海-气CO2通量算法说明 - 海洋遥感在线分析平台

We adopt the commonly used method to calculate the air-sea CO₂ flux, which is based on the multiplication of CO₂ partial pressure difference between surface sea water and atmosphere and CO₂ gas transfer velocity. The equation is as follows:
Flux=∆pCO₂×E=WCP-ACP×KHCO2×ρ×C2×k´24×10-2
where,
1）ACP: 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)
2）KHCO2——Dissolution efficient of CO₂(mol·kg^-1·atm^-1)
ln(K_H^CO2) = -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)
ρ=(ρ_w+A*S+B*S^3/2+C*S²)*10^-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；
A=0.824493-4.0899*10^-3*SST+7.6438*10^-5*SST²-8.2467*10^-7*SST³+5.3875*10^-9*SST⁴；
B=-5.72466*10^-3+1.0227*10^-4*SST-1.6546*10^-6*SST²；
C=4.8314*10^-4
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.
U_j is high-frequency satellite-derived wind speed (e.g. daily), and U_mean is satellite-derived monthly average wind speed, both with unit of m·s^-1.
5）k—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×u₁₀²(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.
Reference
[1]     Bai, Y., Cai, W.-J., He, X., Zhai, W., Pan, D., Dai, M., Yu, P., 2015. A mechanistic semi‐analytical method for remotely sensing sea surface pCO₂ in river‐dominated coastal oceans: A case study from the East China Sea. Journal of Geophysical Research : Oceans, 120, doi:10.1002/2014JC010632.
[2]     Song, X., Bai, Y., Cai, W.-J., Chen, C.-T. A., Pan, D., He, X., Zhu Q., 2016, Remote Sensing of Sea Surface pCO₂ in the Bering Sea in Summer Based on a Mechanistic Semi-Analytical Algorithm (MeSAA), Remote Sensing, 8, 558; doi:10.3390/rs8070558 .
[3]     Ho, D.T., Law, C.S., Smith, M.J., Schlosser, P., Harvey, M., Hill, P., 2006. Measurements of air-sea gas exchange at high wind speeds in the Southern Ocean: Implications for global parameterizations. Geophysical Research Letters, 33(16): L16611, doi:10.1029/2006GL026817.
[4]     Liss, P.S. and Merlivat, L., 1986. Air-sea gas exchange rates: introduction and synthesis. In: P. Buat-Menard (Editor), The Role of Air-Sea Exchange in Geochemical Cycling. Reidel, Hingham, MA, pp. 113-129.
[5]     Millero, 2013, Chemical Oceanography (4th Edition), CRC Press, Taylor & Francis Group, International Standard Book Number-13: 978-1-4665-1255-9 (eBook - PDF).
[6]     Sweeney, C., Gloor, E., Jacobson, A. R., Key, R. M., McKinley, G., Sarmiento, J. L., Wanninkhof, R., 2007. Constraining global air-sea gas exchange for CO₂ with recent bomb C-14 measurements. Global Biogeochemical Cycles, 21(2): GB2015, doi:10.1029/2006GB002784.
[7]     Wanninkhof, R., 1992. Relationship between wind speed and gas exchange over the ocean. Journal of Geophysical Research, 97(C05): 7373-7382.
[8]     Wanninkhof, R., Asher, W.E., Ho, D.T., Sweeney, C. and McGillis, W.R., 2009. Advances in quantifying air-sea gas exchange and environmental forcing, Annual Review of Marine Science. Annual Review of Marine Science, pp. 213-244.
[9]     Wanninkhof, R., Doney, S. C., Takahashi, T., and McGillis, W.: The effect of using time-averaged winds on regional air-sea CO₂ fluxes, in: Gas Transfer at Water Surfaces, edited by: Donelan, M., Geophys. Monogr. Ser., Amrican Geophysical Union,Washington, D.C., 127, doi:10.1029/GM127p0351, 351–357, 2002.
[10] Weiss, E.F. and Price, R.A., 1980. Nitrous oxide solubility in water and sea water. Marine Chemistry, 8: 347-359.

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