TOGA: The Trace Organic Gas Analyzer
Our research group has been deploying fast online gas chromatograph/mass spectrometers (GC/MS) since the 1990s. The HAIS Trace Organic Gas Analyzer (TOGA) was completed in 2012 and is certified for deployment on the NSF/NCAR Gulfstream V (GV), C-130 and the NASA DC-8.
TOGA provides near-continuous real-time atmospheric mixing ratios of an extensive and growing list of volatile organic compounds (VOCs) in the C1-C10 molecular structure range. The list includes alkanes, alkenes, chlorofluorocarbons, halons, nitrates, nitriles, sulfides, alcohols, ketones, aldehydes and ethers. Typically, a subset of some 60-70 unique trace gases are measured, with sufficient sensitivity to measure trace species in the remote background atmosphere and dynamic range to measure in highly polluted regions.
The HAIS TOGA is contained in a standard GV rack and consumes ~1 kW of power. The primary hardware components of TOGA are the pump box, clean air generator/calibrator (CAG), electronics box, mass spectrometer (MS) electronics/flow controllers, MS chamber and high vacuum pumps, and LN2 Dewar assembly. These are shown in Figure 1.
Figure 1. Diagram of the HAIS Trace Organic Gas Analyzer (TOGA).
The TOGA inlet is a constant mass flow design to mitigate the intrusion of aerosols and allow for calibrations and system blanks through the inlet and associated tubing.
A cryogenic preconcentrator, consisting of a custom-built LN2 dewar and a system of traps, is used for water removal, sample enrichment and cryofocusing of trace constituents. Flow switching between traps is done via a heated (50°C) Valco 10-port valve. TOGA's flow system is diagrammed in Figure 2.
Figure 2. Flow system of the Trace Organic Gas Analyzer (TOGA).
A quadrupole mass spectrometer operating in selected ion monitoring mode is used to quantify individual compounds. High speed electronics allow the simultaneous measurement of several peaks with ~ 1-second peak widths. System calibrations and blanks are performed using a catalytic-clean air generator/dynamic dilution system with accurate (±1%) and precise (±1%) calibration gas delivery. The system operates continuously, allowing for frequent calibrations and zeros during flight.
Overall estimate of uncertainty: VOC dependent, typically 20%.
Precision: 3% or less.
Response time: typically 35-second samples on a 2-minute cycle.
TOGA-TOF: High Resolution Time-of-Flight TOGA System
Our TOGA-TOF system paired our TOGA instrumentation with a High Resolution Time-of-Flight (HR-TOF) Mass Spectrometer (Tofwerk, Switzerland). The TOGA-TOF allows for greater selectivity, increased VOC separation in polluted and complex atmospheric mixtures resulting in more accurate measurements of a wider range of hard-to-measure gases including oxygenated VOCs, biogenic VOCs, organic nitrates, nitriles, and halogen-containing VOCs. It has allowed us to identify and quantify new species not previously targeted for analysis.
Figure 3. Diagram of the TOGA-TOF instrument.
WASPP: The Whole Air Sampling Pilotless Platform
The WASPP, described in our paper in Environmental Science & Technology, was developed by Elizabeth (Lizzy) Asher in our laboratory. This system is designed to take advantage of UAV technology that allows access to a region of troposphere between surface measurements and aircraft measurements (0-500 ft) with a significant payload capability. The WASPP payload weighs 6.0 kg and is comprised of an array of miniaturized canisters, onboard sensors and a computer, and is deployed on a commercial drone: the DJI Matrice 600 Pro.
Figure 4 shows the WASPP at a test launchpad in Boulder, CO. The system currently collects 8 whole air canisters per flight but has the capability to collect as many as 15. The system measures ambient temperature, relative humidity, pressure, wind speed and wind direction at 1 Hz. The flights can either be computer programmed (prescribed flight pattern) or manually controlled. The system was designed to work in concert with the TOGA systems, taking advantage of their very small sample size requirements (14 ml). More than 70 individual VOCs can be measured with each WASPP canister with a VOC dynamic measurement range capability of < 10 ppt to ~ 50 ppb.
Figure 4. Photo of Lizzy Asher preparing the WASPP for flight.
Left to right: the interior of the TOGA mass spectrometer; sampling inlets on the belly of the NSF/NCAR C-130 during FRAPPÉ; Alan Hills fills the TOGA LN2 dewar on the NSF/NCAR C-130 before a flight.
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