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An In-situ Sensor Technology for Simultaneous Spectrophotometric Measurements of Seawater Total Dissolved Inorganic Carbon and pH Zhaohui Aleck Wang, Frederick N. Sonnichsen, Albert M. Bradley, Katherine A. Hoering, Thomas M. Lanagan, Sophie N. Chu, Terence R. Hammar, and Richard Camilli Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es504893n • Publication Date (Web): 26 Feb 2015 Downloaded from http://pubs.acs.org on March 3, 2015
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Environmental Science & Technology
An In-situ Sensor Technology for Simultaneous Spectrophotometric Measurements of Seawater Total Dissolved Inorganic Carbon and pH Zhaohui Aleck Wang*1, Frederick N. Sonnichsen2, Albert M. Bradley2, Katherine A. Hoering1 Thomas M. Lanagan2, Sophie N. Chu1, Terence R. Hammar2, and Richard Camilli2 1
Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, McLean 203, MS # 8, 266 Woods Hole Road, Woods Hole, Massachusetts 02543, USA.
2
Department of Applied Ocean Physics & Engineering, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, Massachusetts 02543, USA.
*Corresponding author Email:
[email protected]; Telephone: 1-508-289-3676; Fax: 1-508-457-2183 Keywords: dissolved inorganic carbon, pH, carbon dioxide, in-situ, sensor, ocean acidification, carbon cycle, spectrophotometry, seawater
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ABSTRACT
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A new, in-situ sensing system, Channelized Optical System (CHANOS), was recently
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developed to make high-resolution, simultaneous measurements of total dissolved inorganic
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carbon (DIC) and pH in seawater. Measurements made by this single, compact sensor can fully
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characterize the marine carbonate system. The system has a modular design to accommodate two
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independent, but similar measurement channels for DIC and pH. Both are based on
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spectrophotometric detection of hydrogen ion concentrations. The pH channel uses a flow-
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through, sample-indicator mixing design to achieve near instantaneous measurements. The DIC
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channel adapts a recently developed spectrophotometric method to achieve flow-through CO2
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equilibration between an acidified sample and an indicator solution with a response time of only
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~90s. During laboratory and in-situ testing, CHANOS achieved a precision of ±0.0010 and ±2.5
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µmol kg-1 for pH and DIC, respectively. In-situ comparison tests indicated that the accuracies of
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the pH and DIC channels over a three-week time-series deployment were ±0.0024 and ±4.1 µmol
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kg-1, respectively. This study demonstrates that CHANOS can make in-situ, climatology-quality
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measurements by measuring two desirable CO2 parameters, and is capable of resolving the CO2
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system in dynamic marine environments.
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INTRODUCTION
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The marine carbon dioxide (CO2) system strongly influences the marine carbon cycle. It
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helps regulate the Earth’s climate by controlling the amount of CO2 that exchanges between the
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ocean and the atmosphere. Currently, the ocean absorbs about one quarter to one third of the
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anthropogenic CO2 released to the atmosphere 1-3, therefore reducing the rate of atmospheric CO2
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increase and curbing global warming. However, oceanic uptake of anthropogenic carbon is
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causing a rapid change in seawater carbonate chemistry, a phenomena referred to as ocean
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acidification. In ocean acidification, excess CO2 lowers seawater pH, increases total CO2
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concentration, and decreases calcium carbonate saturation 4-5. Changes in the marine CO2 system
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may result in complicated responses and feedbacks in the ocean including changes in the marine
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carbon and other elemental cycles, as well as changes in marine biology and ecology 4, 6-7. Ocean
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acidification also reduces seawater buffering capacity, which slows oceanic carbon uptake and
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acts as a positive feedback in the increase of atmospheric CO2 3, 8.
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Precise and accurate characterization of the marine CO2 system is critical to studying the
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marine carbon cycle and ocean acidification. The development of in-situ sensor technologies for
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CO2 parameters have been widely recognized as a research priority in the carbon and ocean
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acidification research communities
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system on various time scales ranging from minutes to decades, and spatial scales of meters to
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kilometers. Because of the high costs associated with traditional bottle sampling, it is more
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effective to use autonomous sensors and instruments to achieve high-resolution data 15.
9-14
. It is important to study dynamics of the marine CO2
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The marine CO2 system is described by four measurable, primary parameters: total
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dissolved inorganic carbon (DIC), partial pressure of CO2 (pCO2) or CO2 fugacity (fCO2), pH,
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and total alkalinity (TA). Measurements of any two of the four parameters are required to
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calculate the others and fully resolve the carbonate system using seawater acid-base equilibria.
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Simultaneous measurements of two CO2 parameters are thus attractive. However, different
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measurement pairs will generate a range of calculation errors as a result of analytical errors,
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uncertainties in equilibrium constants, and their non-linear propagation in the calculations. Large
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errors result when pCO2 and pH are used due to their strong co-variation 16. The errors are often
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minimized when DIC and pH, or DIC and pCO2 are used 9. However, only in-situ pCO2 and pH
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sensors are commonly available
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mature but are highly desirable 9.
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. In contrast, in-situ DIC sensor technologies are much less
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Spectrophotometric methods are advantageous for simultaneous measurements of CO2
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system parameters because all of the measurements can be made by absorbance detection of a
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pH sensitive sulfonephthalein indicator in either the water sample or a standard solution
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Moreover, spectrophotometric measurements are highly sensitive and stable, making them well-
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suited for in-situ, underwater deployments. In addition, this method uses a low amount of sample
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and reagent, requires low power consumption, and makes direct measurements of seawater. The
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spectrophotometric method has become the benchmark for high-accuracy seawater pH
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measurements
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water and the pH is determined by measuring the ratio of the absorbance maxima of the
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indicator’s acid and base wavelengths. In-situ applications of this method have achieved a
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measurement uncertainty of 0.001-0.003 18-19, and are relatively immune to instrument drift often
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encountered by potentiometric pH measurements.
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. In this method
15, 23
22
.
, a small amount of pH indicator is added to the sample
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In-situ DIC sensors for seawater applications have recently been developed based on CO2
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equilibration between an acidified sample and a standard solution followed by either
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spectrophotometric or conductometric detection
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gas-sample CO2 equilibration followed by infrared detection, are also under development. The
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established spectrophotometric DIC method
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copolymer) capillary tubing as both an optical cell and a CO2 equilibrator since the material is
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highly permeable to CO2 molecules and can act as a liquid-core waveguide (LCW) for optical
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detection. The spectrophotometric detection occurs after full CO2 equilibration is established
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between the acidified sample and the indicator solution. The time-limiting step of this method is
24-25
22, 24, 26
(Table S1). Other DIC sensors, based on
uses a piece of Teflon AF 2400 (DuPont
TM
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the time that it takes for CO2 to equilibrate across the Teflon AF 2400 tubing, which is about 5
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minutes. An improved spectrophotometric DIC method
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faster response time and to allow for near continuous detection using a dynamic equilibration and
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a countercurrent flow design (Figure 1). The response time of this method varies with indicator
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flow rates, but can be as fast as 22s if partial equilibration27 is used. The improved method also
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overcomes optical signal drifting that can occur when Teflon AF tubing is used as the optical
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cell.
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was recently developed to achieve a
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This paper will describe the development and testing of a new, in-situ carbon sensor,
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Channelized Optical System (CHANOS), which uses a modular design that is capable of making
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simultaneous spectrophotometric measurements of seawater DIC and pH. The DIC channel uses
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the improved spectrophotometric method 27. The sensor design was targeted for high-resolution,
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time-series measurements at a fixed location. CHANOS is one of the first systems that is able to
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fully resolve the carbonate system with a desirable pair of CO2 system parameters in a single
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system. CHANOS has a built-in mechanism for in-situ calibration, which ensures high quality
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measurements throughout a deployment cycle and reduces the need for laboratory calibration. In-
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situ tests indicate that the system is able to make high-resolution, climatology-quality
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measurements 15 to resolve seawater CO2 system dynamics.
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METHODS AND MATERIALS
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Principles.
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(1) pH – The CHANOS pH channel uses a flow-through design in which seawater directly
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and continuously mixes with an indicator solution
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spectrophotometric pH method
15, 23
22
. It is based on the well-established
, where dissociation of the added sulfonephthalein indicator
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KI H+ + I2 − ; KI is the dissociation constant of the (H2I) in seawater is dominated by HI− ←→
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indicator acid species HI-. Combining Beer’s Law, seawater pH can then be expressed as:
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pH = pKI + log
R − e1 , e2 − Re3
(1)
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where R = λ2 A/ λ1 A , and λ1 and λ2 are the wavelengths for the absorbance maxima of HI- and I2-;
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e1, e2, and e3 are indicator molar absorbance ratios at wavelengths λ1 and λ2: e1 =
∈HI , e2 = λ 1 ∈HI
λ2
∈I , e3 = λ 1 ∈HI λ2
∈I , λ 1 ∈HI
(2)
λ1
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where λ1∈I and
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λ2
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work included thymol blue sodium salt 28 (λ1 = 435 nm and λ2 = 596 nm) and m-cresol purple
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sodium salt 29 (λ1 = 434 nm and λ2 = 578 nm). A non-absorbing wavelength (700 nm) was used
λ2
∈I are the molar absorbances of I2- at wavelengths λ1 and λ2, and λ1∈HI and
∈HI refer to the molar absorbances of HI- at wavelengths λ1 and λ2. The indicators used in this
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to correct baseline changes. Calibrations of pKI, e1, e2, and e3 of the two indicators for typical
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seawater temperature and salinity have been established in laboratory experiments 28-29. It has
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been demonstrated that in-situ spectrophotometric pH measurements require infrequent or no
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calibration 19, 22.
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(2) DIC – The CHANOS DIC channel utilizes continuous, countercurrent flow between the
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indicator and acidified sample (Figure 1), instead of CO2 equilibration with static indicator used
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in the previous method
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maintaining a chemical concentration gradient
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AF capillary tubing
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acidified sample as the indicator flows through the Teflon AF tubing that is enclosed in the
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24, 26
. This dramatically improves CO2 equilibration efficiency by 30-32
across the wall of the gas-permeable Teflon
. CO2 fugacity (fCO2) in the indicator equilibrates with fCO2 in the
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sample flow cell. Partial or full CO2 equilibration can be achieved depending on the indicator
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flow rate. After equilibration, DIC (as total CO2) of the acidified sample (denoted by subscript a)
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is proportional to fCO2 of the internal indicator solution (denoted by subscript i) 27: log(p × fCO2)a = log( p ×
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(3)
[ DIC] ) = log(fCO2)i ( K0 ) a
(K0)a is the Henry’s Law constant
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for the acidified sample; p (value 0 – 1) is the
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exchange efficiency or percentage of equilibration of CO2 when the indicator exits the Teflon
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tubing. If the indicator reaches full CO2 equilibration, p has a value of 1 or 100%. Partial
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equilibration (p
