Science in the stratosphere – the mission to measure our atmosphere

“There are a lot of chemical processes in the air, starting close to the ground and ending in the upper troposphere or the stratosphere, and these chemical processes are a huge amount of different things,” says HALO pilot Marc Puskeiler. “We have particles in the air – particles from the sea, salt for example, particles from the land, sand or some organic things.

“There are some chemical reactions between these particles, together with the humidity of the air and lightning from thunderstorms, and this is all in a three-dimensional huge air volume. This was the main goal: to do measurements of these particles, of the change of concentrations, and these developments there.

“CAFE is a worldwide mission. We went to Brazil last year, and to the front of the coast of Africa three years ago. Cairns is a good base to do a sampling of air over the Pacific region, where not many land masses are, where the air is really clear, with no human-made pollution. We flew many hundreds or thousands of kilometres, mostly to the east over the Pacific Ocean.”

Blending aviation and meteorology

Speaking to Australian Aviation, Puskeiler says flying isn’t his only passion – rather, his work for the DLR allows him to blend his twin loves for aviation and meteorology.

“I grew up at an airfield. My father is a pilot, and I started gliding at the age of 14, got my PPL licence at 18, and so on,” he says. “I didn’t join commercial aviation from the beginning, I studied and I did my PhD in meteorology, and beneath that, I worked a lot as a flight instructor and did my commercial pilot’s licence. And after this, I got a job at DLR as a research pilot.

“I was always interested in meteorology and in flying, and then DLR had a job that was able to combine these two things. I’m very happy to do these research flights with this good combination – it’s very interesting to fly in the atmospheric parameters and do the measurements.”

Puskeiler’s knowledge of both subjects came in handy when working with the CAFE scientists to plan the flights, allowing the flight crews and science teams to better communicate.

“We are preparing these campaigns and these flights together with the scientists – the scientists don’t have any idea of aviation and flight planning and performance of the aircraft,” he says. “During the preparations and the briefings of the campaigns and the flights, we’re talking to each other, we’re working on the flight patterns, on the flight tactics and so on, and it’s really interesting to bring these two things together.”

Flying the HALO

The HALO aircraft cuts a striking image: the Gulfstream G550 is bristling with different holes, tubes, and instruments all over the outside, which Puskeiler says are vital to taking accurate samples of the atmosphere at different heights.

“We have different holes in the fuselage to mount the scientific instruments. Scientific instruments are mostly inside the cabin, and we have some air inlets on the top and the bottom of the fuselage to lead ambient air into the aircraft to the instruments where analysis is made,” he says. “We also have some instruments outside on the fuselage, sometimes also under the wings, doing particle measurements, chemical measurements, and we have some optical measurements. For every mission, the plane is modified individually for these chemical measurements, we had a lot of instruments detecting gases and so on.”

One might expect such a heavily modified aircraft to be more unwieldy or difficult to fly. Not so, says Puskeiler: the flights are smooth and comfortable, though the modifications do add some extra factors the pilots need to keep track of.

“The plane would have more range without its modifications because they cause some drag and they are very heavy, so we are not able to take full fuel,” he says. “The plane is a long-range aircraft and is able without modifications to fly 12 hours, and it was bought for this purpose – to fly very long and very high. We’re also certified up to flight 510; with this heavy weight it’s possible, but levels 450, 470 are mostly possible at the end of the flight.

“We have some more limitations than a normal airplane regarding icing, regarding speeds and so on, and you have to calculate very exactly because no flight planning software, for example, is capable of covering these kinds of flights – going up, going down, going up, going down and coming back to your home base. Normally it’s calculated from A to B and constant altitude, so here, we need a little bit more experience than an airline pilot for fuel flow and performance. We need the whole time an overview of how we are with our fuel, given we’re far outside and over the ocean so that we come back safely.”

This is especially true given the length of each flight: around eight or nine hours, which, coupled with preparation and debriefing, meant 12-hour workdays.

“The typical flight is we’re going to the east, for example, flight level 300 or flight level 400. It depends on the scientific interests,” says Puskeiler. “Then mostly we’re descending to do a vertical profile of the air down to 500 feet or 1,000 feet above the ocean and staying there for a minimum of 20 minutes to have a range of all gases from several hundred kilometres. One day we did a 500-kilometre leg along the ocean at 1,000 feet. Then we climb back, we do this perhaps several times, these vertical profiles, and then after eight to nine hours, we’re coming back to home base.”

Memories and measurements

Sitting in the cockpit of the HALO opened up some striking views and unforgettable experiences, says Puskeiler, including one of Australia’s most famous landmarks.

“One day, we had the chance to fly low level at 1,000 feet along the Great Barrier Reef to do measurements of the chlorophyll emissions,” he says. “We also saw islands like Papua New Guinea, and the Solomon Islands – on one occasion we had a fuel stop at the Solomon Islands, which was very nice and interesting for everybody on board.”

On another occasion, Puskeiler had the rare opportunity to fly around a tropical storm off the coast.

“For me as a meteorologist, it was also very interesting to see these huge thunderstorms, these huge clouds,” he says. “It was one of the most memorable flights because it’s a big thing to fly around and see this system. We were not inside the system, so it was a very smooth flight in calm and sunny air, but for me, it was a definitive, very memorable day.”

As for the measurements, while it’s early days yet, the data so far looks promising, with Puskeiler saying scientists have noted some “very unique and special effects regarding condensation of water droplets” due to aerosols emitted by the sea and by tropical forests.

“The atmosphere is a really complex system, so there are a lot of different processes, and we know a lot of these but not all. For example, the forecasts of thunderstorms, forecasts of cloud build-up and so on, are always a big challenge,” he says. “All these aerosols and gases contribute, for example, to this building of clouds and precipitation, and all this data is to better understand the atmosphere, to perhaps improve weather forecasts, to improve forecasts for future climate and so on.”

Not only that, Puskeiler says studies like this, in conjunction with other projects, can help chart the impact of aviation – including aircraft contrails – on a changing climate.

“Our institute was doing some chemical measurements direct in the exhaust gas of airliners, for example the A350 or A321, burning synthetic fuels,” he says. “They also did measurements with other aerosols and other gases, which are not so sensitive for the build-up of contrails and are much better for the environment. So, this is one big part of our work here: to find out what’s going on in this exhaust gas, and what the contribution to the weather and climate is.”