CHARM-F is a newly developed airborne integrated path differential absorption (IPDA) lidar that will be operated on board the HALO research aircraft to quantify concentration gradients and surface-atmosphere fluxes of CH4 and CO2 both over anthropogenic point sources and larger-scale natural sources. Lidar does not require the sun as a light source, and can therefore provide both day and night, all-seasons and all-latitude measurements. Due to similar measurement geometry and weighting function, HALO validation campaigns with CHARM-F will be essential to MERLIN’s success.
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 per trace gas at slightly different wavelengths are emitted. 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.
The CHARM-F system 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 250 µs between the on- and off-line pulses ensures that nearly the same spot on ground is illuminated. The wavelengths have been carefully selected in the frame of sensitivity studies 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 Nd:YAG lasers. To achieve single mode operation both the pump and the OPO are injection seeded. The seed lasers are locked to a gas cell filled with a mixture of CO2 and CH4 which ensures an absolute wavelength calibration.