1,349 research outputs found
Growth of sulphuric acid nanoparticles under wet and dry conditions
New particle formation, which greatly influences the number concentrations
and size distributions of an atmospheric aerosol, is often followed by a
rapid growth of freshly formed particles. The initial growth of newly
formed aerosol is the crucial process determining the fraction of nucleated
particles growing to cloud condensation nuclei sizes, which have a
significant influence on climate. In this study, we report the laboratory
observations of the growth of nanoparticles produced by nucleation of
H<sub>2</sub>SO<sub>4</sub> and water in a laminar flow tube at temperatures of 283, 293
and 303 K, under dry (a relative humidity of 1%) and wet conditions
(relative humidity of 30%) and residence times of 30, 45, 60 and 90 s.
The initial H<sub>2</sub>SO<sub>4</sub> concentration spans the range from 2 × 10<sup>8</sup>
to 1.4 × 10<sup>10</sup> molecule cm<sup>−3</sup> and the calculated
wall losses of H<sub>2</sub>SO<sub>4</sub> were assumed to be diffusion limited. The
detected particle number concentrations, measured by the Ultrafine
Condensation Particle Counter (UCPC) and Differential Mobility Particle
Sizer (DMPS), were found to depend strongly on the residence time.
Hygroscopic particle growth, presented by growth factors, was found to be in
good agreement with the previously reported studies. The experimental growth
rates ranged from 20 nm h<sup>−1</sup> to 890 nm h<sup>−1</sup> at relative humidity (RH) 1% and from
7 nm h<sup>−1</sup> to 980 nm h<sup>−1</sup> at RH 30% and were found to increase
significantly with the increasing concentration of H<sub>2</sub>SO<sub>4</sub>.
Increases in the nucleation temperature had a slight enhancing effect on the
growth rates under dry conditions. The influence of relative humidity on
growth was not consistent – at lower H<sub>2</sub>SO<sub>4</sub> concentrations, the
growth rates were higher under dry conditions while at H<sub>2</sub>SO<sub>4</sub>
concentrations greater than 1 × 10<sup>10</sup> molecule cm<sup>−3</sup>, the
growth rates were higher under wet conditions. The growth rates show only a
weak dependence on the residence time. The experimental observations were
compared with predictions made using a numerical model, which investigates
the growth of particles with three different extents of neutralization by
ammonia, NH<sub>3</sub>: (1) pure H<sub>2</sub>SO<sub>4</sub> – H<sub>2</sub>O particles; (2)
particles formed by ammonium bisulphate, (NH<sub>4</sub>)HSO<sub>4</sub>; (3) particles
formed by ammonium sulphate, (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>. The highest growth
rates were found for ammonium sulphate particles. Since the model accounting
for the initial H<sub>2</sub>SO<sub>4</sub> concentration predicted the experimental
growth rates correctly, our results suggest that the commonly presumed
diffusional wall losses of H<sub>2</sub>SO<sub>4</sub> in case of long-lasting
experiments are not so significant. We therefore assume that there are not
only losses of H<sub>2</sub>SO<sub>4</sub> on the wall, but also a flux of
H<sub>2</sub>SO<sub>4</sub> molecules from the wall into the flow tube, the effect being
more profound under dry conditions and at higher temperatures of the tube
wall. Based on a comparison with the atmospheric observations, our results
indicate that sulphuric acid alone cannot explain the growth rates of
particles formed in the atmosphere
Risico's van bodemverontreiniging in het landelijk gebied; bodemkwaliteitskaarten, risico's voor de voedselveiligheid, actief bodembeheer en beslissingsondersteunende systemen
Dit rapport bevat achtergrondinformatie over de aanpak en resultaten van onderzoek naar bodemkwaliteitskaarten met daarop de gehalten en de risico's van zware metalen; de risico's van bodemverontreiniging voor gewaskwaliteit en diergezondheid; de effecten van veranderend landgebruik op bodemkwaliteit, bodemleven en hogere flora en fauna; de mogelijkheden van fytoremediëring voor het beheersen en terugdringen van risico's van bodemverontreiniging; verificatieonderzoek in verband met de toepassing van actief bodembeheer in de Krimpenerwaard; en ontwikkeling en toepassing van een beslissingondersteunend systeem voor inrichting en beheer van gronden voor natuurontwikkeling. De informatie in dit rapport is gebaseerd op artikelen die reeds toegestuurd zijn dan wel spoedig toegestuurd worden naar populair-wetenschappelijke tijdschriften
An accurate description of quantum size effects in InP nanocrystallites over a wide range of sizes
We obtain an effective parametrization of the bulk electronic structure of
InP within the Tight Binding scheme. Using these parameters, we calculate the
electronic structure of InP clusters with the size ranging upto 7.5 nm. The
calculated variations in the electronic structure as a function of the cluster
size is found to be in excellent agreement with experimental results over the
entire range of sizes, establishing the effectiveness and transferability of
the obtained parameter strengths.Comment: 9 pages, 3 figures, pdf file available at
http://sscu.iisc.ernet.in/~sampan/publications.htm
Increased susceptibility to proactive interference in adults with dyslexia?
Recent findings show that people with dyslexia have an impairment in serial-order memory. Based on these findings, the present study aimed to test the hypothesis that people with dyslexia have difficulties dealing with proactive interference (PI) in recognition memory. A group of 25 adults with dyslexia and a group of matched controls were subjected to a 2-back recognition task, which required participants to indicate whether an item (mis)matched the item that had been presented 2 trials before. PI was elicited using lure trials in which the item matched the item in the 3-back position instead of the targeted 2-back position. Our results demonstrate that the introduction of lure trials affected 2-back recognition performance more severely in the dyslexic group than in the control group, suggesting greater difficulty in resisting PI in dyslexia.Peer reviewedFinal Accepted Versio
Evolution of the electronic structure with size in II-VI semiconductor nanocrystals
In order to provide a quantitatively accurate description of the band gap
variation with sizes in various II-VI semiconductor nanocrystals, we make use
of the recently reported tight-binding parametrization of the corresponding
bulk systems. Using the same tight-binding scheme and parameters, we calculate
the electronic structure of II-VI nanocrystals in real space with sizes ranging
between 5 and 80 {\AA} in diameter. A comparison with available experimental
results from the literature shows an excellent agreement over the entire range
of sizes.Comment: 17 pages, 4 figures, accepted in Phys. Rev.
Multiband theory of quantum-dot quantum wells: Dark excitons, bright excitons, and charge separation in heteronanostructures
Electron, hole, and exciton states of multishell CdS/HgS/CdS quantum-dot
quantum well nanocrystals are determined by use of a multiband theory that
includes valence-band mixing, modeled with a 6-band Luttinger-Kohn Hamiltonian,
and nonparabolicity of the conduction band. The multiband theory correctly
describes the recently observed dark-exciton ground state and the lowest,
optically active, bright-exciton states. Charge separation in pair states is
identified. Previous single-band theories could not describe these states or
account for charge separation.Comment: 10 pages of ReVTex, 6 ps figures, submitted to Phys. Rev.
Theory of Luminescent Emission in Nanocrystal ZnS:Mn with an Extra Electron
We consider the effect of an extra electron injected into a doped quantum dot
. The Coulomb interaction and the exchange interaction between the
extra electron and the states of the Mn ion will mix the wavefunctions, split
the impurity energy levels, break the previous selection rules and change the
transition probabilities. Using this model of an extra electron in the doped
quantum dot, we calculated the energy and the wavefunctions, the luminescence
probability and the transition lifetime and compare with the experiments. Our
calculation shows that two orders of magnitudes of lifetime shortening can
occur in the transition when an extra electron is present.Comment: 15 pages, 2 Figs No change in Fig
- …