Deuterium excess in the atmospheric water vapour of a Mediterranean coastal wetland: regional vs. local signatures
Résumé
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 δ 18 O 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 (δ v) is δ 18 O = −14.66 ‰ and δD = −95.4 ‰, with data plotting clearly above the local meteoric water line on a δ 18 O-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 Mediter-ranean 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 δ v 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 δ v depletion with d remaining stable. Using a two-component mixing model, we calculate the average composition of the locally evaporated vapour (δ E). We find higher d (E) under North Atlantic air mass conditions, which is consistent with lower humidity conditions. We also suggest that δ v 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 (δ E) to the ambient moisture (δ v). 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.
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