Atmospheric Analytical Chemistry

The challenge to chemically characterize relevant components of the Earth’s atmosphere is probably greater than many scientists might expect. This is due to the large number of organic and inorganic compounds present in the atmosphere and the large chemical space these compounds cover, e.g., with respect to volatility, polarity or molecular weight. One obvious reason for the complexity is the fact that the atmosphere is a multiphase system. Not only gas phase compounds have to be considered, but at the same time also solid or liquid particulate matter as well as the atmospheric aqueous phase plays a major role in atmospheric chemistry. Moreover, often the most interesting and relevant compounds are present in ultra trace amounts, such as organic or inorganic radicals (ppt to ppq levels) or the material forming nanometer particles during a typical atmospheric nucleation event. Despite their very low concentrations these components are among the major drivers which shape atmospheric chemistry, i.e. hydroxyl radicals control the oxidizing power of the atmosphere and nanometer particles act as cores to finally form climatically relevant cloud condensation nuclei. As in many scientific fields, also in the atmospheric sciences major progress is often driven by the introduction of new or improved analytical techniques. For example, the recent introduction and frequent use of aerosol mass spectrometers in field measurements greatly improved the understanding of organic aerosol components.

The chemical characterization of atmospheric aerosol particles certainly belongs to the current analytical challenges, in particular for organic components. One reason is the impact of atmospheric aerosols on both climate and health, and the fact that knowledge of the variation of their chemical composition with particle size is key to understanding both effects. Based on the current physico-chemical understanding of atmospheric aerosol formation processes, especially quantitative information about low and semi-volatile, often highly oxidized organic compounds is needed. However, exactly the chemical and physicochemical properties that classify them as interesting target analytes (i.e. volatility, polarity, gas/particle partitioning, reactivity) complicate a reliable measurement of their concentration (analyte losses during sampling, poor recoveries). Nevertheless, the search for techniques to detect suitable tracers for process understanding or source attribution using both traditional off-line as well as on-line techniques for aerosol characterization will continue. For organic compounds to be considered as suitable molecular tracers their potential degradation due to chemical processing in the gas and particle phase (multiphase chemistry) has to be characterized, a topic recently discussed intensively but a not well understood process. In respect to inorganic aerosol components it is today recognized that the determination of the total burden of a specific element, e.g. the total amount of a trace metal in aerosols, provides insufficient information. To understand source and transformation of aerosol constituents the analysis of the distribution of an element among defined chemical species is necessary. Even more important is elemental speciation to predict the relevance and fate of individual aerosol constituents, i.e. the hygroscopicity of particle phase compounds will affect the particle's ability to act as cloud condensation nuclei, its vapor pressure the gas-particle partitioning and its solubility the bioavailability and potential toxicity of the individual species. Therefore, analytical strategies for the separation and quantification of the different elemental species are needed.

The research areas of our group are therefore focused on the development of analytical techniques for the chemical characterization of atmospheric aerosol particles. Two research fields were especially relevant in the last years, the investigation of Secondary Organic Aerosols and Atmospheric Iodine-related chemistry. We make use of sophisticated online mass spectrometric methods (Atmospheric-Pressure-Chemical-Ionisation IT-MS) as well as hyphenated techniques, such as HPLC-ESI-ITMS or Size Exclusion Chromatography-ESI-ITMS. These techniques are used for example to identify single molecules in SOA from chamber experiments or from ambient samples. Likewise, the investigation of the gas-particle partitioning behaviour of ageing SOA particles with the online mass spectrometric techniques is within the focus of the group.