Water vapour 18O/16O isotope ratio in surface air in New England, USA

In this paper, we report the results of the analysis of two high-resolution time-series of water vapour ¹⁸O/¹⁶O ratio (δv) in surface air observed in Connecticut, USA. On an annual time-scale, δv is a linear function of lnw, where w is water vapour mixing ratio, and is approximated by a Rayleigh distillation model with partial (80%) rainout. On time scales a few days, δv shows considerable variations, often exceeding 20 per mil, and is higher in the wetting phase than in the drying phase of a weather cycle. In precipitation events, the vapour in the surface layer is in general brought to state of equilibrium with falling raindrops but not with snowflakes. On a diurnal time-scale, a peak-to-peak variation of 1–2 per mil is observed at a coastal site. At an interior site, evidence of a diurnal pattern is present only on days of low humidity. Our results suggest that the intercept parameter of the Keeling plot is an ambiguous quantity and should not be interpreted as being equivalent to the isotopic signature of evapotranspiration.