Researchers from the DLR Institute of Atmospheric Physics have developed a laser-based system that can be used for airborne measurements of important greenhouse gases with unprecedented accuracy and resolution. The new system might even be useful for monitoring trace gases from space.
An article describing how their lidar instrument was used aboard an aircraft to acquire the first simultaneous measurements of the vertical structure of water vapor and ozone in the tropopause region of the atmosphere has been selected for an editor’s pick in the Applied Optics journal.
“The ability to detect the vertical structure of water vapor and ozone is critical for understanding the exchange of these atmospheric gases between the troposphere and the stratosphere,” said Andreas Fix, who led the research team. “These measurements could help us identify errors and uncertainties in climate models. That would help improve predictions of the future climate, which is one of the central challenges for our society and economy.”
Atmospheric trace gases can be assessed with instruments flown into the atmosphere or with data acquired from satellites. However, these methods either lack the vertical component or don’t provide high enough resolution. Although instruments carried with balloons — known as balloon sondes — can provide highly resolved vertical profiles they don’t offer detailed temporal resolution and can only be used at selected sites.
The new system addresses those problems by providing measurements with a resolution in the order of 250 m vertically and 10 km along the aircraft’s flight track. It uses two slightly different UV wavelengths to measure each gas, in an approach called differential absorption lidar (DIAL). The UV radiation at one wavelength is mostly absorbed by the gas molecules while most of the other wavelength is reflected. Measuring the ratio of the UV signals returning from the atmosphere allows calculation of a detailed gas profile. To perform this method aboard a plane, the researchers used a highly efficient optical parametric oscillator (OPO) they previously developed to convert the laser output to the UV wavelengths needed to measure water vapor and ozone.
Tests of the new lidar system showed that its accuracy matched well with that of balloon sondes. In 2017, the researchers flew the new system aboard the wave-driven isentropic exchange (WISE) mission, which involved multiple long-range flights over the North Atlantic and Northern Europe. They found that the instrument worked remarkably well, remained stable during use and could measure characteristic ozone and water vapor distributions at the tropopause.
The researchers plan to integrate the new vertical-component data acquired during WISE into climate models, and also look forward to deploy the instrument in future aircraft missions to collect additional exiting data.
Researchers developed a new lidar system that was used aboard the German research aircraft HALO (High Altitude and Long-Range Research Aircraft) to acquire the first simultaneous measurements of the vertical structure of water vapor and ozone in the tropopause region of the atmosphere. (Photo: DLR, CC-BY 3.0)