The Flash Spectrum of the Sun – H lines in the solar chromosphere Astronomy Picture of the Day – 9/7/2017 Image Credit & Copyright: Yujing Qin (University of Arizona) Agenda • Optical spectroscopic methods for gas concentration measurement – Physical Fundamentals and Applications to Chemical Measurements • Instrumental Analysis Techniques • Applications: Air Quality • Applications: Satellites • Future perspectives Measuring Gas Concentration with Optical Measurement Optical Methods Gas Concentration Measurement Fundamentals Incident Light Beam Emerging Beam • Light is absorbed by the chemical species of interest Intensity Io Intensity I – Air Quality Gases and Greenhouse Gases • Molecular spectroscopic characteristics pathlength provide identification of chemical species Concentration Measurement Equation • The Beer-Lambert law used for concentration 퐼 퐴푏푠표푟푏푎푛푐푒 = 푙푛( 표) = 푒푥푐 measurement 퐼 – Observable quantities: 퐼 ln 표 • Light Intensities, absorbance pathlength 푐 = 퐼 푒푥 – Reference quantity – Molar absorptivity Where: I = Intensities – initial, Io and final I • Specific to individual chemical species e = Molar absorptivity of the Gas x = Absorption pathlength • Based upon each species spectroscopic properties that c = Concentration of the Gas are fundamental physical quantities Measuring Gas Concentration with Optical Methods Measurement Fundamentals Lambert-Beer-Law 1 1 1 I( , L) I0 exp S ( 0 )n L Partial pressure measurement kT I (1) p ln 0 1 1 1 S(T)( 0 )L I( ) Line intensity measurement kT I (1) S(T) ln 0 1 1 1 p( 0 )L I( ) Form-function (1 1)d1 1 0 Measuring Gas Concentration with Optical Methods Air Quality & Greenhouse Gas Properties • Chemical species have different spectral features both in wavelength and strength • Molecular have so-called band structures • Absorbance strength is at it maximum at absorption line peaks • Concentration measurements rely on spectral reference data for values of the molar absorptivity • The availability of frequency controlled lasers has revolutionized concentration measurements using spectral methods – Wavelength tuning allow analytical instruments to measure properties of individual & groups of absorption features – Spectral reference data capabilities • These are approaching accuracies below 0.1% and precisions to support concentration measurements Measuring Gas Concentration with CO2 Bands: 1.5 ~ 1.64 µm Optical Methods Using Rotational/Vibrational Bands • Non-Dispersive Infrared (NDIR) . Instrument realization is relative simple – Thermal Source – tungsten lamp – An absorption path containing gas to be measured – A bandpass filter selected for the desired wavelength region . Strategies for achieving good precision and accuracy – Use of a reference and sample gas light path – Multiple on-band and off-band filters/single detector • Molecular Selectivity: – Bandpass filters transmit source radiation in a specific radiation band – Different molecules (chemical species) may absorb in overlapping bands • Filter widths and center wavelengths – Cover several absorption lines – Widths of 2% of the center wavelength or larger – Center wavelengths are temperature sensitive – change by substantial fractions of the width Measuring Gas Concentration with Optical Methods High Precision Measurement Approaches • Increasing the absorption pathlength – Long pathlength absorption cells – pathlengths of 10’s to 100’s of meters – Optically-Folded cells – high finesse resonant cavities can have effective pathlengths of several kilometers • Narrow wavelength laser emission sources – Well controlled emission wavelengths, 1,000’s X smaller than thermal sources (lamps) – Wavelength tunable several adjacent absorption features Lock In Amplifier Dual Channel Filter Herriott Cell PC Vacuum chamber Digitizing Oscilloscope CO2 Small Cell Lock Ramp Generator In Amplifier Laser Controller Etalon Chopper Laser Source Monochromator Absorbance vs. Wave-Number 2.5 2.0 N 2500 1.5 N 1 1 k j A 1.0 abs j j α j1 0.5 1 1 I0( ) 1 0.0 S(T ) ln d nL I(1) 0.04 0.06 0.08 0.10 0.12 0.14 0.16 -0.5 -1 ν j (cm ) 1 S(T ) A (T ) 0 nL abs 0 With several measuremens of A(To), n and L, we can measure S(To) as the slope of a linear model: Aabs(T0 ) S(T0 )nL Line intensity of CO2 - R12 for T0=296K (with functional- structural linear analysis): -21 S(T0) = (1.2550 ±0.0062)∙10 cm/molec Relative Standard Uncertainty: 0.49% (26 degrees of freedom) TDLAS Measurement of CO2 Partial Pressure in a Mixture using the CO2 - R12 Line 1.2E+05 PTB Partial u[p ] ) p T ( 1.0E+05 Sample Pressure (%) R12 S / (%) (k = 1) 6 8.0E+04 10 cm) ∙ ∙ T ∙ (Pa k 6.0E+04 ∙ Gravimetric 5.1743 0.0046 ) T ( 4.0E+04 abs A 2.0E+04 TDLAS 5.170 0.042 5.0E+03 1.0E+04 1.5E+04 2.0E+04 2.5E+04 P total ∙L (mBar∙cm) Our result is only 0.08% smaller than the gravimetric reference value. Relative Uncertainty is 0.8% (k = 1, 13 degrees of freedom). BAM Partial u[Pp] Sample Pressure (%) C49286 (%) (k = 1) Gravimetric 0.10680 0.00029 TDLAS 0.1064 0.0021 Our result is only 0.37% smaller than the reference value. High Accuracy MeSpectroscopic Reference Data – Cavity Ringdown Spectroscopy (CRDS) absorption spectral line 200 MHz optical resonator pzt cw probe laser decay signal stabilized comb of frequency -stabilized resonant frequencies reference laser FS-CRDS cavity length Frequency-stabilized cavity stabilization servo ring-down spectrometer frequency 0.44 12 Objective: 250 Torr CO2 Develop advanced measurement techniques and provide reference data that enable quantitative, high-precision, measurements of 0.4 ] 1 greenhouse gases through observations of visible and near-infrared - m c 6 - 0.36 0 absorption and transmission spectra. 1 [ 1 - ) c • Accurate absorption line shape measurements augmented with ( high accuracy absorber number density determination reduced 0.32 13 absorption line intensity uncertainties from 2% to <0.02% CO2 • Update HYTRAN Database 0.28 • Quantified corrections required to accurately describe line 0 4 8 12 16 20 intensities in the O2 A-band (~765 nm) – NASA OCO2 & 3 0.0002 • Reference data requires both absorber number density (Gas ] 1 - m c 6 Metrology) and optical parameters (Ch. Process Measurements) - 0 0 1 [ t i f I - • Advanced measurement tools – Freq. Agile CRDS most recently I -0.0002 • Potential for development of intrinsic standards for main GHGs 0 4 8 12 16 20 and their isotopologues frequency detuning (GHz) • Benchmark data for ab-initio model development High-Spectral Fidelity of Frequency Stabilized-Cavity Ring Down Spectroscopy 12 ppm of CH4 in air, 150 Torr; 6046 cm-1 region Line shape effects in O2 SNR = 160,000 & QF = 61,000:1 (fit quality) Voigt Profile std. dev = 2.2x10-11 cm-1 Galatry Profile Systematic Errors Arise From Overly Simplistic Line Shape Models Earth Viewing, Greenhouse Gas Measuring Satellites Smaller & Cheaper with Frequent Revisit: >200 now on orbit Flotilla of tiny satellites will photograph the entire Earth Advancing Launch Capability with Much Lower Cost every day Rocket Lab’s Electron rocket at the company’s launch site in New Zealand By Mark Strauss, Science, 2/23/2017 (Photo courtesy of “On 14 February, earth scientists and ecologists received a Valentine's Rocket Lab) Day gift from the San Francisco, California-based company Planet*, (Washington Post which launched 88 shoebox-sized 11-12-17) satellites on a single Indian rocket.” Satellite Swarms “With the launch last week of 88 tiny "Doves," the satellite company Planet now has 144 at work, which Ready to book your will permit daily images of the satellite launch entire Earth” online? The rocket industry looks to run more like an airline. Washington Post – 11/12/17 47 ready to go Current Spectrally-Capable Satellites • GHGSat – Canadian Prototype/ Indian Launch – 2016 • EU CarboSat * 4/2017: Equity state in Planet Labs by Google & integrated with Terra Bella. Google • NASA GEOCarb has a multi-year agreement to purchase SkySat Imaging data. • China’s TanSat Internationally-Recognized Protocols for comparing results Mid-2018 – Approximately 500 small satellites are on orbit These are primarily specialized camera instruments that image Earth’s within a common reference frame are not currently available surface in the visible part of the spectrum. or accepted. Earth Viewing, Greenhouse Gas Measuring Satellites Smaller & Cheaper with Frequent Revisit First-of-its-kind satellite gets key data To fully understand the problem – and drive the solutions – we need more and better data about: • How large methane emissions are. • Where they're coming from. • The biggest potential reductions. • Progress of those reductions over time. MethaneSAT will provide global high-resolution coverage, exceeding anything in orbit or on the drawing board today. Because it will focus only on methane, it will be quicker and less expensive to launch than the complex, multi-function satellites built by government space agencies, so we can get data April 2018 Announcement - 2020 to sooner. 2021 Projected Launch … this project is about turning data into action. Information from MethaneSAT is intended to give both countries and companies robust data to spot problem areas, identify savings opportunities and measure their progress over time. https://www.edf.org/climate/space-technology-can-cut-climate-pollution-earth These are Fundamental Issues Requiring Sound Measurement Science and Technology and International Recognition Micro-Satellites with Spectral Measurements GHGSat-D (Claire) • GHGSat’s mission is to become the global reference for remote sensing