Photochemical Exposures: The Scientist's Tanning Booth

Quartz tubes on the back deck

There are two ways to study the role of photochemistry in seawater. Both require comparing dark samples to those exposed to sunlight.  Dark is easy.  Wrap your bottle in aluminum foil and wait.  The sun part can be as easy as setting quartz tubes with your water out on the deck on a sunny day.  But with weather in the Gulf of Alaska  (think about the TV Show, "Deadliest Catch"), depending on sunny days could leave you floating in the middle of the ocean on a VERY expensive research vessel with nothing to do.  Not good. 

To ensure a productive photochemical cruise, we are using two Solar Simulators.  These have a constant radiation output from a 1500W Xenon lamp that is similar to the solar spectrum in the energetic UV wavelengths most important for the reactions we study (take a look at the spectra in Leanne's previous post).  The steady output can be as great as 3 times the intensity of noon summer sun which makes experiments go 3 times faster than real sunlight... more experiments = more data = more "bang for the buck" on a cruise... and there are no clouds, nighttime, or other variables that make light hard to quantify. They also allow extremely careful characterization of the photons absorbed by our samples.  From this we can be very quantitative about the rate of photochemical production in seawater. Carbon monoxide, hydrogen peroxide, and superoxide (molecular oxygen with an extra electron supplied by light-activated organic carbon molecules) are what we are measuring this trip, but similar study of other things like carbon dioxide, and other gases need the same defined light field.

Our plan is to develop quantitative comparisons between photochemical rates for the Dissolved Organic Carbon (DOC) in the deep ocean and that found at the surface.  This will help us define the global lifetime of DOC in the ocean, and predict the relation of this huge pool to the pool of CO2 in the atmosphere.  We are generating piles of data with the "Sun" shining 24/7 in our two boxes.  Time and a great deal of post-cruise analysis and thought will let us know what new and important stories will emerge.  We are excited by the prospects.

- Bill

Water-jacketed beaker under the Solar Simulator with sampling pump for real-time superoxide analysis.