Identification of Different Moisture Sources through Isotopic Monitoring during a Storm Event

Abstract

ain samples were collected for isotopic analyses during the entirety of an extreme rainfall event in Beijing, China, on 21 July 2012, the city?s heaviest rainfall event in the past six decades. Four stages of the storm event have been identified with corresponding isotopic characteristics: 1) isotopes deplete as rain increases, 2) isotopes enrich as rain decreases, 3) isotopes quickly deplete as rain increases, and 4) isotopes remain constant as rain reduces to a small amount. The rainout effect dominates the depletion of isotopic composition in stages 1 and 3. The incursion of a new air mass with enriched heavy isotopes was the main cause for the enriched isotopic composition during stage 2. A Rayleigh distillation model was used to describe the isotopic trends during stages 1 and 3. The Rayleigh distillation model and a binary mixing model were used to estimate the initial isotopic composition of different air masses, which were found to be similar to δ18O of precipitation at nearby Global Network of Isotopes in Precipitation stations representing southwest and southeast trajectories. The results are in agreement with meteorological arrays analysis. This model also indicates that 29% of the initial vapor from the southwest trajectory was precipitated in stage 1, followed by a mixing process between southeast and southwest moisture. In stage 3, up to 56% of mixed moisture was precipitated, among which ~65%?100% was from southeast moisture.