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.