Dongmei Qu

Against the traditional view, a recently published theory argued that isotope ratios are higher in convective precipitation but lower in stratiform precipitation and proposed that isotope ratios reflect rain type proportions. This theory has been widely cited despite some early reservations. Whether the theory represents a faithful reflection of signals of water isotope ratios remains unclear. Here, we reassess its validity from different timescales and broader observations from the pantropics. Unexpectedly, our findings contradict the theory on daily, monthly, and even annual timescales. Pantropical precipitation isotope ratios remain strongly correlated to convection intensity but are independent of rain type proportions because stratiform precipitation isotope ratios cover a large range of values. We find that the theory has many serious weaknesses related to preferential data selection and suggest that new theories need to be validated at more locations on different timescales before gaining widespread acceptance.

Rationale: Measuring δ ¹⁸ O and δ ² H values in water using wavelength‐scanned cavity ring down spectroscopy (WS‐CRDS) requires multiple injections of up to six (and sometimes eight or more) of one sample to remove the memory effect, which decreases the sample throughput and increases the consumables cost. Thus, improved methods for removing the memory effect are required. Methods: We calculated the memory coefficients by sequential WS‐CRDS measurement of two lab standard waters with isotopic differences, and used them to establish calibration equations. We then used these equations to correct the measured δ ¹⁸ O and δ ² H values by removing the memory effect, instead of using multiple injections in the routine daily measurements. Results: By using this method, the number of injections per sample was reduced to one. The reproducibility (one standard deviation) of the δ ¹⁸ O and δ ² H values obtained for quality control sample was less than 0.05‰ and 0.5‰ for an annual average, respectively. Conclusions: By measuring the memory coefficients and establishing the calibration equations, a highly effective method was developed for determining the δ ¹⁸ O and δ ² H values of water, which could significantly improve sample throughput for liquid water dual isotope measurement without sacrificing the precision.