What Was Old is New Again: Monitoring and the Clean Air Act
In the early decades of Clean Air Act implementation, it was common for permitting agencies to require facilities seeking Prevention of Significant Deterioration (PSD) permits to conduct pre-construction ambient air quality monitoring prior to permit issuance.
While less common, it was not unprecedented to require facilities to conduct post-construction ambient air quality monitoring to confirm that the new or modified facility would not cause a violation of a National Ambient Air Quality Standard or consume more of a PSD increment than allowed.
The requirement to conduct post-construction ambient air quality monitoring was in recognition that:
- regulatory dispersion models, while typically (but not always) conservative in their estimation of impacts, are only as good as the data inputs (e.g. emission rates), assumptions, and limitations of the selected model; and
- emission estimation methodologies for fugitive emission sources (sources that cannot easily be exhausted through a stack or vent) were often unreliable.
Today, pre-construction monitoring is seldom required. Permitting agencies rely upon the network of existing air quality monitors to provide reasonable estimates of pre-construction air quality in the area of the proposed facility. Post-construction monitoring is even rarer. Regulatory agencies, for various reasons (cost, schedule, efficiency, resource requirements, etc.) have come to rely almost completely upon quick-and-cheap dispersion modeling analyses to predict post-construction air quality.
Regulatory agencies, for various reasons (cost, schedule, efficiency, resource requirements, etc.) have come to rely almost completely upon quick-and-cheap dispersion modeling analyses to predict post-construction air quality.
To confirm emission rates, permitting agencies continue to rely upon stack testing to demonstrate compliance with point source emission limits. This, however, is not the case for fugitive emission sources such as storage piles, equipment leaks (flanges, valves, pumps, etc.), wastewater treatment facilities, and storage tanks. While a permit may require implementation of operating practices aimed at reducing emissions (e.g. a leak detection and repair program for equipment leaks or spraying water on a storage pile), permits rarely require facilities to prove fugitive emission rates via testing.
Considering the poor reliability and accuracy of many fugitive emission estimation methodologies (often simple emission factors compiled by EPA and other regulatory agencies), it is not surprising that fugitive emission source testing using advanced analytical tools such as Differential Absorption Lidar (DIAL) and Solar Occultation Flux (SOF) has often shown poor correlation between estimated values and actual emissions.
So, if fugitive source emission estimates often show poor correlation to measured actual emission rates, and dispersion modeling analyses are limited by the quality of the data inputs (e.g. estimated emission rates), how reliable are the modeling results? More importantly, are they protective of the downwind communities?
EPA is of the opinion that petroleum refineries underestimate emissions from fugitive sources and that fence-line monitoring will help refiners to detect, identify and fix sources of excess benzene and other hydrocarbons emissions, mainly from fugitive sources.
Regulatory agencies have started to integrate this line of reasoning into the rulemaking process. In December 2015, the EPA finalized rules that require petroleum refineries to conduct continuous fence-line monitoring for benzene. In the rule development documents EPA states that the underlying reason for the fence-line monitoring requirement is not to establish a de facto benzene ambient air quality standard. Rather, EPA is of the opinion that petroleum refineries underestimate emissions from fugitive sources and that fence-line monitoring will help refiners to detect, identify and fix sources of excess benzene and other hydrocarbons emissions, mainly from fugitive sources. More generally, it is broadly accepted within the regulated community that EPA will, going forward, routinely integrate fence-line monitoring requirements into rules for source categories with potentially large fugitive source contributions beyond petroleum refiners, with petrochemical manufacturing plants likely to be the next source category subject to the requirement.
The Bay Area Air Quality Management District (BAAQMD) recently expanded on EPA's fence-line monitoring concept. In a rule finalized in April 2016, the BAAQMD requires the five petroleum refineries in its jurisdiction to install advanced, open-path monitoring technologies that measure the concentration of multiple key pollutants at the boundary between the refinery and the surrounding communities. [An "open path" technology is one that detects and/or measures concentrations of pollutants directly in the air.] Further, the BAAQMD requires that this information be made available to the public in real-time.
The information provided by sophisticated monitoring systems can be used for purposes beyond compliance. These systems can be used as early-warning detection for significant fugitive emission issues (e.g. equipment leaks) that not only represent lost product but also pose potential for fires, explosions and exposure of workers and the local community.
Air dispersion models and emission factors will certainly continue to have a place in air quality management. However, rapid advancements in the reliability and performance of monitoring systems, the ability to access monitoring information in near real-time via the internet, and decreasing costs are helping monitoring to regaining its importance as a valued air quality management tool.
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