New methods to study composition and processes of atmospheric organics in the gas and the condensed phase

Abstract

Volatile organic compounds (VOCs) and organic aerosols (OA) play an important role in the Earth’s atmosphere due to their influence on human health and climate. To be able to control amounts of VOCs and OA in the atmosphere, one has to understand the main sources of them and the effect of external factors on these sources. While many studies on the main VOC source (i.e. emission from plants) have been performed, there has been only a limited number of studies on how external factors such as environmental pollution effect plant emissions. Historically, OA is measured as bulk organic carbon. Measuring the chemical composition of OA is more challenging but nevertheless essential to fully understand secondary sources and climate effects of aerosols. The goal of this work was the development and validation of new methods that aim at providing new tools for innovative research in the abovementioned field. The first part of this work describes a newly developed setup in order to study the effect of pollution on VOCs emitted by plants. Various pollutants can be applied in the setup. The setup allows for in situ detailed measurements of changes in plant emissions and allows distinguishing isomeric compounds emitted by plants in normal conditions as well as under stress. The setup has been tested based on experiments with birch seedlings: their emissions were measured, oxidized and the oxidation products were measured as well. A reasonable agreement with literature data was observed. The second part of this work describes a new method called offline thermal-desorption proton-transfer-reaction mass-spectrometry (TD-PTR-MS), which allows for detailed and relatively cheap measurements of OA composition based on filter samples which can be collected in a broad range of locations. The method has been tested based on the inter-comparison with the proven in situ technique (in situ TD-PTR-MS) (e.g., Holzinger et al., 2010) and in general a good agreement between the two techniques was confirmed. However, it was found that for the filters sampled for one day, positive sampling artifacts caused by the adsorption of semivolatile organic species on the filters were high, which is in accordance with previous findings described in the literature. For the filters sampled for two and three days slight negative artifacts were observed likely caused by not complete desorption of aerosols off the filters at temperatures up to 3500C and potential chemical degradation of aerosols on the filters. The third part of this work shows the ability of the in situ TD-PTR-MS technique to reasonably measure total OA mass concentrations as well as speciated organics with a time resolution of 0.5 hour. The former is shown based on the comparison with aerosol mass spectrometer (AMS) measurements. The latter is done based on the inter-comparison with two-dimensional gas chromatography with a time-of-flight mass spectrometer (GC×GC/TOF-MS) technique. We found that species with mass concentrations above 2 ng/m3, as measured by GC×GC/TOF-MS, were reasonably detected with the in situ TD-PTR-MS technique.