PM Measurement: In-Situ Methods

Heinz Burtscher, W. Addy Majewski, Hannu Jääskeläinen

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Abstract: In-situ instruments perform PM measurements in the aerosol phase, typically from diluted exhaust gas samples. A variety of analyzers have been developed to measure particle sizes and size distributions, including the MOUDI, ELPI, SMPS, and more. These instruments utilize aerodynamic or electrical mobility measuring principles. A number of instruments have been developed to measure other PM properties, such as particle mass, surface, or various optical properties.

Introduction

In-situ PM measuring techniques are defined as those where the analysis—or at least its important steps—are performed in the aerosol phase. Contrary to the more conventional collecting methods, particulates are not collected on a sampling filter and analyzed after collection. This makes in-situ methods very suitable for the measurement of such particle properties as sizes and size distributions—which are impossible to determine from a filter sample. In-situ instruments are being developed to measure a range of other properties, including PM mass emission.

Some in-situ instruments have the capability to measure undiluted, hot exhaust at the tailpipe. In most cases, however, exhaust gas needs to be sampled and conditioned before measurement. Many in-situ instruments work with diluted exhaust, such as from a CVS tunnel or other dilution system, as discussed in the sampling paper. The sampling issues discussed there are pertinent to most in-situ instruments.

Importantly, one needs to remember that particle size distributions are critically influenced by the choice of wet or dry sampling. With few exceptions, particle measurement instruments are sensitive to both solid and liquid particles. Therefore, particles made of hydrocarbon, sulfuric acid and/or water condensates—and perhaps even single large size molecules, such as those from certain lube oil additives—are measured and reported together with solid particles made of carbon or ash.

Particle Number Concentration

Condensation Particle Counter

Condensation particle counter (CPC), also called the condensation nucleus counter (CNC), is the most common instrument used to determine number concentrations of diesel particulates [645].

[schematic]

Figure 1. Condensation Particle Counter

As shown in Figure 1, upon entering the CPC the aerosol stream is saturated with alcohol (typically butanol) vapor. As the mixture is cooled in the condenser tube, the vapor becomes supersaturated and condenses on particles. As a result, the particles grow to a diameter of about 10 µm, allowing for optical detection [2419][645]. The particle size detection limit in the CPC is related to the increasing saturation ratio which is required with decreasing particle diameters. Modern CPCs have detection limits of around 10 nm going down to less than 3 nm for ultrafine particle CPCs. Within a certain range, the lower cutoff can be tuned by setting the temperatures in the saturator and the condenser.

Commercial CPC instruments also exist that utilize water, instead of alcohol, as the condensing fluid. However, there is some evidence that for fresh combustion particles, the performance of butanol based CPC's is superior [2420][2415].

CPCs can be operated in two modes: (1) the counting mode and (2) the total intensity mode. In the counting mode, pulses of scattered light from individual particles are counted. This mode provides the most accurate measurements, but can be used only at low particle concentrations. In the total intensity mode, typically used for concentrations of above 104/cm3, number concentrations are determined from the total light scattering intensity. This mode, generally subject to a larger error, requires that all particles grow to the same diameter and that the optical system is frequently calibrated [646]. The CPC instrument is very sensitive to its inner temperature, which affects the degree of supersaturation, as well as to its positioning and vibration. For these reasons, it is suitable primarily for laboratory measurements; its use in field equipment is more challenging.

The CPC is used for particle detection in many aerosol size distribution measurement instruments. In particular, most mobility analyzers use CPC’s.

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