Update to US EPA Method 325b for Benzene Fenceline Monitoring
In April 2016, Environmental Standards Inc. outlined errors in Equations 12.5 and 12.6 within US EPA Method 325b included in the Final Rule (December 1, 2015) under 40 CFR 63, Appendix A. Those errors were acknowledged by the US EPA which, has been investigating the derivation of these equations. The US EPA has not yet revised the method with updated equations, but the Industry, and ExxonMobil specifically, chose to apply for approval of an alternative test procedure (August, 2017). In the alternative test procedure letter, ExxonMobil requested the following.
In September, 2017, in response to that request by ExxonMobil, the USA EPA provided a revised equation to replace Equations 12.5 and 12.6 in Section 12.2 of Method 325b.
This equation removes the requirement to correct for pressure:
Where Cc = the concentration of target compound at normal ambient temperature and pressure, (µg/m3),
Mmeasure = the 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), and
Tss : Average temperature during the collection period at the sampling site (K).
The US EPA also allowed a 13-15 day sample collection range under circumstances that make collection at 14 days unsafe. However, the reason must be recorded and reported in the next routine reporting cycle. US EPA also provided clarification for the number of co-located and field blank samples as well as instrumentation quality control samples.