Southern Ocean GasEx Blog

Dispatches from the Southern Ocean Gas Exchange Experiment

Tiny Bubbles

Posted by sogasex on April 7, 2008

By Carlos Del Castillo, The Johns Hopkins University-APL

The loud popping sound was immediately followed by pressurized 4°C seawater being sprayed all over the room. We are working inside the wet lab on board the NOAA Ship Ronald H. Brown (see Richard’s blog entry) and one of the clean seawater lines that feeds our instruments just burst. A high-pressure water line does not just burst and calmly spills water. The line swings left and right, up and down, squirting water on everything and everyone. But no worries, we are in the wet lab. It is supposed to be wet. Before the indoor shower, we had settled into an easy, boring routine for our long transit to the proposed research site, so the burst line was almost a welcomed distraction. Almost welcomed because a busted line means some data will be lost, and the inevitable invasion of air bubbles into our system. We do not like bubbles in the wet lab. Air bubbles dramatically change the optical properties of water and create a lot of noise in our data. Bubbles must be dealt with. Bubbles are the enemy. We battle bubbles along three fronts. The water that flows through our optical instruments enters the boat through an intake that is several meters below the sea surface. There are not many bubbles at this depth unless the weather is bad. Weather is almost always bad in the Southern Ocean. The second line of defense is a “debubbler.” This plastic contraption uses a vortex to trap bubbles and send them back to the ocean – where they belong- while tunneling bubble-free water to our instruments. Bubble-free water is good. In our quest for bubble free water we keep all the lines that feed the instruments submerged in a water bath as our third line of defense. By doing this, we keep the water inside the lines very cold to avoid degassing– or the formation of un-welcomed bubbles that will eventually migrate to our instruments. In this case, the water bath is a large sink where we also keep the instruments to avoid temperature fluctuations. The water in the bath is the same 4°C seawater that flows through the instruments.

In this expedition we encountered our first un-welcomed bubbles in bottled water. As in most countries, bottle water in Chile can be found in two varieties, sparkling water and regular water, or “agua con gas y agua sin gas.” Sparkling water seems to be the most popular and the default offering unless otherwise specified. So, if one does not add the “sin gas” modifiers, one may get bubbles. Agua con gas is not all that bad, we are just not used to it. The wet lab gang prefers to drink our bubbles with beer.

Our instruments in the wet lab measure several parameters. We have two acs’s (absorption, attenuation, spectral) that measure light absorption and attenuation from ~400 through ~700 nm at 4 nm resolution. These are the successors to the veritable WET Labs ac9 (same measurements but at 9 wavelengths). Light attenuation is measured along a fixed path length and represents the loss of incident light due to light absorption by chromophores (i.e. colored dissolved organic matter –CDOM- and photopigments), and losses due to light scattered away from a narrow detection angle. The absorption measurements include incident light losses due only to absorption by chromophores. The measurements are achieved by using two cells, or in this case plastic flow through cylinders. The attenuation tube (c) has an opaque inner wall so that scattered light is absorbed by the tube and counted as light loss. The absorption tube (a) has a highly reflective inner wall so that light scattered forward and away from the direction of the incident light field is not absorbed by the inner walls and reaches the detector. The cell in this case works as a waveguide. Clearly, backscattered light is lost, but most of the light scattering is forward scattering. In addition we have a Turner Designs C-6 fluorometer that measures the fluorescence of CDOM and phytoplankton, and a ctd that measures salinity and water temperature.

The color of a substance is an expression of its chemical characteristics. In the case of seawater, its optical properties – or its color – can give us information about the concentrations of chlorophyll and organic matter in seawater. These measurements are very important to further our understanding of the carbon cycle. Our instruments only measure these parameters along the thin line that is the track of the research vessel. However, several NASA research satellites are equipped with ocean color sensors that provide daily coverage over the globe. The data provided by these satellites are essential to our understanding of the global carbon budget and climate change. Data from these sensors, however, has to be interpreted using complex mathematical algorithms. These algorithms are created and validated using field data like the data provided by our instruments in the wet lab. Curiously enough, satellite ocean color sensors can be affected by bubbles. White caps (or “espuma”) formed in the ocean when winds exceed ~ 14 knots, are nothing more than bubbles at the air-sea interface. White caps change the optical properties of surface waters making it more difficult for the satellites to detect the true color of the ocean. Also, bubbles injected into the water column by large braking waves interfere with satellite color measurements. Again, bubble-free water is good.

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So, here we are in our wet lab, happily bubble free and drinking liquids without gas – at least until the next water line bursts.

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The tangle of hoses that is our underway system

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Scott Freeman (standing on the right) and Carlos Del Castillo (with the funny hat) calibrating one of the acs’s using pure water.

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Carlos Del Castillo cleaning the interior of one of the optical tubes of an acs.

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