A common question from new Terrestream users: "Why does my PM2.5 read 18 µg/m³ when AirNow shows 11?" Three reasons.
First, hyperlocal variation. The nearest EPA AQS reference monitor might be 5–30 km from your house. Air quality varies meaningfully over hundreds of meters, traffic corridors, industrial neighbors, valleys with poor ventilation. Your reading captures your block; AirNow captures your county.
Second, time resolution. AirNow reports a "NowCast" running average designed for stability, updated hourly. Your sensor updates every few seconds. During a fast event, neighbor's wood stove firing up, a delivery truck idling out front, your reading captures the spike; AirNow smooths it away.
Third, calibration class. EPA reference monitors are large, expensive, lab-calibrated instruments, gravimetric PM sampling with chemistry-grade gas analyzers. Consumer optical PM sensors (everything in this price range, including the SEN66) are correlated to reference monitors via empirical models but are not equivalent. The PurpleAir correction factor work documents typical biases, most consumer sensors over-read in humid conditions and under-read coarse-dust events.
In practice: treat your sensor as the truth for what your specific room is experiencing right now, treat AirNow as the truth for the regional trend, and use the difference between them as a signal. When your indoor reading climbs but the regional outdoor reading is flat, the source is inside. When both rise together, the regional event is reaching you.
References
- EPA - Air Quality System (AQS) monitoring network www.epa.gov
- AirNow API documentation docs.airnowapi.org
- Open-Meteo - Air Quality API documentation open-meteo.com
- Barkjohn et al. - PurpleAir correction factors doi.org
