Tropospheric Volatile Organic Compounds

Over the past ten years it became obvious that the main sources for volatile organic compounds (VOCs) which act as precursors for tropospheric ozone production are of biogenic origin. Nearly all of these compounds can be readily detected and quantified by means of Proton-Transfer-Reaction Mass Spectrometry (PTR-MS), a fast on- line technique developed over the past seven years at our institute. Most of the investigations proposed here are based on exploiting this technique, which has been used successfully for environmental, medical and food research. It is the aim of this project to extend the inventory of biogenic VOCs, emitted from different sources, such as decaying biomass, an important tropospheric input of acetone and methanol which has been detected at our institute in cooperation with Prof. Paul Crutzen, Mainz. These investigations will be extended to C5 and C6 alcohols, various aldehydes and other VOCs. Furthermore, plant wounding has been recognised recently in collaboration with Prof. Ray Fall, Boulder, as a substantial tropospheric source of short lived C6 compounds, including (Z)-3-hexenal, (E)-2-hexenal, and hexenol, as well as acetaldehyde. These sources will be quantified in the present project by enclosure measurements (using small cuvettes for single leaf emissions and large ones for field emissions) as well as micrometeorological flux systems, based on eddy covariance measurement using the combination of a fast response real time sensor (PTR-MS) and sonic anemometer. Plant emissions and biomass burning will also be investigated extensively. All these VOCs originating from different sources are present in the troposphere showing different densities and density gradients as a function of altitude, depending on their total source strengths and reactivities. During LBA- CLAIRE flight experiments in 1998 over the rain forest in South America, PTR-MS has proven to be most suitable to measure the densities of many VOCs up to elevations of 11 km, thus we will use this technique to study the densities of VOCs as a function of time (over periods of many months) at mountain sites (different elevations) and based in the valley and we will measure height profiles by putting the PTR-MS system onto cable cars at two sites, one in Tirol and one in Bavaria, each of them covering height differences of about 1000 m. The results will provide input data for model calculations on tropospheric ozone chemistry.

Recently, a new method for gas analysis was developed at the Institut für Ionenphysik. Using a novel soft ionization technique this so called proton transfer reaction -mass spectrometry (PTR-MS) allows the quantitative and qualitative analyis of volatile organic compounds (VOC), such as methanol, ethanol, acetone, benzene, tolouol, isoprene and many others. Thus it was possible for the first time to monitor on-line simultaneoulsy all VOC compounds in air down to concentrations of as low as about 10 ppt . In the frame of the present project this method was systematically improved and applied successfully to a number of different fields, including environmental questions, medical applications and food technology. Novel type of results have been published in more than 33 articles in international refereed journals. For instance it is possible to measure with PTR-MS the concentration of acetonitrile in the breathing air of humans. This compound is produced when biomass is burning, thus also during the smoking of a cigarette. Even several days after a human stops smoking cigarettes it is possible to detect minute amounts of concentration of this tracer gas. In the human breath a plethora of VOCs in all kind of small concentrations is present, if the concentration of one of these VOCs deviates from the normal value, this can be an indication of the presence (start) of an illness. It is well known and accepted fact that human activities start to change the complex matrix of the earth`s atmosphere. First negative signs of this can be observed since years and measurements of air with PTR-MS on-line are imperative and important for understanding the complex chemistry going on. First successful campaigns have been carried out in Austria but also in the Indian ocean allowing to pin down specific sources of pollution. Other possible applications have been pointed out here for the first time in the food area. Meat, cheese, fruits (bananas, apples, strawberries), vegetables or root extracts emit a large number of different VOCs. These characteristic patterns of aroma have been shown to yield important information about the quality (it was possible to identify spoilage of meat by bacterial pollution within minutes) or origin (region, clone, etc.) of food. Moreover, rosting of food can lead to the production of hazardous compounds such as acrilamide and these compounds can be monitored during the production cycle.