The 2024 updates to the Hazardous Organic National Emission Standards for Hazardous Air Pollutants (HON MACT) have introduced new fenceline monitoring requirements for petrochemical facilities. These updates aim to enhance emissions tracking for hazardous compounds like benzene, ethylene oxide, and vinyl chloride. Through pilot studies, our team has gained field level customer insights into setting up and executing successful fenceline monitoring programs.
Understanding HON MACT Fenceline Monitoring Requirements
The Environmental Protection Agency (EPA) first introduced fenceline monitoring regulations with the 2018 Refinery MACT updates, requiring ongoing benzene monitoring via Method 325A/B. The 2024 HON MACT updates expand these requirements to additional facilities that use, store, or emit six hazardous air pollutants.
With the compliance deadline approaching in July 2026, continuous fenceline monitoring requirements will be required for six toxic air pollutants (chloroprene, ethylene dichloride, ethylene oxide, vinyl chloride, butadiene, and benzene). However, facilities had the ability to apply for a Presidential exemption for sources of air pollution from compliance for up to two-years for compliance with any standard or limitation under Section 112 of the Clean Air Act. Facilities submitting an extension request had to do so by March 31, 2025.
Unlike traditional emissions limits, fenceline monitoring regulations focus on ΔC (Delta-C)—the rolling 12-month average difference between maximum and minimum concentrations. Facilities exceeding the ΔC limit must take corrective action.
Key Considerations for Fenceline Monitoring Implementation
Successful fenceline monitoring implementation depends on thorough planning and coordination to ensure accurate data collection and regulatory compliance. From site mapping to equipment logistics, each step plays a critical role in establishing a monitoring program.
Project Setup & Logistics
- Site Mapping & Sampling Locations: Accurate site maps help determine the best placement for sampling equipment. Methods 325 and 327 may require different locations due to distinct sampling siting requirements.
- Equipment & Supply Lead Times: Sampling infrastructure—including shelters, canisters, flow controllers, and meteorological stations—often has lead times of 4+ weeks.
- Shipping & Field Staff Coordination: Samples must arrive at the lab promptly to meet hold times, and backup staff should be in place to handle holidays and absences.
- Data Management & Visualization: A successful fenceline monitoring program requires robust data tracking with modern data software, like the Sensible Environmental Data Platform. It integrates passive tube, canister, meteorological, and real-time monitoring data, allowing facilities to track trends, visualize problem areas, and automate CEDRI reporting for quarterly compliance. It also supports root cause analysis (RCA) when ΔC exceedances occur, helping facilities quickly identify and address emission sources.
Field Sampling Insights
- Passive Tube Sampling (M325A/B): Proper tube handling, shelter setup, and sampling documentation are critical to prevent contamination and ensure accurate results.
- Canister Sampling (M327): Pre/post-sample flow rate and vacuum checks using NIST-traceable instruments can take over an hour, requiring careful scheduling. Large facilities use timers to synchronize sampling. Montrose’s lab partner Enthalpy Analytical is the only laboratory that holds Method 327 accreditation for the HON MACT regulations where Method 327 is required.
The Role of Real-Time Monitoring
When facilities exceed ΔC thresholds, they must perform a root cause analysis within 5 days and implement corrective actions within 30 days. Real-time monitoring is a powerful tool to pinpoint emissions sources quickly and reduce reliance on passive tube and canister data. Understanding the instruments that are currently available to accurately measure these compounds in real-time is important. If a facility is required to conduct real-time monitoring as part of the root cause analysis, the instruments must be quickly and easily deployed. Facilities detecting off-site contributions can submit a site-specific monitoring plan (SSMP) to the EPA for approval but must conduct real-time monitoring to account for those off-site contributions Understanding the onsite requirements, and the technologies available to conduct real-time monitoring is critical.
How We Can Help
Through multiple pilot projects, we’ve gained valuable insights into optimizing monitoring strategies—we can guide you on when to deploy real-time monitoring or canisters, provide siting and implementation of EPA Methods 325A/B and 327, laboratory analysis, field services, and data aggregation. With these early lessons from our pilot studies, Montrose Environmental Group and Enthalpy Analytical are helping our customers navigate HON MACT fenceline monitoring requirements effectively.
Need help developing or evaluating your fenceline monitoring program? Contact us to learn more about our solutions.
Jenna Granstra
Gulf Coast Operations Manager
Jenna is a certified air quality scientist who specializes in Leak Detection Sensor Networks (LDSN), air quality data analysis, and HON MACT projects.
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