A Scriptable powered iOS 14 widget (https://docs.scriptable.app/) using JavaScriptCore to get the current AQI from a PurpleNow sensor: https://fire.airnow.gov/

get-local-api.js

const API_URL = "https://www.purpleair.com/json?show=";
// Params: lat, lng, zoom
const MAP_URL = "https://fire.airnow.gov/"
// const CACHE_FILE = "aqi_data.json"
// Find a nearby PurpleAir sensor ID via https://fire.airnow.gov/
// Click a sensor near your location: the ID is the trailing integers
// https://www.purpleair.com/json has all sensors by location & ID.
let SENSOR_ID = args.widgetParameter || "19066"

const HEADER_COLOR = "#222222"
const TEXT_SIZE = {
  aqi: 50,
  title: 15,
  meta: 10
}

// The PM2.5 concentration levels and AQI breakpoints, used to
// calculate the resultant AQI.
const PM2_5BREAKPOINTS = {
	"Good":{conc: [0.0, 12.0], aqi:[0, 50], color: "#48D086 "},
	"Moderate":{conc:[12.1, 35.4], aqi:[51, 100], color: "#F3CF2C"},
 	"Unhealthy for Sensitive Groups":{conc:[35.5, 55.4], aqi:[101, 150], color: "#F59748"},
	"Unhealthy":{conc:[55.5, 150.4], aqi:[151, 200], color: "#F45252"},
	"Very Unhealthy":{conc:[150.5, 250.4], aqi:[201, 300], color: "#A373DF"},
	"Hazardous":{conc:[250.5, 500.4], aqi:[301, 500], color: "#B44868"},
}

// Fetch the PurpleAir sensor data for a given sensor ID.
async function getSensorData(url, id) {
  let req = new Request(`${url}${id}`)
  let json = await req.loadJSON()

  return {
  	"val": json.results[0].PM2_5Value || 0,
  	"lat": json.results[0].Lat,
  	"long": json.results[0].Lon,
  	"location_type": json.results[0].DEVICE_LOCATIONTYPE || "",
   "temp_f": json.results[0].temp_f || "-",
  	"ts": json.results[0].LastSeen
  }
}

// Get the sub-locality (e.g. neighborhood) or locality (city, town)
// of the given latitude & longitude.
// 
// Useful for getting the human-readable name of a sensor's location.
async function getLocation(lat, long) {
  	let loc = await Location.reverseGeocode(lat, long)
  
	return {
  		name: loc[0].subLocality || loc[0].locality,
  		data: loc
	}
}

// Calculates the AQI level based on 
// https://cfpub.epa.gov/airnow/index.cfm?action=aqibasics.aqi#unh
function calculateLevel(input) {
	let res = {
		level: "Unknown",
		aqi: "0",
		color: "#dddddd"
	}

	let conc = parseFloat(input).toFixed(1)
	// Apply AQ&U correction factor
	// PM2.5 (µg/m³) = 0.778 x PA + 2.65
	conc = 0.778 * conc + 2.65
	
	for (let [key, val] of Object.entries(PM2_5BREAKPOINTS)) {
  		if (conc >= val.conc[0] && conc <= val.conc[1]) {
    		console.log(key)
    		// AQI = (aqihi-aqilo)/(conchi-conclo) * (conc - conclo) + aqilo
    		let x = (val.aqi[1] - val.aqi[0])
    		let y = (val.conc[1] - val.conc[0])
    		let z = (conc - val.conc[0])
    
    		res.aqi = Number((x/y) * z + val.aqi[0]).toFixed(0)
    		res.level = key
    		res.color = val.color
		}
	}

	return res
}

async function run() {
	let wg = new ListWidget()
	
	let header = wg.addText("PM2.5 AQI")
	header.font = Font.mediumSystemFont(TEXT_SIZE.title)
	header.textColor = new Color(HEADER_COLOR)

	// Loading/pending/stale state
	wg.backgroundColor = new Color("#bbbbbb")

	try {
  		console.log(`Using sensor ID: ${SENSOR_ID}`)
  		let data = await getSensorData(API_URL, SENSOR_ID)
  		console.log(data)
  
  		let loc = await getLocation(data.lat, data.long)
  		console.log(loc.name)
  		// wg.url = `${MAP_URL}?lat=${data.lat}&lng=${data.long}&zoom=12`

  		let res = calculateLevel(data.val)
  		console.log(res.aqi)
  		wg.backgroundColor = new Color(res.color)
  
  		let content = wg.addText(res.aqi.toString())
  		content.font = Font.mediumRoundedSystemFont(TEXT_SIZE.aqi)
		content.textColor = Color.black()
		
		wg.addSpacer()

		let locality = wg.addText(loc.name)
		locality.font = Font.regularSystemFont(TEXT_SIZE.meta)
  		locality.textOpacity = 50
  		locality.textColor = Color.black()
  
		let temp = wg.addText(`${data.temp_f}F (${data.location_type})`)
		temp.font = Font.regularSystemFont(TEXT_SIZE.meta)
  		temp.textOpacity = 50
  		temp.textColor = Color.black()
  
  		let updatedAt = new Date(data.ts*1000).toLocaleTimeString("en-US", { timeZone: "PST" })
  		console.log(updatedAt)

  		let ts = wg.addText(`Updated at ${updatedAt}`)
  		ts.font = Font.regularSystemFont(TEXT_SIZE.meta)
  		ts.textOpacity = 50
  		ts.textColor = Color.black()
  
  		let id = wg.addText(`Sensor ID: ${SENSOR_ID}`)
		id.font = Font.regularSystemFont(TEXT_SIZE.meta)
  		id.textOpacity = 50
  		id.textColor = Color.black()
  
	} catch (e) {
  		console.log(e)
		// Don't overwrite existing content
		// (there is not an API to inspect the existing widget content)
  		// let err = wg.addText(`error: ${e}`)
  		// err.textSize = TEXT_SIZE.meta
  		// err.textColor = Color.red()
  		// err.textOpacity = 30
	} finally {
		Script.setWidget(wg)
		Script.complete()
	}
}

await run()