Fast in situ airborne measurement of ammonia using a mid-infrared off-axis ICOS spectrometer

A new ammonia (NH₃) analyzer was developed based on off-axis integrated cavity output spectroscopy. Its feasibility was demonstrated by making tropospheric measurements in flights aboard the Department of Energy Gulfstream-1 aircraft. The ammonia analyzer consists of an optical cell, quantum-cascade laser, gas sampling system, control and data acquisition electronics, and analysis software. The NH₃ mixing ratio is determined from high-resolution absorption spectra obtained by tuning the laser wavelength over the NH₃ fundamental vibration band near 9.67 μm. Excellent linearity is obtained over a wide dynamic range (0-101 ppb _v) with a response rate (1/e) of 2 Hz and a precision of ±90 ppt_v (1σ in 1 s). Two research flights were conducted over the Yakima Valley in Washington State. In the first flight, the ammonia analyzer was used to identify signatures of livestock from local dairy farms with high vertical and spatial resolution under low wind and calm atmospheric conditions. In the second flight, the analyzer captured livestock emission signals under windy conditions. Our results demonstrate that this new ammonia spectrometer is capable of providing fast, precise, and accurate in situ observations of ammonia aboard airborne platforms to advance our understanding of atmospheric compositions and aerosol formation.