The high-volume (Hi-Vol) sampler is the workhorse of air sampling and monitoring.The sampler uses a continuous duty blower to suck in an air stream. When fitted with a particle size classifier, it separates particles greater than 10μm size from the air stream. The air stream is then passed through a filter paper to collect particles lesser than 10μm size (PM10). Gravimetric measurements yield values of suspended particulate matter (SPM), as the sum of the two fractions, and PM10, the material retained on the filter paper.The filter paper can be used to determine benzene-soluble organics, metals, such as Pb, Cd, etc., fluorides, radioactive materials and biologically active non- metals, sulphate, nitrate and ammonium.
The sampler can also be used to sample gaseous pollutants. A stream of unfiltered air is bubbled through a reagent, which either reacts chemically with the gas of interest or into which the gas is dissolved. Wet chemical techniques are then used to measure the concentration of the gas.
The following sections describe various components of the sampler, particularly with reference to 'Envirotech Respirable Dust Sampler (RDS), Model APM 460 NL'. The sampler design is based on the know how developed at National Environmental Engineering Research Institute (NEERI), Council of Scientific and Industrial Research (CSIR), Govt. of India. The sampler is widely used in India in the National Air Quality Monitoring Programme (NAMP) of the country.
1(a)Particle size classifier
SPM in air is usually very irregular in shape. While liquid particles present in mists and sprays tend to be spherical, the solid particles making up most dust and fumes fall into one of the three general classes: granular, flaky and needlelike. For describing such agglomeration of particles, the concept of equivalent sizes is used. The equivalent size of a particle is determined on the basis of equivalent behaviour, such as settling velocity, of an ideal spherical particle.Aerodynamic diameter is defined as the diameter of a spherical particle of density 1 g/cm3, having a settling velocity equal to that of the particle in question. As discussed earlier, the respirable or thoracic particles are found to have diameter less than 10μm. This diameter for PM10 refers to the aerodynamic diameter.
The APM 460 NL Hi-vol sampler uses a cyclone type particle size classifier to separate out particles of diameter greater than 10 mm aerodynamic diameter. Ambient air laden with SPM enters the cyclone near its top where the air stream is given a swirling motion. The resulting centrifugal acceleration moves the coarse and heavier air borne particles to the outer wall. These separated particles fall
through the cyclone conical hopper and are collected in the sampling bottle placed at
the bottom. The air containing the respirable dust exits through a cylindrical outlet, mounted
concentrically at the top of the cyclone, and enters the filter assembly.
1(b) Filter assembly
The filter assembly consists of two parts, a top cover connected to the outlet port of the cyclone and a filter adapter with a backing screen for the filter paper. A rectangular 20.3 cm x 25.4 cm (8 in x 10 in) glass fibre filter paper is placed on the backing screen and the top cover is bolted. Suitable gasket is used to obtain an airtight connection. The collected matter on the filter is
classified as PM10.
NEERI has analysed the collected PM1o for a variety of dusts like coal, limestone, cement, fly-ash, etc., for their size. Table below gives a summary of their findings.
S.No. Nature of dust Specific gravity Maximum size range,mm
1. Coal 1.4-1.8 10-12
2. Road side 2.0 - 2.2 8 -10
3. Fly-ash 2.3 - 2.4 8-9
4. Mining limestone/sandstone 2.5 - 2.7 7-9
5. Cement 3.0 - 3.3 6-8
6. Manganese ore 3.5-4.0 5-7
It is seen from the reported data that the particulate matter collected as PM10 has a maximum size range of 7-12 mm, S.No.1 to 4. For heavier particles of specific gravity 3.0 to 4.0, S. No.5 & 6, the maximum size range is 5 to 8 mm. This implies that for these cases, the heavier particles in the size range of 10mm were retained in the cyclone. Taking into account the heterogeneity of the characteristics of SPM, it is possible that the cut-off level is not exactly met in a practical field situation.
1(c) Air blower and flow measurement
A standard regenerative blower is used for sucking the ambient air through the High-Vol sampler. The blower is capable of maintaining a flow rate in the range of 0.9 to 1.4 m3/min through the assembly. Airflow is measured from the pressure drop across a calibrated orifice which is built in the lower part of the filter adapter. The manometer recording the pressure drop is graduated in terms of flow rate in m3/min.
A time totaliser is provided to record the period of sampling. It has been wired in such a way that it operates only when the blower receives power. Thus it records the true time in hours for which the sampler samples the air. Readings must be noted before and after each sampling occasion to determine the duration of the sampling. The total reading on the totaliser also facilitates timely preventive maintenance.
2. Transmissometer
A transmissometer is an instrument for measuring the extinction coefficient of the atmosphere, and for the determination of visual range. It operates by sending a narrow, collimated beam of energy (usually a laser) through the propagation medium. A narrow field of view receiver at the designated measurement distance determines how much energy is arriving at the detector, and determines the path transmission and/or extinction coefficient. Atmospheric extenction is wavelength dependent phenomenon, but the most common wavelength in use for transmissometers is 550 nm, which is right in the middle of the visible waveband, and allows a good approximation of visual range.
3. Crystal Mass Balance
Quartz is one member of a family of crystals that experience the piezoelectric effect. The piezoelectric effect has found applications in high power sources, sensors, actuators, frequency standards, motors, etc., and the relationship between applied voltage and mechanical deformation is well known; this allows probing an acoustic resonance by electrical means. Applying alternating current to the quartz crystal will induce oscillations. With an alternating current between the electrodes of a properly cut crystal, a standing shear wave is generated. The Q factor, which is the ratio of frequency and bandwidth, can be as high 106. Such a narrow resonance leads to highly stable oscillators and a high accuracy in the determination of the resonance frequency. The QCM exploits this ease and precision for sensing. Common equipment allows resolution down to 1 Hz on crystals with a fundamental resonant frequency in the 4 – 6 MHz range. A typical setup for the QCM contains water cooling tubes, the retaining unit, frequency sensing equipment through a microdot feed-through, an oscillation source, and a measurement and recording device. The frequency of oscillation of the quartz crystal is partially dependent on the thickness of the crystal. During normal operation, all the other influencing variables remain constant; thus a change in thickness correlates directly to a change in frequency. As mass is deposited on the surface of the crystal, the thickness increases; consequently the frequency of oscillation decreases from the initial value. With some simplifying assumptions, this frequency change can be quantified and correlated precisely to the mass change using Sauerbrey's equation.
f0 – Resonant frequency (Hz)
Δf – Frequency change (Hz)
n – Harmonic number (n=1, 2, 3... etc. Usually n=1)
Δm – Mass change (g)
A – Piezoelectrically active crystal area (Area between electrodes, m2)
ρq – Density of quartz (ρq = 2.648 g/cm3)
μq – Shear modulus of quartz for AT-cut crystal (μq = 2.947x1011 g/cm.s2)
νq – Transverse wave velocity in quartz (m/s)
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