You can use pH and TAlk (Total Alkalinity) to computed DIC (Dissolved inorganic carbon) and then pCO2 (partial pressure of CO2). This indirect method can be easily used in any field sampling condition and it is far less expensive and more easy to set up than the direct one (using the IR analysers). TAlk is measured by Gran electrotitration. pH is measured with a combined electrode. In seawater, the electrode is calibrated on the Total Hydrogen Ion Concentration Scale using the buffers prepared according to Dickson (1993). In contrast, in estuaries and freshwater environments, the electrode is calibrated on the National Bureau of Standards (NBS) (Frankignoulle and Borges, 2001). I particularly use this method to get CO2 data in coastal environments (e.g. lagoons, mangroves, ...) and in rivers.

Note that in practice, the pH of samples is computed using the Nernst equation.

You can use pH and TAlk (Total Alkalinity) to computed DIC (Dissolved inorganic carbon) and then pCO2 (partial pressure of CO2). This indirect method can be easily used in any field sampling condition and it is far less expensive and more easy to set up than the direct one (using the IR analysers). TAlk is measured by Gran electrotitration. pH is measured with a combined electrode. In seawater, the electrode is calibrated on the Total Hydrogen Ion Concentration Scale using the buffers prepared according to Dickson (1993). In contrast, in estuaries and freshwater environments, the electrode is calibrated on the National Bureau of Standards (NBS) (Frankignoulle and Borges, 2001). I particularly use this method to get CO2 data in coastal environments (e.g. lagoons, mangroves, ...) and in rivers.

Note that in practice, the pH of samples is computed using the Nernst equation.

You do not clarify important practical aspects: will you need it for taken-home samples, for continuous monitoring from a boat... or what?

Mathieu already (briefly) mentioned IRGAs. If continuos monitoring is your aim, this is a good (although expensive) way to go. Have a look at this Application Note by LiCor (a leading company in the business):

http://www.kohsieh.com.tw/PDF_Files/LICOR/pCO2ApplicationNote.pdf

There are references there, which can be useful to you.

Hope it helped.

Best, Andrea

@kone Thanks. are you using CO2Sys for calculation.

@ Andrea these are in situ measurements. I usually do a insitu pH and total alkalinity is determined in lab, teh sample is carried on ice to lab.

Dear saif,

Mathieu and Andrea already answered, you can use measured salinity, temperature, nutrients (phosphate and silicate), pH and DIC. If you need further more details please see the (Millero et al., 2006) and CO2 sys program (Lewis and Wallace, 1998). The second thing is that, underway pCO2 measurement system. This system in seawater, continuously measure using shower type equilibrator and non-dispersive Infra Red (NDIR) detector.

Thanks Naidu can you please share specs of NDIR seawater alkalinity measurement system.

You should check the ocean acidification best practice guide. it covers methods (e.g. correct pH measurements, methods for alkalinity, etc.) for chemistry as well as biology:

http://www.epoca-project.eu/index.php/guide-to-best-practices-for-ocean-acidification-research-and-data-reporting.html

Jason Waters' recent dissertation has a nice discussion of pros and cons of various methods of measuring parameters of the CO2 system:

http://scholarlyrepository.miami.edu/cgi/viewcontent.cgi?article=1903&context=oa_dissertations

As others have mentioned, CO2SYS is a very useful tool to calculate unmeasured parameters from the selected parameters that are measured. Here is the official version of CO2SYS:

http://cdiac.ornl.gov/oceans/co2rprt.html

Here is link to another Excel version of CO2SYS:

http://www.ecy.wa.gov/programs/eap/models.html

@Sam thanks am aware of the document

@Greg Thanks will be very happy to go through it.

See pages 15-16 of Waters for discussion of best CO2 system parameters to measure. Here is an excerpt about potential accuracy of fCO2 calculation from measurement of pH and either TA or DIC citing Millero 2007:

"Calculated fCO2 can also be determined with high accuracy, to within ∼ 2 uatm, from pH and either TA or DIC [Millero, 2007].

You can find useful information about some in situ pCO2 analyzers at :

http://www.act-us.info/evaluations.php#pco2

Thanks @Greg and @Pascale

Your question is the the topic of carbone cycle inside the ocean

For my test - better to ask people who measure fluxes across the

sea-air interface They know such a problems You can ask Int Oceanology

Russian Academy

Regards

Hi Saif,

Where are you thinking of working? Calculated pCO2 data from DIC/TA measurements are typically very accurate in open ocean and some coastal settings, but you can run into trouble in areas that experience a high influx of fresh water. In places like estuaries you can get moderate levels of organics that contribute alkalinity, and this results in a divergence between calculated and directly measured pCO2. The same sort of thing can happen in coastal areas impacted by glacier melt, which can have a high particle load of carbonates. Segr et al (Tellus, 2011, 63B, 815-822) have a nice paper that, in part, discusses this problem. The direct measurement of pCO2 is the way to go, and reliable systems are getting cheaper (see SuperCO2 from Sunburst Sensors).

Wiley

Wiley I am going to use it in Persian Gulf. The fresh water inpur is from three rivers but is not huge. I am currently using the flow injection system for total alkalinity determination. If you are interested can share the results obtained till now.