As an important part of ensuring data availability and quality, PurpleAir does not remove sensors from the Map. When a sensor goes offline, it appears as a gray dot on the map and stays there permanently. The visibility of these gray dots is controlled by a map setting labeled “Reporting or modified within.”
The default option for this setting is “7-days.” This means that, by default, sensor visibility is locked to devices that have reported in last seven days. If you see a gray dot on your map, it will disappear after about a week depending on your settings. If you do not want to wait that long, you can lower the reporting or modified within selection to something like “1-day” or “1-hour.” See the image below listing all selections.
If you change the setting to “All-time,” you will see many gray dots. This is because many of the early PurpleAir sensors in the network have been replaced with new sensors. These older sensors, despite no longer contributing data, will always have their old data available.
When a sensor is registered as “public,” the data retrieved by that sensor (air quality data, temperature and humidity data, metadata, etc.) will be stored in the public PurpleAir database. When a sensor is registered as “private,” the data from the sensor will be stored in the private PurpleAir database; this means that the data will only be accessible to you and those you provide with the required information.