Evaluating precipitation amount and water isotope ratios in the OPEnS and Palmex RS1 passive collectors: implications for high-frequency precipitation sampling

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) passive collectors for water isotope analysis in the Dallas-Fort Worth Metroplex (north-central Texas, USA). Two systems were compared: (1) the low-cost OPEnS collector (version 3.0) and (2) the Palmex RS1 collector combined with a portable autosampler for sub-daily sampling. Our results showed no significant variation in the annual arithmetic mean ( p =0.110 for δ 18 O, p =0.477 for δ 2 H, and p =0.054 for d -excess) between the OPEnS collector and the Palmex-autosampler setup. The annual precipitation weighted means in 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 and -4.05±2.2‰, -20.6±15.3‰ and 11.9±6.7‰ for the Palmex-autosampler setup. Strong linear correlations in δ 2 H ( r 2 =0.96), δ 18 O ( r 2 =0.91), and d -excess ( r 2 =0.83) were found between the OPEnS and Palmex-autosampler setup. However, the best agreement in precipitation amounts was found between the OPEnS and Palmex RS1 (alone) ( 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 bottles carousel. Meteoric water lines suggest a larger influence of secondary evaporation when using the Palmex-autosampler setup (δ 2 H=7.00×δ 18 O+6.72; r 2 =0.84) than the OPEnS collector (δ 2 H=7.33×δ 18 O+8.87; r 2 =0.84). The OPEnS collector is a consistent, cost-effective, and reliable alternative for precipitation sampling similar to the Palmex RS1 collector (alone). Autosampler assemblages to Palmex RS1 or other in-house collectors should be carefully tested and revised regarding precipitation amount and post-sampling evaporation.