May 8: Optics Day 1

Bonnie and I went out today to do some optics measurements at a location close to the sampling site on the Chukchi Sea. We also recruited the BARC Senior Scientist, Karl Newyear, to help us out.

Bonnie wanted to measure the following: the relative amount of sunlight reflected by the snow (albedo), the relative amount of sunlight transmitted through the snow and ice at the bottom of the sea ice (light transmittance), and the amount of sunlight that passes through the snow and ice at regular intervals down a core hole (vertical light profile).

Bonnie and Karl setting up the optical instruments. Photo by Julianne

In the photo above, you can see the spectrophotometer in its casing (not directly visible but protected inside the gray box), a laptop computer, and the albedometer (the attachment at the end of the long pole mounted on a tripod). What do all these instruments do? And why do the measures taken by these instruments matter?

To begin, it’s useful to make a distinction between spectral data and broadband data. Spectral data are collected with a spectroradiometer, which is an instrument that collects light at different wavelengths and reports each wavelength separately. Broadband data are collected with a variety of different instruments, but in this case, we are using a PAR sensor. PAR stands for “photosynthetically available radiation”. Below Bonnie makes some adjustments to her spectrophotometer, a specialized variation on the spectroradiometer.

Squinting in the sun. Not easy to read computer screens in bright sunlight! Photo by Julianne

To capture the total amount of light capable of heating a surface, one measures the light incident on a flat plate, also known as the “planar irradiance.” This measure is what counts for understanding how sunlight heats a surface. This type of sensor is sensitive to the angle of the light– light coming from directly overhead heats more effectively than light that comes from the horizon. This is part of the reason why the sun feels a lot warmer at noon than it does early or late in the day. In fact, this distribution of sunlight follows the cosine of the zenith angle of the sun (measured from overhead). Sensors that measure this quantity are called “cosine collectors”.

It’s also useful to measure the “scalar irradiance,” particularly under the ice. The scalar irradiance is a good measure of the light available for photosynthesis. It is irrespective of direction, since photosynthetic organisms can collect photons equally from all directions. It also depends on the number of photons received, and does not respond to the energy of different wavelengths of light (shorter wavelength light is more energetic than long wavelength light– think x-rays compared to radio waves).

Bonnie and the albedometer. Photo by Julianne

Having set up the albedometer, spectrophotometer, and computer, Bonnie could measure the albedo of the snowy surface. For each measurement, we first turned the apparatus at the end of the albedometer, a.k.a., the cosine collector, towards the sky, and then rotated it down to face the ground. Each pair of measurements provides a ratio of the amount of sunlight reaching the surface versus the amount of sunlight reflected. In total, we took ten pairs of readings, which is easy to lose track of! Karl devised an ingenious way of keeping track with what he could find around him.

Karl’s ingenious method for record-keeping. Photo by Julianne

To measure the transmitted light at the bottom of the sea ice, Bonnie drilled a core hole. Through this hole, she could lower a folding arm fitted with different optics sensors.

Bonnie versus Sea Ice. Photo by Julianne

As you can see here, Karl is holding up the array of sensors. Perhaps it’s an offering to please the Guardians of Science and bestow good fortune on the Rotten Ice Team?

Offerings to the Sea Ice Divinities! Photo by Julianne

Sun dogs. Photo by Julianne

I took this picture just outside of BARC. Here, you can see more neat optical phenomena in the Arctic, known as sun dogs, phantom suns, or parhelia. As sunlight passes through the Arctic atmosphere, it “catches” on ice crystals to produce these brilliant optical phenomena, which are best seen when the sun is close to the horizon. While there may no longer be any sunsets to admire in Barrow (the sun won’t set again beginning May 11 until August 2), there is plenty else to behold!

– Julianne and Bonnie

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