Aerosol layer height constrained by micro-lidar to enhance space borne push-broom spectrometer measurements of CH4 and CO2

The greenhouse gases (GHG) methane (CH4) and carbon dioxide (CO2) have been emitted at an increasing rate since the Industrial Revolution, leading to amplified global warming. The Paris agreement, signed by 175 nations, represents the world’s first sound political framework to regulate GHG emissions. It entails a need to quantify GHG fluxes, ideally with global coverage. 

Since the pioneering missions able to detect and quantify trace gases in the Troposphere, green gas monitoring instrument (GMI) and scanning imaging absorption spectrometer for atmospheric cartography (SCIAMACHY) almost 30 years ago, a number of satellite missions that provide global coverage have been launched and are used to serve that need. There is, however, a significant discrepancy between bottom-up GHG emission estimates from inventories and top-down estimates using a combination of space-borne GHG concentration measurements and atmospheric dispersion modeling. Over the last 12 years or so, a new generation of satellites-borne imaging spectrometers emerged with sub-kilometre pixel resolution, able to map trace gas plumes and thus able to quantify GHG fluxes directly at the source, contributing to improved GHG inventories. Among those are the first commercial Earth observation missions to monitor GHG sources.

The commercial AIRMO mission aims to quantify GHG fluxes, notably CH4, in the planetary boundary layer, over regions that may contain significant aerosol concentrations, such as sulfate, marine and desert aerosols and aerosols from biomass burning. These aerosols exhibit extinction of solar photons by scattering and absorption, which may significantly modify the path of solar photons so that apparent and actual column lengths differ, leading to a possible over or underestimation of GHG column densities from passive spectroscopy. 

To correct for this bias in the retrieval algorithm, employing full physical modeling of light extinction by aerosols in the forward model is envisaged. Sensitivity tests are performed using the GRASP (Generalized Retrieval of Atmosphere and Surface Properties) retrieval and simulation code to assess the sensitivity of three crucial model parameters: The aerosol concentration parametrized as aerosol optical depth (AOD), the bidirectional surface reflectance (BRDF) and the aerosol layer height (ALH). A conceptually straightforward way to constrain the model aerosol parameters is a lidar co-located to the spectrometer, operating in a spectral region where the gases of interest have absorption lines. Consequently, a pulsed micro-lidar is simulated as a tool to constrain ALH. The benefits of this input information in the retrieval against methodologies based on oxygen A-band absorption bands are assessed. Furthermore, work is underway that assesses the overall benefits of the lidar, including those for other model parameters and mission objectives.