Development and Evaluation of Methods in Source Apportionment of the Carbonaceous Aerosol

Sammanfattning: Carbonaceous aerosols are known to affect human health negatively and to affect the climate of Earth. As specific types of aerosols originate from specific sources, it is important to know the amount of aerosols emitted from each source. Aerosol source apportionment is an important tool in mitigating air pollution and improving air quality. The subject of this thesis was the evaluation and development of methods that can be used to improve source apportionment of the carbonaceous aerosol. Wood-burning aerosol emissions have previously been assumed to absorb relatively more light in the ultraviolet region than in the infrared, as compared to traffic aerosol emissions. The research presented in this thesis shows that this assumption may not be valid for modern well-insulated wood stoves. Emissions from these appliances exhibit light absorption patterns similar to that for diesel combustion aerosols. Hence, light absorption source apportionment methods may not be reliable in areas with a high abundance of modern well-insulated wood stoves. A light absorption source apportionment method was compared to a source apportionment method based on radiocarbon and a chemical tracer. Good agreement was found for the apportioned wood-burning and biogenic aerosols, but a small discrepancy was observed in the apportionment of fossil fuel/traffic aerosol mass concentrations. The measurement of the heavier stable isotope of carbon, 13C, is relatively cheap and easy, and was therefore evaluated as an alternative method of source apportionment. 13C, chemical tracers and radiocarbon were measured in aerosol filter samples from a rural background station and source apportionment calculations were performed. These results were then compared to source apportionment results derived only from radiocarbon and chemical tracers. Measuring 13C only led to minor insignificant changes in source allocation, and could not distinguish between biogenic and wood-burning carbon. Thermo-optical analysis is a common method in source apportionment of the carbonaceous aerosol. However, organic carbon can be falsely interpreted as combustion-related elemental carbon due to pyrolysis during the analysis. An attempt was made to remove the fraction of pyrolytic organic carbon using supercritical carbon dioxide. Adding a small amount of methanol during the extraction allowed a large proportion of the pyrolyzed carbon to be removed. The isolation of elemental carbon may improve source apportionment of combustion-derived aerosols. Finally, the impact of air mass exposure to different geographical surface categories on the observed chemical species of biogenic secondary organic aerosols was studied. Aerosol samples were collected on filters at a rural background station in southern Sweden. Chemical tracer compounds in biogenic secondary organic aerosols were measured. Air mass exposure to various geographical surface categories was calculated with the FLEXPART model. The results showed that air mass exposure to coniferous forests could explain many of the observed organic chemical species, while exposure to oceans did not contribute to the observed aerosol mass concentration. Mapping the effect of different geographical surface categories on the chemical composition of aerosols may provide an important tool in predicting how changes in land use may affect air quality.