LHR prototype in use for a field measurement test.

Device physical map

Sun Detector

Photodetector

Beat Signal

Processing

Circuit

Fiber coupler

Fiber splitterSolar radiation

Chopper

Single

mode fiber

Sun tracker

b . LO :Tunics tunable external cavity laser

3642 HE CL(1500 to 1640 nm )

DAQ

a . Sun Tracker: Tracking accuracy 0.01° c . Fiber coupling and signal processing module

Schematic diagram of the instrument integration principle of the dual-channel LHR

Laser heterodyne radiometers (LHR) for in situ ground-based remote sensing of greenhouse gases in the atmospheric column

Jingjing Wang1,2, Fengjiao Shen1, Tu Tan2, Zhensong Cao2, Xiaoming Gao2, Pascal Jeseck3, Yao Te3, and Weidong Chen1

1 Laboratoire de Physicochimie de l’Atmosphère, Université du Littoral Côte d’Opale- 59140 Dunkerque, France 2 Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, 230031, Hefei, China

3 Laboratoire d'Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères Université Pierre et Marie Curie, 75252 Paris, France* Email : chen@univ-littoral.fr

Introduction Measurements of vertical concentration profiles of greenhouse gases (GHGs) is extremely important for our

understanding of regional air quality and global climate change trends. In this context, laser heterodyne radiometer (LHR)technique has been developed[1-5] for ground-based remote measurements of GHGs in the atmospheric. Solar radiationundergoing absorption by multi-species in the atmosphere is mixed with a local oscillator (LO) in a photodetector. Beating note atradio frequency (RF) resulted from this photo mixing contains absorption information of the LO-targeted molecules. Scanning theLO frequency across the target molecular absorption lines allows one to extract the corresponding absorption features from thetotal absorption of the solar radiation by all molecules in the atmospheric. Near-IR (~1.5 µm) and mid-IR (~8 µm) [6] LHRs havebeen recently developed in the present work. Field campaigns have been performed on the roof of the platform of IRENE inDunkerque (51.05°N/2.34°E) to measure the CH4, N2O, CO2 (including13CO2 / 12CO2 ), H2O vapor (and its isotopologue HDO) in theatmospheric column.

[1] R. T. Menzies, and R. K. Seals, Science 197 (1977) 1275-1277.

[2] D. Weidmann, T. Tsai, N. A. Macleod, and G. Wysocki, Opt. Lett. 36 (2011) 1951-1953.

[3] E. L. Wilson, M. L. McLinden, and J. H. Miller, Appl. Phys. B 114 (2014) 385-393.

[4] A. Rodin, A. Klimchuk, A. Nadezhdinskiy, D. Churbanov, and M. Spiridonov, Opt. Express 22(2014) 13825-13834.

[5] J. Wang, G. Wang, T. Tan, G. Zhu, C. Sun, Z. CAO, W. Chen, and X. Gao, Opt. Express 27 (2019) 9600-9619.

[6] F. Shen, P. Jeseck, Y. Te, T. Tan, X. Gao, E. Fertein, and W. Chen, Geophys. Res. Abstracts, 20(2018) EGU2018-79.

Acknowledgments

The authors thank the financial supports from the LABEX CaPPA project (ANR-10-LABX005), the MABCaM (ANR-16-CE04-0009) and the MULTIPAS (ANR-16-CE04-0012)contracts, as well as the CPER CLIMIBIO program. S. F. thanks the program LabexCaPPA and the "Pôle Métropolitain de la Côte d’Opale" (PMCO) for the PhDfellowship support.

Principle of LHR

Dual-channel LHR for the simultaneous measurement of CH4 & CO2

Near-IR LHR for the measurement of CO2 (including 13CO2 / 12CO2), H2O vapor (and its isotopologue HDO)

Conclusions & PerspectivesA dual-channel LHR for the simultaneous measurement of CH4 & CO2 , a broadband LHR for CO2(13CO2 / 12CO2) and a laser heterodyne balance detection device in near IR, anda prototype of mid-IR LHR were developed and tested with ground-based remote measurement of CH4, N2O, CO2 (including 13CO2 / 12CO2), H2O vapor (and its isotopologueHDO) in the atmospheric column, on Dunkerque and Hefei. The LHR spectrums were in good agreement with the FT-IR spectrum (Bruker IFS 125HR FTS) and SNR.

➢ Improve the SNR and ILS of LHR and optimize the laser wavelength correction.

➢Development of a heterodyne spectrum inversion algorithm to determine the vertical concentration profile of the target atmospheric trace gases and analyze the reliabilityof the inversion results.

6235.0 6235.2 6235.4 6235.6 6235.8 6236.0 6236.2 6236.4 6236.6 6236.8 6237.0

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0.8

1.0

1.2

Transmission of CO2 in the atmosphere

Tra

ns

mis

sio

n

Wavenumber (cm-1

)

(12)CO2

6056.2 6056.4 6056.6 6056.8 6057.0 6057.2 6057.4 6057.6 6057.8 6058.0

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0.2

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0.8

1.0

1.2

Transmission of CH4 in the atmosphere

Tra

ns

mis

sio

n

(12)CO2 Solar

CH4

The solar radiation is collected by the sun tracker, thenmodulated by a 1x2 fiber optical switch. The outputs of twolow-cost DFB lasers (1650.9 nm & 1603.6 nm) are combinedwith the sunlight from the fiber optical switch by fibercombiners and mixed in the photodetectors to generatortwo beat signals respectively. These signal are processed bybeat signal processing circuits and collected by a DAQ toproduce the absorption spectrum of sunlight. 5% power ofone output of the optical switches is separated by a fibersplitter and it is used to monitor the sunlight intensity by asun power detector. Among them, the module of lasersdriver and temperature control, the sunlight power detectorand pre-amplifier are self-made, digital lock-in amplifier isdesigned in the acquisition program (LabVIEW software isemployed)Fig 2: Physical picture of integrated instrument.

Fig 1: Schematic diagram of the instrument integration principle of the dual-channel LHR.

Fig 4: Two instrument functions of LHR.

-1000 -800 -600 -400 -200 0 200 400 600 800 1000

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0.2

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1.2

Resp

on

se

Frequency (MHz)

Response of CH4 laser and sunlight

Response of CO2 laser and sunlight

Fig 5: Atmospheric transmittance spectrum of methane and CO2 in the atmosphere.

500 1000 1500 2000 2500 3000 3500 40000.04

0.06

0.08

0.10

0.12

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SN

R

Switching half cycle（us）

Fig 3: SNR at different modulationfrequencies of optical switches.

50 100 150 200 250 300 350 400 450 5000

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4

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8

10

12

14

16

18

20

Vo

lta

ge (

mV

)

Frequency (MHz)

10 MHz - 500MHz

225MHz - 270MHz

27 MHz - 33 MHz

55 MHz - 67 MHz

63 MHz - 77 MHz

6249 6250 6251 6252 6253 6254 6255 6256

0.3

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1.1

(13)CO2

(13)CO2

(13)CO2

(13)CO2

(12)CO2

(12)CO2

(12)CO2

(12)CO2

(12)CO2

(12)CO2

Tra

nsm

issio

n

Wavenumber (cm-1)

(13)CO2

Fig 7: Measurement system RF noise. Fig 8: Transmittance spectrum of CO2in the atmosphere.

This LHR has similar fiber coupling structure (Fig6.c) and beat signal processing circuits with theprevious setup, but a wide band tunable externalcavity laser is used as the LO(Fig 6.b). It has lowRF noise(Fig 7). The radiation from the solartracker (Fig 6.a) and LO is connected to themeasurement system by single-mode fiber.Fig 6: Schematic diagram

of measuring device.

（in Hefei, China, 31.9°N 117.16°E)

In this structure (Fig 9), a balanced photo-detector is used, one of it input ports accept the combination

of sunlight and LO, another input ports only accept LO. A variable fiber attenuator is used to adjust the

optical power in the reference channel of balanced photo-detector. The Spectrums from spectrum

analyzer(Fig 10) show balanced detection can reduce RF noise.

Fig 9:Schematic diagram of laser heterodyne balance detection.

Optical fiber structure

Optical fiber

structure

Fig 10: Spectrum in balanced and unbalanced mode.

Fig 11: Spectrum under different local oscillator powers.

Fig 12: Laser heterodyne signal at different times of one day.

Fig 13: H2O isotope absorption signal measured by LHR.

Beat Signal

Processing Circuit

Fiber coupler

(50:50)

Sun Tracker

(with Optical Chopper)

LO :Tunics tunable external

cavity laser

(Tunics-BT1560)

Fiber coupler

(50:50)

Optical Attenuator

Balanced

photodetector

DAQ

Fig 14: Schematic of the MIR-LHR prototype involving afiber to couple sunlight to the LHR receiver.

Fig 15: Mid-IR LHR prototype inuse for a field measurement test.

Mid-IR LHR for the measurement of N2O

Fig 16: LHR spectrum identification.

As H2O is interfering with the LHR

spectrum of CH4-N2O, and the H2O

distribution changes with time and space

Slightly different

in line-shape

Fig 17: Inter-comparison.

Slightly different in line-

shape due to H2O (diff.

locations; diff. time; diff.

atmospheric conditions)

Consistent

in overall spectral

line-shape