Development and evaluation of a highly effective gas particle partitioner with minimal effect on the gas composition
Particle free air is required in a wide range of scientific and technical applications such as pre-filtering for gas analysers or for the artefact correction of particle mass concentration measurements. Most common processes for the removal of particles from a gas stream are filtration and electrostatic precipitation. However, both mechanisms introduce changes to the thermodynamic conditions and/or the composition of the gas, which might be detrimental to the downstream measurement of gas or particle concentrations. For highly efficient separation of gas and particles with no changes of the thermodynamic conditions and substantially no changes to the gas phase, a coaxial Gas Particle Partitioner (GPP) has been developed and intensively tested. The GPP utilizes corona charging to electrically charge the particles and a strong electric field in a separate unit to take them out of the sample flow when switched on. The corona was optimised with respect to gas formation, i.e. a gold wire of only 25 µm is used as corona electrode along with positive corona polarity. Additionally, the vicinity of the corona wire gets continuously flushed by means of a wash flow, which is spatially separated from the sample flow, to prevent gases formed by the corona, such as ozone and oxides of nitrogen from reaching the sample aerosol flow. Inside the GPP, the total aerosol flow is split into a sample flow and an excess gas flow. The splitting of the flow ensures that evaporation or release of particles, deposited on the outer wall does not affect the sample flow but only the excess gas flow. The flow splitter is designed such that the particle size distribution in the sample flow is identical to the ambient distribution, when the GPP is switched off. When switched on, the sample flow is particle free. The laboratory investigations have shown that the GPP is able to separate gas and particles with an efficiency of near 100% in the particle size range of 25 nm < dp < 10.7 µm. For particle sizes below 25 nm, the separation efficiency decreased with decreasing particle size. The measures for a minimization of the influence of the corona on the gas composition have led to substantially reduced O3 and NOx concentrations in the sample flow. Compared to the worst case (100 µm gold wire, negative polarity, no wash flow) the additional ozone concentration could be reduced by 98.5% and the nitrogen dioxide concentration by approx. 90%.
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