2.2 Sampling and measurements
A 5–L plastic container with a funnel were used to collect the precipitation. The funnel is used to drain water and prevent evaporation. Before sampling, we put the sampler outside and washed it with the rainwater collected at the beginning. The first sample was collected within half an hour after the rainfall began. Once the sampling was completed, each sample was immediately filtered with a 0.45 µm membrane and then poured into 20-mL Labcol bottle using a syringe with no air headspace. These bottles were sealed with rubber plug to prevent evaporation and stored in a refrigerator at 2 to 4 ℃ until stable isotopic analysis could be performed. After sampling, meteorological parameters such as temperature, humidity and air pressure were recorded simultaneously. Due to the different precipitation rate and duration, the samples of precipitation were collected at different time intervals (1 h – 3 h). A total of 162 samples were collected during June 2018 to August 2019, including 33 typhoon-related precipitation samples and 129 normal precipitation samples.
Stable isotope composition (δD and δ18O) of the samples was measured using L-2130i Liquid Water Isotope Analyzer (Picarro, USA). This analysis was conducted at the Third Institute of Oceanography, Ministry of Natural Resources, China. Calibration of the measurements used four internal standards (δ18O: −2.80‰, −7.69‰, −13.10‰, and −16.14‰; δD: −9.5‰, −51.0‰, −96.4‰, and −123.6‰). The results of the stable isotope compositions are expressed as δ-values, relative to the standard V-SMOW (Vienna Standard Mean Ocean Water).
δ (‰) = (Rsample − Rstandard) / Rstandard × 1000,
where R refers to the 18O/16O or D/1H ratio. The measurement accuracy was typically ±1‰ for δD and ±0.2‰ for δ18O.