3.1 Overview of Measurement Approaches
Greenhouse gas monitoring relies on three complementary measurement approaches, each with distinct advantages and limitations:
| Approach |
Spatial Scale |
Temporal Resolution |
Precision |
Coverage |
| In-situ (ground) |
Point measurements |
Seconds to minutes |
Very high (±0.1 ppm CO₂) |
Sparse spatial |
| Remote sensing |
Column/profile |
Revisit of a few days to 16 days (16 days for OCO-2) |
Moderate (±0.5 ppm CO₂) |
Global |
| Inverse modeling |
Regional to global |
Weekly to monthly |
Moderate (depends on data) |
Complete |
Complementary Nature
The ideal GHG monitoring system combines all three approaches:
- Ground stations provide high-precision validation data
- Satellites offer global spatial coverage
- Models integrate observations to estimate fluxes
3.2 In-Situ Analyzers
Ground-based instruments that continuously measure GHG concentrations at a specific location.
Non-Dispersive Infrared (NDIR) Analyzers
NDIR Technology
Principle: CO₂ and other GHGs absorb specific infrared wavelengths. The specification's ground measurement accuracy requirements are ±0.1 ppm for CO₂, ±2 ppb for CH₄, and ±0.1 ppb for N₂O (Phase 1 §3.1).
How it works:
- Infrared light source emits broad spectrum
- Light passes through sample cell containing air
- CO₂ molecules absorb light at 4.26 μm
- Detector measures transmitted light intensity
- Absorption is proportional to concentration (Beer-Lambert Law)
Advantages: Robust, low maintenance, field-deployable
Limitations: Moderate precision (±0.5 ppm), single gas
Cavity Ring-Down Spectroscopy (CRDS)
CRDS Technology
Principle: Measures how quickly light decays in an optical cavity
How it works:
- Laser pulse injected into high-finesse optical cavity (mirrors >99.995% reflective)
- Light bounces back and forth thousands of times (~10 km effective path)
- GHG molecules absorb specific wavelengths, accelerating decay
- Measure "ring-down time" - how fast intensity drops
- Faster decay = higher concentration
Advantages: Very high precision (on the order of ±0.1 ppm CO₂, ±1 ppb CH₄), multi-gas, low drift
Limitations: Expensive equipment, requires stable environment
Example: Picarro G2401 (CO₂, CH₄, CO, H₂O)
Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS)
OA-ICOS Technology
Principle: Similar to CRDS but measures continuous wave absorption
Key difference: Laser enters cavity at slight angle, creating dense pattern of reflections
Advantages: High precision, good stability
Limitations: Slightly lower precision than CRDS
Example: Los Gatos Research analyzers
Gas Chromatography (GC)
GC Technology
Principle: Separates gas mixture components, then detects individually
How it works:
- Air sample injected into carrier gas stream
- Mixture flows through separation column
- Different gases travel at different rates
- Detector identifies gases as they exit (retention time)
- Common detectors: FID (for CH₄), ECD (for N₂O, SF₆)
Advantages: Can measure many trace gases, very sensitive for halocarbons
Limitations: Not continuous (sample-based), requires calibration
Applications: F-gases, N₂O, isotope-specific measurements
3.3 Flask Sampling Networks
Air samples collected in flasks and analyzed in central laboratories - the gold standard for precision:
Flask Sampling Process
- Collection: Air drawn into evacuated glass flasks (typically paired)
- Pressurization: Sample compressed to 2-3 atmospheres
- Sealing: Valves closed to preserve sample
- Shipping: Flasks sent to central laboratory
- Analysis: Multi-instrument analysis (GC, CRDS, mass spec)
- Quality control: Comparison with reference standards
| Network |
Operator |
Sites |
Gases Measured |
Precision |
| NOAA GML |
NOAA (USA) |
50+ (cooperative flask network) |
CO₂, CH₄, N₂O, SF₆, CO, H₂ |
±0.05 ppm (CO₂) |
| AGAGE |
MIT/Scripps |
~14 |
40+ gases (esp. halocarbons) |
Varies by gas |
| ICOS |
Europe |
Several dozen atmospheric stations |
CO₂, CH₄, N₂O, CO |
±0.1 ppm (CO₂) |
| CSIRO |
Australia |
~10 |
CO₂, CH₄, N₂O, O₂/N₂ |
±0.05 ppm (CO₂) |
3.4 Tall Tower Networks
Towers extending 100-300+ meters measure vertical GHG profiles, capturing regional flux signals:
Tall Tower Advantages
- Vertical sampling: Multiple inlets at different heights reveal atmospheric mixing
- Regional signal: Upper levels sample air from larger areas (~100 km)
- Boundary layer: Daytime sampling captures well-mixed regional emissions
- Inverse modeling: Ideal data for estimating regional fluxes
Example Sites:
- WLEF (Wisconsin, USA): 447m tower, 11, 30, 76, 122, 244, 396m inlets
- Cabauw (Netherlands): 213m tower, European flux hotspot
- ZOTTO (Zotino, Siberia): 304m tower, vast boreal forest
3.5 Eddy Covariance Flux Measurements
Direct measurement of GHG exchange between ecosystems and atmosphere:
Eddy Covariance Principle
Concept: Measure vertical wind velocity and GHG concentration at high frequency (10-20 Hz)
Flux calculation:
Flux = ρ × cov(w', c')
where:
ρ = air density
w' = deviation of vertical wind from mean
c' = deviation of GHG concentration from mean
cov = covariance (correlation between w' and c')
Physical meaning: Updrafts carrying high CO₂ = net emission; updrafts with low CO₂ = net uptake
Requirements:
- Fast-response analyzer (10-20 Hz)
- 3D sonic anemometer for wind
- Flat, homogeneous terrain (ideally)
- Continuous operation
Applications:
- Forest carbon uptake
- Agricultural emissions (CH₄, N₂O)
- Urban flux measurements
- Wetland methane emissions
| Network |
Sites |
Ecosystem Types |
Data Availability |
| FLUXNET |
Several hundred or more (federation of regional networks) |
Forests, crops, grasslands, wetlands, urban |
Public database |
| ICOS Ecosystem |
Several dozen |
European ecosystems |
Near real-time |
| AmeriFlux |
Several hundred |
North American ecosystems |
Public database |
3.6 Ground-Based Remote Sensing (FTIR)
Fourier Transform Infrared Spectrometers measure total atmospheric column concentrations:
TCCON - Total Carbon Column Observing Network
Instrument: High-resolution FTIR spectrometer
Measurement: Solar absorption spectra (near-infrared)
Output: Column-averaged dry-air mole fractions (XCO₂, XCH₄, etc.)
How it works:
- Tracks the sun throughout the day
- Records absorption spectrum as sunlight passes through atmosphere
- Each gas has unique spectral fingerprint
- Retrieval algorithm inverts spectrum to concentrations
- Result: Total column amount divided by the dry-air column
Precision: ~0.25% (±1 ppm for XCO₂ of 410 ppm)
Sites: About 30 worldwide (28 as listed in specification Phase 1)
Critical role: Primary validation for satellite missions
3.7 Aircraft and UAV Measurements
Research Aircraft
| Platform |
Altitude Range |
Instruments |
Applications |
| Small aircraft |
0-6 km |
CRDS, flask sampling |
Vertical profiles, city emissions |
| High-altitude |
Up to 15 km |
Multi-gas analyzers |
Stratospheric chemistry, validation |
| Commercial |
10-12 km |
Automated flasks (CONTRAIL) |
Upper troposphere monitoring |
Unmanned Aerial Vehicles (UAVs)
UAV Platforms for GHG Monitoring
Small UAVs (< 25 kg):
- Compact NDIR sensors
- 0-1 km altitude
- Plume mapping (landfills, oil & gas facilities)
- Cost-effective, flexible deployment
Medium UAVs (25-150 kg):
- Miniaturized laser spectrometers
- 0-3 km altitude
- Extended flight time (hours)
- Agricultural emissions, pipeline monitoring
Advantages:
- Fill gap between ground and satellite
- On-demand deployment
- Dangerous area access
- High spatial resolution
3.8 Isotopic Analysis
Measuring isotope ratios provides "fingerprints" to identify emission sources:
Carbon Isotopes in CO₂
| Isotope |
Natural Abundance |
Application |
Signature |
| ¹²C |
98.9% |
Baseline |
- |
| ¹³C |
1.1% |
Source discrimination |
Fossil fuels: -28‰
C3 plants: -27‰
C4 plants: -13‰ |
| ¹⁴C |
trace |
Fossil fuel detection |
Fossil fuels: 0 ¹⁴C
Modern biosphere: ~100 pMC |
Radiocarbon (¹⁴C) for Fossil Fuel Attribution
Fossil fuels are millions of years old - all ¹⁴C has decayed (half-life: 5,730 years)
Application: Measure atmospheric ¹⁴CO₂. Decline indicates fossil fuel CO₂ addition.
Example: Urban Δ¹⁴CO₂ is lower than in background air, and the difference is used to estimate the amount of fossil-fuel CO₂.
Hydrogen and Carbon Isotopes in Methane
| Source Type |
δ¹³C-CH₄ (‰) |
δD-CH₄ (‰) |
Interpretation |
| Thermogenic (fossil) |
-50 to -25 |
-275 to -100 |
Natural gas, coal |
| Microbial (wet) |
-70 to -50 |
-400 to -250 |
Wetlands, rice, landfills |
| Pyrogenic |
-25 to -10 |
-250 to -100 |
Biomass burning |
3.9 Continuous Emission Monitoring Systems (CEMS)
Regulatory-grade instruments for industrial stack monitoring:
CEMS Requirements
Regulatory context: EPA, EU ETS, and other programs require continuous monitoring of large emitters
Measured parameters:
- CO₂ concentration (% or ppm)
- Stack gas flow rate (m³/hr)
- Temperature and pressure
- O₂ or CO₂ for combustion efficiency
Emission calculation:
Emissions (tonnes CO₂/hr) =
(P × Concentration × Flow Rate × MW_CO₂) / (R × T)
where:
P = absolute stack gas pressure
Concentration = CO₂ mole fraction
MW_CO₂ = 44 g/mol
R = gas constant
T = absolute temperature
(unit conversion factor to tonnes/hr omitted)
Quality requirements:
- Relative Accuracy Test Audit (RATA): < 10% error
- Calibration error (drift) check: daily
- Linearity check: quarterly
- Data availability: > 95%
3.10 Mobile and Portable Systems
Vehicle-Based Monitoring
Mobile Survey Applications
- Urban mapping: Street-level CO₂ and CH₄ mapping
- Leak detection: Natural gas pipeline surveys
- Landfill screening: Perimeter CH₄ monitoring
- Agricultural surveys: Field-scale emissions
Example system:
- Vehicle-mounted CRDS analyzer (e.g., Picarro G2301)
- GPS for georeferencing
- Air inlet on vehicle roof
- Real-time data logging and visualization
Handheld Devices
| Device Type |
Technology |
Precision |
Applications |
| Portable NDIR |
Non-dispersive IR |
±10 ppm level |
Indoor air quality (IAQ), safety monitoring |
| Laser CH₄ detector |
Tunable diode laser |
ppm level (ppm·m for remote types) |
Leak detection at oil & gas facilities |
| FID surveyor |
Flame ionization |
ppb to ppm level |
Fugitive emission surveys |
3.11 Emerging Measurement Technologies
Next-Generation Instruments
Quantum Cascade Lasers (QCL):
- Mid-IR lasers enabling compact, multi-gas analyzers
- Target: sub-ppb precision in portable package
- Applications: UAVs, low-cost networks
Photoacoustic Spectroscopy:
- Detects sound waves from light absorption
- Very small sample volumes
- Miniaturization potential for IoT sensors
Dual-Comb Spectroscopy:
- Ultra-fast, broadband spectroscopy
- Measure many gases simultaneously
- Open-path capability (no sampling required)
Low-Cost Electrochemical Sensors:
- Low unit cost enables mass deployment
- Dense urban networks
- Moderate accuracy, needs calibration
- Citizen science applications
3.12 Measurement Quality Assurance
WMO/GAW Quality Framework
WMO/GAW network compatibility goals:
- CO₂: ±0.1 ppm (Northern Hemisphere), ±0.05 ppm (Southern Hemisphere)
- CH₄: ±2 ppb
- N₂O: ±0.1 ppb
- SF₆: ±0.02 ppt
Traceability chain:
- Primary standards: Gravimetrically prepared at WMO Central Calibration Lab (CCL)
- Secondary standards: Distributed to network operators
- Working standards: Used for field instrument calibration
- Target gas: Intermediate checks
QA/QC procedures:
- Daily automated calibrations
- Weekly to monthly target gas checks
- Annual comparison with traveling standards
- Inter-comparison campaigns
- Blind audits by independent teams
Key Takeaways
- CRDS and OA-ICOS provide high precision for continuous in-situ monitoring
- Flask sampling networks offer unmatched accuracy for long-term trends
- Tall towers bridge point measurements and regional fluxes
- Eddy covariance directly measures ecosystem-atmosphere exchange
- TCCON ground-based FTIR is critical for satellite validation
- Isotopic analysis enables source attribution (fossil vs. biogenic)
- CEMS provides regulatory-grade industrial monitoring
- Mobile systems enable flexible spatial surveys
- Emerging technologies promise lower cost and wider deployment
- Rigorous QA/QC with WMO traceability ensures data quality
Review Questions
- Explain the principle behind Cavity Ring-Down Spectroscopy (CRDS).
- Why is flask sampling considered the "gold standard" despite being non-continuous?
- What advantages do tall towers offer over surface measurements?
- Describe how eddy covariance measures GHG fluxes. What is the key equation?
- Why is TCCON critical for satellite missions?
- How can ¹⁴C measurements distinguish fossil fuel CO₂ from biogenic CO₂?
- What are the key components of a CEMS, and what accuracy is required?
- Compare NDIR and CRDS technologies in terms of precision, operating conditions, and applications.
- What role do isotope ratios play in methane source attribution?
- What are the WMO compatibility goals for CO₂ and CH₄ measurements?