Inhaltszusammenfassung

Chemical ionisation mass spectrometers (CIMS) equipped with a radioactive ion source (usually 210Po) to generate iodide ions are frequently used devices for atmospheric trace gas measurement with a broad field of application. Within the scope of this thesis a chemical ionisation quadrupole mass spectrometer (CI-QMS) with a novel electrical discharge ion source was characterised and proved to represent a promising alternative to similar instruments based on a radioactive ioniser, particularly ...Chemical ionisation mass spectrometers (CIMS) equipped with a radioactive ion source (usually 210Po) to generate iodide ions are frequently used devices for atmospheric trace gas measurement with a broad field of application. Within the scope of this thesis a chemical ionisation quadrupole mass spectrometer (CI-QMS) with a novel electrical discharge ion source was characterised and proved to represent a promising alternative to similar instruments based on a radioactive ioniser, particularly in environments where a permission for 210Po is difficult to obtain or transport is not feasible. In addition to the well-established detection of peroxyacetyl nitrate (PAN), nitryl chloride (ClNO2) and peracetic acid via I− primary ions, the instrument is capable of measuring sulphur dioxide (SO2), hydrogen chloride (HCl), acetic acid and pyruvic acid through additional ion-molecule-reactions, unique for the radio-frequency (RF) discharge ion source. The compact design and flexibility of the CI-QMS allow for stationary, airborne and shipborne measurements with limits of detection (2sigma, 1 s) of a few tens of pptv (parts per trillion by volume) for ClNO2 and SO2, which is beneficial for the investigation of chlorineand sulphur-related chemistry in the polluted marine boundary layer. Due to a high variability in the background signal, the application as a PAN detector is limited to polluted conditions or low temporal resolution. The CI-QMS with its discharge ion source was successfully operated in three different field studies under variable atmospheric conditions ranging from mixed urban / rural to remote forested to polluted marine environment. During the NOTOMO campaign, which took place at a forested mountain-site in south-western Germany, generally high levels of primary and secondary pollutants (SO2 and PAN) were measured, which reflected the influence of nearby urban conglomerations and industry on local air quality. In addition, ClNO2 mixing ratios of several hundred pptv were observed at this continental site, indicating the importance of aerosol chloride transported inland from coastal regions. The IBAIRN campaign, which took place at a remote site in the Finnish boreal forest with limited anthropogenic influence, was characterised by large biogenic emissions and low nitrogen oxide (NOx) levels. The first gas-phase measurements of pyruvic acid, a widely unexplored biogenic acid, in the boreal forest revealed its ubiquity and impact on radical chemistry in late summer. During AQABA, a shipborne campaign designed to study air quality and climate around the Arabian Peninsula, a unique dataset of ClNO2, HCl and SO2 in severely understudied regions like the Red Sea and the Arabian Gulf was collected. Mixing ratios of ClNO2 ranged from the limit of detection in the Arabian Sea to several hundred of pptv over the northern Red Sea, the Gulf of Oman, the Gulf of Suez and the Suez Canal, mainly due to high nitrate radical (NO3) production rates and the availability of particulate chloride. However, the overall production efficiency, i.e. the ClNO2 yield per NO3 molecule formed in the reaction of nitrogen dioxide (NO2) with ozone (O3), remained generally low due to exceptionally high nocturnal temperatures, which shifted the equilibrium from nitrogen pentoxide (N2O5) towards NO3. In combination with elevated NO3 reactivity towards volatile organic compounds (VOCs) this resulted in the dominance of direct NO3 losses over the heterogeneous uptake of N2O5 to the particle phase, with the latter representing a crucial step in the formation of ClNO2. The photolysis of ClNO2 and the oxidation of HCl (released by acid displacement) were determined to be important sources of chlorine radicals in polluted parts of the marine boundary layer over the Mediterranean Sea and the Arabian Peninsula, recycling NOx and potentially enhancing oxidation rates of several hydrocarbons. The successful long-term operation in field campaigns confirmed the suitability of the CI-QMS as an alternative to 210Po-based instruments for future measurements of ClNO2, SO2, HCl, PAN and pyruvic acid in regions where observational data is lacking, in order to enhance the quality of atmospheric models and to deepen our understanding of tropospheric trace gases with an impact on air quality and climate.» weiterlesen» einklappen

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Klassifikation

DDC Sachgruppe:
Physik