Black Death - Blue Skies - White Clouds: Water Vapour Uptake of Particles Produced from Traffic Exhaust and their Effect on Climate

Aerosol particles are everywhere in the air around us, regardless of whether you are in a busy city or in the serene Arctic. Airborne particles can be produced naturally or anthropogenically, and their properties changes during the time they spend in the atmosphere. Their sizes range from about 1 nm to 100 μm, and affect us in two ways; firstly, our health by deposition in the respiratory tract, and secondly via pertubation of the climate.The Earth’s climate is affected by the radiation balance, which is in turn affected by the presence of particles and the formation of cloud droplets. Cloud droplets form on pre-existing particles by condensation of watervapour. These particles, which act as seeds for the condensation of water, are called cloud condensation nuclei (CCN). The ability of particles to take up water vapour depends on their chemical and physical properties, and is described by particle hygroscopicity. The theoretical framework used in this work to calculate particle hygroscopicity was first introduced by Kohler in 1936, and has since then been developed to account for nonideal conditions. Particle hygroscopicity of fresh and aged traffic exhaust was investigated in laboratory measurements. The complete transformation of soot particles, from fresh emissions of hydrophobic, aspherical soot agglomerates to compacted soot particles coated with secondary organic aerosol (SOA), which are able to act as CCN, was captured for the first time. The SOA produced from traffic emissions showed differences in water vapour uptake, when measured in the subsaturated compared to supersaturated regimes. Theoretical analysis using modified Kohler theory, indicated that these measured differences could be explained by limitation of the solubility of the SOA that was condensed on the seed particles.Ambient measurements of particle hygroscopicity associated with traffic emissions were performed in urban and rural environments. The urban aerosol showed a clear diurnal variation as well as a dependence on air mass origin. The fraction of particles with low hygroscopicity and the fraction of fresh soot (from traffic) showed good agreement during the daytime. However, during the night-time the fraction of agglomerated soot decreased, probably as a result of soot emissions from further away having undergone ageing, and hence restructured to more dense particles, while the hygroscopicity was not notably improved. Furthermore, observations made by following air masses from the urban to the rural environments showed that soot particle restructuring and changes in their properties may occur much faster than previously thought (within 5 hours), due to particulate nitrate formation coupled to water vapour uptake.Finally, the impact of traffic exhausts on climate was synthetized by combining the results in this thesis with those from the literature. Soot particles lead mainly to global warming. Traffic emissions can also reduce visibility, as the ability to absorb and scatter light may increase with ageing and water vapour uptake. However, with further ageing and increased hygroscopicity, the particles produced by traffic can act as cloud condensation nuclei, thus contributing to cooling. The increased hygroscopicity (due to condensation of organic and inorganic material) will affect the atmospheric lifetime of the soot particles, which also influence climate change. (Less)