NIST Special Publication 1175
 

Greenhouse Gas Emissions and Dispersion
 

3. Reducing Uncertainty in Estimating Source Strength and 

Location through Plume Inversion Models 

Kuldeep Prasad 

Adam Pintar 

Heming Hu 

Israel Lopez-Coto 

Dennis Ngo 

James Whetstone 

This publication is available free of charge from: 

http://dx.doi.org/10.6028/NIST.SP.1175 
 

 

 

 

 

NIST Special Publication 1175 

Greenhouse Gas Emissions and Dispersion
 

3. Reducing Uncertainty in Estimating Source Strength and

Location through Plume Inversion Models 

Kuldeep Prasad 

Heming Hu 

Israel Lopez-Coto 

Dennis Ngo 

NIST Engineering Laboratory

Adam Pintar

Statistical Engineering Division

James R. Whetstone 

NIST Office of Special Programs 

This publication is available free of charge from: 

http://dx.doi.org/10.6028/NIST.SP.1175 

September 2015 

U.S. Department of Commerce 

Penny Pritzker, Secretary 

National Institute of Standards and Technology 

Willie May, Under Secretary of Commerce for Standards and Technology and Director 
Certain commercial entities, equipment, or materials may be identified in this

document in order to describe an experimental procedure or concept adequately.

Such identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose. 

National Institute of Standards and Technology Special Publication 1175

Natl. Inst. Stand. Technol. Spec. Publ. 1175, 27 pages (September 2015) 

CODEN: NSPUE2

This publication is available free of charge from: 

http://dx.doi.org/10.6028/NIST.SP.1175
1

Greenhouse Gas Emissions and Dispersion
 3. Reducing Uncertainty in Estimating Source Strength and Location
 through Plume Inversion Models
 

Kuldeep Prasad, Adam Pintar, Heming Hu, Israel Lopez-Coto, Dennis Ngo,
 

James Whetstone
 

Abstract

Recent development of accurate instruments for measuring greenhouse gas concentrations

and the ability to mount them in ground-based vehicles has provided an opportunity to make

temporally and spatially resolved measurements in the vicinity of suspected source locations,

and for subsequently estimating the source location and strength. The basic approach of using downwind atmospheric measurements in an inversion methodology to predict the source

strength and location is an ill-posed problem and results in large uncertainty. In this report,

we present a new measurement methodology for reducing the uncertainty in predicting source

strength from downwind measurements associated with inverse modeling. In order to demonstrate the approach, an inversion methodology built around a plume dispersion model is developed. Synthetic data derived from an assumed source distribution is used to compare and

contrast the predicted source strength and location. The effect of introducing various levels of

noise in the synthetic data or uncertainty in meteorological variables on the inversion methodology is studied. Results indicate that the use of noisy measurement data had a small effect

on the total predicted source strength, but gave rise to several spurious sources (in many cases

8-10 sources were detected, while the assumed source distribution only consisted of 2 sources).

Use of noisy measurement data for inversion also introduced large uncertainty in the location

of the predicted sources. A mathematical model for estimating an upper bound on the uncertainty, and a bootstrap statistical approach for determining the variability in the predicted

source distribution is demonstrated. The new measurement methodology, which involves using

measurement data from two or more wind directions, combined together as part of a single

inversion process is presented. Results of the bootstrap process indicated that the uncertainty

in locating sources reduced significantly when measurements are made using the new proposed

measurement approach. The proposed measurement system can be significant in determining

emission inventories in urban domains at a high level of reliability, and for studying the role of

remediation measures.

Introduction

There has been renewed interest in recent years, in using inverse atmospheric dispersion methods

as part of a top-down analysis, to estimate the source location and strength of greenhouse gas

(GHG) emissions [1]-[16]. Development of accurate instruments [17]-[19] for measuring temporally

resolved greenhouse gas concentrations and the ability to mount them in mobile vans, airplanes

and towers has enabled us to conduct such inversion analysis. However, the basic approach of

using atmospheric measurements to locate and characterize sources (the inverse problem) is an

inherently ill-posed problem, and can lead to large uncertainty in the predicted results. In this

paper, we present sources of uncertainty in inversion analysis, quantify the error, and put forward

a measurement methodology for reducing the uncertainty in estimating emissions.

Methods for estimating source strength and/or location from measurements of concentration

can be divided into two major categories depending on the physical scale of the problem. For urban,

regional, or continental scale inversions, researchers [1]-[7] have used ground-based observations and

a high-resolution mesoscale atmospheric transport model to determine greenhouse gas emissions.

They combine prior emission inventories and atmospheric observations using a Bayesian statistical

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approach to estimate the emissions and the corresponding uncertainty. Enting et al. [9] has reported

that characterizing and calculating the uncertainties in emissions at regional scale is difficult, and

that the uncertainty structure in space and time is difficult to interpret. When the physical distance

between the sources and sensor measurements are smaller, it becomes difficult to use the mesoscale

atmospheric transport models for inversion. Typical examples include measurements made with

instruments mounted inside ground-based vehicles operating on roads close to a possible source.

For such scales atmospheric inversions are usually done using plume dispersion and surface layer

models [10],[11],[12]. In this report, we are primarily interested in measurements that are made

relatively close to the source and restrict ourselves to studying the uncertainty in inverse modeling

related to plume dispersion models. A large number of inversion studies based on plume inversion

models have appeared in the literature. A few of these studies that deal with uncertainty estimates

are discussed below.

Lushi et al. used a Gaussian plume model to provide inverse estimates of particulate emissions

[14]. They added artificial, randomly generated noise to their measurements at levels of ± 30 %

of the measured values and found that the introduction of noise had little effect on the estimates

of source strength. It was speculated that the positive and negative contributions of noise tend to

cancel out its effect [14]. Their approach does not exhibit the high levels of sensitivity normally

found in ill-conditioned problems, and this was attributed to the fact that measurement locations

were close to the sources (< 1 km). Using monthly deposition data for the inverse estimates, aggregate (year long) estimates were within 10%-20% of reported emissions. However, large variations

in monthly output were estimated and these variations were ascribed to errors in the measurement

data, and the fact that detailed meteorological data for assessing atmospheric stability were not

available. Lushi et al. (unlike the focus of the current study) did not investigate the sources of

uncertainty in their predicted source distribution, nor did they put forth an approach for reducing

the uncertainty. The result of our current research study show that noisy measurement data can

have a large effect on the predicted source strength and location, a result that contradicts the work

reported by [14].

Jeong et al. [15] have reported experiments in which sulfur hexafluoride (SF6) was released at

strengths of 110 kg/h to 115 kg/h from a single point source at 57 m above ground level near a

nuclear power plant. Measurements were made at various heights and at two downwind distance

(3 km and 8 km) from the source. Atmospheric conditions were moderately unstable during both

trials comprising the experiment. They subsequently employed an inversion technique based on the

Gaussian plume model; however, only source strengths, not locations, were estimated. Despite the

fact that the source location was known, source strengths were overestimated by about 47 % using

data from only the 3 km arc, and by about 72 % when data from only the 8 km arc were used

[15]. The experiments reported by Jeong et al. provide a useful way to test the inversion method.

Although they report some uncertainty in the predicted results, Jeong et al. did not investigate

the source of this uncertainty, nor did they provide a scientific basis for reducing or managing the

uncertainty in their predicted results.

Finally, Rudd et al. [16], studied the sensitivities of predicting source strength and location

to the number of downwind sensors, sensor configuration, averaging time and noise in the input

data. Data from wind tunnel experiments, in which a tracer gas (propane) was released into the

air from a single source location, were used to test their inverse model. Using data from the

wind tunnel experiment, dispersion coefficients and wind speed necessary in the Gaussian plume

model were customized to provide the best possible model of downwind concentrations. Rudd et

al. estimated that the Gaussian plume model introduced an error of 1.5 ppmv into estimates of

downwind concentration when the wind tunnel experiment was assumed to have a scale of 1:500,

corresponding to a reference wind speed of 10 m s−1 at 32 m, a boundary layer height of 500 m and

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� �

a source strength of 0.1 m3 s−1 [16]. It was noted that uncertainties in wind speed and dispersion

coefficients will be greater in field applications than in the controlled wind tunnel experiment, and

therefore greater model error would be expected [16].

The literature review presented above clearly shows that plume dispersion models have been

used extensively for estimating the source strength and location from various tracer release experiments. Researchers also reported large uncertainty in their inverse estimates, but have provided

limited explanation for the observed uncertainties. In this report, we use synthetic data to understand the sources of the uncertainty in plume inversion models. There has been limited research

on developing methods for reducing the uncertainties and for studying the variability of the source

distribution. The goal of this report is to propose a new measurement methodology for reducing

the uncertainty in inversion analysis.

In Section 2, we provide a brief description of the physical problem and provide the basic

mathematical framework of an inverse problem. Governing equations for the plume dispersion

model are presented and the numerical methods for solving the inversion problem are discussed.

Section 3 provides a description of the basic source - sensor configuration that is used to test the ideal

(base-case) inversion problem. Several sources of uncertainty such as noise in the measurement data,

lack of accurate information on wind speed and direction, as well as atmospheric stability conditions

are explored, and their effect on inversion are quantified. Section 4 presents a methodology for

estimating the upper bound on the uncertainty in inversion, while section 5.1 provides an approach

for quantifying the variability in predicting source strength and location. Finally, section 6 presents

a measurement approach for reducing the uncertainty in inversion. We conclude by providing

suggestion for incorporating the new measurement approach into future mobile campaigns for

estimating emission inventories.

2 Model Formulation and Description of the Problem

Consider a source of known strength Q (kg s−1) that is releasing a tracer gas into the atmosphere.

We also assume that a sensor is located downwind of the source, where we need to estimate the

concentration of the tracer gas. The concentration C (measured in kg m−3) of the tracer gas at

the sensor location can be modeled using the equation

C Q + Cbg = C (1) Q est

where Cbg is the background concentration, and (C/Q)est, represents the influence of the source

on the sensor (influence function). This ratio can be estimated from a variety of models such as

plume models, computational fluid dynamics models or from tracer experiments. The difficulty in

this seemingly trivial model lies in obtaining an accurate value of the ratio (C/Q)est for the given

configuration of source and measurement point.

2.1 Estimating source strength and location - the inverse problem

The inverse of this problem occurs when concentrations are known, but source strengths, or their

locations, are not. In the above model, a single source strength could be estimated from a single

measurement simply by solving for Q in Equation 1.

C − Cbg Q = (2) C
 

Q est

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Note, that the predicted source strength is a function of the reciprocal of the estimated ratio,

(C/Q)est. Thus, small changes (errors) in the estimated ratio may produce large changes in the

inverse estimate of Q.

The problem becomes more complicated when multiple sources and measurements are involved.

For n sources that generate concentrations measured at m locations, the necessary set of equations

may be written as [11]:

⎛ C1,1 C1,2 C1,n ⎞ ⎛⎞⎛ ⎞ ⎞⎛ Q1 Q2 · · · Qn
 Q1 C1 C1

C2,1 C2,2 C2,n · · · Q1 Q2 Qn

. . . . . . . .
 . . . . Cm,1 Cm,2 Cm,n · · ·

⎜⎜⎜⎜⎝
 

⎜⎜⎜⎝
 

⎟⎟⎟⎟⎠
 

⎟⎟⎟⎠
 

+
 

⎜⎜⎜⎝
 ⎟⎟⎟⎠
 =
 ⎜⎜⎜⎝
 ⎟⎟⎟⎠

Q2

.
 

.
 

C2

.
 

.
 

C2

.
 

.
 ,
 (3)
 .
 .
 .
 

Qn Cm bg Cm Q1 Q2 Qn est

where for visual clarity, the subscript “est” has been applied to the entire matrix instead of its

individual elements. Equation 3 may be written in a more compact notational form as

GQ + Cbg = C, (4)

where G represents a matrix of the estimated ratio terms ( C )est, Q represents a vector of the Q

source terms, and Cbg and C represent the vector of background concentrations and the sensor

measurement, respectively.

The challenge of the inverse problem however, lies in the fact that the coefficient matrix G is

usually ill-conditioned for physically realistic scenarios, rendering the inverse problem extremely

sensitive to very small changes in concentration measurements as well as error introduced by the

dispersion model [8]. Any model relating source strength to downwind concentration could be

used to calculate the elements of G and to subsequently conduct an inverse analysis. Since the

purpose of this report is to identify the sources of uncertainty in inversion analysis, and to develop a

methodology for reducing this uncertainty, we use a simple plume model to construct the inversion

matrix, so that computations can be carried out in a quick and efficient manner. It should be

pointed out that the predicted source strengths will be sensitive to errors introduced in the analysis,

irrespective of the choice of the actual model that is used to construct the elements of the inversion

matrix because of the ill-conditioned nature of the inversion matrix G.

2.2 Gaussian plume model

The time-evolution of the concentration of a passive, conserved scalar in the atmosphere may be

modeled as a diffusion equation:

∂C(x, y, z,t) = Kx,y,z'2C(x, y, z,t), (5) ∂t

where t is time, and the diffusion coefficients, Kx,y,z, represent turbulent (eddy) diffusivity rather

than molecular diffusion Assuming the source strength, Q, as a forcing term, a solution to this

equation in a cartesian coordinate system was provided by Ermak et al. [20]

 C(x, y, z)

Q = 1

2πuσyσz

exp −1

2

y

σy

2

exp −1

2

z − H

σz

2

+ exp −1

2

z + H

σz

2

, (6)

where u is the mean streamwise wind speed; σy and σz (m) are dispersion coefficients in the

crosswind and vertical directions, respectively, y is crosswind distance from the plume centerline
 

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(m), z is height above ground (m) and H is the height of the source (m). This form of the plume

model (Equation 6) includes the portion of the plume “reflected” from the ground (2nd term in the

large brackets).

The diffusion coefficients in the crosswind (Ky) and vertical (Kz) directions are related to the

dispersion coefficients σ through the relation [18].

 

σy	 = 2Ky(x/u) (7)

 σz	 = 2Kz(x/u) (8)

The values of the dispersion coefficients are therefore functions of streamwise (x-axis) distance,

and may be estimated according to atmospheric stability and downwind distance from the source.

For simplicity, we use the Pasquill-Gifford-Turner (PGT) categories which partition atmospheric

stability into six groups, ranging from “A” (very unstable), to “F” (very stable). Formulas relating

σy and σz to downwind distance were determined empirically for each of these classes [21], [22].

Several assumptions are made in the simple dispersion model that is proposed in this section.

Emissions for the sources are assumed to be constant and continuous over time. It is also assumed

that the gases or pollutants are conserved as they disperse (i.e., there is no deposition) and that they

do not react with, or decay into other products. Meteorological conditions (wind speed, direction

and static stability) are assumed to be constant throughout the domain and over the time that it

takes for gases to be transported between sources and receptors.

2.3 Solving the inverse problem

In the inverse simulations below, we assume that source strengths are unknown and that all sources

are contained somewhere within an upwind area defined and represented by a grid of points. Ratios

of downwind concentration to source strength were calculated for each combination of source grid

point and measurement (concentration) location using Equation 6. The values of these ratios

populate the matrix in Equation 3. The dimensions of the resulting matrix are therefore m x n,

where m is the number of measurement points, and n is the number of locations within the source

area.

A solution to the inverse problem was estimated using the non-negative least squares (nnls) algorithm of Lawson and Hanson [23]. This numerical algorithm minimizes the norm of the difference

between the left and right hand sides of Equation 3, i.e., it produces a vector, Q, with all elements

greater than or equal to zero, that minimizes IGQ − CI. Proof of convergence and details of the

algorithm may be found in [23]- [26].

3	 Basic Configuration for Predicting Source Strength and Location

The objective of this study is to estimate and reduce the uncertainty in locating the sources and

their strength from downwind measurement data. In this section, we present the basic configuration

of measurement locations relative to the source area, that is used in our simulations.

Figure 1 illustrates a schematic diagram, in plan view, of the configuration of sensor measurements and source area for evaluating the plume inversion model. The square area shown in Figure

1 has dimensions of 2.5 km × 2.5 km, and represents a domain where the sources are located. It

is assumed that the wind direction is from left to right (parallel to the x-axis). Downwind of the

source are three lines located at x = 4000, 5000 and 6000 m, which schematically represent the

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location of the measurements. Measurement data1 can be obtained from a variety of sources such

as tower measurements, aircraft measurements or from an instrument mounted inside a vehicle.

Synthetic measurements that attempt to mimic measurement made from an instrument mounted

inside a vehicle can also be generated. Since the goal of this report is to estimate and reduce uncertainty in source location and strength, we employ the approach of using synthetic measurement

data. The use of synthetic data has the additional advantage that we can compare the inversion

results with the total known source strength as well as checking the accuracy of the location of the

predicted sources. The lines in Figure 1 each indicate the location of 502 measurement points, with

a total of 1506 measurements points on the three lines. Since the assumed wind direction in this

ideal (base) configuration is from the West, the lines of measurement points are perpendicular to

the flow. The source and measurement domains depicted in Figure 1 have been chosen to roughly

match what is expected in real deployments.

Figure 1: Plan view of source-sensor configuration that is used for estimating and reducing the

uncertainty in prediction of sources and their location. The square (2.5 km × 2.5 km) represents

the location of the sources, while the lines indicate an array of measurement locations downwind

of the source. All units are in meters.

As discussed earlier, we assume a set of known sources (located inside the square area of Figure

1) to create synthetic data that is subsequently used in the algorithm. The location of these known

(assumed) sources and their source strength are shown in detail in the surface plot (Figure 2 left

sub-figure). The source area is represented as a regular grid of 40 x 37 elements (i.e., the area is

resolved into 62.5 m x 67.6 m cells). The elements of the source grid are therefore represented by

a vector of 1480 elements. The surface plot shows the presence of two sources; one that is located

in the leftmost corner of the grid has a source strength of 0.1398 kg/s, while the other located in

the rightmost corner has a source strength of 0.0975 kg/s.

The known (assumed) sources and their strength are also indicated by the bar plot on the right

side of Figure 2, where they are referred to by the index number of the cells within the source area

grid. The spikes shown in the bar plot represent, from left to right, source strengths of 0.1398 kg/s

and 0.0975 kg/s (total = 0.2373 kg/s), respectively. Hereon, the stronger (leftmost) and weaker

(rightmost) source will be referred to as sources “A” and ”B,” respectively. In all simulations

described below, the location of the assumed sources and their strengths remained constant, while

the configuration of measurement locations, the assumed wind direction, measurement noise and

1Certain commercial equipment, instruments, or materials are identified in this report in order to specify the

experimental procedure adequately. Such identification is not intended to imply recommendation or endoresement

by the National Insitute of Standards and Technology, nor is it intended to imply that the materials or equipment

identified are necessarily the best available for the purpose.

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atmospheric stability class were varied to study their effect on inversion.
 

Figure 2: Surface plot (left sub-figure) and bar chart (right sub-figure) showing the location of the

assumed sources and source strength measured in kg/s.

The assumed sources are used to create synthetic data at the measurement locations, as well

as for comparison with the predicted source distribution. Uncertainty in the prediction can be

estimated by evaluating the differences between the predicted and assumed source distribution.

Qualitative comparisons can be made through comparison of the surface and bar plots shown

in Figure 2. Synthetic data is generated by using the plume Equation 6 to calculate downwind

concentrations at the measurement locations (illustrated by the lines in Figure 1). A wind speed of

2.5 m/s and a neutral atmospheric stability (PGT class D, urban) was assumed. The synthetic data

is shown in Figure 3, which shows a 3D line plot of downwind concentrations (density measured in

kg/m3) plotted on the three measurement lines. Synthetic data at measurement lines located at x=

4000 m, 5000 m and 6000 m are shown with red, green, and blue lines, respectively. Note that the

nearest line of measurement points (x = 4000 m) shows two separate peaks corresponding to the

separate sources. As expected, the peak concentrations diminish with distance from the sources.

Figure 3: Synthetic data obtained at the sensor locations using the assumed source location and

strength.

The synthetic data generated above is next used in an inversion framework to identify the

source strength and locations. The synthetic data forms the concentration vector C in Equation

4; the coefficient matrix, G was generated from Equation 6, and the nnls algorithm (discussed in

Section 2) was used to solve the inverse problem by estimating the source vector, Q. The predicted

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source distribution Q obtained after inverting the matrix is shown in Figure 4. Comparison of the

predicted source distribution (Figure 4) and the assumed source distribution (Figure 2) indicates

that the predicted and assumed source distributions are essentially identical. The total predicted

source strength was only 0.001 % greater than the assumed total source strength. The location of

the predicted sources was also identical to that of the assumed sources (as seen clearly in the bar

plots). Since the synthetic data was generated from the plume model, this case represent perfect

inversion. This perfect inversion is a verification of the underlying algorithms that have been used

and the validity of the matrix inversion process.

Figure 4: Surface and bar chart showing the location of the predicted sources and source strength

for the base (ideal) case scenario.

3.1 Effect of measurement distance from the source area

In order to further evaluate the inversion methodology, we study the effect of moving the measurement locations further away from the source. It is well known that dispersion increases with

downwind distance from the source, but it is not clear if moving the measurement locations further

downwind will adversely affect the inversion results.

To study the effects of this phenomenon on the inverse estimates, the ideal case, in which the

array of measurement locations began at x=4000 m, was repeated with the array moved further

downwind. In three separate simulations, the array began at x=8000, 12000 and 16000 m, respectively. The distance between the rows (1000 m) array was held constant. Concentrations at each

measurement location were estimated using Equation 6. These concentration values were then used

in the inverse estimates to find the locations and strengths of the individual sources.

The results are illustrated in Figure 5 below, where distance to the measurement rows increases

from the top to bottom of the page. The top panels are for measurements that begin at 8000 m

downwind of the source, the middle two panels for 12000 m downwind of the source, and bottom

panels for 16000 m from the source. The total source strength for the three cases was 0.23728 kg/s,

0.23729 kg/s and 0.23726 kg/s. Error in the estimate of total source strength was insignificant

(<< 1%), but it is clear from the surface and bar plots, that estimates of source locations become

less accurate with increasing distance to the measurement locations.

In all three cases, multiple source locations were estimated, with a total of 13 sources in the 8000

m case, and 10 sources in each of the other two. Many of these sources were of negligible strength

(∼ 10−3 kg/s) compared to the true source strengths, and are barely, or not at all, visible on either

the surface or bar plots. However, it is clear that the number of non-negligible (∼ 10−1 kg/s)

spurious sources increased with distance to the measurement locations, though these sources tend

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Figure 5: Inverse estimates of source strength and location when the set of measurements begins

at 8000 m (top two panels), 12000 m (middle panels) and 16,000 m (bottom panels) downwind of

the source area.

to be grouped relatively near the true source locations. For example, in the bottom panel of Figure

5, the inverse estimate of source B comprises four sources with a total strength approximately equal

to the true strength of source B.

Since dispersion increases with downwind distance from the source, details of the combined gas

plume such as the two separate peaks of the leftmost line of measurements in Figure 3 become

less distinguishable with increasing distance from the sources. In effect, information that can be

used to estimate separate source locations is lost with increasing downwind distance as seen in our

results.

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3.2 Effect of noise in measurement data

Measurements are usually collected with instruments mounted on a vehicle traversing along a road

that is nominally perpendicular to the wind direction. As the vehicle path intercepts the plume,

the on-board instruments record the local concentration in the plume. Since the plume itself is

continuously evolving in time, the vehicle may intercept different portions of a turbulent plume

at different locations. The microscopic turbulent eddies that are part of a plume can result in

significant noise in the measurement data. Noise in the measurement data can also be attributed

to changes in meteorological conditions during a transect. Usually data from a few transects

are averaged together, but it is conceivable that this averaged concentration may not be a true

representation of a Gaussian plume.

In this section, we investigate the effect of using noisy measurement data on the inverse calculation. Results are shown for two cases with different levels of noise incorporated in the measurement

data. Noisy data was created by adding random variations to the downwind concentrations that

were calculated in the ideal case (base case) synthetic data. The level of noise was chosen based

on observations of real data from mobile campaigns [19]. At the location of each measurement,

a random percentage of its concentration, taken from a uniform distribution was added to the

synthetic data.

Figure 6: Synthetic data modified with 10 % noise

In the first case, 10% noise (i.e., variation of ± 5 % of the signal above the background level) was

added to each of the measurement points. The synthetic data modified with 10 % noise is shown

in Figure 6. The resulting effect of using the noisy data on the inverse calculation is illustrated

in Figure 7, which shows surface and bar charts for the predicted source distribution. The total

predicted source strength was 0.2415 kg/s, which was only about 1.8 % greater than the actual

(assumed) source strength. However the surface and bar charts (Figure 7) indicated the presence

of multiple sources. Results also indicate that the location of the sources was significantly different

than the assumed source distribution shown in Figure 2. Estimates of crosswind locations were

accurate for both of the sources, but the streamwise (x-axis) location of the stronger source (source

A) was estimated to be at the windward edge of the source area, about 300 m upwind of the true

location, and was spread over adjacent source grid locations. Overall, ten source locations were

identified in the inverse calculation, though only four had values of the same order (∼ 10−1) as the

true sources.

In the second case, 30 % noise (variation of ± 15 %) was added to the downwind concentrations

above the base-case levels. The modified synthetic data with 30 % noise is shown in Figure 8,

while the predicted source distribution is shown in Figure 9. Despite tripling the amount of noise

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Figure 7: Predicted source distribution (surface and bar charts) for inversion of synthetic data

modified with 10 % noise.

in the measurements, total source strength was overestimated by only about 3 %, but there was

significant error in the location of the predicted sources. Once again, the larger source location was

spread over two cells, and estimated to be at the windward edge of the domain. The sum of source

strengths for these two locations (leftmost vertical red bars in the lower right panel of Figure 9) was

0.15 kg/s, an overestimate of about 8 % from the value of source A. As before, ten source locations

were identified, although only five were of the same order as the true sources.

Figure 8: Synthetic data modified with 30% noise

In both of these cases, the estimated streamwise position of the source furthest from the measurement locations (source A) was most affected by the random noise added to the measurements.

This is due to the fact that the measurements are much less dense in the streamwise direction,

where the relevant information for the inverse estimate comes essentially from the concentration

gradients between only three rows of data, rather than between closely neighboring points as in the

crosswind case.

3.3 Other sources of uncertainty

The previous section demonstrated the effect of using noisy measurement data on the predicted

source location and strength, following an inversion analysis. Besides noisy measurement data,

there are several other sources of uncertainty that can have a direct effect on the predicted source

strength and location. They include factors such as changes in meteorological conditions, such

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Figure 9: Predicted source distribution (surface and bar charts) for inversion of synthetic data

modified with 30 % noise.

as wind speed and direction, as well as the uncertainty in the ability of the model to simulate

the atmospheric conditions (atmospheric stability). In this section we demonstrate the effect of

uncertainty in using incorrect wind direction information and its effect on the inversion process.

Downwind concentrations estimated by the forward dispersion model (Equations 1) are based

on the assumption that mean wind direction is known to a high degree of accuracy. However, there

are several sources of uncertainty in measurements of wind direction including instrument calibration errors, and accuracy and placement of anemometers or wind vanes. The latter is especially

important since the mean wind direction near a source location may not be the same as that near

the measurement location (instrument is located far away from the sources). A different kind of

uncertainty arises when atmospheric models are used to estimate mean wind direction between

sources and measurement locations.

Figure 10: Effect of uncertainty in wind-direction on the predicted source strength and location.

Synthetic data is created with a wind in the x-direction (270.0 ◦), but a wind direction of 265.0 ◦

is assumed during the inversion process.

We report two specific calculations that were conducted to understand and evaluate the effect

of using an incorrect wind-direction vector in the inversion algorithm. Downwind concentrations

from the ideal case, in which wind direction was parallel to the x-axis, were used in both cases.

However, in the first case, the terms of the inversion matrix G were computed by assuming that

the wind direction was in-correctly estimated to be 5◦ counter-clockwise (CCW) from the original

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� �

4

direction (i.e., from the x-axis). Figure 10 shows the predicted source strength and location when

wind direction was assumed to be 5◦ counter-clockwise (CCW) from the x-axis. The error in the

wind direction had little effect on the estimate of total source strength, with an overestimate of only

0.6 %, but it is clear from Figure 10 that the locations and strengths of the individual sources were

not well-characterized. Figure 11 shows the source strength and location when wind direction was

in error by 5◦ clockwise (CW) from the original direction. In this case, the total source strength

was underestimated by 3.4 %, but the estimated locations of the individual sources again departed

significantly from the true locations.

Figure 11: Effect of error in wind-direction on the predicted source strength and location. Synthetic

data is created with a wind in the x-direction (270.0 ◦), but a wind direction of 275.0 ◦ is assumed

during the inversion process.

The asymmetry in the estimates of source strength for the two cases reflects the lack of symmetry

around the x-axis in the true source and measurement locations. Relative to the true source

locations, the rows of measurement points extend further in the positive y-axis direction than in

the negative (see Figure 1). Inspite of this asymmetry, these cases illustrate the strong effect of error

in wind direction on the location of individual sources within the estimated source area. However,

the results also suggest that total source strength can be estimated with reasonable accuracy as

long as the measurement field intersects the majority of the plume.

Upper Bound on Uncertainty in Source Strength

In the previous section, we have presented several examples that demonstrate the uncertainty in

predictions of source strength and location attributed to noisy measurement data or incorrect meteorological data. This uncertainty in prediction of source strength and location can be mathematical

quantified through the concept of condition number.

In order to predict emission from downwind measurement data, the system of linear Equations

4 are inverted. The relative error, or uncertainty, in the calculation of source strength can be

written mathematically as

IΔQI IΔGI IΔCI IΔCbgI ≤ κ + + (9) IQI IGI ICI ICbgI

where Δ signifies a change or error, the operator I · I represents a norm (usually the Euclidean

or spectral norm), and κ indicates the condition number [10]. If G is square (i.e., m = n) and

non-singular, the condition number κ may be calculated as IGI/IG−1I. For rectangular matrices

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(m = n), the condition number may be calculated as the ratio of the largest and smallest singular

values [26].

Equation 9 provides an upper bound on the relative error in the estimated source strength. This

relative error is proportional to the condition number κ, as well as the relative error in estimating the

coefficient matrix, the measured concentrations and background concentrations [10][25]. Therefore,

a low value of κ implies a low sensitivity to error, but a large value does not necessarily imply a

high sensitivity. The uncertainty in source strength increases, if the configuration of measurement

locations and sources is ill-posed and is reflected in large values of the condition number.

For the ideal case shown in Section 3, the condition number of the coefficient matrix was

calculated to be 2.87 × 1026. Inspite of the large value for the condition number, the errors in the

inversion process were found to be extremely small (Figure 4). This is due to the fact that the

uncertainties in the (prescribed) background concentration, “measurements,” and the coefficient

matrix (see Equation 9) are at the level of machine precision (∼ 2 × 10−16).

In section 3.1, we show the effect of increasing the distance between sources and measurement

locations on the predicted source strength and location. As the distance increases, the condition

number does not change significantly; in-fact it becomes marginally smaller. However, the relative

error in the measurement becomes larger, as the absolute value of the measurement reduce with

distance (plume becomes more diluted further away from the source). As a result of this large

relative error, the uncertainty in the prediction of source strength and location increases with

distance (consistent with Equation 9).

The use of noisy measurement data can result in large uncertainty in the predicted source

strength as seen in section 3.2. Large levels of noise result in large relative errors, which in turn

results in large uncertainty in source location and strength, consistent with Equation 4). Uncertainty in meteorological variables (shown in section 3.3) such as wind direction or wind speed also

results in large uncertainty in the terms of the coefficient matrix IΔGI, which in turn results in

uncertainty in the source prediction.

5 Quantification of Uncertainty due to Noisy Measurement Data

The results shown in Section 4 clearly illustrated that error is introduced in predicting source

strength and location due to noisy measurement data or in-correct meteorological data. The goal

of this section is to quantify the distribution of this uncertainty and to develop a measurement

methodology that reduces the uncertainty. In order to characterize and quantify the measurement

uncertainty, we employ a statistical bootstrap procedure discussed below.

5.1 The bootstrap method

The bootstrap was introduced by Efron et al. [27], [28] and is widely used for estimating uncertainty

in various applications. Generally speaking, the bootstrap proceeds by re-sampling the observed

data with replacement, and the parameters (e.g. Q ̃) are re-computed with the bootstrap datasets.

After this is done many times, the variability in the bootstrap replicates provide an estimate of the

variability in the original parameter estimate.

To begin, we modify Equation 1 so that it explicitly recognizes the measurement error in C.

C = GQ + Cbg + E, (10)

where E is a random vector with mean vector 0 and co-variance matrix σ2Diag(GQ), where Diag(x)

is a diagonal matrix with x on its diagonal. Next, define i ̃ = C − GQ ̃ and  ̃r = i ̃/GQ ̃ , where the

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“/” operator implies element-wise division. Now, carry out these steps:
 

∗ 1. Re-sample with replacement the elements of  ̃r to get r .

2. Calculate i∗ = r∗ × GQ ̃ , where the “×” operator implies element-wise multiplication.

3. Calculate C∗ = GQ ̃ + i∗.

4. Calculate Q ̃ ∗ by solving the inverse problem using C∗ instead of C.

Q∗ Q∗ 5. Repeat 1 – 4 B times to get Q ̃ ∗

1,  ̃ 2, · · · ,  ̃ B

The bootstrap replicates  ̃ 1 Q , · · · , are used to summarize the variability in  ̃ For exam- ∗ Q ̃ ∗

2, Q ̃ ∗

B Q.

ple, if Q ̃ (i) is the ith element of  ̃ Q∗  ̃ 2 Q (i), · , Q∗ , the standard deviation of  ̃ 1(i), Q∗ · ·  ̃ (i) is an estimate B

Q∗  ̃ Q∗ of the standard deviation of Q ̃ (i). Also, the α and 1 − α quantiles of  ̃ 1(i), Q2

∗(i), · · · ,  ̃ (i) are 2 2 B

an approximate (1 − α)100% confidence interval for Q(i), the true source strength.

To choose B, we take an initial guess, B0 and run the bootstrap procedure twice. If the

difference in the results between the two runs is practically trivial, B0 is sufficiently large. If not,

repeat with a larger value of B until the difference between the two runs is as small as required,

i.e. the number of decimal places of accuracy that are required.

5.2 Application of the bootstrap process to estimate uncertainty

The bootstrap iterative process described in section 5.1 was used to iteratively estimate the source

strength with various estimates of the measurement vector. The measurement vector was obtained

by adding an error vector (difference between the predicted concentrations and measurements)

through re-sampling with replacement. Before employing the bootstrap procedure to estimate the

variability, it was important to determine the maximum number of iterations that were needed to

obtain a converged solution. Figure 12 shows bar plots of the maximum source strength at each

index locations at three different points during the evolution of the bootstrap process. Results are

shown after 100 bootstrap replications(left panel), 500 bootstrap replications (middle panel) and

1000 replications (right panel). While convergence has not been achieved after 100 replications,

comparison between the results for 500 replications and 1000 replications show that convergence has

been achieved. Based on Figure 12, results for 1000 replications will be used to quantify uncertainty

in the inversion process.

Figure 13 shows a surface plot of the maximum sources strength at each grid point of the

rectangular domain, obtained over 1000 replications of the bootstrap procedure. This plot shows

the uncertainty in predicting source strength and location due to use of noisy measurement data.

Comparison of the bootstrap results shown in Figure 13 with the assumed source distribution shown

in Figure 1 indicates that there was considerable uncertainty in the location of the sources. The

stronger (leftmost) source ”A” as well as the weaker (rightmost) source ”B” are visible in the plot

shown in Figure 13, but their exact location and strength seems to be unclear. It is important

to note that the estimated locations varies along the x-axis, while the variability along the y-axis

is relatively small. The results of the bootstrap procedure indicate that the uncertainty in source

prediction is not distributed uniformly over the entire domain. Instead the uncertainty is limited

to the streamwise direction (in this case the x-direction) for both the sources.

In order to test if the distribution of uncertainty in source identification is limited to the

streamwise direction, a set of simulations were conducted where the wind direction was shifted to

be parallel to the y-axis (southerly wind). The measurement locations were also shifted so that

they were nominally perpendicular to the wind direction. Figure 14 shows a schematic diagram

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Figure 12: Bar chart showing the maximum source strength plotted as a function of grid index

for various levels of iteration. Results are shown for 100 iterations (left sub-figure), 500 iterations

(middle sub-figure) and 1000 iterations (right sub-figure).

Figure 13: Surface plot showing the maximum source strength at each grid location, plotted at the

end of 1000 iterations of a bootstrap method. The wind is parallel to the x-axis.

of the new configuration. The only difference between the configuration shown in figure 14 and

the one shown in Figure 1 is the wind direction and the measurement locations. The location of

the square source area did not change, nor did the strength and location of the assumed sources

A and B. Synthetic data was generated using the plume model Equation 6 to calculate downwind

concentrations at the locations illustrated by the lines in Figure 14. As for the previous cases, a

wind speed of 2.5 m/s (parallel to the X-axis) and a neutral atmospheric stability (PGT class D,

urban) was assumed.

Using only the downwind measurements and the coefficient matrix, G, generated from Equation

6, the nnls algorithm was used to solve the inverse problem by estimating the source vector, Q.

The resulting effect of using noisy data on the inverse calculation is illustrated in Figure 15, which

shows surface and bar charts for the predicted source distribution. Comparison of the predicted

source distribution (Figure 15) and the assumed source distribution (Figure 2) again indicates that

there are significant differences in the predicted source strength and location due to the noise in

the measurement data. The total predicted source strength was 0.0.2451 kg/s, which is only 3.2%

greater than the actual (assumed) source strength. Surface and bar charts (Figure 15) indicate

the presence of multiple sources, and the location of the sources was significantly different from

the assumed source distribution. Estimates of crosswind locations were accurate for both of the

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Figure 14: Plan view of the model domain used to investigate the effect of a wind in a direction

parallel to the y-axis.

sources, but the streamwise (y-axis) location of the stronger source (source A) was not estimated

accurately. Overall, seven source locations were identified in the inverse calculation, though only

two sources had strengths of the same order as the true sources.

Figure 15: Surface and bar chart showing the location of the predicted sources and their strength,

when the wind is parallel to the y-direction.

As for the case when the wind was in the x-direction, we used the bootstrap procedure to

quantify the uncertainty for the case when the wind is in the y-direction. Figure 16 shows a

surface plot of the maximum source strength over 1000 bootstrap replicates at each grid point of

the rectangular domain, assuming that the wind was in the y-direction. Results indicate that that

the stronger (leftmost) source ”A” as well as the weaker (rightmost) source ”B” are visible in the

plot shown in Figure 16, but their location and strength is not clear. It should be noted that

the uncertainty in the location of the sources and the variability is primarily in the y-direction

(streamwise), while the variability in the x-direction is relatively small. Results of the bootstrap

procedure with a wind in either the x or y-direction, both indicate that the uncertainty in source

prediction is primarily in the streamwise direction. This is an important conclusion, and is the basis

for designing a measurement methodology aimed at reducing the uncertainty in source prediction,

as discussed in the next section.

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Figure 16: Surface plot showing the maximum source strength at each grid location, plotted at the

end of 1000 iterations of a bootstrap method. The wind is parallel to the y-axis.

A Novel Measurement Methodology for Reducing the Uncertainty in Source Prediction

In this section we propose a new measurement methodology for reducing the uncertainty introduced

in the source prediction due to noisy data. The application of the bootstrap procedure demonstrated

the uncertainty in source strength and location when the wind was in the +x-direction (Figure 13),

or in the + y-direction (Figure 16). Results clearly illustrated that the uncertainty is distributed

in the streamwise direction.

In order to reduce this uncertainty, we investigate the feasibility of taking data from two different

wind directions and combining them together in a single calculation. Figure 17 (left sub-figure)

illustrates a typical configuration for collecting data with two different wind directions. The set of

measurement lines parallel to the y-axis, were used to collect data when the wind was blowing in

the +ve x-direction. The set of measurement lines parallel to the x-axis were used to collect data

when the wind was in the +y-direction. For each wind direction, synthetic data was generated on

the measurement lines using an assumed source distribution shown in Figure 2. Note, that the

source area and the location / strength of the assumed sources has not changed in any of these

configurations. Next, we introduced 10% noise in the measurement data collected at each location

for both the wind directions. The noisy measurement data from the two wind directions is combined

into a single data set and is used for inversion. It should be pointed out that there is twice as much

data available for inversion as compared to the earlier cases. The inversion approach was similar

to the one discussed in the previous section, the primary difference being that the data from two

different wind directions was combined in one single calculation.

Figure 17 (right sub-figure) shows the results of the inversion process, where synthetic data

with noise from two different wind directions was utilized. Comparison of the predicted source

strength and location following the inversion process (Figure 17 right sub-figure) and the assumed

source distribution (Figure 2), indicates that the results are quite similar. The total source strength

predicted from the inversion process was computed at 0.2333 kg/s, which compared favorably with

the total assumed source strength of 0.2373 kg/s. Moreover, the location of the predicted sources

also compared favorably with the assumed source distribution. The bootstrap process (described in

section 5.1) was used again on the configuration described in Figure 17 (left sub-figure) to quantify

the variability and uncertainty in source location and strength. Figure 18 left sub-figure) shows

the maximum source strength and location over 1000 bootstrap replications, while Figure 18 right

sub-figure shows the minimum source strength and location over 1000 iterations. The same data is

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shown in bar format in Figure 19.
 

Figure 17: Left sub-figure shows a plan view of the source-sensor configuration that was used for

estimating and reducing the uncertainty in prediction of sources and their location. Data from two

different wind directions was utilized. The right sub-figure shows a surface plot of the predicted

source distributions obtained when noisy data (synthetic data with 10% noise) from two different

wind directions was combined in a single inversion step.

Figure 18: Results of the bootstrap procedure when noisy measurement data from two different

wind directions are used in a single inversion procedure. The left sub-figure shows surface plot of

the maximum predicted values over 1000 iterations, while the left sub-figure shows a surface plot

of the minimum values.

These results clearly demonstrate that the uncertainty due to noisy measurement data can be

reduced by using measurements from two different wind directions combined together as part of

a single inversion procedure. In general, the best results are obtained under conditions when the

two wind directions are perpendicular to each. However, it should be noted that the two wind

directions do not have to be perpendicular to each other, and that the analysis can be conducted

with the two winds at a non-right angle to each other. Furthermore, utilizing data from more than

two different wind directions can further reduce uncertainty in the analysis.

It should also be pointed out that collecting data for two different wind directions is not very

challenging, since the mean wind speed and direction changes several times during the course of a

single day. As a result collecting data from two or more wind directions / speed is not a significant

issue, as long as appropriate roads are available for collecting downwind data. Moreover, the

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Figure 19: Results of the bootstrap procedure when noisy measurement data from two different

wind directions are used in a single inversion procedure. The left sub-figure shows a bar plot of the

maximum predicted values over 1000 iterations, while the left sub-figure shows a bar plot of the

minimum values.

reduction in uncertainty (shown in this section) is due to the presence of wind data in two different

directions, and not due to the availability of more data. This was confirmed by reducing (halving)

the number of measurement points, and finding no change in the computed results or the reduction

in uncertainty. The approach for reducing uncertainty is also independent of the model used for

plume dispersion, and as a result is applicable for all scenarios where one needs to estimate source

location and strength from measurement data.

Conclusions and Suggestions for Future Research

Recent development of accurate instruments for measuring greenhouse gas concentrations and the

ability to mount them in ground-based vehicles has provided an opportunity to make temporally and

spatially resolved measurements in the vicinity of suspected source locations. The basic approach of

using downwind atmospheric measurements in an inversion methodology to predict source strength

and location can be an ill-posed problem and results in high degree of uncertainty, if the wrong

measurement methodology is used.

In this report, we have presented a new measurement strategy for reducing the uncertainty in

predicting source strength and location from downwind measurements. In order to demonstrate the

approach, the basic inversion methodology built around a plume dispersion model was presented.

Synthetic data derived form an assumed source distribution was used to allow us to compare and

contrast the predicted source strength and location. The effect of introducing various levels of

noise in the synthetic data on the inversion methodology was presented. It was concluded that

the presence of noise in synthetic data resulted in large uncertainty in the location of the sources.

The use of noisy measurement data had a small effect on the total predicted source strength, but

gave rise to several spurious sources (in many cases 8-10 sources were detected, while the assumed

source distribution only consisted of 2 sources). Uncertainty in wind direction also resulted in large

uncertainty in the location of the predicted sources. Similar results were obtained when uncertainty

in determining the meteorological variables was incorporated in the inversion process. The new

measurement strategy involved collecting data from two or more wind directions. The data is

subsequently combined together as part of a single inversion process. Results of the bootstrap

procedure indicated that the uncertainty in locating sources reduced significantly when data from

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two or more wind directions is used simultaneously.

Future research will involve collecting data as part of a mobile measurement campaign around

suspected methane sources such as landfills, natural gas transmission and regulating stations, and

waste-water treatment plants. The measurement methodology presented in this report will be

incorporated as part of the mobile campaign to accurately identify sources and their strength.

Such approaches can be very useful in developing an emissions inventory for mega cities and for

evaluating the role of remediation measures on greenhouse gas emissions.

Acknowledgments

The authors acknowledge the suggestions from Drs. Antonio Possolo and Hratch Semerjian at NIST

and thank them for their support of this work. We also appreciate the discussion and suggestions

from various member for the INFLUX and LA Megacities team.

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List of Figures
 

1 Plan view of source-sensor configuration that is used for estimating and reducing

the uncertainty in prediction of sources and their location. The square (2.5 km ×

2.5 km) represents the location of the sources, while the lines indicate an array of

measurement locations downwind of the source. All units are in meters. . . . . . . . 7

2 Surface plot (left sub-figure) and bar chart (right sub-figure) showing the location of

the assumed sources and source strength measured in kg/s. . . . . . . . . . . . . . . 8

3 Synthetic data obtained at the sensor locations using the assumed source location

and strength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4 Surface and bar chart showing the location of the predicted sources and source

strength for the base (ideal) case scenario. . . . . . . . . . . . . . . . . . . . . . . . . 9

5 Inverse estimates of source strength and location when the set of measurements

begins at 8000 m (top two panels), 12000 m (middle panels) and 16,000 m (bottom

panels) downwind of the source area. . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6 Synthetic data modified with 10 % noise . . . . . . . . . . . . . . . . . . . . . . . . . 11

7 Predicted source distribution (surface and bar charts) for inversion of synthetic data

modified with 10 % noise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

8 Synthetic data modified with 30% noise . . . . . . . . . . . . . . . . . . . . . . . . . 12

9 Predicted source distribution (surface and bar charts) for inversion of synthetic data

modified with 30 % noise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

10 Effect of uncertainty in wind-direction on the predicted source strength and location.

Synthetic data is created with a wind in the x-direction (270.0 ◦), but a wind direction

of 265.0 ◦ is assumed during the inversion process. . . . . . . . . . . . . . . . . . . . 13

11 Effect of error in wind-direction on the predicted source strength and location. Synthetic data is created with a wind in the x-direction (270.0 ◦), but a wind direction

of 275.0 ◦ is assumed during the inversion process. . . . . . . . . . . . . . . . . . . . 14

12 Bar chart showing the maximum source strength plotted as a function of grid index

for various levels of iteration. Results are shown for 100 iterations (left sub-figure),

500 iterations (middle sub-figure) and 1000 iterations (right sub-figure). . . . . . . . 17

13 Surface plot showing the maximum source strength at each grid location, plotted at

the end of 1000 iterations of a bootstrap method. The wind is parallel to the x-axis. 17

14 Plan view of the model domain used to investigate the effect of a wind in a direction

parallel to the y-axis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

15 Surface and bar chart showing the location of the predicted sources and their strength,

when the wind is parallel to the y-direction. . . . . . . . . . . . . . . . . . . . . . . 18

16 Surface plot showing the maximum source strength at each grid location, plotted at

the end of 1000 iterations of a bootstrap method. The wind is parallel to the y-axis. 19

17 Left sub-figure shows a plan view of the source-sensor configuration that was used for

estimating and reducing the uncertainty in prediction of sources and their location.

Data from two different wind directions was utilized. The right sub-figure shows a

surface plot of the predicted source distributions obtained when noisy data (synthetic

data with 10% noise) from two different wind directions was combined in a single

inversion step. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

18 Results of the bootstrap procedure when noisy measurement data from two different

wind directions are used in a single inversion procedure. The left sub-figure shows

surface plot of the maximum predicted values over 1000 iterations, while the left

sub-figure shows a surface plot of the minimum values. . . . . . . . . . . . . . . . . . 20

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This publication is available free of charge from: http://dx.doi.org/10.6028/NIST.SP.1175
19	 Results of the bootstrap procedure when noisy measurement data from two different

wind directions are used in a single inversion procedure. The left sub-figure shows

a bar plot of the maximum predicted values over 1000 iterations, while the left

sub-figure shows a bar plot of the minimum values. . . . . . . . . . . . . . . . . . . . 21

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______________________________________________________________________________________________________ 

This publication is available free of charge from: http://dx.doi.org/10.6028/NIST.SP.1175