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 Köhler 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 Köhler 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

Data Descriptor : Collocated observations of cloud condensation nuclei, particle size distributions, and chemical composition

Cloud condensation nuclei (CCN) number concentrations alongside with submicrometer particle number size distributions and particle chemical composition have been measured at atmospheric observatories of the Aerosols, Clouds, and Trace gases Research InfraStructure (ACTRIS) as well as other international sites over multiple years. Here, harmonized data records from 11 observatories are summarized, spanning 98,677 instrument hours for CCN data, 157,880 for particle number size distributions, and 70,817 for chemical composition data. The observatories represent nine different environments, e.g., Arctic, Atlantic, Pacific and Mediterranean maritime, boreal forest, or high alpine atmospheric conditions. This is a unique collection of aerosol particle properties most relevant for studying aerosol-cloud interactions which constitute the largest uncertainty in anthropogenic radiative forcing of the climate. The dataset is appropriate for comprehensive aerosol characterization (e.g., closure studies of CCN), model-measurement intercomparison and satellite retrieval method evaluation, among others. Data have been acquired and processed following international recommendations for quality assurance and have undergone multiple stages of quality assessment.Peer reviewe

Cloud Droplet Forming Potential of Ageing Soot and Surfactant Particles : Laboratory research and Köhler modelling

Aerosol particles affect Earth’s climate system by scattering and absorbing light. The perturbation of the climate system caused by a change in aerosol configuration due to anthropogenic emissions has recently improved. However, the radiation balance and hydrological cycle of Earth are also highly influenced by clouds: a small change in cloud configuration can have a large effect on the climate. Cloud droplets form on preexisting particles. For instance, the ability of black carbon (BC) particles when co-emitted with organic carbon (OC) to form cloud droplets are less well understood. A large fraction of the ambient organic aerosol is formed via oxidation of volatile organic compounds from both biogenic and anthropogenic sources. During atmospheric ageing, the properties of freshly emitted soot nanoparticles will change as volatile organics oxidize and interact with the primary soot emissions. As the particles undergo photochemical processing in the atmosphere they progressively become more hygroscopic and will thereby influence the climate due to their improved ability to act as cloud condensation nuclei (CCN) as well as affecting the human health by altering the uptake in the respiratory system. This work is part of the PhD work of the author of this report, with the aim to evaluate the ability of aerosol nanoparticles to form cloud droplets; of biogenic and anthropogenic origin. To gain better understanding of the cloud forming potential of organic compounds produced from living organisms as well as soot when co-emitted with organic compounds both laboratory research and Köhler modeling has been performed. Different biosurfactants was measured and compared with two different techniques in the first study. In the second study, soot from both a diesel vehicle and a flame soot generator was photochemically processed in a smog chamber, monitored with a comprehensive instrumental set-up. For both studies the measurement technique of the cloud activation properties was improved, gaining better resolution in data. The results in the first study show that the biosurfactants have good cloud forming abilities, however not as good as previously believed. There are discrepancies in the results from the two measurement techniques (on- and off-line), which partly can be explained by surface partitioning. In the second study, the freshly emitted soot particles neither showed any hygroscopic growth (at 90 % relative humidity) nor activated into cloud droplets (at a supersaturation of 2 %). As the emissions are photochemically processed the properties of the particles change and they become progressively more hygroscopic. The enhanced cloud forming abilities of the soot particles are due to changes in the organic fraction, both regarding quantity and quality, as well as a change in size and shape of the particles. Experimental and modeled (κ-Köhler theory) results show good agreement for particles with higher organic content, and with a κ-value of ~0.13. The κ-value derived from the chemical composition and the CCN measurements are consistent. Due to the morphology of the soot particles, predictions of the cloud droplet activity and hygroscopic growth cannot be performed using the measured mobility diameter. Instead, the volume equivalent diameter is a better size measure. This parameter has in this study successfully been estimated from the mobility diameter and the organic aerosol fraction of the particle.In summary, both the biosurfactants and aged anthropogenic particles show cloud droplet forming potential relevant in the ambient air, with the ability to affect the climate. The results from the anthropogenic experiment also imply that the sunlight affect the lifetime of soot in the atmosphere, due to changed cloud droplet activation as the UV radiation was turned on during experiments. Probably, the uptake in human lungs of larger particles increases as the soot particles age

Vavihill, Sweden, submicron aerosol size distribution, 2012-2014

submicron aerosol size distribution, Vavihill, Sweden, 2012 – 201