Novel Keeling-plot-based methods to estimate the isotopic composition of ambient water vapor

Yusen Yuan,Honglang Wang,Taisheng Du,Lixin Wang

doi:10.5194/hess-24-4491-2020

Abstract

Abstract. The Keeling plot approach, a general method to identify the isotopic composition of source atmospheric CO2 and water vapor (i.e., evapotranspiration), has been widely used in terrestrial ecosystems. The isotopic composition of ambient water vapor (δa), an important source of atmospheric water vapor, is not able to be estimated to date using the Keeling plot approach. Here we proposed two new methods to estimate δa using the Keeling plots: one using an intersection point method and another relying on the intermediate value theorem. As the actual δa value was difficult to measure directly, we used two indirect approaches to validate our new methods. First, we performed external vapor tracking using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model to facilitate explaining the variations of δa. The trajectory vapor origin results were consistent with the expectations of the δa values estimated by these two methods. Second, regression analysis was used to evaluate the relationship between δa values estimated from these two independent methods, and they are in strong agreement. This study provides an analytical framework to estimate δa using existing facilities and provides important insights into the traditional Keeling plot approach by showing (a) a possibility to calculate the proportion of evapotranspiration fluxes to total atmospheric vapor using the same instrumental setup for the traditional Keeling plot investigations and (b) perspectives on the estimation of isotope composition of ambient CO2 (δa13C).

Highlights

We proposed two new methods to estimate δa, one based on the intersection of two Keeling plots of two continuous observation moments and the other based on the intermediate value theorem
While at nighttime (19:00 to 07:00 the day), average δET was lower than that at daytime, which was contrary to δv, δa(IP) and δa(IVT)
Point-to-point scale data were based on the 88 points of overlapped δa(IP) and δa(IVT) among all 49 d, which accounted for 7.0 % of δa values using the Intersection point (IP) method and 85.4 % of δa values using the Intermediate value theorem (IVT) method

Summary

Introduction

Stable isotopes of hydrogen and oxygen (1H2HO and H128O) have been widely used in root water uptake source identification (Corneo et al, 2018; Mahindawansha et al, 2018; Lanning et al, 2020) and evapotranspiration (ET) partitioning (Brunel et al, 1997; Wang et al, 2010; Cui et al, 2020) in terrestrial ecosystems based on the Craig–Gordon model (Craig and Gordon, 1965), isotope mass balance and mechanisms of isotopic fractionation (Majoube, 1971; Merlivat and Jouzel, 1979). With the advent of laser isotope spectrometry capable of high-frequency (1 Hz) measurements of the isotopic composition of atmospheric water vapor (δv) and atmospheric water vapor content (Cv) (Kerstel and Gianfrani, 2008; Wang et al, 2009), the number of studies based on high-frequency ground-level isotope measurements was continuously increasing These studies generate new insights into the processes that affect δv, including meteorological factors (Galewsky et al, 2011; Steen-Larsen et al, 2013), biotic factors (Wang et al, 2010) and multiple factors (Parkes et al, 2016). More than δv, several new methods using high-frequency ground-level isotope measurements were devised to directly estimate the isotopic composition of leaf water (Song et al, 2015) and leaf-transpired vapor (Wang et al, 2012).

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Conclusion