Aerosol Optical Thickness, MODIS, 2000-2016

Tiny solid and liquid particles suspended in the atmosphere are called aerosols. Examples of aerosols include windblown dust, sea salts, volcanic ash, smoke from fires, and pollution from factories. These particles are important to scientists because they can affect climate, weather, and people's health. Aerosols affect climate by scattering sunlight back into space and cooling the surface. Aerosols also help cool Earth in another way -- they act like "seeds" to help form clouds. The particles give water droplets something to cling to as the droplets form and gather in the air to make clouds. Clouds give shade to the surface by reflecting sunlight back into space.

People's health is affected when they breathe in smoke or pollution particles. Such aerosols in our lungs can cause asthma or cancer of other serious health problems. But scientists do not fully understand all of the ways that aerosols affect Earth's environment. To help them in their studies, scientists use satellites to map where there were large amounts of aerosol on a given day, or over a span of days.

Aerosol optical depth is a measure of the extinction of the solar beam by dust and haze. In other words, particles in the atmosphere (dust, smoke, pollution) can block sunlight by absorbing or by scattering light. AOD tells us how much direct sunlight is prevented from reaching the ground by these aerosol particles. It is a dimensionless number that is related to the amount of aerosol in the vertical column of atmosphere over the observation location.

A value of 0.01 corresponds to an extremely clean atmosphere, and a value of 0.4 would correspond to a very hazy condition. An average aerosol optical depth for the U.S. is 0.1 to 0.15.