Sub-hourly variability of stable isotopes in precipitation in the marginal zone of East Asian monsoon

Abstract

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 δ18O values of the individual events were analyzed from June to October 2019 in Lanzhou, Northwest China. The Sequential Meteoric Water Line (SMWL) correlation between δ2H and δ18O is ffi2H = 7.33 δ18O + 9.78 (R2 = 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 δ18O values dominated, exhibiting a different pattern. Variations in the δ18O values were about 1-5‰ during most intra-events. We found that δ18O values mainly exhibit three patterns in the intra-event, namely a "V"-shaped pattern, an "L"-shaped pattern, and a decreasing pattern. Positive δ18O values are controlled by re-evaporation in the beginning period. Relative humidity has no effect on the δ18O 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 δ18O 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.