Ammonia Measurements From the NSF/NCAR C-130 Research Aircraft Using a TILDAS Spectrometer with Active Passivation During the WE-CAN Campaign

The Western Wildfire Experiment for Cloud Chemistry Aerosol Absorption and Nitrogen (WE-CAN) is an aircraft campaign focused on measuring the emissions of wildfire smoke in the western U.S. WE-CAN deployed an extensive suite of instrumentation on the NSF/NCAR C-130, and the aircraft was based in Boise, ID for two months during summer 2018. Ammonia (NH₃) is a key molecule to measure in order to understand nitrogen chemistry in wildfires. NH₃ was measured on the aircraft using a quantum cascade tunable infrared laser direct absorption spectrometer from Aerodyne Research (QC-TILDAS) optimized for use on the C-130. The large dipole moment of NH₃ is a basis for its notorious ability to interact with polar molecules on sampling surfaces. This 'stickiness’ increases instrument time response and can compromise rapid in-situmeasurements. To mitigate this ‘stickiness’, several methods were implemented during WE-CAN to improve or maintain a sufficient time response. These included: 1) heating the sampling surfaces in the aircraft inlet and inertial inlet, 2) operating with a high sample flow rate (>10 slpm), and 3) employing active continuous passivation of sample surfaces with 1H,1H-perflourooctylamine. This molecule is a strong perflourinated base that coats the sampling surfaces and prevents the adsorption of water and basic species allowing NH₃ to easily pass through the inlet and instrument. Here, we report on how those techniques were integrated into the instrument system and their benefits with respect to in-situ sampling of NH₃ during WE-CAN.NH₃ was measured with and without passivant addition are compared and contrasted.

The Western Wildfire Experiment for Cloud Chemistry Aerosol Absorption and Nitrogen (WE-CAN) is an aircraft campaign focused on measuring the emissions of wildfire smoke in the western U.S. WE-CAN deployed an extensive suite of instrumentation on the NSF/NCAR C-130, and the aircraft was based in Boise, ID for two months during summer 2018. Ammonia (NH3) is a key molecule to measure in order to understand nitrogen chemistry in wildfires. NH3 was measured on the aircraft using a quantum cascade tunable infrared laser direct absorption spectrometer from Aerodyne Research (QC-TILDAS) optimized for use on the C-130. Thelarge dipole moment of NH3 is a basis for its notorious ability to interact with polar molecules on sampling surfaces. This “stickiness’ increases instrument time response and can compromise rapid in-situmeasurements. To mitigate this ‘stickiness’, several methods were implemented during WE-CAN to improve or maintain a sufficient time response. These included: 1) heating the sampling surfaces in the aircraft inlet and inertial inlet, 2) operating with a high sample flow rate (>10 slpm), and 3) employing active continuous passivation of sample surfaces with 1H,1H-perflourooctylamine. This molecule is a strong perflourinatedbase that coats the sampling surfaces and prevents the adsorption of water and basic species allowing NH3to easily pass through the inlet and instrument. Here, we report on how those techniques were integrated into the instrument system and their benefits with respect to in-situ sampling of NH3during WE-CAN.NH3measured with and without passivantaddition are compared and contrasted.