Thesis (M.App.Sc.) - Carleton University, 2002. / Includes bibliographical references (p. 129-131). Also available in electronic format on the Internet.

Zugl.: Karlsruhe, Univ., Diss., 2008 / Hergestellt on demand

Zsfassung in engl. Sprache. - Würzburg, Univ., Diss., 2008

Zugl.: Köln, Universiẗat, Diss., 2009.

Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 271-296). Available also in a digital version from Dissertation Abstracts.

Universiẗat, Diss., 2002--München.

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.

This thesis forms a NERC funded CASE studentship with the Met Office, whose aim is to investigate the treatment of cloud microphysical processes in numerical models, with a particular focus on exploring the impacts and possible benefits of microphysical complexity for the purpose of simulating clouds and precipitation. The issue of complexity is an important one in numerical modelling in order to maintain computational efficiency, particularly in the case of operational models. The latest numerical modelling tools are utilised to perform simulations of cloud types including idealised trade wind cumulus, orographic wave cloud and wintertime shallow convective cloud. Where appropriate, the modelling results are also validated against observations from recent field campaigns. The Factorial Method is employed as the main analysis tool to quantify the effect of microphysical variables in terms of their impact on a chosen metric. Ultimately it is expected that the techniques and results from this thesis will be used to help inform the future development of cloud microphysics schemes for use in both cloud resolving and operational models. This is timely given the current plans to upgrade the microphysics options available for use within the Met Office Unified Model. For an idealised warm cloud, it is shown that different bin microphysics schemes can produce different results, and therefore additional microphysical complexity does not necessarily ensure a more consistent simulation. An intercomparison of bin microphysics schemes in a 1-D column framework is recommended to isolate the origin of the discrepancies. In relation to the mixed-phase wave cloud, model simulations based on an adaptive treatment of ice density and habit struggled to reproduce the observed ice crystal growth rates, highlighting the need for further laboratory work to improve the parameterization of ice growth by diffusion within the sampled temperature regime. The simulations were also found to be largely insensitive to values of the deposition coefficient within the range of 0.1 to 1.0. Results from a mesoscale modelling study of shallow wintertime convection demonstrate the importance of the representation of dynamical factors that control cloud macrostructure, and how this has the potential to overshadow any concerns of microphysical complexity. Collectively, the results of this thesis place emphasis on the need to encourage more synergy between the dynamics and microphysics research communities in order to improve the future performance of numerical models, and to help optimise the balance between model complexity and computational efficiency.

Anthropogenic aerosol emissions lead to an increase in the amount of cloud condensation nuclei and consequently an increase in cloud droplet number concentration and cloud albedo. The corresponding negative radiative forcing due to aerosol cloud interactions (RFaci) is one of the most uncertain radiative forcing terms as reported in the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Here we show that previous observation-based studies underestimate aerosol-cloud interactions because they used measurements of aerosol optical properties that are not directly related to cloud formation and are hampered by measurement uncertainties. We have overcome this problem by the use of new polarimetric satellite retrievals of the relevant aerosol properties (aerosol number, size, shape). The resulting estimate of RFaci = −1.14 Wm 2 (range between −0.84 and −1.72 Wm 2) is more than a factor 2 stronger than the IPCC estimate that includes also other aerosol induced changes in cloud properties.

The aerosol direct and indirect effects were investigated for three specific cases during the March 2000 Cloud IOP at the SGP site by using a modified WRF model. The WRF model was previously altered to include a two-moment bulk microphysical scheme for the aerosol indirect effect and a modified Goddard shortwave radiation scheme for the aerosol direct effect. The three cases studied include a developing low pressure system, a low precipitation event of mainly cirrus clouds, and a cold frontal passage. Three different aerosol profiles were used with surface concentrations ranging from 210 cm-3 to 12,000 cm-3. In addition, each case and each aerosol profile was run both with and without the aerosol direct effect. Regardless of the case, increasing the aerosol concentration generally increased cloud water and droplet values while decreasing rain water and droplet values. Increased aerosols also decreased the surface shortwave radiative flux for every case; which was greatest when the aerosol direct effect was included. For convective periods during polluted model runs, the aerosol direct effect lowered the surface temperature and reduced convection leading to a lower cloud fraction. During most convective periods, the changes to cloud, rain, and ice water mixing ratios and number concentrations produced a nonlinear precipitation trend. A balance between these values was achieved for moderate aerosol profiles, which produced the highest convective precipitation rates. In non-convective cases, due to the presence of ice particles, aerosol concentration and precipitation amounts were positively correlated. The aerosol threshold between precipitation enhancement and suppression should be further studied for specific cloud types as well as for specific synoptic weather patterns to determine its precise values.