Hero image provided courtesy of the Quezon City Climate Change and Environmental Sustainability Department.

TL;DR — The US EPA’s Enhanced Air Sensor Guidebook outlines several best practices for most effectively leveraging low-cost sensor air quality technology as part of an air pollution monitoring network. The guidebook includes information regarding how best to collocate low-cost sensors alongside reference-grade monitors to ensure accurate air pollution measurements, which can then be leveraged to improve air quality.

The US EPA Enhanced Air Sensor Guidebook

This blog is part of a series that highlights key themes from the US EPA Enhanced Air Sensor Guidebook, which covers a variety of topics involving low-cost air quality sensor use.

Today’s focus will be on highlighting the Guidebook’s recommendations for air quality measurement equipment collocation and how it ensures a more successful air quality monitoring network.

What is air quality measurement equipment collocation?

Collocation is essential to best understand and optimize the performance of low-cost sensors — or any other air pollution measurement equipment — being used for an air quality monitoring project.

Collocation is defined by the US EPA as the operation of a regulatory-grade monitor and a non-reference grade monitor, such as a low-cost sensor, in order to evaluate the sensor’s accuracy. During this evaluation period, the same time, location, and real-world conditions must be maintained to arrive at the most accurate comparison of the data from the two instruments.

Read the US EPA’s Collocation Instruction Guide for more information about the specifics of collocation with federal reference-grade equipment.

Collocation helps to understand how environmental variables — like temperature, humidity, and other meteorological factors —  affect sensor performance in that specific location. Understanding these variables allows the user or the sensor to calibrate (or “tune”) the sensor to best perform in local conditions.

Because these variables can change over time, calibration should ideally be an ongoing process that is continually adjusted during the sensor’s lifetime.

Collocation can be carried out in two main ways. Collocation with a reference monitor means that the low-cost sensor will be installed alongside the reference monitor — between five and ten meters away — so that its data can be compared to reference readings.

To ensure our customers collect the most accurate air quality data possible, Clarity strongly recommends that anyone using our Node-S air quality sensors work with our Customer Success team to perform a collocation in line with EPA recommendations before deploying air quality monitoring equipment to the field — this is a component of our Sensing-as-a-Service approach.

How can collocation be carried out?

The first step in the collocation process is to identify possible reference sites where low-cost sensors can be collocated alongside reliable reference monitors.

It is important to choose reference monitors that measure your pollutants of interest (those that your low-cost air sensors will measure) in order to ensure calibration for all relevant pollutants.

These sites should also be representative of the network locations for your project in terms of their existing environmental conditions, pollutant sources, and the range of pollutant concentrations that typically occur.

It is crucial to start this process early since obtaining approvals for collocation can be a time-consuming process.

For more information about the specifics of carrying out collocation, check out our Collocation Course here.

The image above demonstrates the setup of collocation, specifically as part of the air quality monitoring project under Breathe London. Clarity Node-S devices are being collocated alongside a reference-grade monitor before deployment to ensure the most accurate data measurements.

Why is it important to collocate air pollution sensors with reference-grade air monitoring equipment?

Collocation of low-cost sensors alongside reference monitors allows for the highly important process of calibration to occur.

Calibration is the process of evaluating and adjusting low-cost sensor measurements against those generated by a reference monitor to report the most accurate data. Calibration must be carried out individually for each pollutant being measured.

PM2.5 calibration is important because particulate matter composition and size distribution can vary based on a variety of factors, including local pollutant sources and environmental conditions. Thus, it is important to calibrate the sensor with a reference monitor that deals with a similar particulate matter composition to that the low-cost sensor will be monitoring.

NO2 measurements are also subject to influence from environmental conditions, so it is important to do a local calibration to get the most accurate comparison.

It is important that low-cost sensors are calibrated against a reference monitor with reliable and accurate data. Accurate data means that the measurements from low-cost monitors can be leveraged for policy change, initiatives to decrease air pollution, and other actions to concretely improve air quality.

Read our blogs for more information on generally assessing sensor accuracy and using the measurements of R2 and MAE to do so.

Accurate data means that the measurements from low-cost monitors can be leveraged for policy change, initiatives to decrease air pollution, and other actions to concretely improve air quality.

Brightline Defense, a non-profit organization based in San Francisco, uses its low-cost sensor network to validate residents’ long-standing concerns about air pollution, particularly in historically disadvantaged and SRO communities.

Read more about Brightline Defense’s air quality monitoring work here.

Monterey Bay Air Resources District (MBARD) has also deployed a low-cost sensor network whose collocated and calibrated air sensors supplement existing reference monitors. These low-cost sensors help to collect more high-resolution data points, especially during times of poor air quality such as wildfires.

For more information on MBARD’s air quality monitoring network, read about our work with them here.

Collocating as part of an effective air quality monitoring network

Carrying out a well-planned collocation process allows for the thorough calibration of low-cost sensor data alongside reference-grade equipment, resulting in more accurate air quality measurements.

Interested in collocating our air quality sensors with your reference air pollution measurement equipment? Apply for a free collocation demo here.