Earth's atmosphere is filled with aerosols - tiny particles of liquid or solid, suspended in the air. There are many, many different kinds of aerosols. Some are naturally occurring, like water vapor and dust, and some can be generated from industry, like soot and smoke. These particles affect the sunlight reaching the earth, as light rays are scattered and redirected as they bounce off of aerosols. This effect is measured by aerosol optical depth, or AOD. At very small AOD values of 0.1 or below, the sky appears clear and the sun bright, but at higher levels, the sky can appear hazy due to high concentrations of particles that affect the path of sunlight. But AOD is about more than just how nice of a view we have. Tracking AOD helps scientists monitor pollution and air quality changes, and AOD in itself is a valuable measurement for calibrating and correcting other data products. 

Many NASA instruments help scientists monitor AOD, including satellite missions like the Visible Infrared Imagine Radiometer Suite (VIIRS) and Ozone Monitoring Instrument (OMI). Combining these satellite observations with ground sensing stations like NASA's Aerosol Robotic Network (AERONET) can illustrate how light is redirected throughout Earth's atmosphere, allowing researchers to estimate the concentration and size of aerosols. 

AOD measurements can help researchers detect changes in air quality from emissions or natural events like volcanic eruptions or wildfires. This makes accurate AOD monitoring a priority for long term climate studies as well as public health research.