L-35

Ambient air quality monitoring and High volume sampler

Unit-V

Instruments needed for sampling

Sr. No

Parameter Instrument used

Measuring unit

1 Temperature Thermometer 0C

2 Wind speed Anemometer m/sec

3 Relative humidity

Hygrometer No unit (Expressedin percentage)

4 Arm. pressure

Barometer Millibars or mm of Hg

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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 alterthe chemical or physicalcharacteristics of the sample in anundesirable manner.

The major components of most

sampling systems are:

1) An inlet manifold

2) An Air mover (Blower)

3) A collection medium

4 ) Flow measurement device

(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).

(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 SAMPLING METHODS

• Gravitational method

• For collecting dust particles of 1µ (micron) or larger 40 µ (micron ) in the atmosphere, clean glass jars are kept in the area where dust fall is to be determined and after a few hours or days, the dust is collected from each jar and then weighed.

• The average weight of dust in each jar is estimated and the dust fall is expressed as weight of dust per unit area per unit time. 15-10-2013Page 8

• 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 are collected and brought into the laboratory where their contents are analysed.

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• In most cases only the total particulate matter is determined, and the results are expressed in terms of tons per square kilometer per month or g/sq. m / month.

• Monthly isopleth maps can be constructed showing the variation of dust fall throughout the area.

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Dust fall jar

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Dust depositor

Filtration • A fibrous filter which is woven with randomly

oriented fibers acts as a target for the collection of fine particles. When airborne particles enter a filter and flow around the fibers, they are subjected to aerodynamic forces which result in their collection on the fibers.

• The principal mechanisms operating are gravitation, inertia, interception and diffusion.

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• Many types of filters are available for removal of particulate matter from gas streams.

• The chief variations are in the filter media material and in the shape of the membrane.

• One common type is the thimble filter shown diagrammatically.

• The filter paper thimble is filled with well-fluffed cotton which operates at a sampling rate of 2 cf/min.

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• By determining the change in weight of the dried filter after exposure, dust concentration in the gas stream is determined.

• The paper thimble cannot be used with high temperature gases; an alundum thimble is useful in this case.

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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.

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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). 15-10-2013Page 21

• Cascade impactors often have up to some ten stages ranging from a cut-off diameter on the first stage of 10 – 30 m to a diameter of 0.1 m 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.

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• 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 m).

• 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.

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Electrostatic precipitators • An electrostatic precipitator consists of an

ionizing electrode charged with a high negative potential and a collector to be maintained at positive 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 the electrode and adhere to the positively charged collecting tube.

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• The collected particles are weighed and their number can also be counted accurately by using a microscope.

• This method of collection of particulate matter is very efficient as high flow rates with small pressure drops can be used.

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Thermal precipitator• In thermal precipitators the particles in the

sampled stream move past a very hot wire and are repulsed to the nearby cold plate where they are collected.

• However, it can operate only at very low sampling rates-approximately 50 mililitresper minute and is useful only for R&D work. Figure shows a thermal precipitator of the type 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 basic instruments used to monitor Ambient Air Quality.

• They are in widespread use all over the world to measure air pollution in industrial areas, urban areas, on the shop floor, near monuments and other sensitive areas.

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• The High Volume Sampler is a vital tool for studies relating to impact of industrialisation to the air analysis, and for work related diseases of the respiratory system to air pollution.

• These are very much essential for various Environmental Impact Assessment studies

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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 against voltage fluctuation,

• Detachable Gable roof- allows passage of air and protects filter. 15-10-2013Page 34

PRINCIPLE OF OPERATION• In these samplers, air-borne suspended

particulates (SPM) are measured by passing air at a high flow-rate of 1.1 to 1.7 cubic meters per minute through a high efficiency filter paper which retains the particles.

• The instrument measures the volume of air sampled, while the amount of particulates collected is determined by measuring the change in weight of the filter paper as a consequence of the sampling.

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• The passage for air reaching the filter is designed to prevent heavier settleable dust particles from reaching the filter (by provision of cyclone) thus measuring the concentration of Suspended Particulate Matter (SPM) in atmospheric air.

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• In high volume sampler provisions have been made for simultaneous sampling of gaseous pollutants.

• Here the air is passed through suitable reagents that would absorb specific gases where gaseous pollutants like SO2, NOx, Cl2, H2S, CS2, NH3, etc. are analysed subsequently by simple wet chemistry method to determine the concentration of specific pollutant.

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Applications of HVS

• Routine Monitoring by air monitoring networks

• Open Spaces like forests and national park air monitoring.

• Monitoring around ecologically sensitive monuments

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• Data reporting for monthly and yearly averages by local area air quality 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 with sketch. (May 2011, 8 marks).

Q3. Explain various principles used in particulate matter sampling. (May 2011, 8 marks)

Q4. Explain procedure for determining concentration of SPM in ambient air.

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