Adapting automatic water samplers for the isotopic study of rainfall at
high temporal resolution
Abstract
Rationale: Stable isotopes of water 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 evaporation
after sampling and to allow precipitation sampling during an event.
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 hours 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 to 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.