acid-base system of the Baltic Sea: A pilot study

Karol Kuliński

, Bernd Schneider

, Karoline Hammer

, Detlef Schulz Bull

a)

Institute of Oceanology of the Polish Academy of Sciences, Sopot, Poland

b)

Leibniz Institute for Baltic Sea Research, Warnemünde, Germany

The influence of dissolved organic matter on the acid-base system of the Baltic Sea: A pilot study

IOW Science Symposium 2015 Little Salt and Many Protons:

Acid-Base System Studies in the Baltic Sea

Seawater acid-base system

The measurable parameters:

o C_T – total CO₂ concentration (DIC) o A_T – total alkalinity

o pCO₂ – partial pressure of CO₂ o pH

It is possible to calculate 2 parameters when the following is known:

o other 2 parameters o temperature & salinity

o equilibrium constants for each of the acid dissociation reactions o total concentrations for each non-CO₂ substances

The pairs used in the calculations:

o C_T & A_T – recommended, used in biogeochemical modelling o A_T & pH – measured within the monitoring programs

bases]

[minor ...

- ] [H ] [OH 4 ]

[B(OH) 2 ]

2[CO 3 3 ]

T [HCO

A

The total alkalinity of seawater is defined as the excess of proton acceptors (bases formed from weak acids with a dissociation constant K≤10^-4.5 at 25ºC) over proton donors (acids with K>10^-4.5) and expressed as a hydrogen ion equivalent in one kilogram of sample (Dickson, 1981):

DOM

Organic term is not included in the thermodynamic model of a seawater

A org inorganic

T A

A

Functional groups in DOM

hypothetical structure of humic like substances

A org ...

- ] [H ]

[OH 4 ]

[B(OH) 2 ]

2[CO 3 3 ]

T [HCO

A

The total alkalinity of seawater is defined as the excess of proton acceptors (bases formed from weak acids with a dissociation constant K≤10^-4.5 at 25ºC) over proton donors (acids with K>10^-4.5) and expressed as a hydrogen ion equivalent in one kilogram of sample (Dickson, 1981):

A org inorganic

T A

A

HCl

seawater

2 ]

[CO 2 ]

[CO 3 3 ]

T [HCO

C

r/v Meteor cruise, November 2011 Database

• C

, A

, pH, pCO

Influence of A

on the calculations of pCO

and pH

Arkona Basin Gulf of Bothnia

A_T measured – A_T calculated from C_T and pH A_T measured – A_T calculated from C_Tand pCO₂

Influence of A

on the calculations of pCO

and pH

20 30 40 50 60

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

ΔA T[μmol kg-1 ]

Sampling station

A

– A

calculated from C

and pH or C

and pCO

A

is the difference between measured and calculated A

(ΔA

)

A

= A

+ A

A

= A

- A

Arkona Basin Gulf of Bothnia

A_T measured – A_T calculated from C_T and pH A_T measured – A_T calculated from C_Tand pCO₂

Influence of A

on the calculations of pCO

and pH

20 30 40 50 60

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

ΔA org= ΔA T[μmol kg-1 ]

Sampling station

Influence of A

on the calculations of pCO

and pH

100 200 300 400 500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

pCO 2[µatm]

Sampling station

Arkona Basin Gulf of Bothnia

pCO₂ measured

pCO₂calculated from A_T and pH pCO₂ calculated from A_T and C_T

Arkona Basin Gulf of Bothnia 7,7

7,8 7,9 8,0 8,1 8,2 8,3 8,4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

pH

Sampling station

pH measured

pH calculated from A_T and pCO₂ pH calculated from A_T and C_T

Influence of A

on the calculations of pCO

and pH

(In)consistency of the acid-base system parameters

Org - H

HOrg   

[HOrg]

- ] [Org ]

K _a [H  



Monoprotic acid dissociation

The bulk DOM dissociation constant – K

org org

A -

DOC) (f

A ]

K _DOM [H



 



[H

] – calculated from pH A

– organic alkalinity

DOC – well described method

f – share of DOC providing functional groups

f = 0.12

pK _DOM = 7.34

Conclusions

• A

term is missing in the A

model

• A

is the difference between measured and calculated A

.

• A

was found in the range 25-35 µmol kg

in the Baltic Sea water.

• Ignoring the DOM component in A

model causes significant uncertainty of pH and pCO

in numerical studies, especially for the input data of A

and C

.

• Some 12% of DOC carry the functional groups dissociating in seawater. The pK

in the Baltic Sea water amounts to 7.34

• Tests of „K

” approach in numerical studies are required

• Further studies on DOM acid-base properties are required.

Thank you