Different PM2.5 units being used interchangeably

[gsehrlich]

See issue #237.

Per a Slack conversation (edited for clarity and correctness):

There are not two but three different kinds of units under discussion here:

• PM2.5 mass concentration in milligrams per cubic meter (what the DusTrak records) or micrograms per cubic meter (what the EPA uses in their AQI calculation table).
• PM2.5 particle count in PPM (parts per million), which appears in the wireframe and in (incorrectly) in the "archived" portion of the OP in reference to what the DusTrak records. The difference is that if the particles get more massive (while staying smaller than the 2.5 micron maximum diameter to qualify) but not more numerous, the PM2.5 mass concentration goes up but the PM2.5 particle count/concentration stays the same. PM2.5 particle count and PPM are not relevant to WOAQ and should be discontinued.
• AQI, which specifies the colors that WOEIP wants used to contextualize the numbers. Based on the 24-hour average, each pollutant gets a number linearly in between the break points in the table, and the largest of those numbers becomes the AQI with the corresponding pollutant as the "pollutant of concern". So the colors essentially say "this is what the AQI would be if PM2.5 were the worst pollutant and it had this value averaged over the last 24 hours."

We'll use the format "## PM2.5," where ## is the PM2.5 mass concentration in units of micrograms per cubic meter. To reflect this in design and implementation, we need to correct the text in the hover in:

• [ ] the wireframes https://www.figma.com/proto/IZw8VvX1CsRc764CTvg4aRSU/WOAQ-Wireframes-Parent-File?node-id=753%3A813&scaling=contain&page-id=753%3A0
• [x] the app.

Also,

• [x] Indicate units of measure at bottom of map or somewhere.

(The "moves with cursor" text in the first image corresponds to a different issue.)

[theecrit]

Add criteria:

• [ ] Indicate units of measure at bottom of map or somewhere.

[gsehrlich]

Edited OP to include new criterion.