EarthCARE lidar reveals details of particles in the atmosphere

24 Sep 2024

Atmospheric lidar instrument has now commenced operations, measuring clouds and aerosols.

The EarthCARE satellite, a joint venture between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), was launched in May 2024 to provide a new level of insight into the influence of aerosols on the Earth's climate.

EarthCARE's cloud profiling radar and broadband radiometer devices began their work soon afterwards. The most advanced of the satellite's instruments, the atmospheric lidar or ATLID platform, has also now commenced observations.

ATLID is designed to capture detailed 20-kilometer vertical profiles of both clouds and atmospheric aerosols, the tiny particles and droplets arising from natural sources like wildfires, dust, and sea spray, and from human activities like industrial emissions or burning of wood.

The ATLID device is based around a frequency-tripled Nd:YAG laser operating at 355 nanometers and a 62-centimeter-diameter receiving telescope. The optical design is bistatic, meaning that separate optics are used for the transmitter and receiver elements.

Research into the ATLID design, and how to build a working atmospheric lidar platform capable of the performance metrics envisaged, began nearly two decades ago at Jena-Optronik and Jenoptik Laserdiode, which set out to develop a suitable pumping source for a laser.

In 2017 ESA described how "as a result of high spectral resolution filtering, the lidar is able to separate the relative contribution of aerosol (Mie) and molecular (Rayleigh) scattering, which gives access to aerosol optical depth."

The Leibniz Institute for Tropospheric Research (TROPOS) also contributed to the project, by developing algorithms that derive the aerosol and cloud stratification from measurements made by the ATLID instrument. Fifty ground stations in Europe's Aerosol Clouds and Trace Gases Research Infrastructure (ACTRIS) are involved in the data gathering.

Atmospheric lidar brings completely new insight

The first ATLID observations were of polar stratospheric clouds over Antarctica, crucial contributors to the depletion of stratospheric ozone during winter and spring.

These clouds serve as surfaces for chemical reactions generating chlorine free-radicals, which actively destroy ozone molecules in the stratosphere, commented ESA. This process is a key factor in the formation of the infamous ozone hole over Antarctica.

"The jagged shape of the clouds is likely to be caused by gravity waves," noted the researchers. "Gravity waves form when air is forced upwards into a layer of stable air and gravity drags the air back down, creating a ripple effect. These waves transfer energy and momentum from the lower atmosphere to the upper atmosphere, influencing weather patterns, general atmospheric circulation and impact the formation of polar stratospheric clouds."

ATLID also observed a strip of atmosphere above North America, where the clouds registered the presence of tropical storm Debby over the Gulf of Mexico. The images revealed a striking red layer of aerosols carried by the wind from forest fires that have been burning for a number of weeks in several regions of Canada.

"Following on from the first images from EarthCARE’s other three instruments, we can now also see how well the atmospheric lidar is working," said Simonetta Cheli, ESA Director of Earth Observation Programmes.

"These profiles from ATLID come exactly as we had anticipated. The atmospheric lidar brings us completely new insight into the vertical distribution of clouds and aerosols and, together with the other instruments, puts us on course to gaining new scientific understanding into Earth’s energy balance."