Aerosols include, but are not limited to, smoke, dust, volcanic ash, sea spray, polar stratospheric clouds, and smog. Although cloud particles can be considered as a particular type of aerosol, it is conventional to put them in a separate category.
Liquid water clouds are defined as distinct optically thick particulate features composed of droplets. Cirrus clouds are defined as visible, or sub-visible particulate layers (either natural, or man-made, such as contrails), which reside in the upper troposphere/lower stratosphere and are composed of water ice crystals with sizes ranging from several micrometers to a millimeter.
APS threshold science requirements and science goals are characterized in following sections and described in terms of the specific data products. These data products, sometimes referred to as Environmental Data Records (EDRs), are: