Satellites provide a comprehensive global view of spectral water-leaving reflectances, which is incredible… but this is only where the science begins! We must then use these measurements to understand what is in the ocean and atmosphere. To do this, we use the AOPs measured by the satellite, like Remote Sensing Reflectance, to estimate or “retrieve” IOPs. Essentially this is working our way backwards through the equations relating to radiative transfer.
But before working backward, we must build forward models to learn how different components of the ocean and atmosphere interact with light across wavelengths – i.e., their spectral shape. It is important to know both the absorption and scattering. Why? Some things in the ocean absorb, some scatter, and others do both at different wavelengths! These models also need to define how various boundary conditions impact the light field.
This type of research can be done in the laboratory through controlled experiments. For example, by mixing up a tank with known quantities of sediment, CDOM, and phytoplankton and measuring its AOP. You can then see how your known IOPs (i.e., sediment, CDOM, etc.) combine together to give you the AOP value of the mixture. If you do this with enough different combinations, you can develop a computer model to help determine or "retrieve" what’s in seawater solely from the AOP measurements. Similar experiments and models are created to look at components from the atmosphere.
It is important to remember that working backwards through the equations to go from satellite measurements of AOPs to individual IOPs represent best estimates. Why? There could be several potential solutions for any water-leaving reflectance spectral signature. A general rule of thumb is that the number of unknown IOPs must not exceed the number of spectral bands (wavelengths) observed. So observing more wavelengths provides more clues about what’s in the ocean or atmosphere.
It’s analogous to the classic game of “20 Questions” where one player is thinking of something and the other player guesses it by asking a series of Yes or No questions. The game would be much harder if you could only ask five questions and much easier if you could ask 200 questions. The same principle applies to pulling out components from the AOPs: “Does it reflect at 550nm?” or “Does it absorb at 710nm?”. If you have run out of questions to ask, or don’t have enough wavelengths to test, you might be stuck trying to choose between two answers or two types of phytoplankton that very nearly resemble each other.
This is where the innovations of the PACE Ocean Color Instrument helps take ocean color research to the next level. By measuring a breadth of wavelengths at a fine-scale resolution, PACE dramatically increases our ability to parse out what is in the ocean or atmosphere based on its AOP. With vastly improved spectral information, scientists will be able to better uncover the mysteries of our ocean and atmosphere.
The Ocean Color Instrument alone, however, cannot provide all the information scientists need to decode the complex light signatures received by the PACE satellite. In particular, when deciphering light signatures from components of the atmsophere, it is important to understand the polarization of that light. For that, PACE has two polarimeters SPEXone and HARP2. Understanding polarization involves breaking down the oscillation of light into it's horizontal and vertical planes. A good description of polarized light can be found in The Air Down There e-brochure and StoryMap Something New Under the Sun or for deeper dive into the concept check out the Ocean Optics Web Book .
For more information check out the Optical Constituents of the Ocean and the Remote Sensing sections of the Ocean Optics Web Book.