Using environmental spectroscopy and bio-optics to understand what we see

Yesterday, my mind was blown. A little chat with Steven Loiselle and I will forever think differently about the oceans. If our eyes had better resolution and the sun had a slightly different spatial intensity, oceans would appear purple! Loiselle took us into the crazy world of environmental spectroscopy and gave us the basics on solar radiation. What was supposed to be lecture on the bio-optics of the Great African Lakes actually turned into an intriguing and perhaps novel understanding of solar radiation, its affect on climate change, and ultimately everything we see around us.

He starts with carbon dioxide and gave us some info regarding green house gases and how there has been a rise in global temperatures due to changes in the concentration of atmospheric gases. When we think of greenhouse gases, we tend to think of CO₂ as the bad guy. Yes, it’s the primary gas responsible for green house effect enhancement but it’s a bit more complicated than that. When the sun’s radiation hits the planet, every entity is capable of absorbing, scattering, and transmitting radiation; but how much arrives and how much is emitted depends on a lot of things. Loiselle explains that how much radiation arrives is not linear with distance but squared, and theoretically, it’s a balance between what’s going in and what’s going out. Planck developed an equation that actually calculates spectral distribution of a blackbody using factors like wavelength, temperature, and a few constants.  See formula below:

While ozone absorbs much of the atmospheric radiation emitted from he sun, the rest of our ecosystem plays an equally important role into the manifestation of energy on Earth. Most oceans, for example, also absorb large amounts of radiation while land surfaces like deserts, glaciers, and poles are capable of reflecting it. Clouds, which are essentially condensed and frozen water particles, are responsible for scattering light. How these particles come together and their conditions, give rise to different properties that effect light scattering and how we visualize what’s around us. Have you ever asked yourself why the sky is blue? It’s because lights with smaller wavelengths (blues and purples) scatter more than colors with larger wavelengths. Colors like red simple pass through the atmosphere, and are not scattered as easily. When the sun sets, we see a red/orange glow because the sun is at a much greater distance from the earth, meaning there is a longer optical pathway for light. This optical pathway allows more lights to get scattered, including colors with larger wavelengths which would normally be difficult to scatter. If there were no atmosphere to scatter and reflect light, then everything would be black with the exception of the sun, which would appear white/yellow.

How water particles come together and their conditions give rise to different properties that effect light scattering and how we visualize what’s around us.

Forget the atmosphere though, Loiselle’s work was mostly on bodies of water. He tells us there are four major things that are responsible for absorbing and scattering incident solar radiation in water: tripton, phytoplankton, water, and chromopheric dissolved organic matter (CDOM). A closer look at water and we find that although the water we pour in our glasses to drink is transparent, the water in the ocean appears blue. Why? Well, when the sun’s white light penetrates the surface of the water and hits the white sand at the bottom of the ocean, light scatters and we see a dark rich blue color. In reality, this means that everything except this blue color is being absorbed. Environmental spectroscopy, however, shows the wavelength of light least absorbed in the ocean is purple! The oceans appears blue to us because the our eye pigments have better resolution to blue-like colors, and they only have the capacity to see colors of the ocean around this wavelength. This also depends on the spectral intensity of the light source (the sun), which does not have equal proportions of violet, blue, green and red. Putting this in context and we can also conclude that snow is actually more blue than white because spec analysis shows that it absorbs in red and transmits in blue.

Wavelength least absorbed in the ocean is purple

Using this fundamental understanding and technique, we can measure how far and how much water was been polluted by measuring the optical input of biology and chemistry in water. Loiselle looks at water using 1) in situ measurements and 2) temporal spatial satellites to measure what’s being absorbed, scattered, reflected and emitted from the surface of the water. By determining the extension of pollution in lakes, they can see how lake behavior changes and ultimately what affect this may have on climate. Any variance in the expected behavior of lakes and it can affect hydrology, vegetation, biodiversity, currents, trade winds, rain patterns, ect. The list can go on and on.

Regarding modern day application, using daily satellites has allowed Loiselle to create “modes” of behavior for lakes that serve as a map for regions of the earth and bodies of water that are more sensitive to climate change. If the concentration of some chemical gets too high or the lake becomes polluted to the point where it may negatively affect the climate of surrounding area, they can work in cooperation with government to stop water usage/flow, thereby preventing any of the aforementioned consequences. The Great African Lakes Loiselle has studied amount to about ¼ of the Earth freshwater source, and sitting idle is not worth the potential risk. If the technology exists to monitor changes in such an influential part of the planet, we shouldn't hesitate take to advantage of it. Let’s do our part.

As a student, I have absorbed this info and am ready to emit it back to my peers next week. My only hope is that it scatters enough so that everyone in the class enjoys the presentation and walks away seeing things in a different "light". After all, the ocean IS purple... right?

Sources: Steven Loiselle, his lecture, and his powerpoint (for images).