Along with methane, isoprene is the most emitted organic molecule into the Earth's atmosphere. The unsaturated hydrocarbon is emitted into the air by a variety of tree species (e.g., oaks and poplars) and is typically degraded within an hour. However, secondary air pollutants such as ozone and particulate matter are produced when the highly reactive trace substance is broken down in the presence of traffic and industrial exhaust gases. For this reason, back in 2013, isoprene was in the focus of elaborate satellite and aircraft measurements by US agencies in the southeastern United States, where emissions from oak forests, caloric power plants and traffic together provide the chemical ingredients for poor air quality. Two scientists from the University of Innsbruck, Assistant Professor Martin Graus and Professor Armin Wisthaler, who participated in more than 40 research flights, were also significantly involved in the data collection: On behalf of the U.S. National Oceanic and Atmospheric Administration (NOAA) and the U.S. National Aeronautics and Space Agency (NASA), they carried out isoprene measurements in the atmosphere using the Proton-Transfer-Reaction Mass Spectrometer (PTR-MS) developed at the Institute for Ion Physics and Applied Physics. "Satellite measurements of trace gases in the Earth's atmosphere include many uncertainties. That is why - especially when new types of measurements are carried out and new satellite data products are presented - in-situ measurements are always needed to check their accuracy," explains Prof. Armin Wisthaler from the University of Innsbruck. He has been collecting data for NASA with the support of the Austrian Space Applications Programme (ASAP) of the FFG for over 10 years. Like Assistant Professor Martin Graus, member of the Atmospheric Physics and Chemistry Group at the University of Innsbruck, he took part in the major measurement campaigns of NASA and NOAA in 2013. Both researchers contributed their years of expertise in the field of PTR-MS measurements. "PTR-MS measurements in aircraft are much more complex than in the laboratory. The preparation for such a campaign starts already one year before with the adaptation of the instruments and technical tests for the measurements on board the aircraft", Martin Graus describes the extensive planning work for the up to 8-hour measurement flights.
In-situ data confirm satellite measurements
The data obtained were used to validate a method developed by Prof. Dylan Millet and his team at the University of Minnesota to evaluate the measurement data from the satellite-based Cross-Tracked Infrared Sounder (CrIS). Using a new algorithm based on an artificial neural network, the isoprene concentration in the Earth's atmosphere can be determined globally from the CrIS measurement data. "Our in-situ data, which give precise snapshots of the isoprene concentrations present in the Earth's atmosphere along the flight tracks, have helped our colleagues from Minnesota to validate and publish their novel method and the global isoprene data obtained with it," says Martin Graus. In this way, an initial discrepancy between satellite data and existing air quality models was resolved. "Our in-situ measurements have confirmed the findings from the satellite measurements," says Wisthaler, illustrating the possibilities of the novel observations from space: "In a sense, it is possible to observe the entire Earth's atmosphere as a chemical reactor from space and study atmospheric chemical processes on a global scale.”