Now that you better understand why a collocation is critical for getting more accurate and reliable data, here are some tips on how to start a collocation. From identifying a local reference site to receiving approval, the collocation period is a critical step for the success of your project. We recommend starting this process as early as possible, as approvals may take several weeks to months.

By the end of this lesson, you’ll understand:

• Best practices for identifying and selecting a local reference site.
• Information on what time of year you should collocate your devices.
• Tips on receiving approval to collocate your devices.
• What to do if you’re finding it difficult to establish permission to collocate.

   
Photos of a collocation courtesy of Imperial College London.

Note: If you already have a reference site identified and approvals obtained, please skip to the next lesson to see if your site meets the requirements needed for Clarity to run calibrations for PM2.5 and NO₂

As you have learned in previous lessons, calibration of your Clarity Node-S devices using a local collocation at a reference site is critical to improve the accuracy of the data. Here are the steps you need to take to collocate your devices.

If your project is located within the US, you can check EPA’s Air Data Air Quality Monitors Map. To find the reference site near you using the EPA Tool, navigate to the Map at the provided link and enter your city or zip code in the search box.

From there, use the Layer feature —    — to toggle on PM2.5 - Active and NO2 - Active sites.

In general, you want to use a reference site that is as similar to the locations you want to monitor as possible. That means a similar mix of pollution sources and similar environmental conditions. Some things to consider:

• When in doubt, favor reference sites that are exposed to a wider range of pollutant concentrations
• If you’re not sure about the pollution sources near the reference site, or the sites you’ll be monitoring, select the reference site closest to where you’ll be monitoring.

You’ll also need to make sure that the reference sites regularly see high enough levels of pollution to perform the calibration. For PM2.5, that means at least one daily (24h) average greater than 25 µg/m³ during the collocation period. For NO2, at least one daily (24h) average greater than 15 ppb.

Often regulators maintain websites where you can check historical data from their monitors, which you can use to check whether a site regularly measures daily average concentrations above those thresholds. You can also ask a contact at the agency for a recommendation, or check with your Clarity contact.

If you are calibrating your NO2, it is important to not only pick where to monitor, but also when to monitor. The NO2 sensor in the Node-S is impacted by temperature and relative humidity, so it is important to collocate your devices during a month where temperature and relative humidity are as typical of the ranges typically seen at the site you’re monitoring as possible. Picking the right collocation month is especially important if you are monitoring in a place with big swings in temperature and relative humidity throughout the year.

Check with your Clarity contact about the best time to collocate your devices.

After you have researched your collocation site options, you should have a better understanding of which regulatory agency to contact. Some agencies have defined processes for collocating low-cost sensors, while others do not. 

If you already have a contact at the agency, please reach out to them. Here is an example of an outreach email you are free to use:

If you don’t have an existing relationship, we suggest reaching out to the agency’s air monitoring team. Be sure to copy your Clarity contact into this email. We are here to help and answer any follow up questions that may arise.

Additionally, it might be helpful to attach the following documents to help the agency familiarize themselves with our devices.
◦ How do I collocate my Clarity device with a reference monitor?
◦ Clarity Node-S Specifications Sheet

What if I can’t get permission to collocate?

There have been instances where projects have not been able to gain access to a reference site. Reasons for this may include:
   ◦ Lack of response from the regulatory agency
   ◦ Denial of site access by the regulatory agency
   ◦ No suitable reference monitors within a reasonable distance
   ◦ Tight project timelines

In some locations, researchers or other groups may operate reference monitors outside of the regulatory network. If you know of any such groups, you can reach out to them to see if they will allow permission to collocate.

Clarity devices may be used without collocation for special projects that are focused on PM2.5 data only. In some cases, Clarity may be able to apply a generalized calibration model even if you are not able to collocate your devices (more on this in the next module!). If your project is focused on NO2 data, collocation is required. 

If you are unsure whether you can begin your project without collocation, please reach out to your Clarity contact.