-
Long open path Fourier Transform Spectroscopy measurements of
atmospheric greenhouse gases
David Griffith University of Wollongong, Australia
& Sam Hammer, Sanam Vardag, Ingeborg Levin
Denis Pöhler, Stefan Schmitt, Ulrich Platt, University of
Heidelberg, Germany
2nd ICOS Science Conference Helsinki, Sept 2016
-
History 1: open path FTIR in the mid IR Measuring greenhouse
gases from agriculture
Tracer release (N2O) FTIR beam
wind
FTIR
Retroreflector
FTIR & telescope
-
OP-FTIR-tracer: results
Tracer
CHTracerCH C
CFF 4
4=
-
History 2: TCCON, GOSAT and OCO-2: Near IR remote sensing of
CO2, CH4, N2O, CO, H2O
Long path absorption spectroscopy in the near IR Sun as source,
~8-40 km absorption path TCCON precision/accuracy for CO2 0.1 –
0.2% (0.4 – 0.8 ppm)
TCCON FTIR
Satellite SCIA, GOSAT, OCO-2
-
Open path FTIR meets DOAS in the NIR
The most accurate greenhouse gas measurements are point-based
Open path spectroscopy has some advantages
Spatial averaging Better match to regional-scale model
resolution How spatially representative are point measurements? How
accurate are open path measurements?
TCCON provides precise measurements of GHGs Solar NIR absorption
spectroscopy, 8-20 km-atm atmospheric path,
high spectral resolution Precision/accuracy 0.1 – 0.2% (0.4 –
0.8 ppm for CO2)
How well can we measure GHGs at the ground in an open path? Low
resolution portable FTS Weaker source than the sun (50W quartz
lamp) 2-6 km path
-
Long path FTS setup
FT detector
interferogram source spectrum
FT Interferometer 0.5 cm-1 resoln Bruker IRcube
1.5 km
Illuminating bundle Receiving fibre
φ 30 cm
FL 150 cm
17 x 70mm retroreflectors
-
IUP
Retrorefl.
Heidelberg Commercial centre
-
FTIR-DOAS setup
-
NIR long path spectrum 3.1 km return path IUP - Philosophen
Weg
Weak CO2 Strong CO2
2 1.6 Microns 1.25 2.5
CH4
O2
1.4
H2O H2O
-
“Strong” CO2 band 4900-5000 cm-1
CO2
H2O
CO2
-
CH4 is weaker…
CH4
-
O2
O2
-
Quantitative spectrum analysis Least squares calculated fit:
example CO2
-
Temperatures point vs path averaged
-
O2 – reality check
Stray sunlight
0.21
-
Water vapour and δD
2.0
1.5
1.0H2O
/ %
21-Jul-14 10-Aug-14 30-Aug-14 19-Sep-14 09-Oct-14 29-Oct-14
-200
-100
0 δD / ‰
30
20
10
0
Air t
emp
/ °C
H2O - open path H2O - met station delD Temperature
-
CO2 July-October 2014
-
CO2 – 3 days in July
Germany wins World Cup!
-
CO2 open path – point in situ differences
“1σ” = 7ppm (n=21,800) Slight skew from SE (city)
in situ > open path
-40
-20
0
20
40
CO
2 di
ffere
nce
/ ppm
108642
Wind Speed / m s-1
-50
-25
0
+25
+50
-25 0 +25 +50
0
45
90
135
180
225
270
315
-
CH4 July-October 2014
-
CH4 open path – point in situ differences
“1σ” = 93 ppb (n=21,800)
-300
-200
-100
0
+100
+200
+300
-200 -100 0 +100+200+300
0
45
90
135
180
225
270
315
-400
-200
0
200
400
CH
4 diff
eren
ce /
ppm
86420
Wind speed / m s-1
-
Precision and accuracy - summary
Repeatability limited by signal noise optical throughput
Accuracy/bias limited by HITRAN/MALT ~ 2-4% Temperature ~ 3°C
(1%) Pressure
-
An interesting artefact …
18:15 every sunny day!
-
Solar beam travels through ~17 km path - Apparent increase in
CO2, CH4, O2
-
Potential improvements & future work
More light! Precision is detector noise limited
Brighter source larger telescope/retro reflector area
Pre-modulate IR source before transmission Removes stray
(sun)light artefacts
Remove or co-fit fibre spectral structures Higher
resolution?
Better discrimination against interferences, but… Lower SNR
=> lower precision Less portable
-
Thank you !
Long open path Fourier Transform Spectroscopy measurements of
atmospheric greenhouse gases�History 1: open path FTIR in the mid
IR�Measuring greenhouse gases from agricultureOP-FTIR-tracer:
resultsHistory 2: TCCON, GOSAT and OCO-2: �Near IR remote sensing
of CO2, CH4, N2O, CO, H2OOpen path FTIR meets DOAS in the NIRLong
path FTS setupSlide Number 7Slide Number 8FTIR-DOAS setupNIR long
path spectrum�3.1 km return path IUP - Philosophen Weg“Strong” CO2
band 4900-5000 cm-1CH4 is weaker…O2Quantitative spectrum
analysis�Least squares calculated fit: example
CO2Temperatures�point vs path averagedO2 – reality checkWater
vapour and dDCO2 July-October 2014CO2 – 3 days in July�CO2�open
path – point in situ differencesCH4 �July-October 2014CH4�open path
– point in situ differencesPrecision and accuracy - summaryAn
interesting artefact …Slide Number 25Potential improvements &
future workSlide Number 27Slide Number 28
