*

 Throughout this section, the work duct is meant to include any conduit through which an exhaust gas

flows, including stacks and any ductwork between the source and stack where a PM CEMS probe or detector may be

located.

V-1

V.0 STRATIFICATION

V.1 WHAT IS STRATIFICATION?

Stratification of a gas stream in a duct* is a condition in which one or more characteristics of the

gas stream differ significantly over the cross-section of the duct. Stratification can be

problematic for PM CEMS when establishing the initial correlation and meeting the correlation

criteria of PS-11. In addition, changes in stratification over time can invalidate the correlation

for the PM CEMS, resulting in PM CEMS responses that are not representative of PM emissions

and in failure to meet the acceptance criteria for RCAs. 

Stratification of gas streams can be defined in terms of pollutant concentration, velocity,

temperature, or other characteristics. However, this section focuses on the stratification of PM

concentrations, which is the characteristic of primary concern with PM CEMS. Most of the

published literature on stratification concerns stratification of gases, such as SO2 or NOx. The

EPA has defined gaseous pollutant stratification as a condition in which the concentration of a

gaseous pollutant at a specific point in a duct differs from the average concentration of that

pollutant over the cross-section of the duct by more than 10 percent. Several performance

specifications published by EPA have incorporated a stratification test using this 10 percent

criterion. Examples include PS-2 for SO2 and NOx CEMS and Appendix A to 40 CFR 75,

Specifications and Test Procedures. 

V.2 WHAT CAUSES STRATIFICATION?

The primary factors responsible for PM stratification are the size, mass density, and velocity of

the particles; the duct configuration; and the degree of mixing in the gas stream. Particle shape

also affects stratification, but in most cases, to a much lesser degree than do the other factors

listed above. Stratification of PM usually results from a combination of these factors, so it can

be misleading to discuss these factors independently of one another. 

Stratification of PM is generally more severe than gaseous stratification because of the inertial

forces that act on PM but do not affect gases. For a specific gas stream velocity, the magnitude

of the inertial forces is largely a function of particle size and mass density. For particles with a

uniform mass density, the smaller the particles, the weaker the inertial forces acting on those
V-2

particles, and the more the particles behave like gases. Specifically, particles with diameters that

are less 1.0 micrometers (μm) are assumed to behave as gases.

1

 Stratification is more likely to

occur in gas streams with particles that are larger than 1.0 μm, and the likelihood for

stratification increases with increasing particle size. Any factor that affects particle size can also

affect the severity of PM stratification. For example, in exhaust streams characterized by sticky

particles, the likelihood of stratification downstream increases as the particles agglomerate and

grow in size.

Because the inertial forces acting on particles are a function of particle mass density, mass

density also affects the degree of stratification. Gas streams characterized by particles with

higher mass density are more likely to be stratified because the inertial forces on the PM increase

with increased particle mass density. 

The velocity of the gas stream can affect PM stratification in two ways. For lower velocities in

nonvertical ducts, the relative effect of gravity on particles in the gas stream is greater because

the particles have more time to settle. As the particles settle, the PM concentration along the

bottom or lower side of the duct increases, resulting in increasingly stratified flow. For vertical

ducts, gravity does not affect stratification over the cross-section of a duct because gravity acts in

the same direction (for downward flow) or opposite direction (for upward flow) of the PM as it

flows through the duct.

The inertial forces on particles in an exhaust stream increase with increasing velocity. For

straight ducts with no flow disturbances, the inertial forces on the particle do not induce

stratification. However, these inertial forces become significant following bends, obstructions, or

other disturbances that cause changes in the velocity or direction of the exhaust flow. In such

cases, the higher the velocity of the gas stream, the greater the degree of PM stratification.

As explained in the previous paragraph, duct configuration can have a significant effect on PM

stratification. Stratification of PM may become problematic following bends or changes in

direction of ducts, where the momentum of the larger or more massive particles carries them to

the far side of the duct wall immediately following the bend. The more abrupt the directional

change, the more severe the stratification that is likely to occur. Other duct locations where PM

stratification is likely to occur include the areas immediately following a junction where a

secondary gas stream is introduced into the duct. If the concentration of the secondary stream

differs from the PM concentration in the duct, stratification can occur until a downstream

location in the duct where the combined flows are relatively well mixed. 

For obvious reasons, the degree of mixing can significantly affect PM stratification. 

Stratification will not be a problem in any gas stream that is well mixed. Thus, the presence of

baffles, vanes, or other devices that induce mixing will help to eliminate stratification in the gas

stream.

V.3 WHERE IS STRATIFICATION LIKELY TO OCCUR?

Stratification is more likely to occur immediately downstream of a disturbance in a duct. 

Specifically, stratification can be present in the following locations:
V-3

# Downstream of a bend,

# Downstream of any obstruction that results in changes in the velocity of the exhaust

stream,

# Downstream of a junction in the duct where an additional exhaust or air stream is

introduced to the duct,

# Along any nonvertical section of duct where the exhaust gas velocity is relatively low,

and

# Along long sections of nonvertical ducts.

This is not meant to imply that stratification always is present downstream of a disturbance in the

duct. As noted above, there are several factors that affect the degree of stratification. For

example, if the PM in an exhaust stream consists of very fine particles (< 1.0 μm), the likelihood

and severity of stratification are reduced. The distance downstream of the disturbance is also

important. With the exception of relatively long horizontal ducts, the further downstream, the

less severe the stratification is likely to be. 

The degree of stratification is also highly dependent on particle size and mass density; exhaust

gas streams with larger or more dense particles are more likely to experience stratification than

exhaust streams with smaller or less dense particles. For this reason, knowledge of particle size

distribution and other characteristics may be helpful in determining the likelihood of

stratification in any particular duct.

V.4 HOW DOES STRATIFICATION AFFECT PM CEMS OPERATION?

If the PM concentration in a gas stream is stratified, the location of the PM CEMS probe (for

extractive systems) or detector (for in situ systems) may not be representative of the average

concentration in the cross-section of the duct. For example, in a stratified stack, the

concentration of PM at the PM CEMS probe or detector may be 80 percent of the average PM

concentration across the stack at that point. In such cases, the correlation equation will account

for such a difference if the stratification does not change with changes in PM concentrations. 

That is, if the PM concentration at the probe or detector remains at or above 80 percent of the

average PM concentration across the stack, the stratification will have no adverse effect on the

PM CEMS correlation. Although the correlation equation is likely to be different, the correlation

coefficient, confidence interval half range, and tolerance interval half range do not change.

In general, the correlation will account for the stratification if there is a numerical relationship

between the PM CEMS response in the stratified duct and the PM CEMS response for the

corresponding unstratified duct (i.e., what the PM CEMS responses would have been if the duct

had not been stratified and the PM concentration at the PM CEMS probe or detector equaled the

average PM concentration across the duct). Appendix A includes several examples that

demonstrate this principle. In each of the examples, the correlation equation changes, but the
V-4

correlation coefficient, confidence interval half range, and tolerance interval half range do not

change significantly.

If there is no such numerical relationship between the concentration at the probe or detector of

the PM CEMS and the average PM concentration across the duct, it may not be possible to

develop a correlation equation that satisfies the requirements of PS-11. In such cases, developing

multiple correlations may solve the problem. Otherwise, the only options are to eliminate the

stratification or move the PM CEMS probe or detector to a location where the PM concentrations

are not stratified. The possible presence of stratification reinforces the need to perform the

correlation test over the widest practical range of PM concentrations. The wider the range of

concentrations, the more representative the correlation equation is likely to be. 

Problems also can arise when the stratification changes with time as a result of changes in PM 

characteristics, such as particle size distribution, or changes in the exhaust and emission control

system. For example, replacing an induced draft fan with another fan that changes the exhaust

flow rate can give rise to changes in stratification. In cases where the stratification undergoes

significant change, the original correlation equation for the PM CEMS may no longer be a

reliable means of quantifying emissions, and it may be necessary to conduct a correlation test and

develop a new PM CEMS correlation.

V.5 HOW DO I TEST FOR STRATIFICATION?

If PM stratification is suspected, a stratification test should be performed. Performance

Specification 2 of Appendix B to 40 CFR 60 and Appendix A to 40 CFR 75 include procedures

for evaluating gaseous pollutant stratification. Determining the presence of PM stratification in a

duct can be carried out by following the same general steps specified for gaseous pollutants. For

gaseous pollutants, sampling times as short as 2 minutes at each point may be adequate. To test

for PM stratification, much longer sampling times at each point may be necessary. The basic

procedure is to sample over the cross-section of the duct and compare the concentrations at each

sampling point to the average concentration for the cross-section.2 For rectangular ducts, a

minimum of nine sampling points is recommended, with each point located at the centroid of

similarly shaped, equal area divisions of the duct cross-section. For circular ducts, a minimum of

12 sampling points is recommended, with 6 sampling points along each of two traverses, as

specified in 40 CFR 60, Appendix A, Method 1. 

Isokinetic sampling should be used at all sampling points during a stratification test because

sampling nonisokinetically can disrupt stratification patterns in the duct.3 The minimum

sampling time at each point is a function of the PM concentration and the sampling method. For

example, if EPA Method 5 is used, sampling times of at least 15 minutes to 1 hour may be

required at each point to collect a quantifiable PM sample on the filter. Longer sampling times

help to ensure that the mass of PM collected can be accurately measured. On the other hand, the

longer the sampling times, the more likely there will be variations in PM concentrations over the

duration of the stratification test. Regardless of duration, the sampling times must be the same

for each point. 
V-5

Rather than using a manual reference method to sample PM, it may be possible to use a PM

CEMS. Even if a correlation between the PM CEMS and PM concentrations has not been

established, the responses of the PM CEMS at the various sampling points across the stack can

be compared to determine if there is a significant difference in PM concentrations across the

stack. An advantage to using a PM CEMS instead of a manual method to test for stratification is

that PM CEMS provide immediate results. In addition, much shorter sampling times may be

adequate if a PM CEMS is used. As noted above, regardless of duration, the sampling times

must be the same at each point when testing for stratification.

It is critical that the source is operated consistently (e.g., at constant load or process throughput)

throughout the entire stratification test to ensure that PM concentrations do not vary significantly

during the test. Control device operation also should be monitored carefully. 

To ensure that there are no temporal variations in stratification, it is advisable to take separate

independent PM concentration measurements at a stationary location throughout the entire

stratification test, particularly if the test is scheduled to last several hours. To accomplish this, a

second stationary probe connected to an independent sampling train or measurement system

should be located at or near the centroid of the duct. One option is to use a PM CEMS as the

stationary probe. If a manual method sampling train (e.g., Method 5) is used for the stationary

measurements, the sampling conducted concurrently with each run of the traversing probe must

also be treated as a separate test run for the stationary probe/sampling train. For example, upon

the completion of sampling at each traverse point, the Method 5 stationary probe is removed and

the sample recovery procedures specified in Section 4.2 of Method 5 are followed.

Following completion of the test, the PM concentration is determined for each sampling point. If

a second, stationary probe was used, the PM concentrations also should be determined for each

traverse point sampling period (e.g., if the sampling time was 15 minutes, the concentrations

measured using the stationary probe should be determined for each corresponding 15-minute

period). If the data from the stationary probe indicate that PM concentrations did not vary

significantly over the test, the PM concentrations at each traverse point are then compared to the

average of the PM concentrations for all sampling points to determine the percent stratification

(S). This can be expressed by the following:

S = |ci - cave| × 100%

where

S = percent stratification

ci = PM concentration at sampling point i

cave = average PM concentration for all sampling points.

If the percent stratification is more than 10 percent for any traverse point, the duct is considered

to be stratified at that location. However, as noted above, the presence of stratification does not

necessarily have an adverse effect on the correlation. If the data from the second probe indicate

significant variations in PM concentration over the course of the stratification test, the results
V-6

from the traversing probe are more difficult to interpret, and it may be necessary to repeat the

stratification test.

An alternative method is to conduct a series of test runs using two sampling trains, one of which

traverses the stack and the other is placed at the proposed location of the PM CEMS throughout

the test. For each test run, the average PM concentration measured by the traversing probe is

compared to the concentration measured by the stationary probe. If the concentrations are in

good agreement, the PM CEMS location can be considered representative. However, the test

should be repeated under a range of process operating conditions to provide some assurance that

the location remains representative under normal operating conditions. 

As explained previously, stratification per se may not be a problem because the correlation

equation can account for differences in PM concentrations, provided there is a numerical

relationship between the average concentration and the concentration at the PM CEMS probe or

detector location (e.g., the PM concentration at the probe or detector remains a fixed percentage

of the average PM concentration across the duct). Examples of this situation are provided in

Appendix A. If there is no such relationship, it may not be possible to develop a correlation

equation that satisfies PS-11 at that location. To determine if this is the case, the stratification

test described above can be repeated for a range of PM concentrations. This can be a costprohibitive undertaking and other options may be preferred (e.g., select a different location for

the PM CEMS).

V.6 WHAT CAN I DO IF I HAVE A STRATIFIED GAS STREAM?

If you suspect that PM stratification is occurring at a proposed PM CEMS sampling point, the

primary options are as follows:

# Locate the PM CEMS probe or detector as proposed, assume that the correlation will

account for the stratification, and verify the performance of the PM CEMS over time

through RCAs;

# Eliminate the cause of the stratification;

# Select a location for the PM CEMS probe or detector where stratification is less

likely; or

# If available, use a PM CEMS with a multipoint probe.

If you perform a stratification test and determine that the duct is stratified at the proposed

location, you may decide to proceed with installing the PM CEMS at that location. In that case,

it is recommended that you locate the probe or detector at a point where the PM concentration is

comparable to the average PM concentration over the cross-section of the duct, which can be

determined through a stratification test. Eliminating the cause of the stratification may not be

practical, but it may be possible to modify or eliminate the obstruction or disturbance that causes

the stratification. If you decide to select a different location for the PM CEMS probe or detector,

you should take into consideration the various factors that affect stratification, as described above
V-7

(see Section V.2). It is important to find a location that satisfies the criteria of Method 1 (i.e., at

least eight duct diameters downstream and at least two duct diameters upstream of a disturbance). 

Generally, the farther away from a disturbance, the less severe the stratification is likely to be,

although with relatively long horizontal ducts, stratification can increase with increasing distance

downstream of the disturbance. 

If the stratification appears to change with different process or control device operating

conditions, it may be necessary to develop multiple correlations. Emissions could then be

determined using the correlation equation that matches the operating condition.

V.7 REFERENCES

1. U.S. Environmental Protection Agency. 1997. Handbook: Continuous Emission

Monitoring Systems for Non-criteria Pollutants. EPA/625/R-97/001. U.S. Environmental

Protection Agency, Office of Research and Development.

2. Bivins, D. 1994. Determination of the Presence of Stratification of Gaseous Pollutant and

Diluent Emissions for Continuous Emission Monitor or Reference Method Relative

Accuracy Locations. EMC GD-025. U.S. Environmental Protection Agency, Emission

Measurement Center.

3. U.S. Environmental Protection Agency. 1994. An Operator’s Guide to Eliminating Bias in

CEM Systems. EPA/430/R-94-016. U.S. Environmental Protection Agency, Acid Rain

Division.
A-1

Appendix A

Examples of Correlation with Stratified PM Concentrations
A-2

Example 1

For this example, assume that the PM CEMS is located in an unstratified stack, and the PM

concentration at the probe or detector is the same as the average PM concentration across the

stack. The results of the initial correlation test are summarized in Table A-1, and Table A-2

shows the correlation equation developed from the data and the values for the relevant linear

correlation parameters developed from the regression analysis.

Table A-1. Summary of Correlation Test Data

Run no.

PM CEMS

response, mA

(x)

PM concentration,

mg/acm

(y)

1 4.9 2.1

2 7.6 2.9

3 7.8 5.1

4 9.2 4.8

5 10.1 2.9

6 11.1 6.3

7 18.2 8.4

8 15.7 4.9

9 24.1 8.7

10 24.7 12.6

11 29.1 16.2

12 29.3 13.5

13 30.5 15.9

14 27.1 14.4

15 19.8 12.1

Table A-2. Results of Linear Regression Analysis

Parameter Criterion Results

Correlation equation í = -0.633 + 0.521x

Correlation coefficient (r) $ 0.85 0.94

Confidence interval half range (CI) # 10% 4.7%

Tolerance interval half range (TI) # 25% 14.8%
A-3

Now assume the stack is stratified, and the probe for the PM CEMS is placed in a location where

each PM CEMS response (xN) is 80 percent of the corresponding response (x) for unstratified

conditions (i.e., 80 percent of each of the x values listed in Table A-1). Table A-3 shows the

equivalent data for the correlation test, and Table A-4 shows the parameters for the linear

correlation equation developed from the data with and without the stratification.

Table A-3. Summary of Correlation Test Data at 80% Response

Run no.

PM CEMS response,

mA

(xN = 0.80x)

PM concentration,

mg/acm

(y)

1 3.9 2.1

2 6.1 2.9

3 6.2 5.1

4 7.4 4.8

5 8.1 2.9

6 8.9 6.3

7 14.6 8.4

8 12.6 4.9

9 19.3 8.7

10 19.8 12.6

11 23.3 16.2

12 23.4 13.5

13 24.4 15.9

14 21.7 14.4

15 15.8 12.1

Table A-4. Results of Linear Regression Analysis for 80% Response

Parameter Criterion

Results

No stratification xN = 0.80x

Correlation equation í = -0.633 + 0.521x í = -0.633 + 0.652x

Correlation coefficient (r) $ 0.85 0.94 0.94

Confidence interval half range (CI) # 10% 4.7% 4.7%

Tolerance interval half range (TI) # 25% 14.8% 14.8%
A-4

Although the slope of the correlation equation is different, the correlation coefficient, confidence

interval half range, and tolerance interval half range are unchanged.

Example 2

In the previous example, the PM concentration at the PM CEMS probe varied linearly with the

average PM concentration across the stack at any given time such that the PM CEMS response

was a constant 80 percent of what the response would have been if the probe were located where

the PM concentration was equal to the average PM concentration across the stack. In this

example, assume that a nonlinear relationship exists between the PM CEMS response (xN) and

the response (x) for the corresponding unstratified stack. Specifically, the PM CEMS response

varies according to the following:

xN = x0.8

where

xN = PM CEMS response values during the correlation test

x = PM CEMS response values if the probe had been located where the PM

concentration equaled the average PM concentration across the stack.

Table A-5 shows the equivalent data for the correlation test, and Table A-6 shows the parameters

for the correlation equation developed from the data with and without the stratification. 

Although the correlation equation is quite different for the stratified stack, the other correlation

parameters are essentially unchanged.

Table A-5. Summary of Correlation Test Data for Example 2

Run no.

PM CEMS

response, mA

(xN = x0.8)

PM concentration,

mg/acm

(y)

1 3.6 2.1

2 5.1 2.9

3 5.2 5.1

4 5.9 4.8

5 6.4 2.9

6 6.8 6.3

7 10.2 8.4

8 9.1 4.9

9 12.8 8.7
Table A-5. (continued)

Run no.

PM CEMS

response, mA

(xN = x0.8)

PM concentration,

mg/acm

(y)

A-5

(continued)

10 13 12.6

11 14.8 16.2

12 14.9 13.5

13 15.4 15.9

14 14 14.4

15 10.9 12.1

Table A-6. Results of Linear Regression Analysis for Example 2

Parameter Criterion

Results

No stratification xN = x0.8

Correlation equation í = -0.633 + 0.521x í = -2.51 + 1.14x

Correlation coefficient (r) $ 0.85 0.94 0.94

Confidence interval half range (CI) # 10% 4.7% 4.8%

Tolerance interval half range (TI) # 25% 14.8% 15.2%

Example 3

This example is similar to Example 2 except that in this example, the relationship between the

PM CEMS response and the response for an unstratified stack is expressed as

xN = x1.2

where

xN = PM CEMS response values during the correlation test

x = PM CEMS response values if the probe had been located where the PM

concentration equaled the average PM concentration across the stack.

The correlation test data are summarized in Table A-7. The results of the linear regression

analysis are shown in Table A-8. As indicated in Table A-8, the correlation equation differs
A-6

significantly from the equation developed from the unstratified stack data. However, there are

only minor differences in the two sets of results for the correlation coefficient, confidence

interval half range, and tolerance interval half range.

Table A-7. Summary of Correlation Test Data for Example 3

Run no.

PM CEMS response,

mA

(xN = x1.2)

PM concentration,

mg/acm

(y)

1 6.7 2.1

2 11.4 2.9

3 11.8 5.1

4 14.3 4.8

5 16 2.9

6 17.9 6.3

7 32.5 8.4

8 27.2 4.9

9 45.5 8.7

10 46.9 12.6

11 57.1 16.2

12 57.6 13.5

13 60.4 15.9

14 52.4 14.4

15 36 12.1

Table A-8. Results of Linear Regression Analysis for Example 3

Parameter Criterion

Results

No stratification xN = x1.2

Correlation equation í = -0.633 + 0.521x í = 0.609 + 0.246x

Correlation coefficient (r) $ 0.85 0.94 0.94

Confidence interval half range (CI) # 10% 4.7% 4.6%

Tolerance interval half range (TI) # 25% 14.8% 14.5%