AIR POLLUTION CONTROL TECHNOLOGY HANDBOOK - CHAPTER 6 pps

Ambient Air Quality and Continuous Emissions Monitoring 6.1 AMBIENT AIR QUALITY SAMPLING PROGRAM Air pollution monitoring is conducted to determine either emission concentration or ambi

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Ambient Air Quality and Continuous

Emissions Monitoring

6.1 AMBIENT AIR QUALITY SAMPLING PROGRAM

Air pollution monitoring is conducted to determine either emission concentration or ambient air quality The range of emission concentrations, temperature, and pressures encountered is sometimes many magnitudes greater than found at an ambient air sampling station For this reason, sampling and analysis techniques and equipment are different for each case, even though the same general principles may be employed This chapter deals with both ambient air quality sampling procedures and monitoring and continuous emissions monitoring Lodge1 presents a good dis-cussion in Methods of Air Sampling and Analysis.

6.2 OBJECTIVES OF A SAMPLING PROGRAM

Instrumentation for ambient air quality monitoring is perhaps best described in terms

of the types of measurements that would be made in a typical community by the local air pollution control agency Usually a sample network would be installed that would blanket the area with a series of similar stations The object would be to measure the amount of gaseous and particulate matter at enough locations to make the data statistically significant It is not uncommon to find each station in a network equipped with simple, unsophisticated grab sampling devices However, quite a few sophisticated monitoring networks have been developed which contain continuous monitors with telemetry and computer control Meteorological variables are also monitored and correlated with the concentration data The information is then used:

1 To establish and evaluate control measures

2 To evaluate atmospheric-diffusion model parameters

3 To determine areas and time periods when hazardous levels of pollution exist in the atmosphere

4 For emergency warning systems

6.3 MONITORING SYSTEMS

Ambient air quality data may be obtained through the use of mobile or fixed sampling networks and the use of integrated samplers or continuous monitors Decisions 6

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regarding these monitoring techniques constitute the first important steps in design

of a monitoring network

6.3.1 F IXED VS M OBILE S AMPLING

Fixed-point sampling entails a network of monitoring stations at selected sites, operated simultaneously throughout the study Stations are permanent or, at least, long-term installations In a mobile sampling network, the monitoring/sampling instruments are rotated on schedule among selected locations The monitoring/sam-pling is not conducted simultaneously at all locations, and the equipment is generally housed in trailers, automobiles, or other mobile units An advantage of fixed sampling

is that measurements are made concurrently at all sites, providing directly compa-rable information, which is particularly important in determining relationship of polluting sources to local air quality and in tracing dispersion of pollutants through-out the area The chief advantage of mobile sampling is that air quality measurements can be made at many sites — far more than would be feasible in a fixed sampling program Mobile sampling provides better definition of the geographical variations

if the program is long enough to generate meaningful data

6.3.2 C ONTINUOUS VS I NTEGRATED S AMPLING

Continuous monitoring is conducted with devices that operate as both sampler and analyzer Pollutant concentrations are instantaneously displayed on a meter, contin-uously recorded on a chart, magnetic tape, or disk Integrated sampling is done with devices that collect a sample over some specified time interval after which the sample

is sent to a laboratory for analysis The result is a single pollutant concentration that has been integrated, or averaged, over the entire sampling period This is an older technique and currently in limited use

Continuous or automatic monitoring instruments offer some advantages over integrating samplers; for example, there is a capability for furnishing short-interval data, and there is a rapid availability of data Moreover, output of the instruments can be electronically sent to a central point Also, continuous monitors require less laboratory support They also may be necessary to monitor some pollutants where

no integrating method is available or where it is necessary to collect data over short averaging times, for example, 15 min Automated monitors also have some draw-backs They require more sophisticated maintenance and calibration, and the oper-ators and maintenance personnel have to be more highly technically trained The selection of a monitoring system is influenced by the averaging time for which concentrations are desired, i.e., 15-min, 1-h, 3-h, 8-h, 12-h, or 24-h time intervals It should be consistent with the averaging times specified by air quality standards For example, in assessing SO2 levels, good coverage can be provided by use of integrated samples, widely dispersed over the area, and one or more contin-uous sampler/analyzers situated in heavily populated areas The integrated sampler defines SO2 levels over a broad area, and the continuous devices provide detailed information on diurnal patterns The short averaging time of interest for CO and ozone dictates the use of continuous monitors for these pollutants

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6.3.3 S ELECTION OF I NSTRUMENTATION AND M ETHODS

Choice of instrumentation for an air-monitoring network depends on the following factors:

• Type of pollutants

• Averaging time specified by air quality criteria or standards

• Expected pollutant levels

• Available resources

• Availability of trained personnel

• Presence in the air of interfering materials

Most pollutants may be monitored by a number of different methods and tech-niques The selection of the methodology to be used is an important step in the design of the monitoring portion of the assessment study

6.4 FEDERAL REFERENCE METHODS AND

CONTINUOUS MONITORING

In order to evaluate the current air quality and the effect of air pollution control measures, most larger communities maintain monitoring networks The EPA, as well

as most of the states, maintains its own surveillance networks The ideal objective when installing a monitoring network is to be able to obtain continuous real time data Table 6.1 lists standard methods of measurement Only 3 of these standard methods employ continuous or semicontinuous monitors

No satisfactory device exists as yet for determining suspended particulate on a continuous basis However, Table 6.2 lists continuous methods which are now com-monly employed for the measurement of the five regulated air pollutants

TABLE 6.1

Federal Reference Methods

ionization detector

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Particulate matter emissions can be continuously detected through opacity mea-surements Opacity is a function of light transmission through the plume and is define by the following equation

(6.1) where

OP = percent opacity

I = light flux leaving the plume

I0 = incident light flux

The following information, Documentation of the Federal Reference Methods for the Determination of the Regulated Air Pollutants, can be found in the Code of Federal Regulations, Title 40 (CFR 40) — Protection of Environment, Chapter 1, Environmental Protection Agency, Subchapter C — Air Programs, Part 50, National Primary and Secondary Ambient Air Quality Standards The following is a list of those methods with reference to CFR 40

TABLE 6.2

Continuous Monitor Methods for Federally Regulated Pollutants

Coulometric Flame chemiluminescence

ionization detection, a semicontinuous method

Chemiluminescent reaction with ozone

Flame ionization detector Semicontinuous

Chemiluminescent reaction with ethylene

Documentation of the Federal Reference Methods for the Determination

of Regulated Air Pollutants

Sulfur Dioxide

Appendix A

Reference Method for the Determination of Sulfur Dioxide in the Atmosphere (Pararosaniline Method)

Particulate Matter

Appendix B

Reference Method for the Determination of Suspended Particulate in the Atmosphere — High Volume

OP= −(1 I I0)×100

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6.5 THE “COMPLETE” ENVIRONMENTAL SURVEILLANCE

AND CONTROL SYSTEM

An ideal surveillance and control system can be devised employing continuous monitoring, telemetering, and electronic data processing It is possible to assemble such a system from the hardware components that are now available The major drawback of this automatic system is the limitations of the computer software; there

is little economic information available for formulating the ambient air quality and optimizing models Figure 6.1 illustrates one conception of a surveillance and control system This system would demand instruments which could be calibrated to a known standard, would retain their calibration over long periods of time, would be

Carbon Monoxide

Appendix C

Measurement Principle and Calibration Procedure for the Continuous Measurement of Carbon Monoxide

in the Atmosphere (Non-Dispersive Infrared Spectrometry)

Ozone

Appendix D

Measurement Principle and Calibration Procedure for the Measurement of Photochemical Oxidants Corrected for Interferences Due to Nitrogen Oxides and Sulfur Dioxide

Hydrocarbons

Appendix E (Corrected for Methane)

Reference Method for Determination of Hydrocarbons Corrected for Methane

Nitrogen Dioxide

Appendix F

Measurement Principle and Calibration Procedure for the Measurement of Nitrogen Dioxide in the Atmosphere (Gas Phase Chemiluminescence)

Lead

Appendix G

Reference Method for the Determination of Lead in Suspended Particulate Matter Collected from Ambient Air

Ozone

Appendix H

Interpretation of the National Ambient Air Quality Standards for Ozone

Appendix J

Particulate Matter

Appendix K

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free of electronic drift over these long-time periods, and would possess suitable dynamic response

Many automated environmental surveillance systems employing continuous monitors exist in the U.S and throughout the world None are quite as sophisticated

as would be implied by the system of Figure 6.1

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6.6 TYPICAL AIR SAMPLING TRAIN

A typical air pollution sampling train is applicable to the intermittent collection of

an air sample containing either gaseous or particulate pollutants The sample is retained in the collection equipment which is then removed for the sample train Further processing takes place to prepare the sample for analysis Most of the analysis techniques are standard procedures involving one or more of the following methods:

1 Gravimetric

2 Volumetric

3 Microscopy

4 Instrumental

a Spectrophotometric

i Ultraviolet

ii Visible (Colorimetry) iii Infra-red

b Electrical

i Conductometric

ii Coulometric iii Titrimetric

c Emission Spectroscopy

d Mass Spectroscopy

e Chromatography

6.7 INTEGRATED SAMPLING DEVICES FOR SUSPENDED

PARTICULATE MATTER

Suspended particulate are small particulate that vary in size from less than 1 micron

to approximately 100 microns They remain suspended in the atmosphere for long periods of time and absorb, reflect, and scatter the sunlight, obscuring visibility When breathed, they penetrate deeply into the lungs They also cause economic loss because of their soiling and corrosive properties

The new EPA ambient particulate-matter definition includes only the part of the size distribution that could penetrate into the human thorax This requires a sampling inlet with a 10 µm cutpoint to mimic deposition in the extra thoracic regions Figure 6.2 is a schematic of a sampler designed to meet this requirement There is

a two-stage selective inlet Air is drawn into the inlet and deflected downward into acceleration jets of the first-stage fractionator Larger, non-inhalable particles are removed Air then flows through the first stage vent tubes and then through the second fractionation stage More non-inhalable particles are removed, and the remaining aerosol is drawn through the usual 8″× 10″ sampling filter The new PM10 standard includes only those particles with an aerodynamic diameter less than or equal to 10 µm This standard went into effect on July 31, 1987

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6.8 CONTINUOUS AIR QUALITY MONITORS

Continuous emissions monitoring (CEM) is required by the Clean Air Act Amend-ments to monitor SO2, NOx, CO, CO2, opacity, total hydrocarbons, and total reduced sulfur (TRS) Title IV, which is to ensure compliance with the Acid Rain Program, sets out provisions for CEM in the two-phase utility power industry control strategy Phase I Utilities were required to install CEM by November 15, 1993, and Phase II Utilities by January 1, 1995 Title III focuses on 189 hazardous air pollutants (HAP), some of which will possibly require CEM This should spawn CEM techniques optimized for the chemical compound being monitored Title V will require CEM for compliance assurance The collection of real-time emission data will be the first step to attaining the national mandated reduction in SO2 and NOx emissions Fur-thermore, CEM can be used to track use of allowances in the new market-based SO2 emissions trading program

CEM is carried out by two general methods — in situ and extractive Each of the methods measures on a volumetric basis, ppm for example Therefore, the measurements require conversion to mass emission rates on a lb/h or lb/106 BTU basis Monitoring instrumentation requires at least 10% relative accuracy Perfor-mance specifications and test procedures can be found in Title 40 CFR, Part 60, Appendix B

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The EPA Emission Measurement Center (EMC) at Research Triangle Park and

the Midwest Research Institute have created a data base on CEM You may use

browse techniques or search the data base by HAP or analyzer type The data base

is found on the EPA home page at: www.epa.gov/ttnami1/siteindx.html

Figure 6.3 is a schematic flow diagram of a general continuous air-quality

mon-itor The device contains:

1 Primary air-moving device, usually a vacuum pump, to pull the air sample

through the instrument

2 Flow-control and -monitoring device, usually a constant pressure regulator

and rotameter

3 Pollutant detection by various primary sensing techniques

4 Automatic reagent addition where needed

5 Electronic circuitry for transducing the primary signal to a signal suitable

for recording and telemetering

6 Provisions for automatic calibration, usually several solenoid valves which

can be operated remotely to connect the inlet gas to a scrubbing train for

removal of all pollutants and establishing a chemical zero, or, alternatively,

to one or more span gases for setting the chemical range of the instruments

Many monitors of the general type described above have been developed for all

of the federally regulated gaseous pollutants and others as well The remainder of

this section will provide details of these devices This list is not exhaustive

Further-more, although the devices described below are indicated for a particular pollutant,

they can be used for other types of pollutants as well

6.8.1 E LECTROCONDUCTIVITY A NALYZER FOR SO 2

Electrical conductivity was the basis of the first continuous monitor used to detect

and monitor an air pollutant — SO2 This monitor was built by a Dr Thomas to

monitor SO2 in a greenhouse during a study of the effects of SO2 on plants Later,

it was employed by TVA in its original studies of SO2 from power plant plumes

In an electroconductivity apparatus, a reagent passes through a reference

con-ductivity cell and then into an absorbing column Air is drawn by a vacuum pump

counter-currently to the reagent flow through the absorbing tube, then through a

separator to the exhaust The SO2 is absorbed in the reagent which then passes

through a measuring conductivity cell A stabilized AC voltage is impressed across

the conductivity cells resulting in a current flow that is directly proportional to the

conductivity of the solution The value of this current is measured by connecting a

resistor in series with the cell to obtain an AC voltage which is proportional to the

current This voltage is then rectified to direct current The DC signals from the

rectifiers are connected in opposition, thus resulting in a voltage that induces a

current through a meter which is directly related to the difference in conductivity

between the two solutions To set the zero on the instrument, any SO2 is removed

by passing the air through a soda-lime absorber The conductivity in both cells should

then be the same, and the meter output should be zero

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