Appendix D 

Mendenhall Data Replacement 

Documentation 
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TABLE OF CONTENTS

OVERVIEW ................................................................................................................................... 1

 

Mendenhall PM2.5 Data Imputation for 4Q2006 ............................................................................3 

Letter: December 12, 2006, Hoke P. Kimball, Mendenhall Monitoring Site in Greensboro, North 

Carolina. ...........................................................................................................................................7 

Letter: June 6, 2007, Hoke P. Kimball, Surrogate Data for Mendenhall-Guilford County, NC 

PM2.5 FRM Data 4Q 2006 (37-081-0013)....................................................................................17 

Letter: August 28, 2007, R. Douglas Neeley, Response to NCDAQ letter sent June 6, 2007 .....21 
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OVERVIEW 

 Data from the fourth quarter of 2006 at Mendenhall is incomplete due to construction at the 

monitoring site and complications in relocating the site. The NCDAQ had to move the site since 

the school where the monitoring site is located constructed a two-story field house immediately 

adjacent to the monitoring site (letter to USEPA Region 4, Appendix D). The school had not 

notified the NCDAQ of its intention to build the field house. During a routine site visit, the 

NCDAQ discovered that construction had started within four meters of the monitoring site. At 

that point, the monitor no longer met the ambient monitoring siting criteria and had to be shut 

down and moved. 

A new site was promptly found and appropriate permits were applied for. It took the NCDAQ 

four months (instead of an estimated 3 weeks) to resolve all the necessary permits and complete 

the relocation of the monitor. This unfortunate circumstance resulted in most of the fourth 

quarter to go unmonitored. 

The USEPA guidance does not address missing monitoring data for an entire quarter. The 

NCDAQ decided the best option was to develop regressive analysis between Mendenhall and 

surrounding monitors. 

Since there were no fourth quarter 2006 data with which to calculate a proper Mendenhall design 

value, the NCDAQ decided to present to the USEPA Region, IV an estimate of what the missing 

Mendenhall PM2.5 sample values would have been if they had been properly observed, along 

with the resulting design value summary statistics for their consideration. The estimate is based 

on linear regression using data acquired during the four years, 2002 through 2005 at surrounding 

sites. The monitors examined were Lexington (Davidson County, ID 370570002), Hattie Ave 

(Forsyth County, ID 370670022), Hopedale (Alamance County, ID 370010002), and Cherry 

Grove (Caswell Co., ID 370330001). Of the 4 monitors, a regression using a combination of the 

Hopedale and Lexington monitors proved to be the best predictor for PM2.5 at the Mendenhall 

monitor. 

An estimated fourth quarter 2006 value for Mendenhall is then computed via a regressive 

analysis using the fourth quarter 2006 data from the Hopedale and Lexington monitors. The 

computed fourth quarter average at Mendenhall is 12.92 μg/m3. This value is then used to obtain 

annual design values. The Mendenhall monitor has never violated the PM2.5 standards and has 

consistently had annual averages that were 1 – 2 μg/m3 below the annual PM2.5 NAAQS. The 

NCDAQ is confident that had the monitor been operating during the 2006 fourth quarter, the 

ambient air quality levels would have been consistent with the estimated values the NCDAQ has 
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developed. The USEPA has also proposed to determine that the Greensboro/WinstonSalem/High Point nonattainment area has attained the 1997 PM2.5 NAAQS (74 FR 51249). 

Attached are the correspondence with the USEPA Region IV regarding this issue as well as the 

regression analysis for developing the estimated 2006 fourth quarter average. 
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Memorandum 

To: Hoke Kimball 

CC: Joelle Burleson, George Bridgers 

From: Wayne L. Cornelius 

Date: 2007-05-09 (revised 2008-12-16 and 2009-04-14) 

Re: Mendenhall PM2.5 Data Imputation for 4Q2006

Contents 

Introduction ............................................................................................................................................................1

Methods..................................................................................................................................................................2

Results ....................................................................................................................................................................2

Discussion ..............................................................................................................................................................4

Recommendations ..................................................................................................................................................4

List of Tables 

Table 1 Regression Analysis using the Lexington, Cherry Grove, Burlington and Hattie Avenue PM2.5 data ....3

Table 2 Regression Analysis using the Lexington and Hopedale PM2.5 data .......................................................3

Table 3 Imputed Raw Data.....................................................................................................................................3

Table 4 Quarterly Summaries.................................................................................................................................4

Table 5 Weighted Annual Means and Design Value..............................................................................................4

List of Figures 

Figure 1 Map of Monitor Locations .......................................................................................................................2

Introduction 

The North Carolina Division of Air Quality (DAQ) is in nonattainment for Davidson County for PM2.5 from 

the monitoring station in the city of Lexington (AQS Site ID: 370570002). DAQ has performed design value 

calculations with the PM2.5 data for 2005-2008 for Lexington. These calculations indicate that the design value 

for this monitor will be in attainment for this time period and thus DAQ will be applying for PM2.5 

nonattainment redesignation. Redesignation requires assessing the PM2.5 data from PM2.5 monitors at 

Mendenhall (Guilford Co., ID 370810013), Lexington (Davidson Co., ID 370570002) and Hattie (Forsyth Co., 

ID 370670022). 

The design value calculations for 2005-2008 for the Mendenhall site are incomplete because no valid PM2.5 

data were collected during the fourth quarter of 2006. This happened because of major complications in having 

to move the site. DAQ moved the site about 100 yards because a 2 story field house that was constructed 

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immediately adjacent to the monitoring site (unpublished letter to Artra Cooper, 12 December 2006). The 

construction was started without DAQ’s knowledge. When DAQ realized what was happening it was too late to 

stop the project, the new field house was built, and the site no longer met ambient siting criteria. 

Figure 1 Map of Monitor Locations 

Since there were no 4Q2006 data with which to calculate a proper Mendenhall Design value, the DAQ Planning 

and Ambient staff decided to present to EPA Region IV an estimate of what the missing Mendenhall PM2.5 

sample values would have been if they had been properly observed, along with the resulting Design value 

summary statistics. The estimate is based on linear regression using data acquired during the four years, 2002 

through 2005 at surrounding sites including those in the MSA and also Hopedale (Alamance County, ID 

370010002) and Cherry Grove (Caswell Co., ID 370330001). These monitor locations are shown in Figure 1 

(an extraneous PM2.5 monitoring site at Clemmons, southwest Forsyth County, is also shown for reference but 

was not used in the analysis). 

. 

Methods 

The estimation procedure is as follows: 

1. Fit a linear regression to the 2002-2005 PM2.5 data of the regressors to determine equation coefficients 

2. Estimate missing sample values for Mendenhall by substituting the corresponding observed PM2.5 

data in 4Q2006 into the regression equation 

3. Compute quarterly averages for Mendenhall including the imputed 4Q2006 data using actual data 

where available and imputed data where provided by the regression procedure 

4. Compute weighted averages for each year 

5. Compute the completed Design value for Mendenhall derived by averaging the weighted annual means 

Results 

I applied two regression fits to the data, starting with the most inclusive possible model, using Lexington, 

Cherry Grove, Hopedale and Hattie Avenue all as predictors. Estimates from this model are shown in Table 1. 

In this combination, Lexington, Cherry Grove and Hattie Avenue are not significant predictors for Mendenhall. 

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Table 1 Regression Analysis using the Lexington, Cherry Grove, Burlington and Hattie Avenue PM2.5 

data 

Call: lm(formula = MH ~ LX + UC + HD + HA, data = MH4q.md3, na.action = na.exclude) 

Residuals: 

 Min 1Q Median 3Q Max 

 -3.763 -1.161 -0.3787 0.5814 11.34

Coefficients: 

Regressor Value Std. Error t value Pr(>|t|) 

(Intercept) 0.6105 0.5347 1.1417 0.2565

 LX 0.1615 0.1165 1.3861 0.1690

 UC 0.0791 0.0947 0.8346 0.4061

 HD 0.4782 0.1495 3.1982 0.0019

 HA 0.2466 0.1414 1.7445 0.0844

Residual standard error: 2.111 on 93 degrees of freedom 

Multiple R-Squared: 0.8798 

The second regression removed Cherry Grove and Hattie Avenue from the model. The resulting model had a 

residual standard error of 2.094 and R2

= 0.877. Both Lexington and Hopedale were significant in this 

regression, but the intercept term was not significant, so I fit the model with its intercept forced to zero. This 

model’s estimates are shown in Table 2. The regression equation is shown as equation (1) 

MH = LX + *6322.0*3464.0 HD (Equation 1) 

I fit (1) to the Lexington and Mendenhall data values acquired during 4Q2006. Table 3 shows the regressors for 

the 22 days with valid data for both regressors, and the resulting Mendenhall estimates. The average of the 22 

imputed samples is 12.92. 

Table 2 Regression Analysis using the Lexington and Hopedale PM2.5 data 

Call: lm(formula = MH ~ -1 + LX + HD, data = MH4q.md3, na.action = na.exclude) 

Residuals: 

 Min 1Q Median 3Q Max 

 -4.258 -0.9024 -0.1271 0.915 11.67

Coefficients: 

Regressor Value Std. Error t value Pr(>|t|) 

LX 0.3464 0.0891 3.8873 0.0002

HD 0.6322 0.0957 6.6042 0.0000

Residual standard error: 2.103 on 107 degrees of freedom 

Table 4 shows the quarterly averages for 2004, 2005 and 2006, including the imputed value for 4Q2006 and the 

11 actual values for the remaining quarters. Finally Table 5 shows the 3 annual means and the overall Design 

value result that obtains from them, 14.01. 

Table 3 Imputed Raw Data 

Sampling_Date Mendenhall Hopedale Lexington 

10/02/2006 11.485813 11.1 12.9 

10/05/2006 23.900103 24.6 24.1 

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PM2.5 Data Imputation 

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Sampling_Date Mendenhall Hopedale Lexington 

10/08/2006 6.785218 7.5 5.9 

10/11/2006 13.710670 14.4 13.3 

10/20/2006 11.761229 11.7 12.6 

10/26/2006 11.848665 11.4 13.4 

11/01/2006 16.130242 16.2 17.0 

11/04/2006 12.507779 13.1 12.2 

11/07/2006 10.703944 12.0 9.0 

11/16/2006 4.536147 4.6 4.7 

11/22/2006 3.983631 4.0 4.2 

11/25/2006 16.255683 14.7 20.1 

11/28/2006 18.157377 15.9 23.4 

12/01/2006 4.790717 4.4 5.8 

12/04/2006 9.893171 8.8 12.5 

12/10/2006 16.526730 14.8 20.7 

12/13/2006 25.615500 24.3 29.6 

12/19/2006 18.599245 18.9 19.2 

12/22/2006 8.990912 9.4 8.8 

12/25/2006 6.246493 6.1 6.9 

12/28/2006 14.394680 13.4 17.1 

12/31/2006 17.455206 17.2 19.0

Table 4 Quarterly Summaries 

Period CY2004 CY2005 CY2006 

1Q 11.76 11.45 10.55

2Q 14.40 13.12 13.71

3Q 16.54 19.25 19.07

4Q 13.19 12.21 12.92

 Table 5 Weighted Annual Means and Design Value 

Period CY2004 CY2005 CY2006 2004-2006 D.V. 

Mean 13.97 14.01 14.06 14.01 

Discussion 

I maintain that the estimated Design value presented in Table 5 is an accurate prediction of the result that would 

have been obtained from Mendenhall for 2004-2006, had siting conditions not changed during 4Q2006. The 

imputed average is also the most accurate and appropriate value to use for the 2006-2008 Design value 

calculations at Mendenhall to assist with the redesignation package for the Lexington site. 

Recommendations 

Design value calculations for the Greensboro-Winston-Salem-High Point MSA (or any subsequently redefined 

area that includes Greensboro) for any group of years that includes 2006 should use the imputed 4Q2006 value 

as a surrogate for the missing “actual” 4Q2006 at the Mendenhall site. 

For future consideration, we can apply (1) to data acquired after 2006 from Lexington, Hopedale and 

Mendenhall. We can also repeat the regression fitting exercise using data acquired from the regressor sites in 

2007 and later instead of 2002-2005 Either of these actions can be used to demonstrate how well the moved site 

location “represents” the original location.

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