Airborne ammonia measurements with a fiber-coupled quantum cascade laser

Free tropospheric ammonia plays critical roles in aerosol nucleation and ammonium nitrate formation with significant impacts on the Earth’s radiative forcing and tropospheric photochemistry. Remote sensing measurements on aircraft and satellite report large values (> 1 ppbv) in the upper troposphere in the outflow of deep convection over source regions. Accurate, in-situ “point” ammonia measurements from aircraft in the free troposphere are non-existent because of surface adsorption effects on existing instrument surfaces and inlets. Such higher spatiotemporal resolution measurements are needed to better deduce the processes that impact the transport of ammonia into the free troposphere from biomass burning and deep convection and its subsequent transformation into particulate ammoniated aerosols. To this end, we are developing an open-path, airborne-based ammonia instrument for the NASA DC-8 aircraft in order to measure ammonia without sampling biases throughout the troposphere. Development of such an instrument requires characteristics of fast response (10 Hz) and low detection limits (10 pptv), requiring instrument attributes of high-stability and high sensitivity. Complicating matters, these measurement attributes have to occur under a wide range of temperatures (210-310 K), pressures (150-1013 hPa), absolute humidities (ppmv to %), and environmental sampling challenges (high airspeed, vibrations, aerodynamic stresses) over the flight envelope (e.g. for vertical profiles for satellite validation). To avoid thermal management issues with the laser under the extreme temperatures experienced by the sensor, a 9.06 micron, distributed feedback quantum cascade laser (c-mount) is mounted inside a custom housing and located inside the aircraft cabin. The laser light is coupled into a 200 micron hollow core fiber for single mode operation and passed through the fuselage of the aircraft to a Herriott cell mounted 35 cm above the fuselage. The fiber and laser housing are continuously purged with dry nitrogen filtered by an ammonia scrubber to avoid interstitial absorption of ammonia in the optical path internal to the Herriott cell. Light emanating from the back facet of the laser is passed through a reference cell of ethylene and ammonia at 50 hPa to ensure appropriate linelocking and laser tuning characterization. The Herriott cell (61 m) consists of polished, aluminum mirrors held together by invar rods to minimize thermal effects on the mirror spacing (55 cm).  The mirrors are heated slightly above ambient by 50 W heaters to avoid water and ice condensation. Allan deviation experiments of the instrument show a precision of 80 pptv (1 Hz) and 13 pptv (60 s), and drift of the calibration is much less than these values up to 3000 s. Wavelength modulation spectra are fit to reference conditions over the range of the flight envelope with accuracies of fit to better than 10%. Field tests of the instrument will be shown, particularly at cold temperatures representative of the upper troposphere. The instrument was test fit onto the NASA DC-8 in summer 2019 and test flights are planned for 2020. The design attributes needed for such measurements – particularly in an aircraft platform - and laboratory and field data supporting the instrument performance will be demonstrated.