Adapting Automatic Water Samplers for the Isotopic Study of Rainfall at High Temporal Resolution

Rationale

Stable water isotopes in precipitation are powerful tracers of atmospheric processes. Automatic rain samplers are valuable for high temporal resolution isotopic studies but building them from scratch requires significant financial and material resources. A commercial water autosampler has been modified to prevent post-sampling evaporation and to allow for intra-event precipitation sampling.

Methods

New sampling bottles were created by reducing the original volume and opening area. Evaporation tests were carried out on different volumes of water for 72 h under laboratory conditions. These were used to determine the minimum amount of rain to collect to minimize the impact of isotopic fractionation by evaporation. The impact of the autosampler's air moisture saturation was also tested. Samples were analyzed by dual-inlet isotope ratio mass spectrometry and cavity ring-down spectroscopy.

Results

For samples larger than 10 mL, evaporative heavy isotope enrichment due to Rayleigh distillation remains negligible compared with the overall analytical uncertainty. Intentional saturation of the autosampler's atmosphere significantly reduces post-sampling evaporation but leads to equilibration of the samples with the added water. We have investigated the maximum time that samples must be left for this fractionation to remain negligible. Under these conditions, this autosampler is suitable for intra-event rainfall sampling for isotopic analysis.

Conclusions

It is now possible to perform low-cost high-resolution precipitation sampling for isotopic analysis. The intentional air saturation of the sampler, which effectively prevents post-sampling evaporation, had never been proposed before. All instructions for modifying this sampler are now available in open access so the scientific community can easily repeat them.