How to read air quality observations
Understand what monitoring stations measure, how station type affects readings and why recent and verified records can differ.
Start with the station setting
A monitoring station samples outdoor air at a known point. Traffic stations describe conditions near busy roads. Industrial stations track areas influenced by nearby industrial activity. Background stations describe conditions in a wider setting away from the immediate influence of a major source.
If two nearby stations show different concentrations, first compare their station type, time and pollutant. A roadside nitrogen dioxide reading can differ from an urban background reading taken at the same hour. Both readings can be valid for the places where they were collected.
Reporting delay and verification
The measurement time tells you when the station sampled the air. Publication follows collection and transmission, which can take several hours. Recent records may be provisional and can change after the reporting authority completes its quality checks.
A missing hour remains a gap in the time series. Some stations measure a limited set of pollutants, while others publish daily samples. Use the recorded time, averaging period and quality label when reading or comparing values.
Reading historical records
Look for the dates actually covered, the number of usable values and any changes to the station. A relocation or instrument change can affect how comparable two periods are. A forecast archived after its valid time remains a record of a prediction. A reanalysis is a separate model reconstruction that can incorporate observations.