Formaldehyde Integrated Path Differential Absorption LIDAR

An improved understanding of the coupled chemistry-climate system is a key objective of NASA Earth Science. Central to this objective is the reactive photochemistry that controls the lifetime of greenhouse gases like methane, the production of ozone, and the growth of organic aerosols. Formaldehyde (HCHO) is a critical player in these processes: It is a key measure of the oxidative power of the atmosphere (it is produced in the oxidation of methane), an important intermediate in the production of ozone (it produces the species that make ozone), and an indicator for the abundance of the organic precursors that lead to organic aerosols (it is produced from the same organic precursors). HCHO is an important component of existing (OMI, TROPOMI, OMPS) and planned (TEMPO, GEMS, Sentinel-4) satellite missions. Each of these instruments measures reflected sunlight in the near ultraviolet (300-350 nm) to determine the column of HCHO. The difficulty with this technique is that the high level of scattering makes the retrieval of the column abundance strongly dependent on the assumed shape of the HCHO profile. The combination of the weak absorption signal (HCHO is present at parts per billion in the atmosphere), limited light due to scattering, and the dependence on a model-derived concentration profile, make HCHO difficult to measure accurately with passive spectroscopy. Our motivation is that this accuracy is not adequate to solve emerging science goals. Our team proposes to develop a new method to detect formaldehyde remotely with integrated path differential absorption (IPDA) LIDAR under the IIP instrument concept demonstration (ICD) call. Our concept uses a tunable narrow-linewidth fiber amplified laser to measure the absorbance of single rotational lines of the A-X transition at 339 nm. The concept will measure the column of formaldehyde in the laser path using a simple Beer's law analysis that is largely independent of the a priori assumptions needed in passive systems, providing improved capability in sensitivity and accuracy. In addition, since this is an active system with a small footprint, it can measure at night and in scenes partially obscured by clouds and aerosol. The challenge is to develop the experimental capability to detect the low abundances of formaldehyde in the UV where Rayleigh scattering is large. We now have the technology to meet this challenge. We will modify a new laser developed with ESTO and GSFC IRAD support to provide the laser light at 339 nm. We will demonstrate the capability to detect HCHO with a remote target on the ground using commercially available electronics for data acquisition. We will evaluate and optimize the technique and, at the conclusion of this effort, we will provide our recommendation for the best path towards an instrument design. We will advance the TRL from 2 to 3 over an 18-month period. At the conclusion of this IIP ICD, we plan to pursue the development of an airborne IPDA instrument with the instrument Development and Demonstration (IIP-IDD) or airborne instrument technology transfer (AITT) program. Our short-term (3-5 yr) goal is to demonstrate performance in an operational airborne configuration. Longer term (5-10+ yr), this concept can be applied to space-based measurements.

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