Direct and indirect pCO2 measurements in a wide range of pCO2 and salinity values

Recent improvements in both Infra-red spectroscopy and equilibrator techniques have allowed to determine, for the first time, pCO2 using simultaneously and continuously both the direct and indirect methods in an estuary where pCO2 values range from 500 to 8500 µatm and salinity from 0 to 30. Our results show that both methods are in excellent agreement in the whole estuary (r2 = 0.999, n = 1075, p < 0.0001). Thus, the NBS (US National Bureau of Standards) scale, although inadequate for seawater samples, is appropriate for estuarine waters and can be applied with confidence to calculate pCO2.