Over the last several years, the public and regulatory agencies have become more interested in air quality monitoring along the fencelines of industrial facilities. Regulatory requirements from the U.S. Environmental Protection Agency’s (EPA) Risk and Technology Review process for the refinery sector in 2015 required petroleum refineries to monitor for benzene using two-week passive samplers. Additional regulations from local and state agencies require real-time air monitoring at fenceline perimeters with public reporting. On occasion, consent decrees also prompt facilities to conduct fenceline monitoring for elective reasons. This article provides information about why fenceline monitoring networks are being installed, the available technologies that can be included, and additional factors to consider.
Why Install a Fenceline Monitoring Network?
Most facilities currently conducting fenceline monitoring are doing so to be compliant with state or local air district regulations. For example, California Assembly Bill 1647 (AB-1647) imposed state-mandated fenceline monitoring programs at petroleum refineries to be regulated by local air districts. This subsequently resulted in several local regulations, including Regulation 12 Rule 15 from the Bay Area Air Quality Air Management District (BAAQMD), Rule 1180 from the South Coast Air Quality Management District (SCAQMD), Rule 364 from the Santa Barbara County Air Pollution Control District, and Rule 4460 from the San Joaquin Valley Air Pollution Control District (SJVAPCD). The San Luis Obispo County Air Pollution Control District opted instead for a Memorandum of Understanding (MOU) with the only applicable facility in their district in place of a rule. Elsewhere, the state of Colorado enacted regulations in 2021 for refineries and fuel distribution terminals via HB21-1189, and comparable regulations may arise in other states with similar industrial operations.
While there are small differences in regulations, all include verbiage that requires facilities to “track air emissions and crude oil composition characteristics from Refineries and Support Facilities over time” and “establish air monitoring systems to provide air quality data along refinery boundaries” (Regulation 12 Rule 15). These rules have changed over time to specify which facilities must comply and what compounds should be monitored.
Apart from regulations, some facilities elect to install fenceline monitoring networks to promote positive relationships with nearby communities. Others do so to be able to demonstrate that emissions caused by exceptional events or other emitters did not originate at their facility. A facility may also implement a temporary network to determine the benefit of installing a permanent network.
Available Technologies
Fenceline monitoring networks can include a wide range of complementary monitoring techniques. A key development over the last few years has been the more widespread incorporation of instrumentation that provides real-time (immediate) information, which is often reported to the public and regulatory authorities on a rapid (minutes) basis.
- Open-Path Spectroscopy – Open-path spectroscopy is the fastest-growing technology for fenceline monitoring. An analyzer sends a beam of light along a pathway that is open to the environment, either (1) to a reflector that sends the light back to a detector, referred to as a monostatic configuration, or (2) to a detector at the other end of the pathway, referred to as a bistatic configuration. The amount of light absorbed at specific wavelengths is used to identify and quantify compounds that are present in the air. This spectroscopic technique can provide simultaneous, real-time (sub-five-minute) measurements of dozens of compounds across sampling paths that are hundreds of meters long. While open-path analyzers are expensive, they offer the most complete coverage of fenceline perimeters and may be more cost effective than achieving comparable coverage with point monitors. Shelter and power requirements vary based on the number and length of paths, analyzer configuration, and facility topography.
- Point Monitors – The term ‘point monitors’ broadly describes analyzers that sample at a single geographic point, instead of across a long pathway like open-path analyzers. Cavity ring-down spectroscopy, UV-induced fluorescence, gas chromatography, beta attenuation monitors, Aethalometers, and many other instruments fit into this class. Most U.S. EPA Federal Reference Method (FRM) and Federal Equivalent Method (FEM) monitors are point monitors that are commonly used for regulatory applications. Point monitors are widely used because of their affordability and low minimum detection limits (MDLs). However, multiple point analyzers are needed to cover fenceline perimeters and they usually require climate-controlled shelters with reliable access to power.
- Active Samplers – Active samplers refer to samplers that collect air through a filter or into a container over a fixed period of time, typically 24-hours, for use in regulatory or risk-assessment applications. For example, toxic metal contaminants are collected with a sampler that pulls air through a filter material, such as Teflon, after which the collected material is analyzed in an off-site laboratory. Volatile organic compounds (VOCs) are often collected into specially passivated (i.e., chemically treated to resist corrosion) stainless steel canisters with subsequent laboratory analysis by gas chromatography-mass spectrometry. These canisters are often referred to as ‘summa’ canisters, but can include both summa passivated and silica-coated canisters, which are equivalent in their functionality.
- Passive Samplers – Passive samplers are a type of point monitor that predates other instruments in the class and tend to be used for a variety of applications. There are two types: (1) passive sorbent-tube type samplers and (2) vacuum-driven samplers, such as a stainless-steel canisters. Sorbent-tube samplers contain an absorbent media that is selective for the compound(s) of interest and open to the environment, without any active means to draw air through the sorbent tube. These samplers can be left for extended periods of time, commonly up to two weeks or longer, and analyzed in a laboratory after collection. Vacuum-driven canisters use their internal vacuum to draw the sample via a flow controller, commonly over a period of 24 hours. Because passive samplers are analyzed with compliance-grade instrumentation, they can measure compounds of interest at extremely low levels, beyond the capabilities of open-path analyzers or point monitors. However, because they only provide average concentrations for the entire sampling period, it can be difficult to determine the source of emissions and whether they were evenly dispersed during the timeframe or passed by as a concentrated plume all at once. Passive samplers are often very cost-effective and may be used to complement other fenceline monitoring techniques.
- Next-Generation Monitors, or Low-Cost Sensors – Low-cost sensors are a newer class of instruments that are typically used for indicative measurements, but are not widely used for regulatory applications. They can be used to complement and extend more sophisticated networks, as they cost significantly less than research-grade instrumentation and can identify broad trends or “hotspots” for certain classes of compounds. Their minimum detection limits are higher than with other technologies and they commonly experience interference or cross-sensitivity with other compounds present in the air. This can result in false positive or negative measurements, so collocation with other established techniques is recommended. As low-cost sensor technologies improve, they represent an attractive possibility for some regulatory applications. For example, the Colorado Department of Public Health and Environment (CDPHE) preliminarily approved low-cost sensors for use in air toxics monitoring near oil and gas wells under Regulation 7, which was adopted in May 2021.
- Mobile Monitoring – Mobile monitoring does not itself make up a fenceline monitoring network, but it is a powerful method that can complement these systems and add valuable information for facilities and surrounding communities. Instruments are installed on a mobile platform such as a vehicle, drone, or plane, which samples while moving around a facility or in a nearby community during a potential emissions event. Mobile monitoring represents a rapid response element that can provide real-time, location-specific information about where potential emissions are detected, enabling fast and informed decision-making.
Important Considerations
For industrial facilities implementing fenceline monitoring networks to meet regulatory requirements, compliance is the most important consideration. The definition of compliance is determined by the regulation, but usually requires broad spatial coverage and simultaneous, real-time monitoring of a range of compounds. A combination of open-path analyzers and point monitors is generally a robust solution that meets recent fenceline monitoring regulations.
Another factor to consider is upfront procurement costs vs. ongoing operations and maintenance costs. For example, the capital expense to procure and install large-scale open-path fenceline monitoring networks may be less than the combination of procurement expenses for enough point monitors to provide comparable spatial coverage of all compounds, plus the ongoing operations and maintenance expenses.
Facilities should also consider data acquisition and management systems. From passive samplers corresponding to one data point per site every two weeks, to open-path analyzers generating dozens of data points per site every five minutes, a robust data management system is one that is well-suited for the requirements of the regulation or application. For networks requiring real-time data display with many sampling paths and/or compounds of interest, reliable internet access and data acquisition infrastructure is an important consideration.
Key Takeaways
Whether you are implementing a fenceline monitoring network to be compliant with regulatory requirements or to proactively monitor for potential emissions in your area, there are many different elements to keep in mind. Instrument selection, infrastructure, cost, upkeep, and data management must all be considered and can directly inform the type of fenceline network needed. Our upcoming articles dive deeper into fenceline monitoring approaches and emerging technologies.
Steven R. Schill
