Development of a highly sensitive and versatile mass spectrometer system for laboratory and atmospheric measurements

Trace gases in the upper troposphere and the lower stratosphere (UTLS) strongly influence our climate and even can affect tropospheric weather. Therefore a better understanding of the processes governing trace gas distributions and transport is crucial. Sensitive and accurate atmospheric measurements are thus essential to improve and verify Chemical Climate Models (CCM’s) and ultimately enable more reliable forecasts of future climate. Two especially important trace gases in the UTLS are sulphuric acid, H2SO4 and its precursor SO2. Although H2SO4 is present at sub-ppt mixing ratios it plays a central role in the formation of the stratospheric aerosol layer, which is a crucial component of the global radiative budget [52]. Very selective and sensitive measurements of H2SO4 and many other trace species are possible using mass spectrometers in combination with chemical ionization (CI) at atmospheric pressure (AP). However, to reach a favorable detection limit, improvements of the ion source and the ion transfer stages are needed. Therefore the ion transmission through each transfer element needs to be optimized. Additional sensitivity may be gained using a “brilliant” ion source. The cluster chemistry in the ion source and transfer stage plays an important role for the sensitivity and the reliability of the measurement. In order to gauge the impact of cluster chemistry a transfer stage was developed that provides realistic mass spectrometer measurements of ion bound clusters and their reactions in the ion source. This transfer stage with tunable electrostatic fields provides full control of the ion energy, to the extent that additional reactions, induced by heating the ions in electrostatic fields, can be prevented. A thermally sampling atmospheric pressure ionization mass spectrometer has been constructed. The ion transfer stage offers the capability to sample cluster ions at thermal equilibrium. Fundamental processes in the transfer stage possibly affecting the cluster distribution can be readily identified. The performance of the setup is demonstrated with regard to the proton-bound water cluster system. The omnipresent water cluster chemistry occurs in every AP ion source and is also part of most AP CI reaction cascades.