NASA - National Aeronautics and Space Administration

The NASA GSFC In Situ Airborne Formaldehyde (ISAF) instrument measures formaldehyde (CH2O) on both pressurized and unpressurized (high-altitude) aircraft. Using laser induced fluorescence (LIF), ISAF possesses the high sensitivity, fast time response, and dynamic range needed to observe CH2O throughout the troposphere and lower stratosphere, where concentrations can range from 10 pptv to hundreds of ppbv.

Formaldehyde is produced via the oxidation of hydrocarbons, notably methane (a ubiquitous greenhouse gas) and isoprene (the primary hydrocarbon emitted by vegetation). Observations of CH2O can thus provide information on many atmospheric processes, including:
 - Convective transport of air from the surface to the upper troposphere
 - Emissions of reactive hydrocarbons from cities, forests, and fires
 - Atmospheric oxidizing capacity, which relates to formation of ozone and destruction of methane
In situ observations of CH2O are also crucial for validating retrievals from satellite instruments, such as OMI, TROPOMI, and TEMPO.

The NOAA NOyO₃ 4-channel chemiluminescence (CL) instrument will provide in-situ measurements of nitric oxide (NO), nitrogen dioxide (NO₂), total reactive nitrogen oxides (NO_y), and ozone (O₃) on the NASA DC-8 during the FIREX-AQ project. Different versions of this instrument have flown on the NASA DC-8 and NOAA WP-3D research aircraft on field projects since 1995. It provides fast-response, specific, high precision, and calibrated measurements of nitrogen oxides and ozone at a spatial resolution of better than 100m at typical DC-8 research flight speeds. Detection is based on the gas-phase CL reaction of NO with O₃ at low pressure, resulting in photoemission from electronically excited NO₂. Photons are detected and quantified using pulse counting techniques, providing ~5 to 10 part-per-trillion by volume (pptv) precision at 1 Hz data rates. One detector of the integrated 4-channel instrument is used to measure ambient NO directly, a second detector is equipped with a UV-LED converter to photodissociate ambient NO₂ to NO, and a third detector is equipped with a heated gold catalyst to reduce ambient NOy species to NO. Reagent ozone is added to these sample streams to drive the CL reactions with NO. Ambient O₃ is detected in the fourth channel by adding reagent NO.

The Harvard QCLS (DUAL and CO2) instrument package contains 2 separate optical assemblies and calibration systems, and a common data system and power supply. The two systems are mounted in a single standard HIAPER rack, and are described separately below:

The Harvard QCL DUAL instrument simultaneously measures CO, CH4, and N2O concentrations in situ using two thermoelectrically cooled pulsed-quantum cascade lasers (QCL) light sources, a multiple pass absorption cell, and two liquid nitrogen-cooled solid-state detectors. These components are mounted on a temperature-stabilized, vibrationally isolated optical bench with heated cover. The sample air is preconditioned using a Nafion drier (to remove water vapor), and is reduced in pressure to 60 mbar using a Teflon diaphragm pump. The trace gas mixing ratios of air flowing through the multiple pass absorption cell are determined by measuring absorption from their infrared transition lines at 4.59 microns for CO and 7.87 microns for CH4 and N2O using molecular line parameters from the HITRAN data base. In-flight calibrations are performed by replacing the air sample with reference gas every 10 minutes, with a low-span and a high-span gas every 20 minutes. A prototype of this instrument was flown on the NOAA P3 in the summer of 2004.

The Harvard QCL CO2 instrument measures CO2 concentrations in situ using a thermoelectrically cooled pulsed-quantum cascade laser (QCL) light source, gas cells, and liquid nitrogen cooled solid-state detectors. These components are stabilized along the detection axis, vibrationally isolated, and housed in a temperature-controlled pressure vessel. Sample air enters a rear-facing inlet, is preconditioned using a Nafion drier (to remove water vapor), then is reduced in pressure to 60 mbar using a Teflon diaphragm pump. A second water trap, using dry ice, reduces the sample air dewpoint to less than –70C prior to detection. The CO2 mixing ratio of air flowing through the sample gas cell is determined by measuring absorption from a single infrared transition line at 4.32 microns relative to a reference gas of known concentration. In-flight calibrations are performed by replacing the air sample with reference gas every 10 minutes, and with a low-span and a high-span gas every 20 minutes.

TOGA measures volatile organic compounds (VOCs). TOGA was deployed with an Agilent quadrupole mass spectrometer from 2006 through 2018. Since 2019, the TOGA has been equipped with a TOFWERK high-resolution time-of-flight (HR-TOF) mass spectrometer detector (TOGA-TOF). Specific data will be obtained for radical precursors, tracers of anthropogenic and biogenic activities, tracers of urban and biomass combustion emissions, tracers of ocean emissions, products of oxidative processing, precursors to aerosol formation, and compounds important for aerosol modification and transformation. TOGA measures a wide range of VOCs with high sensitivity (low to sub-ppt), frequency (2 minutes or better), accuracy (20% or better), and precision (<3%). Over 100 species are routinely measured throughout the troposphere and lower stratosphere from the surface to 16 km or higher. See table for list of VOCs that have been quantified using TOGA and TOGA-TOF. The TOGA-TOF is contained in a dual extended HIAPER rack, weighs approximately 225 kg and consumes ~1 kW of power. The major components of the instrument are the inlet, cryogenic preconcentrator, gas chromatograph, time-of-flight mass spectrometer detector, zero air/calibration system, and the control/data acquisition system. All processes and data acquisition are computer controlled.

Principle: The CU aircraft version of the Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) detects non-refractory submicron aerosol composition by impaction on a vaporizer at 600°C, followed by electron ionization and time-of-flight mass spectral analysis. Size-resolved composition can be quantified by measuring the arrival times of the aerosol at the vaporizer.

Aircraft Operation: (1 min cycles, can be adjusted to meet mission goals):
46 s total concentration measurements (1 s resolution, can be increased to up to 10 Hz upon request)
5 s speciated size distribution measurements (with improved S/N detection due to ePToF acquisition)
9 s Background + Overhead
Higher accuracy due to flight day calibrations using built-in system
Custom pressure controlled inlet with confirmed performance up to 45 kft

Real Time Data Products: 
PM1 Aerosol Mass Concentrations:
Organic aerosol (OA) , SO4, NO3, NH4, Chloride 
OA Chemical Markers: f44 (Secondary OA), f57 (hydrocarbon-like OA), f60 (biomass burning OA), f82 (isoprene epoxide-SOA), other fx upon request

More Advanced Products:
- PM1 Seasalt, ClO4, total I, total Br, MSA concentrations
- O/C, H/C, OA/OC, OSc
- Particle organic nitrates (pRONO2)
- Ammonium Balance, estimated pH
- OA components by positive matrix factorization (PMF)
- Particle eddy covariance fluxes of all species
- Speciated Aerosol size distributions

Detection Limits (1s, ng sm-3), (1 min, ng sm-3) from start of the flight (due to custom cryopump):
Sulfate: 40, 15
Nitrate: 15, 6
Ammonium: 3, 1
Chloride: 30, 12
OA: 200, 80
For detailed OA analysis, longer averaging (3-30 s, depending on OA concentration) is needed. A 1 min product is hence provided as well.

The Unmanned Aircraft Systems (UAS) Chromatograph for Atmospheric Trace Species (UCATS) was designed and built for autonomous operation on remotely piloted aircraft, but has also been used on manned aircraft. It uses chromatography to separate atmospheric trace gases along narrow heated columns, followed by precise and accurate detection with electron capture detectors. There are currently three chromatography channels on UCATS, which measure nitrous oxide (N2O) and sulfur hexafluoride (SF6); CFC-11, CFC-12, CFC-113, and halon 1211; and chloroform (CHCl3) and carbon tetrachloride. On an earlier version of UCATS, with only two channels, we also measured methane, hydrogen, and carbon monoxide, along with N2O and SF6. In addition, there is a small ozone instrument and a tunable diode laser instrument for water vapor. Gas is pumped into the instruments from an inlet outside the aircraft, measured, and vented. UCATS has flown on the Altair UAS, the GV during HIPPO, the NASA Global Hawk UAS during the Global Hawk Pacific (GloPac) and ATTREX missions, where a record was set for the longest duration research flight (more than 28 hours), the DC-8 for ATom, and the ER-2 for DCOTSS. UCATS is relatively lightweight and compact, making it ideal for smaller platforms, but it is easily adaptable to a mid-size platform like the GV or Global Hawk. The data are used to measure sources and sinks of trace gases involved in climate and air quality, as well as transport through the atmosphere.

UCATS is three different instruments in one enclosure:

1. 3-channel (formerly 2-channel, up until 2020) gas chromatograph (GC)
2. Dual-beam ozone photometer (OZ)
3. Tunable diode laser (TDL) spectrometer for water vapor (WV)

As part of the measurement team on the NASA DC-8 we operate two related installations: a mist chamber/ion chromatograph (MC/IC) sampling/analysis system providing near real time results for selected species, and a bulk aerosol system that collects particulates onto filters for subsequent analysis. We use ion chromatography on aqueous extracts of the bulk aerosol samples collected on Teflon filters to quantify soluble ions (Cl^-, Br^-, NO3^-, SO4^2-, C2O4^2-, Na⁺, NH4⁺, K⁺, Ca⁺, and Mg⁺). Filters are exposed on all level flight legs. Below 3 km exposure times are 5 minutes or less, increasing at higher altitudes to a maximum sample time of 15 minutes. Aerosols participate in heterogeneous chemistry, impact radiative transfer, and can be detected from space. Our measurements help to validate and extend retrievals of aerosol distributions and properties by MODIS, MISR and CALIPSO. In addition, several of the particle-associated ions are tracers of sources of gas and aerosol pollutants (e.g., SO4^2- from industrial emissions of SO2, enhancements of C2O4^2-, K⁺, and NH4⁺ indicate encounters with biomass burning plumes, Na⁺, and Cl^- are tracers of seasalt, Mg²⁺ and Ca²⁺ are tracers of dust). Our system has two inlets, allowing collection of paired samples simultaneously.