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Monitoring aerosols emitted from mobile sources is a difficult task due to a number of reasons. For example vehicular emissions contain particles with diameters varying from few nanometers up to several microns. Furthermore, the temperature of the exhaust gases is rather high and the exhaust flow is unsteady and non-uniform (Harrison, 1986).

As indicated earlier traffic pollution has been a major public concern and several measuring techniques have been developed for this purpose. These either use the full or the partial flow of the exhaust gas (ISO 11614, 1994). In case o f partial flow, it is important to ensure that the taken sample is representative o f the exhaust aerosol. This

Chapter 3 Review o f Techniques fo r Monitoring Particulate Pollutants

the main gas stream at the inlet of the probe. This technique is called isokinetic sampling (Hinds, 1982).

Furthermore, the sampled stream is usually diluted in a tunnel with dry filtered air in order to simulate the equivalent process that particles undergo after they are emitted into the atmosphere and to prevent condensation o f the hot vapours (Habibi, 1970, Ahlvik et al., 1998). The latter can also be achieved by maintaining the exhaust gas and the measuring equipment in elevated temperatures (60-120 °C) (ISO 3173, 1974).

A brief review o f the most frequently used instruments for measuring diesel aerosols is given below followed by a summary o f their main features in Table 3.1.

3.4.1. International Standard Method : Opacimeter (ISO 11614/ 94)

Opacimeters along with filter-type smokemeter are the earliest methods used to measure the total amount o f black smoke in diesel exhausts. Even though these methods are considered to be outdated, they are still the official methods used for inspection and maintenance purposes (Ahlvik et al., 1998). The operational characteristics o f the filter-type smokemeter (ISO 10054) are similar to those of the BS smokemeter used for ambient sampling and therefore will not be discussed further (see 3.3.2.1. and 3.3.3.2.)

In opacimeters, smoke is directed through a parallel visible light beam of specific length where it absorbs some of the light and thereby diminishing the intensity o f the beam (Hinds, 1982). The fraction of the transmitted light which is absorbed through the smoke obscured path is called opacity and is measured in opacity units. The latter is used as an indication o f the particulate concentrations in the exhaust gases (ISO

11614, 1994).

The instrument comprises a light source (lamp with colour temperature between 2800 to 3250K), a photoelectric receiver (photocell or photodiode) and a smoke chamber. The latter constitutes the casing of the instrument where the light source and the

Chapter 3 Review o f Techniques fo r Monitoring Particulate Pollutants

receiver are embodied. All the instrument’s internal surfaces are painted in matt black in order to minimise reflection and/or diffusion o f stray light which may otherwise result in measurement errors (ISO 11614, 1994).

The biggest disadvantage o f the technique is that many such instruments of established design cannot achieve a response time less than 0.5 sec (ISO 11614,

1994).

3.4.2. Electrical Analysers

Electrical analysers have been the most common instruments used in the US for diesel particulate measurements. These are based on the particle’s electrical mobility Z, which is the ability o f a charged particle to move in an electric field (Kittelson et al., 1999). More specifically, the velocity o f a charged particle towards the opposite charged terminal of an electric field o f strength E is given by:

Vte = ZE (3.1)

Figure 3.9.a demonstrates the operational principle o f the device. It consists of two opposite charged plates o f a certain length, retained at a constant voltage. The aerosol particles are introduced into the constant electric field along a centre line. Equation 3.1 indicates that all particles with mobility greater than a certain value will be attracted and consequently captured onto one plate whereas those with less mobility will pass through the analyser. Therefore by comparing the particles mass entering and leaving the analyser one can determine the particles fraction with a certain electrical mobility (Hinds, 1982).

The particle size distribution measurement is based on the fact that the electrical mobility o f a particle is directly proportional to the carried charge which in turn is proportional to the particle’s diameter. Therefore, a unique value of electrical mobility is associated with every particle size. The analyser’s cut-off mobility and

Chapter 3 Review o f Techniques fo r Monitoring Particulate Pollutants

1979). However, this application is feasible only when the particles have been charged under well defined and controlled conditions (e.g. diffusion charging) so that the charge on the particle o f a given size is known (Ludgen, 1979; Hinds, 1982) .

3.4.2.I. Electrical Aerosol Analyser (EAA)

A commercial EAA (TSI Inc. Model 3030) used for monitoring diesel exhaust particulate matter is shown in Figure 3.9.b (Baumgard et al., 1992). In this case, the aerosol is drawn into the instrument at about 4 1 /min and passes through a unipolar- ion diffusion charger where by using proper control, particles o f a given size receive a predictable number o f charges. Next, the particles flow to the mobility analyser in laminar flow enveloped by a clean air stream. Depending on the applied voltage on the tubes, the particles with mobility less than the cut-off mobility pass through the analyser and are then collected in a high efficiency filter. An electrometer connected to the filter continuously monitors the accumulated charge carried by the particles. The particle numbers can be calculated from the measured charge and the predetermined cut-off size. The instrument operates with preset voltages to provide the number of particles in each of 10 cut-off sizes between 0.003-1 pm (Hinds 1982; Baumgard et al., 1992).

There are three major problems associated with using EAA for sampling diesel emissions. First, EAA requires nearly constant aerosol concentration (± 10%) during an operating cycle (1-3 minutes). However under typical driving conditions aerosol concentration varies more than one order of magnitude within few seconds (Ludgen, 1979). Second, EAA is adversely affected by vapours condensation. However during short period o f acceleration/deceleration the exhaust fumes become supersaturated with vapours which results in condensation (Ludgen, 1979). Finally, the size and weight o f the instrument have caused difficulties in its applications. The model shown in Figure 3.9.b. has a volume of 0.05 m^ and weights approximately 24 kg.

Chapter 3 _{Review o f Techniques for Monitoring Particulate Pollutants}