| Summary: | Since the Industrial Revolution, the anthropogenic emission of greenhouse gases has been increasing, leading to a rise in the global temperature. Particularly in the Arctic, climate change is having serious impact where the average temperature has increased almost twice as much as the global during the past 100 years. In addition to greenhouse gases, aerosols have a large impact on Earth’s climate through their ability to absorb and scatter solar radiation and through interactions with clouds. Aerosols are expected to have an overall cooling effect on the climate. However, at the same time black carbon, which is the most efficient aerosol to absorb radiation, is found to be one of the largest contributors to global warming. Aerosols are emitted from both anthropogenic and natural sources and the major components of atmospheric particulate matter include sulfate, organic aerosols, nitrate, ammonium, black carbon, and trace metals. This PhD dissertation studies Arctic aerosols and their sources, with special focus on black carbon, attempting to increase the knowledge about aerosols’ effect on the climate in an Arctic content. The first part of the dissertation examines the diversity of aerosol emissions from an important anthropogenic aerosol source: residential wood combustion. The second part, characterizes the chemical and physical composition of aerosols while investigating sources of aerosols in the Arctic. The main instrument used in this research has been the state-of-the-art Soot Particle Aerosol Mass Spectrometer, which was deployed both in the laboratory and in the field. Also auxiliary instruments have been applied, including Multi-Angle Absorption Photometer and a seven-wavelength aethalometer. Most of the Arctic data presented in this dissertation was collected at the Villum Research Station, Station Nord in North Greenland. Laboratory studies of a conventional wood stove showed that particle emissions were strongly dependent on the intensity of burn rate. The burning cycle was divided into three phases, where the first phase, the fuel addition, resulted in short-lived but high emissions of levoglucosan and organic aerosols. The second phase, the intermediate phase, was dominated by black carbon and only to a minor extent organic aerosols and levoglucosan. The final burn out phase was generally represented by low concentrations of all species and overall the full cycle was dominated by black carbon. While staying within a realistic and plausible range, different degrees of burn rate showed that aerosol emissions increased with the intensity of burn rate, where especially polycyclic aromatic hydrocarbons showed a large increase. Levoglucosan is a commonly used tracer for wood combustion and this study showed, as expected, a correlation between the tracer and particulate matter during normal burn rate. However, with an increase in combustion intensity the same correlation was not valid. Therefore, the use of levoglucosan as a sole marker compound for determining the organic aerosol contribution from wood combustion will not be sufficient. Arctic aerosols were investigated during several time periods with different instruments and time resolutions. Two years of weekly measurements of black carbon and sulfate at the Villum Research Station showed elevated concentrations during winter and spring explained by expansion of the polar dome enabling long-range transport of aerosols from source regions outside the Arctic. This phenomenon is better known as the Arctic haze. Contrary, the summer and fall concentrations were lower due to the retreat of the polar dome. These seasonal patterns were compared with five other Arctic stations, which all showed the same characteristics of Arctic haze. Black carbon and sulfate were found to be internally mixed and it was thus concluded that the two species undergo comparable transport patterns in the Arctic. The internal mixing of black carbon and sulfate can enhance the absorption of black carbon and lead to further warming. In addition, results suggested that throughout the Arctic the source regions, contributing to the two species, were similar. The observed concentrations were compared with different models and generally models had improved compared to previous studies and succeeded in reproducing the seasonal patterns. The aerosol composition during Arctic haze was investigated with a higher time resolution during a field campaign at the Villum Research Station, showing a highly acidic environment where sulfate was the dominant species. The aerosol concentration decreased during spring as the Arctic haze leveled off. A source apportionment analysis showed that three factors were contributing to organic aerosols. A hydrocarbon-like organic aerosol factor was assigned to fossil fuel combustion and a second factor, less oxygenated organic aerosol, was allocated as secondary organic aerosols. The second factor began to decrease from the end of March and was replaced by the third factor, which was a more oxygenated organic aerosol factor. It could therefore be concluded, that secondary organic aerosols were dominating during the Arctic spring and that the major part of the sub-micrometer aerosol mass was internally mixed and long-range transported. This PhD has helped to increase our current knowledge concerning Arctic aerosols and their sources, which is an important step towards improving the overall understanding of aerosols’ effect on the climate.
|
|---|