NASA - National Aeronautics and Space Administration

Ambient ammonia (NH₃) mixing ratios are measured in-situ using a flight-ready, closed-path, optical-based NH₃ monitoring system. The CSU-NH₃ instrument system consists of a combination of commercially-available and custom-built components including: 1) a commercially-available infrared absorption spectrometer that serves as the heart of the NH₃ monitor, 2) a commercially-available inertial inlet that acts as a filter-less separator of particles from the sample stream, 3) a custom-built aircraft inlet, 4) a custom-designed vibration isolation mounting system for the spectrometer, and 5) an optional system for adding passivant to the sample stream.

The heart of the instrument is a closed-path, commercial (Aerodyne Research, Inc.), single-channel, quantum-cascade tunable infrared laser direct absorption spectrometer (QC-TILDAS) [McManus et al., 2010; McManus et al., 1995; Zahniser et al., 1995]. This spectrometer 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 mixing ratios. The QC-TILDAS is operated with a heated aerodynamic separator (Aerodyne Research Inc., Inertial Inlet) that provides filter-less separation of particles >300 nm from the sample stream [Ellis et al., 2010]. An injection-style aircraft inlet allows calibration gases to be introduced into the sample stream within a few centimeters of the inlet tip. The custom inlet system is also designed to support the option for active continuous passivation of the sampling sufaces by 1H,1H-perflurooctylamine, a strong perfluorinated base that acts to coat the sampling surfaces with nonpolar chemical groups. Injection of this chemical into the aircraft inlet near the inlet tip prevents adsorption of both water and basic species on the sampling surfaces. The coating has been shown to greatly improve the instrument's time response in the laboratory and aboard research aircraft by increasing transmission of NH₃ through the sample flow path [Pollack et al., 2019; Roscioli et al., 2016].

The QC-TILDAS is regularly calibrated on the ground and in flight via standard addition to the sample stream with a known concentration of NH₃ generated from a temperature-regulated permeation tube (Kin-Tech), and zeroed by overflowing the inlet tip with a bottled source of NH₃-free, synthetic air. The emission rate of the permeation device is calibrated before and after every mission by the NOAA ultraviolet optical absorption system [Neuman et al., 2003]. Allan variance analyses indicate that the in-flight precision of the instrument is 60 ppt at 1 Hz corresponding to a 3-sigma detection limit of 180 ppt. Zero signals span ±200 pptv, or 400 pptv total, with fluctuations in cabin pressure and temperature and altitude in flight. The total uncertainty associated with the 1-Hz measurement is ±(12% of the measured mixing ratio + 200 pptv).

The CSU-NH₃ instrument has been successfully deployed (i.e. 100% data coverage) in two prior airborne research campaigns; one on the NSF/NCAR C-130 aircraft during the 2018 Western wildfire Experiment of Cloud Chemistry, Aerosol absorption and Nitrogen (WE-CAN) field campaign and the other aboard the University of Wyoming King Air during the TRANS²Am field campaign in 2019, 2021, and 2022. The aircraft inlet and aerodynamic separator are currently being modified in the laboratory to support lower pressure altitudes such as those anticipated for the full altitude range of the NASA DC-8 aircraft.