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Surface-based instruments

Siuface-based instruments that measure atmospheric greenhouse and air quality gas use the similar classes of techniques as do LEO satellite instruments, dispersive spectrometers coupled with solar viewing collection optics and laser-based sources having both narrow and broadband emission capability with specialized detection. Fourier Transform IR (FTIR) spectrometers are available in a range of wavelength resolutions. Those used in atmospheric observing applications clearly resolve individual lines of absorption bands supporting mole fraction determination. Laser-based sources fall into the two categories discussed previously.

6.4.1 Dispersive, solar-viewing instruments

Solar viewing Fourier Transform IR (FTIR) spectrometers are used for total atmospheric column observation of mole fractions from Earth’s surface over broad wavelength ranges using the sun as the illumination source (Chen, 2016). NASA’s Total Column Carbon Observing Network (Wunch, 2011) has been established to support the calibration and validation of greenhouse gas observing satellites, such as the OCO and GOSAT series. The TCCON instruments are very high-resolution spectrometers located globally near or on satellite overpass tracks. Although this network has stations located globally, coverage of Earth’s surface is sparse. Recently, the Collaborative Carbon Column Observing Network (COCCON) has been established for use in remote regions of the world and utilizes portable sun-observing FTIR instruments (Frey. 2019). If realized, a COCCON observing network could add substantially to existing mole fraction observing capabilities. The portability and lower cost of these devices hold potential for uses ranging from urban to remote settings.

6.4.2 Laser-based, path-integrated methods application

Laser-based mole fr action measurements use two long path methods, integrated path differential absorption (IPDA) and Light Detection and Ranging (LIDAR). Both rely on signals returned to a transceiver set from reflections. In the IPDA case, the reflection comes fr om a reflective surface located at some distance from the transceiver and the laser illumination is constant over the period of the measurement. IPDA methods employ both single-frequency lasers using the ou-line/off-line diode laser timing method (Johnson, 2013; Dobler, Greenhouse Gas Laser Imaging Tomography Experiment (GreenLITE), 2016) and frequency comb methods (Reiker, 2014; Waxmau, 2019). Path-integrated measurements often use retroreflectors to return the incident laser beam to a co-located receiver, often a small telescope. Applications of these methods is spatially limited by the effective range of the laser system used. Some examples of these are given later in the chapter.

6.4.2.1 Single-frequency laser example

An example single-frequency laser application using a tunable, intensity-modulated laser source initially developed to measure CO, mole fraction over pathlengths of several hundred meters covering an area of ~ 1 km2 to generate near-real-time two-dimensional estimates of mole fraction spatial distribution. Sparse tomographic reconstruction analysis was used to compute 2-D, CO, mole fraction maps using chords of pathlength of 50 to 200 meters to demonstrate feasibility of the technique, the GreenLITE10 system (Dobler, Greenhouse Gas Laser Imaging Tomography Experiment (GreenLITE), 2016). Subsequent developments extended the range to more than 5-kilometer pathlengths suitable for application over urban settings involving complex emissions sources.

As a demonstration in urban applications, a prototype system was operated over the center of Paris for approximately a year, beginning in November 2015 (Dobler, 2017), to capture seasonal emissions variations. Using two transceivers separated by к 2 km and 15 retroreflectors separated from each transceiver by 3 to 5 km, the spatial distribution of CO, emissions over the center of Paris was mapped on a continuous basis. This year-long deployment demonstrated the feasibility of using such a system in an urban environment. The top image in Figure 11 illustrates transceiver and retroreflector positions in Paris, showing the chords between each transceiver and the array of reflectors and a mapping of CO, mole fraction. Each transceiver was mounted on a remotely controlled pointing system that acquired a

The disposition of transceivers and retroreflectors placed on top of buildings in Pans roughly auanged in a hemisphere

Figure 11. The disposition of transceivers and retroreflectors placed on top of buildings in Pans roughly auanged in a hemisphere. Orange and Green rays are chords, or paths, between a transceiver and one of the 15 retroreflectors. Images courtesy of J.T. Dobler, Spectral Sensor Solutions, LLC & T.S. Zaccheo, Atmosphenc and Environmental Research.

“Certain commercial equipment, mstmments, or matenals are identified in this paper m order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose.

return from individual reflectors. Each transceiver cycled through the reflector array with at a rate of ~ 10 seconds per reflector, transiting the complete array in less than 4 minutes including a calibration observation.

An integrated column mole fraction measurement is measured along each chord. These IDPA data are input for a sparse 2-D tomographic reconstruction algorithm of the mole fraction field. Each chord crossing the reconstruction returns a mole fraction estimate at that location. These values then allow interpolation between chord crossings, thereby giving continuous estimation of mole fraction over the measurement domain. Because the cost of additional chords is that of a retroreflector and its installation, a relatively small additional cost, a system architecture containing many mole fraction values could be realized where doing so with discrete instruments could be prohibitive in both cost and access to chord crossing locations. In this 30-chord design, ~ 98 chord crossings result. Modification of transceiver design for 360° rotation with similar pointing capability, rather than the я 180° rotation used in this demonstration, could support a substantial increase in reflectors and chords depending upon reflector placement, which is strongly influenced by local building/structure access in a given urban area. Such a design could significantly increase the size of the sampled domain.

Initial comparisons with an in situ CRDS instrument calibrated relative to WMO standards indicates that the system precisely tracks the CO, concentrations within an urban environment. However, measurements made both before and after the Paris demonstration indicate that although the precision of the system is ~ 0.5 pmol/mol or smaller, offsets of the order of 2 pmol/mol to 3 pmol/mol may be present. Systems such as these continue to evolve as research continues to improve their performance and methods developed to better establish standards needed to assess long-path, laser-based methods.

6.4.2.2 Dual-frequency comb example

Dual-frequency comb spectroscopy (DCS) methods have recently been demonstrated over the city of Boulder, Colorado (Waxman, 2019). The DCS makes simultaneous measurements of multiple species and path-integrated temperature with low systematic uncertainty and without the need for instrument calibration. Additionally, the eye-safe, high-brightness, single transverse-mode DCS output allows for beam paths exceeding 10 km, while the speed and parallelism of the measurement suppress any spectral distortion from the inevitable turbulence-induced power fluctuations over such a path. These measurements were made along two paths, as shown in Figure 12, one a reference path and the other over the city itself. The reference path traversed a very low population/indus trial activity region to the west-soutliwest of the city while the over-city beam traversed a path covering a substantial fraction of the city. Data at 5-minute resolution for CO„ CH4. and H.O and its isotopologues were acquired for IVi weeks as a demonstration of the method for emissions quantification as a means to estimate traffic emissions of CO,. The data were filtered for conditions where wind direction was from the west at wind speeds sufficiently low to allow the increase in mole fraction fr om traffic in the over-city path to have a measurable enhancement relative to the reference path. Using a Gaussian plume model, combined with anticipated traffic emissions and accounting for non-traffic sources, the measured emission values relative to a bottom-up city inventory estimate agreed within 25% to 32% relative to the measured value. As discussed above, IDPA methods of higher chord coimt provide spatial resolution.

6.4.2.3 Laser-based, spatially resolved methods

Spatially resolved methods are generally based on Lidar techniques that use short laser pulses reflected back to a receiver by atmospheric particulates. These pulses are much shorter that the round-trip time necessary for a reflected pulse to return to the receiver. For example, a laser pulse of~ 50 ns width travels approximately 300 meters in a microsecond. A detected rouudtrip time, measured fr om the beginning of the pulse from the laser to the time of detection, determines the reflecting particle’s distance from the laser source. Temporal binning of detected signals effectively segregates the range of the laser pulse into path segments between the source and reflecting particles located along the laser’s path. This temporal gating, range gating, effectively segments the laser beam’s path into segments over its practical range.

Reference and over-city DCS IDPA chords for Boulder, CO

Figure 12. Reference and over-city DCS IDPA chords for Boulder, CO. Green diamonds indicate weather stations providing wmd speed and direction. Colored circles are turning movement traffic counts used as a proxy for traffic source locations. Color and size represent traffic counts. Dominant wind directions are shown for prevailing and test days (22 Oct.-purple and 25 Oct.-blue). The dominate wmd direction is indicated by the aqua arrow. Image courtesy of E. Waxman, NIST.

The combination of a short pulse-length laser tuned to frequencies either coincident or non-coincident with an absoiption line (on-line/off-line) with range gating of signals reflected from atmospheric particulates is often termed Differential Absorption Lidar (DIAL). Recent developments of CO, DIAL have shown promising demonstrations of this method while complying with eye safety requirements for laser beams (Wagner, 2018). DIAL is free of constraints of IPDA methods that rely on retroreflectors located across a domain at fixed locations. Applying CO, DIAL for real-time vertical mole fraction profiling has the potential to add considerably to our knowledge of vertical mole fraction distribution in the PBL and perhaps above it. Applied in a horizontal or near horizontal mode, DIAL functions similarly to the IDPA methods described above. However, DIAL does not depend upon detection of signals reflected from fixed objects. Rather DIAL detects photons reflected back to its receiver/detector from atmospheric particles, much the same as particle and Doppler LIDARs. Although, current embodiments of eye-safe CO, DIAL show' good agreement with CRDS measurements made near the beam path in research level experiments, DIAL detector sensitivity limitations must be overcome before broader applications can be supported.

 
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