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L-35
Ambient air quality monitoring and
High volume samplerUnit-V
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The Aim of Sampling:The principal requirement of asampling system is to obtain a samplethat is representative of theatmosphere at a particular place andtime and that can be evaluated as amass or volume concentration.
The sampling system should not alter the chemical or physicalcharacteristics of the sample in an
undesirable manner.
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The major components of most
sampling systems are:
1) An inlet manifold
2) An Air mover (Blower)
3) A collection medium4 ) Flow measurement device
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(1)The inlet manifold transports the
material from the ambient atmosphere to
the collection medium or analytical
device in an unaltered condition, all inlet
of ambient air must be rainproof.
(2)The air mover (Blower) provides the force
to create a vacuum or lower pressure at
the end of the sampling system (pumps).
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(3) The collecting medium, may be
solid or liquid sorbent for dissolving gases a filter surface for
collecting particles.
(4)The flow device measures the
volume of air associated with the
sampling system.
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PARTICULATE SAMPLINGMETHODS
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Gravitational method For collecting dust particles of 1
(micron) or larger 40 (micron ) inthe atmosphere, clean glass jars are keptin the area where dust fall is to be
determined and after a few hours or days,the dust is collected from each jar andthen weighed.
The average weight of dust in each jaris estimated and the dust fall isexpressed as weight of dust per unitarea per unit time. 15-10-2013Page 8
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Containers, generally conical plastic jars,10-15 cm in diameter , open at the top
are used. The jars are kept in strategic locations
throughout a community or in the vicinity
of particulate sources under study. Grit and dust fall into the jars which
sometimes have water to hold the dust. After a one month exposure, the jars arecollected and brought into the laboratorywhere their contents are analysed.
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In most cases only the totalparticulate matter is determined, and
the results are expressed in terms oftons per square kilometer permonth or g/sq. m / month .
Monthly isopleth maps can beconstructed showing the variation of
dust fall throughout the area.
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Dustfall
jar
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Dustdepositor
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Filtration A fibrous filter which is woven with randomly
oriented fibers acts as a target for thecollection of fine particles. When airborneparticles enter a filter and flow around thefibers, they are subjected to aerodynamicforces which result in their collection on thefibers.
The principal mechanisms operating aregravitation, inertia, interception and diffusion.
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By determining the change inweight of the dried filter after
exposure, dust concentration in thegas stream is determined.
The paper thimble cannot be usedwith high temperature gases; analundum thimble is useful in this
case.
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Cascade Impactors
Cascade impactors are consists of anumber of impactor stages connectedin series with smaller and smaller cut-off diameter.
The cut-off diameter in each stage
depends on the air velocity andgeometry of the stage (i.e. thedistance from the nozzle to theimpaction plate). 15-10-2013Page 21
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Cascade impactors often have up tosome ten stages ranging from a
cut-off diameter on the first stageof 10 30 m to a diameter of 0.1
m or lower on the backup filter inthe end.
This gives the opportunity to
analyse (e.g. chemical orgravimetrical) a number of smallsize intervals.
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Some drawbacks are the risk of bounce offfrom one stage to the next (i.e. particles ofwrong size at some of the stages) as well asthe problem of obtaining sharp cut-offdiameters in the last stages (cut-off diameterless than 0.1 0.2 m).
Coating the impaction plates with oil or someother sticky substance, which catches theparticles more effectively, can reduce the risk
of bounce of. This will then prevent or severely complicate
direct mass concentration calculations of the
different stages. 15-10-2013Page 23
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Electrostatic precipitators An electrostatic precipitator consists of an
ionizing electrode charged with a high negativepotential and a collector to be maintained atpositive potential produced by a special
transformer and rectifier. A known volume of air is allowed to pass
through the precipitator where the incoming
particles become negatively charged by theelectrode and adhere to the positively chargedcollecting tube.
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The collected particles are weighed andtheir number can also be countedaccurately by using a microscope.
This method of collection of particulatematter is very efficient as high flowrates with small pressure drops can beused.
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Thermal precipitator In thermal precipitators the particles in the
sampled stream move past a very hot wireand are repulsed to the nearby cold platewhere they are collected.
However, it can operate only at very lowsampling rates-approximately 50 mililitresper minute and is useful only for R&D work.
Figure shows a thermal precipitator of thetype supplied by Casella Company Ltd.,London, England.
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L-36High Volume Sampler
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INTRODUCTION
High Volume Samplers are the basicinstruments used to monitor Ambient AirQuality.
They are in widespread use all over theworld to measure air pollution in industrialareas, urban areas, on the shop floor,
near monuments and other sensitiveareas.
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Th Hi h V l S l i i l l
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The High Volume Sampler is a vital toolfor studies relating to impact of
industrialisation to the air analysis, andfor work related diseases of therespiratory system to air pollution.
These are very much essential forvarious Environmental Impact Assessment studies
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C t f HVS
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Components of HVS Heavy Duty Blower,
Orifice flow meter- measures flow, Time Totaliser-records time, Programmable Timer- measures time,
Instrument Cabinet- acts as protection, Filter Holder assembly- holds the filter in
position,
Voltage Stabiliser- guards againstvoltage fluctuation,
Detachable Gable roof- allows passage of
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The passage for air reaching the filter isdesigned to prevent heavier settleable dust
particles from reaching the filter (by provisionof cyclone) thus measuring the concentration ofSuspended Particulate Matter (SPM) in
atmospheric air.
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I high l l i i h
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In high volume sampler provisions havebeen made for simultaneous sampling of
gaseous pollutants. Here the air is passed through suitablereagents that would absorb specific gases
where gaseous pollutants like SO 2, NOx,Cl2, H2S, CS 2, NH3, etc. are analysedsubsequently by simple wet chemistry
method to determine the concentration ofspecific pollutant.
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Applications of HVS
Routine Monitoring by air monitoringnetworks
Open Spaces like forests and national parkair monitoring.
Monitoring around ecologically sensitivemonuments
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Data reporting for monthly andyearly averages by local area airquality networks.
Site Monitoring by industries. Evaluative Studies
Lab Applications Research Studies
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Calculation of SPM concentration
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Theory QuestionsQ1. State principles used for sampling of
particulate matter. Explain any one in detail.(Nov 2008, May 2009, 8 marks)
Q2. Explain working of high volume sampler withsketch. (May 2011, 8 marks).
Q3. Explain various principles used in particulatematter sampling. (May 2011, 8 marks)
Q4. Explain procedure for determiningconcentration of SPM in ambient air.
