DESCRIPCIÓN DE EQUIPOS DE UTILERÍA Y VESTUARIO DEL CENTRO DE

Charger Insulated Impactq Stages High Voltage HV M ul tic han ne l E le ct r o m e te r Amp lifier

A/D & Control Electronics PC/laptop Data treatment (a) SPRING-LOADED CONTACT ISOLATION RING IMPACTOR STA G E \

i

Figure 3.10. a) Schematic diagram o f the Electrical Low Pressure Impactor b) Detail: Isolation and electrical connections o f the impactor stages (Keskinen, 1992)

Chapter 3_________________________Review o f Techniques fo r Monitoring Particulate Pollutants

This time is subsequently compared with predetermined calibration curves created by using particles o f a known size. Finally, the particles enter into the ion source region where an Nd:YAG laser with a 266-nm pulse output generates the mass spectra o f the individual particles from which composition information are inferred. The operational size range o f the instrument is between 0.2 to 10 pm (Gard et al. 1997; Silva et al., 1997).

Preliminary studies suggest that ATOFMS can be used potentially for monitoring vehicular particulates on a continuous basis. However the system dimensions (1.8m long X 0.7 m wide x 1.5 m high and weigh 250 kg) makes it unsuitable for on-board monitoring of automotive emissions (Gard et a l . , 1997).

3.4.7. Epiphaniom eter

Epiphaniometer is an instrument which continuously measures aerosol surface concentrations. The sampled air is pumped into a chamber containing short lived radioactive lead isotopes (^" Pb) produced by an actinium source (^^^Ac). The lead atoms are attached onto the aerosol particles and are transported through a capillary tube which acts like a diffusion trap for non attached lead particles. At the end o f the capillary tube the aerosol particles with the attached lead atoms are deposited on a filter where they are detected by an a-detector which measures the decay o f lead. The measured signal is proportional to the exposed surface o f the aerosol particles (Gaggeler et al, 1989a; Gaggeler et al., 1989b).

The device requires very small sampling rates (around 1 1 /min) and can detect particle concentrations as low as 100 ng/m^ in the size range between 0.08 to 7 pm. Furthermore due to the relatively short half life o f the lead isotopes the system allows continuous monitoring of aerosol without changing or transporting the filter (Gaggeler et al, 1989a ; Gaggeler et al., 1989b).

Chapter 3 Review o f Techniques fo r Monitoring Particulate Pollutants

3.5. Conclusion: Overview of the state of the art, problems identified, way to go!

Over the last two decades, a wide range o f instruments have been developed for measuring ambient and source particulate concentrations. Table 3.1 presents a league table of these various techniques.

However, data acquired from the on going research reveal the need for new techniques that respond to new requirements.

The current emissions legislation requires new car models to comply with the correspondent emissions standards. However, it is important to note that for many of these vehicles very little is known about their on-road emissions which can differ significantly from those in lab dynamometers. To obtain realistic emissions data it is necessary to measure emissions from a range o f vehicles that are in service. Subsequently valuable information can be inferred about the effect o f age o f the vehicle as well as poor maintenance on the resulting emissions. Therefore, measuring emissions as vehicles are being driven along public roads provides a test of real emissions. However, in order to achieve this the measuring instrument should be small enough to be portable and capable of taking measurements on line and on real time b a s is .

Finally, since 1993 smoke tests have been used as random road side test as well as part o f the annual MoT inspection. However, a number o f car engines in poor conditions were damaged as a result of the test and the test was temporary withdrawn after three months. At the moment, the Department o f Transport and Environment has three on going projects (S026F/VB, S140AATB, S260H/VB) investigating alternative means o f testing smoke emissions from exhaust pipes of vehicles, on an on line basis. It should however he home in mind that the final decision about the suitability o f a specific instrument for a defined task should also take into account both physical and economic factors which include:

Chapter 3 Review o f Techniques fo r Monitoring Particulate Pollutants

• accuracy, reproducibility and sensitivity o f the measurement • the number and frequency o f sampling and analysis operations • cost o f the equipment as well as installation and operational cost.

Chapter 3 Review o f Techniques fo r Monitoring Particulate Pollutants

Table 3.1, Instrument summary (Kittelson, 1999)

Instrument Parameter Range Comments

Cascade Impactors M O U D I Aerodynamic

diameter

0.056-10 (pm)

- Not real-time, gravimetric and chemical analysis performed after collection

E L P I - I I -

0.030-10 (pm)

- Near real time with resolution 2 to 15 s - Also used for ambient measurements but with potential volatile particles losses Electrical Mobiliity Instruments

E A A Electrical mobility equivalent diameter 0.003-1 (pm)

- time resolution : 2-3 mins, 11 size stages - inaccurate results when concentration and

size fluctuate during scan

- requires high dilution (>200:1) to avoid the saturation o f the electrometer

- results for particles < 10pm are not very accurate

- pre-classification stage required to remove particles < 1pm for diesel aerosol

measurements

D M P S - I I -

0.01-0.5 (pm)

- discrete electric field changes makes the whole sampling slower than SMPS

- inaccurate results when concentration and size fluctuate during scan

- pre-classification stage required to remove

particles < 1pm for diesel aerosol measurements

S M PS —//— 0.01-0.7

(pm)

- 2-4 m in scan time for 100 size intervals - time resolution o f a few seconds for a single

channel

- inaccurate results when concentration and size fluctuate during scan

- pre-classification stage required to remove

particles < 1pm for diesel aerosol measurements

Miscellaneous Instruments P A S surface area < 1

(pm)

- condensation o f vapours on particles interfere with measurement

- response time < 1 sec

- particles 1 > pm are not charged efficiently

A T O F M S aerodynamic diameter

0.3 -50 (pm)

- real time measurements o f up to 600 particles per min

Epiphaniometer

diffusion mobility

0.01-0.5 (pm)

- slow sampling rates i.e. long sampling times - requires steady state conditions

- inaccurate results when concentration and size fluctuate during scan