Total alkalinity (TA) is an essential variable for the study of physical and biogeochemical processes in coastal and oceanic systems, and TA data obtained at high spatiotemporal resolutions are highly desired. The performance of the current in situ TA analyzers/sensors, including precision, accuracy, and deployment duration, cannot fully meet most research requirements. Here, we report on a novel high-precision in situ analyzer for surface seawater TA (ISA-TA), based on an automated single-point titration with spectrophotometric pH detection, and capable of long-term field observations. The titration was carried out in a circulating loop, where the titrant (a mixture of HCl and bromocresol green) and seawater sample were mixed in a constant volume ratio. The effect of ambient temperature on the TA measurement was corrected with an empirical formula. The weight, height, diameter, and power consumption of ISA-TA were 8.6 kg (in air), 33 cm, 20 cm, and 7.3 W, respectively. A single measurement required ∼7 min of running time, ∼32 mL of seawater, and ∼0.6 mL of titrant. ISA-TA was able to operate continuously in the field for up to 30 days, and its accuracies in the laboratory and field were 0.5 ± 1.7 μmol kg–1 (n = 13) and 10.3 ± 2.8 μmol kg–1 (n = 29) with precisions of 0.6–0.8 μmol kg–1 (n = 51) and 0.2–0.7 μmol kg–1 (n = 8), respectively. This study provides the research community with a new tool to obtain seawater TA data of high temporal resolution.
Abstract Carbon dioxide partial pressure ( p CO 2 ) in surface water was continuously measured every 3 h from July 2012 to June 2013 using an autonomous p CO 2 system (MAPCO 2 ) deployed on a moored buoy on the East China Sea shelf (31°N, 124.5°E). Sea surface p CO 2 and pH had the largest variations in summer, ranging from 215 to 470 μ atm, and 7.941 to 8.263 (averagely 8.084 ± 0.080), respectively. They varied little in winter, ranging from 328 to 395 μ atm, and 8.003 to 8.074 (averagely 8.052 ± 0.010), respectively. The seasonal average sea surface p CO 2 was respectively 335 ± 70 μ atm, 422 ± 43 μ atm, 362 ± 11 μ atm, and 311 ± 59 μ atm in summer, autumn, winter, and spring, and was overall undersaturated with respect to atmosphere on a yearly basis. Although the average sea surface p CO 2 in summer was below the atmospheric level, the net CO 2 flux has suggested a CO 2 source status due to the influence of typhoon. Our observation thus demonstrated the significant, even dominant impact of episodic typhoon events on surface ocean CO 2 chemistry and air–sea CO 2 gas exchange, which would be impossible to capture by shipboard observation. The high wind stress and curl associated with the northward movement of typhoon induced complex sea surface water movement, vertical mixing, and subsequent biological drawdown, which differed in pre‐, onset, and post‐typhoon stages. Based on our estimates, the degassing fluxes during typhoon reached as high as 82 mmol m −2 CO 2 and 242 mmol m −2 CO 2 in summer and autumn, respectively, accounting for twice as large as the summer CO 2 sink during non‐typhoon period, and 28% of the total CO 2 source in autumn.
We examined the sub-seasonal to interannual variability and multi-year trend of sea surface CO2 partial pressure (pCO2) and air-sea CO2 flux at a coastal site of the East China Sea (31⁰N, 122.8⁰E) based on high-frequency time-series data collected by a buoy since 2013. Seasonal average sea surface pCO2 was highest in autumn, but the lowest value can appear in winter or spring, depending on the biological productivity in spring. The seasonal amplitude of pCO2 was up to 123 μatm. Based on property-property relationships and a simple mass budget model, we found that temperature change, biological activity, water mixing and air-sea CO2 exchange all made significant contributions to the seasonal variation of pCO2. From winter to summer, seasonal warming and atmospheric CO2 uptake elevated the pCO2, while net biological production, weakened vertical mixing and the retreat of the Yellow Sea Coastal Water (YSCW) lowered the pCO2. Conversely, from summer to winter, seasonal cooling and CO2 emission lowered the pCO2, while respiration, enhanced vertical mixing and the YSCW intrusion raised them up. Over short-term timescale, biological production and respiration frequently drew down or elevated the pCO2 by 150-400 μatm within 5-10 days during warm months. When biological activity was suppressed during cold months, such short-term variations were dominated by water mixing with a smaller pCO2 amplitude of 5-60 μatm within 2-6 days. This site was a sink of atmospheric CO2 in winter and spring, but a CO2 source in summer and autumn. Annually, it was a moderate CO2 source in 2014 (air-sea CO2 flux was 2.88 ± 11.02 mmol m−2 d−1), a weak CO2 sink in 2016 (-0.21 ± 12.23 mmol m−2 d−1), and a weak CO2 source in the combined year of the first half of 2017 and the second half of 2018 (0.40 ± 9.11 mmol m−2 d−1). The relatively high CO2 source in 2014 was likely due to the weaker biological production in spring and more typhoon passage in autumn. From 2013 to 2019, the wintertime sea surface pCO2 didn’t follow the increasing trend of the atmospheric pCO2, leading to an enhancing carbon sink in winter.
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