Jyrki Ristimäki

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.

Sampling Conditions for the Measurement of Nucleation Mode Particles in the Exhaust of a Diesel Vehicle

A novel porous tube diluter was characterized to define sampling parameters for repeatable measurements of nucleation-mode particles (NMPs) in the exhaust of a modern diesel passenger car at moderate engine load. This porous tube diluter permits the variation of sampling parameters independently and in a wide range. We investigated the sampling parameters: primary dilution temperature (PDT; 15–55°C), primary dilution ratio (PDR; 8–45), residence time (RT; 0.5–4.0 s), and relative humidity of primary dilution (PRH; 5–90%). Decreased PDT and increased PRH led to a growth of particle number and size in the nucleation mode. While a maximum number of NMPs was found at moderate PDRs between 20 and 30, a maximum volume of NMPs was achieved at PDRs below 20. Coagulation explains the number reduction of NMPs under sampling conditions of prolonged RT and decreased PDR. However, the size growth of the nucleation mode can only partly be attributed to coagulation, and thus growth due to organic compounds from the exhaust probably plays an important role. Sensitivity analysis was conducted as a function of PDR-PDT and revealed two optimal sampling conditions for repeatable NMPs in number or repeatable maximum NMPs in volume. In addition to the sampling parameters, exhaust line conditioning was found to affect NMPs strongly and thus needs to be controlled to minimize effects on the history of measurements.

Performance Evaluation of a Novel Sampling and Measurement System for Exhaust Particle Characterization

This paper presents a novel partial flow sampling system for the characterization of airborne exhaust particle emissions. The sampled aerosol is first conditioned in a porous dilutor and then subsequent ejector dilutors are used to decrease its concentration to the range of the instrumentation used. First we examine the sensitivity of aerosol properties to boundary sampling conditions. This information is then used to select suitable sampling parameters to distinguish both the nucleation and the accumulation mode. Selecting appropriate sampling parameters, it is demonstrated that a distinct nucleation mode can be formed and measured with different instruments. Using these parameters we examine the performance of the system over transient vehicle operation. Additionally, we performed calculations of particle losses in the various components of the system which are then used to correct signals from the instruments. Several quality characteristics are then discussed, such as the repeatability and reproducibility of the measurements and the potential to derive total emission rate with a partial flow sampling system. Comparisons in different laboratories show that repeatability (intra-laboratory variability) is in the order of 10% for accumulation mode particles and 50% for nucleation mode ones. Reproducibility (inter-laboratory variability) values are in the range of ±20-30%. Finally, we compared laboratory size distributions with ambient samples obtained chasing a vehicle. This demonstrated that the sampling system accurately reproduced the accumulation mode particles as well as the potential for nucleation mode formation. This sampling system has been used in the framework of a European project for measurement of emissions of a number of light duty vehicles and heavy duty engines.

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.

Method for Measuring Effective Density and Fractal Dimension of Aerosol Agglomerates

A method to find particle effective density and the fractal dimension, based on simultaneous size distribution measurements with SMPS and ELPI, is introduced. A fitting procedure is used to find the particle density as a function of particle size and the fractal dimension. The method was tested by simulation and by experimental measurements of particles with varying morphology. For fractal dimension values between 2.2 and 3.0, fractal dimension was measured with an accuracy of 0.1, and effective density was measured with 15% relative accuracy.

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.

Mass Measurement of Non-Spherical Particles: TDMA-ELPI Setup and Performance Tests

An ELPI was introduced to the TDMA setup for measurement of effective density and particle mass. This allows measurement of particle mass change also in cases when the particles are non-spherical or have voids. In addition to mass change of a particle, the density of transferred matter can be calculated if either unconditioned or conditioned particles are spherical and the bulk (material) density of the core particle is known. The performance of the system was tested by numerical simulations and laboratory experiments. According to the results, the smallest detectable particle mass change is approximately ± 17%. The measured density for the condensing species was within 15% of the bulk density value. Particle mass change caused by condensation of semi-volatile components of exhaust gas on diesel soot particles was also demonstrated.

Evaluation of thermal optical analysis method of elemental carbon for marine fuel exhaust

ABSTRACT The awareness of black carbon (BC) as the second largest anthropogenic contributor in global warming and an ice melting enhancer has increased. Due to prospected increase in shipping especially in the Arctic reliability of BC emissions and their invented amounts from ships is gaining more attention. The International Maritime Organization (IMO) is actively working toward estimation of quantities and effects of BC especially in the Arctic. IMO has launched work toward constituting a definition for BC and agreeing appropriate methods for its determination from shipping emission sources. In our study we evaluated the suitability of elemental carbon (EC) analysis by a thermal-optical transmittance (TOT) method to marine exhausts and possible measures to overcome the analysis interferences related to the chemically complex emissions. The measures included drying with CaSO4, evaporation at 40–180ºC, H2O treatment, and variation of the sampling method (in-stack and diluted) and its parameters (e.g., dilution ratio, Dr). A reevaluation of the nominal organic carbon (OC)/EC split point was made. Measurement of residual carbon after solvent extraction (TC-CSOF) was used as a reference, and later also filter smoke number (FSN) measurement, which is dealt with in a forthcoming paper by the authors. Exhaust sources used for collecting the particle sample were mainly four-stroke marine engines operated with variable loads and marine fuels ranging from light to heavy fuel oils (LFO and HFO) with a sulfur content range of <0.1–2.4% S. The results were found to be dependent on many factors, namely, sampling, preparation and analysis method, and fuel quality. It was found that the condensed H2SO4 + H2O on the particulate matter (PM) filter had an effect on the measured EC content, and also promoted the formation of pyrolytic carbon (PyC) from OC, affecting the accuracy of EC determination. Thus, uncertainty remained regarding the EC results from HFO fuels. Implications: The work supports one part of the decision making in black carbon (BC) determination methodology. If regulations regarding BC emissions from marine engines will be implemented in the future, a well-defined and at best unequivocal method of BC determination is required for coherent and comparable emission inventories and estimating BC effects. As the aerosol from marine emission sources may be very heterogeneous and low in BC, special attention to the effects of sampling conditions and sample pretreatments on the validity of the results was paid in developing the thermal-optical analysis methodology (TOT).

Comparison of filter smoke number and elemental carbon from thermal optical analysis of marine diesel engine exhaust

The interest on contribution of shipping to global warming and especially on polar ice melting has increased. The International Maritime Organization is working toward reporting and estimation of black carbon emissions from shipping. The filter smoke number method is discussed as one possible candidate for onboard determination of black carbon/soot concentration of the engine exhaust gas, and it has recently been considered as one of the best candidates for further evaluation in the International Council on Clean Transportation 4th workshop on marine black carbon emission. Proven, standardized technology and small size and simple operation of the filter smoke meter make it a potential choice for actual onboard use. In our study, we evaluated the validity of the filter smoke number method for measuring soot emission by looking at correlations between the filter smoke number and elemental carbon analyzed using thermal optical transmittance analysis. Until now the conversion of the filter smoke number to black carbon /soot emission has been performed with equations derived from high-speed engines operating with distillate fuels. We introduce optimized calculation parameters for filter smoke number to black carbon/soot conversion, which are derived from light and heavy fuel oil measurements. These new parameters can be utilized with improved accuracy for the estimation of the black carbon emission from filter smoke number measurement with marine fuel qualities.