Atmosphere Vapor Isotope Change in Monsoon Region at Lhasa in Southern Tibetan Plateau

Abstract:

Daily atmosphere vapor samples were collected by using a cold trap maintaining in -70℃ during the 2 years of Nov 2011—Nov 2013. The oxygen isotope and hydrogen isotope were measured for all these samples, and were compared with daily precipitation isotopes. We first present the seasonal variations of vapor d¹⁸O and deuterium-excess (d-excess). A stable increasing of vapor d¹⁸O was found from beginning of the year to the maximum of -10‰ in May, followed by a sharp drop in June with the monsoon incursion. Large fluctuation of d¹⁸O marked the monsoon season of June-September, with the lowest d¹⁸O reached approximately to -37‰, even lower than in the winter season. The large fluctuation of vapor d¹⁸O is related to rainfall events in the rainfall season, which sharp decrease following the precipitation events. D-excess fluctuations in vapor differ from d¹⁸O. However, there is an obvious lower d-excess in monsoon precipitation season during June-September. In average, vapor d-excess did not correlate well with d¹⁸O. However, in monsoon season (June-September), the correlation is obvious. Vapor d¹⁸O shows an positive correlation with vapor pressure in non-monsoon season, but a clear inverse correlation with vapor pressure in monsoon season. Comparing the atmospheric vapor and concurrent precipitation isotopes within one year of parallel observation shows that vapor isotope is 13.0‰ lower than precipitation for d¹⁸O, 98.6‰ lower for dD, and 15‰ higher for d-excess.