Measurement
Methods II
How to detect,
collect and analyse aerosol particles.
Content
The concept of TSP, PM10, PM2.5 and UFP
The concept of TSP, PM10, PM2.5 and UFP
TSP: Total Suspended Particulate
As the name suggests, TSP are all aerosol particles (sometimes even
above 100 mm sizes) suspended in the air.
PM10 and PM2.5
PM10 means particulate mass of particles smaller than 10 mm in diameter and consequently PM2.5 means particulate mass of particles smaller than 2.5 mm in diameter.
The reason for these two size categories is that particles smaller than 10 mm are respirable, that is, they can be inhaled below the nasopharynx area (nose and mouth), and particles below 2.5 mm travels down below the tracheobronchial region (into the lungs). The PM2.5 cut off size also coincides with both the local minimum in particle concentration between coarse and fine particles and the natural and anthropogenic fractions.
The fraction of
PM10 larger than PM2.5 is often called coarse particles and PM2.5 is often
called fine particles.
Ultra Fine Particles (UFP)
Ultra fine
particles are particles with an aerodynamic diameter less than 0.1 mm. In recent years this size fraction
has come into focus due to the new found link between these particles and
health effects in form of respiratory problems such as asthma, inflammatory
diseases and reduced oxygen uptake efficiency.
Particle Collection
Filter cups
Using filter cups to collect
particles is the simplest way of collecting particles. Weighing the filters
before and after the sampling gives the amount of particulate mass collected,
and by knowing the amount of air drawn through the filter the mass
concentration can be calculated. The downside is that if no size-limiting inlet
has been used, the particle size limit is not known.
Cyclones
Cyclones are a special type of impactors where the impaction (on the side walls of the cyclone) is combined with the gravitational settling of the large particles. The effect is that the large particles will get trapped before they reach the collecting filter and thus giving a cut off size dependent of the size, geometry and air flow velocity through the cyclone.
Cyclones (often) have the advantage of being both small and cheap but still accurate enough for most modern requirements.
Virtual Dichotomous Impactor
This type of impactor uses a
virtual impaction stage to separate between sizes, letting the particles
continues down to be collected on a filter. It is a two-stage impactor (hence
the name dichotomous) dividing the particles into two size ranges, 10 – 2.5 mm and less than 2.5 mm.
Cascade Impactors
Cascade
impactors are consists of a number of impactor stages connected in series with
smaller and smaller cut-off diameter. The cut-off diameter in each stage
depends on the air velocity and geometry of the stage (i.e. the distance from
the nozzle to the impaction plate). Cascade impactors often have up to some ten
stages ranging from a cut-off diameter on the first stage of 10 – 30 mm to a diameter of 0.1 mm or
lower on the backup filter in the end. This gives the opportunity to analyse
(e.g. chemical or gravimetrical) a number of small size intervals. Some
drawbacks are the risk of bounce off from one stage to the next (i.e. particles
of wrong size at some of the stages) as well as the problem of obtaining sharp
cut-off diameters in the last stages (cut-off diameter less than 0.1 – 0.2 mm). Coating the impaction plates with oil or some other sticky
substance, which catches the particles more effectively, can reduce the risk of
bounce of. This will then prevent or severely complicate direct mass
concentration calculations of the different stages.
Particle Counters
Aerodynamic Particle Sizer (APS)
The APS is used for measurement of size distributions of particles in the size range 0.5–20 mm aerodynamic diameter. The instrument both counts and measures the size of the particles using the properties of the particle (light scattering and settling velocity).
Condensation Particle Counter (CPC)
The CPC counts particles with a diameter of a few nanometres up to about one micrometer. It is very hard to optically detect sub micron particles since the diameter is equal or smaller than the wavelength of light; the diameter of the particles has to be increased before detection. This is accomplished by letting the particles pass through a “cloud” of evaporated alcohol witch attach to the particle and makes much larger and easy to detect/count. This uncontrolled increase makes it impossible to size classify the particle.
Differential Mobility Particle Sizer (DMPS)
Scanning Mobility Particle Sizer (SMPS)
The DMPS and SMPS are basically the same type of instrument with only different electronic control mechanism. It consists of a DMA (Differential Mobility Analyser), a CPC and some electronic controlling devices, everything controlled by a PC. As mentioned before, the CPC is unable to size classify the particles since it cannot determine the original size before the “cloud”. This is now taken care of by the DMA, an apparatus capable of selecting only a very narrow size fraction of particles that then enters the CPC. The CPC counts the number of particles as usual and since the size is already determined, information of both concentration and size is obtained. By changing the setting of the DMA, scanning over the whole particle size interval (3 nm to 1 mm) is possible.
Other methods
Electrical Low Pressure Impactor (ELPI)
The ELPI enables real time particle size distribution and concentration measurement in the size range from 30 nm up to 10 mm. It is intended for any application where a wide size range and fast response time are required.
The ELPI is based on combining electrical detection principles with
low-pressure impactor size classification. The electric current carried by
charged particles into each impactor stage is measured in real-time by a
sensitive multichannel electrometer. By using this instrument it is possible to
measure size distribution and concentration even when the concentration varies
rapidly.
Tapered-Element Oscillating Microbalance (TEOM)
The TEOM is a real-time instrument for measuring the mass concentration of particulate matter smaller than 10 mm in ambient air.
The instrument uses a vibrating collection substrate to collect the particles. The change in “load” or sample mass will change the frequency of oscillation and that is used to calculate the mass concentration.
The TEOM can measure particulate concentration in the range from 5 mg/m3 up to several mg/m3.
Aerosol Time Of Flight Mass Spectrometer (ATOMFS)
ATOFMS is the first instrument to offer the
ability to determine size and chemical composition of individual particles in
near real-time.
The instrument detects and analyse particles in
the size range from 0.3 to 3 mm. It can
be described as an APS followed by a time-of flight mass spectrometer.
A selection of links
http://www.anderseninstruments.com/
http://www.tsi.com/particle/homepage/particlehome.htm
Aerosol Analysis
When the aerosol is collected a number of different methods and techniques exists to get more information about the aerosol. The simplest is to weigh the sample to get the mass concentration, but often more information is needed. Depending of the questions addressed, the way of collection method, available instruments and a number of other aspects (money!!!) different analysis options are of interest.
Below is as short list of some of the more common analytical techniques in the field.
|
Analytical method |
Acronym |
Type of Information |
|
Energy Dispersive X-Ray Fluorescence
spectrometry |
EDXRF |
Elemental |
|
Total Reflection X-Ray Fluorescence
spectrometry |
TXRF |
Elemental |
|
Atomic Absorption Spectrometry |
AAS |
Elemental |
|
Mass Spectrometry |
MS |
|
|
Inter
Coupled Plasma MS |
ICP-MS |
Elemental |
|
Gas
Chromatography (-MS) |
GS or GC-MS |
Organic Compounds |
|
Neutron Activation |
NA |
Elemental |
|
Anodic Stripping Voltammetry |
ASV |
Elemental |
|
Electron Spectroscopy (for chem.. analysis) |
ESCA |
Elemental |
|
Ion Chromatography |
IC |
Inorganic compounds |
|
Selective Ion Electrodes |
- |
Elemental |
|
Colorimetry |
- |
Inorganic compounds |
|
IR-, UV- Spectroscopy |
- |
compounds |