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DEPARTMENT OF MINING ENGINEERING

NATIONAL INSTITUTE OF TECHNOLOGY, ROURKELA-8

Seminar and Technical Writing

Topic: Monitoring and Control of Air Pollution

________________________________________________________________________________

Submitted by:

Balaji Vemana

Roll : 108MN025 

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Contents

Topic1.  Introduction : Air pollution

2.  Moitoring of Air Pollution

a)  Introduction b)  Monitoring devices

i)  Manual methods

ii)  Instrumental methods

c)  Suspended particulate monitoring methodsd)  Special monitoring methods

3.  Control of air pollution

a)  Introductionb)  Control devices

i)  Methods for control of gaseous pollutantsii)  Control of Soxiii)  Control and Treatment of VOC (Volatile Organic Compounds) and

Hydrocarbons

4.  References

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Air pollution 

Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harmor discomfort to humans or other living organisms, or cause damage to the natural environment or builtenvironment, into the atmosphere. 

The atmosphere is a complex dynamic natural gaseous system that is essential to support life on planetEarth. Stratospheric ozone depletion due to air pollution has long been recognized as a threat to humanhealth as well as to the Earth's ecosystems

Pollutants

A substance in the air that can cause harm to humans and the environment is known as an air pollutant.Pollutants can be in the form of solid particles, liquid droplets, or gases. In addition, they may be naturalor man-made.

Pollutants can be classified as primary or secondary. Usually, primary pollutants are directly emitted from

a process, such as ash from a volcanic eruption, the carbon monoxide gas from a motor vehicle exhaust orsulfur dioxide released from factories. Secondary pollutants are not emitted directly. Rather, they form inthe air when primary pollutants react or interact. An important example of a secondary pollutant is groundlevel ozone — one of the many secondary pollutants that make up photochemical smog. Some pollutantsmay be both primary and secondary: that is, they are both emitted directly and formed from other primarypollutants.

Major primary pollutants produced by human activity include:

  Sulphur oxides (SOx) - especially sulfur dioxide, a chemical compound with the formula SO 2.SO2 is produced by volcanoes and in various industrial processes. Since coal and petroleum oftencontain sulfur compounds, their combustion generates sulfur dioxide. Further oxidation of SO2,

usually in the presence of a catalyst such as NO2, forms H2SO4, and thus acid rain.[2] This is oneof the causes for concern over the environmental impact of the use of these fuels as powersources.

  Nitrogen oxides (NOx) - especially nitrogen dioxide are emitted from high temperaturecombustion, and are also produced naturally during thunderstorms by electrical discharge. Can beseen as the brown haze dome above or plume downwind of cities. Nitrogen dioxide is thechemical compound with the formula NO2. It is one of the several nitrogen oxides. This reddish-brown toxic gas has a characteristic sharp, biting odor. NO 2 is one of the most prominent airpollutants.

  Carbon monoxide (CO)- is a colourless, odorless, non-irritating but very poisonous gas. It is aproduct by incomplete combustion of fuel such as natural gas, coal or wood. Vehicular exhaust isa major source of carbon monoxide.

  Carbon dioxide (CO2) - a colourless, odorless, non-toxic greenhouse gas also associated withocean acidification, emitted from sources such as combustion, cement production, andrespiration. It is otherwise recycled in the atmosphere in the carbon cycle. 

  Volatile organic compounds - VOCs are an important outdoor air pollutant. In this field they areoften divided into the separate categories of methane (CH4) and non-methane (NMVOCs).Methane is an extremely efficient greenhouse gas which contributes to enhanced global warming.Other hydrocarbon VOCs are also significant greenhouse gases via their role in creating ozoneand in prolonging the life of methane in the atmosphere, although the effect varies depending onlocal air quality. Within the NMVOCs, the aromatic compounds benzene, toluene and xylene are

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suspected carcinogens and may lead to leukemia through prolonged exposure. 1,3-butadiene isanother dangerous compound which is often associated with industrial uses.

  Particulate matter - Particulates, alternatively referred to as particulate matter (PM) or fineparticles, are tiny particles of solid or liquid suspended in a gas. In contrast, aerosol refers toparticles and the gas together. Sources of particulate matter can be manmade or natural. Someparticulates occur naturally, originating from volcanoes, dust storms, forest and grassland fires,

living vegetation, and sea spray. Human activities, such as the burning of fossil fuels in vehicles,power plants and various industrial processes also generate significant amounts of aerosols.Averaged over the globe, anthropogenic aerosols — those made by human activities — currentlyaccount for about 10 percent of the total amount of aerosols in our atmosphere. Increased levelsof fine particles in the air are linked to health hazards such as heart disease,[3]  altered lungfunction and lung cancer.

  Persistent free radicals connected to airborne fine particles could cause cardiopulmonary disease.  Toxic metals, such as lead, cadmium and copper.   Chlorofluorocarbons (CFCs) - harmful to the ozone layer emitted from products currently banned

from use.  Ammonia (NH3) - emitted from agricultural processes. Ammonia is a compound with the formula

NH3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes

significantly to the nutritional needs of terrestrial organisms by serving as a precursor tofoodstuffs and fertilizers. Ammonia, either directly or indirectly, is also a building block for thesynthesis of many pharmaceuticals. Although in wide use, ammonia is both caustic andhazardous.

  Odors — such as from garbage, sewage, and industrial processes  Radioactive pollutants - produced by nuclear explosions, nuclear events, war explosives, and

natural processes such as the radioactive decay of radon. 

Secondary pollutants include:

  Particulate matter formed from gaseous primary pollutants and compounds in photochemicalsmog. Smog is a kind of air pollution; the word "smog" is a portmanteau of smoke and fog.Classic smog results from large amounts of coal burning in an area caused by a mixture of smoke

and sulfur dioxide. Modern smog does not usually come from coal but from vehicular andindustrial emissions that are acted on in the atmosphere by ultraviolet light from the sun to formsecondary pollutants that also combine with the primary emissions to form photochemical smog. 

  Ground level ozone (O3) formed from NOx and VOCs. Ozone (O3) is a key constituent of thetroposphere. It is also an important constituent of certain regions of the stratosphere commonlyknown as the Ozone layer. Photochemical and chemical reactions involving it drive many of thechemical processes that occur in the atmosphere by day and by night. At abnormally highconcentrations brought about by human activities (largely the combustion of fossil fuel), it is apollutant, and a constituent of smog. 

  Peroxyacetyl nitrate (PAN) - similarly formed from NOx and VOCs. 

Monitoring of air pollution

Two main problems are faced by the analytical chemist: what has to be monitored and how themonitoring has to be carried out. It is well established that air quality is usually evaluated through anumber of indices relative to the species listed. These species are found in all areas of the industrializedworld, the only differences between various places being the relative concentration of the species. It must,however, be emphasized that these indices do not fully represent the status of the air, as the indicatedspecies are in dynamic equilibrium with the environment and according to meteorological, energetic andlocal conditions a variety of reactions may take place among them and with the components of the air.

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Urban pollutantsSulphur oxides SO (SO2, SO3)Nitrogen oxides NO (NO, NO2)Carbon monoxide COHydrocarbons Alkanes, olefmsOxidants O3 peroxides, peroxy-acetylnitrate, etc.

Particulate matter

The analytical chemist engaged in the study of the environment has the task of determining these indicesand improving the present knowledge of air chemistry to obtain representative data. To monitor an airpollutant it would be desirable to have an ideal sensor which would be able to yield an instantaneous andspecific response for it which might be integrated and recorded with time. With few exceptions, however,such sensors are not available and in most cases the monitoring of a pollutant, sampled by eithermechanized or automatic techniques, is carried out by bubbling air in a medium to bring into solution acertain species. The integrated sample collected over a long-term period (day, hours) or a short-term(minutes), as in automatic systems, is thus analysed and the concentration of the pollutants is evaluated bya variety of analytical procedures. For the sake of classification these instruments are now designated asfirst-generation instruments. If the pollutant to be measured is a gas, it can be directly determined as such

or through a gas phase reaction with a gaseous reagent partially specific. The detection of a pollutant cantherefore be carried out through the formation of an excited species. The characteristic of this species issuch that its emission spectrum has features which permit one to discriminate it from other compoundspresent or formed in the reaction. Instruments of this type, defined as second-generation instruments,differ from their first-generation progenitors primarily in that they generate their signals as a result of ahomogeneous gas phase reaction rather than by dissolving the substance to be analysed in a solution. It isquite obvious that as the circuitous route a gas has to follow to dissolve and react in solution iseliminated, these instruments offer defmite advantages of sensitivity, specifity and simplicity of operation. A third-generation instrument might also be conceived'. If a pollutant acts upon thechemoelectric properties of a transducer, a direct sensor can be obtained. Variations of such propertiesmay yield an electrical analogue signal directly related to a certain pollutant concentration.

 MODERN METHODS FOR AIR POLLUTION MONITORINGThe development of the instruments for air pollution monitoring is shownin Table 2.

The trend to improve instrumentation aims to achieve the following features: (1) simplicity of concentration and operation; (2) reliability and reproducibility of operation over long time intervals; (3)

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adequate sensitivity to meet the requirements of specific applications; (4) specific response to pollutant orpollutants of interest.Additional criteria apply to continuous air monitoring operations:(1) unattended operation; (2) real-time data output; (3) systems capable of monitoring two or morepollutants concurrently by the same measuringtechnique; (4) ability to monitor an increasingly wide range of pollutants; (5) capability for direct input to

computers. The monitoring of the urban pollutants according to the present developmentof analytical instrumentation is examined.

POLLUTION MONITORING DEVICESA wide range of methods is available for the measurement of air pollutants, from the very simple to thehighly sophisticated, and with a corresponding variation in costs. Descriptions of the most commonprocedures are given below. The manual procedures described are relatively labour intensive, and limitedin the amount of information provided. As a result these have gradually been phased out where possible infavour of the more sophisticated direct-reading instruments. Apart from periodic maintenance, theoperational requirements of the latter are minimal. However, they do require considerably more effort indata processing and analysis, because of the much greater volumes of data produced.

Manual methods

 Passive samplersThe simplest approach to sampling of gaseous air pollutants involves passive collection onto a chemicallytreated surface or material. The driving force for collection is diffusion through the air and/or movementdue to wind. Once in contact with the collector, the pollutant is retained by chemical reaction. Resultsgenerated by these methods are useful in a relative sense, but because of the variability of the factorsaffecting collection and retention of the pollutant it is difficult to establish any simple relationships to theairborne concentrations.

 Paper tape samplersThe usefulness of the above systems can be extended if air is forcibly drawn through or overthe treated surface by means of a pump. An example of this is the paper tape sampler shownin Figure 1. Numerous applications have been reported for gases such as H2S, HCN, NH3,NO2, SO2, Cl2, COCl2, amines and isocyanates.

In most commercial systems the tape is automatically advanced at selected time intervals to produce aseries of discrete spots or samples. Some units also incorporate a direct measurement system for use whenstain development takes place in situ. In others it is necessary to remove the tape for measurement oranalysis of each spot.

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A paper tape system has also been developed into a continuous monitoring system for particulate matter,using the attenuation of low energy radiation. This is described under the section on suspended particulatemonitoring methods given below.

 Bubbler SystemsOne of the most universal approaches for the collection of gaseous pollutants is to bubble the air through

a solution designed to absorb or react with the contaminants. Most gases and vapours can be collected inthis way, followed by an appropriate laboratory analysis of the resulting solutions.

Instrumental Monitoring MethodsA wide range of instrumental methods have been reported for the monitoring of air pollutants. Many of the systems are based on photometric techniques, and the most common examples of these are describedbelow.

 Non-Dispersive Infra-Red (NDIR)NDIR analysers have been developed to monitor SO2, NOx, CO, and other gases that absorb in the infra-red, including CO2 and hydrocarbons. However it is probably true to say that this is the "preferred"technique only for CO monitoring of pollutants in ambient air. The technique is of relatively low

sensitivity, and is more applicable to the concentrations found in source emissions than in ambient air.An NDIR analyser is basically an instrument that does not disperse the light emitted from an infra-redsource - i.e. the light is not split up into its component wavelengths by means of a prism or grating.Instead a broad band of light is produced by means of a bandpass filter, which is chosen to coincide withan absorption peak of the pollutant molecule. The IR band centres for some common gases are shown inTable 1.

The layout of a typical NDIR analyser is shown in Figure 2. Infra-red radiation passes through a

reference cell, usually containing clean dry air, and a separate cell containing the sample. The detector isreferred to as a "microphone" type. It consists of two chambers separated by a thin metal diaphragm andfilled with gas of the species being measured. As the molecules in the detector absorb the IR radiationtheir kinetic energy increases, causing the pressure in each chamber will increase. If, however, absorbingmolecules are present in the sample cell, the amount of energy reaching that side of the detector willdiminish. Thus a pressure differential develops between the two chambers, resulting in displacement of the diaphragm. This is sensed as a change in capacitance by the instrument electronics. As shown in thefigure the instrument also includes a beam chopper. This serves to create an alternating signal in thedetector, which makes it easier to detect and amplify.

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A common problem with this type of analyser is that other gases that absorb light in the same spectralregion as the pollutant will cause a positive interference in the measurement. For CO analysers watervapour and CO2 are potential interferents. Water can be readily removed from the sample by means of aninlet filter containing a desiccant, such as silica gel. In ambient air monitoring the effect of CO 2 is usuallynot significant.

ChemiluminescenceChemiluminescence is the emission of light energy that results from some chemical reactions. Thereaction between NO and O3 is an example:

NO + O3 → NO2* + O2 NO2* → NO2 + hv

These reactions produce a continuum of radiation in the range 500 to 3000 nm. The reaction between O 3 and ethylene is also chemiluminescent, with an emission in the region of 435 nm. Both of thesephenomena have been used to produce continuous monitors for NOx and O3, respectively. A typicallayout of a chemiluminescence analyser for NOx is shown in Figure 3. Ozone is generated by the UVirradiation of clean air and mixed in a reaction chamber with the sample air. Light from the reaction

passes through an optical filter and is detected with a photomultiplier tube.

Clearly, NO2 in the sample will not be detected in this system. However, this can be reduced to NO bymeans of a heated catalyst, such as a stainless steel or molybdenum. If this is included in the system theinstrument can respond to NO and NO2, i.e. NOx. In commercial analysers the converter is eitherincorporated as shown in Figure 3, with automatic valves to switch continuously between operation withand without the catalyst, or two separate channels with individual reaction chambers and detectors areused.

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 Flame Photometric AnalysersGas chromatographers will be familiar with the flame photometric detector (FPD) which is used for theanalysis of sulfur compounds. In this detector, samples eluting from a GC column are passed through ahydrogen-rich flame. If any sulfur-containing compounds are present the sulfur is reduced to a diatomicmolecule, S2, which is initially in an excited state. On decay to the ground state light is emitted over awavelength range of 300 to 425 nm, centred at about 394 nm, i.e.

S2* → S2 + hvThe light emitted from the chamber is viewed by a photomultiplier filled with a narrow-bandpass filter. Aschematic of the FPD is shown in Figure 4. This has been incorporated into air monitoringinstrumentation for the detection of sulfur. The first way in which it has been used involves passingsample air directly into the detector as part of the air/fuel mixture. The system then becomes a continuous"total" sulfur monitor. Some degree of specificity can be achieved by the use of appropriate pre-filters(e.g. to remove H2S in the presence of SO2, and vice versa). Commercial sulfur gas monitors are alsoavailable that are essentially custom-built gas chromatographs. Sample injection is by means of automaticgas sampling valves which are operated at regular intervals. The components of each discrete sample arethen separated on an appropriate GC column, or columns, to give a continual series of individualchromatograms.

Thus the flame photometric detector can provide a sensitive and selective monitoring system for thesulfur-gases. Its major limitation is the dependence on hydrogen (which fuels the flame), as thisintroduces strict safety requirements in both the instrument and its installation.

 Fluorescence MonitorsAnother form of monitor using the principle of fluorescence has been developed, and this provides asatisfactory alternative to the FPD sulfur analysers. Fluorescence is a process whereby light energy of agiven wavelength is absorbed and then reemitted at a different wavelength, i.e.AB + hv → AB* → AB + hv’ The change in wavelength occurs because the molecule that is excited remains in that state for some finiteperiod of time (ca 10-8 - 10-4 s). This is sufficient for some of the energy to be dissipated in the form of 

vibration or rotation within the molecule. This results in the emission of light of a lower energy, andhence a longer wavelength.

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 This phenomenon has been utilised in the development of a monitor for SO2. As shown in Figure 5 a UVlamp provides a source of radiation, either continuous or pulsed, which is filtered to admit a narrow bandof light into the cell, centred at about 210 nm. The fluorescent radiation is measured at right angles to theincident beam, using a photomultiplier. Unlike the FPD analyser, the fluorescence system is specific forSO2. Sample air must be dry and free of dust to avoid fouling of the cell. There is also the potential forinterference from fluorescent organic compounds that may be present in the air, but these can also beremoved with an appropriate pre-filter.

Suspended Particulate Monitoring Methods.The term ’suspended particulate matter’ refers to particules less than 20 microns (µm) in size, which canremain suspended in air for significant periods of time, ranging from a few minutes for the larger particlesthrough to several days for very fine material (ca. < 0.1 µm). These particles can effect visual air qualityand can have effects on human health. Traditionally this material was measured by sucking air through afilter and determining the weight of dust collected. The equipment used was known as a High-VolumeAir Sampler, and collected all particles below 20 µm plus a proportion of larger particles as well. Theresults were referred to as total suspended particulate (TSP). In more recent times the equipment has been

modified to collect only particules below 10 µm, which are the ones most likely to be inhaled andtherefore have an effect on respiratory health. This measurement is known as inhalable particulate, orPM-10.The most common is the High-Volume Air Sampler, fitted with a size selective inlet. Other systems inuse are the β-attenuation tape sampler and the Tapered-Element Oscillating Microbalance (TEOM).The High Volume Air Sampler operates by drawing air at a rate of about 1.5 m3/ min through a 25 cm x20 cm glass fibre filter, which is weighed before and after sampling under conditions of constanthumidity. Samples are normally collected over 24 hours. The β-attenuation unit operates by drawing air ata rate of 15 to 20 litres per minute through a continuous glass-fibre or teflon tape. A source of β-particlesis used to sense the build-up of particles on the tape by changes in the amount of absorption.Measurements are normally averaged over one hour to obtain sufficient sensitivity, and the tape isadvanced either at the end of each cycle or some other pre-set interval. In the TEOM monitor air is drawn

through a filter which is attached to a sensitive oscillating microbalance. Changes in the frequency of oscillation are directly related to the mass of material on the filter, and this is computed electronicallyonce every few minutes. The sampling rate is 16.7 litres per minute and the unit operates continuously.The microfilters need to be changed every 1 to 4 weeks depending on particle loadings.

SPECIAL MONITORING METHODSMore detailed explanations of some of the air monitoring methods are given below. These are mainly forthe older non-instrumental methods which are no longer used routinely in this country. However the

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procedures are still of interest for the range of different chemistries involved. Also some of the methodsmay be suitable for use at secondary school level with appropriate modifications to suit the availableequipment.

Particulate MatterSolid particulate pollutants may be arbitrarily classified into the following categories which reflect both

the physical characteristics of the materials and the procedures commonly employed in theirmeasurement.

 Dust DepositionLarge particules that settle readily out of the atmosphere are collected by deposition. Particles larger thanabout 10µm in diameter may be collected in this way, although it is those 100µm and above which are themost significant in terms of both visual impact and overall mass. Monitoring of dustfall is carried out bydetermining the amount of solid matter deposited over an exposed surface in a period of time. The devicecommonly used for dustfall monitoring is shown in Figure 6, although other types of collectionequipment are available. This arrangement utilises readily available equipment, consisting of a 100 - 150mm diameter glass funnel held inside a 4.5 litre bottle. The supporting wooden box provides stability andhelps to keep the funnel level. The collectors are normally exposed for periods of up to a month. At the

end of this time the samples are filtered and analysed for any or all of the following: weight of insolublematerial and ash content, quantity of liquid collected and pH, weight of dissolved solids, chemicalanalysis for trace metals, or anions such as sulfate, nitrate, and chloride. Results are reported in terms of weight of material collected over unit area and in unit time, i.e. mg m-2 day-1. It should be noted thatresults produced by different systems will not necessarily be comparable. Results are best interpreted in arelative sense with one type of collector only.

Suspended Particulate MatterThis includes particles in the approximate range of 0.1 to 20µm, which may remain suspended in theatmosphere for periods of a few minutes through to a few days or even weeks. SPM was formerlymonitored using a glass fibre filter through which air was pumped, which collected solid matter that

could then be weighed and measured. The SPM concentration is calculated by dividing the weight of dustby the volume drawn through the filter. The first of these is a $-attenuation monitor, in which the air isdrawn through a glass fibre or paper filter, and the mass of dust collected measured by the attenuation of $-rays passed through it. The other is based on the scattering of light by the dust present in the aircontained in a measurement chamber, giving results in kilometres of visibility.

SmokeSmoke consists of fine particles, ca. 10µm and smaller. These have the greatest impact on atmosphericvisibility, and are evaluated using optical techniques with or without prior collection (by e.g. filtration).

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Two main methods can be used. The first procedure used for smoke monitoring is as described in aBritish Standard (B.S. 1747, part 2). Air is drawn through a filter paper held between two brass blocks,and the smoke stain produced is measured by light reflectance, but could also be assessedsemiquantitatively against a set of ’colour’ standards. The equipment is normally incorporated into asampling train for the measurement of both smoke and sulfur dioxide. The normal sampling period is 24hours. For convenience, sampling units containing multiple sampling trains are sometimes used to allow

continuous monitoring over extended periods. An alternative method that has been used in Christchurch ismeasuring the amount of matter collected on a paper tape that is advanced every two hours, which gives apicture of changes in pollution level throughout the day. The measurement of smoke is based on the lightreflectance of the pollutant, and the measurements are sometimes referred to as a "soiling index". Theresults are converted from reflectance to ’smoke  units’ on the basis of a calibration curve included in theBritish Standard, giving and indicator of the relative "dirtiness" of the smoke. The following pollutantsare all gaseous, and there are some general issues relevant to all such pollutants, in particular the need forsensitivity and selectivity. The concentrations of most pollutants in air are of the order of tens of micrograms per cubic metre, and thus unless large volumes of air are sampled the quantities of materialavailable for analysis are quite low. Since many of the pollutants occur together in the air it is alsoimportant that the procedures used for any one component are not subject to interference from others.

Sulphur dioxideA typical bubbler system is the combined sampling unit for smoke and SO2. In this case the absorbersolution is dilute hydrogen peroxide adjusted to a pH of 4.5. This retains the SO2 by converting it tosulfuric acid, i.e.SO2 + H2O2 → H2SO4 The quantity of pollutant collected can be determined by titration, with the endpoint at the original pH,using dilute base (e.g. 0.004 N sodium borate). Alternatively one can calculate the result directly from thepH change. Obviously this method will not be specific for SO2. Interferences can occur due to othergases, such as NO2 or NH3. The use of a pH of 4.5 is specifically designed to counter one potentialinterferent, CO2, which under these conditions is not absorbed. If the absorber solution is analysed forsulfate ion rather than acidity the method can be made quite specific, but at the expense of increasedanalytical requirements of time and/or cost.

Oxides of nitrogenThe most common bubbler procedures for NOx are based on the Griess-Ilosvay method for thedetermination of nitrite ion. This involves diazotisation of an aromatic amine in acid solution, followed bycoupling of the diazo compound with an aromatic amine to form an intensely-coloured azo dye. One of the first applications of this method to the determination of NO2 in air was reported by B.E. Saltzman, andthis has since come to be known as the Saltzman or Griess-Saltzman method. The absorber used in theSaltzman method is a mixture of sulfanilic acid and N-(l-naphthyl)- ethylenediamine in acetic acidsolution. After fifteen minutes a pink solution, which can be measured at 550 nm. There are no majorinterferences in the Saltzman method, other than from excessively high concentrations of SO 2 or O3. Themajor difficulty with the method arises out of the conversion of NO2 to nitrite ion prior to analysis. Thisis expected to occur according to the reaction:

2NO2 + H2O → NO2-

+ NO3-

+ 2H+

and an equivalence of 0.5 moles of nitrite ion is expected for each mole of NO 2. In practice one finds anequivalence of between 0.5 and 1.0, typically about 0.7, this is referred to as the Saltzman factor. There isno general agreement in the literature as to the "correct" value of the Saltzman factor, or whether in fact aconstant value should be expected. To some extent the problem has become essentially a "non-issue",with the advent of instrumental methods for the direct determination of NOx in air. Two variations to theSaltzman method should be mentioned before concluding this section, as these have been used quiteextensively in the past in this country. One difficulty with the Saltzman method is that the colour formedin the absorber is likely to fade after a few hours, and thus the method is mainly suitable for short-term

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measurements. Most interest has been in 24-hour sampling, and for this a modification of the method inwhich the NO2 is absorbed in aqueous triethanolamine solution has been used. Colour development iscarried out as before, but with the addition of the Saltzman reagents at the end of the sampling period. In afurther modification, it is possible to move to a liquid-free sampling system by the use of triethanolamine(TEA) absorbed on to granulated pumice or firebrick. This has advantages in the development of semi-automated multi-day sampling units, as well as removing the need for fragile and expensive glassware.

Here the air is drawn over an absorber of TEA on pumice, removing the NO2, and then over CrO3 (also onpumice) to oxidise the NO to NO2. This NO2 is then collected on second TEA absorber. After samplingthe pollutants are washed from the solid substrate with water and the solution is analysed for nitrite ion asbefore.

O 3 and Total OxidantsMost of the bubbler methods for ozone are based on the oxidising properties of the gas, andhence the methods are indicative of total "oxidants" in the air sample.The most commonly used procedure involves the reaction with neutral-buffered potassiumiodide solution (NBKI). The reaction with ozone is approximated by:O3 + 3KI + H2O → KI3 + O2 + 2KOHThe liberated iodine is measured at 352 nm.

The most significant interferences in the method are from SO2 and NO2, both of which willalso liberate iodine. The former can be removed by a prefilter treated with CrO3.

Control of Air Pollution

1.  Natural Controls of the Human Body eg. Nose 2.  Controls at Source 3.  Control Devices and Techniques of Fixed Sources 4.  Control Devices and Techniques for Mobile Sources 

Controls at SourceSource controls work in one of two ways,

  Prevent the formation of the pollutant  Destroy, alter or trap the pollutants before they reach the atmosphere.  Source relocation  If a source is found to disperse harmful pollution levels, the surrounding area is reviewed

attempting to find a location with less population and using the wind patterns as an advantage.  Source shutdown  Fuel and energy substitution  Process change – but can be very expensive and time consuming  Good operating practices  Using control devices  Control Devices and Techniques of Fixed Sources

Control devicesThe following items are commonly used as pollution control devices by industry or transportationdevices. They can either destroy contaminants or remove them from an exhaust stream before it is emittedinto the atmosphere.

For Gaseous Pollutants  The individual properties of the specific gas to be controlled dictates which control device will be

used.

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  There are primarily three major classifications of control methods for gaseous pollutants –   Absorption –   Adsorption –   Combustion.

Gas Absorption  Gas Absorption : In this technique, the polluted gases are passed through absorbers, also known

as scrubbers, which contain liquid absorbents that remove, treat, or modify one or more of thepollutants in the gas stream.

  Absorbers are the devices that physically contain the liquid absorbent material. Absorbers can bearranged and designed differently with the goal of maximizing the removal efficiency of thepollutant. Some of the most common absorber types include a packed tower, a plate tower, aspray tower and a liquid jet scrubber.

Wet packed bed scrubbers are designed typically for absorption of emissions that are present in anexhaust in the form of a gas (HCL – NH3, NOX, SO2, etc.), but they can also lower mist emissions.Packed bed scrubbers use a packing media to distribute a scrubbing solution that contacts the gas and

 promotes ―mass transfer‖ of gas into the liquid (scrubbant). The scrubbant may be water, but often is achemical solution that has greater affinity for absorption of the undesirable gas. Chromium emissions

from plating processes are in the form of a mist not a gas; therefore, wet scrubbers work mechanically aswet collectors, not gas absorbers, on such emissions.

Wet scrubbers consist of:

  Vessel designed to handle a flow velocity of 500 fpm or below

  Bed of random packing (media) typically 1 – 3 feet in depth

  Spray chambers and a recirculation system to continuously saturate the packing

  Mist elimination system (usually a mesh pad) to remove re-entrained scrubbant

The most effective designs of packed bed scrubbers use either cross flow (horizontal flow) or countercurrent (vertical flow). The principle behind removal of gases in a wet scrubber is ―gas absorption‖ (i.e.,the absorption of specific constituents of the gas stream by dissolving them into the scrubbing solution).

The final stage of the scrubber must be an effective mist eliminator to prevent reentrained scrubbingsolution from entering the atmosphere. The two most common mist eliminator designs are chevron baffleangles and mesh pads. The chevron baffles are designed to create abrupt directional changes in theairstream, thereby impinging wet particles onto the baffles surface or catchment. Chevron baffles aretypically effective on the larger range of particle sizes (i.e., 12 – 100 microns), and because their pluggagepotential is minimal they are considered low maintenance. Mesh pad eliminators, used in wet scrubbers,usually consist of woven or knitted polypropylene monofilament, either in a random or specificconfiguration. They work by mechanical impingement and are velocity dependent. The particle to becollected must be traveling with sufficient velocity to impact and adhere to the fiber. These are designedfor 506 specific particle size removal depending on the diameter of the monofilament, the designed voidspaces, and the number of mesh layers used. Smaller monofilaments and tighter void spaces are obviouslymore efficient in removing smaller particle sizes (1 – 3 microns), but the down side is they have a greater

potential for pluggage and require more maintenance and care. Therefore, the most logical scrubber meshpad eliminator design is a filament diameter and void space that is large enough to minimize the pluggagepotential and have a functional liquid-handling drainage capacity. This type of mesh pad configuration isusually effective on particle sizes 5 – 10 microns in diameter.

Gas Adsorption

  Works by passing the polluted gas through solid adsorbents contained in an adsorption collectingdevice.

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  can be a chemical or physical process, each relying on different factors for maximum efficiency.  Solid adsorbents are ordinarily capable of adsorbing organic and inorganic gases.  Examples of solid adsorbents include activated alumina and silica gel.  Both are generally used in an industrial setting to dry gases.  Activated carbon is the most frequently used adsorbent and most often used to remove organic

solvent vapours.

Combustion  Combustion uses the proper proportions of oxygen, temperature, turbulence and time to convert

 polluted organic compounds into ―safe‖ carbon dioxide and water.   With the presence of turbulence, oxygen can mix with the combustion substance at all times.  The amount of time must be long enough to allow for efficient burning.  There are three types of combustion including furnace combustion, flare combustion, and

catalytic combustion.

 Mechanical devices consist of settling chambers and cyclones. 1. Settling chambers2. A cyclone device forces a particle-filled gas into a cone-shaped compartment and with the help

of a fan, the gas is rapidly swirled resembling a cyclone.As the gas moves downward, the particles spiral upward and stick to the surrounding walls.

Filter Systems force particle-filled gases through bags of woven fabric. The rough surface of the fabricenables the particles to be collected. The particles remain there indefinitely until the filtersare changed. This must occur frequently in order to prevent an overload of particles.

Fig: A Filter Cartridge

ElectrostaticprecipitatorElectrostatic precipitator (ESP), or electrostatic air cleaner is a particulate collection device that removesparticles from a flowing gas (such as air) using the force of an induced electrostatic charge. Electrostatic

precipitators are highly efficient filtration devices that minimally impede the flow of gases through thedevice, and can easily remove fine particulate matter such as dust and smoke from the air stream .[1]  Incontrast to wet scrubbers which apply energy directly to the flowing fluid medium, an ESP applies energyonly to the particulate matter being collected and therefore is very efficient in its consumption of energy(in the form of electricity). In the smokestacks of industrial plants, the gases carrying air pollutants passthrough negatively charged wires or go past a negatively charged central rod. By coming in direct contactwith the negatively charged wires or rod, the gas particles acquire a negative charge, and are then repelledby those negative wires or rod and attracted to positively charged collecting plates or walls, where they

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are deposited. Electrostatic precipitators typically remove 98 percent or more of the particulates. Thisprocess is especially useful when dealing with very small particles.

Methods for cleaning gaseous pollutants 

Control of Sox   The control of SO2 is largely based on chemical means. The sulphur present in organiccompounds can be converted to various forms by oxidation or reduction.

  Sulphur oxidizes to Sulphur Dioxide (SO2) and then Sulphur Trioxide (SO3). In the atmosphereSO3 reacts with water to form sulphuric acid, which then reacts with ammonia or other cat-ions toform particles of ammonia sulphate or some other sulphate.

  These small particles are responsible for urban particulate and visibility problems. Reductionmeans the removal of oxygen or the addition of hydrogen.

Control and Treatment of VOC (Volatile Organic Compounds) and HydrocarbonsControl and treatment of VOC and organic hazardous air pollutant emissions are generally accomplishedby adsorption, incineration, condensation and gas absorption. The methodology is usually chosen

depending upon the temperature, composition and volumetric flow rate of the emission stream, spaceconstraints and allowable installation and operational costs. Most commonly used for small sources. Itcan be physical adsorption or chemisorption. The later is rarely used for the VOC emission controlbecause, it involves a less-reversible chemical bonding of the adsorbate(pollutant) and the adsorbing solid(packing) and is relatively expensive. Physical adsorption uses the van der waals force, giving theadvantage of reversibility and regeneration due to the weaker bonding of the gas and adsorbentmaterial. In general, activated carbon and other adsorbents such as hollow aluminium spheres coatedwith a catalyst can be employed in a fixed, moving or fluidized bed system.

IncinerationIncineration or combustion is another common VOC control technology. Complete combustion oroxidation of pure hydrocarbons produces carbon dioxide and water. Sulphur and nitrogen compounds

produce acid gases and limited air supply results in the formation of carbon monoxide.

CondensationCondensation and gas absorption are most commonly used for highly concentrated VOC streams that areadvantageous to recover. It employs a drop in temperature and/ or increase in pressure to cause the VOCsin the emission stream to condense. The cleaned air stream is separated from the condensate containingtarget pollutants. In many cases, very large temperature drops are required to achieve effectivecondensation, requiring significant energy investment to accomplish cooling.

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 Gas AbsorptionIt involves the absorption of a gas into a liquid. Water can be used for recovery of water-solublecompounds such as acetone and low molecular weight alcohols, which can later be separated from waterusing distillation. Additives are used to increase the effective mass transfer rate of the pollutant from thegas phase into the liquid phase, affecting the surface tension, reducing interfacial resistance. Gasabsorption can be expensive, however it is generally used only to recover VOCs that have a secondarymarket value. Gas absorption techniques are used for the recovery of a variety of chemicals in the cokemanufacturing industry. They are often called scrubbers

Particulate ControlThe control of particulate matter is an important aspect of industrial air pollution engineering. Particlesare collected by a combination of several mechanisms. The six available mechanisms are gravitationalsettling, centrifugal impaction, inertial impaction, direct interception, diffusion and the electrostaticattraction. The physical phenomenon of gravitational settling, centrifugal impaction and electrostaticattraction are known to engineers. The other three mechanisms are described below.

 Inertial Impaction The large particles in the gas stream have too much inertia to follow the gas streamlines around theimpactor and are impacted on the impactor surface,while the small particles and the gas tend to divergeand pass around the interceptor.

 Direct InterceptionIn case of direct interception, the particles have less inertia and barely follow the gas streamlines aroundthe fiber. If the distance between the center of the fiber and the outside of the fiber is less than the particleradius, the particle will graze or hit the fiber and be "intercepted". Inertial impaction and directinterception mechanisms account for 99% collection of particles greater than 1 micrometer aerodynamicdiameter in fabric filter systems.

 DiffusionIn diffusion, small particles are affected by collisions on a molecular level. Particles less than 0.1micrometer have individual or random motion. The particles do not necessarily follow the gasstreamlines, but, move randomly throughout the fluid. This is known as "Brownian Motion". There arefive basic types of dust collectors in use: i) gravity settling chambers, ii) cyclones, iii) fabric filters, iv)electrostatic precipitators, and v) scrubbers.

1)  Gravity Settling ChambersThis is a simple particulate collection device using the principle of gravity to settle the particulatematter in a gas stream passing through its long chamber. The primary requirement of such a

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device would be a chamber in which the carrier gas velocity is reduced so as to allow theparticulate matter to settle out of the moving gas stream under the action of gravity. Thisparticulate matter is then collected at the bottom of the chamber. The chamber is cleanedmanually to dispose the waste. The velocity of the particles in the settling chamber can beobtained by Stokes’ law as follows:Vs = (g(rp – r ) D2 ) /18 µ 

Where,D = Diameter of the particle.g = acceleration due to gravityrp = density of the particler = density of the gasµ = viscosity of the gas

The advantages of settling chambers are:i) low initial cost,

ii) simple construction,iii) low maintenance cost,iv) low pressure drop,v) dry and continuous disposal of solid particles,

vi) use of any material for construction, andvii) temperature and pressure limitations will only depend on the nature of the construction material.The disadvantages of this device arei) large space requirements and

ii) only comparatively large particles (greater than 10 micron) can be collected.

2)  CyclonesSettling chambers discussed above are not effective in removing small particles. Therefore, one needs a

device that can exert more force than gravity force on the particles so that they can be removed from thegas stream. Cyclones use centrifugal forces for removing the fine particles. They are also known ascentrifugal or inertial separators.

3)  Electrostatic Precipitators

The step by step process of removing particles using ESPs is:i) Ionizing the gas.ii) Charging the gas particles.iii) Transporting the particles to the collecting surface.iv) Neutralizing or removing the charge from the dust particles.v) Removing the dust from the collecting surface.The major components of electrostatic precipitators are:i) A source of high voltageii) Discharge and collecting electrodes.

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iii) Inlet and outlet for the gas.iv) A hopper for the disposal of the collected material.v) An outer casing to form an enclosure around the electrodes.

The important applications of ESPs in different industries throughout the world are givenasbelow:

i) Cement factories:a) Cleaning the flue gas from the cement kiln.b) Recovery of cement dust from kilns.ii) Pulp and paper mills:a) Soda-fume recovery in the Kraft pulp mills.iii) Steel Plants:a) Cleaning blast furnace gas to use it as a fuel.b) Removing tars from coke oven gases.c) Cleaning open hearth and electric furnace gases.iv) Non-ferrous metals industry:a)  Recovering valuable material from the flue gases.Collecting acid mist.

b) Collecting acid mist.v) Chemical Industry:

a) Collection of sulphuric and phosphoric acid mist.

b) Cleaning various types of gas, such as hydrogen, CO2 , and SO2.

c) Removing the dust from elemental phosphorous in the vapor state.

vi) Petroleum Industry:

a) Recovery of catalytic dust.

vii) Carbon Black industry:

a) Agglomeration and collection of carbon black.

viii ) Electric Power Industry:

a) Collecting fly ash form coal-fired boilers.

4)  ScrubbersScrubbers are devices that remove particulate matter by contacting the dirty gas stream with liquid drops.Generally water is used as the scrubbing fluid. In a wet collector, the dust is agglomerated with water andthen separated from the gas together with the water. The mechanism of particulate collection and removalby a scrubber can be described as a four-step process.i) Transport : The particle must be transported to the vicinity of the water droplets which are usually10 to 1000 times larger.ii) Collision : The particle must collide with the droplet.iii) Adhesion : This is promoted by the surface tension property.iv) Precipitation: This involves the removal of the droplets, containing the dust particles from the gasphase.

The scrubbers are used in a variety of applications. Some of the situations are:i) They’re particularly useful in the case of a hot gas that must be cooled for some reason.  ii) If the particulate matter is combustible or if any flammable gas is present, even in trace amounts, in thebulk gas phase, a scrubber is preferred to an electrostatic precipitator.iii) Scrubbers can be used when there are waste water treatment systems available on the site, withadequate reserve capacity to handle the liquid effluent.iv) Scrubbers are also used when gas reaction and absorption are required simultaneously with particulatecontrol.

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5)  Fabric Filters Fabric filtration is one of the most common techniques to collect particulate matter from industrial wastegases. The use of fabric filters is based on the principle of filtration, which is a reliable, efficient andeconomic methods to remove particulate matter from the gases. The air pollution control equipment usingfabric filters are known as bag houses.A bag house or a bag filter consists of numerous vertically hanging, tubular bags, 4 to 18 inches in

diameter and 10 to 40 feet long. They are suspended with their open ends attached to a manifold. Thenumber of bags can vary from a few hundreds to a thousand or more depending upon the size of the baghouse. Bag houses are constructed as single or compartmental units. In both cases, the bags are housed ina shell made of rigid metal material.

Operation of baghouse

The gas entering the inlet pipe strikes a baffle plate, which causes larger particles to fall into a hopperdue to gravity. The carrier gas then flows upward into the tubes and outward through the fabric leavingthe particulate matter as a "cake" on the insides of the bags. Efficiency during the pre-coat formation islow, but increases as the pre-coat (cake) is formed, until a final efficiency of over 99% is obtained. Onceformed, the pre-coat forms part of the filtering medium, which helps in further removal of the particulate.Thus the dust becomes the actual filtering medium. The bags in effect act primarily as a matrix to support

the dust cake. The cake is usually formed within minutes or even seconds.  The accumulation of dustincreases the air resistance of the filter and therefore filter bags have to be periodically cleaned. They canbe cleaned by rapping, shaking or vibration, or by reverse air flow, causing the filter cake to be loosenedand to fall into the hopper below. The normal velocities at which the gas is passed through the bags is at0.4-1m/min. There are many types of "filter bags" depending on the bag shape, type of housing andmethod of cleaning the fabric.

The applications of a fabric filter are:Fabric filters find extensive application in the following industries and operations:i) Metallurgical industryii) Foundriesiii) Cement industry

iv) Chalk and lime plantsv) Brick worksvi) Ceramic industryvii) Flour mills

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References

1.  Jes Fenger & Jens Christian Tjell, ―Air Pollution- From a Local to a

Global Perspective‖, RSC Publishing, PP. 65-163, 2009.2.  Werner Strauss, ―Air Pollution Control‖, part-iii, John Wiley and

Sons, PP. 1-49, 217-317, 1978.3.  Arnaldo liberti, ―Modern Methods for Air Pollution Monitoring‖. 4.  http://en.wikipedia.org/wiki/Air_Pollution 

5. Air Pollution Control Permitting Handbook, State of VermontDepartment of Environmental Conservation, Air Pollution ControlDivision, 1999.