Particulate matter fields without a specified density estimation automatically select one based on the registered location_type. This mirrors the PurpleAir Map, which uses the same method to obtain a value for the Raw PM2.5 datalayer. “atm” is used for outdoor sensors and “cf_1” is used for indoor sensors.
Fields that do this are currently only queryable from the real-time API, as the location_type field is not available in historical data.
We’ll update the API documentation so this is more clearly reflected.