CO2 profiling by space-borne Raman lidar

As clearly reported in the IPCC fifth Assessment Report, CO2 emissions are already producing destructive effects to the plant ecosystem through the alteration of soil-atmosphere interaction mechanisms.
Although the space and ground network for CO2 monitoring has regularly expanded over the past 50 years, it does not guarantee the necessary spatial and temporal resolution needed for a quantitative analysis of sources and sinks. For the purpose of estimating forests’ carbon capturing capabilities, accurate measurements of CO2 gradients between the forest floor and the top of the canopy, which ultimately translates into the capability to measure CO2 concentration profiles. Space sensors provide CO2 measurements above forest canopies, which do not allow to properly estimate Gross Primary Production (GPP).
These observational gaps could be addressed with an active remote sensing system in space based on the vibrational Raman lidar technique. CO2 profile measurements are possible, together with simultaneous measurements of the temperature and water vapour mixing ratio profile and a variety of additional variables (aerosol backscatter profile, aerosol extinction profile, PBL depth, cloud top and base heights, cloud optical depth). An assessment of the expected performance of the system has been performed based on the application of an analytical simulation model developed at University of Basilicata.


ISTP 2019, Toulouse, 20-24 May 19
• CO 2 mixing ratio in the atmosphere has substantially increased from values around 300 ppm in the fifties of previous century to a current value of 415-420 ppm, with an annual increasing rate of approximately 2 ppm.
• Approximately 50 % of CO 2 amount produced through fossil fuel combustion and other human activities is injected in the atmosphere and accumulates in it, while the remaining 50 % is absorbed by the oceans and the terrestrial biosphere.
• Forests cover ~30% of the Earth's global land area and store large amounts of carbon captured from the atmosphere.

Motivation
• In order to properly quantify this sink mechanism and its contribution to the carbon cycle, accurate measurements of the CO 2 gradients between the forest floor and the top of the canopy, and their temporal variations, are urgently needed.
• This ultimately translates into the capability to perform accurate and high vertical and horizontal resolution measurements of CO 2 mixing ratio profiles.
Vertical variability in CO 2 Seasonal and annual mean of CO 2 vertical profiles reflect the combined influences of surface fluxes and atmospheric mixing.
During the summer in the Northern Hemisphere, atmospheric CO 2 concentrations are generally lower near the surface than in the free troposphere, reflecting the greater impact of terrestrial photosynthesis over industrial emissions at this time (Stephens et al., 2007).
Conversely, during the winter, respiration and fossil-fuel sources lead to high low-altitude atmospheric CO 2 concentrations at northern locations. The gradients are comparable in magnitude in the two seasons, this being of the order of 10 ppm.
CO 2 gradients between the forest floor and the top of the canopy are in the range 75-100 ppm in daytime and 10-50 ppm at night. Both ranges represented spring conditions when canopy leaf area development is not completed.

ISTP 2019, Toulouse, 20-24 May 2019
In order to properly design and size a Space Raman lidar, observational requirements have to be assessed and defined.

Observational requirements for CO 2 profiling
• Vertical resolution: 200 m • Horizontal resolution : 50 km • Accuracy: 2-5 ppm The Space Raman lidar has to be designed and sized with the goal of allowing measurements of these gradients.

Mission Characteristics
Instrumental concept benefits from recent advances in solid-state laser, largeaperture telescope, and detector technologies.
• frequency-tripled diode-laser pumped Nd:YAG laser • average power of 250 W at 355 nm • new generation of pump chambers and diode lasers (demon. under dev. at UHOH) • electrical-to-optical efficiency > 5 % (by improving power supply and diode laser efficiencies, reducing electrical losses, optimizing pump chambers and the laser conversion efficiency). • Inclusion of several amplification stages (3-4), each one embedding highdensity stacks of pumping diodes • Use of diode laser pumping determines radiative cooling to be sufficient.
• Far less complex than, e.g., for ADM or EarthCARE. The lidar setup considered in the present research effort heavily relies on the mission concept of ATLAS proposed to the European Space Agency in response to the Call for Explorer-10 Mission Ideas. Simulations consider a sun-synchronous low Earth orbit, with an orbiting height and speed of 450 km and 7 km/s, respectively. A dawn-dusk orbit with overpasses at 6/18 h local time has been selected for the simulations.

Receiver
• large-aperture, lightweight telescope, 4 m diameter • Different technological solution:  rigid primary mirror (single physical element or segmented optics),  Rigid central mirror with folded deployable outer sections  inflatable optics • several glass materials (e.g. Zerodur, SiC, etc.) with appropriate low weight and thermal stability properties for this type of space application • no astronomical quality needed (no diffraction-limited performances (swe<l/14), surface wavefront error< λ RMS) • receiving field-of-view = 25 mrad

Mission Characteristics
Large aperture primary mirrors (with a total surface of ~ 10 m 2 ), with adequate rigidity, low weight (primary mirror areal density ~ 15 kg/m 2 ), high wave-front quality (< l/3) and sufficient temporal and thermal stability, have been demonstrated (Mazzinghi et al., 2006) to be developable based on the use of segmented mirrors, including a very rigid carbon-fibre composite back-plane and a thin Zerodur glass shell, supported by a set of high efficiency electromagnetic, actively-controlled actuators [102].
• In the present mission concept, the Raman lidar collects five primary lidar signals:  CO 2 vibrational Raman signal  water vapour roto-vibrational Raman signal  O 2 -N 2 high-and low-quantum number rotational Raman signals An Input CO 2 mixing ratio profile data for the Simulator was generated which accounts for different vertical gradients associated with the conflicting contributions of terrestrial photosynthesis and industrial emissions, and CO 2 capturing within forest canopy. This input profile includes a : • 10 ppm increase at an altitude of 2 km from a value of 400 ppm to one of 410 ppm, introduced in order to simulate the daytime CO 2 depletion taking place within the mixed layer. In addition to this; • 50 ppm decrease is considered at an altitude of 50 m above the surface level, which is intended to represent CO 2 capturing within forest canopy. • Other atmospheric quantities considered in the simulation include:  vertical profiles of pressure, temperature, and humidity from the U.S. Standard Atmosphere 1976 atmospheric reference model;  Aerosol optical properties from the median aerosol extinction data from the ESA ARMA Model. The Space Raman lidar has to be designed and sized with the goal of perfomimg measurements allowing to resolve these gradients.

Simulation results
An assessment of the expected system performance has been performed based on the application of an analytical simulation model developed at University of Basilicata.
Considering a vertical and horizontal resolution of 200 m and 50 km, respectively, the statistical uncertainty (precision) affecting CO 2 mixing ratio profile measurements is not exceeding 5 ppm at night and 60 ppm in daytime from the surface up to an altitude of 5 km.
Night-time performance is acceptable, but daytime is NOT.
However, so far we have been exploiting only the Raman signal in the anti-Stokes 2v 2 vibrational band. Dx (ppm) Fermi resonance results in the splitting of two vibrational bands that have nearly the same energy and symmetry in both IR and Raman spectroscopies. The two bands are usually a fundamental vibration and either an overtone or combination band.
As a result, two strong bands are observed in the spectrum, instead of the expected strong and weak bands. It is not possible to determine the contribution from each vibration because of the resulting mixed wave function.

Anti-Stokes branch
Thus, measuring the Raman lidar echoes from the two bands v 1 and 2v 2 both in the Stokes and anti-Stokes branches, the intensity of the CO 2 Raman lidar signal can be increased by a factor of 4 to 5.  • Ground-based demonstrators of the present instrument concept are under development both at Univ. of Basilicata and Univ. of Hohenheim.
• An accurate quantitative assessment of the various components of the carbon cycle requires accurate measurements of the different sources and sinks.

Summary
• For the purpose of estimating forests' carbon capturing capabilities, accurate measurements of CO 2 gradients between the forest floor and the top of the canopy are needed, which ultimately translates into the capability to measure CO 2 concentration profiles.
• Simulations reveal that a space-borne Raman lidar based on the roto-vibrational technique, if properly conceived (i.e. exploiting both v 1 and 2v 2 vibrational bands in both the Stokes and anti-Stokes branches), may provide CO2 mixing ratio profile measurements with the level of accuracy needed to quantitatively asses the different sources and sinks carbon dioxide measurements based on the application of the Raman lidar technique have been carried out since the early nineties (Riebesell, 1990;Ansmann et al., 1992). The technique is based on the measurement of anelastically retro-diffused laser radiation (retro-diffusion Raman roto-vibrational) from the carbon dioxide molecules present in the atmosphere.
Despite its important potential scientific applications, the Raman lidar technique for the measurement of carbon dioxide has received little attention in the last 25 years both at theoretical and experimental level, mainly due to the limited precision that characterizes this measure, due to the limited section d impact of the scattering phenomenon on which the technique is based and the low concentration of the species of interest.