Add AQI

[djjudas21]

The ecowitt app gives numerical measurements from the WH41 air quality sensor, but it also generates an Air Quality Index (AQI) and a "smiley face" rating system. Only the numerical measurements are made available by the ecowitt base station, so this exporter should generate some indices to export, too.

We should make it clear that this is specifically the PM 2.5 air quality index as the WH41 can't measure other pollutants.

Australia

The ecowitt app seems to calculate the AQI as described by the Australian government in NEPM

The NEPM standard calculates the AQI by

        pollutant reading
AQI =  ---------------------   * 100
            standard

where the reading for PM 2.5 is averaged over 24 hours, and the "standard" reference is 25 µg/m3.

The resultant index is then binned into the following categories:

AQI	Category	Colour
0-33	Very Good	Blue
34-66	Good	Green
67-99	Fair	Yellow
100-149	Poor	Orange
150-200	Very Poor	Purple
200+	Hazardous	Red

UK

As I'm British, this is the standard I propose making the default in the ecowitt exporter.

There is also a UK standard called DAQI which assigns the index directly based on the concentration of PM 2.5. It also uses a rolling 24-hour average.

Index	Band	µg/m3	Colour
1	Low	0-11	Light green
2	Low	12-23	Green
3	Low	24-35	Dark green
4	Moderate	36-41	Yellow
5	Moderate	42-47	Amber
6	Moderate	48-53	Orange
7	High	54-58	Pink
8	High	59-64	Red
9	High	65-70	Maroon
10	Very High	71+	Purple