| 1. |
- Swietlicki, Erik, et al.
(författare)
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Hygroscopic properties of aerosol particles in the northeastern Atlantic during ACE-2
- 2000
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Ingår i: Tellus. Series B: Chemical and Physical Meteorology. - : Stockholm University Press. - 0280-6509 .- 1600-0889. ; 52:2, s. 201-227
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Tidskriftsartikel (refereegranskat)abstract
- Measurements of the hygroscopic properties of sub-micrometer atmospheric aerosol particles were performed with hygroscopic tandem differential mobility analysers (H-TDMA) at 5 sites in the subtropical north-eastern Atlantic during the second Aerosol Characterization Experiment (ACE-2) from 16 June to 25 July 1997. Four of the sites were in the marine boundary layer and one was, at least occasionally, in the lower free troposphere. The hygroscopic diameter growth factors of individual aerosol particles in the dry particle diameter range 10-440 nm were generally measured for changes in relative humidity (RH) from <10% to 90%. In the marine boundary layer, growth factors at 90% RH were dependent on location, air mass type and particle size. The data was dominated by a unimodal growth distribution of more-hygroscopic particles, although a bimodal growth distribution including less-hygroscopic particles was observed at times, most often in the more polluted air masses. In clean marine air masses the more-hygroscopic growth factors ranged from about 1.6 to 1.8 with a consistent increase in growth factor with increasing particle size. There was also a tendency toward higher growth factors as sodium to sulphate molar ratio increased with increasing sea-salt contribution at higher wind speeds. During outbreaks of European pollution in the ACE-2 region, the growth factors of the largest particles were reduced, but only slightly. Growth factors at all sizes in both clean and polluted air masses were markedly lower at the Sagres, Portugal site due to more proximate continental influences. The frequency of occurrence of less-hygroscopic particles with a growth factor of ca. 1.15 was greatest during polluted conditions at Sagres. The free tropospheric 50 nm particles were predominately less-hygroscopic, with an intermediate growth factor of 1.4, but more-hygroscopic particles with growth factors of about 1.6 were also frequent. While these particles probably originate from within the marine boundary layer, the less-hygroscopic particles are probably more characteristic of lower free tropospheric air masses. For those occasions when measurements were made at 90% and an intermediate 60% or 70% RH, the growth factor G(RH) of the more-hygroscopic particles could be modelled empirically by a power law expression. For the ubiquitous more-hygroscopic particles, the expressions G(RH) = (1 - RH/100)-0.210 for 50 nm Aitken mode particles and G(RH) = (1 - RH/100)-0.233 for 166 nm accumulation mode particles are recommended for clean marine air masses in the north-eastern Atlantic within the range 0 < RH < 95%, and for wind speeds for which the local sea-salt production is small (< ca. 8 m s-1).
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| 2. |
- Andersson, Helene, et al.
(författare)
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Hydrophobic valves of plasma deposited octafluorocyclobutane in DRIE channels
- 2001
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Ingår i: Sensors and actuators. B, Chemical. - 0925-4005 .- 1873-3077. ; 75:1-2, s. 136-141
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Tidskriftsartikel (refereegranskat)abstract
- The suitability of using octafluorocyclobutane (C4F8) patches as hydrophobic valves in microfluidic biochemical applications has been shown. A technique has been developed to generate lithographically defined C4F8 hydrophobic patches in deep reactive ion-etched silicon channels. Some of the advantages of this process are that no specific cleaning of the substrate is required, C4F8 is deposited on the sidewalls and the bottom of the channels, a standard photoresist mask can be used to define the patches, and that it is a fast and convenient dry chemical process performed by a standard inductively coupled plasma etcher using the Bosch process. Different patch lengths (200-1000 mum) of C4F8 were deposited in 50 mum wide channels to evaluate which size is most suitable for microfluidic biochemical applications. The valve function of the hydrophobic patches was tested for the following liquids: DD water, acetone, propanol, bead solution and a mixture used for pyrosequencing of DNA. Patch lengths of 200 mum of C4F8 successfully stopped each solution for at least 20 consecutive times. The C4F8 film resists water for at least 5 h. The hydrophobic valve also resists very high concentrations (25%) of surfactants (Tween 80). C4F8 shows a much higher resistance towards water and surface active solutions than previous hydrophobic patches. However, 50% Tween 80 was not stopped at all by the hydrophobic patch. An applied pressure of 760 Pa at the inlet was needed for water to over-run the hydrophobic patch.
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| 3. |
- Flynn, Michael J., et al.
(författare)
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Modelling cloud processing of aerosol during the ACE-2 HILLCLOUD experiment
- 2000
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Ingår i: Tellus. Series B: Chemical and Physical Meteorology. - : Stockholm University Press. - 0280-6509. ; 52:2, s. 779-800
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Tidskriftsartikel (refereegranskat)abstract
- A numerical model has been used to simulate the conditions observed during the ACE-2 Hillcloud experiment and to study the processes which may be taking place. The model incorporates gas phase chemistry of sulphur and nitrogen compounds upstream of the cloud, and the interaction of aerosol, precursor trace gases and oxidants within the cloud. Gas phase and aerosol inputs to the model have been provided from measurements made in the field. Dynamics of the air flow over the hill consisted of simple prescribed dynamics based on wind speed measurements, and also for some cases modelled dynamics. In this modelling study, it was found that during clean case studies particles down to 40-55 nm diameter were activated to form cloud droplets, the total number of droplets formed ranging from 200 to 400 drops/cm3. Significant modification of the aerosol spectra due to cloud processing was observed. In polluted cases particles down to 65-80 nm diameter were activated to form cloud droplets, the total number of droplets ranging from 800 to 2800 drops/cm3. Modification of the aerosol spectra due to cloud processing was slight. In all cases, changes in the aerosol spectra were due to both the uptake of HNO3, HCl, NH3 and SO2 from the gas phase, (the SO2 being oxidised to sulphate) and the repartitioning of species such as HNO3, HCl, and NH3 from larger particles onto smaller ones. Modelling results have been compared with observations made. Modelled droplet numbers are typically within 20% of the best measured values. The mode of the droplet distribution typically around 10-20 μm for clean cases and 4-8 μm for polluted cases was found to be in good agreement with the measured values of 10-25 μm for clean cases, but not in such good agreement for polluted cases. Measurements of upwind and interstitial aerosol distributions showed that the smallest particles activated were 30 and 50 nm for clean and polluted cases respectively, slightly smaller than the model values quoted above. Measured upwind and downwind aerosol spectra showed similar modification to that predicted by the model in eight out of the eleven model runs carried out. Chemistry measurements also give general evidence for both the uptake of species from the gas phase, and repartitioning of species from large particles onto smaller ones, though comparisons for individual cases are more difficult. From this modelling study, it can be concluded that in general, in the remote environment the exchange of hydrochloric acid, nitric acid and ammonia between aerosol particles and take up from the gas phase in the vicinity of cloud may be a very important mechanism in regulating the evolution of the aerosol spectrum. Further, the much more linear relationship between cloud droplet and accumulation mode aerosol number, which was observed in the measurements made during the ACE-2 HILLCLOUD project is supported by these modelling results. The implications of this for the indirect effect will be explored in future work.
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| 4. |
- Frank, Göran, et al.
(författare)
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Characterisation of a unipolar charger for droplet aerosols of 0.1-20 mu m in diameter
- 2004
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Ingår i: Journal of Aerosol Science. - 0021-8502. ; 35:1, s. 117-134
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Tidskriftsartikel (refereegranskat)abstract
- A charging unit for unipolar charging of cloud and fog droplets has been developed and calibrated regarding ion current, angular distribution of ions, mean droplet charge and droplet charge distribution. The charger is part of the droplet aerosol analyser, which is an instrument used for characterisation of droplets and interstitial particles in atmospheric clouds and fog. Three different calibration methods have been used for the charge calibration, whereof the third and newest method makes use of a differential mobility analyser, especially developed for large particles, up to at least 20 mum in diameter. The number of charges acquired is a strictly increasing function of the droplet diameter. Two charging models have been used for comparisons with the measurements and both models show good agreement with the data regarding the size dependency of the charge level. Both models can thus be used to describe the performance of the charger. (C) 2003 Elsevier Ltd. All rights reserved.
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| 5. |
- Frank, Göran
(författare)
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Experimental Studies of the Interaction of Atmospheric Aerosol Particles with Clouds and Fogs
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this work, cloud and fog droplet formation was studied in three joint field experiments. The instrument, the droplet aerosol analyser (DAA), has also been further developed and verified as part of this work. The DAA is an instrument especially developed for studies of cloud and fog droplet formation and growth. It measures the ambient size of individual droplets and interstitial particles in a fog or a cloud. It then measures the size of the individual dry residual particles after evaporation of the water. It also counts the number of dry particles of each size, which gives a unique three-parameter data set that connects ambient size to dry size and to the number of particles. Having access to these parameters, a number of related aerosol/cloud parameters can be determined, whereof the microstructure, i.e. the size and number of droplets and their size distribution, characterisation of the droplet activation, as defined by the Köhler equation, and the size-dependent cloud droplet nucleation scavenging of particles due to activation, are the most important. The results from one fog experiment showed that, most of the time fog consisted of unactivated droplets with a continuous size distribution in the size region 1-47 µm in diameter. There were no gaps generated by the droplet activation process, as has been observed in clouds. In the two ground-based cloud experiments, cloud droplet number concentrations increased almost linearly with particle number concentration. Droplet concentrations of up to 2000 per cubic centimetre in an experiment in northern England and up to almost 3000 per cubic centimetre in an experiment on Tenerife, Spain, were observed, thus demonstrating the influence of air pollution on cloud microstructure.
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| 6. |
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| 7. |
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| 8. |
- Haasl, Sjoerd, et al.
(författare)
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Arrays of monocrystalline silicon micromirrors fabricated using CMOS compatible transfer bonding : IEEE The sixteenth annual international conference on Micro Electro Mechanical Systems
- 2003
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Ingår i: MEMS-03. - New York : IEEE. - 0780377443 ; , s. 271-274
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Konferensbidrag (refereegranskat)abstract
- In this paper, we present CMOS compatible fabrication of monocrystalline silicon micromirror arrays using membrane transfer bonding. To fabricate the micromirrors, a thin monocrystalline silicon device layer is transferred from a standard silicon-on-insulator (SOI) wafer to a target wafer (e.g. a CMOS wafer) using low temperature adhesive wafer bonding. In this way, very flat, uniform and low stress micromirror membranes made of monocrystalline silicon can be directly fabricated on top of CMOS circuits. The mirror fabrication does not contain any bond alignment between the wafers; thus, the mirror dimensions and alignment accuracies are only limited by the photolithographic steps. Micromirror arrays with 4x4 pixels and a pitch size of 16mum x 16mum have been fabricated.
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| 9. |
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| 10. |
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International Perspectives on Learning and Teaching Mathematics
- 2004
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Samlingsverk (redaktörskap) (övrigt vetenskapligt/konstnärligt)abstract
- This book provides examples from around the world of research insights and approaches to teaching mathematics that recognize the crucial role of the teacher in facilitating learning. The book has contributions from 14 countries and more than 50 mathematics educators and researchers. The scope of their expertise clearly illustrates how varied, complex and stimulating the study of mathematics education is. It also demonstrates that teachers and researchers around the world share a powerful interest in improving mathematics learning for all students. This book is written for classroom teachers, for pre-service and in-service education and for students in postgraduate courses.
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