Purple US EPA Index Seems Strange

User Experience
1

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I have a Purple Flex installed outside and I monitor it from my phone’s home screen using the “widgets” offered on the Purple Map. I like to know the current readings so I use the “real time” overlay along with the US EPA conversion and showing the US EPA PM 2.5 data layer. I also want to know both PM2.5 and AQI values at any given time. So I actually use a widget for each and just combine them into a single html page that gets shown on my phone’s home screen.

The attached screen show shows this at 2 adjacent points in time: 8:11am and 8:16am this morning. As you can see, the PM2.5 level is exactly the same at 2.

What I cannot understand is how/why at exactly the same PM2.5 level the computed/displayed AQI should be different: 9 and 11? From what I understand, the PM2.5 value is simply put through a fixed formula to produce the index. So how can the same input produce two different outputs?

Can anyone help me understand this? Thanks

2

Someone over on Reddit said: For US EPA AQI, 0-50 = 0-9ug/m3. So this is LIKELY a product of PA only showing the whole concentration value (I.e., 2.1 vs. 2.2 ug/m3).

So I decided to start running pm2.5 values between 1.5 and 2.5 thru the EPA formula I have (got elsewhere here). I found that at pm2.5 = 1.6 I get AQI = 9. At 2.0 I get 11, at 2.2 I get 12 and so on. So I guess the Reddit answer is correct - the display of the pm2.5 value is being rounded while the computation for AQI is using the raw value.

A bit confusing but…