Développement d’instruments photoniques pour les mesures de polluants à l’état de traces et des propriétés optiques des aérosols
Developments of photonic instrumentation from the UV to the mid-IR for optical metrology of key atmospheric species
Photonic sensing of atmospheric trace species by absorption spectroscopy
Our research aim is to develop state-of-the-art photonic instrumentation for optical metrology of key atmospheric species (chemically reactive short-lived atmospheric species and short-lived climate pollutants) in atmospheric simulation chamber, in laboratory investigation or in intensive field campaigns to help the detailed and deeper understanding of the physico-chemical atmospheric processes related to the scientific and societal questions in air quality, environment science and climate change.
Chemically reactive short-lived atmospheric species : atmospheric oxidation capacity
Chemically reactive short-lived atmospheric species (such as OH, NO3, HONO, O3, halogen, etc) play a crucial role in tropospheric processes that affect regional air quality and global climate change. Measurements of such atmospheric species are essential for the investigation of tropospheric chemistry in field campaigns and simulation chamber experiments. The measured data allow us to test chemical models simulating their concentrations in the atmosphere, and help to improve chemical mechanisms used in regional and global models for predictions of the atmospheric chemical composition and to develop strategies for mitigation of climatic impacts.
Contrary to long-lived species (such as greenhouse gases), fast, interference-free, accurate, and precise in situ monitoring of such strongly reactive species represents a real challenge owing to their very high reactivity resulting in short atmospheric lifetimes (1-100 s) and ultralow concentrations in the atmosphere (ppbv-ppqv).
Short-lived climate pollutants (SLCPs) : contributing to both climate forcing and air quality
Anthropogenic emissions of black carbon (BC), methane (CH4) and precursors of ozone (O3) and aerosols are important compounds for climate change. These atmospheric constituents are relatively short-lived, with atmospheric lifetimes from days to about a decade. Although carbon dioxide (CO2) emission is responsible for 55-60% of current anthropogenic radiative forcing on warming impact, because of its long lifetime (~130 years) in the atmosphere, long-lasting CO2 will remain the primary driver of long-term temperature rise even if new CO2 emissions dropped to zero. A « fast-action » climate mitigation strategy is therefore strongly needed to provide more sizeable short-term benefits than CO2 reductions. Short-lived climate pollutants (SLCPs) like BC, CH4 and O3, have the potential of providing relatively rapid atmospheric response to policy intervention and multiple visible benefits. As such, reduction of SLCP emissions would allow us to quickly lower their concentrations in the atmosphere and hence slow climate change over the next several decades.
we overview our recent progress in the development of absorption spectroscopy-based photonic instruments involving modern photonic light sources (quantum cascade lasers, interband cascade lasers, diode lasers, light emitting diodes) combined with high-sensitivity spectroscopic measurement techniques, such as incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS), Faraday rotation spectroscopy (FRS), wavelength modulation enhanced off-axis integrated cavity output spectroscopy (WM-OA-ICOS), tuning fork- or microphone-based photoacoustic spectroscopy (PAS), and open-path multipass absorption spectroscopy.
Measurements of chemically reactive short-lived atmospheric species
Nowadays accurate measurement of such short-lived atmospheric reactive species remains a highly challenging task and requires persistent care. The major challenge lies in the need for very high detection‐sensitivities, the demand for calibration standards with accurately known concentrations, and prevention of possible interferences, in particular artificial production in the measurement instruments. Validation and calibration of the developed photonic instruments could be usually performed by side-by-side inter-comparison with measurements from available reference instruments or/and with chemical reaction simulations. In this section, illustrative examples of monitoring some key atmospheric reactive species (such as HONO, NO3, N2O5, NO2 and OH) are presented for applications in intensive field campaigns, in instrumented atmospheric simulation chamber or in laboratory investigation.
1. Nitrous acid (HONO)
Gaseous nitrous acid (HONO) is an important source of hydroxyl radical (OH) and plays a significant role in the atmosphere, especially in the polluted troposphere. Typical HONO concentrations vary from 30 pptv to a few ppbv in the atmosphere (early morning) with an atmospheric lifetime of about 10-20 minutes. Despite its importance and several decades of research, the sources and sinks of HONO as well as their formation mechanism in the atmosphere are still not completely defined and understood. Limitations existed in current measurement techniques (spectroscopic and chemical techniques) include detection limit, sampling artifact and chemical interference effects, which affects our understanding of the budget of HONO and its relevance for the OH photochemistry. In our group, two spectroscopic techniques have been developed for HONO monitoring : incoherent broad-band cavity enhanced absorption spectroscopy (IBBCEAS) and quartz-enhanced photoacoustic spectroscopy (QEPAS).
1.1. Field monitoring of HONO & NO2 by LED-IBBCEAS in the UV (358-378 nm)
Dr. Tao Wu (now with Nanchang Hangkong University, China) in collaboration with Prof. Tao Wang of Hong Kong Polytechnic University
(1) Open-path configuration [Appl. Phys. B 106 (2012) 501]
1 σ detection limits : 430 pptv for HONO and 1 ppbv for NO2 using a 90 s acquisition time.
Total measurement uncertainty : ~ 10%.
Figure 1 (a) Schematic of the used open-path IBBCEAS set-up. A PTFE mobile tube was used for system calibration (inserted between the cavity mirror mounts) which was removed out of the cavity for open-path measurement. (b) Picture of the IBBCEAS infrastructure deployed for test in ambient air measurements.
Figure 2 Simultaneous concentration measurements of HONO and NO2: Measured (grey) and fit (red) absorption spectra of a mixture of 3.1±0.3 ppbv HONO and 22.2±0.5 ppbv NO2 in ambient air. The used cross-sections of NO2 (blue) and HONO (purple), the background (green) deduced from the fit, and the fit residual spectrum (lower panel) showing a standard deviation of 5.7 × 10-9 cm-1 are attached as reference.
(2) Closed-cavity in Hong Kong campaign [Atmos. Environ. 95 (2014) 544]
1 σ detection limits : 300 pptv for HONO and 1 ppbv for NO2 using a 120 s acquisition time.
Figure 3 (a) Schematic diagram of the cavity-closed LED-IBBCEAS set-up; (b) picture of the instrument.
Figure 4 Correlation plot of time series of : HONO concentration measurements from the IBBCEAS and the LOPAP instruments (a); NO2 measurement results using IBBCEAS and BLC-NO2 analyzer (b).
Figure 5 Field measurements in Hong Kong. left : Time series measurements of environmental HONO and NO2 concentrations; right : HONO concentrations in relation to wind sector which indicates that most of high HONO levels (up to ~ 2 ppbv) was associated with the wind coming from the airport and from the urban area involved in busy traffic activity.
1.2. HONO measurement by QEPAS using EC-QCL in the mid-IR near 8 µm [Appl. Phys. Lett. 106 (2015) 101109]
Dr. Hongming Yi (now with NIST, USA) and Dr. Rabih Maamary (now with Reims University)
Although optical methods based on long-path absorption spectroscopy can provide sensitive and direct monitoring of HONO without any sample preparation and chemical conversion, the use of optical cavity or multipass cell for achieving the required sensitivity not only makes the instrumental setup oversized but also results in a large residence time of gas sample inside the absorption module due to limited pumping rate, which affects the measurement accuracy because of additional loss and potential generation of HONO in heterogeneous reaction with the cell wall. While hunting for high-sensitivity and small-size optical sensors, QEPAS (quartz enhanced photoacoustic spectroscopy) technique is one of the best choices, especially when coupled to high optical power light sources, which allows one to perform gas measurement in a very small sensing module and sample volume (of a few mm3 to ~ 2 cm3). We developed an off-beam coupled QEPAS spectrophone in conjunction with an external cavity quantum cascade laser (EC-QCL) operating at 1254.85 cm-1 (Fig. 6(a)) for high-sensitivity monitoring of HONO within a very small gas-sample volume of ~ 40 mm3 (Fig. 6(b)), allowing a significant reduction (of about 4 orders of magnitude) of air sampling residence time, down to less than 10 ms (using a pumping rate of 300 ml/min), compared to ~ 7 min for a conventional 210 m multipass cell with a typical volume of 2 L.
1 σ detection limits : 66 ppbv HONO at 70 mbar using a laser power of 50 mW and 1 s integration time, and down to 330 pptv in 150 s using 1 W laser power.
Figure 6 (a) Schematics of the developed QEPAS spectrophone for HONO detection; (b) picture of the quartz tuning fork (upper) used for acoustic signal detection in the QEPAS spectrophone cell (lower).
2. Hydroxyl radical (OH)
The hydroxyl radicals play a central role in atmospheric chemistry due to its high oxidation capacity of volatile organic compounds (VOCs) and other pollutant species (CO, CH4, SO2, NO2, etc), which determine the lifetime of these pollutants in the atmosphere. Because of its very short lifetime (~ 1 s or less) and very low concentration in the atmosphere (down to106 OH/cm3, corresponding to 0.1 pptv at standard temperature and pressure), interference-free high sensitivity in-situ direct measurements of absolute concentration of OH radicals by laser absorption spectroscopy is highly challenging. Nowadays, two spectroscopic methods are well established for OH monitoring in the atmosphere with a detection limit down to ~ 105 OH/cm3 : (1) long-path differential optical absorption spectroscopy (LP-DOAS) providing concentration averaged over the light path at atmospheric pressure (hence the operation is sensitive to visibility conditions); (2) fluorescence assay by gas expansion (FAGE) working at very low pressure (~ 1 mbar) which involves a powerful pumping and complex external calibration systems. In an attempt to lifting the technological barrier to realize absolute OH concentration assessment with a compact and high spatial resolution laser spectroscopic system, two highly sensitive spectroscopic techniques have been developed in the present work based on Faraday rotation spectroscopy (FRS) and wavelength modulation enhanced off-axis integrated cavity output spectroscopy (WM-OA-ICOS).
2.1. Faraday rotation spectroscopy (FRS) at 2.8 µm [Opt. Express 19 (2011) 2493, Appl. Phys. B 109 (2012) 511]
Dr. Weixiong Zhao (now with AIOFM, China) and Tong Nguyen Ba in collaboration with Prof. Gerard Wysocki of Princeton University (USA) and Prof. Xiaoming Gao of AIOFM (China)
With respect to the currently used FAGE and LP-DOAS techniques, FRS (Fig. 7) offers the ability to make direct measurements of OH concentration at any arbitrary pressure including atmospheric conditions (limited primarily by the maximum strength of magnetic field provided by the coil for optimum Zeeman splitting). In addition, as diamagnetic molecules do not produce any significant Faraday rotation effect, FRS can efficiently mitigate spectral interferences from the major atmospheric diamagnetic constituents H2O and CO2.
1σ detection limits (using the OH Q(1.5e) lines around 3568.5 cm−1) :
(1) 25-cm long single pass gas absorption cell : 5.5×108 OH/cm3 in 1 s.
(2) Leff = 12-m multipass call : 107 OH/cm3 in 25 s.
Figure 7 (left) FRS instrument scheme. (Right) Picture of the FRS setup (upper) and the used multipass cell (lower). DFB laser: interband cascade laser (ICL); PM: parabolic mirror; F: lens, PC: personal computer.
2.2. Wavelength modulation enhanced off-axis integrated cavity output spectroscopy (WM-OA-ICOS) at 1434 nm [J. Quant. Spectrosc. Rad. Transfer 113 (2012) 1300]
Christophe Lengignon in collaboration with Weixiong Zhao of AIOFM (China)
As an alternative to the FRS technique, OH radical detection by OA-ICOS was developed (Fig. 8) in combination with wavelength modulation spectroscopy (WMS). The spectroscopic measurement of OH radicals was carried out by probing the OH line at 6965.1939 cm-1 (with a line intensity of 6.5×10-21 cm2.cm-1/molecule).
Figure 8 (left) WM-OA-ICOS setup; (Right) OH absorption spectra obtained with OA-ICOS approach (a), in comparison with its 1f spectrum (b) from WM-OA-ICOS which yielded a detection limit of ~ 109 OH/cm3.
3. Nitrate radical (NO3) monitoring in smog chamber by LED-IBBCEAS at 662 nm [J. Quant. Spectrosc. Rad. Transfer 133 (2014) 199, Opt. Express 24 (2016) A781]
Dr. Tao Wu (now with Nanchang Hangkong University, China) and Dr. Hongming Yi (now with NIST, USA) in collaboration with Dr. Cécile Coeur, Dr. Andy Cassez and Mr. Thomas Fagniez of the LPCA
The nitrate radical (NO3), formed in the atmosphere by reaction of NO2 with O3, is a very important atmospheric oxidant and plays a dominant role in the nighttime oxidation chemistry. Its atmospheric concentration, exhibiting large temporal and spatial variability, is at sub-pptv levels during daytime and in the range of 10-50 pptv in urban environment during nighttime. An LED-based IBBCEAS instrument operating in the visible at 662 nm was developed (Fig. 9 left) for study of NO3 radical formation from reaction of O3 + NO2 in an atmospheric simulation chamber of the laboratory. As neither standard NO3 gas reference nor other NO3 measurement instrument is available, in order to validate the NO3 concentration measurement by IBBCEAS, the measured temporal profiles of NO3, NO2, N2O5 and O3 concentrations were used in simulation of the kinetic chemistry of the NO3 / N2O5 system using FACSIMILE software (Fig. 9 right).
1 detection limits : 7.9 pptv for NO3 and 9 ppbv for NO2 with an integration time of 60 s.
The NO3, NO2, N2O5 and O3 concentrations simulated from the used chemical reaction system are displayed (line curves) in Fig. 9 together with the experimental data. The good accordance between experimental and simulated concentrations allows us to validate the IBBCEAS measurements. This approach involving simulation of chemical kinetic reactions not only provided a method for calibration of the emerging photonic instrument for reactive species measurements for which no alternative instrument available for validation, but also demonstrated its high capacity of non-intrusive, in-situ, real-time « watching » chemical kinetic processes (Table 1) in smog chamber, without sampling and perturbing the chemical reaction.
Figure 9 (left) Smog chamber equipped with optical sensing instruments; (Right) Time series measurements (dots) of NO3, NO2 concentrations simultaneously measured by the IBBCEAS, as well as N2O5 (by EC-QCL, see the following section) and O3 (by an O3 analyzer).
Table 1 Fitted reaction rate constants using the measured temporal profiles of NO3, NO2, N2O5 and O3, in comparison with the referenced data
4. Dinitrogen pentoxide (N2O5) detection in smog chamber by laser multipass absorption in the mid-infrared near 8 µm[Analyst 142 (2017) 4638]
in collaboration with Prof. Tao Wu of Nanchang Hangkong University, Dr. Hongming Yi (now with NIST, USA), Dr. Cécile Coeur and Dr. Andy Cassez of the LPCA
Dinitrogen pentoxide (N2O5) is formed through the reaction of NO3 with NO2 in a reversible equilibrium and is a key intermediate component in atmospheric nitrogen chemistry. N2O5, as a large reservoir for NO3, affects directly the oxidation capacity of the atmosphere through reaction of NO3 with volatile organic compounds (VOCs). Because of its reactivity and its very low concentration (ranging from tens of pptv up to a few ppbv), direct measurement of N2O5 concentration remains challenging since its first observation in the stratosphere by infrared remote sounding in 1985 [Nature 319 (1986) 570]. We recently developed an open-path multipass direct absorption method for in situ direct concentration measurements of N2O5 in a smog chamber (Fig. 10) without thermal conversion. The spectroscopic measurement was performed using a widely tunable continuous-wave external cavity quantum cascade laser (EC-QCL) to scan the full broadband absorption spectrum of N2O5 in the mid-infrared of 1223-1263 cm-1 (Fig. 11(a)).
1 detection limit : 15 ppbv N2O5 in 25 s integration time and down to 3 ppbv in 400 s.
Figure 10 (left) Photographs of the QCL-multipass cell set-up and (Right) the multireflection spots on the mirror of the multipass cell.
Figure 11 (a) Measured and fitted broadband absorption spectra (1223 – 1263 cm-1) of 544 ppb N2O5 (upper panel), and the corresponding fit residual (lower panel); (b) on-line traced temporal concentration profiles of NO3, NO2, and N2O5 during the study of the equilibrium constant Keq, with temporal resolutions of 1 s for NO3 – NO2 (by IBBCEAS) and 25 s for N2O5 (by multipass direct absorption).
The real-time measured time-concentration of N2O5, NO3 and NO2 using IBBCEAS (Fig. 11(b)) allows for study of the reversible equilibrium reaction NO3 + NO3 : N2O5. The equilibrium constant Keq can be determined by the following equation :
A value of Keq=(3.0±0.3)×10-11 cm3.molecule-1 (296 2 K) was determined based on the direct optical measurements of N2O5, NO2 and NO3 concentrations, which is within the range of the reference value of (2.84-5.73)×10-11 cm3.molecule-1 at 296 K [Phys. Chem. Chem. Phys. 9 (2007) 5785].
5. Nitrogen dioxide (NO2) monitoring in ambient air by photoacoustic spectroscopy (PAS) using a blue laser diode at 444 nm [Photonic Sensing of Reactive Atmospheric Species in Encyclopedia of Analytical Chemistry]
Gaoxuan Wang in collaboration with Prof. Markus W. Sigrist of ETH (Zurich), Dr. Hongming Yi (now with NIST, USA), Dr. Fabrice Cazier and Ms. Dorothée Dewaelle of the CCM
Nitrogen dioxide (NO2) is not only a regulated air pollutant but also plays an very important role in tropospheric chemistry affecting the oxidation capacity of the atmosphere through its direct participation in the formation of ozone (O3), nitrous acid (HONO) and nitrate radical (NO3). The reference method used in monitoring networks for NO2 concentration measurement, recommended by the US EPA and by European legislation, is the chemiluminescence technique. However this indirect NO2 measurement method may be strongly affected by either positive (due to non-selective molybdenum catalyzing of NOy) or negative (due to the photolysis of VOCs in the photolytic converter and the consecutive peroxyradical reactions with NO in the sampled air) interferences in a heavily polluted urban environment. In the present work, a PAS-based NO2 sensor operating at 444 nm (Fig. 12) has been developed for direct measurement of NO2 in urban air, which was tested and validated via side-by-side measurements of outdoor and indoor NO2 concentrations (Fig. 13) using the PAS sensor and a referenced NOx analyzer (AC 31M, Environment S.A).
1 detection limit : 0.4 ppbv in 1 min time resolution using a laser power of ~ 700 mW.
Figure 12 (a) Picture of the developed PAS-based NO2 sensor; (b) experimental set-up. Laser diode controller: Arryo instrument 6340; power meter: Field Master TM GS (Coherent); lock-in amplifier: SR 830 (Stanford Research Systems).
Figure 13 (a) Side-by-side inter-comparison measurements of indoor and (b) NO2 concentrations using a NOx analyzer (red) and the developed PAS sensor (black).
Measurements of short-lived climate pollutants (SLCPs)
The main SLCPs are black carbon (BC), methane (CH4) and tropospheric ozone (O3), which are the most important contributors to the human enhanced global greenhouse effects after CO2 (Fig. 14). Monitoring of such climatically and environmentally active SLCPs, exerting together greater influence than CO2, is important not only for policy-based reporting, but also for basic process-based understanding of climate related processes in the atmosphere. Further, each of these SLCPs has other environmental impacts, especially air quality (health) impacts, such that they could be a part of win‐win strategies for climate‐air quality policy making.
Figure 14 Main perturbation agents forcing the climate since 1850 : CO2, the single most important agent, associated to other important agents (CH4, black carbon, tropospheric ozone, etc.) exerting together greater influence than CO2.
In this section, we overview our recent progress in the developments and applications of laser-based spectroscopic instruments for the measurements of black carbon (BC), atmospheric and livestock emitted methane (CH4).
1. Measurements of BC by µ-phone based photoacoustic spectroscopy (PAS)
Gaoxuan Wang in collaboration with Prof. Markus W. Sigrist of ETH (Zurich), Prof. Denis Petitprez of University of Lille 1, Dr. Hongming Yi (now with NIST, USA), Dr. Dean Venables of University College Cork (Ireland), Salah Khardi of French IFSTTAR, Pr. Pascal Flament, Dr. Karine Deboudt, Dr. Marc Fourmentin and Mrs Asma Beji of the LPCA
Black carbon (BC) is a primary aerosol emitted directly from incomplete combustion processes such as fossil fuel and biomass burning and is characterized by its strong absorption in the visible and near-infrared. The uncertainties associated to BC radiative forcing are nowadays still larger than 70% (see Fig. 15), which is mostly related to the actually used filter-based techniques for the measurements of its optical properties. These techniques do not directly determine the aerosol absorption, but the attenuation coefficient through the deposit filter that may strongly affect the particle-light interaction via the effects such as shadowing, multiple and total scattering, and filter loading, which cause the overestimate measurement of absorption with uncertainties of 20-35%. Such large uncertainties may confound our understanding of trends, spatial and temporal variability of BC and its impacts on climate.
Figure 15 Quantitative estimates of black carbon climate forcing, which indicates that the direct effects due to black carbon are nearly twice the number reported in the 2007 IPCC Fourth Assessment.
In the present work, microphone-based photoacoustic (PA) spectrophones were developed for filter-free direct measurement of the optical properties of black carbon, volcanic ash and secondary organic aerosols, in particular the mass absorption coefficient (MAC) and the absorption Angström coefficient (AAC).
Figure 16 Developed 3-color PAS analyzer in the present work.
Measurements of wavelength-dependent MAC and AAC of BC and Eyjafjallajökull volcanic ash samples from the eruption in 2010 have been performed using a 3-color PAS analyzer (Fig. 16). The results (Fig. 17) were in good agreement with the results from filter-based methods, but with higher precision (~10%).
The 1-σ minimum detectable absorption coefficient of the PAS sensor at 440 nm is derived to be 1.2 Mm-1 in 1 s which corresponds to a normalized noise equivalent absorption coefficient (NNEAC) of 1.5×10-8 Wcm-1Hz-1/2 and a minimum detectable BC mass concentration of 0.11 μg/m3. The minimum detectable absorption coefficient can be further lowered down to 0.28 Mm-1 with an integration time of about 200 s, which allows for real time monitoring of atmospheric aerosol absorption.
Figure 17 PA-based measurements of the MAC and the deduced AAC of BC and volcanic ash samples.
The developed PA-sensor was deployed to field measurements of environmental particles and NO2 around highway and at the city center of Lyon and Grenoble (France). Its performance has been evaluated in comparison with referenced instruments such as aethalometer (Magee Scientific, AE33) for particles and NOx analyzer (Environment S.A, AC31 M) for NO2.
A single-wavelength PAS sensor was deployed, for the first time to our knowledge, to optical monitoring (Fig. 18) of the formation of secondary organic aerosols (SOA) produced from the photolysis of 2-nitrophenol in a smog chamber at University College Cork (Ireland).
Figure 18 Optical monitoring of SOA formation by the photolysis of 2-nitrophenol.
2. Measurements of environmental CH4
CH4, with an atmospheric lifetime of about 12 years, is considered as one of the most powerful climate-forcing agents in the atmosphere after CO2. A recent study revealed that the U.S. Environmental Protection Agency’s inventory of greenhouse gases is undercounting total CH4 emissions in the U.S. by roughly 50%. Moreover, CH4 emission from animals, rice cultivation, and energy-related sources may be prone to future increases due to demands of increasing human populations. In the present work, we developed a quantum cascade laser (QCL) based optical sensor for in situ real-time monitoring of CH4 in the environment by direct absorption spectroscopy (DAS) in a multipass cell (Fig. 19 left), with the objective of understanding and inventorying CH4 sources and sinks from various anthropogenic activities.
2.1.Identification of CH4 source monitored by direct absorption spectroscopy (DAS) [Sensors 16 (2016) 224]
Dr. Rabih Maamary (now with Reims University) in collaboration with Ms. Dorothée Dewaele and Dr. Fabrice Cazier of the CCM, Dr. Patrick Augustin and Dr. Marc Fourmentin of the LPCA
Figure 19 (left) QCL-based CH4 sensor; (right) Time‐series CH4 concentrations (upper panel) from 9th to 22nd January. Lille air quality data of NOx (middle) and PM10 (lower), recorded by the ATMO observation station in Lille for the same period. Significant increases of NOx and PM10 levels near 7:00 at Lille correspond to an arrive at Dunkirk at about 16:00 where the CH4 concentration getting started raising, as indicated by the air mass back trajectory (red curve in Fig. 20 right).
During a campaign measurement of environmental CH4 concentration (Fig. 19 right, upper panel) from 9th to 22nd January, a high CH4 concentration of up to ~ 3 ppm was observed starting from ~15:00 (Paris winter local time) on 17th January. Based on the measured time‐series concentrations of NOx, NO2, NO, CO, SO2 and H2O vapor, associated with the measured meteorological parameters, this episode of high temporal variability of CH4 has been identified and attributed to urban pollution transported by air mass through local urban area and the district of Lille, which was confirmed by the pollution wind rose of CH4 concentrations in relation to wind sector (Fig. 20 left) and by the analysis of air mass back trajectories using the NOAA HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) model (Fig. 20 right) as well as by the analysis of the air quality monitoring data from a regional network site in Lille (Fig. 19 right).
Figure 20 (left) Frequency distribution rose of CH4 concentrations depending on the blowing wind sectors on 17th January; (right) Air mass back trajectory (NOAA HYSPLIT) from Lille to Dunkirk on 17th January.
2.2.Identification of CH4 source monitored by direct absorption spectroscopy (DAS) [Sensors 16 (2016) 224]
in collaboration with Dong Chen of Hefei University of Technology, China
Livestock emission is responsible for 18% of the GHGs which may be prone to future increases due to demands of increasing human populations. The largest share of livestock-related GHG emissions is from methane (CH4) and nitrous oxide (N2O). Domestic livestock such as cattle, buffalo, sheep, goats, and camels produce large amounts of CH4 as part of their normal digestive process. CH4 is also produced when animals’ manure is stored or managed in lagoons or holding tanks. CH4 emissions from livestock account for about 80% of agricultural CH4 and 35% of the total anthropogenic CH4 emissions.
In the present work, a sensing platform based on a mid-IR EC-QCL (Fig. 21 left) has been deployed to a stable in a farm (Fig. 21 middle) for in-situ measurement of CH4 emission from manure management with a 1σ detection limit of 30 ppbv and a relative measurement precision of 0.8% in 1 min. CH4 concentration of more than 10 times higher than its level in the atmosphere (~ 1.8 ppm) was observed (Fig. 21 right).
Livestock emissions represent therefore a large and growing share of anthropogenic GHG emissions, which should be carefully monitored and inventoried. This study demonstrates the importance of proper data collection (ex. better measurements of ground level emissions) to find the missing methane and hence to efficiently reduce CH4 emissions once the right sources can be targeted.
Figure 21 (left and middle) Field measurements of CH4 emission from ruminant in a stable; (right) Two weeks’ time-series measurements of CH4 concentration variation
3. Measurement of vertical concentration profile of GHGs by laser heterodyne radiometry (LHR)
Fengjiao Shen, in collaboration with Dr. Pascal Jeseck and Dr. Yao-Veng Te of University of Paris 6, Dr. Tan Tu and Prof. Xiaoming Gao of AIOFM (China)
Monitoring of vertical concentration profiles of key atmospheric trace gases, in particular greenhouse gases (GHGs), is essential for our understanding of regional air quality and global climate change trends. In this context, an mid-infrared (mid-IR) laser heterodyne radiometer (LHR) has been developed for ground-based remote measurements of GHGs in the atmospheric column.
The sunlight traverses the Earth’s atmosphere and undergoes absorption by atmospheric species. The shape of the ground-measured absorption spectrum of the molecular absorber contain information on its vertical concentration distribution. By de-convoluting this spectral signal (absorption line shape and depth) through a retrieval algorithm, it is possible to retrieve the target gas abundance at different altitudes. In a LHR (Fig. 22), the sunlight containing atmospheric absorption features is mixed in a photomixer with a laser radiation from a local oscillator (LO). The beat note at radio-frequency (RF), produced by the photomixing, is fed to a heterodyne receiver and then demodulated with a lock-in amplifier. Target atmospheric species absorption spectrum (changes in intensity of the resulting RF beat signal) can thus be recovered from the total absorption of the sunlight by scanning the LO frequency across its specific absorption feature.
Figure 22 Principle schematic of a LHR. ES is the sunlight electric field and EL the laser LO electric field.
A LHR prototype is recently developed and tested in collaboration with the QualAir platform of the Université Pierre et Marie Curie (Paris 6) in Paris, which is part of Total Carbon Column Observing Network (TCCON-Paris station) using ground-based Fourier Transform Spectrometer (FTS) to record direct solar spectra in the near-infrared spectral region. In the present work, solar radiation was captured by an external heliostat installed on the roof terrace of the lab building and directed to the LHR instrument. The sunlight was mixed with the laser beam from an EC-QCL local oscillator in a VIGO photomixer, atmospheric CH4 absorption signal (Fig. 22 black) was field extracted by scanning the EC-QCL frequency across a CH4 absorption line around 1242 cm-1. As can be seen in Fig. 23, the LHR spectrum of atmospheric CH4 absorption (black) was in good agreement with the corresponding TCCON FTS spectrum (red). Development is ongoing in order to render the developed mid-IR LHR a truly portable instrument.
Figure 23 Atmospheric absorption spectra of CH4 recorded with a : Bruker IFS 125HR (a) and our LHR (b).
Research programs (since 2010)
 GDRI (Groupement de Recherche International) FRANCE-RUSSIE-CHINE (CNRS) : » Spectroscopie d’absorption de molécules d’intérêt atmosphérique et planétologique: de l’innovation instrumentale à la modélisation globale et aux bases de données (SAMIA) » (2009-2012)
 CPER-IRENI (Institut de Recherche en Environnement Industriel) : » 1.B. Instruments laser de terrain pour la détection de polluants gazeux à l’état de traces dans l’atmosphère » (2006-2013)
 ANR Blanc International NexCILAS : » Next generation of Compact Infrared Laser based Sensor for Environmental Monitoring » (2011-2014)
 ANR-LABEX CaPPA : » Chemical and Physical Properties of the Atmosphere » (2012-2020)
 CPER-CLIMIBIO : » Development of a Laser Heterodyne Radiometer for Atmospheric Remote Sensing » (2016-2020)
 EU-INTERREG SAFESIDE : » Système d’Analyse de Feux et Emanations par Spectroscopie Infrarouge à Distance et Embarquée » (2016-2020)
 ANR MABCaM : » Multi-channel wavelength-resolved Albedometer for Black Carbon Measurement » (2016-2019)
 ANR MULTIPAS : » MULTI-gas and multi-source Photo Acoustic Spectroscopy Measurement Platform » (2016-2020)
Colorado State University, USA
Princeton University, USA
ETH Zurich, Switzerland
University College Cork, Ireland
Anhui Institute of Optics & Fine Mechanics, Hefei, Chinav
University of Bari, Italy
University of Cambridge, UK
Université de Montpellier 2, France
Laboratoire des sciences du climat et de l’environnement (ICOS-Integrated Carbon Observation System), France
Hong Kong Polytechnic University, Hong Kong
Rice University, USA
Ghent University, Belgium
University Lille 1, France
Tampere University of Technology, Finland
Moscow Institute of Physics and Technology, Russian
La Voix du Nord (12 Sept. 2016)
Invited paper in the Encyclopedia of Analytical Chemistry
One of the most downloaded papers of JQSRT during one year since its publication
Cover article in Analyst
Installation d’une technique de spectroscopie de type NICE-OHMS
Au sein du LPCA, nous démarrons l’installation d’une technique de spectroscopie de type NICE-OHMS, dans le domaine de l’Infra-rouge (1.3–1.5 mm). Cette technique est particulièrement bien adaptée aux espèces faiblement absorbantes en phase gazeuse. Les techniques ultra-sensibles permettant d’étudier les absorptions faibles sont cruciales pour sonder, les systèmes moléculaires offrant de faible section efficace d’absorption, et les espèces seulement observables en faible quantité. Fondamentalement, la technique NICE-OHMS combine, une cavité de très haute finesse, une détection hétérodyne dans le domaine radio-fréquence. Ceci permet d’optimiser le rapport signal-à-bruit. C’est une technique de détection ultime car la limite quantique de détection (bruit de photons) peut-être approchée.
Figure 1 : Schéma récapitulatif de l’expérience NICE-OHMS
La technique NICE-OHMS a été inventée au NIST (National Institute of Standard and Technology à Boulder, États-Unis d’Amérique) à la fin des années 1990 par John Hall (Prix Nobel de Physique). Cette technique offre la meilleure limite de détection (LOD) parce qu’elle combine de grandes longueurs équivalentes d’absorption atteignables par l’emploi de cavités de haute finesse, et une haute fréquence de modulation/détection : cela permet de s’affranchir des fluctuations résiduelles d’intensité de la source laser (voir Figure). Moins de 10 laboratoires de par le monde ont mis en œuvre cette technique. L’avènement de composants optiques basés sur des fibres optiques a ouvert de nouvelles perspectives. L’extension à de nouveaux domaines spectraux est activement considéré.
Les cavités à haute finesse peuvent être obtenues par des réflecteurs très efficaces (super-miroirs) qui sont maintenant disponibles dans plusieurs gammes spectrales. Cependant, les cavités à haute finesse induisent, 1) un affinement spectral des modes de résonances (inversement proportionnel à la finesse), et 2) une intensification du champ électromagnétique piégé intra-cavité. L’affinement spectral nécessite d’asservir efficacement la source laser sur la cavité optique alors que le fort champ électromagnétique peut induire la saturation des transitions moléculaires. Ceci est à la base d’une spectroscopie de type sans élargissement Doppler. Cependant, la forme inhabituelle du champ électromagnétique intra-cavité, résultant de la modulation radio-fréquence, est à l’origine de nouveaux effets physiques (comme l’émission laser sans inversion) observable sur des systèmes moléculaires.
De nombreux défis en physique et chimie-physique nécessitent de pousser la technique NICE-OHMS à ses limites en termes de sensibilité. Nous espérons obtenir une bruit d’absorption équivalent (NEA) de l’ordre de 10-14 cm-1 Hz-1/2.
D’un point de vue de l’intérêt spectroscopique, nous souhaitons développer la technique NICE-OHMS dans le domaine crucial de l’infra-rouge. La spectroscopie à haute-résolution, la métrologie et la détection de trace sont les thématiques que nous commençons à regarder d’un point de vue expérimental alors que les formes précises des résonances nécessitent des développements additionnels.
La figure 2 montre des simulations du signal NICE-OHMS en dispersion obtenues pour la transitions R0 d’une bande de combinaison de l’acétylène dans l’infra-rouge moyen à température ambiante (1m Torr). La résonance étroite (sans élargissement Doppler), au centre de la transition est nettement visible. En plus, des résonances apparaissent à la fréquence de modulation (ici 380 MHz), de part et d’autre de la transition centrale. De plus, des résonances croisées apparaissent à la moitié de la fréquence de modulation (190 MHz). Ces résonances étroites (MHz) contrastent avec l’élargissement Doppler (~0.0159 cm-1). La forme des résonances varient légèrement avec la puissance intra-cavité (ici, jusqu’à 20 W), ou bien avec le coefficient de saturation (jusqu’à ~ 275), alors que le profile Doppler ne varie que très faiblement.
Figure 2 : Simulation d’une transitions NICE-OHMS d’une bande de combinaison de l’acétylène
Systèmes moléculaires considérés
Les systèmes moléculaires qui seront initialement étudiés au LPCA pour qualifier l’instrument sont CO2, C2H2, H2, NH3 et OH.
Une diode à cavité étendue (ECDL, Toptica) fonctionnant dans la gamme 1.34–1.45 mm, a été récemment acquise par le laboratoire.
Supports Financiers : LPCA, ULCO, CNRS, Région Nord-Pas de Calais.