Some of the questions we get asked
We are a grassroots group with a technical background that wanted to measure the pollution coming from a mining operation in our backyard. The website and map was started in November 2015. Sharing the data and our research with others created a need for more monitors, to expand the network and to test the sensors by collocating them with other monitors.
The PurpleAir Map displays the points using the Federal Environmental Protection Agency (EPA) Air Quality Index (AQI) scale. The AQI allows comparison for different pollutants with an easy to visualize color scheme. The air goes from good green air to yellow to orange to red to purple air, or even hazardous maroon air. The graphs give further information on different counts, relative weights and sizes of particles in the air.
PurpleAir operate under an llc. We reached this point by using our own resources, some donations and help from friends. Due to numerous requests for sensors, in late February 2016 PurpleAir accepted pre-orders of a small batch of prototype sensors. Since that time, we have made over 500 air quality sensors.
They use laser beams to detect the particles going past by their reflectivity, like dust shimmering in a sunbeam. The PM 2.5 and PM 10 micro-gram weights are calculated from the counts. The values are averaged every 20 seconds and graphically displayed on our website.
PurpleAir displays the particulate matter numbers, in both the PM 2.5 and PM 10 range, We also track particle counts in six sizes between 0.3um and 10.0um in diameter.
PurpleAir sensors read very similar numbers to other sensors of the same type (laser counters). During a recent inversion, PurpleAir sensors were reading correlated but slightly lower than University of Utah sensors on the MesoWest map. Some sensors use very different methods to measure particles and so we expect some disagreement with their numbers. It is the amount of disagreement we are trying to figure out and will let you know when we have more information from the University of Utah and SCAQMD tests.
Every care is taken to place monitors in locations that are representative of the air we breathe every day. They are placed in neighborhoods, on the side of houses, a few feet above our heads. They use the homeowner’s Wi-Fi and power. Local pollution like cigarette smoke, BBQ’s, fireplaces and idling cars can cause spikes in the short term graphs.
Quality techniques like an initial decay test and correlation testing with new and older monitors are used. One of the issues that faces these devices, and others, is what is called “drift”. This happens with optical particle sensors because of dust that may settle inside the device, and may cause an offset or drift over time. A maintenance schedule is necessary with any measuring device, but ours are relatively easy to maintain and cheap to service.
PurpleAir sensors can be used indoors, please contact us for more information or availability on indoor sensors..
The simplest way to view our data is the PurpleAir Map. Data is also available on MesoWest's network and others. We frequently share raw data with researchers. If you want access to data for whatever reason, please feel free to contact us.