When placing your AirVisual Pro outdoors, it is possible that your Pro will show a different value than the nearest outdoor official government station. There are several reasons that this can happen: The air quality may vary greatly even within a few miles/km. The official government stations usually report measurement as an average of the past hour. In practice, this means the official government data reported might already be 1 to 2 hours old. By the time the data is published, some government data may already outdated by a few hours. The Pro has the advantage of showing real-time data. The government station doesn't actually report PM2.5. In this case, the Pro would display a note below the outdoor data: "The main pollutant is xyz." The Pro or the government station may actually show an inaccurate measurement. Government stations require frequent and expensive maintenance, which is not always performed in time.