Calibration via a collocation is a critical step in establishing any air sensor network and ensuring the data from the network are able to meet the project goals.

By the end of this lesson, you’ll be able to:

• Understand the fundamentals of collocation
• Understand the basics of calibration and why it is important. We’ll dive into the details of Clarity’s calibration method in the next module.

What is a calibration?

Sample particulate matter measurements before (gray) and after (blue) calibration using reference monitor (black) data to adjust. After calibration, the Node-S devices match the reference monitor measurements much better, meaning more accurate data.

Clarity Node-S devices require a local collocation of all devices with a reference monitor for calibration.

Why calibrate?

PM2.5 calibration — Calibration is important for particulate matter because particulate profiles (i.e. particle composition and size distribution), vary based on local pollutant sources and environmental conditions. The types of sources (roadways, industrial facilities, wildfire smoke) can have significant impact on what the particulate matter ‘looks like’ to the sensor, so ideally we need to calibrate the sensor at a site with a similar PM composition so that we can improve how accurate the sensor is.

NO2 calibration — The electrochemical NO2 sensor in the Clarity sensor is sensitive to its environmental conditions, so a local calibration is required to provide more reliable readings. Important: At this moment, raw Clarity Node-S NO2 data cannot be used for data interpretation without calibration and we are only making the data available for projects that meet certain requirements. Please reach out to your Clarity contact if you’d like to learn more.

What is a collocation?

A Clarity Node-S collocated at a reference site in London.

Why collocate our devices?

Collocating your Clarity devices with a reference monitor serves two important purposes:

• Calibration of the Clarity Node-S to improve the accuracy
• Evaluation of performance (i.e., accuracy) of the Clarity Node-S

We can assume that collocated devices are measuring air with the same pollutant concentration, so if the devices are well calibrated they should each report the same pollutant concentration. If two collocated devices are reporting different concentrations of pollutants, that indicates that one or both of the devices’ measurements are incorrect.

Because reference monitors, especially those maintained by a regulatory agency, have a high degree of accuracy, we can assume that the measurements they report are fairly close to the actual concentration we are hoping to measure. So, collocating a Clarity device with a reference monitor allows us to check whether the Clarity device is measuring accurately. And if it is not, we can use data from the reference monitor to develop a calibration model that we use to adjust the data, improving its accuracy.

FAQ: How far away from the reference monitor can we install our Clarity Node-S devices for a collocation? 

We recommend installing the Node-S as close as possible to the reference monitor inlet so both are monitoring similar air and therefore pollutant concentrations, ideally within 5 meters and no more than 10 meters away(16-32 feet). The further away the most likely there will be variability in concentrations especially for gasses like nitrogen dioxide (NO2) that tend to vary more across locations.

We’ll dive into more operational details on how to collocate your devices in Lesson 4.

For more information on air sensors and calibration, check out these third party resources:
EPA’s Enhanced Air Sensor Guidebook
Community in Action A Comprehensive Guidebook on Air Quality Sensors