Testing a portable infrared CO2 gas system for the coastal region

Despite their reduced area (7%), the biogeochemical processes that controls the CO₂ system are more intense in coastal oceans compared to open ocean regions, due to the continental influence or physical processes. Such interactions contribute with the spatial and temporal variability, leading to controversy the role of coastal oceans in the global carbon budget as a net sources or sinks. To determine the latter, a common practice is to derive the partial pressure of the CO_2gas (pCO₂, µatm) from discrete sampling of dissolved inorganic carbon (DIC, µmolkg-1), total alkalinity (TA, µmolkg-1) and/or pH. However, in order to obtain to obtain a continuous sampling, the most reliable way to quantify the partial pressure of this gas is to measure it directly in the field. In this work we propose two objectives: (1) evaluate the precision of an portable system using a infrared CO₂ gas analyzer (LICOR 840-A, ± 2 ppm, MBARI) to measure the molar fraction of CO_2gas (xCO₂, ppm), and (2) define if Bahia de los Angeles, a Mexican coastal region influenced by upwelling, as a CO₂ source or sink. In order to evaluate the performance of the portable system, the xCO₂ measured values were contrasted with discrete samples of DIC (± 3 µmol kg^-1), TA (± 3 µmol kg^-1) and pH (±0.001 pH units). Our results showed that the internal calibration using dry standard gases (475 ± 0.3 ppm and CO₂-free air) worked properly to match discrete samplings, showing a discrepancy of ± 11.6 ppm (± 2.3%) with xCO₂ values derived from TA-DIC, while the use of pH showed a offset around 100 ppm. Finally, the pCO₂ transect measured with the infrared portable system along the bay showed that the study area was a source of CO₂ (3.15 to 4.73 mmol C m^-2 d^-1).