Marko Marjamäki

Performance evaluation of the electrical low-pressure impactor (ELPI)

The performance of the Electrical Low-Pressure Impactor (ELPI) has been evaluated using monodisperse aerosols. ELPI is a near real-time size analyzer consisting of an aerosol charger and a cascade impactor. Particle cut sizes of the cascade impactor and the charging efficiency of the charger were determined experimentally in the designed range of the instrument, i.e. 0.03–10 μm. The Stokes numbers of 50% particle cut sizes of all the impactor stages were found to vary from 0.421 to 0.483 with an average of 0.456 and standard deviation of 0.017. The collection efficiency curves were found to be steep with an average steepness (ratio of 70–30% and 30% collection efficiency) of 1.19. The charging efficiency of the charger was close to that specified by the manufacturer for particles smaller than 2 μm in diameter. For larger particles, a deviation from the manufacturer’s specifications was observed. It results from the high particle loss in the charger because of the small size of the charger, strong trapping electric field, and the perpendicular directions of the aerosol stream and the electric field in the charging zone. A comparison with SMPS for size distribution measurements was made in this study. The good agreement between the two measured size distributions shows the capability of the ELPI for near real-time particle size measurements.

On-line measurement of size distribution and effective density of submicron aerosol particles

Abstract An on-line method is presented for simultaneous size distribution and particle density measurement, based on parallel measurements made by SMPS and ELPI. The measured SMPS number distribution is integrated with the ELPI response functions to produce calculated current response. By varying the density value, the best fit is sought between the calculated and measured current response. Simulation tests made showed relatively good stability against small uncertainties in the distribution and the response functions. Test measurements of well-defined aerosols with known density were conducted. Density values of 0.86, 1.1, and 1.9 g / cm 3 were measured for liquid particles of DOS, Santovac vacuum oil, and Fomblin vacuum oil, respectively. These values are within 8% of the accepted bulk values. For solid particles of NaCl, Zn, and Ag, slightly larger experimental errors in the range of 4–18.2% were found.

Fine particle losses in electrical low-pressure impactor

Abstract The fine particle losses in the individual impactor stages of the electrical low-pressure impactor (ELPI) were studied. Three different loss mechanisms were considered: diffusion, space charge, and image charge deposition. Diffusion losses were determined experimentally in particle size range of 10– 400 nm . The measured values varied from 0.1 to 6% depending on particle size and impactor stage. In the measurement range of the instrument, i.e. above 30 nm , the losses were below 2%. Image charge losses exceeded the diffusion losses when particle size was larger than 200 nm , but even the combined loss in this size range was below 0.5%. Space charge losses were determined both experimentally and through calculations. The space-charge effect was found to be a dominant loss mechanism in ELPI when measured concentrations were high.

Sampling method for particle measurements of vehicle exhaust

This paper describes a new sampling concept for particle emission measurements. The purpose is to produce repeatable and reproducible conditions for nucleation phenomena. The exhaust is sampled and instantaneously diluted by inserting a porous tube diluter inside the tailpipe. This is carried out in order to prevent uncontrolled sample transformations in sampling lines. The sampling system was tested in size distribution measurement of light duty diesel vehicle. The tests showed a clearly bimodal size distribution with distinguished nuclei and accumulation modes.

ELPI Response and Data Reduction I: Response Functions

Response functions of the ELPI impactor with normal and porous substrates are presented together with fit functions to describe particle collection. In addition to primary collection efficiency, fits for the secondary collection mechanisms, diffusion, image charge force, and space charge field are presented. Charging efficiency for different configurations is also presented. Presented response functions can be used in data reduction and inversion of ELPI data.

Electrical calibration method for cascade impactors

Abstract A new method for measuring collection characteristics of cascade impactors is described. The method is based on challenging the impactor with monodispersed charged particles and measuring electrical currents on all the impactor stages plus a backup filter. From the current values, collection efficiency at a certain particle diameter can be calculated simultaneously for each impactor stage. To test the method, monodispersed dioctyl sebacate particles were generated using an atomiser, evaporation/condensation generator and an electrostatic classifier. The particle collection characteristics of the six lowest stages of a Berner-type low-pressure cascade impactor were measured. Comparison measurements were made with a real-time stage efficiency measurement system for single impactor stages. The results of the two methods were in good agreement.

Size-selected agglomerates of SnO2 nanoparticles as gas sensors

Financial support was provided by ETH Zurich FEL-04 08-3, Finnish Academy, Tekes The Finnish National Technology Agency, and Nanoprim.

Instrumentation for measuring fluorescence cross sections from airborne microsized particles

An experimental instrument for measuring a laser-induced fluorescence spectrum from a single aerosol particle is described. As a demonstration of instrument capabilities, the results of monodisperse 4.7 microm sodium chloride particles doped with fluorescent riboflavin, produced with an inkjet aerosol generator, are presented. The fluorescence of the aerosol particles is excited in the wide range from 210 to 419 nm using a pulsed, tunable optical parametric oscillator laser. The maximum of the fluorescence emission of separately measured particles is detected at 560 nm. The dependence of the fluorescence on the excitation wavelength is studied and fluorescence cross sections are estimated. Agreement between the measured fluorescence data and the literature data for riboflavin is observed.

Fluorescence cross sections of bioaerosols and suspended biological agents

Laser-induced fluorescence is used to investigate fluorescence properties of unwashed Bacillus thuringiensis and Bacillus subtilis spores, ovalbumin, and washed bacteriophage MS2. A fluorescence detector is calibrated to obtain absolute fluorescence cross sections. Fluorescence maps of biological aerosols and suspensions are measured at a wide excitation range from 210 to 419 nm and a wide detection range from 315 to 650 nm. The dominant features of the measured spectra are the amino acid peaks, having excitation maxima at 220 and 280 nm. The peaks are similar for the bacterial spores, both for aerosols and suspensions, whereas the peaks are shifted toward the shorter emission wavelengths for the suspended ovalbumin and MS2. Moreover, the fluorescence emission, excited above 320 nm is more intensive for the aerosols than the suspensions.

The ELPI Response and Data Reduction II: Properties of Kernels and Data Inversion

The first part of this article reports the analytical form of the Electrical Low Pressure Impactor (ELPI) kernel functions. In this latter part, the numerical quality of ELPI response matrices is studied and an example of an inversion algorithm is given. The ELPI assemblies with and without an electrical filter stage and with smooth or sintered impaction plates are studied and compared with basic impactor kernels and the kernels for the calculation of the aerosol mass distribution. It is shown that the ELPI assembly with the electrical filter stage and smooth impaction plates should be the best choice for the inversion of data if no bounce occurs. The comparison to a mass impactor shows that the devices are on par in data inversion. The inversion ELPI data is studied with a Bayesian algorithm assuming a bimodal lognormal size distribution of the aerosol. The algorithm includes a novel procedure for obtaining an initial guess of the distribution parameters. To our knowledge, it is also the first algorithm to use ELPI current readings as its input. Simulations and diesel emission measurements show that the proposed algorithm is a useful tool in the study of ELPI data.