Evaluating Precipitation Amount and Water Isotope Ratios in the <scp>OPEnS</scp> and Palmex <scp>RS1</scp> Passive Collectors: Implications for High‐Frequency Precipitation Sampling

ABSTRACT For decades, precipitation passive collectors for water stable isotopes (δ 2 H, δ 18 O 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 δ 18 O, 0.789 and 0.886 for δ 2 H and 0.410 and 0.937 for d ‐excess, respectively. The annual precipitation weighted means of δ 18 O, δ 2 H 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 (δ 18 O: r 2 = 0.97, δ 2 H: r 2 = 0.98 and d ‐excess: r 2 = 0.97) and Palmex‐autosampler (δ 18 O: r 2 = 0.87 and δ 2 H: r 2 = 0.93), while the correlation for d ‐excess was weaker ( r 2 = 0.72) with the autosampler setup. The best agreement in precipitation amounts was found between OPEnS and Palmex RS1 ( r 2 = 0.91). Combining the Palmex with an autosampler resulted in a lower precipitation amount performance ( r 2 = 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 (δ 2 H = 6.55 × δ 18 O + 5.36; r 2 = 0.84) compared to the OPEnS collector (δ 2 H = 7.33 × δ 18 O + 8.87; r 2 = 0.84) and Palmex RS1 (δ 2 H = 7.56 × δ 18 O + 8.52; r 2 = 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.