom-designed vibration isolation plate were successful in minimizing motion sensitivity of the instrument during flight. Allan variance analyses indicate that the in-flight precision of the instrument is 60 ppt at 1 Hz corresponding to a 3σ detection limit of 180 ppt. Zero signals span ±200, or 400 pptv total, with cabin pressure and temperature and altitude in flight. The option for active continuous passivation of the sample flow path with 1H,1H-perfluorooctylamine, a strong perfluorinated base, prevented adsorption of both water and basic species to instrument sampling surfaces. Characterization of the time response in flight and on the ground showed that adding passivant to a “clean” instrument system had little impact on the time response. In contrast, passivant addition greatly improved the time response when sampling surfaces became contaminated prior to a test flight. The observations further show that passivant addition can be used to maintain a rapid response for in situ NH3 measurements over the duration of an airborne field campaign (e.g., ∼2 months) since passivant addition also helps to prevent future buildup of water and basic species on instrument sampling surfaces. Therefore, we recommend the use of active continuous passivation with closed-path NH3 instruments when rapid (>1 Hz) collection of NH3 is important for the scientific objective of a field campaign (e.g., sampling from aircraft or another mobile research platform). Passivant addition can be useful for maintaining optimum operation and data collection in NH3-rich and humid environments or when contamination of sampling surfaces is likely, yet frequent cleaning is not possible. Passivant addition may not be necessary for fast operation, even in polluted environments, if sampling surfaces can be cleaned when the time response has degraded.