The PurpleAir Map displays the United States Environmental Protection Agency (US EPA) Air Quality Index (AQI) by default, however there are other options you can choose to see also. An AQI is a color-coded scale intended to convey the severity of unhealthy air. The US EPA AQI ranges from 0 to 500, and is partitioned into six categories to report levels of five major pollutants; including particulate matter (PM) such as \text{PM}_{2.5} and \text{PM}_{10}. (The subscript refers to the diameter of the particle in microns.)
The US EPA AQI for PM is calculated from a 24-hour average: this is based on hourly observations of PM concentrations and is updated daily. The EPA also reports a real-time AQI called the NowCast. The NowCast scale for PM is calculated using a weighted 12-hour average. This is based on hourly averages and rate of change of PM concentrations, and is updated hourly depending on data availability.
Visitors to the PurpleAir Map have the option to view different averaging time periods in terms of the visitor-chosen AQI on the PurpleAir map legend. This allows visitors to see data for current conditions in the familiar terms of the AQI, yet at a finer resolution of changes in air quality throughout the day. This finer resolution is not a replacement for the correlations to public health concerns described by an AQI’s default time interval.
It takes a full 24 hours to obtain an AQI value (that’s 24 hourly values for PM or the max 1-hour or 8hour value in a 24-hour period for other pollutants),
This is because the AQI is based on EPA standards, which are based on averaging times. For example, for PM2.5 the averaging time is 24 hours. A 24-h average of 35 ug/m3 would correspond to a value of 100 on the AQI
scale because the value of 35 ug/m3 is the EPA standard for PM2.5
However, it appears that PurpleAir and perhaps Air Now are allowing 1-hour averages to be attributed to AQI. This is wrong, as you can see from the definition above. So a 1-hour average of 35 ug/m3 gets an AQI of 100, but the actual AQI over 24 hours including that 1-hour average could be much less than or even greater than 35 ug/m3. THERE IS NO AQI ASSOCIATED WITH AN AVERAGE LESS THAN 24 HOURS.
This could perhaps be corrected using the AirNow or EPA AQI values but requiring 24-h averages.
Just to echo what Doug (@dwhitemv) said, PurpleAir AQ data is most commonly found as mass concentration (µg/m3). On the PurpleAir map, you can change your data layer to “Raw PM2.5 (µg/m3).” However, if you’re pulling PM2.5 data from the PurpleAir API, it is already in µg/m3.
Thank you all for sharing this! I see now I can download the raw concentration data in ug/m^3 for PM2.5 and PM1.0. Is there a way to calculate or download the data for PM10 in ug/m^3?
You can, yes. I’m assuming you’re looking at the map. In that case, the data layer appears lower in the list, under “Air Quality (obsolete).” The reason it is located there is that while PurpleAir sensors report PM10, scientific evaluations have shown that they significantly underestimate PM10 pollution.
We still make the PM10 data available, because our ethos is one of openness, but we want to make sure it’s clear that the data is not accurate. PurpleAir PM2.5 and PM1 data, on the other hand, has been demonstrated to correlate well with reference instruments.
Thank you again Andrew! I appreciate the transparency. We had hoped for better PM10 accuracy only because our reference instruments in rural AZ only report Ozone and PM10.
It’s something we would love to deliver on in the future. The founder of PurpleAir, Adrian, created the first PurpleAir sensor because he wanted to monitor dust (primarily larger particles >2.5µm) coming out of the gravel pit near his home.
If you find a low-cost nephelometer that performs well with PM10, we’d love to hear about it!
