Use of Sensors in Air Quality Measurements​

Over the past few years there has been an enormous surge in interest in the application of low-cost sensors to measurements of air pollutants by educators, citizen scientists and members of various groups interested in air pollution levels in their own communities. This interest has stemmed from a combination of factors that include: 1) development and marketing of low-cost electrochemical sensors for gas-phase species; 2) application of low-cost optical particle counters (OPCs) originally developed for monitoring of HVAC systems to measurements of ambient particle density and inferred mass concentrations; 3) introduction of the hobbyist microcontroller circuit boards such as the Arduino board; and 4) recent advances in mobile phone app technology that provides for easy display and mapping of data. As a result, dozens of groups, including many university groups, NGOs and small companies, have developed sensor-based devices for measurements of a wide variety of air pollutants. Such devices typically measure various combinations of PM (PM₁, PM_2.5 & PM₁₀), CO, CO₂, O₃, NO, NO₂, SO₂, VOCs and black carbon. The US EPA has responded to the growing public interest in sensors by developing an Air Sensor Toolbox for Citizen Scientists with an abundance of information about available sensors, how to use them and how to interpret the measurements. Programs to evaluate sensors have been established by both the US EPA (Feinberg et al., 2018; Jiao et al., 2016) and by the Air Quality Sensor Performance Evaluation Center (AQ-SPEC) of California’s South Coast Air Quality Management District.

Sensors vs Miniaturized Instruments

The term “sensor” is commonly used to refer to any measurement device that produces an electrical signal related to a chemical concentration, i.e., a transducer. However, in the air monitoring community, sensors have come to refer to small, inexpensive devices based on a variety of sensing technologies (electrochemical, resistance change in a semiconductor, light scattering from particles, etc.). Sensors are distinguished from traditional “instruments” which are much larger, more expensive, require much more power to operate, but are based on measurement principles that are much more specific to individual chemical species and less susceptible to baseline and sensitivity drift. The approach of 2B Technologies over the past two decades has been to miniaturize instruments, i.e., reduce the size, weight and power requirements while retaining the advantages of high accuracy. We see miniaturized instruments as an important means of validating and calibrating sensors in the real atmospheric environment.

A majority of the ~50 commercially available sensors tested by the EPA and AQ-SPEC have performed very poorly, with coefficients of determination (R²) values of 0.5 or less, and several sensors having R² values of ~0.0 (i.e., “random noise generators”). The 2B Tech Personal Ozone Monitor (POM) had the best performance in these independent tests by SCAQMD with measured R² values of 0.99 in the lab and 1.00 in the field when compared with FEM reference instruments. Of course, the POM is a miniaturized instrument (and a US EPA Federal Equivalent Method) – not a sensor. These independent tests have led to sensor improvements, however, and newer versions of some sensors have performed better in retesting. Those sensors packages that make use of multiple sensors calibrated using multivariate methods upon co-location in the real atmospheric environment perform the best, and, partly as a result of external testing, it is now generally agreed that for most sensors lab calibrations simply do not work; sensors must be frequently calibrated in the field against accurate instruments.

Sensible Applications of Sensors

According to the Merriam Webster dictionary, the definitions of a Sensor and an Instrument are [emphasis ours]:

Sensor: “a device that responds to a physical stimulus (as heat, light, sound, pressure, magnetism, or a particular motion) and transmits a resulting impulse (as for measurement or operating a control).”

Instrument: “a measuring device for determining the present value of a quantity under observation.”

It is important to remember that AQ sensors respond to air pollutant in a way similar to our ability to touch an object and respond to whether it is hot or cold or taste a food and give a sweet, bitter, salty, etc. response. Although the electrical signals produced by sensors can be measured with high accuracy, it is often difficult to quantify the concentration of the air pollutant (“determine the present value”) using most low cost sensors. There are several reasons for this, which vary for different types of sensors, as discussed in some detail in the section below. Some of the difficulties in quantifying sensor signals to obtain atmospheric concentrations include:

• Cross Sensitivity: A sensor may respond with varying degrees to different chemical species. For example, a NO₂ sensor signal may be partly due to NO₂, partly due to O₃, partly due to SO₂, etc.

• Environmental Effects: Sensor response often depends on temperature, pressure and humidity.

• Sensitivity Drift: The response (sensitivity) of the sensor may vary over time. This can be due to depletion of redox species and/or electrolytes in an electrochemical sensor, adsorption of contaminants to solid-state sensors, etc.

• Baseline Drift: The offset (signal in the absence of the analyte being measured) may become positive or negative over time for a variety of reasons such as contamination of active sensor surfaces, accumulation of particles inside a PM sensor, etc.

• Linearity and Dynamic Range: Ideally, a sensor would have a linear response over several orders of magnitude, as do nearly all traditional instruments. Sensors tend to respond linearly over only one to two orders of magnitude, compared to approximately five order of magnitude for the measurement of ozone by UV absorbance, for example. However, the lack of a wide dynamic range is seldom a serious limitation for AQ sensors since the concentration range of interest is not that wide. All of these factors affect instruments as well, but generally to a much lesser extent, and correction methods, such as frequent zeroing to eliminate baseline drift and calibration with gas standards, are much better established.

This is not to say that sensors should not be used. Indeed, it is now recognized that sensors can fill important gaps that are virtually impossible to fill with conventional or even miniaturized instruments because of their low cost, small size and ease of deployment. However, the limitations of sensors must be recognized and care must be taken to obtain acceptable results. Some examples of where the use of sensors is meeting important needs include but are not limited to:

• Air Quality Education: Many groups are employing AQ sensors in hands-on educational projects, such as our AQTreks project, that allow K-12 and college students to learn about air pollution. Advantages of sensors for this application include low cost, mobility and fast response time. Students can explore where different air pollutants are high or low – for example near or far away from a busy street. Here, learning is facilitated by the air pollution monitoring experience and qualitative data may be acceptable. Even here, it is important that students be made aware of the accuracy of their measurements. One way is to compare their results to nearby monitoring stations. The AQTreks mobile app allows students see the locations of all nearby stations and their most recent 1-hour averages. Fixed-base stations making use of sensors is valuable education as well. Students can variations with time of day (traffic hours, diurnal changes, etc.) and seasonal variations. Here, it is preferable to have a method to periodically check the sensor calibrations.

• High Density Sensor Arrays: There are only a handful of monitoring stations dedicated to making accurate measurements of air pollutants in each of the US states, somewhere around 2,000 stations to monitor the air breathed by more than 300 million people. Furthermore, these stations are located to provide approximately average air pollutant concentrations in a given area. The low capital cost of sensor deployments make it possible to carry out detailed studies that show how air pollutants are distributed spatially. This is important, especially in terms of environmental justice, since lower income housing is frequently located close to air pollution sources such as highways, factories and power plants. The use of hundreds of low cost monitoring stations would allow us to better map the distribution of air pollutions throughout city or region and to better identify air pollution sources. Here, it is important that the calibrations of sensors at individual stations be well maintained, especially in relation to one another, but also in relation to accurate instrumental measurements. This can be accomplished by co-locating sensor packages with State and Local Air Monitoring Stations (SLAMS) before and after deployment and by frequent checking of individual deployed stations using portable instruments and/or calibrators.

• Mobile Monitoring: Mobile monitoring of air pollutants for high resolution mapping has recently been successfully demonstrated using Google Street Cars (ref). Although the measurements of CO, CO₂ and black carbon in that study made use of highly accurate traditional instruments, future fleets of monitoring vehicles (delivery trucks, etc.) are likely to make use of AQ sensors because of their small size and low cost. Such sensors will require frequent calibration. One promising approach that 2B Tech is pursuing is the development of AQSync drive-by calibration stations. The AQSync stations, which will be mounted to lamp posts and traffic lights along city streets, will contain miniaturized instruments, including our Model 106 Ozone Monitor, Model 405 nm NO₂/NO/NO_x Monitor, Black Carbon Photometer and other instruments to be developed for CO and CO₂. When vehicles carrying sensor packages pass by the AQSync stations, their measurements of air pollutants can be compared to those of accurate instruments.