By Bo Call
You may have heard about the low-cost air quality sensors that people have been using to collect personal air quality data during high pollution periods. The question I often get is, “Do they really work?” And all I have to say is, “Well, it depends.”
The regulatory monitors we use at DEQ have to meet stricter requirements than portable air quality sensors. DEQ’s stationary air monitoring system is designed to measure pollutant levels, ensure compliance with the National Ambient Air Quality Standards (NAAQS), and provide data for air quality modeling. The equipment we use in our monitoring network has to meet Federal Reference Method (FRM) or Federal Equivalency Method (FEM) requirements. These requirements ensure that our data are accurate and representative of the conditions in the airshed. We follow a prescribed set of procedures that include proper siting, precise calibration, regular maintenance, data validation, and periodic audits to ensure proper equipment functioning. Our quality control/quality assurance procedures certify the accuracy of the data we report to the Environmental Protection Agency (EPA) and the public.
While the technology is promising, this new generation of low-cost, portable air quality sensors is in the early stages of development, and these sensors are not held to the same strict requirements as FRM and FEM monitors. The quality of the data from these sensors is dependent upon a number of factors, including the quality and performance of the sensor, its location relative to pollution sources, and the way the user operates it.
The data collected by these sensors are not “information” in and of themselves; they have to be interpreted. That can be tricky when data are collected over a short period of time. The PM2.5 standard, for example, is based on the average of hourly monitoring measurements over a 24-hour period — the measurements our continuous monitors track over time. A single PM2.5 measurement taken over a few minutes or hours that registers above the standard is not necessarily a cause for immediate concern.
Sometimes, all that the user learns from these data are that pollutant levels have increased or decreased, because proper calibration remains an issue with many of these sensors. Calibration — the process of checking and adjusting an instrument’s measurements against a reference standard — is important, because a sensor’s sensitivity and responsiveness can change over time. In addition, because sensors are manufactured in batches, it’s difficult to know whether an individual sensor will or won’t provide accurate readings. One sensor from a batch may be calibrated correctly, another may not; there is no way for the buyer to know. Even if the sensor is measuring pollutants accurately at first, there is no guarantee that it will continue to provide accurate data if the user can’t recalibrate it or verify in some way that everything still works.
The South Coast Air Quality Management District (SCAQMD)established the Air Quality Sensor Performance Evaluation Center (AQ-SPEC) to characterize the actual performance of low-cost air quality sensors and educate the public about the advantages and drawbacks of these devices. AQ-SPEC conducts field testing to provide side-by-side comparison with EPA-approved instruments and methods and uses laboratory testing to evaluate sensor performance under different environmental conditions.
The EPA has developed an Air Sensor Guidebook to assist the public in the appropriate use of low-cost air quality sensors. The guidebook provides advice on what to look for in a sensor, including selecting a target pollutant, looking at the detection range and detection limit for the pollutants being measured, considering the precision and bias that could affect the accuracy of the sensor’s measurements, identifying calibration requirements, understanding response time (is it quick or slow to measure?), and verifying durability, construction, and ease of use.
The price tag for these sensors makes them attractive: prices range from $30 to $5,000, with a number of sensor models available in the $300-$500 range. Although low-cost sensors are not as accurate or reliable as EPA-validated methods, the technology is developing quickly enough that in the near future some of these sensors will provide accurate air quality data for less than the $30,000 to $40,000 FRM/FEM monitoring equipment we use. At some point, you may even be able to buy a personal-exposure monitor that measures particulate matter levels in your neighborhood or along the route your kids use to walk to school. Portable monitors won’t replace our FRM/FEM regulatory monitors, but they may eventually provide data that the public and scientists can use to identify short-term pollution spikes and complement our regulatory monitoring.
Are you wondering how your portable sensor stacks up? The AQ-SPEC will evaluate your sensor free of charge. To be considered for evaluation, your sensor must be able to store data internally or log data to a computer or a cloud-based server. Because sensor evaluations are run in triplicate, you will need to send three sensors for testing. The EPA has developed an Air Sensor Toolbox for Citizen Scientist that provides information and guidance on the purchase of low-cost air quality sensors and the ways that citizens can collect, analyze, and interpret these air quality data.
I have been with the Division of Air Quality (DAQ) for 23 years, managing the air monitoring section since 2009. Prior to that I worked in DAQ’s compliance branch and conducted source inspections, specializing in asbestos rules and enforcement.I have a BA in Biology from Utah State University. I am a retired member of the Air Force Reserve as a Transportation Specialist. On my own time, I have a hobby farm and recently entered into the realm of beekeeping.