ACT pH Sensor Verification Protocols, PV12-01
4
3.
Introduction to Technology
There are four important reasons for measuring pH continuously from coastal moorings.
The first is to monitor pH as a naturally changing environmental parameter and as measure of
CO
2
as a pollutant developing acidification in coastal and open ocean ecosystems. Near
continuous measurements of pH can provide some understanding of CO
2
fluxes, their variability
and their forcing parameters. The second reason involves estimating the changes in saturation
state of the water with respect to carbonate minerals and the impact on calcifying ecosystems.
Surface pH measurements in conjunction with direct measurements of one other parameter of the
marine CO
2
system (pCO
2
TA or total DIC) can be used to calculate saturation state (for CaCO
3
minerals such as calcite and aragonite). A third important use is to infer net community
production in shallow waters, and thus further understanding how the carbon cycle is affect by
climate changes parameters such as temperature, and CO
2
.
Changes in pH can occur on a wide
range of time scales ranging from hourly, to seasonal and inter-annual. All the DIC parameters
are potentially being altered by progressive ocean acidification. Thus it is vital to further
promote, develop and improve measurement capabilities for seawater pH. A fourth reason is to
measure and monitor pH for enforcement of effluent discharge into natural waters.
Participants at an earlier ACT workshop on
DIC parameters in natural waters
recognized
that the required certainty and reproducibility of pH measurements varies 100-fold, depending on
the application (items 1-4 above). Specifically, workshop participants concluded that an
accuracy of 0.1 pH unit was typically required for compliance issues but 0.01 would improve
data for regulation. Furthermore, an accuracy of 0.01for field pH measurements was generally
considered adequate for assessing dissolved inorganic carbon changes and speciation in natural
waters; however, there was interest in improving the precision and reducing the uncertainty of
these measurements for ongoing research applications. At the time of the workshop, participants
also concluded that in-situ measurements of pH, TA and DIC were not yet ready for moored
applications; however, in the ensuing 6 years there has been a dramatic increase in the use of pH
instruments for moored applications. Based on this increase in the use of pH sensors, we believe
it is now appropriate to conduct a performance verification to help to identify useful
technologies.
4.
Objectives of the pH Sensor Performance Verification
The fundamental objectives of this Performance Verification are to: (1) highlight the
potential capabilities of particular in situ pH sensors by demonstrating their utility in a range of
coastal environments, (2) verify the claims of manufacturers on the performance characteristics
of commercially available pH sensors when tested in a controlled laboratory setting, and (3)
verify performance characteristics of commercially available pH sensors when applied in real
world applications in a diverse range of coastal environments.
ACT recently performed a customer needs and use assessment on pH sensors. Scientists,
resource managers, and other users of these technologies were asked about their current use or
application of these instruments, their perceptions of limitations or problems with the
technology, and the most important criteria they use when selecting a sensor or instrument
package. The results of these assessments, conclusions from the past DIC workshop, and
discussions with the Technical Advisory Committee were used to identify the main applications
and key parameters that ACT will evaluate in this Technology Verification.
