In concentration methods, the PM concentration can be in mass (m), number (N) and surface area (S). These instruments are based in different measuring principles, and can be gravimetric, optical, microbalance, and electrical charge.
Figure 1.
Methods and instruments for PM measurement.
3.1.1. Gravimetric Method
In the gravimetric method, the particle mass concentration is determined by weighing the filters before and after the sampling period.
Nussbaumer
et al. [
21] mentioned that the basic method to measure off-line PM mass concentrations, in combustion gases, is the gravimetric sampling in filters.
Giechaskiel
et al. [
24] described that the filter collects PM in all granulometric fractions (nucleation, accumulation, and coarse modes), unless there is a cyclone or impactor to remove larger particles. Nussbaumer
et al. [
21] cited the use of a set with pre-cyclones, which cut-off is of 10 µm or 2.5 µm, with an option to determine mass concentration.
Particle sampling, in filters, results in a resolution time of 15 min or more, therefore the identification of fast processes is not possible. However, particles collected in the filter can be analyzed chemically, as affirmed by Nussbaumer
et al. [
21].
Determination of PM mass can be altered, depending on the conditioning conditions of the filter. For this reason, Nussbaumer
et al. [
21] and Giechaskiel
et al. [
24] emphasized that the filters are typically packed under controlled conditions of temperature and relative humidity.
The gravimetric method is based on filters and Cascade Impactors. It can collect particles and evaluate their concentration. For more detailed analysis, other techniques are necessary, such as Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM).
Among the gravimetric instruments for measuring PM mass, Giechaskiel
et al. [
24] mentioned the Cascade Impactor. According to these authors such equipment are most used in the investigation of particle size distribution in mass. Cascade Impactors are frequently used as components of the systems that involve Size Distribution Methods. In this way, more information about these instruments were made available in
Section 3.2.2.
3.1.2. Optical Methods
In the optical detection methods, aerosol particles are lit by a light beam and irradiate this light in all directions (scattering). Part of this light is simultaneously transformed in other energy forms (absorption), according to the description by Giechaskiel
et al. [
24]. As stated by these authors, extinction of the light can be calculated by the addition of scattering and absorption.
Optical instruments used for measuring particle concentration, in real time, can be based in the principles of scattering, absorption, and light extinction.
(a) Light scattering
Light scattering are classified as light dispersion by single particles, and scattering photometer by an ensemble of particles.
According to Giechaskiel
et al. [
24], the instruments of light scattering by an ensemble of particles include dispersion photometers which measure the intensity of scattered light in one or more angles. In a scattering photometer, the scattering light is measured by employing a photometer detector. Hinds [
29] described that the light scattering photometers measure the scattered light from a combination of all the particles present in the optical detection volume. The authors emphasized that most commercial light scattering instruments use visible light (~600 nm) and measuring angles of 90°, 45°, or less than 30°. Among the scattering photometers, Vincent [
26] mentioned the Respirable Aerosol Monitor (RAM). In the original version, the aerosol was aspirated with the help of a pump, and then passed through a cyclone that separated the respirable aerosol fraction. The aerosol entered the optical sensor zone, where the infrared light scattered in an angle of 45° to 90°, and the aerosol was detected by a photodiode. The author cited a new automated digital version of the instrument, the DataRam 4, which has the ability of data registry. This equipment presents concentration measurements and particle average size, besides other environmental information such as temperature and relative humidity.
Costa
et al. [
17], in their laboratory and field experiments, used a DataRam 4 to sample PM
2.5. According to them, this equipment is compact and performs sampling with data storage made continuously. DataRam 4 showed a good correlation with other instrument used to measure size and particle concentration. Chowdhury
et al. [
30] used a scattering photometer developed by University of California. This equipment is known as UCB-PATS (University of California Berkeley-Particle and Temperature Sensors). According to their description, the UCB-PATS use smoke detector technology, which combines chambers of photoelectric sensors (of light dispersion) and ionization (loss of ions by particles in suspension). This combination guarantees precise measurements of fine particles. The light dispersion chamber uses a light emitting diode (LED), with a wavelength of 880 nm, and a photodiode that measures the light intensity scattered in an angle of 45°. Even though the UCB does not select particles, using a device of traditional cut-off size as the cyclones, the photoelectric sensor is more sensitive to particles smaller than 2.5 µm aerodynamic diameter and the ionization sensor is more sensitive to PM
1, as emphasized by the authors. Another scattering photometer is the DustTrak. This equipment was employed by Prado
et al. [
31] and Chowdhury
et al. [
30]. Chowdhury
et al. [
30] stated that this equipment is a portable light dispersion photometer by laser. In the DustTrak, the measurement of particle mass is obtained in real time. The aerosol is isolated in the optical chamber. In this way, the chamber is kept clean, which guarantees greater reliability in the measurements and low equipment maintenance.
With regards to the method of light scattering by single particles, Giechaskiel
et al. [
24] mentioned the Optical Particle Counter (OPC) as being the most used instrument. OPCs use a light source, normally a diode laser, to light a sample of particles in a given angle. A photodetector measures the light that scattered from the particles. Based on the intensity of the flash, particles can be counted and measured at the same time. For Giechaskiel
et al. [
24] OPCs are similar to Scattering Photometer. According to them, the main difference is that the OPC optical detection volume is smaller in relation to the Scattering Photometer, in such a way that only one particle is lit at once. The scattered light is detected by a photodetector as an electric pulse. The particle size is determined from the height of the electric pulse, using a calibration curve. The minimum limit for detection size is of particles with diameter above 100 nm.
An example of OPC is the SidePak Personal Aerosol Monitor model AM510 (TSI, Shoreview, MN, USA) used by Jiang and Bell [
32] to evaluate PM
2.5 concentration. According to these authors, the aerosol sample is aspirated to inside the collection chamber, in a continuous sequence. A laser lights one part of the aerosol flow. One 90° lens collects the light scattered by the particles and focus this light over a photodetector. The detection circuit converts the light in voltage, which is proportional to the aerosol concentration in mass.
The Condensation Particle Counters (CPCs) are also classified as light scattering counters. These counters are employed to measure the concentration of small particles. These particles do not scatter light sufficiently, in a way that conventional optical counters cannot detect this scattering. In the CPCs, small particles have their size increased by condensation of the produced vapor, from a working fluid, according to what is described by Giechaskiel
et al. [
24]. When the particles are enlarged by condensation, the CPC becomes similar to optical particle counters. In this manner, individual drops go through the focal point of a laser beam, with a flash of light. Each light flash is counted as one particle. The complexity of CPCs is in the technique to condensate vapor over the particles. When the vapor around the particles reach a certain degree of supersaturation, the vapor starts to condensate in the particles. The magnitude of supersaturation determines the minimum detectable particle of the CPC. During sampling of particles from burning wood chips, Leskinen
et al. [
20] and Torvela
et al. [
33] used a particle counter by condensation.
(b) Light absorption
Measuring instruments based in the principle of light absorption are used to measure the concentration of black carbon (BC), which composes the aerosol.
BC strongly absorbs light and is therefore a positive radiative agent, which contributes to climate changes and has been broadly investigated in atmospheric studies as described by Giechaskiel
et al. [
24]. Lack
et al. [
34] affirmed that light absorption by aerosols is one of the most uncertain parameters associated with direct and indirect effects of aerosols in the climate. According to the authors, light absorption is one of the most difficult parameters to measure.
Among the employed techniques that are based on aerosol absorption measurement, Giechaskiel
et al. [
24] mentioned (i) the difference method, in which the absorption is obtained from the difference between extinction and scattering, (ii) the methods based in filters that measure light attenuation by the PM collected in a filter, (iii) the methods based on photoacoustic spectroscopy and (iv) the methods based on Laser Induced Incandescence (LII). The last two methods measure BC through particle heating. The heating is caused due to light absorption by the particles.
(i) Spotmeters—These equipments are also known as reflectometers or smoke filter meters, due to the light absorption measuring principle based on light reflection over a filter. In a Spotmeter, the concentration of particles is obtained by filtering the exhaust gas in a paper filter, and recording of the ratio between the light reflected by this exposed spot and a non-exposed spot, as explained by Giechaskiel
et al. [
24].
(ii) Aethalometer—As with Spotmeters, the Aethalometers are instruments used to determine BC concentrations. According to Giechaskiel
et al. [
24] and Krecl
et al. [
35], PM is collected in Aethalometers using a filter of quartz fiber. A change in light transmission (absorption) is measured in the filter, in several wavelengths. For Krecl
et al. [
35], the Aethalometer is one of the optical instruments based in filters, which is most used to determine the content of light absorbing carbon (LAC), besides the Particle Soot Absorption Photometer (PSAP). However, Giechaskiel
et al. [
24] pointed out that the conventional Aethalometers have a time resolution of several minutes, which is useful for environment monitoring, but slow for transitory emission tests. Versions with time resolution of 1 to 10 s became available more recently.
PSAP and Aethalometer measuring principle is the same, as stated by Krecl
et al. [
35]. According to the authors, these equipments are based on properties of light absorption by carbonated aerosols. They measure the attenuation of light transmitted through particles that are continuously collected in a filter. In the experiments by Krecl
et al. [
35], an Aethalometer series 8100 (Magee Scientific, Berkeley, CA, USA) was used to determine the concentration in mass of LAC in PM
1. The aerosol deposited in the filter were lit by a 880 nm LED and a 525 nm LED, for measures using the Aethalometer and the PSAP, respectively.
Gong
et al. [
5] used a seven-wavelength aethalometer to measurement of the mass concentration of BC and aerosol absorption coefficient.
(iii) Photoacoustic Soot Sensor (PASS)—Light absorbing particles contained in the aerosol samples are periodically heated by absorption of amplitude-modulated light. According to Giechaskiel
et al. [
24], the heat conducted from the particles to the surrounding gas generates acoustic pressure waves that are registered by a microphone. The registered signal is proportional to the concentration in volume of light-absorbing particles <300 nm, and it is proportional to the surface of larger particles (>300 nm). Lack
et al. [
34] developed a very sensitive method to measure aerosol absorption in 532 nm, with an excellent response time, and they used photoacoustic absorption spectroscopy.
(iv) Laser Induced Incandescence (LII)—According to Giechaskiel
et al. [
24], in LII, particles are heated right below the carbon sublimation temperature. Particle heating is done by a short laser pulse. After heating, particles reach incandescence and are decomposed. Particle decomposition is measured by a photomultiplier. According to Santoro and Shaddix [
36] the incandescence intensity and decomposition rate are analyzed to derive the number and average size of primary particles, and then the soot volume.
(c) Light extinction
In order to measure light extinction in aerosols, Mellon
et al. [
37] and Pettersson
et al. [
38] developed in laboratory a system for (i) Cavity Ring Down (CRD). Other equipment that measure light extinction is the (ii) Opacity Meter. These meters are broadly used to measure particles in diesel engines, as Öztürk [
39] studies.
In the CRD experimental system developed by Pettersson
et al. [
38], aerosols were generated by atomization and drying. Dried particles were selected in size using a Differential Mobility Analyzer (DMA). Selected particles were counted in the CRD cell exit, with a counter for condensing particles (TSI, model 3022A).
Giechaskiel
et al. [
24] described that Opacity Meters measure the light fraction transmitted through an exhaust volume. Light extinction (opacity) occurs due to adsorption and scattering, in such a way that opacity is the difference between incident and transmitted light. As stated by the authors, measurements based on light extinction quantify particle concentration depends on path length and light wavelength, as well as the particle shape and its composition.