Evaluating Precipitation Amount and Water Isotope Ratios in the OPEnS and Palmex RS1 Passive Collectors: Implications for High-Frequency Precipitation Sampling

Abstract

For decades, precipitation passive collectors for water stable isotopes (δ²H, δ¹⁸O and d-excess) analysis have been debated regarding secondary fractionation, sample integrity over time and sampling costs. This study compares the performance of precipitation (N = 36 events) passive collectors for water isotope analysis in the Dallas-Fort Worth Metroplex (north-central Texas, USA). Three systems were compared: (1) the low-cost OPEnS collector (version 3.0), (2) the Palmex RS1 collector alone and (3) the Palmex RS1 collector combined with a portable autosampler for sub-daily sampling. Our results showed no significant variation in the annual arithmetic mean between OPEnS and the other sampling systems. Statistical evaluations between the OPEnS and Palmex RS1 and Palmex-autosampler yielded p values of 0.545 and 0.921 for δ¹⁸O, 0.789 and 0.886 for δ²H and 0.410 and 0.937 for d-excess, respectively. The annual precipitation weighted means of δ¹⁸O, δ²H and d-excess were −4.25‰ ± 1.94‰, −21.2‰ ± 15.1‰ and 12.8‰ ± 7.1‰ for the OPEnS collector; −3.59‰ ± 1.59‰, −18.5‰ ± 13.7‰ and 10.2‰ ± 5.4‰ for the Palmex RS1; and −4.69‰ ± 2.36‰, −23.4‰ ± 16.7‰ and 14.1‰ ± 7.6‰ for the Palmex-autosampler setup. Strong linear correlations were observed between OPEnS and both Palmex RS1 (δ¹⁸O: r² = 0.97, δ²H: r² = 0.98 and d-excess: r² = 0.97) and Palmex-autosampler (δ¹⁸O: r² = 0.87 and δ²H: r² = 0.93), while the correlation for d-excess was weaker (r² = 0.72) with the autosampler setup. The best agreement in precipitation amounts was found between OPEnS and Palmex RS1 (r² = 0.91). Combining the Palmex with an autosampler resulted in a lower precipitation amount performance (r² = 0.59). Precipitation amount discrepancies may occur because of (i) the vertical instability in the OPEnS collector during windy and heavy rainfall conditions, and (ii) the Palmex-autosampler mechanical challenges related to ponding or residual water in the connecting vinyl tubing and distributor's arm movement across the 24-bottle carousel. Meteoric water lines suggest a larger influence of secondary evaporation when using the Palmex-autosampler setup (δ²H = 6.55 × δ¹⁸O + 5.36; r² = 0.84) compared to the OPEnS collector (δ²H = 7.33 × δ¹⁸O + 8.87; r² = 0.84) and Palmex RS1 (δ²H = 7.56 × δ¹⁸O + 8.52; r² = 0.85). The OPEnS collector is a consistent, cost-effective and reliable alternative for precipitation sampling that is similar to the Palmex RS1 collector. Autosampler assemblages to Palmex RS1 or other in-house collectors should be carefully tested and revised regarding precipitation amounts and post-sampling evaporation.