Improved Instrumentation for the Detection of Atmospheric CO2 Concentration using an Airborne IPDA LIDAR for 2014 NASA ASCENDS Science Campaign

Abstract:

NASA-GSFC is developing a twin-channel, Integrated-Path, Differential Absorption (IPDA) lidar to measure atmospheric CO₂ from space as a candidate for NASA’s ASCENDS mission (Active Sensing of CO₂ Emissions over Nights, Days, and Seasons).

This lidar consists of two independent, tuned, pulsed transmitters on the same optical bench using a common 8” receiver telescope. The system measures CO₂ abundance and O₂ surface pressure in the same column to derive the dry volume mixing ratio (vmr). The system is being tested on an airborne platform up to altitudes of 13 Km.

The lidar uses a cw scanning laser, externally pulsed and a fiber amplifier in a Master Oscillator Power Amplifier (MOPA) configuration to measure lineshape, range to scattering surfaces and backscatter profiles. The CO₂ operates at 1572.335 nm. The O₂ channel uses similar technology but frequency doubles to the O₂ A-band absorption, around 765nm. Both lasers are scanned across the absorption feature measuring at a fixed number of discrete (~30) wavelengths per scan around ~300 scans/s. Each output pulse is slightly chirped <12MHz as the laser is tuning. Removing this chirp will improve our ability to infer vertical CO₂ distribution from a more accurately measured line shape.

A Step Tuned Frequency Locked (STFL) DBR diode laser system has been integrated into the CO₂ lidar. Tuning and locking takes a ~30µs and the laser is locked to < ±100KHz. We have the ability to position these pulses anywhere on the absorption line other than within a few MHz of line center.

While the telescope and fiber coupling scheme remains unchanged the detectors have been upgraded. The O₂ system now uses eight SPCMs in parallel to improve count rates and increase dynamic range. Especially useful when flying over bright surfaces. This will improve our ability to measure the O₂ pressure at cloud tops and aid in the determining the vmr above clouds.

An HgCdTe e-APD detector with a quantum efficient of >80%, linear over five decades is now used for CO₂ channel. We have increased spatial resolution by an order of magnitude using 0.1s along track integration. The increased signal, negligible nonlinear response and increased spatial resolution will improve our CO₂ retrievals.

We will discuss the improvements and effects on the retrievals from the Summer 2014 ASCENDS Science campaign.