First-Look Ammonia Measurements in Wildfire Smoke from the 2018 WE-CAN Field Campaign

The Western wildfire Experiment for Cloud chemistry, Aerosol absorption and Nitrogen (WE-CAN) in July and August 2018 aimed to understand fixed nitrogen, absorbing aerosols, cloud activation and chemistry in wildfire smoke plumes from the NSF/NCAR C-130 research aircraft. The evolution and partitioning of fixed nitrogen is central to within-plume free radicals and oxidant production, and thus secondary pollutant formation and removal. It also impacts the timescale for plume aging and the eventual form of nitrogen deposited to the surface. Gas-phase ammonia (NH_3(g)) measurements with high precision and rapid time response are critical for understanding the proportion of reduced versus oxidized nitrogen and gas-phase versus particulate phase ammonium species in smoke. NH_3(g) was primarily measured using a commercially-available quantum cascade tunable infrared laser direct absorption spectrometer (QC-TILDAS; Aerodyne Research Inc.) configured for use aboard the NSF/NCAR C-130 aircraft. The QC-TILDAS was further optimized for sampling smoke in flight by using a heated inertial inlet that allows for filter-less separation of particles from the sample stream, and fast time response by actively and continuously passivating the sampling surfaces with 1H,1H perfluorooctyamine. Particulate ammonium and nitrate species were measured with an aerosol mass spectrometer. NH_3(g) mixing ratios could also be measured in flight using a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) that was concurrently deployed aboard the C-130 aircraft during WE-CAN, although the primary objective of the PTR-ToF-MS was to measure volatile organic compounds in smoke. Here, we present a first-look at gas-phase versus particle-phase NH₃ measurements collected in smoke during WE-CAN. We then evaluate the performance of the QC-TILDAS instrument for collecting measurements in smoke with respect to instrument sensitivity, detection limit, and time response. Lastly, we compare NH_3(g) measurements collected by the QC-TIDAS with those acquired by the PTR-ToF-MS.