Evaluation of Ambient Ammonia Measurements from a Research Aircraft Using a Fast-Response TILDAS Spectrometer with Active Passivation

Ammonia (NH₃) is notorious for being a “sticky” molecule, and a difficult gas-phase species to measure in situ. Owing to its ability to readily adsorb and/or desorb from surfaces, it can be particularly difficult to measure large, rapid changes in NH₃ mixing ratios and to determine accurate in situ background, or zero, levels. Measurements from research aircraft have the added challenge of motion sensitivity, changing environmental conditions that require complex inlets, and limitations in power, weight and space available for research instrumentation. Here, we report airborne observations of gas-phase NH₃ collected using a commercially-available instrument during 10 test-flight hours aboard the NSF/NCAR C-130 aircraft during fall 2017. The NH₃ detector is a single channel, tunable infrared laser direct absorption spectrometer (model TILDAS-CS, Aerodyne Research Inc.), and uses a direct absorption technique combined with a high sample flow rate (> 10 SLPM) to achieve fast (up to 10 Hz) collection of absolute NH₃ concentrations. An inertial inlet (Aerodyne Research Inc.) positioned upstream of the TILDAS provides filter-less separation of particles > 300 nm from the sample stream. A heated aircraft inlet constructed of perfluoroalkoxy fluoropolymer (PFA) allows for maximum transmission of NH₃ in ambient air to the inertial inlet and TILDAS, and is configured such that calibration gases can be introduced in to the sample flow path within 1-2 cm of the inlet tip. The instrument is also outfitted at the aircraft inlet with the option for active and continuous passivation of the sample flow path during flight with 1H,1H-Perfluorooctylamine, a strong perfluorinated base that coats the inner surface of the flow path with nonpolar chemical groups that prevent adsorption of both water and basic species. Previous laboratory experiments have shown passivation to be successful in decreasing the instrument response time by a factor of 1.5. We report on the instrument precision, detection limit, stability, and time response measured during flight with and without active passivation and compare these results with measurements collected in the laboratory prior to instrument installation on the aircraft. We also evaluate the performance of the heated aircraft inlet for transmission of NH₃ and motion sensitivity of the instrument during flight, and compare and contrast several different approaches for establishing an in situ zero level.