The development of new analytical techniques is one of the driving forces in the advancement of scientific understanding. The measurement of the properties of aerosol particles is an active area of research due to the impact aerosol has on atmospheric processes. Single particle size and composition are key properties that govern many atmospheric processes, but the measurement of these properties is challenging due to the large dynamic range of size and composition that exists in the environment. Mineral dust represents a significant fraction of the global aerosol mass loading and has a profound impact on the earth's radiative budget through the direct interaction with solar and terrestrial radiation, and by affecting microphysical properties of clouds. In addition, mineral dust is involved in the geochemical cycling of many compounds that are vital for the health and vitality of ecosystems. The importance of the chemical and crystallographic properties of a material, or mineral phase, has been highlighted recently. Measurements of the elemental composition of single particles is possible with off-line analysis of dust collected on filters , but mineral phase is usually obtained from X-ray diffraction of bulk samples. These techniques are labour intensive and the lack of ambient measurements is a limiting factor in the development of models that attempt to resolve the complexity of atmospheric processes. Time-of-flight mass spectrometry (TOF-MS) is well suited to on-line single particle composition measurements due its sensitivity and high temporal resolution. Single particle mass spectrometry (SPMS) is a class of TOF-MS technique that is able to identify mineral dust particles from their chemical signature in the mass spectrum. Analysis of refractory mineral dust by mass spectrometry requires laser desorption ionisation (LDI) by high energy pulsed lasers, a process that renders the composition measurement non-quantitative due to incomplete ionisation and matrix effects. Consequently, the identification of mineral phase is not possible because the reproducibility of the measurement is lower than the natural variation between common minerals. This thesis reports the development of a commercially available single particle mass spectrometer for the measurement of the physiochemical properties of mineral dust. The optical particle detection system is improved for the more efficient detection of single particles in the size range relevant to the ambient measurement of mineral dust aerosol, and a model is developed that will aid the further development of particle detection in SPMS. A novel method for the on-line differentiation of mineral phase in single particles is presented which exploits differences in ion arrival times at the TOF-MS detector of a silicate molecular ion species, that arise from the influence of mineral phase on the ion formation process during the LDI process. The efficacy of the technique is demonstrated with the differentiation of mineral phase in laboratory generated mineral dust from clay mineral standards. The deployment of the improved instrument to measure Saharan dust outflow resulted in the first ever on-line identification of the clay mineral fraction in ambient mineral dust.
|Date of Award||1 Aug 2017|
- The University of Manchester
|Supervisor||Hugh Coe (Supervisor) & James Allan (Supervisor)|
- Mass Spectrometry
- Mineral Dust