Investigating GLOBE Air Quality Using PurpleAir

Lectures, lessons, and learning opportunities of all kinds are available online these days… luckily, air quality investigations can be, too! Publically available sources of air quality data, which can be accessed any time and anywhere, can be used in conjunction with or in lieu of the GLOBE Aerosols Protocol for students and teachers participating in the US Air Quality Student Research Campaign. This blog post will introduce a website which can be used to conduct air quality investigations with a ground-based instrument, called the PurpleAir.

The PurpleAir is a small, commercially available sensor that measures fine particulate matter (PM1.0, PM2.5, and PM10). These measurements, which are reported every 2 minutes, are communicated wirelessly to PurpleAir and published, in near real time, on an interactive map. You can access this map by going to and clicking on ‘View the Map’ , or going directly to the map’s URL here. Check out the screenshot below – PurpleAir sensors have been set up and are making measurements all over the world!

The type of data displayed on the map, the method being used to estimate the PM2.5 concentration, and the time period over which the data that is displayed on the map are averaged can be changed from the menus in the ‘Map Data Layer’ box in the lower left-hand corner of the map. (Shown in the screenshot to the left.) One thing that cannot be changed is the color scale on the map, which is based on the US Environmental Protection Agency’s (EPA) Air Quality Index (AQI) color scheme.

The chart below shows the the US EPA’s Air Quality Index, which color-codes the AQI by the hourly average PM2.5 concentration (in micrograms per cubic meter).

A note on conversions: you may select different methods to estimate the concentration of PM2.5 displayed on the map, which use different conversion factors. These three conversion factors are based on studies done by different research agencies; more information is available by clicking the ‘?’ in the Map Data Layer box. Typically, the default conversion factor of ‘none’ should be used.

Data on the map can be displayed in real-time (updated every two minutes) or averaged over time periods as long as a week. (See the screenshot to the left.) PurpleAir instruments can be used to measure particulates either  indoors or outdoors; the PurpleAir map displays both by default. For the GLOBE Air Quality Campaign, the focus will be on outdoor air quality, so the checkbox beside ‘Inside Sensors’ should be un-checked. (As shown in the animation below and to the left.)

Data from the PurpleAir instruments can be displayed on the map in multiple ways. The default data displayed on the map when it loads is the Air Quality Index, which is calculated automatically, based on the amount of PM2.5 in the air. You can change this to show the actual amount of PM2.5 using the drop down menu (as shown in the animation below and to the right).

One drawback of the PurpleAir map is that it updates continuously, which means that you cannot go “backwards in time” to see historical data on the map. If you’re using the PurpleAir map to investigate air quality over time, a best practice is to take screenshots at the same time every day. If you haven’t done this, data from individual PurpleAir instruments can be downloaded by clicking the download button on the lower right hand side of the map – which will be covered in-depth in a future blog post!

Another best practice when using the PurpleAir map to investigate air quality is to set up the map the way you want it: zoomed into your area of interest, with ‘Raw PM2.5’ selected as the type of data displayed on the map and only ‘Outside Sensors’ selected. Then, copy the map’s URL from your browser and save it; this will load the PurpleAir map exactly the way you want it every time, just with updated data!

One way to use the PurpleAir map to investigate air quality is to compare local particulate matter concentrations to the color and visibility of the sky. The following pictures of the New York City skyline were taken from the same location, at the same time of day; the corresponding screenshots of the PurpleAir map, zoomed into New York City, were taken at approximately same time as the pictures.

July 9, 2020, approximately 12:00PM EST

July 14, 2020, approximately 12:00PM EST

Notice the differences in the color of the sky and the visibility in the two pictures – the sky is much bluer in the July 14 picture as compared to the July 9 picture, and the buildings in the distance are much ‘fuzzier’ in the July 9 picture. The corresponding PurpleAir maps offer an explanation. PM2.5 concentrations on July 9 were much higher – in most cases, over twice as high – on July 9 than on July 14.

If you have questions or need assistance, please contact the GLOBE AQ team at:

GLOBE Air Quality Team:

  • Dr. Margaret Pippin, NASA
  • Angie Rizzi, NASA/SSAI
  • Samantha Adams, Bronx Collaborative High School

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