High-Frequency Spectrophotometric Measurements of Total Dissolved

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High-Frequency Spectrophotometric Measurements of Total Dissolved Inorganic Carbon in Seawater Zhaohui Aleck Wang,* Sophie N. Chu, and Katherine A. Hoering Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, McLean 203, MS no. 8, 266 Woods Hole Road, Woods Hole, Massachusetts 02543, United States ABSTRACT: A new spectrophotometric method was developed to achieve continuous measurements of total dissolved inorganic carbon (DIC) in seawater. It uses a countercurrent flow design and a highly CO2-permeable membrane (Teflon AF 2400) to achieve flow-through CO2 equilibration between an acidified sample and an indicator solution with a fast response time of ∼22 s. This method improves the spatiotemporal resolution by more than 1 order of magnitude compared to the existing spectrophotometric method. The flow-through equilibration allows for continuous (∼1 Hz) detection and real-time data smoothing. The method had a short-term precision of ±2.0 μmol kg−1 for a given flow-through sample. It achieved a field precision of ±3.6 μmol kg−1 and successfully captured high DIC variability down to minute scales. Measurements by the new method over the typical range of oceanic DIC showed good agreement with measurements made by an established method (mean differences −1.6 to 0.3 μmol kg−1 with 1σ ± 6.0−6.7 μmol kg−1). This level of precision and accuracy is comparable to that of the existing spectrophotometric method. The characteristics of the new method make it particularly suitable for high-frequency, submerged measurements required for mobile observing platforms in the ocean. It can also be adapted for high-frequency, spectrophotometric measurements of seawater CO2 fugacity.

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INTRODUCTION The seawater carbonate (CO2) system plays a critical role in regulating CO2 fluxes into and out of the world’s oceans. The four primary parameters used to characterize this system are total dissolved inorganic carbon (DIC), CO2 fugacity (f CO2), pH, and total alkalinity (TA). DIC is defined as the sum of all carbonic acid species in water: DIC = CO2* + HCO3− + CO32−, where CO2* is the sum of dissolved CO2 and carbonic acid (H2CO3). DIC is a master carbon parameter frequently used to study, identify, and differentiate many processes linked to the marine carbon cycle (e.g., biological uptake of CO2, ocean acidification, and anthropogenic CO2 penetration in the ocean1−3). The assessment of these processes ultimately relies on high-quality measurements of seawater DIC. In addition, to fully characterize the CO2 system through thermodynamic calculations, at least two CO2 parameters must be measured. CO2 calculations made using DIC data as one of the parameters yield results that are often more consistent with measured values.4,5 Because of its important role in the CO2 system, DIC was measured during all of the major ocean carbon expeditions, such as the Climate Variability and Predictability (CLIVAR) Hydrography Program and the Joint Global Ocean Flux Study (JGOFS). Traditional bottle sampling and analysis of DIC can only achieve limited spatiotemporal coverage mainly because of associated high costs and low throughput. Development of methodologies that are suitable for high-resolution in situ measurements of CO2 parameters have been widely recognized © XXXX American Chemical Society

as a research priority in the carbon and ocean acidification research community.6−11 Among various methods (e.g., coulometry,12,13 potentiometry,14,15 nondispersive infrared (NDIR) method,16−18 and conductimetry 19) developed for high-precision DIC measurements, the spectrophotometric method20,21 offers high sensitivity, good stability, and direct measurements of water-phase samples. It can be “calibrationfree” in theory,20,22 thus reducing maintenance requirements. These attributes make it well suited for in situ underwater applications. The existing spectrophotometric DIC method20,21 is based on spectrophotometric pH measurements where observed absorbances of a sulfonephthalein indicator and its equilibrium properties are used to quantify sample pH.23 A piece of Teflon AF 2400 (DuPont copolymer) capillary tubing is used as both an optical cell and a CO2 equilibrator as it is highly permeable to CO2 molecules and can act as a liquid-core waveguide (LCW) for optical detection.20,21,24 The spectrophotometric detection occurs after full CO2 equilibration is established between the acidified sample and indicator solution across the Teflon AF tubing. The indicator solution is motionless during the equilibration process. This method is similar in principle to the spectrophotometric f CO2 method,24,25 but the sample is Received: February 4, 2013 Revised: May 23, 2013 Accepted: May 28, 2013

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dx.doi.org/10.1021/es400567k | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

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Figure 1. Schematic drawing of the optical cell with a countercurrent flow design in the new spectrophotometric DIC method. Blue and black arrows indicate acidified sample and indicator flow directions, respectively.

where (K0)a is the Henry’s Law constant26 for the acidified sample. The chemical and optical properties of the internal indicator solution can be expressed as

not acidified, and a different indicator is used. Because the indicator does not directly mix with the sample in either of these methods, no dilution or perturbation to the seawater sample occurs. The response time (i.e., the time required to obtain a stable reading for detection) of the existing spectrophotometric method is about 5 min, which is the CO2 exchange time required to reach full CO2 equilibration. This method has been used for underway measurements of flow-through seawater,21 and actual measurements are intermittent. Such a response is sufficient for stationary measurements, such as bottle samples and buoy deployments. However, it is not ideal for measurements made on mobile platforms, such as automated underwater vehicles (AUVs), remotely operated vehicles (ROVs) or water-column profilers. At the 5 min sampling interval, the spatiotemporal resolution on these mobile platforms may be limited for studying rapid changes on a scale down to minutes or meters and fine-scale features such as those encountered in coastal oceans and water-column profiling. In this paper, we describe a new spectrophotometric DIC method capable of attaining a much faster response time (∼22 s) using flow-through (dynamic) CO2 equilibration by introducing countercurrent, continuous flow between the indicator solution and the sample (Figure 1). This new design allows for continuous measurements as compared to intermittent measurements made with the existing spectrophotometric method (referred to as the intermittent method hereafter). The new method (referred to as the continuous method hereafter) has achieved good measurement stability and repeatability, similar to those of the intermittent method. During field tests, the continuous method produced highresolution DIC data that were in good agreement with measurements made by the established NDIR-based method. These characteristics make the continuous method particularly suitable for expanding observational capabilities of the CO2 system on mobile observing platforms.

⎛ R−e ⎞ 1 log(f CO2 )i = B(t ) − log(K 0)i − log⎜ ⎟ ⎝ 1 − Re3/e 2 ⎠

such that log

⎛Ke ⎞ B(t ) = log(TA + [H+] − [I2−])i + log⎜ I 2 ⎟ ⎝ K1′ ⎠i

(3)

(4) +

where TA is the alkalinity of the indicator solution, [H ] is the internal proton concentration, KI is the indicator dissociation constant, and K1′ is the carbonic acid first dissociation constant for the internal solution. B(t) describes the chemical and optical properties of the indicator solution. It is an experimentally derived constant for a given temperature, calibrated using Certified Reference Material (CRM) obtained from A.G. Dickson at Scripps Institution of Oceanography.20,21 For this work, eq 3 has been rearranged from the expression in the intermittent method20 by combining (K0)a with the DIC concentration such that all sample-related terms are on one side of the equation, while all indicator-related terms are on the other. Bromocresol purple was used as the pH indicator, where λ1 = 432 nm and λ2 = 589 nm. A nonabsorbing reference wavelength (λref = 700 nm) was used to correct baseline drift in absorbance measurements. The governing equations or values for all of the constants and coefficients in eqs 1−4 were previously described by Byrne and colleagues.20,21 Equation 3 quantitatively links DIC (or f CO2) in the acidified sample to f CO2 (or pH) of the internal indicator solution at full CO2 equilibration. To make high-frequency DIC measurements possible, the new continuous DIC method uses a dynamic, partial equilibration process instead of a static, full equilibration that occurs in the intermittent method. A countercurrent flow design (Figure 1) was adopted to maintain fast and stable CO2 exchange between the indicator and acidified sample. Countercurrent flow has been found extensively throughout nature in

METHODS Principle. The intermittent spectrophotometric DIC method relies on 100% f CO2 equilibration between acidified samples and a motionless indicator solution across the wall of Teflon AF tubing.20,21 After f CO2 equilibrium, DIC (as total CO2) of the acidified sample (denoted by subscript a) is proportional to f CO2 of the internal indicator solution (denoted by subscript i): [DIC] = log(f CO2 )i (K 0)a

⎛ R−e ⎞ [DIC] 1 = B(t ) − log(K 0)i − log⎜ ⎟ (K 0)a ⎝ 1 − Re3/e 2 ⎠

where (K0)i is the Henry’s Law constant26 for the internal indicator solution. The coefficients e1, e2, and e3 are indicator molar absorbance ratios at wavelengths λ1 and λ2, where λ1 and λ2 are the wavelengths for the absorbance maxima of the indicator acid (HI−) and base (I2−) species. These coefficients are laboratory determined optical constants. R is the ratio of the indicator absorbance (A) measured at wavelengths λ1 and λ2; R = λ2A/λ1A. B(t) can be expressed as

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log(f CO2 )a = log

(2)

(1) B



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Figure 2. Schematic drawing of the DIC system for continuous, spectrophotometric DIC measurements.

biological systems, such as in lungs and fish gills,27−29 and has been imitated in engineering applications to achieve the maximum transfer of heat or chemicals.30,31 In this case, it maximizes the transfer rate of CO2 between the indicator solution and samples. In the continuous method, Teflon AF tubing was used only as a CO2 equilibrator, not as both an equilibrator and a LCW as in the intermittent method. Instead, optical detection occurs in an optical “Z” cell after the indicator solution passes through the Teflon AF tubing (Figure 2). As the indicator solution travels the length of the Teflon AF tubing, partial CO2 equilibration is attained between the indicator solution and the acidified sample. For a given sample, if such an equilibration process is repeatable each time the indicator solution passes through the Teflon tubing and the optical detection is stable and sensitive, then the method can achieve continuous, highquality DIC measurements. If desired, a slow indicator flow rate, combined with a long piece of Teflon AF tubing, will allow the indicator solution to reach 100% CO2 equilibration. The countercurrent flow design allows for dynamic, efficient exchange of CO2 across the Teflon AF tubing. If the indicator flows at a fast speed, by the time it reaches the end of the flow cell, it has attained partial CO2 equilibration with an exchange efficiency or percentage of equilibration, p (value 0−1), which can be included in eq 3 to describe the continuous method:

tometer, and a white LED light source (LE-1W-CE; WT&T Inc., Canada). The countercurrent flow cell was assembled with a 120 cm piece of Teflon AF 2400 capillary tubing (0.5 mm o.d. by 0.4 mm i.d.) and various commercial PEEK fittings and tubing (1.6 mm o.d. and 0.5−1.0 mm i.d.; Upchurch Scientific). The optical signals were monitored and recorded using a laptop PC and the Ocean Optics SpectraSuite software. The system, with all of its reagents and seawater samples or standards, was thermostated at 25.0 ± 0.1 °C with a water bath and a custommade, air-circulated Peltier device. Flow-through seawater was pumped through a PEEK tubing coil to facilitate temperature equilibration. Reagents. Bromocresol purple sodium salt (Sigma− Aldrich) was used to make 4 mM indicator stock solutions that were stored in opaque glass bottles at 4 °C. Working indicator solutions were prepared from the indicator stock solutions with a final concentration of 20−30 μM. This concentration, about 10 times that of the previous work (2−3 μM),20,21 was required to produce optimal absorbances with the short path length “Z” cell. The alkalinity of the indicator solutions was established by adding extra-pure Na2CO3 (Acros Organics). Final TA concentrations of ∼700−800 μmol kg−1 were chosen so that the final indicator pH for measurements of typical seawater DIC concentrations fell within the range of ∼5.6−6.4, where the indicator absorbance change is sensitive. This is similar to what has been achieved in the intermittent method.21 For each liter of indicator solution, 0.5 mL of 10% lauryl sulfate sodium salt solution was added to serve as a surfactant for cleaning purposes. Reference solutions were prepared using an identical procedure to the indicator solutions but without added indicator. The working indicator and reference solutions were enclosed in 2 L gas-impermeable laminated bags (Calibrated Instruments, Inc.). Bagged solutions can last several months without any appreciable changes in composition.19 Hydrochloric acid (HCl, 2.5 M) was used to acidify the samples. Sodium carbonate solutions and CRMs were used as DIC standards. The former were made with ultrapurified sodium carbonate (Acros Organics) in appropriate ionic strength sodium chloride solutions corresponding to various seawater salinities. They were stored in 1 L borosilicate glass bottles and poisoned with saturated mercuric chloride (HgCl2).32 The DIC values of these standards were ascertained to within ±2.0 μmol kg−1 using a NDIR-based DIC autoanalyzer (AS-C3, Apollo SciTech) that was calibrated with CRMs. The DIC

⎛ R−e ⎞ ⎛ [DIC] ⎞ 1 log⎜p × ⎟ ⎟ = B(t ) − log(K 0)i − log⎜ (K 0)a ⎠ ⎝ ⎝ 1 − Re3/e 2 ⎠ (5)

where log(p × f CO2)a = log(p × ([DIC]/(K0)a)). In eq 5, the right side still represents ( f CO2)i while p is added to the left side of the equation to characterize partial f CO2 equilibration. The variable p is used to characterize the equilibration process and is affected by operational conditions such as flow rate, temperature, indicator composition, and the f CO2 gradient between the internal indicator solution and the external sample. It can be empirically built into the calibration (see the Results and Discussion section) and does not need to be explicitly defined for actual measurements. When p = 1, eqs 3 and 5 are equivalent. DIC System. The continuous DIC system (Figure 2) consists of a countercurrent flow cell (Figure 1), four highprecision digital peristaltic pumps (Ismatec SA, Switzerland), a microvolume, 10 mm optical “Z” cell (SMA-Z-10-uvol; FIAlab Instruments Inc.), an Ocean Optics USB4000 spectrophoC



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during a cruise in the North Pacific. The samples were collected in 1 L borosilicate glass bottles and poisoned with saturated mercuric chloride.32 Each sample was pumped through the DIC system for continuous measurements over a period of 15−20 min. Duplicate bottle samples were also collected into 250 mL borosilicate glass bottles following the same sampling procedure32 for the NDIR-based DIC measurements to gauge the performance of the new system. All bottle samples were analyzed within two weeks.

concentrations were corrected for the dilution effect of HgCl2 and density. The NDIR-based DIC analyzer (AS-C3, Apollo SciTech) uses an inert gas (nitrogen) to purge CO2 gas from a known amount of acidified seawater sample; the CO2 in the resulting gas stream is quantified by a NDIR CO2 analyzer (LI-7000, LICOR). The calibration of the analyzer was conducted using CRMs on a 12 hour interval. This instrument has a precision and accuracy of better than ±2.0 μmol kg−1. Measurement Procedure. The DIC measurement procedure is summarized as follows: (1) Seawater samples or DIC standards were acidified with HCl at a water-to-acid mixing ratio of ∼700:1 and then directed to flow through the countercurrent flow cell outside of the Teflon AF tubing at a flow rate of ∼4.0 mL min−1 (Figure 2). (2) The optical cell was flushed with reference solution and a reference spectrum was taken. (3) Indicator solution was pumped at a selected flow rate (see the Results and Discussion section) through the countercurrent flow cell (inside the Teflon AF tubing) in the opposite direction as the seawater. The indicator solution exited the countercurrent cell after CO2 exchange and flowed through the optical cell for absorbance detection at a frequency of ∼1 Hz. (4) Reference was retaken regularly to correct any potential absorbance baseline drift. Calibration. Calibration of the DIC system was necessary to establish a quantitative relationship between ([DIC]/(K0)a) and ( f CO2)i under the selected running conditions. It involved two steps for each batch of bromocresol purple indicator working solution. First, the system was calibrated with CRMs to obtain the B(t) constant in eq 5 by running the indicator solution at a slow speed (5 min) inside the Teflon AF tubing to achieve 100% f CO2 equilibration (p = 1 in eq 3). B(t) was later used to calculate ( f CO2)i (the right side of eq 5) for standard runs at the higher selected indicator flow rate. Note that B(t) reflects chemical and optical properties of the indicator solution (eq 4) and does not change with indicator flow rate. Second, more than five DIC standards were measured at the same, faster indicator flow rate to obtain the absorbance ratios in eq 5 corresponding to partial f CO2 equilibration of each standard. (f CO2)i was then calculated from eq 5 to establish a ( f CO2)i versus ([DIC]/(K0)a) curve. Sample water was run at the same conditions as the DIC standards to obtain R. The sample DIC concentrations were calculated using B(t), R, and the calibration curve. In this procedure, the variable p is built into the calibration curve (see the Results and Discussion section). Testing and Groundtruthing. Laboratory testing was conducted to establish calibration and measurement characteristics of the new method as well as to try to optimize running conditions. Thereafter, the continuous DIC system was tested at the Environmental Systems Laboratory at Woods Hole Oceanographic Institution (WHOI), Woods Hole, Massachusetts, USA, for measurements of flow-through seawater pumped from a mile offshore. This test was conducted in June 2012 over 3 days. To groundtruth the new DIC method, traditional discrete DIC bottle samples were collected concurrently with continuous DIC measurements. The samples were poisoned and measured using a NDIR-based DIC autoanalyzer. The new DIC system was further tested using discrete bottle samples that were collected from three hydrographic stations up to 3000 m in depth using a conductivity−temperature-depth (CTD) Rosette Niskin bottle package in August and September 2012

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RESULTS AND DISCUSSION Measurement Characteristics. Percentage of CO2 equilibration (p) is a function of indicator flow rate or travel time through the 120 cm long Teflon AF tubing (Figure 3). Travel

Figure 3. Percentage of equilibration (p) and indicator flow rate as a function of indicator CO2 exchange time (travel time in the Teflon AF tubing). CO2 exchange time = internal volume of the Teflon tubing/ indicator flow rate. The data were generated with the same indicator solution at temperature 25.0 ± 0.1 °C using seawater samples with DIC of ∼1950 μmol kg−1 and salinity of ∼32.5.

time is the amount of time that it takes for the indicator solution to travel the length of the Teflon AF tubing. This is also equivalent to CO2 exchange time, the amount of time that the indicator solution exchanges CO2 with the acidified sample. The variable p increases nonlinearly with an increase in CO2 exchange time. A higher indicator flow rate would allow for less travel time in the Teflon tubing for CO2 exchange, resulting in lower CO2 equilibration, faster response time, and greater indicator consumption. At very high flow rates, the optical detection becomes noisy probably due to increased pulsing from the peristaltic pump, causing unsteady flow in the optical cell. Travel time or CO2 exchange time inside the Teflon AF tubing with a fixed internal volume is proportional to the reciprocal of indicator flow rate (Figure 3). Currently, we have chosen an indicator flow rate of ∼1 mL min−1, equivalent to a 9 s CO2 exchange time, which is an effective balance between indicator consumption, response time, and detection stability. A further increase in indicator flow rate would not significantly decrease CO2 exchange time (Figure 3). At the current settings, it takes ∼35−60s to achieve a steady response at 25.0 °C while varying between two samples with DIC concentrations in the range ∼1800−2400 μmol kg−1 (e.g., Figure 4A). The response time only varies by a few seconds for a given change in absorbance ratio under the same running condition. There is a significant linear relationship between response time and changes in absorbance ratios when switching between two DIC samples (Figure 4B). The response time is much longer than the CO2 exchange time of ∼9 s under the current settings. The discrepancy between the two is likely due D



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same total signal change of 92%.21 Therefore, CO2 equilibration in the continuous method is 6 times faster than that in the intermittent method. It is impractical to use partial equilibration in the intermittent method, since the signal associated with a short exchange time (e.g., 9 s) would fall on a sharp changing slope,21 resulting in an unstable and inconsistent recording, and would have large measurement errors. For partial dynamic equilibration in the continuous method, a stable and consistent signal is reached before recording (Figure 4A). Under the current settings, the variability in absorbance ratio (R) when measuring a stream of water with a constant DIC is only ∼± 0.0017 (1σ), which translates to a DIC analytical uncertainty (short-term precision) of ±2.0 μmol kg−1. This precision is similar to that of Wang et al.21 Calibration curves for the continuous DIC method (Figure 5) were derived over the DIC range encountered in samples

Figure 4. (A) Indicator solution response (absorbance ratio, R) as a function of time, when two DIC samples (∼2000 and ∼2200 μmol kg−1) were switched back and forth in lab experiments. (B) Response time as a function of changes in absorbance ratio (ΔR). Running conditions: temperature, 25.0 ± 0.1 °C; indicator flow rate, 1.0 mL min−1; sample flow rate, 4.0 mL min−1. Figure 5. Calibration data for the continuous DIC method using standards with three different salinities (S). The solid line is the best fit of all data y = −0.8197x2 + 0.7315x − 0.0031; R2 = 0.9984. See Figure 4 for running conditions.

to the time that is required to flush the Teflon tubing and the optical cell with new indicator solution. Because of laminar flow throughout the flow path, the volume needs to be replaced several times before it is completely flushed. This explanation is consistent with the fact that the response time becomes shorter when ΔR, or the concentration difference between the two samples, decreases (Figure 4B). The intercept in Figure 4B thus approximately represents an actual response time of ∼22 s during flow-through measurements, when sample concentration change is incremental as opposed to large changes as shown in Figure 4A. The response time can be further improved by reducing the internal volume in the indicator flow path to reduce the effect of laminar flow. The current response time (∼22 s) is more than 1 order of magnitude faster than that in the intermittent method (∼5 min) with static, full equilibration.21 The data does not show that there is an apparent difference in response time between the countercurrent and concurrent flow under the current settings. This may be because a large portion of the response time results from the time that it takes to flush the system. However, the countercurrent flow can achieve a slightly higher CO2 diffusion efficiency by a few percentages for a 9 s CO2 exchange time. In the current method, the signal change for a 9 s CO2 exchange time is ∼92% of the total signal change if the indicator reached full equilibration. However, the same exchange time using static equilibration in the intermittent method only allows for ∼65% of the total signal change.21 As such, the dynamic equilibration can achieve a 40% increase in equilibration efficiency as compared to static equilibration. It would take about 60 s with static equilibration to reach the

with an indicator flow rate of 1.0 mL min−1 and a sample flow rate of 4.0 mL min−1 at a temperature of 25 °C. The data in Figure 5 were obtained from three series of calibrations at three different salinities using the same indicator solution and running conditions. Each series of calibration generates a polynomial equation, with a standard error of ±1.0−3.0 μmol kg−1, comparable to the measurement precision (±2.0 μmol kg−1). The effect of varying the salinity of the DIC standards has no measurable effect on the calibration curves (Figure 5). This is because the salinity effect on (f CO2)a has been accounted for, since (f CO2)a was calculated from DIC values and (K0)a (eq 1), and the latter is a known function of salinity.26 Internally, salinity for a given indicator solution is low (S ∼ 0.05) and constant. Beyond the effect on (K0)a, salinity did not have a measurable effect on the calibration curves in the salinity range encountered (Figure 5). Three individual calibration curves and the calibration curve containing all of the data in Figure 5 had a pooled mean difference of 0.5 ± 3.4 μmol kg−1. This is within the 95% confidence interval of measurement uncertainties. Under fixed running conditions with a particular indicator solution, the variable p is a function of the f CO2 gradient between the acidified sample and the indicator solution. The slight convex of the calibration curve in Figure 5, demonstrates that p varied over the ( f CO2)a, or DIC, range (p represents the slope of the curve as defined in eq 5). As ( f CO2)a in samples increases, p decreases under the same running conditions E



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slightly (31.5−31.8), while temperature showed a ∼4 °C variation (16.5−20.5). DIC concentration varied moderately (1929−2035 μmol kg−1). Salinity and DIC sometimes showed a strong correlation, while at other times, no correlation was observed, which suggests complicated tidal mixing. Each DIC data point in Figure 7A represents a mean of 1 Hz measurements over 1 min intervals. The measurements captured substantial variability on both short (minutes to a few hours) and longer (hours to days) time frames. To evaluate the precision of the continuous DIC measurements during the testing, the data in Figure 7A were smoothed by taking running averages (n = 5; ∼5 min interval; gray line in Figure 7A). The mean residual of individual observations relative to the running average was 0.1 ± 3.6 μmol kg−1 (N = 2332). This uncertainty is likely an upper limit, since the estimate includes DIC variability within a few minutes in the flow-through seawater, the variability that may occur in coastal oceans. It may explain the slightly lower precision in the field testing compared to that in the laboratory experiment. This estimated precision is comparable to that (∼3.0 μmol kg−1) of the intermittent method under field testing.21 The accuracy of the continuous method was assessed by directly comparing the differences between continuous measurements and the NDIR-based bottle measurements (Figure 7). Both methods used standards traceable to CRMs for system calibration. Residuals between the continuous and the discrete bottle measurements did not show systematic trends (Figure 7B). This suggests that any systematic errors in our measurements were minor. The continuous DIC measurements differ from the bottle measurements by −1.6 ± 6.7 μmol kg−1 (N = 23). Such accuracy is similar to that in previous development.21 The new method thus achieved high-frequency

(Figure 6). This can be explained conceptually as follows: the f CO2 gradient across the Teflon AF tubing increases as

Figure 6. Percentage of equilibration, p, across the Teflon AF tubing as a function of f CO2 concentration for acidified DIC standards, (f CO2)a. See Figure 4 for running conditions.

(f CO2)a increases; for a given indicator flow rate (thus a fixed time for CO2 exchange inside the Teflon AF tubing), p decreases with an increase in the sample−indicator f CO2 gradient. However, this effect is relatively small at the selected running conditions (Figure 6): p only changes by ∼1.0% in the (f CO2)a range corresponding to a DIC range of 1780−2370 μmol kg−1. This effect can be fully accounted for in the system calibration (Figure 5). Field Testing. The testing conducted at WHOI Environmental Systems Laboratory was designed to demonstrate highfrequency, high-quality measurements using the new DIC method (Figure 7A). During the 3 day period, which spanned multiple tidal cycles, salinity of the flow-through water varied

Figure 7. (A) Continuous seawater DIC measurements along with flow-through salinity and discrete DIC bottle measurements made at the WHOI Environmental Systems Laboratory on June 19−21, 2012. The continuous DIC measurements were binned to 1 min intervals. The gray line represents the running average (n = 5 or ∼5 min bin) of the measurements. (B) Residuals between continuous (DICc) and discrete DIC bottle (DICLi) measurements. See Figure 4 for running conditions. F



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external sources. These may include discrete sampling and NDIR-based analytical uncertainties. It is important to note that the level of measurement uncertainty achieved with the continuous DIC method is comparable to those of replicate bottle sample measurements using conventional DIC methods during major carbon cruises (http://cdiac.ornl.gov/oceans/ ).5,32 Implications. For any continuous measurement, if the response is instantaneous, then continuous detection reflects the true variability of the measured parameter and has the highest spatiotemporal resolution. Otherwise (if response time is > 0), the measurement reflects a running average of the true variability and has reduced resolution. For shorter response times, the running average better represents the true sample variability. Currently, we achieved an estimated ∼22 s response time with the continuous DIC method. If such a method is used on a CTD package with a lowering rate of 0.5 m s−1 (30 m min−1) to make continuous DIC measurements in the water column, each measurement would represent an average concentration over 11 m of water depth (0.5 m s−1 × 22 s); while for the intermittent method with a response time of 5 min, the resolution would be 150 m (0.5 m s−1 × 300 s). This represents more than 1 order of magnitude improvement in spatial resolution. The current system makes continuous measurements at a constant temperature. Future work will determine the temperature effects on DIC measurements. This may eventually allow all measurements to be performed at in situ temperature. The new countercurrent flow design and dynamic CO2 equilibration can also be adapted for continuous, spectrophotometric f CO2 measurements. This would allow for simultaneous, highfrequency submerged measurements of DIC and f CO2 in the ocean that would otherwise be difficult to make. Our new method development thus constitutes an important first step in the direction of developing high-frequency, in situ carbon sensor technologies.

measurements as well as accuracy and precision comparable to the existing spectrophotometric method. The DIC system was also used to make measurements of discrete bottle samples collected from three stations in the North Pacific at depths up to 3000 m. This test effectively captured the large DIC concentration range that may be encountered in the ocean (Figure 8). In this case, DIC

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Figure 8. (A) Comparison of the water-column DIC profiles between the measurements made by the new DIC method and the NDIR-based method over discrete bottle samples collected in the North Pacific in August and September 2012. (B) Residuals between the two measurements. See Figure 4 for running conditions.

AUTHOR INFORMATION

Corresponding Author

*Phone: (508) 289-3676; fax: (508) 457-2183; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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concentration varied from ∼1990 μmol kg−1 at the surface to ∼2370 μmol kg−1 at depth, a nearly 400 μmol kg−1 difference (Figure 8A). The DIC measurements by the continuous method were also in good agreement with NDIR-based analyses. The mean difference between the two methods was 0.3 ± 6.0 μmol kg−1 (N = 31). This level of accuracy is comparable to that achieved in high-frequency measurements shown in Figure 7. No systematic errors were observed over the DIC measurement range, as evidenced by the random distribution of the residuals between the spectrophotometric and NDIR-based measurements (Figure 8B). This test suggests that the new DIC method can attain good precision and accuracy over a wide range of seawater DIC concentrations. The estimated field precision (±3.6 μmol kg−1) was about 53−60% of the field agreement estimates (±6.0−6.7 μmol kg−1; Figures 7 and 8). The measurement variability resulting from the inherent noise of the new DIC system therefore accounts for ∼53−60% of the variability observed in Figures 7B and 8B. The rest of the variability may be attributed to various

ACKNOWLEDGMENTS This work was supported by WHOI Innovative Technology Award and National Institute of Science and Technologies (NIST no. 60NANB10D024). The authors thank Drs. F. Sayles and W. Martin at WHOI for their insightful inputs and comments.

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High-Frequency Spectrophotometric Measurements of Total Dissolved

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