For a prediction of the rate of climate change during the 21<sup>st</sup> century, there is an urgent need to better understand the global carbon cycle, in particular the processes that control the carbon flows between the various reservoirs, and their interactions with the climate system. Atmospheric carbon dioxide (CO<sub>2</sub>) represents the main atmospheric phase of this biogeochemical cycle. Due to human activities, the concentration of this most important of the Earth’s greenhouse gases has grown from a pre-industrial average atmospheric mole fraction of about 280 parts per million volume (ppm) to 390.5 ppm in 2011 which is an increase of 40%. CO<sub>2</sub> contributes to ~63% to the overall global radiative forcing.
For the CO2 and CH4 IPDA lidar CHARM-F two single frequency Nd:YAG based MOPA systems were developed. Both lasers are used for OPO/OPA-pumping in order to generate laser radiation at 1645 nm for CH4 detection and 1572 nm for CO2 detection. By the use of a Q-switched, injection seeded and actively length-stabilized oscillator and a one-stage INNOSLAB amplifier about 85 mJ pulse energy could be generated for the CH4 system. For the CO2 system the energy was boosted in second INNOSLAB-stage to about 150 mJ. Both lasers emit laser pulses of about 30 ns pulse duration at a repetition rate of 100 Hz.
Carbon dioxide (CO2) and methane (CH4) are the most important of the greenhouse gases that are directly influenced by
human activities. The Integrated Path Differential Absorption (IPDA) lidar technique using hard target reflection in the
near IR (1.57μm and 1.64μm) to measure the column-averaged dry air mixing ratio of CO2 and CH4 with high precision
and low bias has the potential to deliver measurements from space and air that are needed to understand the sources and
sinks of these greenhouse gases. CO2 and CH4 IPDA require tunable laser sources at 1.57 μm and 1.64 μm that coincide
with appropriate absorption lines of these species having high pulse energy and average power as well as excellent
spectral and spatial properties.
Within this study we have realized more than 50mJ of pulse energy in the near IR coincident with appropriate absorption
lines using an injection-seeded optical parametric oscillator-amplifier system pumped at 100 Hz. At the same time this
device showed excellent spectral and spatial properties. Bandwidths of less than 100 MHz with a high degree of spectral
purity (> 99.9 %) have been achieved. The frequency stability was likewise excellent. The M2-factor was better than 2.3.
Owing to these outstanding properties optical parametric devices are currently under investigation for the CH4 lidar
instrument on the projected French-German climate satellite MERLIN. A similar device is under development at DLR
for the lidar demonstrator CHARM-F which will enable the simultaneous measurement of CO2 and CH4 from an
We report on a lidar approach to measure atmospheric CO2 column concentration being developed as a candidate for
NASA's ASCENDS mission. It uses a pulsed dual-wavelength lidar measurement based on the integrated path
differential absorption (IPDA) technique. We demonstrated the approach using the CO<sub>2</sub> measurement from aircraft
in July and August 2009 over various locations. The results show clear CO<sub>2</sub> line shape and absorption signals, which
follow the expected changes with aircraft altitude from 3 to 13 km. The column absorption measurements show
altitude dependence in good agreement with column number density estimates calculated from airborne in-situ
measurements. The approaches for O<sub>2</sub> measurements and for scaling the technique to space are discussed.
Active remote sensing using lidar appears to be very attractive for the measurement of atmospheric greenhouse gases
like carbon dioxide from spaceborne platforms. Feasibility studies are currently being performed to demonstrate the
required measurement performance. Due to the high precision required (less than 0.3 %) for climate studies, space-borne
IPDA (Integrating Path Differential Absorption) Lidar is preferred over the range resolving DIAL technique which uses
atmospheric backscatter. This is due to the larger Lidar echoes from hard target when using systems of comparable size.
Applying the IPDA Lidar method, magnitude and variability of the ground reflectance becomes an important issue in
terms of instrument sizing and pointing requirements of space-borne systems. Because of the stringent sensitivity
requirements, even small gradients of the ground reflectance could introduce noticeable retrieval errors in the CO2
column content, when the laser transmitter does not point on the same ground spot for the on- and off-line measurement.
However, the current knowledge on the variability of the ground reflectance both in the appropriate wavelength range
and on small spatial scales is insufficient for an accurate error assessment. In order to address these deficiencies, airborne
lidar measurements at 1.6 µm wavelength were performed. The wavelength range around 1.6 µm provides suitable
absorption lines for the measurement of carbon dioxide. A pulsed optical parametric oscillator (OPO) system (5 mJ at
1573 nm, 10 Hz pulse rate) was deployed on the DLR Cessna Caravan aircraft to measure the variations of the ground
return. In order to simulate a satellite system, statistical analyses on the data including upscaling to a larger ground spot
size of a space-borne system and different averaging ranges are being performed. The focus of this study is on the
investigation of the characteristics of typical surface types including the open sea.