On the Quantitative Tripartite Allocation of Atmospheric Vapor, Oqtav, by Oxygen 18
Authors: Hussein Mohamed Fahmy , Zouari Kamal , Anter Mohamed , Nosser Mariam 
Published: August 2020

The quantitative distribution of atmospheric vapor mixture, AVM, into three distinguished vapor endmembers is lacking in the literature. This work fills such a gap. The isotope ratio, d18OL, of rainwater in Winter, and artificial condensates in Summer, gave the 18OV contents of local AVMs at temperature-dependent equilibrium, downtown Cairo city, Nile Delta apex. We used our models, TIMAM, CLAW, and SIGNALS to process the d18OV and the commensurate specific humidity, S, values in several AVM data sets for determining the percent and mass contributions of three moisture origins and their temporal waveforms. The proportions and masses revealed Marine vapor dominance, followed by evapotranspiration. By far, the free Troposphere source showed a slight input. The quota of each constituent manifests a delayed waveform vs. AVM d18O influx, which shows a diurnal peak and a nocturnal tunnel. The moderate ET percent inputs in Winter, and by daytime, impose significant AVM 18O enrichment. In contrast, the high Maritime vapor inputs in Summer, and by night, stand behind the depleted AVM 18O content. The relationships between the mass input of each source and the AVM isotope ratio show significant dispersion for the negative trend of the diurnal-nocturnal Marine vapor in the two seasons. Such a high scattering is due to the mingling of northern wind-gust diurnal convection (marked by low Marine vapor input) and northern steady nocturnal advection (characterized by high Marine vapor input). Marine vapor waveform has a 12-hour time-lag by the intertwining of turbulent diurnal transmission, and steady nocturnal transport, through the long trajectory (180 km) from the Mediterranean coast to Cairo. In contrast, the relationships between ET mass input and AVM isotope ratio, on the one hand, and between the Troposphere vapor mass input and AVM isotope ratio, on the other hand, manifest low-dispersion positive and negative regressions, respectively. Such a low dispersion is due to the short transport pathway, the narrow range of the biological input (that increases only by daytime), and sharp Troposphere downdraft (moving northward in Winter but southward in Summer). Also, the ET waveform has a Zero-hour time-lag, like that of the Tropospheric vapor. Albeit the low S value of the Troposphere vapor pole, its impact on the AVM isotopic depletion is significant due to its extremely shallow 18O content. The increase of the Tropospheric input at low AVM S values is related to regional drought, as expected. The high S values, of Marine and biotic origins, usually go with temperature apogees, especially in Summertime, as anticipated. The used models help in improving the time-series simulation of evaporation runs, since using seasonal d18OV and S markers is better than using a snapshot. The ternary-vapor-source allocation procedure is a breakthrough in isotope hydrology. This thoroughly useful procedure will prove its ultimate benefits when the users get CRDS laser-controlled devices for the continuous measurements of the isotopic ratios in the local AVMs.