Final Report Summary - METHANE ISOTOPES (Real-Time Monitoring of Methane Isotopes in Ambient Air)
Determination of the isotopic composition of atmospheric and industrial gases provides valuable information about the origin of the gas species. With the growing requirements to validate changes in the emission of greenhouse gases, there is a need for fast, high-precision instruments capable of determining isotopic ratio data for important greenhouse gases. Modern optical detection techniques, such as cavity ring-down spectroscopy (CRDS), provide an option for high-precision, real-time stable isotope analysis of atmospheric and industrial gases, whilst permitting instrumentation that is field deployable. Methane is one of the three most important greenhouse gases and is considered responsible for 20% of the observed enhanced greenhouse effect. As one of six gases targeted under the Kyoto protocol, EU member states require accurate knowledge of their methane emissions from sources and removals from sinks to meet their annual reporting requirements. Whilst total global emissions are well constrained, emissions by source sector are less well known and current observation networks are unable to verify emission reductions at a national level.
The purpose of this project was to develop an optical instrument capable of real-time, high precision detection of 12CH4, 13CH4 and CH3D in atmospheric samples. Optical detection of 13CH4 and CH3D in natural air is challenging due to their low concentrations. One way to overcome the resulting low levels of absorption is utilisation of a long absorption path length, hence the application of the CRDS technique. The mid-IR region contains a number of strong optical transitions for the different isotopologues of methane, with the strongest absorption band around 3.3µm.
In this project, suitable spectral regions to monitor 13C/12C and D/H line pairs were identified in the HITRAN database. These transitions were confirmed experimentally through measurements obtained with a multipass apparatus designed within VTT. The chosen spectral regions avoid spectral interference due to absorption from other atmospheric species and overlapping methane lines.
The instrumentation was designed and constructed in a compact arrangement, to allow for placement in a transportable field unit. Custom software was developed using LabView to allow for instrument automation. Laser properties, sample gas flow, temperature and pressure measurements, as well as data collection and analysis can be controlled by the software. FPGA technology was utilised to permit faster response times and sampling capabilities for the instrument.
A newly commercially available mid-IR DFB laser diode was employed as the laser source. These lasers have the benefit of small in weight and footprint, as well as relatively low energy requirements. Their output power, however, can be relatively low, with < 1 mW typically reaching the sample optical cavity. Combined with a laser linewidth around 10 MHz, it can be challenging coupling sufficient laser power into the sample cavity. Towards the end of the project, DFB interband cascade lasers became available at a suitable wavelength. Though the power output was higher (~7 mW), the laser wavelength was much more sensitive to the input current. As a result, the current noise of most commercial laser current drivers was large enough to cause a laser wavelength uncertainty of the order of one free spectral range of the sample optical cavity, introducing a significant source of error to the cavity ring-down times and strongly limiting the sensitivity of the instrument. A further source of error to the ring-down times arose from insufficiently fast tuning of the laser off resonance to initiate the ring-down event. To maintain the field capability of the instrument, rapid stepping of the laser current is the most effective way to stop light coupling into the cavity. However, limitations in the bandwidth of commercial laser drivers limit the rate at which the laser stops coupling. This issue is now being address in new laser drivers that are coming to market.
Due to the experimental limitations in the determination of the cavity ring-down time listed above, the sensitivity of the instrument was lower than what was aimed for. The cavity ring-down time could be measured with 1.15% precision in 1 second, yielding a noised equivalent detection sensitivity of 6.7 x 10-7 cm-1 Hz-1/2. Detection limits for CH3D and 13CH4 were, respectively, 170 and 2000 ppm of methane. D/H ratios could be measured with 8% precision in 100 seconds, and 13C/12C ratios with precisions over 10%. As the detection limits were above ambient methane concentrations, field trials could not be performed within the time-frame of the project. However, completion of the optical setup and measurement automation in a field-capable arrangement, combined with the ability to acquire both D/H and 13C/12C ratio measurements, met the main objectives of the project.
Overall, this project provided the first report of the use of a compact mid-IR DFB diode laser for CRDS measurements of methane isotopologues. The implementation of new, compact commercial mid-IR technology presented a number of experimental challenges. However, with interest in the mid-IR region growing and the continual development and improvement of commercial optical and electrical components, it is foreseeable that many of the experimental challenges encountered in this project will be overcome in the coming years, allowing the necessary improvements in instrument sensitivity.
The purpose of this project was to develop an optical instrument capable of real-time, high precision detection of 12CH4, 13CH4 and CH3D in atmospheric samples. Optical detection of 13CH4 and CH3D in natural air is challenging due to their low concentrations. One way to overcome the resulting low levels of absorption is utilisation of a long absorption path length, hence the application of the CRDS technique. The mid-IR region contains a number of strong optical transitions for the different isotopologues of methane, with the strongest absorption band around 3.3µm.
In this project, suitable spectral regions to monitor 13C/12C and D/H line pairs were identified in the HITRAN database. These transitions were confirmed experimentally through measurements obtained with a multipass apparatus designed within VTT. The chosen spectral regions avoid spectral interference due to absorption from other atmospheric species and overlapping methane lines.
The instrumentation was designed and constructed in a compact arrangement, to allow for placement in a transportable field unit. Custom software was developed using LabView to allow for instrument automation. Laser properties, sample gas flow, temperature and pressure measurements, as well as data collection and analysis can be controlled by the software. FPGA technology was utilised to permit faster response times and sampling capabilities for the instrument.
A newly commercially available mid-IR DFB laser diode was employed as the laser source. These lasers have the benefit of small in weight and footprint, as well as relatively low energy requirements. Their output power, however, can be relatively low, with < 1 mW typically reaching the sample optical cavity. Combined with a laser linewidth around 10 MHz, it can be challenging coupling sufficient laser power into the sample cavity. Towards the end of the project, DFB interband cascade lasers became available at a suitable wavelength. Though the power output was higher (~7 mW), the laser wavelength was much more sensitive to the input current. As a result, the current noise of most commercial laser current drivers was large enough to cause a laser wavelength uncertainty of the order of one free spectral range of the sample optical cavity, introducing a significant source of error to the cavity ring-down times and strongly limiting the sensitivity of the instrument. A further source of error to the ring-down times arose from insufficiently fast tuning of the laser off resonance to initiate the ring-down event. To maintain the field capability of the instrument, rapid stepping of the laser current is the most effective way to stop light coupling into the cavity. However, limitations in the bandwidth of commercial laser drivers limit the rate at which the laser stops coupling. This issue is now being address in new laser drivers that are coming to market.
Due to the experimental limitations in the determination of the cavity ring-down time listed above, the sensitivity of the instrument was lower than what was aimed for. The cavity ring-down time could be measured with 1.15% precision in 1 second, yielding a noised equivalent detection sensitivity of 6.7 x 10-7 cm-1 Hz-1/2. Detection limits for CH3D and 13CH4 were, respectively, 170 and 2000 ppm of methane. D/H ratios could be measured with 8% precision in 100 seconds, and 13C/12C ratios with precisions over 10%. As the detection limits were above ambient methane concentrations, field trials could not be performed within the time-frame of the project. However, completion of the optical setup and measurement automation in a field-capable arrangement, combined with the ability to acquire both D/H and 13C/12C ratio measurements, met the main objectives of the project.
Overall, this project provided the first report of the use of a compact mid-IR DFB diode laser for CRDS measurements of methane isotopologues. The implementation of new, compact commercial mid-IR technology presented a number of experimental challenges. However, with interest in the mid-IR region growing and the continual development and improvement of commercial optical and electrical components, it is foreseeable that many of the experimental challenges encountered in this project will be overcome in the coming years, allowing the necessary improvements in instrument sensitivity.