An OpenPath Tunable Diode Laser Sensor for Measurement

An Open-Path Tunable Diode Laser Sensor for Measurement of Greenhouse Gases at the Bonanza Creek Long Term Ecological Research Site near Fairbanks, Alaska D. Michelle Bailey, Erin M. Adkins, J. Houston Miller The George Washington University, Department of Chemistry, Washington, DC 20052 Abstract Field Results – Comparison Open-Path Design Permafrost makes up one-quarter of the Earth’s terrestrial surface and, as global temperatures continue to increase, it is at risk of thawing. Thawing permafrost has the potential to release twice the amount of carbon than is currently in the atmosphere. A multi-year field campaign has begun in collaboration with the University of Alaska – Fairbanks, NASA Goddard Space Flight Center, and our group at George Washington University to study carbon feedbacks during a springtime thaw in Fairbanks, Alaska. 800 750 Carbon Dioixide Concentration, ppm Here we present initial results from our near-infrared open-path instrument for the detection of ambient concentrations of carbon dioxide. The optics launch-box portion of the instrument couples a nearinfrared distributed feedback laser for carbon detection with a visible laser for alignment purposes. The outgoing beam is directed through a 3. 2 -mm hole in a parabolic mirror and the launch-box is oriented using a two axis, alt-azi telescope mount so that the beam will hit the retroreflector target at a set distance downfield. The beam then retraces the path back to the launch-box where the light is collected on the surface of the parabolic mirror and focused onto a multi-mode fiber for detection. Using a National Instruments data acquisition system we are able to collect 500 scans per second which allows for long-term data averaging and subsequently increases the signal-to-noise ratio of our signal. In June 2015, the instrument was deployed to a thermokarst collapse scar bog in the Bonanza Creek Experimental Forest. With a 90 meter total pathlength we were able to resolve carbon dioxide absorption signals on the order of 0. 5%. Data from our Pi. Oxide sensor was compared to flux tower measurements maintained by the University of Alaska – Fairbanks. 700 650 600 550 500 450 400 350 6. 18. 15 12: 00 6. 19. 15 12: 00 6. 20. 15 12: 00 6. 21. 15 12: 00 6. 22. 15 12: 00 6. 23. 15 12: 00 6. 24. 15 12: 00 6. 25. 15 12: 00 Date/Time Above: Carbon dioxide concentrations, corrected for ambient temperature and pressure conditions, from the flux towers in the black spruce forest (blue), thermokarst bog (orange), fen (grey), and our Pi. Oxide sensor (yellow) in the newly formed thermokarst bog. In our initial system design, we use a National Instruments Data Acquisition System (Ni. DAQ) for laser control and data acquisition, which replaces a function generator and oscilloscope used in the laboratory. There is also potential for replacing the line-powered laser current and temperature controllers with low-source voltage OEM units. Below: Temperature Measurements from each of the respective flux towers and Pi. Oxide sensor. 50 45 Permafrost and Climate Change 40 35 Temperature, C Permafrost is classified as ground that is frozen all year round. Present beneath 25% of the Earth’s terrestrial surface, permafrost has been known as a carbon sink due to the carbon-rich soils that comprise it. 30 25 20 15 10 5 0 6. 18. 15 12: 00 6. 19. 15 12: 00 6. 20. 15 12: 00 6. 21. 15 12: 00 6. 22. 15 12: 00 6. 23. 15 12: 00 6. 24. 15 12: 00 6. 25. 15 12: 00 Credit: Hugo Ahlenius, UNEP/GRID-Arendal Date/Time However, permafrost in Alaska is now just below freezing (-1° C) and at risk of thawing. Thawing permafrost can result in ecosystem and landscape changes, as well as, potential release of significant amounts of carbon dioxide and methane (prominent greenhouse gasses) as carbon is broken down by microorganisms. Wavelength Modulation Spectroscopy Field Experiments Field Results WMS entails applying a small amplitude modulation (on the order of the width of a spectral feature) to a laser’s emitted frequency as it tunes through a spectrum. By sampling the detector's signal at a multiple of the modulation frequency and recording the 2 f, 2 nd derivative, signal , the low frequency noise is effectively removed. 0, 004 0, 003 Open-Path Instrument Signal, V Shown here is a time-averaged spectrum for CO 2 from the collapsed scar bog along with a modeled spectrum under field conditions. Total pathlength was 90 meters. 0, 002 0, 001 0 0 0, 1 0, 2 0, 3 0, 4 0, 5 0, 6 0, 7 0, 8 0, 9 1 -0, 002 -0, 003 -0, 004 Ramp, V These figures show signal obtained via direct absorption spectroscopy (top) and wavelength modulation spectroscopy (bottom) for ambient carbon dioxide at 25 meters of pathlength in a long-path cell. 0, 03 0, 02 In addition to the open-path instrument, a non-dispersive infrared (NDIR) sensor was used to collect CO 2 data at the bog site. The sensor uses a raspberry pi to collect data from the on-board temperature and pressure sensors, as well as, the CO 2 sensor. Results show a daily trend in CO 2 emission levels. Field Sites Bog flux tower Other Measurements – Pi. Oxide Black spruce flux tower Fen flux tower Signal, V A: High spatial resolution remote-sensed data, B: NASA suborbital aircraft, C: covariance flux towers, D: ground penetrating radar, E: open path tunable diode laser absorption spectroscopy, F: Atmospheric column measurements, G: Earth System Modeling , Credit: Emily Wilson 0, 01 0 0 0, 1 0, 2 0, 3 0, 4 0, 5 0, 6 0, 7 0, 8 0, 9 1 -0, 02 -0, 03 -0, 04 Ramp, V On-Going Work Moving forward, a second unit will be constructed for deployment in June 2016 to the rich fen site at the Bonanza Creek Experimental Forest. All units will be equipped with lasers that operate in the 1608 nm and 1654 nm regions to allow for simultaneous CO 2 and CH 4 measurements. These instruments will be programmed for auto-optimization which will allow continuous data collection without external support. Acknowledgements Three different permafrost sites will be studied: a black spruce forest with stable permafrost, a thermokarst collapse scar bog undergoing significant thaw, and a rich fen with no active permafrost remaining. All sites are within a 0. 5 km radius. Funding for this collaborative research project (partnered with NASA Goddard Space Flight Center, the University of Alaska Fairbanks, and the George Washington University) is provided by the NASA Hydrospheric and Biospheric Science Research Program.