Earth's atmosphere is filled with aerosols: tiny liquid or solid particles suspended in the air. Aerosols come from naturally occurring sources — like volcanic ash, sea spray, and desert dust — and from human activities and industry — such as soot and smoke from the burning of fossil fuels or vegetation. These particles scatter and redirect sunlight in the atmosphere, and the extent of this effect is measured by aerosol optical depth, or AOD. 

At AOD values of 0.1 or below, the sky appears clear and the Sun bright. But at higher AOD levels, the sky appears hazy due to high concentrations of particles that affect the path of sunlight. Tracking AOD helps scientists monitor pollution and air quality changes, and AOD is also a valuable measurement for calibrating and correcting other satellite data products. 

Many NASA instruments help scientists monitor AOD, including satellite instruments like the Visible Infrared Imagine Radiometer Suite (VIIRS) and Ozone Monitoring Instrument (OMI). Combining these satellite observations with ground-based stations such as NASA's Aerosol Robotic Network (AERONET) allows researchers to estimate the concentration, extent, and size of airborne particles. 

AOD measurements can help researchers detect changes in air quality due to emissions from human activity or from natural events like dust storms or wildland fires. This makes accurate AOD monitoring a priority for public health research and for studies of climate.