Airborne Lidar Provides More Complete Picture of Atmospheric Gases

Researchers from Deutsches Zentrum für Luft- und Raumfahrt eV (DLR), Germany’s national center for aerospace, energy, and transportation research, have developed a laser-based system for airborne measurement of atmospheric gases. The distribution of these gases in the troposphere, the lowest layer of Earth’s atmosphere and where weather takes place, plays a central role in Earth’s climate.

The new lidar system uses laser light to measure the vertical structure of ozone and water vapor simultaneously. In an approach called differential absorption lidar (DIAL), the system uses two slightly different UV wavelengths to measure each gas. Measuring the ratio of the UV signals returning from the atmosphere allows calculation of a detailed gas profile. The gas profiles created using DLR’s lidar system exhibited a vertical resolution of around 250 m and a horizontal resolution of about 10 km below the aircraft’s flight track.

Researchers developed a new lidar system that was used aboard the German high-altitude and long-range research aircraft (HALO) to acquire the first simultaneous measurements of the vertical structure of water vapor and ozone in the tropopause region of the atmosphere. Courtesy of DLR.

“This vertical capability is a significant advancement in studying exchange processes at the tropopause,” Andreas Fix, who led the research team, said. “It helps overcome significant shortcomings in resolving the fine-scale distribution that have made it difficult to understand processes responsible for exchange at the tropopause.”

To perform the DIAL measurement method aboard a plane, the researchers used an optical parametric oscillator (OPO) that they had previously developed to convert the laser output to the UV wavelengths needed to measure water vapor and ozone. The conversion achieved a continuous UV tuning range of ∼297 to 317 nm. “The conversion needs to be very energy efficient to generate UV radiation with adequate pulse energies and high average power from the limited energy available onboard an aircraft,” Fix said.

The new lidar system was used for airborne atmospheric measurements during the wave-driven isentropic exchange (WISE) mission, which involved multiple long-range flights over the North Atlantic and northern Europe. Courtesy of DLR.

Tests of the new lidar system showed that its accuracy matched well with that of balloon sondes. The researchers flew the new system aboard the wave-driven isentropic exchange (WISE) mission in 2017, and found that the instrument showed high potential as a tool for measuring characteristic ozone and water vapor distributions at the tropopause.

The research was published in Applied Optics, a publication of OSA, The Optical Society (https://doi.org/10.1364/AO.58.005892).