GB2324695A

GB2324695A - Monitoring atmospheric pollution

Info

Publication number: GB2324695A
Application number: GB9708215A
Authority: GB
Inventors: Denis Lee Henshaw; Adrian Peter Fews
Original assignee: University of Bristol
Current assignee: University of Bristol
Priority date: 1997-04-23
Filing date: 1997-04-23
Publication date: 1998-10-28
Anticipated expiration: 2017-04-23
Also published as: GB2324695B; GB9708215D0

Abstract

A method of monitoring pollution by determining the number of small aerosol particles in an atmosphere, includes the steps of counting the number of radioactive decays of a first type having a particular energy level occurring on the surface of a detector, counting the number of decays of the first type occurring away from the surface of the detector, and comparing the counts obtained. Since the small aerosol particles are mobile and carry radioactive particles in the atmosphere onto the detector surface, the relative number of decays on and away from the surface is a measure of the number of small aerosol particles present. The method may also provide an indication of particle size distribution. A plastic track detector may be used in the method, and the radioactive decay process may be alpha emission by polonium-214.

Description

POLLUTION MONITORING This invention relates to pollution monitoring, and in particular to a method and apparatus which allows the monitoring of the relative number of ultrafine aerosol particles in the atmosphere.

The concentrations of aerosol particles in the atmosphere vary widely. Moreover, the relative concentrations of relatively small aerosol particles (sizes of the order of tens of nanometres) and larger aerosol particles (sizes of the order of hundreds of nanometres) can also vary. This can for example be expressed as a ratio P < 30:P < 500, which represents the fraction of the particles smaller than 500nm which are smaller than 30nm. It is the very small aerosol particles which are considered of particular importance in lung disease because of their 100% retention when inhaled. Therefore, it would be particularly useful to be able to obtain a measure of the concentration of such particles.

Plastic track radioactivity detectors are known, for example from GB-A-2254990. Such a detector provides a measure of the a-radioactivity in air.

Also, the plastic track detector allows more detailed measurements to be made concerning the detected emissions. Specifically, analysis of the tracks allows measurements to be made concerning the energy of the emissions.

Each a-particle is emitted with an energy which depends upon the type of nucleus whose decay causes it to be emitted. In practice, in many common situations, the most common source of radioactivity in the atmosphere is background radiation, arising from the decay of radon and thoron, that is 222Rn and 220Rn and their decay products.

In the atmosphere the radioactive radon decay product ions quickly attract trace water molecules growing into small aerosol particles which may go on to attach to larger aerosols or particulates in air.

Decays which are recorded by the plastic track detector can then result from radioactive nuclei which are present in the atmosphere around the detector, or from radioactive nuclei which have become attached to aerosol particles which have attached themselves to the detector.

Alpha-particles emitted in decay events which occur on the surface of the detector will be seen to have the characteristic energy level determined by the parent nucleus. Particles arising from the decays which take place away from the detector surface will arrive at that surface with a reduced energy.

Thus, analysing the energies of the a-particles detected by the plastic track detector can provide an indication as to whether those particles were present on the surface of the detector, or in the atmosphere around the detector.

It has now been determined that this itself can be used to obtain a measure of the number of small aerosol particles in the atmosphere, because these small aerosol particles are more mobile, and thus more likely to attach themselves to the surface of the detector.

According to a first aspect of the present invention, there is provided a method of determining the number of small aerosol particles in an atmosphere, comprising analysing the radioactive particles detected by a detector placed in the atmosphere; assessing the relative numbers of radioactive decays which have taken place on the surface of the detector and in the atmosphere surrounding the detector; and relating this assessment to a measure of the number of small aerosol particles present in the atmosphere.

Reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 illustrates a series of radioactive decays; Figure 2 is a representation of radioactivity measurements taken in relatively unpolluted air; and Figure 3 is a representation of radioactivity measurements taken in more polluted air.

Figure l(a) shows a radon (222Rn) atom, in air.

Radon is radioactive, and Figure l(b) shows the decay of the radon atom. An a-particle is emitted, and the resulting 218po ion recoils.

Over the course of the next microsecond or so, this ion attracts water molecules in the air, and grows to become an ultrafine aerosol particle, having a size of the order of 10nm, as shown in Figure l(c). As shown in Figure l(d), this aerosol loses its positive charge, typically taking about one second to do so.

The ultrafine aerosol particle may then become attached to a larger aerosol, which may carry an electrical charge, or may be uncharged, and typically has a size of the order of 100nm, as shown in Figure l(e). The likelihood of this attachment depends on the number of larger aerosol particles in the atmosphere.

An aerosol particle, whether large or small, may attach itself to (become "plated out" on) a solid body present in the atmosphere. Of particular interest is the case when an aerosol attaches itself to the surface of a radioactivity detector placed in the atmosphere.

Aerosol particles, including ultrafine particles, are also present in large numbers in, for example, motor vehicle exhaust emissions. These pollutant ultrafine aerosols may similarly become attached to larger aerosols.

Returning to Figure 1, the 218Po nucleus is highly radioactive and, in due course, it too decays, giving rise to a further a-particle and a 214Pb atom, which detaches from the aerosol particle due to recoil, as shown in Figure l(f).

214Pb is itself radioactive, and so the decay chain continues. 214Pb decays by ss-emission to 214Bi and then to 214po, which in turn decays by a-particle emission. (The final decay product is 206Pb, which is stable.) If any a-particle decay takes place on, or near, the surface of a plastic track radioactivity detector, it will give rise to a track in the detector, and the decay can be counted. Moreover, the size of the track will be a measure of the energy of the a-particle. All a-particles produced by the decay of a particular nucleus, e.g. 218po, are emitted with the same energy, but particles which are produced by decay taking place away from the detector surface will lose some of their energy before they strike the detector surface. By contrast, particles which are emitted on the detector surface will be detected with their full energy.

Therefore, analysis of the energies of the aparticles gives a measure of the relative number of decays on the surface of the detector, and in the atmosphere around the detector.

More details of the treatment and analysis of the detector can be found in, for example, "High Resolution Alpha Particle Spectroscopy using CR-39 Plastic Track Detector", Nuclear Instruments and Methods 197 (1982) 517-529; "Automated Image Analysis of Alpha Particle Autoradiographs of Human Bone", Nuclear Instruments and Methods in Physics Research A263 (1988) 504-514; and "Fully Automated Image Analysis of Etched Tracks in CR39", Nuclear Instruments and Methods in Physics Research B71 (1992) 465-478.

The process described above with reference to Figure 1 is especially typical of the process which occurs indoors, where 214po particles on a detector surface are nearly always found attached to larger aerosol particles, of the order of 100nm in size or larger.

However, in outdoor air there are far fewer larger aerosols available, to which ultrafine aerosols, including both the aerosols which form around radon decay product ions and pollutant particles, can become attached. The result is that the majority remain in the ultrafine form. Ultrafine aerosol particles are far more mobile than the larger aerosol particles, and are correspondingly more likely to come into contact with, and hence remain upon, the surface of a plastic track detector. Thus, when there are many ultrafine aerosol particles in the atmosphere, the number of decays taking place on the detector surface becomes much higher, relative to the number taking place in the atmosphere surrounding the detector.

Analysis of these relative numbers therefore provides a measure of the number or relative proportion of ultrafine particles present in the air. This is of particular significance because it is these ultrafine aerosols which are of particular importance as regards health issues. Specifically, when a pollutant is inhaled, ultrafine aerosol particles thereof are retained in the lungs and thus are far more damaging to health than larger particles, which can be exhaled.

Figure 2 is a scatter plot of the longest axis versus the minor axis of etch tracks recorded on a plastic track detector exposed to an indoor atmosphere.

The band marked by arrow A in Figure 2 represents the a-particles with one particular energy, namely those emitted by the decay of polonium-214 present on the surface of the detector. As is known, it is equally possible to measure the number of decays of other nuclei, including polonium-218. The band marked by arrow B represents a-particles resulting from the decays of radioactive nuclei present in front of the detector. This is therefore a measure of the total alpha-radioactivity in the air. In Figure 2 the ratio of the relative numbers of decays in these two bands, which can be expressed as 214Po/air, is about 3.

This ratio can be seen as a measure of the mobility of the aerosol particles, or a measure of the number of small aerosol particles in the atmosphere.

Thus, the detected energies of the alpha-particles can be used to derive an estimate of the size distribution of the radioactive aerosols, and this in turn can be assumed to be similar to the size distribution of pollutant aerosol particles in the atmosphere.

Figure 3 is a similar graph, with the band C representing the polonium-214 plated out decays, and the band marked by arrow D representing the decays in air. In this case, the plastic track detector was exposed to an outdoor atmosphere. It will be noted that, in Figure 3, the plated out band is much more pronounced, and, indeed, in this case the ratio 214po/air is about 58.

This is a good indication of the higher level of pollution, in the form of ultrafine aerosol particles, in the outdoor atmosphere under examination.

Other useful parameters may also be derived from the same detector exposure. For example, the parameters 218Po/air, and 214Po/218Po can also reveal information about the atmosphere.

There is thus provided an easy way of monitoring the number of ultrafine particles present in an atmosphere.

Claims

1. A method of determining the number of small aerosol particles in an atmosphere, comprising analysing the radioactive particles detected by a detector placed in the atmosphere; assessing the relative numbers of radioactive decays which have taken place on the surface of the detector and in the atmosphere surrounding the detector; and relating this assessment to a measure of the number of small aerosol particles present in the atmosphere.

2. A method of analysing a plastic track radioactivity detector, comprising counting the number of decays of a first type, having a particular energy level, occurring on the surface of the detector, counting the number of detected decays of the first type occurring away from the surface of the detector, and comparing the counted numbers.

3. A method as claimed in claim 2, further comprising obtaining the ratio of the counted numbers.

4. A method as claimed in claim 2 or 3, further comprising using the counted numbers to obtain a measure of the mobility of aerosol particles present in an atmosphere under test.

5. A method as claimed in claim 2, 3 or 4, comprising counting the number of a-particles emitted by the decay of 214Po occurring on the surface of the detector.

6. A method of assessing the mobility of aerosol particles in an atmosphere, comprising placing a radioactivity detector in the atmosphere, the detector being able to distinguish between first radioactive decays occurring on the surface thereof and second radioactive decays occurring away from the surface thereof, and obtaining a measure of the mobility of the aerosol particles from the relative numbers of first and second radioactive decays.

7. A method as claimed in claim 6, wherein the detector is a plastic track detector and is able to distinguish between the first and second radioactive decays by allowing measurements of the energies of aparticles.

8. A method as claimed in claim 6 or 7, wherein the number of a-particles emitted by the decay of 214polonium is used as the number of first radioactive decays.

9. A method of assessing the mobility of aerosol particles in an atmosphere, comprising placing a plastic track radioactivity detector in the atmosphere, the method comprising measuring the number of first aparticle decays having energies characteristic of the decay of 214-polonium occurring on the surface of the detector, measuring the total number of second aparticle decays occurring away from the surface of the detector, and obtaining a measure of the mobility of the aerosol particles from the relative numbers of first and second decays.

10. A plastic track detector, for use in a method as claimed in any preceding claim.

11. An analysis system, comprising a plastic track detector, and means for carrying out the method as claimed in any preceding claim.

12. A method of monitoring numbers of pollutant aerosol particles in air, comprising: taking measurements of the numbers of decays of radioactive particles in the air, at least some of the radioactive particles being assumed to be attached to radioactive aerosol particles; deriving a measure of the size distribution of the radioactive aerosol particles in the air; and converting the derived size distribution of radioactive aerosol particles in the air to a measure of the size distribution of pollutant aerosol particles in the air.