The slow aggregation process of a concentrated silica dispersion (Bindzil 40/220) in the presence of alkali chlorides (LiCl, NaCl, KCl, RbCl, and CsCl) was investigated by means of mobility measurements. At intervals during the aggregation, particles and aggregates were transferred from the liquid phase to the gas phase via electrospray (ES) and subsequently size selected and counted using a scanning mobility particle sizer (SMPS). This method enables the acquisition of particle and aggregate size distributions with a time resolution of minutes. To our knowledge, this is the first time that the method has been applied to study the process of colloidal aggregation. The obtained results indicate that, independent of the type of counterion, a sufficient dilution of the formed gel will cause the particles to redisperse. Hence, the silica particles are, at least initially, reversibly aggregated. The reversibility of the aggregation indicates additional non-DLVO repulsive steric interactions that are likely due to the presence of a gel layer at the surface. The size of the disintegrating aggregates was monitored as a function of the time after dilution. It was found that the most stable aggregates were formed by the ions that adsorb most strongly on the particle surface. This attractive effect was ascribed to an ion-ion correlation interaction.

Abstract. The Sub-millimetre and Millimetre Radiometer (SMR) is the main instrument on the Swedish, Canadian, Finnish and French spacecraft Odin. It consists of a 1.1 metre diameter telescope with four tuneable heterodyne receivers covering the ranges 486−504 GHz and 541−581 GHz, and one fixed at 118.75 GHz together with backends that provide spectral resolution from 150 kHz to 1 MHz. This Letter describes the Odin radiometer, its operation and performance with the data processing and calibration described in Paper II.

Context. Molecular oxygen, O 2 , has been expected historically to be an abundant component of the chemical species in molecular clouds and, as such, an important coolant of the dense interstellar medium. However, a number of attempts from both ground and from space have failed to detect O 2 emission. Aims. The work described here uses heterodyne spectroscopy from space to search for molecular oxygen in the interstellar medium. Methods. The Odin satellite carries a 1.1 m sub-millimeter dish and a dedicated 119 GHz receiver for the ground state line of O 2 . Starting in 2002, the star forming molecular cloud core ρ Oph A was observed with Odin for 34 days during several observing runs. Results. We detect a spectral line at v LSR = +3.5 km s −1 with ∆v FWHM = 1.5 km s −1 , parameters which are also common to other species associated with ρ Oph A. This feature is identified as the O 2 (N J = 1 1 −1 0 ) transition at 118 750.343 MHz. Conclusions. The abundance of molecular oxygen, relative to H 2 , is 5 × 10 −8 averaged over the Odin beam. This abundance is consistently lower than previously reported upper limits.

An instrument for on-line measurements of alkali components in hot flue gas streams is presented. The instrument is based on surface ionization technique and is equipped with a hot sampling line used to extract flue gas for continuous alkali measurements at pressures up to 30 bar. The instrument, its support system and the calibration procedure is described. Field campaigns with the instrument have been performed at fluidized-bed combustion facilities operating under pressurized and atmospheric conditions and using several fuels, including coal, biomass, and demolition waste. The instrument performance has been satisfactory both during pressurized circulating fluidized-bed coal combustion with alkali concentrations downstream of a hot gas filter in the ppb range, and in particle-laden conditions with alkali levels up to about 10 ppm. The measured alkali concentration corresponds to the concentration of alkali components present as vapors and fine-mode particles. In systems with high levels of fluidized-bed material or fly ash, a contribution to the signal from alkali bound to coarse particles is also expected. Under these conditions, the instrument is suggested to be operated in a pulse counting mode where the concentration of coarse particles can be estimated. The instrument is concluded to provide a durable, sensitive, and reliable alkali measurement method with a high time resolution and with a lower detection limit of around 1 ppb, which should be suitable for a large range of applications.