Emerging Airborne Active and Passive Remote Sensing in Support of Multi-Scale Top-Down Inventories

Routine observations of accurate methane (CH4) concentrations over a range of spatial and temporal scales are required to constrain both anthropogenic and biogenic CH4 emissions. To-date, focus has been directed towards better quantifying strong CH4 point sources, but regional diffuse fluxes are still the largest contributor to the anthropogenic CH4 budget. To accurately constrain diffuse regional anthropogenic CH4 fluxes, accurate remote sensing observations with sensitivity to partition anthropogenic from biogenic signal are required. To cover the range of spatial, solar zenith angle, albedo, and sensitivity/accuracy scales needed to constrain emissions from different sectors (including urban), routine top down airborne active/passive remote sensing along with bottom-up long-term ground networks are required.

During summer 2023 we had the opportunity to demonstrate the synergy of combined active and passive remote sensing. The NASA Langley High Altitude Lidar Observatory (HALO) and the Airborne Visible InfraRed Imaging Spectrometer - Next Generation were flown on the GIII as a part of the STAQS mission to compliment an air quality focused mission with city-scale GHG observations. Independently, the MethaneAir (MAIR) imaging spectrometer was flying on the Lear Jet for mapping of different oil and gas sectors. HALO and MAIR mapped XCH4 over similar New York City domains, albeit on separate days. Despite the large temporal difference, both observed similar emission rates from point sources and trends in city scale enhancements. Here we show examples of these spatially co-located observations, estimates of landfill emission rates, and comparisons of city scale observations. These comparisons elucidate how accurate small-foot print lidars without swath can complement and help to constrain retrievals from passive imaging spectrometers with swath but are susceptible to potential sources of bias from aerosols, clouds and low albedo conditions. We also discuss the performance of the HALO instrument from previous campaigns and present a concept to combine MAIR and HALO on one platform for diurnal measurements spanning multiple days, in tight coordination with airborne and ground based in-situ and space-based observations, to create the first comprehensive datasets for which regional emissions can be tightly constrained.