Hydrochemical and isotopic characteristics on the southern and northern slopes of the Himalayas: spatio-temporal controls and source apportionment

Abstract

Water-soluble ions, inorganic nitrogen, and stable isotopes in precipitation were assessed from the southern (Koshi Tappu and Khandbari) and northern slopes (Lhasa and SET) of the Himalayas to understand the sources, chemistry of regional precipitation, and climatic processes. Water soluble ions showed distinct seasonal variation, with higher concentrations in the non-monsoon. The concentration of ionic species was highest in Koshi Tappu, followed by Lhasa, SET, and Khandbari. The sources were from the terrigenous (Ca2+, HCO3−), marine (Na+ and Cl−), anthropogenic (SO42−, NO3−, and NH4+), terrigenous and marine (Mg2+), and biomass-burning (K+). The southern slope, relative to the northern, was more prone to anthropogenic emissions with higher deposition. Among all sites, inorganic nitrogen deposition at Koshi Tappu was higher than the threshold value (10 kg ha−1 y−1). The isotopic composition during the study period was higher in non-monsoon, started declining from June, and depleted in July and August compared to other months, i.e., the monsoon mature phase, along the south-to-north transect. The diminished value of stable isotopes in precipitation with increasing altitude underlines the evidence of the orographic effect in isotopic composition. Our study delineated that the higher/lower d-excess value increased with altitude on the southern/northern slope of the Himalayas. The backward trajectory analysis and the National Centers for Environmental Prediction's Final (NCEP FNL) datasets identified that most of the trajectories arrived from warm and humid low-latitude regions during monsoon and westerlies in non-monsoon. Thus, the chemical characteristics and stable isotopic composition of precipitation differed on the southern and northern slopes of the Himalayas by orographic effect and various sources. This study provides new insights into the atmospheric environment and climatic control of stable isotopes in the Himalayan Tibetan Plateau and facilitates monitoring of transboundary air pollution.