Low-Cost Air Quality Sensors (LCS)
What are Low-Cost Sensors?
Low-cost sensors (LCS) are often handheld devices that offer a relatively affordable way to measure air pollution. LCS can cost in the hundreds to low thousands of dollars. They are often small and portable meaning they can be relocated. Some LCS store the data internally while others push the air quality data to the cloud. The latter require access to WIFI. LCS run on batteries and/or electricity.
Low-cost sensors can measure particulate matter. Others measure ozone, nitrogen oxides (NOx), carbon dioxide, and so on. Generally, most LCS measure relative humidity and temperature in addition to the primary parameter like particulate matter or ozone, etc.
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Low-cost sensors can measure ambient (outdoor) or indoor air quality. If you use an indoor LCS outside, then be certain to shelter it from rain or snow so it avoids getting wet and breaking down.
Why use Low-Cost Sensors?
Often, low-cost sensors are used for educational or informative purposes. They are not research-grade sensors and are limited in terms of the conclusions from which we can draw from their data.
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Teachers may wish to use low-cost sensors to explore sources of pollution in order to help students understand from where air pollution comes. This kind of activity makes air quality tangible, encourages students to learn new technology, and collaborate as a team to brainstorm ways to reduce air pollution and improve air quality and related public health of stakeholders on their campus or in their community.
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Communities may wish to use low-cost sensors to characterize air pollution in order to describe what is in their air, when the air pollution increases, and where the air pollution is more concentrated. These types of data help a community better understand the nature of their concerns about air quality, offer educational opportunities, and lead to identifying solutions to reduce air pollution and improve air quality in their neighborhoods.
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Low-cost sensors offer stakeholders, like teachers, students, and communities, technology that is relatively affordable and easy to use locally. In contrast, the closest federal reference monitor may be miles away and not necessarily representative of the air quality on a school campus or in a neighborhood near an air pollution source.
How to Select Low-Cost Sensors?
Selecting a low-cost sensor can be a challenge. There are many options and vendors. One way to begin this process is to visit the South Coast Air Quality Management District's AQ-SPEC: Air Quality Sensor Performance Evaluation Center.
The AQ SPEC compares the accuracy of LCS to EPA federal reference method instruments. It focuses on two categories, particulates and gases. It produces charts and tables with detailed performance measures and costs with links to the vendors.
As a first step, decide which type of sensor (particulate or gas) you wish to buy. Often particulate sensors are less expensive than gas sensors. Next, you can sort the sensors by correlation or coefficient of determination (R squared) to get a sense how well the LCS tracks the FRM instrument. The correlation is often reported as a range for laboratory and field conditions. A low correlation (closer to 0) means the LCS may not be reliable or accurate. A high correlation (closer to 1) means the LCS may be more reliable or accurate.
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The AQ SPEC offers stakeholders, like teachers, students, and communities with data to help identify the more accurate and less expensive LCS options. After selecting an LCS, I recommend contacting the vendor to get more information or even educational discounts or discounts for purchasing multiple LCS for your science lab or community AQ science project.
How to use Low-Cost Sensors?
Low-cost sensors can measure air pollutants continuously. Some can store the measurements internally on a micro data card and run on battery for a limited time; however, other LCS plug into an electric wall outlet (inside or outside) and push the measurements to the cloud. WIFI and electricity are considerations for using LCS. If there is a power outage then the battery may allow the LCS to continue to collect data. The bottom line is find a location with access to electricity. You may have to purchase a weather-proof extension cord if the outlet is far.
Besides electricity and WIFI, you have to decide an ideal site to deploy a fixed or stationary LCS. Avoid spaces that obstruct air flow. Also take into consideration security of the LCS. Perhaps identify a place where the LCS cannot be easily touched or tampered with yet in a space where people typically occupy. The LCS should not be set on the ground. Instead set the LCS about six feet above the ground. Some LCS come with the supplies to affix or attach the LCS to a wall or frame. If you set the sensor on a crate or table in a conspicuous location, then consider making a sign letting stakeholders know what the LCS is and why it is sited there. For example, 'Please do not touch the air quality sensor. This an air quality science experiment' may keep the LCS intact for the duration of your experiment, and it may serve as a conversation starter--piquing curiosity into campus air quality.
Finally, think about how easy it is for you to visit the LCS to check on it as needed. Closer to your classroom is more convenient than across the campus. You spent at least a few hundred dollars on the LCS so it is in your best interest to protect your investment. You can take your students on a 'field trip' to the LCS to learn about the technology and how to maintain and secure it.
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If the LCS is portable or mobile and meant to be worn and traveled with you, then you need to ensure it is exposed to continuous air flow and avoid obstructions to the air flow. Make sure the LCS battery is fully charged and learn how to connect it to WIFI or a hotspot. Also download apps you may need to view the measurements. This will be true for the fixed or stationary LCS too. Each LCS comes with specifications and instructions. Be sure to become very familiar with these documents so you can trouble shoot issues if they occur.
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Low-cost sensors vary in how they display data. Some have a LCD display with measurements, averages, date and time. Others display data on a dashboard and use the EPA's AQI or Air Quality Index to convey the level of air pollution concern. The AQI uses numbers and colors to communicate the amount of pollution and how to respond. For example, green (0-50) is 'good' and poses little risk; however, yellow (51-100) is 'moderate' and poses some risk to more sensitive individuals.
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You can use LCS to measure air pollution temporally or spatially. You can observe air pollution by time of day. For example you can compare rush hour traffic in the morning and evening to a time with low traffic (between rush hour or overnight). If you have access to a map then you may wish to compare your data in location A to data from location B. For example, you can compare a dense urban area to a less dense suburban area. Or you may compare a busy road to a quiet park near your campus. Another way to use LCS is to compare the air pollution by season. Ozone is a seasonal air pollutant and reaches higher levels in the warmer months with less wind and humidity. Perhaps you can compare summer to winter ozone.
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Students can collect data in a table and graph it. They can analyze their data using the Claim-Evidence-Reasoning method. Or perhaps students can download the LCS data from the vendor's dashboard and graph it. If the data is voluminous then consider averaging it so it is manageable. Compare the data to the NAAQS, National Ambient Air Quality Standards. Contact the vendor to see how they can help students access, visualize, and interpret the data collected by the LCS.
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Most LCS come calibrated; however, if you buy two of the same LCS then you can collocate them. Collocating LCS helps you see how well they track each other or if there is variation between the two sensors. You don't have to do this if your budget cannot allow you to purchase more than one LCS. For most secondary level science labs or experiments with LCS, you can accept the measurements as qualitative and educational. They will indicate the air quality has changed. Your students can speculate the reason or reasons why the change occurred.
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For communities looking to characterize air quality, I would strongly recommend contacting your closest state air quality managers to collocate your LCS with their federal reference method instrument for about a week. This way you can calibrate your LCS so it is more reliably accurate and can draw more powerful conclusions about the data it collects. Your goal is to reduce as much uncertainty as possible if you are seeking solutions to reduce local air pollution and related public health risks.