Chemo-isotopes (18O &2H) signatures and HYSPLIT model application: Clues to the atmospheric moisture and air mass origins

Abstract

The first ever systematic study on spatio-temporal variation of the chemo-isotopic compositions of the precipitation in Iran was carried out in a hydrological year (2016–2017) at 7 sites located in Bojnourd area, northeast of Iran. The main aims were to determine the moisture and air mass sources and, in particular, to evaluate the effect of atmospheric conditions on isotope compositions. The isotopic value of the precipitation fluctuates seasonally with higher values in summer and lower values in winter at the all sites. Stable isotope values of the precipitation samples within the local scale resulted a Bojnourd meteoric water line (BMWL): δD = 6.4 × δ18O+1.2 (R2 = 0.93, n = 75), with a lower slope and intercept than the global meteoric water line (GMWL) due to isotope kinetic fractionation effects during precipitation in semi-arid region with lower humidity. The air temperature can be suggested as the main controlling factor that impacts the stable isotope compositions of precipitation in the study area. The large rainfall events totally displayed the highest slope, intercept and d-excess values nearly similar to the snowfall samples, indicating the re-evaporation effects on the low rainfall amounts. Precipitations during summer season (Jun–August) with higher isotopes and lower d-excess values mainly originated from the polar and Caspian Sea air moistures. While, moisture contributing to high d-excess and low isotope precipitation during January was mainly transported by the Mediterranean air moisture. The air-mass trajectories that affect the precipitation in autumn (September–December) and spring (March–May) displayed multiple sources, originating from the polar, Mediterranean and Caspian Sea air moistures. Trajectory analysis by HYSPLIT modeling allowed the association of moisture sources with the isotopic compositions of the precipitation. Based on the sea salt contribution for Na+ and Cl−, a sea effect is recognized especially from Caspian Sea and Mediterranean region, despite the long distance at the present study. The correlation between EC and the ions indicate that ion compositions in the precipitation and atmosphere of the study area are mainly effected by dust of soil and sea salt and to some extent from secondary evaporation during rainfall. The hydrochemical investigations confirmed the air masses trajectories which resulted from the isotopic characteristics.