Performance of Nano-DMA Operated with Different Gases for Sheath and Aerosol Carrier Flows

摘要

Two new experimental procedures are proposed to evaluate the performance of DMAs operated with different gases for sheath and polydisperse-aerosol-carrier flows.The first procedure evaluates the potential flow mixing in DMAs.An organic compound vapor of heavy molecular weight is doped,and then its concentration in the DMA aerosol-carrier streams is detected by gas chromatography.DMAs that strongly separate the doped vapor from the carrier are qualified as the separation tools to extract particles of desired sizes from an organic-vapor-rich gas flow.The separation is of importance for the particle composition characterization using existing chemical analytic instruments.The second procedure investigates the sizing accuracy of DMAs operated under the conditions of interest.A tandem DMA (TDMA) setup was applied to achieve the test objective.The first DMA was operated with different gases for sheath and polydisperse-aerosol-carrier flows.The same gas used for the first DMA sheath flow was used for both aerosol-carrier and sheath flows in the second DMA.This study evaluated the performance of Nano-DMA using different gas pairs (Ar,N_2,CO_2,and He).In the first test,at sheath flow rates of 7.5 and 15 lpm,approximately 0.1 % or less of the organic vapor in the polydisperse-aerosol-carrier stream reached the DMA monodisperse-aerosol-carrier stream for gas pairings of Ar,N_2,and CO_2.For He with other gases,more organic compound vapor was detected at the Nano-DMA downstream,probably because He has high diffusivity when paired with other gas media.Either N_2 or Ar gas was used as the Nano-DMA sheath flow in the second test to reduce the DMA's operational cost.The second test shows that the classified particle size can be estimated from the measured electrical mobility using the gas property of sheath flow for cases of N_2-Ar pairs.For the cases of He as polydisperse-aerosol-carrier flow with an other gas (N_2/Ar) as the sheath flow,the classified particle size is less than estimated using the sheath flow gas property,possibly due to the diffusiophoresis effect.The experimental validation of the diffusiophoresis effect was further carried out by switching the roles of He-N_2/Ar as the sheath and aerosol carrier flows in Nano-DMA.