Optical properties of atmospheric aerosols using filter-based absorption photometers

by John Backman

Institution: University of Helsinki
Department: Department of Physics, Matemaattis-luonnontieteellinen tiedekunta
Year: 2015
Keywords: ilmakehätiede
Record ID: 1144840
Full text PDF: http://hdl.handle.net/10138/144440


Aerosol particles are part of the Earth's climatic system. Aerosol particles can significantly impact the climate. The ability of aerosol particles to do so depends mainly on the size, concentration and chemical composition of the particles. Aerosol particles can act as cloud condensation nuclei (CCN) and can therefore mediate cloud properties. Aerosol particles can thus perturb the energy balance of the Earth through clouds. Aerosol particles can also directly interact with solar radiation through scattering, absorption, or both. The climatic implications of aerosol radiation interactions depend on the Earth s surface properties and the amount of light scattering in relation to light absorption. Light absorbing aerosol particles, in particular, can alter the vertical temperature structure of the atmosphere and inhibit the formation of convective clouds. The net change in the energy balance imposed by perturbing agents, such as aerosol particles, results in a radiative forcing. Globally, aerosol particles have a net cooling effect on the climate, but, not necessarily on a local scale. Accurate measurements of the optical properties of aerosol particles are needed to estimate the climatic effects of aerosols. A widely used means of measuring light absorption by aerosol particles is to use a filter-based measurement technique. The technique is based on light-transmission measurements through the filter when the aerosol sample is drawn through the filter and particles deposit onto the filter. As the sample deposits, it will inevitably interact with the fibres of the filter and the interactions needs to be taken into account. This thesis investigates different approaches to dealing with filter-induced artefacts and how they affect aerosol light absorption using this technique. In addition, the articles included in the thesis report aerosol optical properties at sites that have not been reported in the literature before. The locations range from an urban environment in the city of São Paulo, Brazil, an industrialised region of the South African Highveld, to a rural station in Hyytiälä in Finland. In general, it can be said that sites that are distant from urban areas tend to scatter more light in relation to light absorption. In urban areas, the aerosol particle optical properties show the aerosol particles to be darker. Aerosolpartiklar är små luftburna partiklar som är burna av en gas. Dessa partiklar kan antingen vara i fast form eller i vätskeform. Luftburna aerosolpartiklar är större än gasmolekyler (större än 0.001 mikrometer) och mindre än 100 mikrometer i diameter. Som jämförelse kan tas ett hårstrå som är ca 50 mikrometer i diameter. Trots deras ringa storlek kan de inverka på jordens klimat nämnvärt. Hur aerosolpartiklar inverkar på jordens klimatet beror främst på deras storlek och antal. Aerosolpartiklar spelar en central roll då vattenånga kondenseras för att bilda moln eftersom vattenångan behöver en yta för att kondens skall kunna bildas. Antalet luftburna partiklar kan därmed ändra på molns…