Aerosol Optical Depth (AOD) is a satellite-derived measurement of how much sunlight is scattered or absorbed by particles in a vertical column of atmosphere. A value of 0.0 means a clean atmosphere; 0.1 is slightly hazy; 0.3 is meaningfully dusty or smoky; 0.5 and above usually indicates a serious event like a wildfire plume or a major dust storm; 1.0 and above is extreme. The NASA Earth Observatory AOD maps are the canonical visualization.
AOD is not directly comparable to ground-level PM2.5, but it correlates strongly. The relationship varies with the height of the aerosol layer: a smoke plume aloft can show high AOD while ground PM stays moderate, and vice versa. Levy et al. documented MODIS-based retrieval algorithms and the correlation patterns. AOD is a satellite product, fusing satellite and ground data into a unified estimate.
For understanding regional events, AOD provides context that ground sensors cannot. When local AOD jumps from 0.1 to 0.5 over a few hours (on the public satellite maps above), a large aerosol event is moving through the region, and outdoor PM2.5 typically rises within hours even before an indoor sensor sees it. AOD trends over days catch slow-developing events like prolonged dust transport that ground-station networks may underreport.
The dashboard itself does not display AOD: it is a satellite product, not one of the fields the Google Air Quality API reports. What the dashboard acts on is the outdoor PM2.5 forecast, which captures the ground-level effect of these aerosol events; when it climbs, the dashboard suggests closing windows and running filtration. AOD is the wider-angle satellite view behind that same story. See related: wildfire smoke, mineral dust events, visibility as air quality.
References
- NASA Earth Observatory - Aerosol Optical Depth earthobservatory.nasa.gov
- Open-Meteo - Air Quality API documentation open-meteo.com
- Copernicus CAMS - Global atmosphere monitoring atmosphere.copernicus.eu
- Levy et al. - MODIS aerosol retrieval doi.org
