Aerosol optical depth
Air quality AOD Atmospheric aerosol optical depth
Aerosol optical depth, or AOD, describes how particles in a column of air weaken light. It is not a direct measurement of particulate pollution at ground level.
How much light the particles remove
Dust, smoke, sea salt and other small suspended particles are aerosols. They can scatter or absorb light. Aerosol Optical Depth (AOD) adds up that effect through the relevant air column. A higher value means stronger attenuation of the direct beam.
No unit, but a wavelength
AOD is dimensionless. Its wavelength belongs with the number, often 550 nanometres in visible light. Different wavelengths or air columns cannot automatically be compared. An AOD of 1 means neither one per cent nor one hundred per cent particulate matter.
Dust aloft, breathing air below
A dust layer can lie several kilometres up, hazing the sky while surface air is less affected. Conversely, a local source can worsen breathing air without strongly altering the whole column.
What the value helps explain
AOD helps interpret haze, long-distance transport of smoke or dust and available direct sunlight. A milky sky may be consistent with elevated AOD, but does not identify the cause on its own. Assessing air at breathing height also requires surface concentrations such as PM10 and PM2.5.