PACE – It All Adds Up!
Breaking Down Light Makes it All Add Up
A photon of light from the sun (along with millions of its friends) slams into science sensors onboard the PACE satellite 676.5 km (420 mi) above the Earth. This may not seem like a significant event, but we can use information from these photons to learn more about our ocean and atmosphere. We do this by using various tools and algorithms to retrace the steps of photons along paths through the Earth system to the satellite. Some travel only through Earth's atmosphere, some reflect off the surface of the ocean or land, and a small fraction travel into the ocean before making their way back through the atmosphere to the PACE satellite. For ocean studies, we must isolate this water-leaving component of the light that reaches the satellite, which makes up only about 10% of the total light the satellite receives.
But how can we use light to learn about ocean health? Or learn about tiny particles in the atmosphere that can affect our own health, known as aerosols? The answer lies in understanding radiative transfer.
Radiative transfer helps us understand how sunlight interacts with the atmosphere's and ocean's optical constituents (e.g., clouds, air pollution, breaking waves on the sea surface, phytoplankton or sediment in the water, etc.). In other words, we can connect various puzzle pieces together to determine what kind of "stuff" in Earth's ocean and atmosphere interacted with the photons of sunlight before it reached the PACE satellite.
For a closer look at exactly what "stuff" is interacting with light, check out How Does the Atmosphere Interact With Light? and How Does the Ocean Interact With Light?
In the following sections, we will skim the surface of radiative transfer and its related equations that define the field of ocean optics. Why? These provide a critical foundation for analyzing the data collected by the PACE Satellite. If you would like to take a deeper dive into this field, check out Mobley (2022) “The Ocean Optics Book ."
Radiative Transfer – The Ins and Outs
Processes that change either the energy or wavelength of light as it passes through a medium such as air or water are covered by radiative transfer. At the core of radiative transfer is a set of equations that merge these processes to identify the net change in radiance (i.e., incoming sunlight) as light moves through a medium (i.e., the ocean or atmosphere). It's complicated because the same process can result in both losses and gains in radiance along any light path. For example, losses occur when light is scattered out of the radiant path or is absorbed within the radiant path. Similarly, gains occur when light is scattered into the radiant path.
How much light is scattered and absorbed – including at which wavelengths – defines the optical properties of a medium (e.g., sea water) or an object (e.g., a banana). Furthermore, optical properties fall into two categories – Inherent and Apparent.
Inherent Optical Properties (IOPs) are absorption and scattering properties of a medium or an object that rely on its physical and chemical makeup. To put it another way, IOPs are how light is absorbed and/or scattered at different wavelengths (colors) by the medium or object. These properties rely solely on the medium or object itself, independent of ambient light conditions. Is a banana in the dark still yellow? Yes, it is inherently yellow! Unless it’s not ripe, of course ;)
Apparent Optical Properties (AOPs) are, on the other hand, dependent on the properties of the medium or object as well as the incoming light. So in terms of its AOP, a banana would appear yellow in a bright room, while in a dark room it would appear black. Q: “There is apparently no banana in here?” A: “Turn on the light switch and see for yourself!”
At its essence, radiative transfer seeks to relate AOPs to IOPs. Why is that important? It allows us to take a key AOP estimate known as Remote Sensing Reflectance to infer IOPs such as the absorption of light by phytoplankton. This can then be used to infer the rate of photosynthesis and associated carbon fixation in the ocean, key objectives of the PACE mission.
To do this, we need an understanding of Radiometric Variables, that is how much sunlight is reaching an area and at which wavelengths. As mentioned above, these measurements are important for calculating AOPs. Radiometric variables will change depending on where you are measuring them – at the top of the atmosphere, at the sea surface, below the sea surface.
Boundary Conditions are another important piece of the puzzle. For example, they tell us how light is reflected and transmitted by the surfaces bounding our medium of interest. So for the ocean this would be the sea surface (e.g., flat calm versus choppy waves) to the sea floor (e.g., reflective sand versus dark vegetation) and for the atmosphere, this would be the boundary between open space and the top of the atmosphere to the sea surface.
This diagram depicts how key pieces of radiative transfer line up. The arrows at left and right indicate that, in order for the central equations of radiative transfer to be useful for satellite measurements, it must be applicable in two directions.
Heading from top to bottom, the equations allow us to build “forward models” that derive the radiance within a medium from known IOPs and boundary conditions. How do we know the suite of possible IOPs and boundary conditions found in nature? By collecting data in the laboratory (e.g., growing cultures of phytoplankton and measuring their shapes, sizes, and colors) and in situ (e.g., instruments in the ocean that measure how sunlight varies with depth). From these types of investigations and forward model results, we can build data lookup tables, arrays of related numbers that save computing time.
The opposite direction – from bottom to top – represents how observations (AOPs) collected by PACE will be applied to understand IOPs in the ocean and atmosphere – what is present and how much. Another way we can think about this is by mapping the components of radiative transfer back onto our diagram of light reaching the PACE satellite. In this way, we see that the light that reaches the PACE satellite represents an AOP (i.e., Remote Sensing Reflectance) that can then be used to retrieve component IOPs - for example any bananas in the ocean or atmosphere - while taking into consideration boundary conditions, such as sunlight reflecting off the surface. In reality we won’t be looking for bananas, instead these IOPs will focus on Phytoplankton, Aerosols, Clouds, and ocean Ecosystems… as "advertised" in the PACE acronym!
Now that you're familiar with the overall flow of radiative transfer, we will discuss each of its elements, how they fit together, and what they help us understand about our ocean and atmosphere. For a more detailed description and equations, check out the Introduction to the Ocean Optics Web Book .
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