MEMORANDUM

DATE: August 2, 2013

SUBJECT: Approach for Hydrogen Fluoride Continuous Emission Monitoring and 

Compliance Determination with EPA Method 320

FROM: Steffan Johnson

Measurement Policy Group, SPPD

TO: Residual Phosphoric Acid Manufacturing and Phosphate Fertilizer Production 

RTR and Standards of Performance for Phosphate Processing

A. INTRODUCTION

Technological advances in stationary source emissions monitoring allow for direct measurement 

of fluoride emissions (hydrogen fluoride) as an alternative to the measurement of a total fluoride.

A total fluoride test method is currently used for the demonstration of compliance with a total

fluoride standard in the Phosphate Fertilizer and Phosphoric Acid rules (40 CFR, Part 63,

Subparts AA and BB). Since promulgation of the existing rules in 1999, new developments in 

emissions measurement using Fourier Transform Infrared (FTIR) instrumentation have made 

direct, real time measurement of specific hazardous air pollutants (HAPs) possible. The use of 

such technologies requires consideration of instrument stability, instrument calibration, 

calibration standards, daily quality assurance/quality control (QA/QC) procedures as well as 

reporting and recordkeeping requirements. The purpose of this memo is to discuss and highlight 

appropriate measurement and monitoring requirements for the use of this technology in regards 

to compliance demonstration testing and continuous emissions monitoring.

B. DISCUSSION

Accurate measurement of hydrogen fluoride (HF) with FTIR technology, in real time, is now a 

possibility with an EPA Method 320 approach. In the early 2000’s the availability of testers who 

could perform such testing, as well as the costs of performing these tests, presented challenges to 

requiring use of this technology. In recent years, more instrumentation and expertise have 

become available, as well as a general trend in lower emissions testing costs using these 

instruments. The EPA is beginning to propose their use as an alternate compliance 

demonstration, as well as continuous monitoring approaches in various rules. The use of such 

instrumentation can be expected to provide benefits to the regulated source as well as the 

regulatory community. Emission measurement results of the specific HAP are available, on site,

with good accuracy and precision at very low levels. Real time data provides for a greater 

understanding of process to pollutant correlation and provides sources with valuable process 

control feedback that is not provided with a manual stack test. This same instrumentation can be 

used for monitoring of other flue gas constituents if needed.
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Concurrent with requiring such monitoring in rules, a number of other supporting requirements 

must be addressed to provide reliable and quality assured data from these systems. Such 

underlying concepts are: selection of appropriate equipment, instrument calibration and drift 

evaluation, relative accuracy determination and performance specifications, appropriate

calibration standards, and data QA/QC.

Selection of Equipment:

In order to continuously measure HF with a continuous emissions monitor (CEMS) equipment 

that satisfies basic source CEMS measurement and control equipment criteria needs to be

installed according to the manufacturer’s specifications and the requirements set out in the soonto-be-proposed Performance Specification 18 (PS-18) for hydrogen chloride (HCl) CEMS.

Extractive HF CEMS would be expected to include a sample extraction system, sample 

conditioning module, HF analyzer, diluent analyzer, system controller, data recorder, reference 

gas system and moisture measurement system. An in-situ CEMS would not need a sample 

extraction system or sample conditioning module, but would need the other above listed 

components.

The current draft of PS-18 specifies that, for extractive CEMS, the system must be designed to 

be able to introduce reference gas flow sufficient to flood the sampling probe and prevent entry 

of gas from the effluent stream. It also specifies that the sampling conditioning module must be 

operated in such a way as to keep the particle free gas sample above the dew point temperature 

of its components.

Typical EPA performance specifications require that CEMS be installed at an accessible location 

where the pollutant concentration or emission rate measurements are directly representative of 

the emissions or can be corrected to be representative of the emissions from the affected facility. 

With regards to HF CEMS emissions measurement location, the current draft of PS-18 specifies 

that an HCl CEMS should be (1) at least two equivalent diameters downstream of the nearest 

control device, point of pollution generation or other point at which a change of pollutant 

concentration may occur, and (2) at least half an equivalent diameter (calculated according to 

Method 1 in appendix A-1 to part 60) upstream from the effluent exhaust. The draft PS-18, 

which provides such requirements for HCl CEMS will address a majority of what is necessary to 

measure HF continuously.

Performance Specification Criteria:

A number of QA/QC requirements are necessary to validate the data from any installed CEMS 

system. With HF CEMS we expect the following to QA/QC requirements to apply:

HF CEMS Measurement Range

An applicable performance specification will require that a documented check of the continuous 

emissions measurement range of the HF CEMS be performed to verify that the instrument is 

functioning correctly following installation. For example, the draft Performance Specification 18 
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requires that the data collection device output range include zero and the upper limit of the 

measurement range for HCl CEMS, and it is likely to be similar for HF CEMS.

Interference Test. 

A test to detect analyzer response to potential interferences that may cause measurement bias that 

are not adequately accounted for in the calibration procedure is required. For the current draft of

PS-18 the combined interference response for the analyzer used for the test must not be greater 

than ± 3.0 percent of the equivalent HCl concentration used for the interference test.

Level of Detection (LOD) Determination.

A determination of the minimum amount of HF that can be detected above the background in a 

representative gas matrix (the LOD) is necessary. LOD determination is independent of the span 

required in applicable regulations. If the LOD determination test is performed in a controlled 

environment, verification of the LOD during the initial field certification test using the DS test 

procedure (as included in Appendix A of the soon-to-be-proposed PS 18) would be required. The 

field verified detection level would replace the controlled environment LOD and would become 

the site or installation specific LOD.

Response Time (RT) Determination.

The response time of an extractive HF CEMS must be determined based on the average upscale 

and downscale response time as the response time for the system (the RT). This is the time it 

takes for the measurement system, while operating normally at its target sample flow rate, 

dilution ration, or data collection rate to response to a known step change in gas concentration 

(from a low or zero level to high-level gas concentration or vice versa). The RT must read within 

five percent of the stable gas response (i.e., the calibration error of the HF CEMS must be less 

than 5 percent).

Calibration Error (CE) Test.

A CE test is the mean difference between the HF calibration gas value and the CEMS response at 

each calibration point expressed as a percentage of the span. The CE of an HF CEMS must be 

less than 5 percent.

Seven-Day Calibration Drift (CD) Test. 

Prior to conducting an RA test on an HF CEMS, a seven-day CD test must be conducted. The 

purpose of the seven-day CD test is to verify the ability of the CEMS to maintain calibration for 

each of seven, 24-hour periods. The zero-level and high-level drift for each day must be less than 

5 percent of the span value. Each daily drift check must be performed for seven consecutive days 

to meet this requirement and each drift check must be recorded and reported for the seven-day 

drift check tests.

Relative Accuracy (RA) Test.

Determination of the RA for an HF CEMS is a requirement. The RA must be established against 

an RM (Reference Method), in this case EPA Method 320. The RA is the absolute mean 

difference between the gas concentration determined by the CEMS and the value determined by 

the RM, plus the 2.5 percent error confidence coefficient of a series of tests divided by the 

average of the RM or the applicable emission standard.
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There are also specific requirements to record and report supporting data for test procedures and 

calculations set out in the draft PS-18. Similar requirements would apply to an HF CEMS 

installation. For example, for systems that use a gas blender and/or liquid evaporative calibrator 

to deliver HF gas standards, the draft PS-18 specifies that your record and report supporting data 

for these devices, including liquid feed calibrations, liquid standard(s) concentration, feed rate 

and gas flow calibrations for all diluent and HCl gas flows. Such systems, when used for HF 

calibration standards would also require documentation. The draft PS-18 also specifies that 

owners and operators record and report summaries (in tabular form) of the results of CD tests, 

beyond calibration span linearity tests, RT tests, CE tests, RA tests and optional spike and 

recover (dynamic spiking or DS) procedures. Additionally, the draft PS-18 specifies that owners 

and operators record and report supporting dilution system data and LOD and system limitation 

verification data for installed HCl CEMS. We expect these specifications would also apply to an 

HF CEMS installation.

Daily QA/QC Requirements:

Procedure 6:

The draft Performance Specification 18 specifies the minimum QA requirements necessary for 

the control and assessment of the quality of CEMS data submitted to the EPA. EPA plans to 

propose QA procedures for HCl (Procedure 6) at the same time it proposes PS-18. The draft

Procedure 6 would directly applicable to HF CEMS. The procedure assesses the quality of the 

CEMS data produced by estimating accuracy. The procedure also assists in the control and 

improvement of the quality of the CEMS data by implementing QC policies and corrective 

actions. Both of these purposes work together to ensure that data quality is acceptable. 

Who would comply with Procedure 6?

Source owners and operators required to install HCl or HF CEMs for compliance monitoring 

would be required to meet the minimum requirements of Procedure 6 once finalized and would 

be encouraged to develop and implement a more extensive QA program or to continue such 

programs where they already exist.

The draft Procedure 6 specifies that the program include written procedures and manufacturer’s 

information that describes in detail, complete, step-by-step procedures and operations for 

specified activities (e.g., calibration drift checks of all HF CEMS and temperature and pressure

measurement accuracy for in-situ CEMS). The draft Procedures specify that records of these 

written procedures be maintained and available for inspection by enforcement agencies.

Calibration Drift Assessment. 

Source owners and operators required to install HF CEMS would be required to check, record 

and quantify the CD at two concentration values at least once daily in accordance with the 

method prescribed by the manufacturer and by the applicable performance specification. The HF 

CEMS calibration must, at a minimum, be adjusted whenever the daily zero (or low-level) CD or 

daily high-level CD exceeds two times the drift limits of the applicable performance 

specification (e.g., PS-18).
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The draft Procedure 6 specifies that the accuracy of each CEMS be audited at least once each 

calendar quarter by a relative accuracy test audit (RATA), DS audit (DSA), a cylinder gas audit 

(CGA) or other acceptable alternative approved by the Administrator. Hydrogen fluoride audit 

gases are expected to be National Institute of Standards and Technology (NIST) certified or 

NIST-traceable. The draft Procedure 6 also specifies that RATAs be conducted once every fourcalendar quarters unless the affected facility is off-line. 

Excessive Audit Inaccuracy. 

The draft Procedure 6 defines what constitutes an out-of-control CEMS and specifies corrective 

actions to eliminate problem(s) when a CEMS is deemed out-of-control. The draft procedure also 

specifies that owners and operators of the out-of-control CEMS verify that the problem(s) has 

been eliminated by documenting and reporting the results of the audit following corrective action 

showing the CEMS to be operating within specifications. For purposes of excessive audit 

inaccuracy, a CEMS is considered out of control when (1) RATA is less than 5 percent of span, 

(2) the DSA allowable RA is ± 15 percent of the average spike value or ± 20 percent of the 

applicable emission standard, whichever is greater, or (3) CGA is less than 5 percent of span. 

CEMS data collected during out-of-control periods must not be used in calculating emission 

compliance nor be counted towards meeting minimum data availability as required under an 

applicable regulation or permit.

Criteria for Acceptable QC Procedures. 

In situations where a CEMS experiences excessive out-of-control conditions resulting from the 

quarterly audits for two consecutive quarters, according to the draft Procedure 6 source, owner or 

operators would need to revise their QC procedures, or modify or replace their CEMS.

Criteria for Optional QA Test Frequency. 

The draft Procedure 6 states that if a CEM is determined to be in-control for eight consecutive 

quarters that include a minimum of two RATA, owners or operators of the CEMS may revise 

their auditing procedures to use CGA or DSA each quarter for seven subsequent quarters. Under 

this proposed scenario, the CEMS owner would only be required to perform a RATA that meets 

the acceptance criteria once every two years. Conversely, an owner or operator of a CEMS that 

fails a RATA, CGA, or DSA would need to revert to their original auditing schedule until the 

audits results meet in-control criteria to start re-qualifying for the optional QA test frequency 

again. 

Calculations for CEMS Data Accuracy. 

The draft Procedure 6 specifies RATA calculation, CGA accuracy calculation and DSA accuracy 

calculation requirements.

Owners and operators of affected HCl CEMS must report for each CEMS the accuracy and CD 

assessment results and the drift and accuracy information as a Data Assessment Report (DAR) 

(an example of a DAR format is provided in Procedure 6; section 9.0, Figure 1). At a minimum, 

the DAR must contain source owner and operator information; identification and location of 

monitors in the CEMS; manufacturer and model number of each monitor in the CEMS; and 

assessment of CEMS data accuracy and date of assessment. The DAR is required to be submitted 

with the report of emissions required under the applicable regulation or permit that requires 
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continuous emission monitoring.

C. SUMMARY

The above discussion outlines our expectations for what constitutes an acceptable HF CEMS 

monitoring system, as well as QA/QC criteria for assessing the performance of the system and 

quality of the reported data. HF CEMS technology is currently on the cusp of development, and a 

number of technical requirements have yet to be put in place to fully implement such systems 

(i.e. specific performance specifications, NIST calibration standards, and daily QA/QC 

requirements). Yet at least one appropriate technology exists, as described in EPA Method 320, 

and we envision a growing need for these systems in the future. This measurement approach has 

significant advantages in terms of being able to directly and continuously measure HF, versus 

current compliance demonstrations that require annual testing for total fluoride as a surrogate for 

HF. The EPA will be soliciting and accepting public comment on the proposed PS-18 and 

Procedure 6, therefore this is subject to future change.

We expect that where HF CEMS will be installed, the points discussed in this memo will need to 

be addressed and resolved.