Tracking the climate by airplane

3 September 2015

Professor Peter Hoor and his Airborne Measurements and Transport Processes work group at the Institute of Atmospheric Physics at Johannes Gutenberg University Mainz (JGU) are studying the processes that occur in the tropopause. The events that take place in this boundary layer between the stratosphere and the troposphere are still puzzling scientists but have significant effects on the climate of our planet.

A world map decorates one wall of Professor Peter Hoor's office. He directs our attention to a picture frame next to it containing a colorful patchwork of photographs that show snapshots taken during field measurement campaigns and images of aircraft. "On eight occasions during a two-year period, we used a Learjet to measure the atmosphere over Europe from the Cape Verde Islands to Spitsbergen," explains the atmospheric physicist. In front stands the model of another jet plane with what looks like a pole emerging from its nose. This, the new German research aircraft HALO, actually circled the whole of the African continent in 2012. "That was really a very complex project," recalls Hoor. The plane is set to start on another flight in the winter of 2015. This time, the researchers will be spending three months based in Kiruna in northern Sweden.

HALO is an acronym for High Altitude and Long Range Research Aircraft and this jet plane enables measurements to be taken at altitudes of up to 15 kilometers and over considerable distances. This is the field the Airborne Measurements and Transport Processes work group of the Institute of Atmospheric Physics at Mainz University specializes in. The focus of the work of Hoor and his team is an important boundary layer in the earth's atmosphere known as the tropopause.

Highly complex system

The troposphere extends upwards to a height of 6 to 18 kilometers. "This is the region in which - putting it simply - weather occurs, in which thunder clouds develop and storms are generated. There's a lot of turbulence here." Above the troposphere is the stratosphere with its ozone layer. "The stratosphere, in contrast, is very stable." This is because the higher you go in the stratosphere, the warmer the air becomes. So the presence of vertical air flows in this situation would appear to be highly unlikely.

Despite this, troposphere-stratosphere exchange processes do occur and these have their origin in the intermediate layer, the tropopause. The ozone, clouds, and water vapor in this region have a particularly extensive effect on our climate. "It's a highly complex system," says Hoor. "We know that exchange processes occur there and that materials are transported but all too often we do not understand how this happens. We're exploring what really is a terra incognita."

But the exploring itself is no easy matter. In order take measurements at these altitudes, specialized aircraft and instruments are required. Each field campaign is an expensive undertaking. Thus, several institutions need to collaborate each time a stratosphere-bound jet takes to the air. Hoor's work group cooperates with teams based in Canada and the USA. "We all work closely together," stresses Hoor. "Each team supplies a piece of the puzzle."

The tropopause mystery

Hoor searches for suitable similes to illustrate how the tropopause works. "To say it's like a lid on a saucepan wouldn't be quite right – that's too static. The lid would have to have a lot of holes." Perhaps a membrane then? "No, the tropopause is much more permeable and variable than that. More like a membrane with differently sized pores. And the location of the tropopause varies both in time and space. Just imagine two fluids with different densities. When you try to mix them, you get structures and fluctuations that are similar to those that occur in the case of the tropopause."

Hoor's work group measures levels of trace gases in order to analyze these fluctuations. They have four different pieces of equipment. "They're all prototypes." They measure levels of nitrous oxide, carbon monoxide, carbon dioxide, and methane. "By determining the concentrations of these gases at a particular location at a particular time we can make specific inferences on the transport processes within the tropopause. The atmosphere is our laboratory."

The measurement processes are astonishingly accurate. "For example, we use lasers to detect molecules at very specific wavelengths." Again Hoor attempts to find an appropriate analogy: "Our instruments are incredibly precise. It would be like observing the earth's population and being able to pick out one individual with a particular characteristic."

Accurate measurements

The aircraft used to take these measurements look rather like hedgehogs, but with tubes in place of the spines. Samples of the air are drawn in through these tubes and passed to the assay equipment on board the aircraft where they are directly analyzed for their content of substances such as nitrous oxide. "Our equipment can undertake three measurements per second. That means we can collect, as it were, snapshots of the composition of the atmosphere at every 70 meters during a flight." That is equivalent to more than 100,000 data points during a 10-hour flight. The result is a detailed map.

"Satellite images are all well and good. We find them useful, but they are simply not precise enough for us. A satellite image will show me what is happening but we want to know how it is happening. So we need to consider the finer points."

The work group decided to look closely at the effects of the late summer Asian monsoon. Here the troposphere can rise up to a height of 18 kilometers, causing the tropopause to create an extensive bulge deep into the stratosphere. "A member of our team was able to demonstrate in his doctoral thesis that these processes influence the composition of the tropopause region over Europe."

Influence is the keyword here: Hoor and his team are undertaking what is essentially fundamental research. They are trying to identify the laws that govern the transport processes between troposphere and stratosphere. Once these laws are understood, it may be possible to find answers to some burning questions. Will the Arctic ozone hole get bigger? Will the ice cap continue to shrink in the Arctic, where the effects of global warming are particularly apparent? And what will be the effects worldwide of the increasingly dense smog that tends to develop over major Asian cities?

The hallmark of Atmospheric Physics in Mainz

If it was down to him, Hoor would be taking measurements around the world and, if possible, at every season of the year. The troposphere at the two poles is only about 8 kilometers thick, much thinner than at the equator, where it can extend to heights of up to 18 kilometers during monsoon periods. Very different processes occur and there is much that differs with the various seasons.

The professor enthusiastically tells of his research: "Using aircraft to take measurements of the atmosphere has become the hallmark of our Institute of Atmospheric Physics at Mainz University. There are very few others in the world operating at a comparable level. We're researching in what is a very multi-layered discipline. Our scientists not only need to be versed in fields such as atmospheric physics and atmospheric chemistry, they also have to have knowledge in electronics, optics, and engineering." In addition, the groups conducting experimental and theoretical research at the JGU Institute of Atmospheric Physics work closely together to try and solve the problems associated with the tropopause. This bringing together of experiment and theory within a single institute makes Mainz one of the leading research hubs in the field.

Students are offered the opportunity to participate as much as possible. "We try to ensure that everyone who writes a Master's thesis here gets to take part in a field campaign and, if they're lucky, even gets a place on-board during a flight. That's a great opportunity for them. We're at the cutting-edge of our discipline."

Hoor peers from the window of his office at the blue summer sky outside. "There, you see those clouds with the wave-like structure? They're probably at an elevation of roughly 12 kilometers. You'll be able to see them for the next 15 minutes, then they'll have dispersed." Looking at him, you know he would like to be up there, measuring exactly what is happening.