1. New technologies and architectures for sensing gases and particles in air are emerging for criteria pollutants, air toxics, and greenhouse gases. These technologies are generally designed to be mass-fabricated through innovative processes (e.g., microfabrication) and are (1) small; (2) able to autonomously and directly measure pollutant levels; (3) have low power consumption; and (4) support flexible deployment options [White et al. Environmental Manager, pp. 36-40, May 2012]. These traits translate to reduced total cost of ownership, consideration of a higher spatial density of air pollution measurements and personal exposure applications. Measurements from the existing national air monitoring network can be used to infer community-wide concentrations of ozone and fine particle (< 2.5 µm) mass. However, other pollutants directly emitted into the air, such as elemental carbon, coarse particles (between 2.5 and 10 µm), carbon monoxide (CO), nitrogen oxides (NOx), and various air toxics have been shown to have much higher variability between regional air pollution monitors compared to pollutants formed in the atmosphere like ozone. Portable air pollution sensors can potentially provide more location-specific information desired by individuals and communities, and needed by exposure and health scientists to improve air quality risk assessments. There is also a desire to use portable sensors to monitor indicator pollutants or classes of pollutants, like methane and total volatile organic compounds (VOCs), as well as nuisance pollutants, like hydrogen sulfide (H2S) and ammonia, which indicate the presence of pollution sources. Current technologies for portable sensors for air pollutant gases reviewed include: electrochemical, metal oxide, spectroscopic, ionization, and pellistor sensors. Current technologies for portable particle sensors include: light scattering, light absorption, and those based on a change in frequency of an oscillating impaction surface. Attributes that indicate the appropriateness of these technologies for sensing the pollutants mentioned above are presented including: estimated range of measurement, selectivity (when applicable), appropriate deployment platforms, response and recovery time, battery lifetime, and expected range of acceptable operating conditions. Information also will be provided about the potential of emerging sensor technologies to address the deficiencies in the abilities of existing sensor technologies to measure these pollutants. New technologies and architectures for sensing gases and particles in air are emerging for criteria pollutants, air toxics, and greenhouse gases. These technologies are generally designed to be mass-fabricated through innovative processes (e.g., microfabrication) and are (1) small; (2) able to autonomously and directly measure pollutant levels; (3) have low power consumption; and (4) support flexible deployment options [White et al. Environmental Manager, pp. 36-40, May 2012]. These traits translate to reduced total cost of ownership, consideration of a higher spatial density of air pollution measurements and personal exposure applications. Measurements from the existing national air monitoring network can be used to infer community-wide concentrations of ozone and fine particle (< 2.5 µm) mass. However, other pollutants directly emitted into the air, such as elemental carbon, coarse particles (between 2.5 and 10 µm), carbon monoxide (CO), nitrogen oxides (NOx), and various air toxics have been shown to have much higher variability between regional air pollution monitors compared to pollutants formed in the atmosphere like ozone. Portable air pollution sensors can potentially provide more location-specific information desired by individuals and communities, and needed by exposure and health scientists to improve air quality risk assessments. There is also a desire to use portable sensors to monitor indicator pollutants or classes of pollutants, like methane and total volatile organic compounds (VOCs), as well as nuisance pollutants, like hydrogen sulfide (H2S) and ammonia, which indicate the presence of pollution sources. Current technologies for portable sensors for air pollutant gases reviewed include: electrochemical, metal oxide, spectroscopic, ionization, and pellistor sensors. Current technologies for portable particle sensors include: light scattering, light absorption, and those based on a change in frequency of an oscillating impaction surface. Attributes that indicate the appropriateness of these technologies for sensing the pollutants mentioned above are presented including: estimated range of measurement, selectivity (when applicable), appropriate deployment platforms, response and recovery time, battery lifetime, and expected range of acceptable operating conditions. Information also will be provided about the potential of emerging sensor technologies to address the deficiencies in the abilities of existing sensor technologies to measure these pollutants. Next-Generation Air Monitoring - A Review of Portable Air Pollution Sensors Emily Snyder 1, Paul A. Solomon 2, Margaret MacDonell 3, Ronald Williams 2, Eben Thoma 4, Dena Vallano 5, Michelle Raymond 3, and David Olson 2 1 Environmental Protection Agency, Office of Research and Development, National Homeland Security Research Center, 2 Environmental Protection Agency, Office of Research and Development, National Exposure Research Laboratory, 3 Argonne National Laboratory, 4 Environmental Protection Agency, Office of Research and Development, National Risk Management Research Laboratory, 5 American Association for the Advancement of Science Fellow at EPA’s Office of Research and Development, Washington, D.C. Abstract Motivation for Next Gen Portable Air Pollution Sensor Technologies Technology Review Approach Current Technology – Fundamental Approaches Acknowledgements Contact: Paul Solomon, solomon.paul@epa.gov orsolomon.paul@epa.gov Ronald Williams, williams.ronald@epa.gov Scope of the Review: Real time or continuous monitoring technologies involving select gases or particles of interest (did not include sensors that detect classes of pollutants only). Only lower-cost (<10 K) portable systems were included. Pollutants of Interest Approach:  Reviewed publications, vendor websites, and patents  Detection limits were taken from independent evaluations (Environmental Technology Verification, peer reviewed publications) where available; otherwise vendor provided information was included  Cost estimates were based on a equipment AND required infrastructure  Tried to capture many different sensor systems for the gas phase pollutants Scope of the Review: Real time or continuous monitoring technologies involving select gases or particles of interest (did not include sensors that detect classes of pollutants only). Only lower-cost (<10 K) portable systems were included. Pollutants of Interest Approach:  Reviewed publications, vendor websites, and patents  Detection limits were taken from independent evaluations (Environmental Technology Verification, peer reviewed publications) where available; otherwise vendor provided information was included  Cost estimates were based on a equipment AND required infrastructure  Tried to capture many different sensor systems for the gas phase pollutants Spatially varying criteria pollutants CO, SO2, NO2, PM A subset of Hazardous Air Pollutants (HAPs) formaldehyde, acetaldehyde, benzene, 1,3-butadiene Indicator pollutants ammonia, total VOCs, hydrogen sulfide, and methane Electrochemical Sensors Metal Oxide Gas Sensors Gas Phase Pollutants* * Particulate Phase Pollutants Spectroscopic Sensors Light Scattering Light Absorption Results of Review (Gases) Sensor Type Pollutants Measured from List RangeSelectivity Response times, seconds Range of operating conditions Other Considerations Electrochemical Sensors Benzene*, H 2 S, NH 3, CO, SO 2, NO 2, O 3 single ppb /1 ppm to up to 10 /1200 ppm Not selective but characterized 1-70 15 -90 % 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 25 -100 ppm. Not selective and not well characterized 60-180 10-90% RH, - 10°C to +50°C, sensitive to changes in RH, T, and P Issues with sensor drift Spectroscopic Sensors NO (chemiluminscence), CH 4, VOCs (NDIR) DL is 9 ppb for NO, NDIR 1- 100 % range Selective for chemiluminscence 20-60 -40/-20°C to +50/55 °C and 0/10 % RH - 95%RH Limitations on ability to make selective sensors inexpensive Results of Review (PM)  Mass concentration and physical properties are measured. There are no commercially available direct particle mass sensors.  Performance  Light scattering sensors - lower particle size detected ranges from 0.3 to 3.0 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  Mass concentration and physical properties are measured. There are no commercially available direct particle mass sensors.  Performance  Light scattering sensors - lower particle size detected ranges from 0.3 to 3.0 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 Technology Gaps and Potential Emerging Technologies  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 *Ionization is another fundamental sensing approach but was not included because it is not specific unless used in concert with a sorbent tube. The authors would like to acknowledge Michelle Raymond, David Wyker, Molly Finster, Young-Soo Chang, Thomas Raymond, Marcienne Scofield, and Bianca Temple for the exposure bench-mark plot. *uses a pyrolizer to convert benzene to CO for electrochemical detection. Personalized Medicine Education Hot Spot Measurements (by individuals or communities) Informational (higher spatial and temporal data) Emerging Sensors for Gas Phase Pollutants - Examples of Emerging Technologies and their Potential Performance for Benzene Sensing Emerging Sensors for PM - Two systems are currently being developed for direct mass PM measurement  Quartz Crystal Microbalance (QCM) – Measures mass through Impaction on piezoelectric sensors  MEMS-PM - Employs a MEMS virtual impactor with Film- Bulk Acoustic Resonator to measure mass  Impaction on an oscillating micro tuning fork MEMS PM Mass Sensor