Development of the Carbon Cycle Column Radiometer (C3R) for ASCENDS CO and CH4 Measurements

This poster describes the development of a new passive remote sensor, the Carbon Cycle Column Radiometer (C3R), for measuring CO and CH4 column abundance in support of the ASCENDS NRC Decadal Survey mission. In order to understand sources and sinks of CO2, as well as the carbon cycle in general, measurements of CO and CH4 are required with high sensitivity in the planetary boundary layer. No passive sensor concept has demonstrated, through either analysis or actual measurement, the ability to sense CO or CH4 to the spatial and temporal quality demanded by the ASCENDS mission. To meet the ASCENDS requirements, we are developing a non-conventional gas filter correlation radiometer (GFCR), the C3R, which will provide high sensitivity all the way to the surface. C3R allows these measurements to be made for a very small footprint (~250 m dia.) concentric with ASCENDS laser observations of CO2. Dominated by reflected sunlight, nadir measurements in the 2.3 µm region have relatively constant sensitivity with altitude, thus offering sensitivity in the important planetary boundary layer, the primary source region of these gases. Measurement challenges at 2.3 µm include: (1) high surface albedo variability (2) CO and CH4 bands overlapping with water vapor (H2O) and each other; and (3) a weak CO band (~100 times weaker than its fundamental mid-IR band). The C3R instrument differs from a conventional GFCR in three primary ways: (1) the use of a focal plane array (FPA) detector in a non-imaging configuration which allows photon noise-limited detection, field of view scrambling and signal flat-fielding; (2) long-path correlation cells improve sensitivity throughout the atmosphere, including high sensitivity in the boundary layer; and (3) removal of interfering gas and spectral albedo effects by a novel technique. The poster will illustrate the optomechanical design (shown in Figure 1) and current state of hardware development, instrument performance modeling results, and plans for continuing development and testing of the instrument. Figure 1. Optical/physical design of the C3R sensor prototype.