Evaluation of ambient ammonia measurements from a research aircraft using a closed-path QC-TILDAS operated with active continuous passivation
Authors: Pollack Ilana B. , Lindaas Jakob , Roscioli J. Robert , Agnese Michael , Permar Wade , Hu Lu , Fischer Emily V. 
Published: July 2019

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 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.