AIR CLIMATE & ENERGY RESEARCH PROGRAM B U I L D I N G A S C I E N T I F I C F O U N D A T I O N F O R S O U N D E N V I R O N M E N T A L D E C I S I O N S U.S. Environmental Protection Agency Office of Research and Development Low Cost Sensors - Current Capabilities and Gaps Emily Snyder, Paul A. Solomon, Ronald Williams, Eben Thoma, Dena Vallano, and Timothy Buckley ISEE Environmental Health Conference Basel, Switzerland

2 U.S. Environmental Protection Agency Office of Research and Development Sensor applications Personal Exposure and Health Monitoring Community Monitoring Near sources and Ambient Air Near Source/Fenceline Monitoring Ambient Air Monitoring Researchers Future: Individual Citizens and Students Government and Communities Future: More Communities and Organized Individual Citizens and Students Government What type of monitoring is being conducted? Who is doing the monitoring? Government and Industry

3 U.S. Environmental Protection Agency Office of Research and Development Purpose of this presentation Present the findings from a recent paper based technology review of gas and particulate phase sensors Focus on the lower-cost sensors Discuss current capabilities, estimated range of measurement, selectivity, deployment platforms, response time, and expected range of acceptable operating conditions Discuss related gaps and opportunities AirCasting air monitor and data visualization app.

4 U.S. Environmental Protection Agency Office of Research and Development Scope of review Real time or continuous monitoring technologies of gases and particulates of interest (no laboratory analysis) Only lower-cost (<10 K) systems and supporting infrastructure presented from overall review Lower-cost gas sensors employ electrochemical, metal oxide, spectroscopic sensing principles Lower-cost PM sensors employ light scattering or light absorption sensing principles Specifications are for selected systems (ranging from sensors sold alone to sensor systems)

5 U.S. Environmental Protection Agency Office of Research and Development List of Pollutants for Technology Review Spatially varying criteria pollutants [CO, SO 2, NO 2, PM] A subset of HAPs (formaldehyde, acetaldehyde, benzene, 1,3-butadiene) Indicator pollutants (ammonia, total VOCs, hydrogen sulfide, and methane) Air Quality Egg

6 U.S. Environmental Protection Agency Office of Research and Development Electrochemical Sensors Pollutants measured: H 2 S, NH 3, CO, SO 2 Range: Varies typically 0.01 ppm to up to 10 /1200 ppm  Some sensors claim single ppb detection limits (DLs) Selectivity: well characterized (e.g., for the SO 2 sensor isobutylene, propane, toluene, hydrogen, chlorine, and CO are interferences) Response times: 1-70 seconds Range of operating conditions: % RH (some have lower upper RH tolerances), 0 to 40 °C Available in fixed, portable, or handheld systems Handheld electrochemical sensor

7 U.S. Environmental Protection Agency Office of Research and Development Metal Oxide Sensors Pollutants measured: non-methane hydrocarbons, benzene, methane, CO, NO 2, NH 3, SO 2, total VOCs, NOx Range: typically 0.1 ppm to ppm.  Some sensors claim DLs down to single ppb (for NO 2, CH 4, benzene) Selectivity: not well characterized but hydrogen is a known interferent for the CO sensor Response times: seconds Range of operating conditions: % RH, -10°C to +50°C, sensitive to changes in RH, T, and pressure – it is recommended to recalibrate the sensors as they age to establish impacts particularly when they are in use outside of the laboratory Available in fixed, portable, and handheld systems Metal oxide sensors

8 U.S. Environmental Protection Agency Office of Research and Development Spectroscopic Sensors Pollutants measured NO (chemiluminscence), CH 4, VOCs (NDIR) Range – chemiluminscence DL is 9 ppb for NO, NDIR % range Selectivity – characterized and depends on what spectroscopy is employed Response times seconds Range of operating conditions: -40/- 20°C to +50/55 °C and 0/10 % RH - 95%RH Available in fixed and handheld systems Sensotran RAEGuard IR hydrocarbon monitor

9 U.S. Environmental Protection Agency Office of Research and Development Summary of Gas-Phase Sensors Sensor Type Pollutants Measured from ListRangeSelectivity Response times, seconds Range of operating conditions Other Considerations Electochemical SensorsH 2 S, NH 3, CO, SO 2 single ppb /0.01 ppm to up to 10 /1200 ppm Yes but characterized % RH (some have lower upper RH tolerances), 0 to 40 °C Short sensor lifetimes (1-2 years)* Metal Oxide Sensors non-methane hydrocarbons, benzene, methane, CO, NO 2, NH 3, SO 2, total VOCs, NOx typically single ppb/0.1 ppm to ppm. Yes but not characterized % RH, - 10°C to +50°C, sensitive to changes in RH, T, and P Issues with sensor drift Spectroscopic Sensors NO (chemiluminscenc e), CH 4, VOCs (NDIR) DL is 9 ppb for NO, NDIR % range Yes for chemiluminscence /-20°C to +50/55 °C and 0/10 % RH - 95%RH Limitations on ability to make selective sensors inexpensive * Determined through EPA sensor evaluation studies.

10 U.S. Environmental Protection Agency Office of Research and Development PM Sensors Mass concentration and physical properties are measured. There are no commercially available direct particle mass sensors Light scattering sensors determine particle size and indirectly determine mass One light absorption sensor is available (to estimate black carbon concentrations) Light scattering sensors - lower particle size detected ranges from 0.1 to 0.5 µm Light absorption sensor - limit of detection of 0.16 µg/m 3 Accuracy for the light scattering systems (where available) range from ±5-10 % relative to the calibration aerosol Light scattering sensor and laser particle counter

11 U.S. Environmental Protection Agency Office of Research and Development Gaps Many of these sensor systems do not have the detection limits required to measure ambient levels of these pollutants Many of the sensors suffer from selectivity issues and/or impacts of high RH There are no direct mass PM sensors and the light scattering sensors do not measure ultrafine PM Very few of these systems have been rigorously tested An example of a DIY sensor system courtesy Joe Saavedra.

12 U.S. Environmental Protection Agency Office of Research and Development Opportunities Under EPA’s Air Sensors Evaluation Project the following are being determined for lower-cost O 3 and NO 2 sensors:  Linearity (range)  Precision of measurements  Lower detectable limit  Resolution (noise)  Response time (lag and rise time)  RH and temperature influence  Interference equivalent CairClip electrochemical sensor evaluated under the Air Sensors Project

13 U.S. Environmental Protection Agency Office of Research and Development Opportunities continued Beginning evaluations of lower cost (<$2500) particulate matter and volatile organic compound (VOC) sensors Results from the evaluation of CairClip an electrochemical sensor for O 3

14 U.S. Environmental Protection Agency Office of Research and Development Conclusion Gas and particle pollutant lower-cost sensors show promise for some air monitoring applications Still many gaps remain EPA is conducting research to characterize these lower-cost sensors and is looking to collaborate with others conducting similar activities