Marine Carbon Fluxes
Marine Carbon
We adopted the commonly used method to calculate air-sea CO2 flux, which is based on the multiplication of CO2 partial pressure differences between surface seawater and atmosphere and CO2 gas transfer velocity.The equation is as follows:
1ACP: partial pressure of CO2 in atmosphere (μatm)
WCP: wpartial pressure of CO2 in seawater(μatm)
SST: sea surface temperature (oC)
SSS: sea surface salinity (psu)
SSW: sea surface wind speed (m/s)
2KHCO2——Dissolution efficient of CO2(mol·kg-1·atm-1)
ln(KHCO2) = -60.2409 + 93.4517×(100/T) + 23.3585×ln(T/100) + SSS×[0.023517 - 0.023656×(T/100) + 0.0047036×(T/100)2];
T = SST (°C) + 273.15
3)ρ——Sea water density, which can be calculated by the function of surface water temperature and salinity(Millero, 2013)(kg·m-3)
ρw=999.842594+6.793952*10-2*SST-9.09529*10-3*SST2+1.001685*10-4*SST3-1.120083*10-6* SST4+6.536332*10-9* SST5
4)C2——Wind speed coefficient C2, which has been calculated and uploaded in the SOED database
To calculate the monthly average flux, it is often necessary to consider the influence of the high-frequency wind speed change (e.g. daily) on the monthly average wind speed, using a coefficient of C2 (Wanninkhof, 2002).The C2 coefficient is not needed when calculate daily flux.
Uj is high-frequency satellite-derived wind speed (e.g. daily), and Umean is satellite-derived monthly average wind speed, both with unit of m·s-1.
5k—Gas transfer velocity(cm·h-1)
Based on the relationship between gas transfer velocity (k) and the wind speed at 10m above sea level(U10), the commonly used equations for calculating k are shown in the table below.

No. Equation References
1 k660 = 0.31×u102(Instantaneous wind speed)
k660 = 0.39×u10(Long-term average wind speed)
Wanninkhof (1992)( Instantaneous)
Wanninkhof (1992)( Long-term)
2 k660 = 0.27×u102 Sweeney et al. (2007)
3 k600 = 0.266×u102 Ho et al. (2006)
4 k660 = 0.24×u102 Wanninkhof et al. (2009)
5 k600 = 0.17×u10 (u10< 3.6 m/s) Liss and Merlivat (1986)
6 k660 = 0.0283×u103 Wanninkhof and McGillis (1999)
7 k600 = 2.85×u10 -9.65 (3.6 < u10< 13 m/s) Liss and Merlivat (1986)
8 k600 = 5.9×u10 -49.3 (u10> 13 m/s) Liss and Merlivat (1986)
k660 and k600 mean the k with the the Schmidt number (Sc) of 660 and 600, respectively.
k = k600 × (Sc/600)-0.5 and k = k660 × (Sc/660)-0.5
When sea suface temperature (sst) ranging 0-30℃, the Sc can be calculated with the following equation(Wanninkhof, 1992),
Sc = 2073.1−125.62×sst + 3.6276×sst2−0.043219×sst3
where, sst is the sea suface temperature, with the unit of oC.
For the calculation of the satellite-derived air-sea CO2 flux in the Open Ocean, the k-u10equation of #1 (long-term wind speed) and #2 in the above table are commonly used.
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