Posts by Mike Vardaro

Global carbon dioxide (CO₂) concentrations are increasing in the atmosphere, largely due to the use of fossil fuels. The oceans are absorbing about 25-30 percent of the atmospheric CO₂, resulting in a shift in seawater acid-base chemistry and a decrease in ocean pH, making seawater more acidic. To help scientists assess this changing ocean chemistry, the Ocean Observatories Initiative (OOI) uses the Sunburst SAMI-CO₂ instrument to measure the partial pressure of carbon dioxide (pCO₂) from 150-700 microatmospheres (μatm) in the upper 200 meters of the water column.

The distribution of pCO₂ in seawater is dependent on gas exchange with the atmosphere at the ocean surface, the breakdown of plant material by microbial processes, and removal by photosynthesis, calcium carbonate formation, and rising temperatures. Increases in pCO₂ can also be caused by dissolution of calcium carbonate, which is of particular importance because calcium carbonate minerals are the building blocks for the skeletons and shells of many marine organisms, such as oysters.

Using the SAMI-CO₂ instrument, OOI researchers determine the partial pressure of CO₂ by equilibrating a pH sensitive indicator solution (Bromothymol Blue) to sampled seawater. Aqueous carbon dioxide in seawater diffuses across a permeable silicone membrane equilibrator within the instrument, which changes the color of the indicator solution from blue to yellow. The equilibrated indicator solution is then pumped through a chamber where light passes through the liquid and into a receptor that uses the wavelength to determine the amount of color change, and thus the amount of CO₂ dissolved in the water.

“The OOI system parses the raw data from the instrument, applies a ‘blank’ value to correct for instrument drift, and then delivers calibrated pCO₂ data to users on demand,” explained Michael Vardaro, OOI Research Scientist at the University of Washington. “We recently created a fix to apply the correct ’blank’ values to the pCO₂ data to improve data accessibility and data quality.”

Blank values (e.g., optical absorbance ratios in the pco2w_b_sami_data_record_cal data stream) are used to calculate the data product “pCO₂ Seawater (µatm)” at a specific timestamp. Blank values, however, are recorded intermittently to correct for drift of the electro-optical system, about once a week, which is a longer interval than the instrument sampling rate of one sample per hour.

The recent correction will ensure that any pCO₂ data request will use a linearly interpolated value from the closest blanks if no blank value is found within the requested time range. This means that for an hourly pCO₂ measurement that falls between weekly blank values the system will calculate the appropriate drift correction to apply based on the surrounding blank values, instead of trying to find a specific blank value that might be outside the date range of the requested data. In addition to improving data quality, this fix prevents the system from returning fill values or empty datasets. Additional restrictions were put on data delivery to prevent interpolation across deployments, which could pull blanks from different instrument serial numbers, potentially creating bad data. These fixes apply to all OOI pCO₂ data.  Users who have pCO₂ data products generated prior to 4 February 2020 are encouraged to re-request their data to ensure that the correct interpolation code is applied.

Any questions about this data fix, or any other OOI data issues, should be directed to help@oceanobservatories.org.

Images

Top: Alex Andronikides, a VISIONS’17 student from Queens College, New York helps clean a Regional Cabled Observatory Shallow Profiler Mooring science pod that was installed off the Washington-Oregon coast. Credit: M. Elend, University of Washington, V17.

Bottom: Pre-deployment photo of a Sunburst SAMI-CO₂ sensor attached to the Oregon Offshore Cabled Shallow Profiler pod, which moves up and down in the water column between 200 meters and near the surface off the coast of Newport, OR. Credit: M. Elend, University of Washington, V19.