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Frequently Asked Questions

How many sensors are active in Temescal Valley as part of the Star Grant Program?
As of October 20220, twenty-three sensors are active.

How long have the sensors been active?
The sensors were distributed on July 26, 2018 and most were operational within two weeks.

Where can I see the sensors?
You can see the real-time sensor data on a the PurpleAir Map.

What do the numbers on the map mean?
The default values are PM2.5 Air Quality Index (AQI) which is a rolling average calculated index biased to recent readings. Readings are scaled in ranges of "Good", "Moderate", "Unhealthy" etc.

What do the numbers included in the historical data downloaded from PurpleAir mean?
The values included in the downloaded historical data correspond to actual particle concentrations in various ranges. These values do not correspond to the same "Good", "Moderate", "Unhealthy" AQI scale.

Can you tell me more about the Air Quality Index (AQI)?
Particle sensors measure real time particle counts. To express these raw data values in a way that is more closely related to potential health consequences the AQI was developed by EPA as a national standard for presentation of air quality information. The AQI is calculated to take into account that pollutant concentrations will vary over time and that the duration of exposure is a significant contributor to any health impact. The AQI is scaled into ranges such as "Good", "Moderate, "Unhealthy" etc and is based on an average of data measured over 24 hours.

While the AQI is useful for the analysis of general trends it is less useful for real time reporting. One way to think of AQI is "if I stay out for 24 hours, and the AQI is 190 during those 24 hours, then the health effect is Unhealthy". In answer to this the EPA developed the NowCast algorithm which is a 12 hour average but strongly biased to recent readings and is arguably a better real time indicator of air quality.

Another approach is to simply use the real time particle measurement and convert it to AQI. Some call this "InstantCast". Purple Air seems to use this approach for reporting real time AQI. The map data layer can be changed between various averages and real time and the AQI values will change in kind.

It's important to know that the true AQI considers other pollutants such as Ozone, CO, SO2 and NO2 and will report the highest concentration from all of these pollutants as the AQI. PA sensors only measure particulates so you may wish to consult a site such as AirNow.gov if you are concerned about these other pollutants.

For more information see the following document: NACAA NowCast Presentation

Can I download historical data for the sensors?
Yes. You may download the data from PurpleAir directly or you may download precomplied data from the following links. Pre-compiled may be the better option.
Download Instructions

Pre-compiled Data

I've noticed that when it is windy out I can see a lot of dust but the map says the readings are low.
Yes, it has been confirmed that the sensors seem to read low when the wind is above approximately 10 MPH.

What is your role in this?
I'm a local resident and one of the original participants in the program. I have an interest in distributed technology, remote sensing and computer programming. I'm not a pollution activist or associated with any regulatory agency. My understanding of how to properly analyze this type of data is very limited.

What are the benefits of the program?
- We now have local data where none existed before and a map of current air quality based on hyper-local data.
- We've had some reports of the local mines stepping up their dust control efforts when they see low air quality on the PurpleAir map.
- Aside from that I wish I more knowledgable about how to best analyze the historical data to answer basic questions such as: