Attachment	B TSRIA	Petition	for	Reconsideration Page B	- 1

*Analysis	by	Bison	Engineering,	Inc.

ATTACHMENT B*

COBURN ROAD DATA QUALITY ISSUES

The Coburn Road sulfur dioxide (SO2) monitoring station was initially established to monitor 

compliance with the NAAQS of 500 ppb, and other short-term averaging periods in effect at that 

time. More recently, SO2 data from Coburn Road was used to determine compliance with the 

new 75 ppb standard, promulgated by EPA in June 2010. EPA reviewed data collected at Coburn 

Road between 2009 and 2011, and ultimately designated a small portion of the Billings area 

nonattainment for SO2 based on that dataset. Bison Engineering, on behalf of TSRIA and the 

Billings Laurel Air Quality Technical Committee (BLAQTC), conducted its own independent 

evaluation of the Coburn Road data from that period to determine whether that designation is 

defensible.

This document identifies data quality issues that cast doubt on the accuracy of the Coburn Road 

data used for that calculation. (See Attachment A, detailing more recent Coburn Road data that 

indicates the area is in compliance with the 75 ppb standard). 

The data quality issues are related to the failure to consistently operate the Coburn Road site in 

accordance with requirements specified by the Montana Department of Environmental Quality

(MDEQ), federal guidance documents and codified regulations. More specifically, many of the 

high hourly SO2 values used to calculate the 2009 – 2011 Design Value must be considered 

suspect, because the quality assurance activities associated with those data were either 1) not 

performed as required, or 2) produced unacceptable results and were not followed by specified 

corrective actions. Those issues are presented in this document and are outlined below as 

follows: 

 Section 1 identifies the applicable MDEQ and federal guidance documents, and federal 

regulations governing quality assurance requirements for ambient SO2 monitoring 

programs – with an emphasis on requirements that were in effect at the time the 2009-

2011 data were collected,

 Section 2 discusses specific data quality assurance requirements in each applicable 

document, and summarizes them with respect to three critical quality assurance 

activities,

 Section 3 presents general data quality issues found with the 2009-2011 dataset as a 

whole, and

 Section 4 identifies specific data quality issues (when applicable) associated with each 

daily maximum hourly SO2 concentration of 75 ppb or higher, particularly during 2010 

(which year’s data is solely responsible for the SO2 nonattainment designation), since 

both 2009 and 2011 (and the 2 years since) had 99th-percentile SO2 maxima below the 

75 ppb standard.
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1. APPLICABLE GUIDANCE DOCUMENTS AND REGULATIONS

Data quality is an essential consideration for any ambient air monitoring program. To facilitate 

the collection of high-quality ambient monitoring data (both for SO2 and other parameters), state 

and federal guidance documents and regulations have been issued addressing the collection of 

ambient monitoring data, and specifically SO2. The purpose of this section is to outline 

documents and regulations that govern quantitative measures of data quality, and arrive at a 

synthesized compilation of data quality requirements that should be applied to the Coburn Road 

dataset.

In the case of conflicting requirements, particularly state versus federal, state requirements must, 

as a rule, be at least as stringent as federal requirements. However, states are free to mandate 

stricter requirements than those identified at the federal level. 

1.1 MDEQ Requirements

1.1.1 MDEQ QAPP

MDEQ formally issued an update to a Quality Assurance Project Plan (QAPP) for the Montana 

Ambient Air Monitoring Program in 1996. The 1996 QAPP incorporated new State and Federal 

air monitoring requirements. Draft updates were prepared in 2005 and 2009, but neither was ever 

finalized and issued as a formal document. Conceptually, both were similar to the 1996 QAPP 

but included expanded detail. A formal QAPP update was issued in 2013, after the close of 

public comments for the 2010 SO2 designation.

1.1.2 MDEQ Quality Assurance Form Requirements

To facilitate MDEQ QAPP requirements, MDEQ has also developed a number of QA-related 

data forms used by station operators and auditors. In most cases, these forms also show 

acceptance criteria for each type of QA check performed and appear to be consistent with the 

1996 QAPP. In some cases the data form criteria are actually stricter than those found in the 

1996 QAPP. While the forms themselves do not constitute “regulations” per se, these stricter 

criteria appear to have been listed in response to evolving federal requirements. The criteria on 

these forms should be applied to the Coburn Road data under the general principle that MDEQ 

requirements must be at least as strict as federal requirements.

1.2 EPA Guidance Document Requirements

Data quality requirements are also specified in various EPA guidance documents, the most 

important being the Quality Assurance Handbook for Air Pollution Measurement Systems:

Volume II, Ambient Air Monitoring Program. That document was originally issued in the late 

1970s and has also been updated periodically. The most recent update for purposes of the 2009-

2011 data was issued in December 2008 (EPA-454/B-08-003). This is referred to as “EPA 2008” 

throughout this document
Attachment	B TSRIA	Petition	for	Reconsideration Page B	- 3

As will be shown later in this document, there is considerable variance between the 1996 MDEQ 

QAPP requirements and the 2008 EPA requirements. 
Attachment	B TSRIA	Petition	for	Reconsideration Page B	- 4

2. QUALITY ASSURANCE REQUIREMENTS IN REGULATORY DOCUMENTS

The 2009 – 2011 Coburn Road data (and particularly the station’s quality assurance records) 

were examined to determine compliance with the applicable regulations and guidance documents

discussed in Section 1. Three types of station records were examined for this data quality 

evaluation. While this review was by no means exhaustive, it did address the three types of 

quantitative quality assurance checks most critical to data integrity:

 Zero/span/precision checks,

 Calibrations, and

 Performance audits

Each activity is described in Section 2.1. In Section 2.2, state and federal quality requirements 

are outlined, along with some discussion of their differences. That section also identifies quality

assurance requirements that were applicable to the Coburn Road site during the 2009-2011 

period EPA chose to use for design value calculations (mostly from the 1996 QAPP, with a few 

exceptions).

2.1 Definition of Quality Assurance Activities

2.1.1 Manual Zero/Span/Precision (ZSP) Checks

Manual ZSP checks are conducted on a frequent basis to demonstrate that an SO2 analyzer is 

correctly measuring three known gas concentrations. The guidance documents often differentiate 

between zero/span checks and precision checks (which are sometimes referred to as QC checks). 

In practice, they are typically conducted as a single activity, and include the following:

 The zero check is performed to show that the analyzer responds accurately to a 

pollutant-free sample (hence the term “zero”). It is accomplished by introducing zero 

air to the analyzer, recording its response, and comparing the result to applicable 

evaluation criteria;

 The span check is performed to show that the analyzer responds accurately to a gas 

concentration that is between 80% and 90% of the analyzer’s operating range. For 

example, an analyzer designed to measure ambient concentrations of up to 500 ppb 

would be checked at a concentration of 400-450 ppb. The corresponding 

concentration for an analyzer operating at a 0 - 1,000 ppb range would be checked at 

800 – 900 ppb.

 A precision check is performed to show that the analyzer responds accurately to a 

lower gas concentration that is more representative of expected ambient 

concentrations (typically below 100 ppb). 
Attachment	B TSRIA	Petition	for	Reconsideration Page B	- 5

If the ZSP check produces acceptable results, no adjustments are made to the analyzer. If any of 

the check results exceed applicable limits (termed warning limits and control limits, and defined 

in Table 1 of this Attachment), the operator proceeds to a calibration as described in Section 

2.1.2.

2.1.2 Calibrations

A calibration is performed to ensure that an SO2 analyzer produces accurate responses to known 

gas concentrations over its entire operating range, and at concentrations relevant to existing 

ambient standards. The basic sequence is that:

1. Zero air is introduced to the analyzer, and the analyzer’s “zero” is adjusted so that its 

response is very close to 0 ppb.

2. Span gas is introduced to the analyzer, and the analyzer’s span response is adjusted so 

that its response is very close to the known span gas concentration.

3. Steps 1 and 2 are repeated as necessary until both the zero and span responses are very

close to the known values.

4. Finally, at least 3 additional non-zero gas concentrations in specified ranges are 

introduced to the analyzer, and its responses are recorded. 

As with ZSP checks, calibration results must meet acceptance criteria for the associated data to 

be considered valid. The calibrations also must be performed at specified frequencies.

2.1.3 Audits

A performance audit is an independent check of an instrument by a person not involved in its 

routine operation, using separate standards from those used to operate or calibrate the instrument. 

The sequence is similar to that for a calibration, except that no adjustments are made by the 

auditor. As with calibrations, audits of an analyzer are conducted using gas concentrations in 

specified ranges.

2.2 Specific Quality Assurance Requirements in Regulatory Documents

In general the applicable regulatory documents discussed in Section 1 provide specific 

requirements for each of the quality assurance activities discussed in Section 2.1. They are 

summarized in Sections 2.2.1 through 2.2.3. 

2.2.1 Zero/Span/Precision (ZSP) Checks

As mentioned above, zero/span checks and precision checks are usually performed as a single 

activity. In some cases precision checks are referred to as single-point quality control (QC 

checks). Guidance on ZSP checks is summarized in Table 1.
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Based on a review of the Coburn Road QA records, MDEQ has clearly used the 1996 QAPP 

requirements as its goal, with one exception: Starting in 2010, MDEQ appears to have adopted 

the EPA 2008 “Critical Criteria” span and precision point error limits of ± 10%, which are in 

effect control limits as defined in Table 1. EPA 2008 does not give corresponding warning limits 

for those parameters, but MDEQ quite logically adopted a warning limit of ± 7% – since the 

purpose of a warning limit is to alert the operator to the need for adjustments before an out-ofcontrol condition occurs. Therefore, the 2009 Coburn Road span and precision check data were 

evaluated with respect to the MDEQ 1996 warning and control limits of ± 9% and ± 12%. The 

2010 – 2011 Coburn Road span and precision data were evaluated with respect to the more 

stringent limits of ± 7% and ± 10% adopted in 2010.

TABLE 1 – REQUIREMENTS FOR ZERO/SPAN/PRECISION CHECKS

Check Type 1996 MDEQ QAPP 2008 EPA Volume II

Zero Check

Frequency Once every 2 weeks Once every 2 weeks

Concentration Range Zero ppb Zero ppb

Warning Limit1± 0.67% of full scale3 None specified

Control Limit2± 1% of full scale4± 3% of full scale

Span Check

Frequency Once every 2 weeks Once every 2 weeks

Concentration Range 400 – 450 ppb5

800 – 900 ppb6

Not explicitly stated

Defined as >120% of 

applicable NAAQS and >99% 

of data.

Warning Limit ± 9% error None specified

Control Limit ± 12% error ± 10% error

Precision Check

Frequency Once every 2 weeks Once every 2 weeks

Concentration Range 80 – 100 ppb 10 – 100 ppb

Warning Limit ± 9% error None specified
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Control Limit ± 12% error ± 10% error

1Denotes limit that, if exceeded, requires recalibration of the analyzer.

2Denotes limit that, if exceeded, requires recalibration of the analyzer and invalidation of associated data

3Equal to 6.7 ppb for analyzer operated at 0 – 1,000 ppb range, 3.4 ppb for 0 – 500 ppb range.

4Equal to 10 ppb for analyzer operated at 0 – 1,000 ppb range, 5 ppb for 0 – 500 ppb range.

5Applies to analyzer operated at 0 – 500 ppb range

6Applies to analyzer operated at 0 – 1,000 ppb range 

2.2.2 Calibrations

Guidance on calibrations is summarized in Table 2, and differs markedly between the 1996 

MDEQ QAPP and EPA 2008. In general MDEQ applied the 1996 QAPP criteria except, as noted 

in Table 2, the more stringent calibration acceptance criteria proposed in the 2005 and 2009 

Draft QAPPs had been adopted by 2009 (as evidenced by MDEQ’s own calibration forms). 

Therefore, MDEQ’s calibration results have been evaluated with respect to those more stringent 

criteria, with one exception: MDEQ has increased its use of a very low calibration concentration 

(e.g. ~5-10 ppb) – apparently in response to EPA’s recent emphasis on accurately measuring low 

ambient concentrations. 

TABLE 2 – REQUIREMENTS FOR CALIBRATIONS

Parameter 1996 MDEQ QAPP 2008 EPA Volume II

Frequency Every 90 days at a minimum Once per six months1

Once per year2

Number of 

concentrations

Total of five

(Four non-zero points, plus zero)

Total of five

(Four non-zero points, plus zero)

Concentration ranges3800 – 900 ppb

400 – 450 ppb

150 – 200 ppb

30 – 80 ppb

Four “upscale” (non-zero points)

Evenly spaced over operating 

range

Concentration ranges4400 – 450 ppb

250 – 350 ppb

150 – 200 ppb

30 – 80 ppb

Four “upscale” (non-zero points)

Evenly space over operating 

range
Attachment	B TSRIA	Petition	for	Reconsideration Page B	- 8

Acceptance limits None specified5

± 2% for span point6

± 5% for other non-zero points6

± 0.5% of full scale for zero

All points within 2% of full scale 

of best-fit straight line7

1

Frequency applies if manual zero/spans are performed every 2 weeks

2

Frequency applies if continuous zero/spans are performed daily

3

For instrument operated at 0 – 1,000 ppb range

4

For instrument operated at 0 – 500 ppb range

5

1996 MDEQ QAPP only requires confirmation of points in error by > ± 2%

6Limits in 2005 and 2009 Draft MDEQ QAPPs, and on MDEQ calibration form

7This equates to ±10 ppb for an analyzer operated at 0-500 ppb range, and ±20 ppb for an analyzer 

operated at 0 – 1,000 ppb scale.

2.2.3 Audits

Guidance on performance audits is summarized in Table 3. Based on a review of the Coburn 

Road QA data, MDEQ used the 1996 QAPP requirements as its goals in 2009 and 2010. 

However, in 2011 MDEQ began using significantly lower audit concentrations. This appears to 

have been done in response to the EPA 2008 guidance document recommendations. Those 

recommendations identified several “audit levels” quite different from historical practice. 

Therefore, the 2009 and 2010 audit data are evaluated based on the 1996 MDEQ QAPP 

requirements, while the 2011 data are evaluated based on EPA 2008.

Another nuance of this evaluation concerns the audit criteria for very low known gas 

concentrations. EPA 2008 set an audit performance criterion of ± 15%, regardless of gas 

concentration. However, it was soon recognized that this criterion was virtually impossible to 

meet at very low concentrations. EPA officially modified the criterion in 2013 so that the 

acceptable error for very low audit points was ± 1.5 ppb. Therefore, that more lenient criterion 

was used to evaluate the Coburn Road data where applicable, since that was ultimately the EPA 

standard. 

TABLE 3 – REQUIREMENTS FOR AUDITS

Parameter 1996 MDEQ QAPP 2008 EPA Volume II

Frequency Each site 1/year

25% of sites 1/quarter

Each site 1/year

25% of sites 1/quarter

Concentration ranges1800 – 900 ppb 3 consecutive audit 
Attachment	B TSRIA	Petition	for	Reconsideration Page B	- 9

350 – 450 ppb

150 – 200 ppb

30 – 80 ppb

concentrations not including zero2

Concentration ranges3350 – 450 ppb

150 – 200 ppb

30 – 80 ppb

3 consecutive audit 

concentrations not including zero2

Acceptance limits ± 15% at each point

± 3% of full scale for zero

Plus linear regression criteria4

± 15% of reading at each point5

± 1.5 ppb at lowest audit levels6

1

For instrument operated at 0 – 1,000 ppb range

2Refers to audit levels described in more detail in EPA 2008

3

For instrument operated at 0 – 500 ppb range

4Audit results also evaluated on basis of linear regression results. In most cases the limit of ± 15% at each 

point is the most restrictive.

5This limit was set before the widespread use of very low audit gas concentrations

6This limit appears in EPA’s 2013 modification (see section 2.2.3 above), and recognizes the difficulty of 

meeting the ± 15% criterion at very low audit gas concentrations.
Attachment	B TSRIA	Petition	for	Reconsideration Page B	- 10

3. GENERAL COBURN ROAD DATA QUALITY ISSUES

The 2009 – 2011 Coburn Road data records were evaluated with respect to the acceptance 

criteria presented in Tables 1, 2, and 3. The types of data quality issues identified are discussed 

below. 

 Zero/Span/Precision (ZSP) check results exceeding warning limits and control limits were 

often not followed by appropriate corrective actions. 

Paragraph 6 of QAPP Section 9.8.4 states that “If the analyzer's response exceeds either 

the upper or lower control limits for either the zero or span it shall be deemed "out of 

control" and shall be recalibrated immediately (within one working day).” Paragraph 7 

of QAPP Section 9.8.4 states that “If the analyzer's response exceeds either the upper or 

lower warning limits for either zero or span, the point shall be repeated immediately 

(same working day). If the analyzer's response to the second point also exceeds the 

warning limits, the analyzer shall be deemed "out of control" and recalibrated 

immediately (within one working day). If the response does not exceed the warning limits, 

the analyzer shall be deemed "in control" and no adjustments are necessary.” Finally, 

Part B of QAPP Section 15.1 shows warning and control limits for precision point checks 

of 9.0% and 12.0%, respectively (identical to those for span checks). Over the 2009-2011 

period, there were 9 occasions where one or more unsatisfactory ZSP result was not 

followed up by corrective action. Eight of these events involved the precision point—the 

concentration most relevant to the 75 ppb standard.

 Calibrations were often not performed at the prescribed 90-day intervals. 

The 1996 QAPP in Section 9.8.7, Item 5 states that “Each continuous gas analyzer shall 

receive a multi-point calibration at least every 90 days. (This is a minimum 

requirement.)” [Emphasis added]. The interval between successive calibrations 

associated with the 2009 – 2011 data exceeded 90 days five times. These excessive 

intervals ranged from 99 days, to as many as 162 days.

 Calibrations were sometimes performed using an insufficient number of “upscale” (nonzero) gas concentrations. 

Both EPA 2008 and the 1996 MDEQ QAPP clearly require a zero concentration, plus 

four upscale points. In three instances, only three non-zero concentrations were used; and 

in those instances, no concentrations in the vicinity of 75 ppb were checked. The nearest 

higher concentration in those instances was 197 ppb, while the nearest lower 

concentration was 23 ppb. 

 Calibration results were outside of acceptance limits

This occurred on one occasion, when the calibration error at the high (“span”) point 

exceeded the prescribed limit. 
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 Specified gas concentration ranges were not used for several performance audits. 

The 1996 QAPP clearly identifies required concentration ranges for performance audits, 

which were not adhered to for seven audits.

 In one instance, an unacceptable audit result was not followed by specified corrective 

action.

 Expired gas cylinders were occasionally used for both audits and routine site operations. 

Part B of QAPP Section 9.8.7 clearly states that “If cylinders are used in the calibration, 

they must have been certified within the time frame specified in Table 9.8B [i.e. 24 

months]. Check the certification date on the cylinder's label or tag to verify the 

certification is current.” An expired gas cylinder was used for one performance audit, 

and for one multipoint calibration. Additionally, calibration records strongly indicate that 

an expired gas cylinder was used for ZSP checks conducted from May 25, 2009 through 

at least July 21, 2009 (since the same gas standard is typically used for both ZSP checks 

and calibrations). 

An exhaustive review and evaluation of each of the more than 25,000 hours of Coburn Road data 

collected between 2009 and 2011 is beyond the scope of this document. However, a very small 

fraction of those hourly readings (<0.01%) were entirely responsible for the SO2 nonattainment 

designation, and data quality issues associated with each reading are discussed in Section 4 of 

this document. 
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4. DATA QUALITY ISSUES ASSOCIATED WITH HIGH SO2 READINGS

The fifteen high (at or above 75 ppb) SO2 readings between 2009 and 2011 each had one or more 

associated data quality issue, and each issue listed in Section 3 was found for at least one of the 

high SO2 readings. 

Table 4 lists each of the high (at or above 75 ppb) SO2 readings observed between 2009 and 

2011, along with their associated data quality issues. Table 4 illustrates that the fifteen high 

readings being used to justify the SO2 nonattainment designation were not collected in strict 

accordance with the 1996 MDEQ QAPP and other EPA guidance documents. 

Additionally, the SO2 analyzer itself has inherent accuracy limitations. MDEQ records indicate 

that the Coburn Road site employed an API Model 100A SO2 analyzer during the 2009 – 2011 

monitoring period. A review of the manufacturer’s data sheet for this analyzer[1] reveals a 

linearity specification of 1% of full scale, which has important consequences for data precision. 

The 4 ppb difference between the Coburn Road site design value and the 75 ppb standard 

represents only 0.4% of the instrument’s full-scale range when operated at a 0-1,000 ppb range. 

This difference increases to 0.8% when the instrument is operated on a 0-500 ppb range. The 

Coburn Road instrument was operated on the higher scale during 2009 and most of 2010; it was 

re-scaled in later 2010 to 500 ppb. 

The concept of linearity is best illustrated with an example: consider an analyzer operated at a 0-

1,000 ppb full scale range. If the instrument actually read 1,000 ppb at a known gas 

concentration of 1,000 ppb, and zero ppb at a known concentration of zero ppb, perfect linearity 

would exist if, at a known concentration of 75 ppb the instrument read 75 ppb. The 1% of full 

scale linearity specification indicates that at any known gas concentration between 0 and 1,000 

ppb, the instrument response could differ from the expected value by up to 10 ppb (even if the 

instrument were in perfect calibration at 0 and 1,000 ppb). Thus, the instrument response to a 

known concentration of 75 ppb could range between 65 and 85 ppb, and still satisfy the 

instrument’s linearity specification. Even for an instrument operated at a 0-500 ppb range, the 

response could range from 70 to 80 ppb.

Both potential linearity errors are greater than the 4 ppb difference between the Coburn Road site 

monitor’s 79 ppb design value and the 75 ppb standard, and call into question whether it is 

known with certainty that the measured 79 ppb design value is in fact greater than 75 ppb. 

 

[1] Instruction Manual Teledyne API Model 100A Sulfur Dioxide Analyzer – Rev. H. Teledyne Advanced Pollution 

Instrumentation, 24 February 2006.
Attachment	B TSRIA	Petition	for	Reconsideration Page B	- 13

TABLE 4 

QUALITY ISSUES FOR HIGH SO2 READINGS AT COBURN ROAD

DATE

SO2

(ppb) REMARKS

2009

Feb 5 83  Interval between preceding (9/10/2008) and following (2/19/2009) 

analyzer calibration was 162 days

 The audit gas concentration ranges specified in (MDEQ 1996) were not 

adhered to in the preceding (12/12/2008) and following (2/25/2009) 

audits

Sep 22

Sep 25

107

83

 Interval between preceding (9/17/2009) and following (1/11/2010) 

analyzer calibration was 116 days

 The audit gas concentration ranges specified in (MDEQ 1996) were not 

adhered to in the preceding (8/12/2009) audit

2010

Feb 10 91  The audit gas concentration ranges specified in (MDEQ 1996) were not 

adhered to in the following (3/23/2010) audit

 The preceding precision point check (2/9/2010) error of -7.2% exceeded 

the warning limit, but no corrective action was taken.

Jul 8

Jul 9

111

92

 The audit gas concentration ranges specified in (MDEQ 1996) were not 

adhered to in the preceding (6/22/2010) audit

 The error for the lowest audit point (+48.8% / +3.9 ppb) exceeded the 

acceptance criterion, but no corrective action was taken.

 Interval between preceding (4/15/2010) and following (8/23/2010) 

analyzer calibration was 130 days

 The following calibration (8/23/2010) was performed using only 3 

upscale points, rather than the four specified in (MDEQ 1996). None of 

the calibration points were in the vicinity of 75 ppb.

 The preceding zero check (6/28/2010) error of +0.7% of full scale 

exceeded the warning limit, but no corrective action was taken.

Sep 26

Sep 28

Oct 2

Oct 3

78

83

75

87

 The audit gas concentration ranges specified in (MDEQ 1996) were not 

adhered to in the following (12/1/2010) audit

 The preceding (8/23/2010) and following (11/5/2010) calibrations were 

performed using only 3 upscale points, rather than the four specified in 

(MDEQ 1996). None of the calibration points were in the vicinity of 75 

ppb./
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TABLE 4 – Continued

QUALITY ISSUES FOR HIGH SO2 READINGS AT COBURN ROAD

DATE

SO2

(ppb) REMARKS

2010

Oct 22 89  The preceding (8/23/2010) and following (11/5/2010) calibrations 

were performed using only 3 upscale points, rather than the four 

specified in (MDEQ 1996). None of the calibration points were in the 

vicinity of 75 ppb.

 The interval between the preceding (10/4/2010) and following 

(10/25/2010) ZSP checks exceeded two weeks

 The audit gas concentration ranges specified in (MDEQ 1996) were 

not adhered to in the following (12/1/2010) audit

Dec 24 101  The audit gas concentration ranges specified in (MDEQ 1996) were not 

adhered to in the preceding (12/1/2010) audit

 The preceding (11/5/2010) and following (1/5/2011) calibrations were 

performed using only 3 upscale points, rather than the four specified in 

(MDEQ 1996). None of the calibration points were in the vicinity of 75 

ppb.

2011

Sep 5

Sep 11

142

113

 An expired gas cylinder standard was used for the following (12/7/2011) 

performance audit

Oct 30 85  An expired gas cylinder standard was used for the following (12/7/2011) 

performance audit

 The following (11/17/2011) analyzer calibration had a span error of 

-3.47%.