Abstract
Abstract. Stable isotopes of water vapour represent a powerful tool for tracing atmospheric vapour origin and mixing processes. Laser spectrometry recently allowed high time-resolution measurements, but despite an increasing number of experimental studies, there is still a need for a better understanding of the isotopic signal variability at different time scales. We present results of in situ measurements of δ18O and δD during 36 consecutive days in summer 2011 in atmospheric vapour of a Mediterranean coastal wetland exposed to high evaporation (Camargue, Rhône River delta, France). The mean composition of atmospheric vapour (δv) is δ18O = −14.66 ‰ and δD = − 95.4 ‰, with data plotting clearly above the local meteoric water line on a δ18O-δD plot, and an average deuterium excess (d) of 21.9 ‰. Important diurnal d variations are observed, and an hourly time scale analysis is necessary to interpret the main processes involved in its variability. After having classified the data according to air mass back trajectories, we analyse the average daily cycles relating to the two main meteorological situations, i.e. air masses originating from North Atlantic Ocean and Mediterranean Sea. In both situations, we show that diurnal fluctuations are driven by (1) the influence of local evaporation, culminating during daytime, and leading to an increase in absolute water vapour concentration associated to a δv enrichment and d increase; (2) vertical air mass redistribution when the Planetary Boundary Layer collapses in the evening, leading to a d decrease, and (3) dew formation during the night, producing a δv depletion with d remaining stable. Using a two-component mixing model, we calculate the average composition of the locally evaporated vapour (δE). We find higher d(E) under North Atlantic air mass conditions, which is consistent with lower humidity conditions. We also suggest that δv measured when the PBL collapses is the most representative of a regional signal. Strong, cold and dry winds coming from the north bring an isotopically depleted vapour, while light, warm and wet winds coming from the south bring an isotopically enriched vapour. Under northern conditions, a strong advection rate dilutes the contribution of the locally evaporated vapour (δE) to the ambient moisture (δv). The higher d values measured under northern conditions, compared to the Mediterranean situation, thus results from the combination of a higher d in both local and regional vapour. This depiction of typical daily cycles of water vapour isotopic composition can be used as a framework for further quantitative analyses of vapour sources during specific days.
Highlights
At the global scale, an acceleration of the hydrological cycle is expected in response to climate change, with an intensification of both precipitation and evaporation
This technology offers new insights into processes that affect the isotopic composition of atmospheric water vapour and the number of studies based on continuous ground level isotope measurements over multiweek periods is continuously increasing, either using a tunable diode laser (TDL) (Lee et al, 2006; Wen et al, 2010, 2012; Griffis et al, 2011; Welp et al, 2012), off-axis integrated cavity output spectroscopy (OA-ICOS) (Sturm and Knohl, 2010; Sunmonu et al, 2012; Farlin et al, 2013; SteenLarsen et al, 2013) or wavelength-scanned cavity ring-down spectroscopy (WS-CRDS) (Galewsky et al, 2011; Noone et al, 2011; Tremoy et al, 2012; Steen-Larsen et al, 2013, 2014)
Our results show that both local and regional isotopic signatures are recorded in the ground level atmospheric vapour composition, and that a sub-daily analysis is necessary to decipher these influences
Summary
An acceleration of the hydrological cycle is expected in response to climate change, with an intensification of both precipitation and evaporation. Whereas cryogenic trapping provides grab samples generally averaging a couple of hours, laser spectrometry performs continuous high frequency (up to ∼ 1 Hz) measurements of δv (Kerstel and Gianfrani, 2008) This technology offers new insights into processes that affect the isotopic composition of atmospheric water vapour and the number of studies based on continuous ground level isotope measurements over multiweek periods is continuously increasing, either using a tunable diode laser (TDL) (Lee et al, 2006; Wen et al, 2010, 2012; Griffis et al, 2011; Welp et al, 2012), off-axis integrated cavity output spectroscopy (OA-ICOS) (Sturm and Knohl, 2010; Sunmonu et al, 2012; Farlin et al, 2013; SteenLarsen et al, 2013) or wavelength-scanned cavity ring-down spectroscopy (WS-CRDS) (Galewsky et al, 2011; Noone et al, 2011; Tremoy et al, 2012; Steen-Larsen et al, 2013, 2014).
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