Air Sensors: EPA Performance Targets and Test Protocol Development

In recent years, the utilization of air sensors for measuring air quality has witnessed a significant upsurge, marking a pivotal shift in how environmental conditions are monitored and understood. This increase in the deployment of air sensors has heralded progress, allowing for more localized and dynamic data collection. However, amidst this wave of advancement, a pronounced challenge remains: the variability in the quality of sensor data. This variability, ranging from exceptional to suboptimal data, poses a substantial hurdle when attempting to draw meaningful conclusions and make informed decisions based on the data generated by these sensors.

By Foster Voelker II, Director of Engineering – Williams Valve

One of the underlying factors contrib­uting to this challenge is the absence of standardized testing methods that would otherwise facilitate a coherent assessment of the data provided by air sensors. This absence translates into a lack of consistency in the way sensor data can be compared and correlated with the outputs of es­tablished regulatory air monitoring systems. The consequences of this situation are twofold: first, it hinders the credibility and reliability of sensor data, and second, it limits the capac­ity to make accurate inferences from this data.

The Environmental Protection Agen­cy (EPA) has taken steps to enhance the understanding of air sensors. One of these measures involves organizing workshops that bring together a diverse array of stake­holders. These workshops provide a platform for experts, sensor manu­facturers, regulatory authorities, and other relevant parties to share their knowledge and insights. The focus of these workshops encompasses various critical aspects, including exploring the intricacies of air sen­sor technologies, formulating ro­bust performance benchmarks, and gleaning lessons from similar initia­tives undertaken globally.

These workshops, by addressing topics ranging from fine particulate matter (PM_2.5) to ozone (O₃), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), and carbon monoxide (CO), provide a comprehensive perspective on the challenges and opportunities asso­ciated with air sensor deployment. The collaborative nature of these workshops facilitates the cross-pol­lination of ideas, nurtures a deeper understanding of the nuances within sensor technologies, and collectively informs the establishment of effective testing protocols.

To aid non-regulatory monitoring, the EPA released performance tar­get reports in February 2021. These reports offer consistent testing pro­tocols, metrics, and target values for evaluating air sensor performance outdoors and at fixed locations. The reports encompass field and laboratory evaluations and provide detailed guidelines for calculating performance metrics. The reports in­tend to benefit testing organizations, sensor manufacturers, and various consumers, aiding in selecting suit­able sensor technologies and com­prehending their performance.

Ozone Sensor Report

This report aims to establish stan­dardized testing procedures, metrics, and target values for assessing ozone air sensor performance in non-regu­latory supplemental and information­al monitoring (NSIM) scenarios in outdoor, fixed-site environments. It suggests two testing protocols: base testing and enhanced testing, with a recommendation for all testers to un­dertake base testing and encourages enhanced testing using laboratory exposure chambers.

The report identifies performance metrics and target values based on current scientific understanding, lit­erature reviews, evaluations by other organizations, ongoing sensor stan­dardization initiatives, and U.S. EPA sensor research expertise. These are summarized in tables within the re­port. For base testing, an additional data visualization termed ‘exploring meteorological effects’ is suggested, involving graphing meteorological data’s impact on sensor performance. The report advises having at least one day during base testing with a one-hour average O₃ concentration of 60 parts per billion by volume (ppbv) or more at the test site(s).

Further performance metrics and test conditions for enhanced testing are detailed. The report explains how to calculate performance metrics for O₃ sensors and offers templates for both base and enhanced testing reports for consistent result presentation. Notably, target values for enhanced testing are omitted due to limited fea­sibility and data constraints.

The report acknowledges the evolv­ing nature of O₃ sensor technologies and anticipates updating the provided information as advances occur. The report’s audience includes potential testing bodies, sensor manufactur­ers, and developers. Various consum­ers such as government agencies, community groups, and academia are expected to benefit from the standardized presentation of testing results. However, testing outcomes do not imply U.S. EPA certification or endorsement, and testers are encour­aged to make their reports public to inform consumers.

Particulate Matter

Similar to the ozone report, the objec­tive of this report is to establish stan­dardized testing methods, metrics, and target values for the comprehen­sive evaluation of PM_2.5 air sensors. Specifically, the focus is on their per­formance within the realm of NSIM applications, primarily conducted in outdoor, fixed-site environments. Within this context, the report delin­eates two distinct testing protocols: base testing and enhanced testing. It strongly encourages all practitioners to undertake the foundational base testing, while also proposing the adoption of enhanced testing using controlled laboratory exposure cham­bers, particularly in scenarios char­acterized by elevated PM_2.5  concen­trations, such as those encountered during wildfire smoke incidents.

The report derives its performance metrics and corresponding target values through an amalgamation of multiple sources. These include cur­rent scientific understanding, com­prehensive literature reviews, evalua­tions conducted by reputable external organizations, the ongoing endeavors of sensor standardization programs, and the invaluable expertise amassed by the U.S. EPA in the realm of sensor evaluation research.

These metrics and targets are sum­marized in tables. For base testing, an ‘exploring meteorological effects’ data visualization is suggested to graph meteorological data’s impact on sensor performance. Additional­ly, the report recommends having at least one day of base testing with a 24-hour average PM_2.5  concentration of 25 micrograms per cubic meter (μg/m³) or higher at the test sites.

Additional performance metrics and test conditions for enhanced testing are provided. The report explains how to calculate performance met­rics for PM_2.5  sensors and includes templates for base and enhanced testing reports to ensure consistent result reporting. However, as in the ozone report, target values for en­hanced testing are omitted due to feasibility constraints and variations in sensor evaluation outcomes based on limited data collection and PM surrogate selection.

Acknowledging the continuous im­provement of PM_2.5  sensor technolo­gies, the report anticipates updating the provided information to reflect advancements in the field and new knowledge gained from sensor eval­uations. The report’s target audience includes potential testing organiza­tions, sensor manufacturers, and de­velopers. Various consumers, such as government agencies, community groups, and academia, are expected to benefit from standardized testing result presentations. Again, however, it is emphasized that testing outcomes do not signify U.S. EPA certification or endorsement, and testers are encour­aged to share their reports on their websites to inform consumers.

Conclusion

The proliferation of air sensors for assessing air quality has ushered in a new era of monitoring environ­mental conditions. This surge in sen­sor deployment has led to valuable localized data collection. However, the challenge of varying data quality remains a significant hurdle. The ab­sence of standardized testing methods impedes the effective comparison of sensor data with regulatory air mon­itoring outputs, impacting data credi­bility and decision-making.

To address these challenges, the EPA has initiated workshops involving experts, manufacturers, and regula­tors. These workshops delve into air sensor technologies, performance benchmarks, and lessons from sim­ilar projects, encompassing pollutants such as PM_2.5, O₃, NO₂, SO₂, and CO. To aid non-regulatory monitoring, the EPA released performance target re­ports with consistent testing protocols, metrics, and target values for assess­ing air sensor efficacy in outdoor and fixed locations.

The report focusing on ozone sensors seeks to establish standardized test­ing methods, performance metrics, and target values for non-regulatory air quality monitoring. It introduces base and enhanced testing proto­cols, outlining performance metrics and advising on base testing condi­tions. It acknowledges the evolving nature of ozone sensor technologies and emphasizes the report’s utility for potential testers, manufacturers, and consumers. A similar report targeting PM_2.5  sensors establishes testing pro­tocols, metrics, and base testing con­ditions. It highlights the importance of meteorological effects on sensor per­formance and encourages enhanced testing for specific scenarios. Both reports anticipate regular updates to reflect advancements and underline that testing outcomes do not imply EPA certification or endorsement. The dissemination of testing results is en­couraged to inform consumers.

References:

1. Duvall, R., A. Clements, G. Hagler, A. Kamal, Vasu Kilaru, L. Goodman, S. Frederick, K. Johnson Barkjohn, I. VonWald, D. Greene, AND T. Dye. Performance Testing Protocols, Metrics, and Target Values for Ozone Air Sensors: Use in Ambient, Outdoor, Fixed Site, Non-Regulatory and Informational Monitoring Applications. U.S. EPA Office of Research and Development, Washington, DC, EPA/600/R-20/279, 2021.
2. Duvall, R., A. Clements, G. Hagler, A. Kamal, Vasu Kilaru, L. Goodman, S. Frederick, K. Johnson Barkjohn, I. VonWald, D. Greene, AND T. Dye. Performance Testing Protocols, Metrics, and Target Values for Fine Particulate Matter Air Sensors: Use in Ambient, Outdoor, Fixed Site, Non-Regulatory Supplemental and Informational Monitoring Applications. U.S. EPA Office of Research and Development, Washington, DC, EPA/600/R-20/280, 2021.