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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 08 Mar 2019

Research article | 08 Mar 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

Evaluation of ambient ammonia measurements from a research aircraft using a closed-path QC-TILDAS spectrometer operated with active continuous passivation

Ilana B. Pollack1, Jakob Lindaas1, J. Robert Roscioli2, Michael Agnese2, Wade Permar3, Lu Hu3, and Emily V. Fischer1 Ilana B. Pollack et al.
  • 1Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, 80523, USA
  • 2Aerodyne Research Inc., Billerica, Massachusetts, 01821, USA
  • 3Deparment of Chemistry and Biochemistry, University of Montana, Missoula, Montana 59812, USA

Abstract. 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, a custom-designed aircraft inlet, a custom-built vibration isolation mounting plate, and a custom-built system for optionally adding active continuous passivation for gas-phase measurements of ammonia (NH3) from a research aircraft. The flight-ready 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 flight-ready 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. 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 addition of 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 flight-ready instrument is 60 ppt at 1 Hz corresponding to a 3-sigma detection limit of 180 ppt. 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 a useful tool for maintaining a rapid response for in-situ NH3 measurements over the duration of an airborne field campaign (e.g., ∼2 months for WE-CAN test and research flights) since passivant addition also helps to prevent future build-up 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., measuring fluxes, sampling from aircraft or another mobile research platform). Passivant addition can be useful for maintaining optimum operation and data collection in NH3-rich/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.

Ilana B. Pollack et al.
Interactive discussion
Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Ilana B. Pollack et al.
Ilana B. Pollack et al.
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Short summary
A closed-path infrared absorption spectrometer was outfitted with the option for active continuous passivation for measuring large, rapid gradients in atmospheric NH3 from a research aircraft. In-flight and ground observations show utility in passivant addition for recovering instrument time response when sampling surfaces are contaminated, and cannot be cleaned in a timely manner, and for maintaining rapid time response in an NH3-rich/humid environment over a several-week-long field campaign.
A closed-path infrared absorption spectrometer was outfitted with the option for active...