IPDA Lidar Method

MERLIN makes use of an IPDA (Integrated Path Differential Absorption) Lidar technique to measure the column abundance of methane.

The methodology and basic measurement concept has been developed in the framework of two dedicated ESA studies to support the Advanced Space Carbon and Climate Observation of Planet Earth (A-SCOPE) study for CO2, and subsequent studies focusing on CH4.

The Lidar instrument measures the reflected or scattered radiation from the Earth’s surface and from cloud tops along the satellite footprint as depicted schematically in Figure.

For each measurement, the ground spot will be illuminated by spectrally narrow-band laser pulses having slightly different frequencies in the 1.64 µm spectral domain, commonly denoted as online and offline frequencies, respectively.

Both signals from online and offline pulses are digitized and corrected for the energy of the emitted laser pulses which are monitored within the instrument. Many such pulse-pairs are accumulated along the instrument’s ground track to increase the measurement precision. The ground reflectivity and beam attenuation from contributors other than the trace gas are typically constant at the very narrow spectral scale of a molecular absorption line, as shown by dashed lines in the right-hand corner.

To be sensitive to the CH4 concentration changes close to the Earth’s surface, the online frequency will be accurately positioned on the wing of a pressure-broadened CH4 absorption line. The spectral precision is possible since the spectral width of the laser pulses is set at about 60 MHz. Hence, it can be regarded as being quasi-monochromatic compared to the pressure-broadened CH4 absorption line of about 3 GHz.

Schematic of IPDA method

Schematic diagram of a two-frequency Integrated Path Differential Absorption (IPDA) lidar measurement.

Definition of XCH4

For modelling purposes it is convenient to introduce pressure coordinates using the hydrostatic equation in combination with the ideal gas law.

By this convention, the column-integrated dry-air mixing ratio of CH4 ,commonly denoted as XCH4, is calculated by normalisation of the DAOD using spectroscopic and NWP data for the molecular absorption cross sections in combination to the individual surface pressure data at the footprint where the laser spot hits the ground (or cloud top). The latter can be derived from laser ranging means in combination with the hydrostatic equation. The normalisation parameter is called the Integrated Weighting Function (IWF) which is a measure for the total air mass.

Unique features

Possible water vapour interference will be significantly reduced by applying the so-called water vapour compensation mode. It is based on the idea of choosing the offline position within a water vapour line such that a measurable water vapor DAOD appears, which then partly compensates the water vapour contribution related to the dry-air mixing ratio calculation.

The scattering and extinction properties of the atmosphere are identical for both frequencies and will not contribute to a measurement bias. A further advantage of using pulsed lasers refers to the known light path, which is indisputably defined by the viewing geometry of the Lidar instrument and the round-trip time of the transmitted laser pulses with typical pulse lengths of a few tens of nanoseconds. Multiple scattering from penetration of optically thick aerosol layers and optical thin cirrus clouds are suppressed by selection of a narrow transmitter divergence in conjunction with a narrow receiver field of view. The laser ranging approach for the total air mass replaces the need for an additional sensor for the surface pressure which helps to reduce costs and instrument resources significantly.