“How many air quality monitors do I need?” is a question we regularly hear, and the easy responses – “it depends”, “how big is your budget?” – are not very helpful.
To give a better idea, it will depend on all these factors:-
Which pollutants you want to measure
Some pollutants are mixed better / are more homogeneous / more background in ambient air, such as PM2.5 and O3. Others are less so, and can be affected by a local source, like NO. Background pollutants can be measured with fewer measurement points than those which will vary greatly over short distances. For example, O3 may range between 50ppb and 60ppb across a city, but NO could vary from 0ppb to 1,000ppb within just 100m.
Your analysis capability
If you have the resources or skills to carry out detailed analysis, you will get more information out of fewer measurement points. For example, if using wind speed and direction data alongside air quality readings, you can look at an area in terms of pollution sources and areas potentially affected by air quality. The resulting plots and mapping allow reading levels to be visualised across space. Taking it further, measurement points can be linked to emissions inventories and modelling can fill the gaps to give an estimated reading for every geographical point. The more measurement points, the more accurate the estimates are likely to be. Additionally, some analysis techniques – such as long distance scaling or network calibration – require a minimum number of measurement points in order to work, which will therefore determine how many pods you might need. As an example, the long distance scaling method offered by AQMesh requires a minimum of 6 different locations.
The area you’re monitoring in
Multiple pollution sources (think busy city vs. a factory in open countryside) create a more complex air quality situation, as do canyons (naturally confined air corridors or streets between high buildings). A single source within an open environment could achieve a lot with just one pod upwind and one downwind, but a city environment means that NO or NO2 readings could be massively different just other sides of a road junction.
Local conditions
If your air quality monitoring location is generally windy you will have to work harder (install more measurement points to pick up plumes) to capture pollutant bursts before they are swept away.
Environmental justice
We have seen customers distribute pods based on one per ZIP code, to achieve fairness to local communities. This is a good idea in itself, but a ZIP code can include a wide range of pollution levels so all the factors about choosing a precise monitoring point still apply.
And, of course, budget!
Seriously, small sensor systems are described as monitoring ‘hyperlocal’ air quality for a reason and even the densest networks will be leaving some gaps where air quality variation is not recorded. So, measurement points can be added infinitely – air quality mapping of an area will improve in accuracy, but there are obviously diminishing returns.
Because, even after all this, “it depends”, just talk to us about your air quality monitoring requirements and we will be more than happy to share our recommendations and give you a more helpful answer to “how many air quality monitors will I need?”