| Summary: | Isotope data at mid-latitude locations are commonly available on annual, monthly, and daily timescales; however, they are rarely available for the variations occurring on the sub-hourly scale within individual precipitation events. To fill this gap, sub-hourly (every 10 min) sequential samples were collected for nine precipitation events, and the <i>δ</i><sup>18</sup>O values of the individual events were analyzed from June to October 2019 in Lanzhou, Northwest China. The Sequential Meteoric Water Line (SMWL) correlation between <i>δ</i><sup>2</sup>H and <i>δ</i><sup>18</sup>O is <i>δ</i><sup>2</sup>H = 7.33 <i>δ</i><sup>18</sup>O + 9.78 (R<sup>2</sup> = 0.97, N = 170). All events had a similar decreasing variability pattern in the initial period (before the vertical gray dotted line), while during the later period (after the vertical gray dotted line), negative <i>δ</i><sup>18</sup>O values dominated, exhibiting a different pattern. Variations in the <i>δ</i><sup>18</sup>O values were about 1–5‰ during most intra-events. We found that <i>δ</i><sup>18</sup>O values mainly exhibit three patterns in the intra-event, namely a “V”-shaped pattern, an “L”-shaped pattern, and a decreasing pattern. Positive <i>δ</i><sup>18</sup>O values are controlled by re-evaporation in the beginning period. Relative humidity has no effect on the <i>δ</i><sup>18</sup>O values of precipitation events, mainly because dry and warm conditions are conducive to the evaporation of rainwater in the study region. The changes in the isotopic characteristics of precipitation are closely linked to the regional climate. The continuous analysis of precipitation samples revealed that the rapid change of <i>δ</i><sup>18</sup>O values is related to different moisture sources and transport paths. A new air mass with enriched heavy isotope intrusion can change the isotopic composition in the intra-event.
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