CHARM-F is an airborne integrated path differential absorption (IPDA) lidar for operation aboard the German research aircraft HALO. This unique lidar system is capable of measuring the column concentration of the two most important anthropogenically influenced greenhouse gases, carbon dioxide (CO2) and methane (CH4), below the aircraft at the same time. CHARM-F was developed as a scientific instrument to help understanding the distribution of those greenhouse gases on local and regional scales and their cycles, but also as an airborne demonstrator for the German-French methane mission MERLIN. As such it furthermore constitutes an essential component for the validation of MERLIN.
Measurement principle of CHARM-F (Graphics: ©DLR)
IPDA uses the lidar backscatter from the Earth’s surface and cloud tops to measure the column concentrations of CO2 and CH4 with high accuracy and low bias. Two spectrally narrowband laser pulses are emitted per trace gas at slightly different wavelengths. The online wavelength is positioned in the vicinity of a CO2 or CH4 absorption line in the near-infrared (NIR) spectral region. Another measurement with negligible absorption by atmospheric constituents is denoted offline and serves as reference. The on- and off-line wavelengths are spectrally close enough to consider the atmospheric and surface properties to be identical with the exception of the greenhouse gas absorption. The column concentration is related to the ratio of both lidar echoes. Since the return signals are very weak, it is necessary to accumulate several single measurements along the flight track in order to achieve the required measurement sensitivity. Lidar does not require the sun as a light source, and can therefore provide both day and night, all-seasons and all-latitude measurements.
CHARM-F, integrated in HALO (Photo: ©DLR)
CHARM-F on HALO consists of an optical unit and two standard racks. The optical unit houses the receiving system and two custom-built laser systems (one for each trace gas) emitting on-/offline pulse pairs with nanosecond duration. It allows for a precise ranging and a clean separation of atmospheric influences from the ground return, and leads to an unambiguously defined column. The close temporal separation of 500 µs between the on- and offline pulses ensures that nearly the same spot is illuminated on the ground. The wavelengths have been carefully selected to minimize temperature and pressure dependent effects and to avoid interference with other constituents, particularly water vapour.
The radiation is generated by optical parametric oscillators (OPOs) pumped by diode-pumped Nd:YAG lasers. To achieve single mode operation, both pump and OPO are injection seeded. The respective seed lasers are locked to a gas cell filled with a mixture of CO2 and CH4 which ensures an absolute wavelength calibration.
After having passed its flight tests, CHARM-F was deployed within its first scientific campaign in 2018: CoMet 1.0. During this intensive field campaign targeting anthropogenic greenhouse gas emissions in Europe, CHARM-F successfully measured individual CO2 and CH4 plumes from, e.g. coal-fired power plants and coal mine ventilation shafts, regional distributions of the greenhouse gases over industrialized areas such as the Upper Silesian coal basin in Poland, and large-scale, sub continental gradients.
Upcoming deployments will go beyond anthropogenic emissions and will concentrate on the largely unknown part of the global greenhouse gas budget, the large natural wetlands in the Arctic and tropics.