Performance Evaluation and Community Application of Low-Cost Sensors for Ozone and Nitrogen Dioxide

1 Office of Research and Development, U.S. Environmental Protection Agency, 109 T.W. Alexander Drive, Research Triangle Park, NC 27711, USA; vog.ape@llessur.gnol (R.W.L.); vog.asan@namkyzs.j.semaj (J.J.S.)

Russell W. Long

1 Office of Research and Development, U.S. Environmental Protection Agency, 109 T.W. Alexander Drive, Research Triangle Park, NC 27711, USA; vog.ape@llessur.gnol (R.W.L.); vog.asan@namkyzs.j.semaj (J.J.S.)

Melinda R. Beaver

2 Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, 109 T.W. Alexander Drive, Research Triangle Park, NC 27711, USA; vog.ape@adnilem.revaeb

Keith G. Kronmiller

3 Jacobs Technology Inc., 600 William Northern Boulevard, Tullahoma, TN 37388, USA; vog.ape@htiek.rellimnork (K.G.K.); vog.ape@leahcim.releehw (M.L.W.)

Michael L. Wheeler

3 Jacobs Technology Inc., 600 William Northern Boulevard, Tullahoma, TN 37388, USA; vog.ape@htiek.rellimnork (K.G.K.); vog.ape@leahcim.releehw (M.L.W.)

James J. Szykman

1 Office of Research and Development, U.S. Environmental Protection Agency, 109 T.W. Alexander Drive, Research Triangle Park, NC 27711, USA; vog.ape@llessur.gnol (R.W.L.); vog.asan@namkyzs.j.semaj (J.J.S.)

4 NASA Langley Research Center, 11 Langley Boulevard, Hampton, VA 23681, USA

1 Office of Research and Development, U.S. Environmental Protection Agency, 109 T.W. Alexander Drive, Research Triangle Park, NC 27711, USA; vog.ape@llessur.gnol (R.W.L.); vog.asan@namkyzs.j.semaj (J.J.S.)

2 Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, 109 T.W. Alexander Drive, Research Triangle Park, NC 27711, USA; vog.ape@adnilem.revaeb

3 Jacobs Technology Inc., 600 William Northern Boulevard, Tullahoma, TN 37388, USA; vog.ape@htiek.rellimnork (K.G.K.); vog.ape@leahcim.releehw (M.L.W.)

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).

Abstract

This study reports on the performance of electrochemical-based low-cost sensors and their use in a community application. CairClip sensors were collocated with federal reference and equivalent methods and operated in a network of sites by citizen scientists (community members) in Houston, Texas and Denver, Colorado, under the umbrella of the NASA-led DISCOVER-AQ Earth Venture Mission. Measurements were focused on ozone (O₃) and nitrogen dioxide (NO₂). The performance evaluation showed that the CairClip O₃/NO₂ sensor provided a consistent measurement response to that of reference monitors (r 2 = 0.79 in Houston; r 2 = 0.72 in Denver) whereas the CairClip NO₂ sensor measurements showed no agreement to reference measurements. The CairClip O₃/NO₂ sensor data from the citizen science sites compared favorably to measurements at nearby reference monitoring sites. This study provides important information on data quality from low-cost sensor technologies and is one of few studies that reports sensor data collected directly by citizen scientists.

Keywords: nitrogen dioxide, ozone, low-cost sensors, electrochemical sensor, performance evaluation, citizen science

1. Introduction

Under the United States Clean Air Act, nitrogen dioxide (NO₂) and ozone (O₃) are regulated as criteria pollutants, or commonly found air pollutants known to cause harmful effects on human health and the environment, as part of the National Ambient Air Quality Standards (NAAQS). These pollutants are routinely monitored by state and local agencies using Federal Reference Methods or Federal Equivalent Methods (FRM/FEM) for NAAQS compliance and other purposes [1]. A number of small, low-cost (~$100–$5,000 USD) sensor technologies for the measurement of criteria gases and other pollutants have recently emerged. These devices can provide near real-time, continuous measurements. Sensors have the potential for use in various applications such as outdoor and indoor air pollution monitoring, source or fence line monitoring, emissions inventory characterization, personal exposure monitoring, and community or individual monitoring activities [2,3,4]. Of these applications, community and individual monitoring has gained popularity as sensor devices are highly accessible, inexpensive compared to traditional air monitoring equipment, straightforward to use, portable, and have software, web interfaces, or smartphone applications to easily view and retrieve data. In addition, the public has a strong desire to know more about what air pollutants and corresponding levels they are being exposed to. Citizen science, which refers to public (citizens) involvement in “collecting, categorizing, transcribing, or analyzing scientific data” [5] is an example of community and individual monitoring. Citizen science can play an important role in augmenting scientific studies and non-regulatory monitoring by increasing the spatial coverage and time resolution of data, offering data for locations or groups that are adversely impacted by pollution, and helping to leverage resource burdens that are typically required for monitoring activities.

The performance of sensors has been an area of focus as there is still a need to determine the accuracy of data to support different applications of sensors. Numerous field and laboratory performance evaluations of low-cost sensors have been conducted. While many sensors have shown good performance in comparison to traditional regulatory monitoring equipment, there are still known issues with data quality and a need to understand long-term (12 months or more) sensor performance, performance in areas with poor air quality, cross interferences with other pollutants, and influences of temperature and relative humidity on the measurements [6,7,8,9,10,11,12,13,14,15]. Although these issues exist, air sensor technologies are quickly being adopted to measure air pollution, especially by individuals and communities who are eager to understand their exposures to air pollutants [2,3,4,16,17,18,19].

The goal of this study was to obtain information on the data quality of measurements from the CairClip O₃/NO₂ and CairClip NO₂ sensors in real-world conditions. This study also sought to understand the feasibility of incorporating citizen science to expand spatial coverage of O₃ and NO₂ measurements. In this study the performance of electrochemical-based CairClip sensors was evaluated and the sensors were operated by citizen scientists in an ambient monitoring network during two month-long field campaigns under the umbrella of the NASA-led DISCOVER-AQ Earth Venture Mission [20]. The goal of DISCOVER-AQ was to understand how satellites can be used to better predict air quality near the earth’s surface using a combination of ground-based and aircraft measurements. Air quality measurements collected on the ground are critical for validating satellite measurements. The use of low-cost sensors combined with citizen science-led data collection can offer unique opportunities to supplement air quality monitoring locations for a host of applications. As such, gaining a better understanding of both the performance and accuracy of low-cost sensors and the elements needed to conduct an effective citizen science study is important.