Resources >> Literature >> Patching the Holes in the Fenceline - Updates to EPA 325 Requirements

Patching the Holes in the Fenceline - Updates to EPA 325 Requirements

Sidebar Image

Heidi C. Hayes, Eurofins Air Toxics, 180 Blue Ravine Rd. Suite B, Folsom, CA 95630

J. Derek Reese, Exxon Mobil Corporation, 22777 Springwoods Village Pkwy, Spring, TX 77389


As facilities began to implement fenceline monitoring requirements in preparation of meeting the regulatory mandated February 1, 2018 timeline, specific gaps in the methodology and prescribed work practices were identified. In response, corrections and clarifications to EPA Method 325 were addressed in an Alternative Test Method recently approved by the EPA. Issues addressed in the Alternative Test Method include updates to sample concentration equations, definition of allowable sample collection periods, and clarification of field duplicate requirements.


After several drafts and multiple public comment periods, the final update to the Refinery Sector Rule was published in the Federal Register on December 1, 20151.  To support the rule’s fenceline monitoring provisions, EPA Methods 325A2 and 325B3 were promulgated concurrent with the rule update.  While the final rule addressed many of issues brought up during the comment period, several sections in EPA Methods 325A and 325B contained technical errors or ambiguous requirements.  Modifications to the method were proposed to the EPA per the General Provisions of Part §63.7(e)(2)(ii) and include: 

  • Correction of concentration formulas to adjust uptake rates for local conditions and to convert concentrations to standard conditions;
  • Clarification of sample collection timing to address conditions when extenuating circumstances interrupt continuous 14-day schedules; and
  • Definition of how to incorporate field duplicate concentrations in site monitoring results and clarification of field duplicate frequency requirements.

This paper provides detailed descriptions and justifications for the Alternative Test Method request submitted and approved by the EPA.


Concentration Formulas – Temperature and Pressure Adjustments


EPA Method 325B currently requires the calculation of sample concentration using Equations 12.5 and 12.6.  Section 12.2.3 directs the laboratory to correct target concentrations determined at the sampling site temperature and atmospheric pressure to standard conditions (25°C and 760 mm Hg).  However, Equation 12.6 utilizes the uptake rate calculated for local conditions to calculate the measured concentration which means that the reported concentrations are based on site conditions.  Furthermore, equation 12.5 to adjust the uptake rate for local conditions is incorrect, and no equation to convert concentrations to standard conditions is included in the method.

To simplify and standardize the calculations required in Method 325B, the derivation of the correction to these equations is provided, assuming normal ambient temperature and pressure conditions of 25°C and 760 mm Hg. 

The air concentration at the sampling site is calculated using Equation 1.

Where Cm =     Concentration measured at the site (µg/m3)
mmeas =           Mass of compound as measured on the sorbent tube (µg)
USS =               Diffusive uptake rate at sampling site conditions (ml/min)
t =                   Sampling duration (minutes)

From Maxwell’s equation, the diffusion coefficient is a function of absolute temperature and pressure, specifically D = f(T3/2, P-1).4  Since the uptake rate is proportional to the diffusion coefficient, the uptake rate published at standard conditions can be adjusted for field temperature and pressure conditions as described in OSHA T-005 5 using Equation 2.

Where UNTP = Uptake Rate at normal ambient temperature and pressure (ml/min)
TSS =                Average temperature during the collection period at the sampling site (K)
PSS =                Average pressure during the collection period at the sampling site (mm Hg)

Substituting USS for UNTP, in Equation 1, the concentration in mass per volume under field conditions, Cm, is calculated using Equation 3.

Applying the ideal gas law, PV = nRT, Cm is converted to Cc using Equation 4, where Cc is the concentration at normal ambient temperature and pressure.

Substituting Cm into Equation 4 generates Equation 5, which simplifies to Equation 6.

Alternative Test Procedure Requested

The Alternative Test Method replaces Equations 12.5 and 12.6 in section 12.2 of EPA 325B with:

Furthermore, the variable definitions are updated to the following:

Cc =     Concentration of target compound at normal ambient temperature and pressure (µg/m3)
mmeas = Mass of compound as measured on the sorbent tube (µg)
UNTP= Diffusive uptake rate at normal ambient temperature and pressure (ml/min)
t =        Sampling duration (minutes)
TSS =    Average temperature during the collection period at the sampling site (K)

Implications to Monitoring Programs

Applying the corrected equations and reporting fenceline concentrations at normal ambient temperature and pressure conditions eliminates the need for average barometric pressure for laboratory calculations. Additionally, the influence of site temperature on the reported concentrations becomes less significant. 

To quantify the effect of the corrected equation on previously reported data generated during pre-compliance sample collection, the magnitude of the concentration adjustments over a range of temperatures was calculated using several possible laboratory interpretations of the current method equations and compared to the Alternative Test Method concentration equation.  The effect of temperature on the calculated concentration is presented as an Adjustment Factor (AF) in Table 1.  The calculation scenarios include the following:

  • Concentrations per current EPA 325B equations (AF325B): Adherence to equations 12.5 and 12.6 (Incorrectly adjusting uptake rate for temperature and pressure with no conversion to normal ambient temperature and pressure);
  • Concentrations adjusted to sampling site conditions (AFSS): Applying the correct uptake adjustment equation in lieu of 12.5 and calculating concentrations using equation 12.6; and,
  • Concentrations adjusted to normal temperature and pressure using Alternative Test Method equation (AFNTP)

Table 1.  Adjustment Factor (AF) to fenceline concentrations as a function of site temperature

Temperature (deg C) AF325B AFSS AFNTP
-10 0.828 1.21 1.06
0 0.876 1.14 1.04
10 0.924 1.08 1.03
20 0.974 1.03 1.01
25 1.00 1.00 1.00
30 1.02 0.976 0.992

Table 1 shows that regardless of the calculation scenario applied to the reported concentrations, the concentrations are generally adjusted by less than 10% as a result of site specific temperature conditions.  In most cases, the differences between the concentrations reported using any of the three calculation scenarios are well within the expected analytical accuracy and precision limits of EPA 325B.  Only at lower temperature conditions do concentration adjustments using calculation scenarios 1 (AF325B) and 2 (AFSS) become significant.  At an average site temperature of -10°C, concentration adjustments are on the order of 20%, introducing a bias compared to AFNTP of -28% and 17% for calculation scenarios 1 and 2 respectively. 

While temperature adjustments to fenceline concentrations may be small under most site temperature conditions, adoption of the Alternative Test Method concentration equation removes any bias due to differences in the calculations applied, providing both a clear and accurate equation to convert mass measured on a passive sorbent tube to a fenceline concentration normalized to conditions of 25°C and 760 mm Hg.

Sample Collection Timing


Section 8.4 of Method 325A states that monitoring may be performed for a period up to 14 days with no clarification of what a facility should do when adverse circumstances make it impossible to meet an exact 14-day sampling period.  There is some guidance provided by §63.658(e)(1) and (2) of Subpart CC which specify the basic sampling period and sampling frequency requirement.  It reads:

(1) Sampling period.  A 14-day sampling period shall be used, unless a shorter sampling period is determined to be necessary under paragraph (g) or (i) of this section.  A sampling period is defined as the period during which sampling tube is deployed at a specific sampling location with the diffusive sampling end cap in-place and does not include the time required to analyze the sample.  For the purpose of this subpart, a 14-day sampling period may be no shorter than 13 calendar days and no longer than 15 calendar days, but the routine sampling period shall be 14 calendar days.

(2) Base sampling frequency.  Except as provided in paragraph (e)(3) of this section, the frequency of sample collection shall be once each contiguous 14-day sampling period, such that the beginning of the next 14-day sampling period begins immediately upon the completion of the previous 14-day sampling period.

Unfortunately, it is not always possible to comply with the 13 to 15-day requirement.  If safe access to one or more sampling locations is restricted by bad weather (e.g., hurricane, flooding, blizzard) during that three day-period or a power failure or other emergency occurs, sample change out may have to be delayed to assure personnel safety.  It is critical the rule reflects this inevitable situation.  Since the action level is a yearly average, occasional deviations in the sampling period duration will have no impact.

Furthermore, the last sentence of §63.658(e)(1) says the sampling period may be 13, 14 or 15 days, but then states that the sampling period must be 14 days. This wording requires correction to provide consistent direction.

Alternative Test Procedure Requested

To clarify this requirement, the alternative procedure states that routine sample collection is performed continuously over 14 day periods with a minimum and maximum allowable duration for routine sampling of 13 to 15 days. When extenuating circumstances do not permit safe deployment or retrieval of passive samplers (i.e., adverse weather, power failure), sampler placement or retrieval may occur later or earlier when safe access to sampling sites is possible.  When the sampling period is adjusted outside the 13 to 15 calendar day range, the reason must be recorded and reported in the next routine reporting cycle.

Implications to Monitoring Programs

This change provides pragmatic and reasonable response options for sites faced with adverse sampling conditions or scenarios posed by unforeseen events.  It will be important for sites to clearly indicate reasons in their monitoring data as well as any averaging period differences.

Field Duplicates


Section 9.3 of EPA Method 325A requires the collection of one co-located sample for every 10 field samples as a measure of method precision.  It is unclear from the method language how many duplicate samples are required for facilities with monitoring locations that are not even multiples of 10.  Furthermore, although EPA Method 325B provides acceptance criterion for field duplicate precision in section 9.9, no guidance is provided as to how to incorporate field duplicate concentrations into the fenceline monitoring reports and the subsequent determination of delta C.  While the sample and its duplicate collected at a monitoring station are expected to generate similar concentrations, the question of which value to use to represent the monitoring location is unclear.  If the duplicate station represents either the maximum or the minimum concentration for a given monitoring period, the value used to calculate of delta C is not defined.

Alternative Test Procedure Requested

To clarify the 10% field duplicate collection frequency requirement and to simplify field blank collection requirements, the Alternative Test Method requires the collection of at least one co-located duplicate sample per monitoring period for sites with 19 of fewer monitoring locations.  If a facility has 20 or more monitoring locations, at least two co-located duplicate samples per monitoring period are collected.  Furthermore, at least one field blank per sampling period is collected regardless of the number of monitoring stations.  This alternative procedure is reflected in updates to sections 8.5.5 and 9.3.1 of Method 325A and section 8.3.2 of Method 325B.

The Alternative Test Method requires averaging the concentrations generated from the field duplicate pair for the purpose of determining delta C, providing a consistent approach without introducing a systematic bias in the calculations.  This additional clarification is added to section 9.4 of EPA 325B.

Implications to Monitoring Programs

The use of the average field duplicate concentration is not expected to significantly impact data sets based on the precision observed in the pre-compliance fenceline monitoring programs.  Evaluation of field duplicates collected over the past several years under a range of environmental conditions and concentrations has shown that method acceptance criterion of <30% RPD is consistently met for benzene with precision often in the single digit %RPD range.  Reviewing the precision of hundreds of field duplicates in our laboratory’s database shows that the precision criterion was exceeded for only 1.8% of the measurements.  Of this small number of exceedances, approximately 21% were attributable to sample concentrations near or below the method reporting limit, 36% were attributed to obvious sample collection errors such as switching the field duplicate and field blank tubes, and 43% had no assignable cause based on the information available to the laboratory.  The latter category of unexplained precision outliers represents only 0.78% of the field duplicates evaluated. 

In all cases, as well as in the rare case that field precision exceeds 30%RPD and sample collection or lab errors are ruled out, the use of the average concentration is appropriate to minimize unintended bias from selecting either the high or the low concentration measured in the co-located samples.


The listed changes were important clarifications to the test method to enable facilities clear guidance as they begin fenceline monitoring program implementation.  EPA was helpful and responsive in expediting approval of the Alternative Test Method.  However, several issues remain for fenceline monitoring programs which are unclear and are outside the scope of the sampling and analytical method.  The most important ones include clarification of what triggers additional sampling locations, flexibility of sampling location placement, and details on the use of site-specific monitoring plans. Additional discussions with EPA will be required to resolve these remaining issues.


  1. Petroleum Refinery Sector Risk and Technology Review and New Source Performance Standards; Final Rule, 40 CFR Parts 60 and 63; Federal Register, Vol. 80, No. 230; U.S. Environmental Protection Agency: Washington D.C., December 2015.

  2. Method 325A-Volatile Organic Compounds from Fugitive and Area Sources, Sampler Deployment and VOC Sample Collection; 40 CFR Appendix A to Part 63; Federal Register, Vol. 80, No. 230; U.S. Environmental Protection Agency: Washington D.C., December 2015.

  3. Method 325B-Volatile Organic Compounds from Fugitive and Area Sources, Sampler Preparation and Analysis; 40 CFR Appendix A to Part 63; Federal Register, Vol. 80, No. 230; U.S. Environmental Protection Agency: Washington D.C., December 2015.

  4. Indoor Ambient and Workplace Air-Sampling and Analysis of Volatile Compounds by Sorbent Tube/Thermal Desorption/Capillary Gas Chromatography-Part 2: Diffusive Sampling; ISO 16017-2:2003, International Standards Organization 2003.

  5. Validation Guidelines for Air Sampling Methods Utilizing Chromatographic Analysis; OSHA T-005, Version 3.0, May 2010.