From a National Oceanography Centre, Southampton (NOCS) press release
Measuring carbon dioxide over the ocean
Reliable measurements of the air-sea flux of carbon dioxide – an important greenhouse gas – are needed for a better understanding of the impact of ocean-atmosphere interactions on climate. A new method developed by researchers at the National Oceanography Centre, Southampton (NOCS) working in collaboration with colleagues at the Bjerknes Center for Climate Research (Bergen, Norway) promises to make this task considerably easier.
Infrared gas sensors measure carbon dioxide based on its characteristic absorption spectra and are used to evaluate the air-sea flux of the gas¬¬. So-called closed-path sensors precondition air before measurements are made, while open-path sensors can be used to measure the air in situ.
One advantage of using open-path sensors at sea is that wind measurements can be taken contemporaneously in the same place. Moreover, because they are small and don’t use much power they can be used on buoys.
“Open-path sensors have the potential greatly to increase our understanding of the variability of air-sea carbon dioxide fluxes,” said John Prytherch of NOCS.
However, a long-standing concern has been that the values from open-path sensors do not tally with those from closed-path sensors, or with measurements made using other techniques.
“Other scientists have been sceptical about the reliability of carbon dioxide flux measurements taken at sea using open-path sensors,” says Prytherch: “However, we now believe that we understand the reason for the discrepancy and that we can correct for it.”
The problem turns out to be that the sensors are sensitive to humidity, meaning that fluctuations in the amount of water vapour in the sample air skew the carbon dioxide measurements. This is probably caused by salt particles on the sensor lens that absorb water.
Having identified the problem, Prytherch and his colleagues developed and rigorously tested a novel method for correcting the data for the cross-sensitivity to humidity.
Data were collected aboard the Norwegian weather ship Polarfront, equipped with a battery of instruments to measure wind speed, humidity and carbon dioxide. Even the motion of the ship was monitored.
The researchers noted that carbon dioxide fluxes calculated from open-path sensor data were clearly too high and affected by humidity. They were also very variable, suggesting that the effect is caused by salt on the optics, which accumulate before being washed off by rain. Indeed, the researchers were able to mimic this effect in the laboratory.
However, after correction using their newly developed method, the calculated carbon dioxide fluxes were in line with previous studies that used different sensors or techniques.
“This robust method opens the way for widespread use of open-path sensors for air-sea carbon dioxide flux estimation,” said Dr Margaret Yelland of NOCS: “This will greatly increase the information available on the transfer of carbon dioxide between the air and sea – information crucial for understanding how the interaction between the oceans and the atmosphere impacts climate.”
The work was supported by the United Kingdom’s Natural Environment Research Council and is part of the UK SOLAS project HiWASE (High Wind Air-Sea Exchanges).
The researchers are John Prytherch, Margaret Yelland, Robin Pascal and Bengamin Moat (NOCS), and Ingunn Skjelvan and Craig Neill (Bjerknes Center for Climate Research, Bergen, Norway).
Prytherch, J., et al. Direct Measurements of the CO2 flux over the ocean: development of a novel method. Geophys. Res. Lett. (published on-line, 2010) doi:10.1029/2009GL041482.
www.agu.org/journals/pip/gl/2009GL041482-pip.pdf
www.noc.soton.ac.uk/ooc/CRUISES/HiWASE/
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Computers on floating buoys use floating point numbers.
Ocean bottom sensors use sinking point numbers. And on ocean bottoms where the instruments are held in place fixed point numbers are appropriate.
Conversion algorithms for inter conversion will be necessary. If the conversions are round up ready a few conversions back and forth will show a steadily increasing temperature in no more time than it takes to run the algorithm.
And you know the result. The unprecedented warming is much worse than we thought.
There is some confusion here:
The measurements are about the sea-air CO2 flux, not about absolute CO2 levels. The CO2 levels in the atmosphere must be measured and the absolute humidity, as that gives the real pCO2 in the atmosphere. Also the pCO2 of the surface water must be measured, to see how high the driving force (pCO2 air vs. pCO2 water) is and in what direction the flow is. With wind speed and wave forms one can calculate the CO2 flux between air and oceans (whatever the direction), with still some large margin of errors.
Thus “sea-air” flux is not meant to be one-way, but simply that all CO2 movements are measured between sea and air or reverse. This is of direct interest for the carbon cycle models, to know where the main natural sources and sinks of CO2 are situated. Not of direct interest for AGW. Only indirect, if the oceans should decline in sink capacity, of which there is no sign until now.
The problem they have now is not directly with water vapor (which is measured), but with liquid water around salt crystals which interfere with the CO2/water vapor measurements. I have no idea what they use as compensation, but some extra measurement of an absorption line in the liquid water spectrum may be of help?
It’s funny that these “adjustments” to the data could be avoided if they could just come down off their high horse and ask NASCAR for some of their nifty rotating/self-cleaning camera protectors.
Git’er Done!
From what I gather, none of the “heavily used” CO2 measurement techniques actually measure CO2. Rather, they measure a “proxy signal” that, in every case, must be “corrected”. Corrected to what? Why corrected to “what we know the answer must be”.
Personally, I think the “official” CO2 measurements used to support AGW arguments are not to be trusted. They “may” be reasonable. However, how would we know — besides taking it on “faith”?
Allen63 (06:41:51) :
From what I gather, none of the “heavily used” CO2 measurement techniques actually measure CO2. Rather, they measure a “proxy signal” that, in every case, must be “corrected”. Corrected to what? Why corrected to “what we know the answer must be”.
Allen, it is not because the measurement is indirect, that it must be wrong. We don’t measure temperature directly, we measure it with a proxy: either the expansion of a liquid or the millivoltage of a thermocouple or the current through a resistor,… In all cases we need calibration of the instrument to known values. In the case of CO2 the absorption of a beam of IR at a certain frequency is calibrated with different calibration gases of known composition.
Once the calibration is done, one can calculate the composition of the atmosphere. No corrections are necessary. See e.g. the calibration procedures at Mauna Loa: http://www.esrl.noaa.gov/gmd/ccgg/about/co2_measurements.html
On buoys, similar procedures are practically impossible, therefore one need a different technique to compensate for water vapor and temperature changes. The results are less accurate than these at MLO, but that is less important for their purpose, which needs the differential partial pressure (pCO2) between the air and the ocean’s surface. In polar waters, that is around +240 microatm difference, while near the equator it is around -360 microatm. Thus a lot of CO2 is coming out the oceans near the equator, and a lot (more) sinks with the THC in the deep oceans near the North Pole.
Yesterday I was sitting on my local beach (not freezing here in Natal), contemplating the waves and wondering how much Co2 was hovering above the surface and what the sea was doing about sequestering it. I noticed that the sea doe not reach up its arms and grab the stuff, but that countless little organisms along the waterline were busy growing their shells, presumably sequestering CO2 to make calcium carbonate. I was quite unable to determine the level of CO2 available at that time, or to measure satisfactorily the metabolic rate of these diverse organisms. However, if some kind group would be willing to fund me (modest need for apparatus and rehydrating beverage) I would be happy enough to spend three or four afternoons a week down there, water vapour permitting, to take measurements, which, I am sure, would be very useful to NOCS. My hand may not be too steady, and my eyes are a little dim, but I am still a whiz bang expert at mathematical extrapolation. Any offers?
Ferdinand,
I agree with your comment that the “proxy” method does not mean the answer is wrong. And, the “official” CO2 measurement may, in fact, be accurate.
My concern is that “proxy” measurement of CO2 seems to be “confounded” by several factors (i.e. not as straight forward as the rise of mercury in a thermometer due to temperature — which is a simple physical situation that is easy to understand and feel confident about).
Its not clear to me that the confounding factors have been adequately addressed — to guarantee accuracy that adequately supports spending trillions of dollars/euros. The magnitude of the topic and the cost of solving AGW enters into my judgment regarding the adequacy of the measurement techniques.
And, as usual, in Science, its the person making the claim (in this case that they’ve got CO2 measurement figured out) that has to provide a clear explanation and logically sound proof.
So, while we can probably agree on the generalities of this topic, I am concerned that “official” historical CO2 measurements may not be that accurate once one digs into the details. However, I do not know.
Thanks for the response.
For a few hundred euro’s you may obtain a hand held CO2 measurement device with sufficient accuracy, but don’t drink before or while at work: one burp of CO2 from the beverage may give a “peak” of several tenthousands ppmv… And hold your breath for the same reason…
“”” Phil. (20:48:41) :
George E. Smith (16:30:44) :
Well not surprisingly, the article is about as uninformative as it could be. Not a mention of the operating wavelength range of the CO2 detector..
No real point in using the 15 micron CO2 band, since water vapor overlaps pretty much all of that to some extent, and other issues.
But CO2 also has a strong absorption line at around 3.5-4.0 microns, whcih is right in a water hole.
Not much overlap in the 15μm band really, there are plenty of lines to work with. Check out the spectra below: H2O top, CO2 below.
http://i302.photobucket.com/albums/nn107/Sprintstar400/H2OCO2.gif “””
Phil, thanks for the reference. Looks like a useful site to know. Of course it begs more questions than it answers; such as under exactly what physical conditions are these spectra calculated; and how would they be modified in a real atmosphere situation. But very cool place to know of.
Thanks again.
George
Allen63 (11:25:22) :
My concern is that “proxy” measurement of CO2 seems to be “confounded” by several factors (i.e. not as straight forward as the rise of mercury in a thermometer due to temperature — which is a simple physical situation that is easy to understand and feel confident about).
Indeed one need to be aware of confounding factors: While there is a nice logarithmic relationship between level of CO2 and IR absorption at certain wavelengths, other gases/vapors may interfere at the same wavelength. That is the case for N2O and some CFC’s, but these are present in such low quantities that the influence is negligible. The main problem is water vapor. In the case of fixed stations like Mauna Loa, water vapor is almost all freezed out at -70 C. And the pressure in the measurement chamber must be kept constant, as pressure/density differences influence the measurement too.
If these circumstances are kept within strict limits, the measurements have an accuracy of +/-0.2 ppmv. That is independently verified by inline and outside flask sampling which are measured in different laboratories (NOAA and Scripts) with different methods (again NDIR but also manometric).
The NDIR method nowadays is used worldwide for near all CO2 measurements. For special purposes GC and/or mass spectrometers are used, the latter if one also wants to know the isotopic composition.
Hmmm, I’m suspicious. Perhaps they are correct. But, they found the CO2 flux between ocean and atmosphere was “too high”?
Wouldn’t a higher than expected flux mean a much shorter residence time in the atmosphere for a CO2 molecule, which is contrary to AGW theory, which re
Hmmm, I’m suspicious. Perhaps they are correct. But, they found the CO2 flux between ocean and atmosphere was “too high”?
Wouldn’t a higher than expected flux mean a much shorter residence time in the atmosphere for a CO2 molecule, which is contrary to AGW theory, which requires that all man-made CO2 molecules hang around doing theoir evil work for hundreds of years.
So, going against “established theory” they must adjust the data; rather like the Argo sea temperatures.
Robert of Ottawa (17:08:27) :
Hmmm, I’m suspicious. Perhaps they are correct. But, they found the CO2 flux between ocean and atmosphere was “too high”?
It is about the local CO2 measurements in air, where the open path devices give higher CO2 values than the closed path devices. That should mean less influx to the atmosphere from where the oceans have a higher pCO2 (equator) and more outflux to the oceans at the poles (lower pCO2 in the oceans). But as this is a clear error, the figures don’t add up with what is measured in the atmosphere as global CO2 levels and sink rates: about halve of the 8 GtC emitted by humans is absorbed by oceans and vegetation.
The measurements are only important to know the partitioning in sink rate between oceans and vegetation and where most of the degassing and absorption in the oceans is situated. These are not important for the overall sink rate of extra CO2 in the atmosphere, which has an about 40 years half life time.
George E. Smith (13:54:44) :
“”” Phil. (20:48:41) :
George E. Smith (16:30:44) :
Well not surprisingly, the article is about as uninformative as it could be. Not a mention of the operating wavelength range of the CO2 detector..
No real point in using the 15 micron CO2 band, since water vapor overlaps pretty much all of that to some extent, and other issues.
But CO2 also has a strong absorption line at around 3.5-4.0 microns, whcih is right in a water hole.
Not much overlap in the 15μm band really, there are plenty of lines to work with. Check out the spectra below: H2O top, CO2 below.
http://i302.photobucket.com/albums/nn107/Sprintstar400/H2OCO2.gif “””
Phil, thanks for the reference. Looks like a useful site to know. Of course it begs more questions than it answers; such as under exactly what physical conditions are these spectra calculated; and how would they be modified in a real atmosphere situation. But very cool place to know of.
Thanks again.
You’re welcome George, using Spectracalc you can calculate spectra for any conditions you want. I don’t recall the conditions used in that case but it’s main purpose was to show the relative scarcity of H2O lines in that region os the spectrum. If you use MODTRAN (free) you can get an ‘real atmosphere’ view but without sufficient resolution to see the individual lines.