Penelitian Masalah Lingkungan di Indonesia 2010 ... - AR As-syakur

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Jun 12, 2011 ... Abstrak: Peningkatan karbondioksida (CO2) di atmosfer yang berpotensi ... manusia dari pembakaran bahan bakar fosil dan proses industri.
CO2 Flux in Indonesian Sea (N.W. Ekayanti)

PERHITUNGAN FLUKTUASI CO2 DI LAUT INDONESIA MENGGUNAKAN ALGORITMA NIGHTINGALE CO2 FLUX IN INDONESIAN SEA DETERMINE BY NIGHTINGALE ALGORITHM N. W. Ekayanti1) and A. R. As-syakur2) Technical High School (SMK) of Tampaksiring, Physic Department, Gianyar, Bali 1,2) Center for Remote Sensing and Ocean Science (CReSOS), Udayana University, Bali 2) Environmental Research Center (PPLH), Udayana University, Bali Email: 1)[email protected]; 2)[email protected] 1)

dikirim 3 Agustus 2010, diterima setelah perbaikan 3 Maret 2011 Abstract: The increase of atmospheric carbon dioxide (CO2) and the potentially resulting global warming has been a great concern for human society. The oceans currently absorb approximately one-half of the anthropogenic CO2 emitted from fossil fuels and industrial processes. Ocean contains more than fifty times carbon in the atmosphere and as a buffer limiting the concentration of CO2 in atmosphere. The amount of CO2 flux between atmosphere and ocean or CO2 concentration in ocean is controlled by physical, chemical, and biological process. It can be determined from air-sea CO2 concentration differences and CO2 exchange process between the atmosphere and ocean. Measurement of the ability of sea in uptake or emitting CO2 could be determined by measure the CO2 exchange coefficient on sea interface and the measuring the different partial pressure of CO2 between the air and sea. In this study, CO2 flux distribution of Indonesia Sea in 2007 to 2009 is computed using monthly CO2 exchange and the different partial pressure of CO2 obtained using wind speed, salinity, SST, and sea characteristic by satellite data. The CO2 flux thus estimated and discussed by Nightingale (2000) relationship transfer velocity of CO2. The result indicated that generally, Indonesia Sea is emitting the CO2 to the air. Keywords: CO2 flux, CO2 transfer velocity, Indonesian Sea, and Nightingale Algorithm. Abstrak: Peningkatan karbondioksida (CO2) di atmosfer yang berpotensi menghasilkan pemanasan global telah menjadi perhatian bagi kehidupan manusia. Lautan secara alami mampu menyerap hingga setengah limbah emisi CO2 yang dihasilkan manusia dari pembakaran bahan bakar fosil dan proses industri. Lautan mengandung lima puluh kali lebih besar konsentrasi karbondioksida (CO2) daripada atmosfer dan mampu menjadi penyangga yang membatasi konsentrasi CO2 dalam atmosfer. Konsentrasi CO2 mengalami fluktuasi secara terus-menerus antara udara-lautan. Konsentrasi CO2 di dalam laut dikendalikan oleh beberapa parameter yaitu, fisika, kimia, dan biologi. Hal tersebut dapat dijelaskan dengan perbedaan konsentrasi CO 2 udara-lautan dan proses pertukaran CO2 antara udara dengan lautan. Pengukuran kemampuan lautan dalam menyerap ataupun melepaskan CO2 dapat dijelaskan dengan mengukur koefisien pertukaran CO2 di permukaan laut dan melalui pengukuran perbedaan tekanan parsial CO2 antara udara dengan lautan. Dalam penulisan ini, dijelaskan tentang fluktuasi CO2 (CO2 flux) di Indonesia pada tahun 2007 sampai dengan 2009 dengan menghitung koefisien pertukaran CO2 di permukaan laut dan melalui pengukuran perbedaan tekanan parsial CO2 yang diperoleh dari pengukuran kecepatan angin, salinitas, suhu permukaan laut, dan karakteristik lautan dengan satelit data. Nilai CO2 flux secara kualitatif dan kuantitatif dihitung dengan menggunakan algoritma Nightingale (2000). Hasil menunjukkan bahwa secara umum lautan Indonesia melepaskan CO 2 ke atmosfer. Kata kunci: flutuasi CO2, kecepatan transfer CO2 Laut Indonesia, dan algoritma Nightingale.

INTRODUCTION The world ocean plays an important role in the earth climate. It is not only absorbs heat from the sun, but plays a major role in carbon cycle processes (Akiyama, 2002). For long term climate forecasts, knowledge of the heat, momentum, and substance exchange 101

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between the atmosphere and the ocean is essential because the time constants and capacities of the ocean are much larger than those of the atmosphere (Baliño et al., 2001). The greenhouse phenomenon, for example, is very serious for the global environment and mainly cause by carbon dioxide in the atmosphere. The exchange rate of CO2 between the atmosphere and ocean is therefore of great importance. The exchange of carbon dioxide between air and sea can be determined from air-sea CO2 concentration differences and CO2 gas exchange between air and sea (Takahasi et al., 2009). According to Baliño et al. (2001), the oceans currently absorb approximately onethird to one-half of the anthropogenic CO2 emitted from fossil fuels and industrial processes. However, preliminary results from ocean-atmosphere models suggest that we will not be able to rely on the oceans to mop up excess CO2 in the future if current global warming trends continue. If the surface ocean becomes warmer, it may alter the nature and intensity of the ocean circulation or the availability of nutrients. The study of air-sea gas exchange is multifaceted (Wanninkhof et al., 2009). Understanding exchange processes at the molecular level has improved our understanding of how environmental factors control the rate of exchange at the air-water interface. Some of the first studies on transfer across liquid-gas surfaces were focused on industrial applications (Danckwerts 1970). For environmental applications, a significant effort has been devoted to techniques for determining gas transfer rates, processes that control them, theories, and techniques to quantify gas fluxes. For determination of gas exchange over the ocean, many efforts have been of an opportunistic nature and take advantage of O2 disequilibria that arise from biological productivity (Redfield, 1948) or utilize 14C excesses in the atmosphere from nuclear bomb tests, called bomb 14C, that invaded the ocean (Broecker et al., 1985, 1995, Sweeney et al., 2007). Of particular note are the improvements in meteorological flux techniques that measure air-sea gas fluxes in the atmospheric boundary layer on minute timescales (McGillis et al., 2001) and waterside, deliberate tracer techniques that can provide gas exchange estimates in the field with timescales on the order of 24 h (Ho et al., 2006). Much effort has gone into determining a relationship between gas transfer and wind speed, such that gas fluxes between ocean and air can be determined from air-water concentration differences and wind speed over the ocean (Liss and Merlivat, 1986; Wanninkhof 1992; Takahasi et al., 2002; Susandi et al., 2006; Ekayanti et al., 2009). Indonesia as an archipelago country in equatorial area has a high annual sea surface temperature. The annual sea surface temperature (SST) in equatorial area, affected the partial pressure on sea interface become higher than the partial pressure in the atmosphere which could affected the emission CO2 from sea to the air (Takahasi et al., 2002). High annual sea surface temperature in equatorial, also affected the solubility, which is a physic-chemical process that transports carbon (as dissolved inorganic carbon) from the ocean's surface to its interior. The solubility of carbon dioxide is a strong inverse function of seawater temperature (i.e. solubility is greater in cooler water and lower in warmer water) (Raven and Falcowski 1999). Qualitative and quantitative study of the ability of Indonesia Sea as an absorbent or emission carbon dioxide (CO2) is very important for Indonesia and in the effort improving Indonesia negotiation in international consultation about global climate policy (Susandi et al., 2006). In recent year, there are very rare researches in describing and informing CO2 flux and the ability of Indonesia Sea in absorb or emitting CO2 in qualitative and quantitatively. In this paper, we present CO2 flux distribution of Indonesia Sea in 2007 to 2009 in qualitative and quantitatively, computed using monthly CO2 exchange and the different partial pressure of CO2, ∆pCO2 obtained using wind speed, salinity, SST, and sea characteristic by satellite data. The carbon dioxide flux thus estimated is estimated and discussed by Nightingale (2000) relationship. That design is quadratics function of wind, to 102

CO2 Flux in Indonesian Sea (N.W. Ekayanti)

derive CO2 transfer velocity in other to get a variation of CO2 flux distribution and caused by the data which is use in this research is long-term averaged winds data.

RESEARCH METHOD CO2 flux distribution of Indonesia Sea in 2007 to 2009 computed using monthly CO2 exchange and ∆pCO2 obtained using wind speed and SST monthly by satellite data. CO2 transfer velocity with The Nightingale et al. (2000) algorithm CO2 transfer velocity derived from satellite wind speeds that computed k from Quickscat satellite wind speed using k-U Nightingale (2000). The Nightingale et al. (2000) relationship deduced from in situ tracer measurements (SF6, 3He) performed at sea and assuming a second order polynomial k-U relationship. 2 k N  (0.222 U 10  0.333U10 )  (600/Sc) 0.5 (1) Where: kN = CO2 gas transfer velocity (cm hr-1), Sc = Schmidt Number of radon gas = kinematic viscosity of water divided by diffusion coefficient of gas (dimensionless) U10 = wind speed at 10 meters above mean sea level (ms-1) The transfer of inert gas through the air interface is controlled by the aqueous viscose boundary layer (Wanninkhof, 1992). Furthermore, viscosity and diffusivity show an opposite temperature dependence. Through this, diffusion coefficient fitted to a temperature relationship as following formula (Zhao, 1995): (2) Sc   8.3  10 2 T 3  6.7954T 2  224.30T  3412.8 Where, Sc = Schmidt Number of radon gas T = Sea Surface Temperature (Celsius) Carbon dioxide (CO2) flux calculation The carbon dioxide flux can be calculated as following formula (Akiyama 2002): (3) F  k  L  pCO2 Where, F = CO2 flux (mol m−2 year−1) ΔpCO2 = CO2 partial pressure between ocean and atmosphere (µatm) LCO2 = CO2 gas solubility (mol liter−1 atm−1). Weiss (1974) provided an empirical formula to estimate CO2 gas solubility, L on the basis of data fitting between the solubility, temperature, and salinity as following formula: ln L  A1  A2(100/Tabs )  A3 ln(Tabs /100)  S B1  B2(Tabs /100)  B3(Tabs /100) 2  (4) Where, Tabs = absolute temperature (K) = (273.15 + To Celsius) K, S = salinity (psu) The value of active coefficient A1-A3 and B1-B3 are shown in table 1 (Weiss, 1974).

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Table 1. Coefficient for calculation of the solubility of CO2 in molar and gravimetric units. No A1 A2 A3 B1 B2 B3

L with unit (moles liter−1 atm−1) -58.0931 90.5069 22.2940 0.027766 -0.025888 0.0050578

Carbon dioxide partial pressure between the atmosphere and ocean, ΔpCO2 distribution derived from sea surface temperature (SST). The ΔpCO2 is a function of temperature, total inorganic CO2 concentration (T CO2), alkalinity, and salinity. Metzl et al. (1995) tried to derive pCO2water from the relation with sea surface temperature (SST) in Indian Ocean. As pCO2 air in the atmosphere varies slowly in both spatial and temporal scale, Zhao (1995) derived the difference of CO2 partial pressure between air and sea (ΔpCO2) from sea surface temperature. The relationship between ΔpCO2 and SST is investigated using in situ data and MODIS data monthly in year 2007 to 2009. Due to data limitation, related to summer and winter period are derived using least square fit method and the relation between ΔpCO2 and SST can be expressed as: (5) pCO2  0.0147 T 3 - 0.1241 T 2 - 12.3453 T  115.5879 Where, T = Sea Surface Temperature (Celsius) ΔpCO2 = CO2 partial pressure different between atmosphere and ocean. When, seawater pCO2 is less than the atmospheric pCO2, seawater takes up CO2 from the overlying air (indicated by negative ΔpCO2). When, it is greater than the atmospheric pCO2, it emits CO2 to the air (positive ΔpCO2) (Takahasi et al., 2002).

RESULTS AND DISCUSSION Monthly pattern of wind speed, sea surface temperatures (SST), and CO2 flux in Indonesia Sea from 2007 to 2009 is shown by figure 1. In general, CO2 flux in Indonesia Sea follows the monsoon of Indonesia. December to January is maximum winter in the northern hemisphere and maximum summer in southern hemisphere. July is maximum winter in southern hemisphere and maximum summer in northern hemisphere (Clift and Plumb 2008). According to Takahasi et al. (2002), when seawater pCO2 is less than the atmospheric pCO2, seawater takes up CO2 from the overlying air (indicated by negative CO2 flux). When, it is greater than the atmospheric pCO2, it emits CO2 to the air (positive CO2 flux). The research of annual CO2 flux data in figure 1 indicated by positive CO2 flux indicated by positive value of CO2 flux). In general, Indonesia Sea is emitting the CO2 to the air (see figure 2). Even though, in some part of Indonesia Sea, carbon dioxide is absorbed from air to the sea, but the CO2 absorb become lower in higher sea surface temperature (SST), in higher SST the CO2 is emitted from sea to the air (Baliño et al., 2001, Takahasi et al., 1997, 1999, 2002, 2008 and 2009). Average carbon dioxide emitted from sea to the air for recently year in 2007 to 2009 is 2.85 (mol m−2year−1). This mean, for the total Indonesia Sea which is approximately 3.3 x 106 km2, the total average carbon dioxide emitted from sea to the air in 2007 to 2009 is almost 0.12 (PgC year−1). This result has a similar value with Susandi et al. (2006) research in Indonesia Sea, which the CO2 emission from sea to the air is variant from 2.64 (mol m−2year−1) to 3.78 (mol m−2year−1). 104

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Figure 1. Monthly pattern of wind speed, sea surface temperatures (SST), and CO2 flux in Indonesia Sea from 2007 to 2009.

According to Baliño et al. (2001) the equatorial ocean, such as equatorial Pacific, is the largest continuous natural source of CO2 in the ocean. This is due to the combination of a strong upwelling of CO2-rich waters and low biological activity. Also, due to a strong physical pump combined with a sub-optimal biological pump. Vigorous upwelling along the equator brings cold, CO2-enriched water to the surface. As this water warms up during its journey upwards, it holds less CO2 and the gas trapped in the water escapes to the air. Although this region is rich in nutrients, phytoplankton do not produce dense “blooms” of large, fast sinking cells and the export of carbon from organic material produced in surface water is generally low.

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Figure 2. Mean CO2 FluxN in 2007 to 2009. The annual sea surface temperature (SST) in equatorial area, affected the partial pressure on sea interface become higher than the partial pressure in the atmosphere which could affected the emission CO2 from sea to the air (Takahasi et al., 2008). The annual sea surface temperature (SST) in Indonesia Sea as an equatorial area, which annual value, is 29.34°C until 31.94°C, affected the different partial pressure on sea interface, ∆pCO2, between the atmosphere and ocean in positive value (mean ∆pCO2 = +12.97 µatm to +39.29 µatm) which could affected the emission CO2 from sea to the air in a such value, also supported by the high wind speed, from 6.02 ms−1 to 8.44 ms−1 could affected the annual CO2 transfer velocity become higher (10.55 cmhr−1 to 18.61 cmhr−1). High annual sea surface temperature in Indonesia Sea, also affected the solubility, which is a physic-chemical process that transports carbon (as dissolved inorganic carbon) from the ocean's surface to its interior. The solubility of carbon dioxide is a strong inverse function of seawater temperature (i.e. solubility is greater in cooler water and lower in warmer water) (Raven and Falcowski, 1999). High annual SST in Indonesia Sea could affected the solubility of carbon dioxide become lower (annual solubility = 0.026 mol liter−1 atm−1), so the dissolved inorganic carbon from the ocean's surface to its interior is pursued. In northwest monsoon, in northern hemisphere, the sea surface temperature (SST) becomes lower than in southern hemisphere. Lower SST affected the partial pressure on the sea interface become smaller than partial pressure in the atmosphere; in this case, sea could uptake CO2 from the atmosphere. Maximum CO2 absorb from air to the sea could be shown in the area of South China Sea in December 2008 as shown by figure 3, with maximum CO2 flux is 36.14 (mol m−2year−1). The sea surface temperature in that time is 22.51°C to 29.32°C could affected the different partial pressure on sea interface, ∆pCO2, between the atmosphere and ocean in negative value (mean ∆pCO2 = −40.01 µatm) which is mean the sea could absorb CO2 from air in a maximum value, also by the high wind speed, from 6.13 ms −1 to 12.0 ms−1 could make the CO2 transfer velocity higher (10.14 cmhr−1 to 32.49 cmhr−1).

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Figure 3. Maximum CO2 uptake in northern hemisphere, December 2008. Meanwhile, in southern hemisphere of Indonesia Sea, although the CO2 transfer velocity is smaller than in northern hemisphere (1.34 cmhr−1 to 10.14 cmhr−1), summer season give a big affect in southeastern part of Indonesia Sea for sea surface temperature (SST) from 29.32°C to 35.31°C which could affected the different partial pressure on sea interface, ∆pCO2, between the atmosphere and ocean in positive value (mean ∆pCO2 = +79.94 µatm) which mean CO2 emitted to the air. The maximum CO2 flux in southeastern part of Indonesia Sea is 27.14 (mol m−2year−1). In southeast monsoon, in northern hemisphere, the sea surface temperature (SST) becomes higher than in southern hemisphere. High SST affected the partial pressure on the sea interface become higher than partial pressure in the atmosphere; in this case, sea emitted CO2 to the atmosphere. July is maximum winter in southern hemisphere and maximum summer in northern hemisphere. This affected the CO2 flux in Indonesia Sea. Maximum CO2 emitted from sea to the air could be shown in the northern hemisphere of Indonesia (area of South China Sea) in July 2009 as shown by figure 4. The maximum CO2 flux in the northern hemisphere of Indonesia is 35.22 (mol m−2year−1). The sea surface temperature in that time is 29.80°C to 35.20°C could affected the different partial pressure on sea interface, ∆pCO2, between the atmosphere and ocean in positive value (mean ∆pCO2 = +78.56 µatm), which is mean the sea is emitting CO2 to the air in a maximum value, also by the high wind speed, from 6.83 ms−1 to 14.80 ms−1 could make the CO2 transfer velocity higher (12.79 cmhr−1 to 55.90 cmhr−1). Meanwhile, in southern hemisphere of Indonesia Sea, although the CO2 transfer velocity is smaller than in northern hemisphere (kW92 = 3.23 cmhr−1 to 15.45 cmhr−1 and by kN = 3.31 cmhr−1 to 12.79 cmhr−1), winter season give a big affect in southeastern part of Indonesia Sea for sea surface temperature (SST) from 25.86°C to 29.80°C which could affected the different partial pressure on sea interface, ∆pCO2, between the atmosphere and ocean in negative value (mean ∆pCO2 = −17.05 µatm) which is mean in those area, sea could absorb CO2 from air. The maximum CO2 flux in southern hemisphere of Indonesia Sea is 9.37 (mol m−2year−1).

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Figure 4. Maximum CO2 uptake in northern hemisphere, July 2009.

CONCLUSION Generally, Indonesia Sea is emitted the CO2 to the air. SST and wind speed influence for the absorbing or emissions of CO2 between the atmosphere and ocean in Indonesia Sea. The CO2 fluxes occur between northern hemisphere and southern hemisphere is following monsoon of Indonesia. Average CO2 emitted from sea to the air for recently year in 2007 to 2009 is almost 2.85 (mol m−2year−1). For the total Indonesia Sea, the total average CO2 emitted from sea to the air in 2007 to 2009 is almost 0.12 (PgC year−1). Maximum CO2 absorb from air to the sea could be shown in northern hemisphere (the area of South China Sea) in December 2008 (36.14 mol m−2year−1). Meanwhile in southern hemisphere, CO2 is emitting to the air, maximum CO2 flux is 27.14 (mol m−2year−1). Maximum CO2 emitted from sea to the air could be shown in the northern hemisphere of Indonesia (area of South China Sea) in July 2009 (35.22 mol m−2year−1). Meanwhile in southern hemisphere, CO2 is absorb from air to the sea (9.37 mol m−2year−1).

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