Calculating the AQI based on the Purple Air API for a sensor#

Purple Air sensors have an API at https://www.purpleair.com/map.json?show=SENSOR-ID-HERE, which returns JSON that looks something like this:

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{
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  "mapVersion": "0.26",
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  "baseVersion": "7",
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  "mapVersionString": "",
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  "results": [
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    {
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      "ID": 123,
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      "Label": "Sensor label",
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      "DEVICE_LOCATIONTYPE": "outside",
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      "THINGSPEAK_PRIMARY_ID": "123",
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      "THINGSPEAK_PRIMARY_ID_READ_KEY": "xxx",
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      "THINGSPEAK_SECONDARY_ID": "1234",
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      "THINGSPEAK_SECONDARY_ID_READ_KEY": "xxx",
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      "Lat": 37.5,
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      "Lon": -122.4,
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      "PM2_5Value": "8.75",
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      "LastSeen": 1630438756,
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      "Type": "PMS5003+PMS5003+BME280",
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      "Hidden": "false",
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      "Flag": 1,
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      "DEVICE_BRIGHTNESS": "15",
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      "DEVICE_HARDWAREDISCOVERED": "2.0+OPENLOG+15476 MB+DS3231+BME280+PMSX003-B+PMSX003-A",
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      "DEVICE_FIRMWAREVERSION": "6.01",
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      "Version": "6.01",
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      "LastUpdateCheck": 1630436115,
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      "Created": 1622588142,
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      "Uptime": "2701447",
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      "RSSI": "-56",
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      "Adc": "0.01",
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      "p_0_3_um": "849.53",
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      "p_0_5_um": "251.67",
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      "p_1_0_um": "78.49",
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      "p_2_5_um": "17.79",
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      "p_5_0_um": "6.95",
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      "p_10_0_um": "5.54",
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      "pm1_0_cf_1": "3.58",
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      "pm2_5_cf_1": "8.75",
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      "pm10_0_cf_1": "14.04",
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      "pm1_0_atm": "3.58",
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      "pm2_5_atm": "8.75",
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      "pm10_0_atm": "14.04",
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      "isOwner": 0,
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      "humidity": "48",
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      "temp_f": "74",
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      "pressure": "1005.56",
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      "AGE": 1,
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      "Stats": "{\"v\":8.75,\"v1\":9.4,\"v2\":10.22,\"v3\":10.96,\"v4\":14.17,\"v5\":15.51,\"v6\":12.53,\"pm\":8.75,\"lastModified\":1630438756184,\"timeSinceModified\":120110}"
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    }
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  ]
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}

There’s just one problem with this: it doesn’t give you the AQI number that is displayed on their site! Instead it gives you the raw numbers that can be used to calculate that AQI number.

I figured someone must have solved this, so I ran a GitHub code search for purpleair.com map json and found zakj/scriptable with code for decoding that. I adapted it to the following:

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// Adapted from https://github.com/zakj/scriptable by Zak Johnson
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async function fetchAqi(sensorId) {
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  const response = await fetch(
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    `https://www.purpleair.com/json?show=${sensorId}`
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  );
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  const json = await response.json();
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  const stats = json.results
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    .filter((r) => !(r.Flag || r.A_H))
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    .map((r) => JSON.parse(r.Stats));
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  const pm2_5 = stats.reduce((acc, { v }) => acc + v, 0) / stats.length;
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  const trend = stats[0].v1 - stats[0].v3;
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  return {
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    current: aqiFromPm(pm2_5),
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    trend: Math.abs(trend) > 5 ? trend : 0,
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    details: json,
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  };
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}
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function aqiFromPm(pm) {
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  const table = [
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    [0.0, 12.0, 0, 50],
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    [12.1, 35.4, 51, 100],
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    [35.5, 55.4, 101, 150],
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    [55.5, 150.4, 151, 200],
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    [150.5, 250.4, 201, 300],
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    [250.5, 500.4, 301, 500],
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  ];
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  const computeAqi = (concI, [concLo, concHi, aqiLo, aqiHi]) =>
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    Math.round(
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      ((concI - concLo) / (concHi - concLo)) * (aqiHi - aqiLo) + aqiLo
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    );
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  const values = table.find(([concLo, concHi, aqiLo, aqiHi]) => pm <= concHi);
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  return values ? computeAqi(pm, values) : 500;
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}

I used this to build a simple demo at https://til.simonwillison.net/tools/aqi

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Calculating the AQI based on the Purple Air API for a sensor#