The What and Why of "Dark Target"
When we view the Earth from space we observe a very complex mixture of many features at the same time. In the image below you can see clouds, smoke, land and ocean surfaces. Generally our eyes and brains do a very good job of recognizing theses features but this is a very complex task to do automatically using a computer algorithm applied to the raw data.
These features reflect solar radiation back to the sensor in varying amounts depending on the wavelength. If we wish to observe only aerosols we need to separate out the non-aerosol signal. Assuming we can avoid clouds (a big if further explained in the "pixel masking and selection" section) we are left with a mixed signal of aerosols and surface which can be either land or ocean.
Essentially, Dark target refers to a method of satellite data analysis that allows us to take advantage of bright aerosols against a dark surface (hence the dark target name) to separate the aerosol and surface signals. Some strengths and limitations of this method is illustrated by a few examples below.
Image from MODIS Rapid Response September 4, 2017
The image above right is a blow up of a portion of the image at left showing smoke over both bright and dark land surfaces.
If you look carefully at the smokey regions of the image at right you will notice that the smoke generally looks bright. When is it over a dark vegetated surface it is fairly easy to pick out with our eyes where the smoke is located due to the contrast of light over dark. There are some areas of bright land surface in the image where it can be very difficult to make this distinction.
Below are another two examples of aerosols over different types of land surfaces. On the left is an image of smoke over land and ocean. The image at the right shows Saharan dust blowing from Africa out over the Atlantic Ocean
In both of these images it is relatively easy to pick out the aerosols over the much darker ocean surface. In the image to the left you can also see the smoke over the darker land surfaces. However in the dust image it is almost impossible to pick out the dust over the bright desert surfaces.
So the strength of the dark target method is when we observe aerosols over dark surfaces. This method cannot be used effectively over bright land or ocean glint.