Add Air Quality Index (AQI) 'sensor fusion' to the PM2.5 sensor

[tylerdcooper]

Using the same setup as this guide, it would be nice if the PM2.5 sensor gave you the option to show AQI.

It is more human readable, and much easier to understand than a simple PM value. Would be nice to have both the AQI value and AQI category (Unhealthy, Healthy, etc).

[tyeth]

I love the AQI, but as each metric is often required to be over a 24hr average, and related method varies per country, I'm not sure what makes sense. I'm from the UK, but previously was using the American model for a basic on-device webpage.

This bit highlights the issue for the USA AQI (the UK is just above and similar): https://en.wikipedia.org/wiki/Air_quality_index#Computing_the_AQI:~:text=For%20example%2C%20calculation%20of%20the%20ozone%20AQI%20requires%20computation%20of%20an%208%2Dhour%20average%20and%20computation%20of%20the%20PM2.5%20or%20PM10%20AQI%20requires%20a%2024%2Dhour%20average

I'm only chiming in because I'm about to want the same thing after buying the SEN55 (pm0.5, 1, 2.5, 4, 10) and was amused to see this recent ticket :)

[brentru]

@tyeth So, calculating an AQI without averaging will result in higher-than-expected AQI values compared to the 24-hour rolling average. If you and other users are OK with this, I don't mind adding it as a type.

Otherwise - we'd need to have this calculated by IO (as IO is storing the values) over the period.

[tyeth]

Yeah to be honest as a "clean air guide" it's very useful, stick it on the list!

[tyeth]

Update noting that there's an open PR waiting for changes on the PM25AQI repo, if no changes forthcoming I'll modify and merge. See https://github.com/adafruit/Adafruit_PM25AQI/pull/12