K. E. J. Lehtinen

Production of Nanometer-Sized Metal Oxide Particles by Gas Phase Reaction in a Free Jet. I: Experimental System and Results

ABSTRACT In a study of the effect of process conditions and material properties on aerosol characteristics, nanosized metal oxide particles were produced by injecting precursors as a free jet into a methane-air flame. Primary particle size increased with volume loading, solid state diffusion coefficient, and maximum temperature. Larger particles were also obtained by decreasing the jet velocity. The number of particles per agglomerate increased with volume loading and decreased with solid state diffusion coefficient and maximum temperature. Metal oxides with diffusion coefficients ranging over several orders of magnitude produced different sized particles under the same process conditions (temperature profile and aerosol volume loading). Niobium oxide (largest diffusion coefficient) formed the largest particles with geometric volume mean diameters between 5.7 and 33.7 nm. Titania (mid-range diffusion coefficient) and alumina (lowest diffusion coefficient) formed particles with geometric volume mean diamet...

Production of Nanometer-Sized Metal Oxide Particles by Gas Phase Reaction in a Free Jet. II: Particle Size and Neck Formation—Comparison with Theory

ABSTRACT Experimental measurements of nanosized primary particle diameters were compared with calculated values based on a collision-coalescence model. The method of analysis permits calculation of the primary particle size when growth is collision limited (individual particles colliding), coalescence limited (primary particles coalescing in agglomerates), or in a transition regime (particles coalescing about as fast as they collide). Calculated particle sizes compared well with experimental measurements. Particle characteristics were studied along the jet axis for the following conditions: exit velocity = 27.8 m/s, volume loading = 3.2 × 10−7, flame gas flow rate = 33 1/min. The growth of niobium oxide particles (largest diffusion coefficient) was collision limited, yielding particles that are large and nonagglomerated. The growth of titania particles (mid-range diffusion coefficient) occurred in the collision limited and coalescence limited regimes to form mid-sized particles in agglomerates. The growth...

VAPORISATION RATES OF CsOH AND CsI IN CONDITIONS SIMULATING A SEVERE NUCLEAR ACCIDENT

Abstract The vaporisation rates of volatile fission product compounds are critical parameters for modelling aerosol formation following a severe nuclear accident. The vaporisation of CsOH and CsI was studied in a pure steam atmosphere at ambient pressure (85– 89 kPa ) by increasing the temperature of the flow furnace up to 1000°C. For this purpose, samples were doped with a small amount of radioactive tracer. The vaporisation rate was then determined from the decrease in sample activity with time, using a germanium gamma detector placed outside the furnace. Calculated vaporisation rates obtained by solving complete velocity, temperature and vapour concentration profiles surrounding the sample with FLUENT CFD-software, were in reasonable agreement with the data. A simple engineering calculation agrees almost perfectly with the FLUENT results, if a constant value, Sh≈8, for the Sherwood number is used.

The effect of early growth dynamics on determining particle formation rates of a nucleating burst

We studied growth and coagulation in the early stages of a nucleating particle population using a numerical model that solves the for particle concentration on the highest possible size resolution. We found that commonly used methods for deriving particle formation rates from nanoparticle concentration measurements may not result in correct estimates for the formation rate. This influences reliability of the observed vapour concentration dependency of the nucleation rate, which is often used to draw conclusions of the nucleation mechanism.

Log-log slope analyses of simulated particle formation events at different conditions

The use of the first nucleation theorem on atmospheric particle formation events is studied under simulated conditions trying to represent the varying conditions of a typical field campaign. The aerosol dynamics model UHMA is used to produce synthesized DMPS data, which are analyzed using typical techniques to obtain particle formation rates, and with back calculation, nucleation rates. The results show that under conditions that cause time and/or size dependent growth rates, standard techniques can result in serious errors when estimating nucleation rates. In addition, the validity of obtaining information about nucleation mechanisms by plotting data at different conditions into a single log J vs. log [H2SO4] plot to deduce mechanisms based on slope values, seems questionable.

Determination of the size distribution of recombination products from atmospheric measurements

The role of ion-ion recombination in new particle formation is still unclear. In this work, we present a new method to estimate the size distribution of recombination products from atmospheric measurements. By applying our method to size distributions of charged and neutral clusters measured in Hyytiala, Finland, we show that only a minor fraction of all sub-2nm neutral clusters originate from ion-ion recombination in boreal forest conditions.

Model Studies of Nitric Acid Condensation in Mixed-phase Clouds

4 Centre for Atmospheric Science, School of Earth, Atmospheric and Environmental Science, University of Manchester, P.O. Box 88 M60 1QD, Manchester, UK Abstract In the upper troposphere, clouds are mainly composed of ice particles which work as a platform for heterogeneous chemistry. They also have a crucial role in distributing compounds, like nitric acid, between different altitudes in troposphere. In this study we use air parcel model to investigate the adsorption of HNO 3 and the possibility that some of HNO 3 remains in the ice particles during the freezing of supercooled liquid particles. We compare the results from our model simulations to airborne measurements of nitric acid partitioning between ice particles and gas phase. The results shows that it is likely that atleast some of HNO 3 is captured to particles during freezing, but the results are highly dependent on the form of Langmuir’s isotherm used.