When we study Apparent Optical Properties (AOPs) and Inherent Optical Properties (IOPs), we are usually trying to learn more about matter contained in the ocean or atmosphere. To get the full picture, we need to take into account the boundaries that may impact the light field in our area of interest (i.e., ocean or atmosphere). There are three boundaries in the ocean-atmosphere system: top of the atmosphere, sea surface, and seafloor. In terms of exploration using PACE data, our primary concern for both atmospheric and oceanic studies is the sea surface.
Anyone who has been to the coast or on a ship can tell you the ocean surface is constantly changing, from a flat calm "mirror" to wind-strewn waves with choppy foam. Also, depending where you are standing while looking at the choppy ocean – with sun behind your back or shining on your face, the same choppy ocean might look different. This behavior is explained by the bi-directional reflectance distribution function (BRDF). BRDF defines how much light of a specific wavelength is reflected off the ocean surface, while taking into consideration the position of the sun and the viewing angle (our eyes or the PACE satellite). Scientists use sea surface slope statistics to model the BRDF of the surface. These calculations are important because having good estimates for how light is reflected off the ocean surface allows scientists to better understand how much light has entered the ocean.
Why don’t we care about the other boundaries? It’s not that the top of the atmosphere and the bottom of the ocean aren't important, it's just that the top of the atmosphere is a straight-forward measure of Earth’s incoming light field measured by PACE and the bottom of the ocean is, well zero. Certainly in coastal areas light reaches all the way to the bottom, but the vast majority of the ocean is deep enough where light does not reach the sea bottom. The ocean’s average depth is 3.7 km (2.3 mi)... that’s about twice the depth of the Grand Canyon (1.83 km or 1.14 mi)! Sunlight only penetrates the top kilometer or less. So, in most locations, the ocean is so dark, you wouldn’t see a banana… but some deep-water denizens might!
For more details and equations check out the "Surfaces" section of the Ocean Optics Web Book .