UNIT I AIR POLLUTION Transfer of harmful and/or of Natural/Synthetic materials into the atmosphere as direct/indirect consequences of human activity.
Types of Air Pollution •
Personal air exposure -It refers to exposure to dust, fumes and gases to which an individual exposes himself when he indulge himself in smoking
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Occupational air exposure -It represents the type of exposure of individuals to potentially harmful concentration of aerosols, vapors, and gases in their working environment.
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Community air exposure
-This is most serious, complex, consists of varieties of assortment of pollution sources, meteorological factors, and wide variety of adverse social, economical, and health effects. Sources of air pollution
So Natural Sources –Volcano, forest fire, dust storms, oceans, plants and trees •
Anthropogenic Sources - created by human beings
-Stationary sources u
Atmosphere
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It is a mixture of gases that forms a layer of about 250 miles thick around the earth. - Bottom 10-12 miles (Troposphere) is most important part in terms of
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Weather Other aspects of Biogeochemical cycle
- The lowest 600 meters of Troposphere: Air Quality Studies
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Composition of Air - 78% nitrogen, 21% oxygen, 1% carbon dioxide, water, other gases
Divided into four zones: - Troposphere - Stratosphere - Mesosphere - Thermosphere
Source of Air Pollution • •
Natural Sources –Volcano, forest fire, dust storms, oceans, plants and trees Anthropogenic Sources - created by human beings -Stationary sources
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Point sources (Industrial processing, power plants, fuels combustion etc.) Area sources (Residential heating coal gas oil, on site incineration, open burning etc.)
- Mobile sources
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Line sources (Highway vehicles, railroad locomotives, channel vessels etc.)
Air Pollutants Any substance occurring in the atmosphere that may have adverse effects on humans, animals, plant life, and/or inanimate materials.
Air pollutants have known or suspected harmful effects on human health and tironment.
Criteria Air Pollutants • •
Based on health effects with measured air quality levels that violate the National Ambient Air Quality Standards (NAAQS) (NAAQS) -CO -NOx -SOx -VOCs -Particulates -Pb
Hazardous Air Pollutants • •
Predecessor: National Emission Standards for Hazardous Air Pollutants (NESHAPs) Clean Air Act Amendments of 1990 directed EPA to establish emission controls for 189 chemicals listed in the Act. -NOT based on health criteria -Based on Maximum Achievable Control Technology (MACT)
Non-Criteria Pollutants • •
In essence, all pollutants not included in the NAAQS and HAP lists Examples: -CO -NaCl
Air Pollutants
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Primary air pollutants - Materials that when released pose health risks in their unmodified forms or those emitted directly from identifiable sources.
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Secondary air pollutants - Primary pollutants interact with one another, sunlight, or natural gases to produce new, harmful compounds
Primary Air Pollutants •
Five major materials released directly into the atmosphere in unmodified forms. -Carbon monoxide -Sulfur dioxide -Nitrogen oxides -Hydrocarbons -Particulate matter
Carbon Monoxide •
Produced by burning of organic material (coal, gas, wood, trash, etc.)
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Automobiles biggest source (80%)
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Cigarette smoke another major source
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Toxic because binds to hemoglobin, reduces oxygen in blood
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Not a persistent pollutant, combines with oxygen to form CO2
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Most communities now meet EPA standards, but rush hour traffic can produce high CO levels
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Not a persistent pollutant, combines with oxygen to form CO2
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Most communities now meet EPA standards, but rush hour traffic can produce high CO levels
Sulphur Dioxide
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Produced by burning sulfur containing fossil fuels (coal, oil)
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Coal-burning power plants major source
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Reacts in atmosphere to produce acids
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One of the major components of acid rain
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When inhaled, can be very corrosive to lung tissue
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London -1306 banned burning of sea coal -1952 “killer fog”: 4,000 people died in 4 weeks o tied to sulfur compounds in smog
Nitrogen Oxides
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Produced from burning of fossil fuels
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Contributes to acid rain, smog
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Automobile engine main source
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New engine technology has helped reduce, but many more cars
Hydrocarbons •
Hydrocarbons - organic compounds with hydrogen, carbon
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From incomplete burning or evaporated from fuel supplies
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Major source is automobiles, but some from industry
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Contribute to smog
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Improvements in engine design have helped reduce
Particulates
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Particulates - small pieces of solid materials and liquid droplets (2.5 mm and 10 mm)
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Examples: ash from fires, asbestos from brakes and insulation, dust
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Easily noticed: e.g. smokestacks
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Can accumulate in lungs and interfere with the ability of lungs to exchange gases.
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Some particulates are known carcinogens
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Those working in dusty conditions at highest risk (e.g., miners)
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Respirable Suspended Particulate Matter (RSPM) -PM1 having size <= 1µm: effects in alveoli -PM2.5 having size <= 2.5µm: effects trachea -PM10 having size <= 10µm: effects in nasal part only<
Secondary Pollutants • • • •
Ozone PAN (peroxy acetyl nitrate) Photochemical smog Aerosols and mists (H2SO4)
Ozone
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Ozone (O3) is a highly reactive gas composed of three oxygen atoms.
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It is both a natural and a man-made product that occurs in the Earth's upper atmosphere (the stratosphere) and lower atmosphere (the troposphere).
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Tropospheric ozone – what we breathe -- is formed primarily from photochemical reactions between two major classes of air pollutants, volatile organic compounds (VOC) and nitrogen oxides (NOX).
PAN Smog is caused by the interaction of some hydrocarbons and oxidants under the influence of sunlight giving rise to dangerous peroxy acetyl nitrate (PAN).
Photochemical smog Photochemical smog is a mixture of pollutants which includes particulates, nitrogen oxides, ozone, aldehydes, peroxyethanoyl nitrate (PAN), unreacted hydrocarbons, etc. The smog often has a brown haze due to the presence of nitrogen dioxide. It causes painful eyes.
Aerosols and mists (H2SO4) Aerosols and mists are very fine liquid droplets that cannot be effectively removed using traditional packed scrubbers. These droplets can be formed from gas phase hydrolysis of halogenated acids (HCl, HF, HBr), metal halides, organohalides, sulfur trioxide (SO3), and phosphorous pentoxide (P2O5).
Assignments 1. Can you explain the word ‘episode’ used in air pollution? 2. Can you think why ‘mountains in a basin like area’ make the pollutants susceptible to accumulation? 3. Can you tell two words making the word ‘smog’? 4. Do you know that ‘soot’ is unburnt/burnt carbon particle? 5. Why Earth Day is celebrated? Explain. 6. Can you explain the significance of World Environment Day? 7. What does ‘Earth Summit's means? 8. Are CO and NOx ‘indicators or ‘pollutants’? 9. Can you list direct/indirect consequences of human activity causing air pollution? 10. Differentiate among personal/occupational/community air exposure. 11. Is environmental tobacco smoke (ETS) personal/occupational / community exposures? 12. Explain various spheres of the Earth. 13. Explain various sources of air pollution. 14. Differentiate between troposphere/stratosphere/mesosphere. Which one is ideal for air pollution studies effecting living beings? 15. Differentiate between criteria/non-criteria/hazardous pollutants . Why O3 is not taken as criteria pollutants?
Ambient Air Pollution Monitoring
Introduction Most frequently occurring pollutants in an urban environment are particulate matters (suspended particulate matter i.e. SPM and respirable suspended particulate matter i.e. RSPM), carbon monoxide (CO), hydrocarbons (HC), sulfur dioxide (SO2), nitrogen dioxide (NO2), ozone (O3) and photochemical oxidants.
The recommended criteria for siting the monitoring stations • • • • •
The site is dependent upon the use/purpose of the results of the monitoring programs. The monitoring should be carried out with a purpose of compliance of air quality standards. Monitoring must be able to evaluate impacts of new/existing air pollution sources. Monitoring must be able to evaluate impacts of hazards due to accidental release of chemicals. Monitoring data may be used for research purpose.
Type of ambient monitoring stations Station type Type A
Type B Type C
Description Downtown pedestrian exposure station- In central business districts, in congested areas, surrounding by buildings, many pedestrians, average traffic flow > 10000 vehicles per day. Location of station- 0.5 m from curve; height 2.5 to 3.5 m from the ground. Downtown neighbor hood exposure stations- In central business districts but not congested areas, less high rise buildings, average vehicles < 500 vehicles per day. Typical locations like parks, malls, landscapes areas etc. Location of station- 0.5 m from curve; height 2.5 to 3.5 m from the ground. Residential population exposure station – In the midst of the residential areas
Type D
Type E
Type F
or sub-urban areas but not in central business districts. The station should be more than 100 m away from any street. Mesoscale stations – At appropriate height to collect meteorological and air quality data at upper elevation; main purpose to collect the trend of data variations not human exposure. Non-urban stations – In remote non-urban areas, no traffic, no industrial activity. Main purpose to monitor trend analysis. Location of station- 0.5 m from curve; height 2.5 to 3.5 m from the ground. Specialized source survey stations – to determine the impact on air quality at specified location by an air pollution source under scrutiny. Location of station- 0.5 m from curve; height 2.5 to 3.5 m from the ground.
Frequency of data collection • •
Gaseous pollutants: continuous monitoring Particulates: once every three days
Number of stations • • • •
Minimum number is three. The location is dependent upon the wind rose diagram that gives predominant wind directions and speed. One station must be at upstream of predominant wind direction and other two must at downstream pre dominant wind direction. More than three stations can also be established depending upon the area of coverage.
Components of ambient air sampling systems Four main components are:
• • • •
Inlet manifold Air mover collection medium flow measurement device
Inlet manifold transports sampled pollutants from ambient air to collection medium or analytical device
in an unaltered condition. The manifold should not be very long. Air mover provides force to create vacuum or lower pressure at the end of sampling systems. They are pumps. The collection mediums are liquid or solid sorbent or dissolving gases or filters or chamber for air analysis (automatic instruments). The flow device like rotameters measure the volume of air sampled.
Characteristics for ambient air sampling systems Five main characteristicss are:
• • • • •
collection efficiency sample stability recovery minimal interference understanding the mechanism of collection
The first three must be 100% efficient. For e.g. for SO2, the sorbent should be such that at ambient temperature it may remove the SO2 from ambient atmosphere 100%. Sample must be stabled during the time between sampling and analysis. Recovery i.e. the analysis of particular pollutant must be 100% correct.
Basic considerations for sampling • • • • • •
Sample must be representative in terms of time, location, and conditions to be studied. Sample must be large enough for accurate analysis. The sampling rate must be such as to provide maximum efficiency of collection. Duration of sampling must accurately reflect the fluctuations in pollution levels i.e. whether 1hourly, 4-hourly, 6-hourly, 8-hourly, 24-hourly sampling. Continuous sampling is preferred. Pollutants must not be altered or modified during collection.
Errors in sampling by HVS • •
Particulates may be lost in sampling manifold – so not too long or too twisted manifold must be used. If ’isokinetic’ conditioned are not maintained, biased results may be obtained for particulate matters.
Advantages of HVS • • • • • • • •
High flow rate at low pressure drop High particulate storage capacity No moisture regain high collection efficiency Low coast Not appreciable increase in air flow resistance Filter is 99% efficient and can collect the particles as fine as 0.3 µm Absorption principle is 99% efficient in collecting the gases
Stack Monitoring: techniques & instrumentation
Sampling The sample collected must be representative in terms of time and location. The sample volume should be large enough to permit accurate analysis. The sampling rate must be such as to provide maximum efficiency of collection. The contaminants must not be modified or altered in the process of collection.
Diagrammatic view of stacksampling
Impingers are glass bubble tubes designed for the collection of airborne particles into a liquid medium (Figure 1).
When using an air sampler, a known volume of air bubbles is pumped through the glass tube that contains a liquid specified in the method. The liquid is then analyzed to determine airborne concentrations.
Figure 1: Glass Impinger
Selection of sampling location The sampling point should be as far as possible from any disturbing influence, such as elbows, bends, transition pieces, baffles. The sampling point, wherever possible should be at a distance of 5-10 diameters down-stream from any obstruction and 3-5 diameters up-stream from similar disturbance.
Size of sampling point The size of the sampling point may be made in the range of 7-10 cm, in diameter.
Traverse points For the sample become representative, it should be collected at various points across the stack. The number of traverse points may be selected with reference to Table 1.
Table 1: Traverse Points Cross-section area of stack sq. m
No. of Points
0.2
4
0.2 to 2.5
12
2.5 and above
20
In circular stacks, traverse points are located at the center of equal annular areas across two perpendicular diameters as shown in Figure 2
Figure 2 In case of rectangular stacks, the area may be divided in to 12 to 25 equal areas and the centers for each area are fixed. (Figure 3)
Figure 3
Isokinetic conditions
Isokinetic conditions exist when the velocity in the stack ‘Vs’ equals the velocity at the top of the probe nozzle ‘Vn’ at the sample point (Figure 4).
Stack Monitoring: techniques & instrumentation
Sampling
The sample collected must be representative in terms of time and location. The sample volume should be large enough to permit accurate analysis. The sampling rate must be such as to provide maximum efficiency of collection. The contaminants must not be modified or altered in the process of collection.
Diagrammatic view of stacksampling
• • •
Impingers are glass bubble tubes designed for the collection of airborne particles into a liquid medium (Figure 1). When using an air sampler, a known volume of air bubbles is pumped through the glass tube that contains a liquid specified in the method. The liquid is then analyzed to determine airborne concentrations.
Figure 1: Glass Impinger
Selection of sampling location •
The sampling point should be as far as possible from any disturbing influence, such as elbows,
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bends, transition pieces, baffles. The sampling point, wherever possible should be at a distance of 5-10 diameters down-stream from any obstruction and 3-5 diameters up-stream from similar disturbance.
Size of sampling point •
The size of the sampling point may be made in the range of 7-10 cm, in diameter.
Traverse points • •
For the sample become representative, it should be collected at various points across the stack. The number of traverse points may be selected with reference to Table 1.
Table 1: Traverse Points Cross-section area of stack sq. m
No. of Points
0.2 0.2 to 2.5 2.5 and above
4 12 20
In circular stacks, traverse points are located at the center of equal annular areas across two perpendicular diameters as shown in Figure 2
Figure 2 In case of rectangular stacks, the area may be divided in to 12 to 25 equal areas and the centers for each area are fixed. (Figure 3)
Figure 3
Isokinetic conditions •
Isokinetic conditions exist when the velocity in the stack ‘Vs’ equals the velocity at the top of the probe nozzle ‘Vn’ at the sample point (Figure 4).
Learning Objectives • • •
Ambient air pollution monitoring: techniques and instrumentation; monitoring stations Stack monitoring: techniques and instrumentation. Experimental analysis: gaseous and particulates; standards and limits.
Experimental analysis: Gaseous & particulates; standards & limits Principles of Sampling and Analysis •
• • • • •
The components of an air pollution monitoring system include the -collection or sampling of pollutants both from the ambient air and from specific sources, -the analysis or measurement of the pollutant concentrations, and -the reporting and use of the information collected. Emissions data collected from point sources are used to determine compliance with air pollution regulations, determine the effectiveness of air pollution control technology, evaluate production efficiencies, and support scientific research. The EPA has established ambient air monitoring methods for the criteria pollutants, as well as for toxic organic (TO) compounds and inorganic (IO) compounds. The methods specify precise procedures that must be followed for any monitoring activity related to the compliance provisions of the Clean Air Act. These procedures regulate sampling, analysis, calibration of instruments, and calculation of emissions. The concentration is expressed in terms of mass per unit volume, usually micrograms per cubic meter (µg/m3).
Particulate Monitoring
• • •
•
Particulate monitoring is usually accomplished with manual measurements and subsequent laboratory analysis. A particulate matter measurement uses gravimetric principles. Gravimetric analysis refers to the quantitative chemical analysis of weighing a sample, usually of a separated and dried precipitate. In this method, a filter-based high-volume sampler (a vacuum- type device that draws air through a filter or absorbing substrate) retains atmospheric pollutants for future laboratory weighing and chemical analysis. Particles are trapped or collected on filters, and the filters are weighed to determine the volume of the pollutant. The weight of the filter with collected pollutants minus the weight of a clean filter gives the amount of particulate matter in a given volume of air. Chemical analysis can be done by atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), inductively couple plasma (ICP) spectroscopy, and X-ray fluorescence (XRF) spectroscopy.
Atomic Absorption Spectrometry (AAS) • •
AAS is a sensitive means for the quantitative determination of more than 60 metals or metalloid elements. Principle: This technique operates by measuring energy changes in the atomic state of the analyte. For example, AAS is used to measure lead in particulate monitoring.
Figure: Atomic absorption spectrometry • • • • • • •
Particles are collected by gravimetric methods in a Teflon (PTFE) filter, lead is acid-extracted from the filter. The aqueous sample is vaporized and dissociates into its elements in the gaseous state. The element being measured, in this case lead, is aspirated into a flame or injected into a graphite furnace and atomized. A hollow cathode or electrode less discharge lamp for the element being determined provides a source of that metal's particular absorption wavelength. The atoms in the unionized or "ground" state absorb energy, become excited, and advance to a higher energy level. A detector measures the amount of light absorbed by the element, hence the number of atoms in the ground state in the flame or furnace. The data output from the spectrometer can be recorded on a strip chart recorder or processed by computer. Determination of metal concentrations is performed from prepared calibration curves or read directly from the instrument.
Gaseous pollutant monitoring •
Gaseous pollutant monitoring can be accomplished using various measurement principles.
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• • • •
Some of the most common techniques to analyze gaseous pollutants include -Spectrophotometry, -Chemiluminescence, -Gas chromatography-flame ionization detector (GC-FID), - Gas chromatography-mass spectrometry (GC-MS), and - Fourier transform infrared spectroscopy (FTIR). With all sampling and analysis procedures, the end result is quantitative data. The validity of the data depends on the accuracy and precision of the methods used in generating the data. The primary quality control measure is calibration. Calibration checks the accuracy of a measurement by establishing the relationship between the output of a measurement process and a known input.
Table 1. Methods of Measuring and Analyzing Air Pollutants
Method
Variable Measured
Gravimetric
PM10, PM2.5
Principle Particles are trapped or collected on filters, and the filters are weighed to determine the volume of the pollutant.
more than 60 metals or This technique operates by measuring energy changes in the Atomic absorption metalloid atomic state of the analyte. Emitted radiation is a function of spectrometry (AAS) elements (e.g. Pb, atoms present in the sample. Hg, Zn) Measure the amount of light that a sample absorbs. The Spectrophotometry SO2, O3 amount of light absorbed indicates the amount of analyte present in the sample. Based upon the emission spectrum of an excited species that is Chemiluminescence SO2, O3 formed in the course of a chemical reaction. Gas chromatography (GC) - flame Responds in proportion to number of carbon atoms in gas VOC ionization detector sample. (FID) Gas Mass spectrometers use the difference in mass-to-charge ratio chromatographyVOC (m/z) of ionized atoms or molecules to separate them from each mass spectrometry other. (GC-MS) Fourier Transform Sample absorbs infrared radiation and difference in absorption Infrared CO, VOC, CH4 is measured. Spectroscopy (FTIR)
Spectrophotometry • • • •
A spectrophotometer measures the amount of light that a sample absorbs. The instrument operates by passing a beam of light through a sample and measuring the intensity of light reaching a detector. Spectrophotometry commonly used to measure sulfur dioxide (SO2) concentrations. The amount of light absorbed indicates the amount of sulfur dioxide present in the sample.
Figure: Schematic of a UV-VIS spectrophotometer
Chemiluminescence •
An ambient air sample is mixed with excess ozone in a special sample cell. A portion of the NO present is converted to an activated NO2 which returns to a lower energy state and in the process emits light. This phenomenon is called chemiluminescence.
Figure: Chemical reaction to determine oxides of nitrogen by chemiluminescence •
Chemiluminescence methods for determining components of gases originated with the need for highly sensitive means for determining atmospheric pollutants such as ozone, oxides of nitrogen, and sulfur compounds.
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The intensity of this light can be measured with a photomultiplier tube and is proportional to the amount of NO in the sample. A second reaction measures the total oxides of nitrogen in the air sample and in turn, the concentration of NO2 can be calculated.
Gas Chromatography (GC) • •
Gas chromatography (GC) coupled with a flame ionization detector (FID) is employed for qualitative identification and quantitative determination of volatile organic compounds (VOCs) in air pollution monitoring. The GC, consists of a column, oven and detector. In the gas chromatograph, a sample goes to the column, separates into individual compounds and proceeds through the hydrogen flame ionization detector.
Figure: Schematic gas chromatography • • •
The flame in a flame ionization detector is produced by the combustion of hydrogen and air. When a sample is introduced, hydrocarbons are combusted and ionized, releasing electrons. A collector with a polarizing voltage located near the flame attracts the free electrons, producing a current that is proportional to the amount of hydrocarbons in the sample. The signal from the flame ionization detector is then amplified and output to a display or external device. Gas chromatography-mass spectrometry (GC-MS) instruments have also been used for identification of volatile organic compounds. Mass spectrometers use the difference in mass-tocharge ratio (m/z) of ionized atoms or molecules to separate them from each other. Mass spectrometry is useful for quantification of atoms or molecules and also for determining chemical and structural information about molecules.
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Fourier Transform Infrared Spectroscopy • •
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FTIR can detect and measure both criteria pollutants and toxic pollutants in ambient air FTIR can directly measure more than 120 gaseous pollutants in the ambient air, such as carbon monoxide, sulfur dioxide, and ozone. The technology is based on the fact that every gas has its own "fingerprint," or absorption spectrum.
Figure: FTIR can directly measure both criteria pollutants and toxic pollutants in the ambient air.
• • • •
The FTIR sensor monitors the entire infrared spectrum and reads the different fingerprints of the gases present in the ambient air. Carbon monoxide is monitored continuously by analyzers that operate on the infrared absorption principle. Ambient air is drawn into a sample chamber and a beam of infrared light is passed through it. CO absorbs infrared radiation, and any decrease in the intensity of the beam is due to the presence of CO molecules.
• • •
This decrease is directly related to the concentration of CO in the air. A special detector measures the difference in the radiation between this beam and a duplicate beam passing through a reference chamber with no CO present. This difference in intensity is electronically translated into a reading of the CO present in the ambient air, measured in parts per million.
National Ambient Air Quality Standards POLLUTANTS sulphur dioxide (SO2) Oxides of Nitrogen (NO2) Suspended Particulate Matter (SPM) Lead Carbon Monoxide Respirable Particulate Matter (RPM)
AVERAGE TIME CONCENTRATION Annual average 24 hour
60 µg/cubic m 80 µg/cubic m
A.A 24H A.A 24H A.A 24H A.A 24H A.A 24H
60 µg/cubic m 80 µg/cubic m 140µg/cubic m 200µg/cubic m 0.75 µg/cubic m 1.0 µg/cubic m 2.0 µg/cubic m 84.0 µg/cubic m 60 µg/cubicm 100 µg/cubic m
NAAQS by USEPA 2006 Pollutant
Primary Stds.
Averaging Times
9 ppm (10 mg/cubic 8-hour(1) m) Carbon Monoxide 35 ppm (40 mg/cubic 1-hour(1) m) Lead 1.5 µg/cubic m Quarterly Average 0.053 ppm (100 Nitrogen Dioxide Annual (Arithmetic Mean) µg/cubic m) Particulate Matter (PM10)
Particulate Matter (PM2.5)
Revoked(2)
Annual(2) (Arith. Mean)
150 µg/cubic m
24-hour(3)
15.0 µg/cubic m
Annual(4) (Arith. Mean)
35 µg/cubic m
24-hour(5)
0.08 ppm
8-hour(6)
Ozone
None None Same as Primary Same as Primary
Same as Primary
Same as Primary
0.03 ppm
1-hour(7) (Applies only in limited areas) Annual (Arith. Mean)
0.14 ppm
24-hour(1)
-------
-------
3-hour(1)
0.5 ppm (1300 µg/cubic m)
0.12 ppm
Sulfur Oxides
Secondary Stds.
(1)Not to be exceeded more than once per year.
Same as Primary -------
(2)Due to a lack of evidence linking health problems to long-term exposure to coarse particle pollution, the agency revoked the annual PM10 standard in 2006 (effective December 17, 2006). (3) Not to be exceeded more than once per year on average over 3 years. (4) To attain this standard, the 3-year average of the weighted annual mean PM2.5 concentrations from single or multiple community-oriented monitors must not exceed 15.0 µg/cubic metre. (5) To attain this standard, the 3-year average of the 98th percentile of 24-hour concentrations at each population-oriented monitor within an area must not exceed 35 µg/cubic metre (effective December 17, 2006). (6) To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within an area over each year must not exceed 0.08 ppm. (7) (a) The standard is attained when the expected number of days per calendar year with maximum hourly average concentrations above 0.12 ppm is < 1, as determined by appendix H. (b) As of June 15, 2005 EPA revoked the 1-hour ozone standard in all areas except the fourteen 8hour ozone nonattainment Early Action Compact (EAC) Areas.
WHO Air Quality Guidelines Value POLLUTANTS
AVERAGE TIME
Particulate matter PM2.5 PM10
1 year 24 hour(99th percentile) 1 year 24 hour(99th percentile)
Ozone, O3
8 hour, daily maximum
Nitrogen dioxide, NO2
1 year 1 hour
Sulfur dioxide, SO2
24 hour 10 minute
AQG value 10 µg/cubic metre 25 µg/cubic metre 20 µg/cubic metre 50 µg/cubic metre 100 µg/cubic metre 40µg/cubic metre 200µg/cubic metre 20 µg/cubic metre 500 µg/cubic metre
References USEPA, 2007. Online literature from www.epa.gov WHO, 2005. WHO air quality guidelines global update 2005, WHOLIS number E87950. CPCB 2006, Central Pollution Control Board. http://www.cpcb.nic.in/standard2.htm
Air pollution effects : On living and non living beings Human Health Effects •
Exposure to air pollution is associated with numerous effects on human health, including pulmonary, cardiac, vascular, and neurological impairments.
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The health effects vary greatly from person to person. High-risk groups such as the elderly, infants, pregnant women, and sufferers from chronic heart and lung diseases are more susceptible to air pollution.
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Children are at greater risk because they are generally more active outdoors and their lungs are still developing.
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Exposure to air pollution can cause both acute (short-term) and chronic (long-term) health effects.
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Acute effects are usually immediate and often reversible when exposure to the pollutant ends. Some acute health effects include eye irritation, headaches, and nausea.
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Chronic effects are usually not immediate and tend not to be reversible when exposure to the pollutant ends. - Some chronic health effects include decreased lung capacity and lung cancer resulting from long-term exposure to toxic air pollutants.
Effects on Human respiratory system
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Both gaseous and particulate air pollutants can have negative effects on the lungs.
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Solid particles can settle on the walls of the trachea, bronchi, and bronchioles.
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Continuous breathing of polluted air can slow the normal cleansing action of the lungs and result in more particles reaching the lower portions of the lung.
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Damage to the lungs from air pollution can inhibit this process and contribute to the occurrence of respiratory diseases such as bronchitis, emphysema, and cancer.
Table 1: Sources, Health and Welfare Effects for Criteria Pollutant Carbon Monoxide
Description Colorless, odorless gas
Sources Motor vehicle exhaust, indoor sources include
Health Effects Headaches, reduced mental alertness, heart
Welfare Effects Contribute to the formation of smog.
(CO)
kerosene or wood burning stoves.
attack, cardiovascular diseases, impaired fetal development, death. Sulfur Colorless gas that Coal-fired power plants, Eye irritation, wheezing, Dioxide dissolves in water petroleum refineries, chest tightness, (SO2) vapor to form acid, manufacture of sulfuric shortness of breath, lung and interact with other acid and smelting of damage. gases and particles in ores containing sulfur. the air. Nitrogen Reddish brown, highly Motor vehicles, electric Susceptibility to Dioxide reactive gas. utilities, and other respiratory infections, (NO2) industrial, commercial, irritation of the lung and and residential sources respiratory symptoms that burn fuels. (e.g., cough, chest pain, difficulty breathing). Ozone (O3) Gaseous pollutant Vehicle exhaust and Eye and throat irritation, when it is formed in certain other fumes. coughing, respiratory the troposphere. Formed from other air tract problems, asthma, pollutants in the lung damage. presence of sunlight. Lead (Pb) Metallic element Metal refineries, lead Anemia, high blood smelters, battery pressure, brain and manufacturers, iron and kidney damage, steel producers. neurological disorders, cancer, lowered IQ. Particulate Very small particles of Diesel engines, power Eye irritation, asthma, Matter (PM) soot, dust, or other plants, industries, bronchitis, lung damage, matter, including tiny windblown dust, wood cancer, heavy metal droplets of liquids. stoves. poisoning, cardiovascular effects.
Contribute to the formation of acid rain, visibility impairment, plant and water damage, aesthetic damage. Contribute to the formation of smog, acid rain, water quality deterioration, global warming, and visibility impairment. Plant and ecosystem damage.
Affects animals and plants, affects aquatic ecosystems.
Visibility impairment, atmospheric deposition, aesthetic damage.
Table 2: Sources, Effects of Air Pollutants on Vegetables Pollutants Aldehydes
Sources Photochemical reactions
Effects on Vegetables The upper portions of Alfalfa etc. will be affected to Narcosis if 250 ppm of aldehydes is present for 2 hrs duration. Ozone (O3) Photochemical reaction of hydrocarbon All ages of tobacco leaves, beans, grapes, pine, and nitrogen oxides from fuel pumpkins and potato are affected. Fleck, stipple, combustion, refuse burning, and bleaching, bleached spotting, pigmentation, growth evaporation from petroleum products. suppression, and early abscission are the effects. Peroxy Acetyl The sources of PAN are the same as Young spongy cells of plants are affected if 0.01 Nitrate (PAN) ozone ppm of PAN is present in the ambient air for more than 6 hrs. Nitrogen dioxide High temperature combustion of coal, Irregular, white or brown collapsed lesion on (NO2) oil, gas, and gasoline in power plants intercostals tissue and near leaf margin. and internal combustion engines. Suppressed growth is observed in many plants. Ammonia & Thermal power plants, oil and petroleum Bleached spots, bleached areas between veins, Sulfur dioxide refineries. bleached margins, chlorosis, growth suppression, early abscission, and reduction in yield and tissue collapse occur. Chlorine (Cl2) Leaks in chlorine storage tanks, If 0.10 ppm is present for at least 2 hrs, the hydrochloric acid mists. epidermis and mesophyll of plants will be affected. Hydrogen Phosphate rock processing, aluminum Epidermis and mesophyll of grapes, large seed fluoride, Silicon industry, and ceramic works and fruits, pines and fluorosis in animals occur if 0.001 tetrafluoride fiberglass manufacturing. ppm of HF is present for 5 weeks.
Pesticides & Herbicides Particulates
Mercury (Hg)
Agricultural operations
Defoliation, dwarfing, curling, twisting, growth reduction and killing of plants may occur. Cement industries, thermal power Affects quality of plants, reduces vigor & hardness plants, blasting, crushing and and interferences with photosynthesis due to processing industries. plugging leaf stomata and blocking of light. Processing of mercury containing ores, Greenhouse crops, and floral parts of all burning of coal and oil. vegetations are affected; abscission and growth reduction occur in most of the plants.
UNIT II
Learning Objectives To make the student aware of effects of gaseous and particulate air pollutants on humans, plants and materials; principles of air pollution control and various control equipments at source.
Air pollution control : Principles of controls, source control Source Control Technology • • • •
Air quality management sets the tools to control air pollutant emissions. Control measurements describes the equipment, processes or actions used to reduce air pollution. The extent of pollution reduction varies among technologies and measures. The selection of control technologies depends on environmental, engineering, economic factors and pollutant type.
Settling Chambers • •
Settling chambers use the force of gravity to remove solid particles. The gas stream enters a chamber where the velocity of the gas is reduced. Large particles drop out of the gas and
are recollected in hoppers. Because settling chambers are effective in removing only larger particles, they are used in conjunction with a more efficient control device.
Figure: Settling chambers
Cyclones •
The general principle of inertia separation is that the particulate-laden gas is forced to change direction. As gas changes direction, the inertia of the particles causes them to continue in the original direction and be separated from the gas stream.
•
The walls of the cyclone narrow toward the bottom of the unit, allowing the particles to be collected in a hopper.
•
The cleaner air leaves the cyclone through the top of the chamber, flowing upward in a spiral vortex, formed within a downward moving spiral. Cyclones are efficient in removing large particles but are not as efficient with smaller particles. For this reason, they are used with other particulate control devices.
Venturi Scrubbers
• • • • • • • •
• •
•
Venturi scrubbers use a liquid stream to remove solid particles. In the venturi scrubber, gas laden with particulate matter passes through a short tube with flared ends and a constricted middle. This constriction causes the gas stream to speed up when the pressure is increased. The difference in velocity and pressure resulting from the constriction causes the particles and water to mix and combine. The reduced velocity at the expanded section of the throat allows the droplets of water containing the particles to drop out of the gas stream. Venturi scrubbers are effective in removing small particles, with removal efficiencies of up to 99 percent. One drawback of this device, however, is the production of wastewater. Fabric filters, or baghouses, remove dust from a gas stream by passing the stream through a porous fabric. The fabric filter is efficient at removing fine particles and can exceed efficiencies of 99 percent in most applications. The selection of the fiber material and fabric construction is important to baghouse performance. The fiber material from which the fabric is made must have adequate strength characteristics at the maximum gas temperature expected and adequate chemical compatibility with both the gas and the collected dust. One disadvantage of the fabric filter is that hightemperature gases often have to be cooled before contacting the filter medium.
Electrostatic Precipitators (ESPs)
Figure: Fabric filter (baghouse) components
•
An ESP is a particle control device that uses electrical forces to move the particles out of the flowing gas stream and onto collector plates.
•
The ESP places electrical charges on the particles, causing them to be attracted to oppositely charged metal plates located in the precipitator.
•
The particles are removed from the plates by "rapping" and collected in a hopper located below the unit.
•
The removal efficiencies for ESPs are highly variable; however, for very small particles alone, the removal efficiency is about 99 percent.
•
Electrostatic precipitators are not only used in utility applications but also other industries (for other exhaust gas particles) such as cement (dust), pulp & paper (salt cake & lime dust), petrochemicals (sulfuric acid mist), and steel (dust & fumes). Figure: Electrostatic precipitator components
Control of gaseous pollutants from stationary sources •
The most common method for controlling gaseous pollutants is the addition of add-on control devices to recover or destroy a pollutant. There are four commonly used control technologies for gaseous pollutants: - Absorption, - Adsorption, - Condensation, and - Incineration (combustion)
•
Absorption
• • • • • •
The removal of one or more selected components from a gas mixture by absorption is probably the most important operation in the control of gaseous pollutant emissions. Absorption is a process in which a gaseous pollutant is dissolved in a liquid. As the gas stream passes through the liquid, the liquid absorbs the gas, in much the same way that sugar is absorbed in a glass of water when stirred. Absorbers are often referred to as scrubbers, and there are various types of absorption equipment. The principal types of gas absorption equipment include spray towers, packed columns, spray chambers, and venture scrubbers. In general, absorbers can achieve removal efficiencies
grater than 95 percent. One potential problem with absorption is the generation of waste-water, which converts an air pollution problem to a water pollution problem.
Adsorption •
When a gas or vapor is brought into contact with a solid, part of it is taken up by the solid. The molecules that disappear from the gas either enter the inside of the solid, or remain on the outside attached to the surface. The former phenomenon is termed absorption (or dissolution) and the latter adsorption. The most common industrial adsorbents are activated carbon, silica gel, and alumina, because they have enormous surface areas per unit weight. Activated carbon is the universal standard for purification and removal of trace organic contaminants from liquid and vapor streams. Carbon adsorption systems are either regenerative or non-regenerative. - Regenerative system usually contains more than one carbon bed. As one bed actively removes pollutants, another bed is being regenerated for future use. - Non-regenerative systems have thinner beds of activated carbon. In a non-regenerative adsorber, the spent carbon is disposed of when it becomes saturated with the pollutant.
• •
Condensation • • • • •
Condensation is the process of converting a gas or vapor to liquid. Any gas can be reduced to a liquid by lowering its temperature and/or increasing its pressure. Condensers are typically used as pretreatment devices. They can be used ahead of absorbers, absorbers, and incinerators to reduce the total gas volume to be treated by more expensive control equipment. Condensers used for pollution control are contact condensers and surface condensers. In a contact condenser, the gas comes into contact with cold liquid. In a surface condenser, the gas contacts a cooled surface in which cooled liquid or gas is circulated, such as the outside of the tube. Removal efficiencies of condensers typically range from 50 percent to more than 95 percent, depending on design and applications.
Incineration • • •
Incineration, also known as combustion, is most used to control the emissions of organic compounds from process industries. This control technique refers to the rapid oxidation of a substance through the combination of oxygen with a combustible material in the presence of heat. When combustion is complete, the gaseous stream is converted to carbon dioxide and water vapor. Equipment used to control waste gases by combustion can be divided in three categories: - Direct combustion or flaring, - Thermal incineration and - Catalytic incineration.
Direct combustor • •
Direct combustor is a device in which air and all the combustible waste gases react at the burner. Complete combustion must occur instantaneously since there is no residence chamber. A flare can be used to control almost any emission stream containing volatile organic compounds. Studies conducted by EPA have shown that the destruction efficiency of a flare is about 98 percent. In thermal incinerators the combustible waste gases pass over or around a burner flame into a residence chamber where oxidation of the waste gases is completed. Thermal incinerators can destroy gaseous pollutants at efficiencies of greater than 99 percent when operated correctly.
Thermal incinerator general case Catalytic incinerators are very similar to thermal incinerators. The main difference is that after passing through the flame area, the gases pass over a catalyst bed. A catalyst promotes oxidation at lower temperatures, thereby
reducing fuel costs. Destruction efficiencies greater than 95 percent are possible using a catalytic incinerator.
Catalytic incinerator
•
References • • • • •
USEPA, 2007. Online literature from www.epa.gov Rao, M.N. and Rao, H. V. N., 1993. Air Pollution, Tata Mc-Graw Hill, New Delhi. Murty, B. P., 2004. Environmental Meteorology, I.K. International Pvt. Ltd., New Delhi. Nevers, N.D. 2000. Air Pollution Control Engineering, Second Edition, Pub., McGraw Hill, New York. Cheremisinoff, N.P., 2002. Handbook of Air Pollution Prevention and Control, Pub., Butterworth-Heinemann, Elsevier Science, USA.
UNIT III Air pollution meteorology Learning Objectives To make the student aware of dispersion phenomenon of air pollutants covering diffusion and advection, meteorological components, stability of atmosphere and corresponding plume shapes.
THE ECOLOGICAL CRISIS - A Philosophical Perspective
Transport and diffusion from source to receptor
Air Pollutant Cycle
Dispersion • • • • •
General mean air motion Turbulent velocity fluctuationsTurbulent velocity fluctuations Diffusion due to concentration gradients – from plumes Aerodynamic characteristics of pollution Particles - Size - Shape - Weight
• • •
Not always completely understood Two types: Atmospheric heating - Causes natural convection currents --- discussed - Thermal eddies Mechanical turbulence - Results from shear wind effects - Result from air movement over the earth’s surface, influenced by location of buildings and relative roughness of terrain.
•
Lapse Rate • • •
Important characteristic of atmosphere is ability to resist vertical motion: stability Affects ability to disperse pollutants When small volume of air is displaced upward - Encounters lower pressure - Expands to lower temperature
- Assume no heat transfers to surrounding atmosphere - Called adiabatic expansion
Adiabatic Expansion •
To determine the change in temp. w/ elevation due to adiabatic expansion . - Atmosphere considered a stationary column of air in a gravitational field - Gas is a dry ideal gas - Ignoring friction and inertial effects
( dT/dz)adiabatic perfect gas = - (g M/ Cp) • • • • •
T = temperature z = vertical distance g = acceleration due to gravity M = molecular weight of air Cp = heat capacity of the gas at constant pressure
Adiabatic Expansion ( dT/dz)adiabatic perfect gas = -0.0098°C/m or ( dT/dz)adiabatic perfect gas = -5.4°F/ft Change in Temp. with change in height
Lapse rate • •
Lapse rate is the negative of temperature gradient Dry adiabatic lapse rate = Metric:
Metric: G = - 1°C/100m or SI: G = - 5.4°F/1000ft • • •
Important is ability to resist vertical motion: stability. Comparison of G to actual environment lapse rate indicates stability of atmosphere. Degree of stability is a measure of the ability of the atmosphere to disperse pollutants.
Atmospheric Stability • • •
•
Affects dispersion of pollutants Temperature/elevation relationship principal determinant of atmospheric stability Stable - Little vertical mixing - Pollutants emitted near surface tend to stay there - Environmental lapse rate is same as the dry adiabatic lapse rate 4 common scenarios
Stability Classes • • •
Developed for use in dispersion models Developed for use in dispersion models Stability classified into 6 classes (A – F) A: strongly unstable B: moderately unstable C: slightly unstable D: neutral E: slightly stable F: moderately stable
Vertical Temperature Profiles Environmental lapse rate (ELR) Dry adiabatic lapse rate (DALR) If,
• • • •
ELR > DALR =sub adiabatic condition, atmosphere is stable. ELR >> DALR= Inversion conditions. Very stable atmosphere. ELR= DALR= atmosphere is neutral. ELR< DALR = super adiabatic condition, atmosphere is unstable.
Shapes of plumes depends upon atmospheric stability conditions.
Mixing Height of atmosphere The height of the base of the inversion layer from ground surface.
MORNING AND AFTERNOON MIXING DEPTH CALCULATIONS
General Characteristics of Stack Plumes • • • • • •
Dispersion of pollutants Wind – carries pollution downstream from source Atmospheric turbulence -- causes pollutants to fluctuate from mainstream in vertical and crosswind directions Mechanical & atmospheric heating both present at same time but in varying ratios Affect plume dispersion differently
Plume Types •
Plume types are important because they help us understand under what conditions there will be higher concentrations of contaminants at ground level.
Looping Plume
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High degree of convective turbulence
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Superadiabatic lapse rate -- strong instabilities
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Associated with clear daytime conditions accompanied by strong solar heating & light winds
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High probability of high concentrations sporadically at ground level close to stack.
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Occurs in unstable atmospheric conditions.
Coning Plume •
Stable with small-scale turbulence
•
Associated with overcast moderate to strong winds
•
Roughly 10° cone
•
Pollutants travel fairly long distances before reaching ground level in significant amounts
•
Occurs in neutral atmospheric conditions
Fanning Plume •
Occurs under large negative lapse rate
•
Strong inversion at a considerable distance above the stack
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Extremely stable atmosphere
•
Little turbulence
•
If plume density is similar to air, travels downwind at approximately same elevation
Lofting Plume
•
Favorable in the sense that fewer impacts at ground level.
•
Pollutants go up into environment.
•
They are created when atmospheric conditions are unstable above the plume and stable below.
Fumigation •
Most dangerous plume: contaminants are all coming down to ground level.
•
They are created when atmospheric conditions are stable above the plume and unstable below.
•
This happens most often after the daylight sun has warmed the atmosphere, which turns a night time fanning plume into fumigation for about a half an hour.
References • • • • • •
USEPA, 2007. Online literature from www.epa.gov Meteorology and Air Quality Modeling Support for Measurement Projects http://files.harc.edu/Sites/TERC/About/Events/ Other200503/MeteorologyAndAirQuality.pdf Rao, M.N. and Rao, H. V. N., 1993. Air Pollution, Tata Mc-Graw Hill, New Delhi. Murty, B. P., 2004. Environmental Meteorology, I.K. International Pvt. Ltd., New Delhi. Nevers, N.D. 2000. Air Pollution Control Engineering, Second Edition, Pub., McGraw Hill, New York. Cheremisinoff, N.P., 2002. Handbook of Air Pollution Prevention and Control, Pub., Butterworth-Heinemann, Elsevier Science, USA.
UNIT IV Learning Objectives To make the student aware of the air quality model, its definition, types and description of Gaussian based air quality model for point source along with its application.
Air Quality Modelling What does model means? Models reflects a mathematical description of hypothesis conveying the behavior of some physical process or other. Not exact replica but contain some of nature’s essential elements.
What is mathematical modelling? When the process of problem reduction or solution involves transforming some idealized form of the real world situation into mathematical terms, it goes under generic name of mathematical modelling. “Mathematical modelling is an activity which requires rather more than the ability just to solve complex sets of equations difficult through this may be”. Mathematical modelling utilizes ANALOGY to help understand the behavior of complex system.
What is physical modelling? In physical modelling nature is simulated on a smaller scale in the laboratory by a physical experiment. When detailed mathematical models and/ or experimental field measurements become very costly, laboratory simulation using scaled down models in wind tunnels or water channels is often the best approach.
Concept of mathematical modelling applied to air pollution
Source : Point, Line, Area. Receptors : Humans. Transport : Decides fate of air pollution Re-entertainment : Re suspension of air pollutants.
Air Quality Models Analogy - helps in explaining / understanding unfamiliar situations. Ex: Children playing father/ mother game Expectant mothers: practice nappy changing on dolls.
Models -Not exact replica but contain some of nature’s essential elements.
-Ex: When expectant mother practice nappy changing to dolls, dolls are laying still while in reality, babies do not lie still!. -Hence, models reflects a mathematical description of hypothesis conveying the behavior of some physical process or other.
What is air quality model A mathematical relationship between emissions and air quality that incorporates the transport, dispersion and transformation of compounds emitted into the air.
Model objective •
Models are not a unique representation as they never completely replicate a system.
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But models are useful tool in the design of new, large or otherwise modified existing processes or systems.
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Conventional method of designing physical models replicating a process or system is time consuming and cumbersome process.
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Physical models sometime can not replicate a system which is having complicated heat and mass transfer processes.
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Mathematical models therefore is able to cope reasonably well with such processes or systems provided each is built into the set of mathematical equations.
Model categories
Suggested readings:
M. Crossal A.O. Moscardini, “Learning art of mathematical modelling”,Ellis Harmood Publication
Air Quality Models
Suggested readings:
Weber, E., “Air pollution assessment modelling methodology”, NATO, challenges of modern society, vol.2, Plenum press, 1982
What is deterministic approach? The deterministic mathematical models calculate the pollutant concentrations from emission inventory and meteorological variables according to the solution of various equations that represent the relevant physical processes.
Deterministic modelling is the traditional approach for the prediction of air pollutant concentrations in urban areas.
Deterministic approach: Basics What is Transport ? o o o
It is also termed as advection Most obvious effect of atmosphere on emission Advection: implies transport of pollutant downwind of source
What is Dilution? o o o
It is also termed as “mixing”. It is accomplished through “turbulence” Mainly atmospheric turbulence is active
What is Dispersion? Dispersion = Advection (Transport) + Dilution = Advection +Diffusion
Basic Mathematical Equation
Deterministic based AQM The deterministic based air quality model is developed by relating the rate of change of pollutant concentration in terms of average wind and turbulent diffusion which, in turn, is derived from the mass conservation principle.
where C = pollutant concentration; t = time; x, y, z = position of the receptor relative to the source; u, v, w =wind speed coordinate in x, y and z direction; Kx, Ky, Kz = coefficients of turbulent diffusion in x, y and z direction; Q = source strength; R = sink (changes caused by chemical reaction). The above diffusion equation is derived in several ways under different set of assumptions for development of air quality models Gaussian model is one of the mostly used air quality model based on ‘deterministic principle’ Reference: Cheremisinoff, P.N.,1989. Encyclopedia of environmental control technology: air pollution control. Volume 2, Gulf Publishing Company, Houston.
Gaussian plume Dispersion model: Assumptions • • •
Steady-state conditions, which imply that the rate of emission from the point source is constant. Homogeneous flow, which implies that the wind speed is constant both in time and with height (wind direction shear is not considered). Pollutant is conservative and no gravity fallout.
• • • • •
Perfect reflection of the plume at the underlying surface, i.e. no ground absorption. The turbulent diffusion in the x-direction is neglected relative to advection in the transport direction , which implies that the model should be applied for average wind speeds of more than 1 m/s (> 1 m/s). The coordinate system is directed with its x-axis into the direction of the flow, and the v (lateral) and w (vertical) components of the time averaged wind vector are set to zero. The terrain underlying the plume is flat All variables are ensemble averaged, which implies long-term averaging with stationary conditions.
Gaussian Plume Dispersion Model
Application: Gaussian Based Vehicular Pollutant Dispersion Model The basic approach for development of deterministic vehicular pollution (line source) model is the coordinate transformation between wind coordinate system (X1, Y1, Z1) and line source coordinate system (X, Y, Z). A hypothetical line source is assumed to exist along Y1 that makes the wind direction perpendicular to it (Figure 1). The concentration at receptor is given by Csanady (1972):
Reference: Csanday, G.T., 1972. Crosswind shear effects on atmospheric diffusion. Atmospheric Environment, 6,221-232.
Numerical approach Numerical models also comes under deterministic modelling technique which are based on numerical approximation of partial differential equations representing atmospheric dispersion phenomena.
Basic mathematical equation
The term Ft in the above equation is unknown and diffused equation is not in close form. Reference Juda, K., 1986. Modelling of the air pollution in the Cracow area. Atmospheric Environment, 20 (12), 2449-2458.
Basis for numerical approach First order closure models, also called K- models, have their common roots in the atmospheric diffusion equation derived by using a Ktheory approximation for the closure of the turbulent diffusion equation. The first order closure models are time dependent.
Numerical based AQM Eulerian grid model (Danard, M.B., 1972) Lagrangian trajectory model (Johnson, 1981) Hybrid of eulerian-lagrangian model (Particle-in-cell) (Sklarew et al., 1972) Random walk (Monte-Carlo) trajectory particle model (Joynt and Blackman, 1976)
Mostly used numerical based AQM Gaussian puff model (Hanna et al., 1982) Reference
• • •
Danard, M.B., 1972. Numerical modelling of carbon monoxide concentration near a Highway. Journal of Applied Meteorology, 11, 947-957. Johnson, W.B., 1981. Interregional exchanges of air pollution: model types and application. In Air pollution modelling and its application-I, Edited by Wispelaere, C. De., Plenum Press, New York. Sklarew, R.C., Fabrick, A.J. and Prager, J.E., 1972. Mathematical modelling of photochemical smog the using PIC method. Journal of Air Pollution Control Association, 22, 865-
• •
Joynt, R.C. and Blackman, D.R., 1976. A numerical model of pollutant transport. Atmospheric Environment, 10, 433-. Hanna, S.R., Brigs, G.A. and Hosker, Jr. R.P., 1982. Handbook on atmospheric diffusion. National Technical Information Centre, U.S. Department of Energy, Virginia.
Unit V Statistical Approach noise pollution Statistical models calculate pollutant concentrations by statistical methods from meteorological and emission parameters after an appropriate statistical relationship has been obtained empirically from measured concentration
Basis for statistical approach Regression and multiple regression models (Comrie, 1997) •
Regression models describes the relationship between predictors (meteorological and emission parameters) and predictant (pollutant concentrations)
Time series models (Box and Jenkins, 1976) • • •
Time series analysis is purely based on statistical method applicable to non repeatable experiments. Box-Jenkins approach extracts all the trends and serial correlations among the air quality data until only a sequence of white noise (shock) remains. The extraction is accomplished via the difference, autoregressive and moving average operators.
Reference: • •
Comrie, A. C., 1997. Comparing neural networks and regression model for ozone forecasting. Journal of Air and Waste Management Association, 47, 653-663 Box, G.E.P. and Jenkins, G.M., 1976. Time series analysis forecasting and control. 2nd Edition, Holdenday, San Francisco.
Basic mathematical equation The Box –Jenkins (B-J) models are empirical models created from the historical data. Statistical graphs of the autocorrelation function (ACF) and partial autocorrelation function (PACF) to identify an appropriate time series model. The general class of univariate B-J seasonal models, denoted by ARIMA (p, d, q) X ( P, D, Q)s can be expressed as:
Where
= regular and seasonal autoregressive parameters, B = backward shift operators,
=difference operators, d and D =
= observed data series, = regular and seasonal moving average order of regular and seasonal differencing, s= period/span, parameters, at = random noise, p, P, q and Q represent the order of the model and c = constant.
Mostly used stochastic based AQM
- 24 h avg.. CO model - Max. daily 1-h avg.CO model - Max. daily working hours (8 AM - 8PM) 1-hour COmodel - Hourly average CO model
- 24 h avg.. CO model with wind speed as input - 24 h avg.. CO model with temperature as input - Max. daily 1-h avg.. CO model with wind speed as input - Max. daily 1-h avg.. CO model with temperature as input - Max. daily working hours 1-hour avg.. CO model with wind speed as input - Max. daily working hours 1-hour avg.. CO model with temperature as input - Hourly average CO model withwind speed as input - Hourly average CO model with temperature as input
- 24 h avg.. CO model with temperature and wind speed as inputs - Max. daily 1-h avg.. CO model with wind speed and temperature as inputs - Max. daily working hours 1-hour avg.. CO model with wind speed and temperature as inputs
Reference: • •
Khare, M. and Sharma, P., 2002. Modelling urban vehicle emissions. WIT press, Southampton, UK. Sharma, P. and Khare, M., 2001. Short-time, real – time prediction of extreme ambient carbon monoxide concentrations due to vehicular exhaust emissions using transfer function noise models. Transportation Research D6, 141-146.
Physical modelling approach – Wind Tunnel
•
26 m long, suction type, low wind speed, 16 m test section, 8 panels, 2 m each
•
EWT consists of entrance section, honeycomb section, wire mesh screen filters, test section, exit contraction section, transition and diffuser section
•
Turntable of 1.8 m diameter
•
Plenum chamber for prevention of surge and other disturbances, 6mx5m wall
ENVIRONMENTAL WIND TUNNEL- IIT DELHI
Basis for physical approach • • •
The physical simulation studies using wind tunnels have shown high potential to understand complex urban dispersion phenomenon. The pollutant concentrations measured within the physical model can be converted to equivalent atmospheric concentrations through the use of appropriate scaling relationship. In the physical simulation studies of exhaust dispersion, the model vehicle movement system (MVMS) plays a vital role. The vehicle-induced turbulence can be understood accurately by using MVMS.
Design consideration for MVMS* •
maintenance of ‘‘no slip’’ boundary condition in atmospheric boundary layer (ABL) flow,
• • • •
variations in traffic volume and traffic speed for two-way traffic, operation of MVMS for various street configurations, variation in approaching wind directions and wind speed, operation of vehicles in different lanes.
Reference:
• •
*Ahmad, K., Khare, M. and Chaudhry, K.K. 2005. Wind tunnel simulation studies on dispersion at urban street canyons and intersections- a review. Journal of Wind Engineering and Industrial Aerodynamics, 93, 697-71 Eskridge, R.E. and Hunt, J.C.R., 1979. Highway modelling-I: prediction of velocity and turbulence fields in the wake of vehicles. Journal of Applied Meteorology, 18 (4), 387- 400.
Plan of MVMS for urban street
Plan of MVMS for Urban Intersection
Wind tunnel based AQM •
Development, testing and validation of atmospheric dispersion models through EWT generated database in a variety of atmospheric conditions.
•
Systematic understanding of the pollutants dispersion characteristics for line source (automobile exhaust emissions), point source (stack emissions) and area source (low level areal emissions) in plain and complex terrains, such as, hills and valleys.
•
Understanding of the dispersive behavior of toxic gases from accidental releases.
•
Studies on the effects of pollutants on plants and buildings under dynamic environmental conditions for various geographical conditions.
•
Simulation of ‘heat islands’ and its effect on pollutant dispersion.
•
Location of ‘hot spots’ at the urban intersections.
Reference
• •
Eskridge, P.E. and Thompson, R.S., 1982. Experimental and theoretical study of the wake of a block-shaped vehicle in a shear-free boundary flow. Atmospheric Environment, 16 (12), 2821-2836. Snyder, W.H., 1972. Fluid models for the study of air pollution meteorology: similarity facilities, review of literature and recommendations, U.S. Environmental Protection Agency, Washington.
Limitations of Models * Deterministic models • • • • • • •
Inadequate dispersion parameters Inadequate treatment of dispersion upwind of the road Requires a cumbersome numerical integration especially when the wind forms a small angle with the roadways. Gaussian based plume models perform poorly when wind speeds are less than 1m/s. Numerical models have common limitations arising from employing the K-theory for the closure of diffusion equation. The Ktheory diffusion equation is valid only if the size of the ‘plume’ or ‘puff’ of pollutants is greater than the size of the dominant turbulent eddies. The Gaussian puff model relative diffusion parameters are derived from very few field experiments, which limits its applicability. The other limitations of numerical models are large computational costs in terms of time and storage of data. It also requires large amounts of input data.
Statistical models • • • • • • • • • •
Require long historical data sets and lack of physical interpretation. Regression modelling often underperforms when used to model non-linear systems. Time series modelling requires considerable knowledge in time series statistics i.e. autocorrelation function (ACF) and partial auto correlation function (PACF) to identify an appropriate air quality model. Statistical models are site specific. Hybrid model prediction accuracy depends on the selection of suitable deterministic model and identification of appropriate statistical distribution parameter. Application of hybrid approach to strongly auto correlated and/or non-stationary data requires specific treatment for auto correlation /non stationary to increase prediction accuracy. In ANN based vehicular pollution model, the main problem facing when training neural network model, is deciding upon the network architecture (i.e., number of hidden layers, number of nodes in hidden layers and their interconnection). At present, no procedures has been established for selecting proper network architecture, rather than training several network architecture and choose the best out of them. Multilayer neural network performs well when used for interpolation, but poorly, if used for extrapolation. No thumb rules exist in selection of data set for training, testing and validation of neural network based model.
Physical models: wind tunnel • • •
The major limitations of wind tunnel studies are construction and operational cost. Simulation of real time air pollution dispersion is expensive. Real time forecast is not possible.
* Reference: • •
Juda, K., 1989. Air pollution modelling. In: Cheremisinoff, P.N. (Eds.), Encyclopedia of Environmental Control Technology, Vol. 2: Air Pollution Control, Gulf Publishing Company, Houston, Texas, USA, pp.83-134. Nagendra, S.M.S. and Khare, M., 2002. Line source emission modelling- review. Atmospheric Environment, 36 (13), 20832098.
Box Model •
Application : Area source
•
Principle : (i) It assumes uniform mixing throughout the volume of a three dimensional box. (ii) Steady state emission and atmospheric conditions. (iii) No upwind background concentration.
•
Model description
Suggested reading:
Lyons, T.J. and Scott, W.D. “Principles of air pollution meteorology”, Behavan press, 1990
Line source model Application •
motor vehicle travelling along a straight section of highway OR agricultural burning along the edge of a field OR line of industrial sources on the bank of a river
Assumption • • •
Model:
Infinite length source continuously emitting the pollution Ground level source Wind blowing perpendicular to the line source
