Chemical compositions of fog and precipitation at Sejila Mountain in the southeast Tibetan Plateau, China.

Chemical compositions of fog and rain water were measured between July 2017 and September 2018 at Sejila Mountain, southeast Tibet, where fog events frequently occurred in original fir forests at altitude 3950 m. Fog water samples were collected using a Caltech Active Strand Cloud Collector (CASCC), and rain samples were collected using a precipitation gauge. Differences were observed between fog water and rain composition for most analyzed ions. Ion abundance in fog water was Ca2+ > Cl- > Na+ > SO42- > Mg2+ > NH4+ >K+ > NO3- whereas an order of Ca2+ > Na+ > Cl- > Mg2+ > SO42- > NO3- > K+ > NH4+ was observed for rain water. All ion concentrations were higher in fog water than in rain water. Additionally, Ca2+ was the dominant cation in both fog and rain samples, accounting for more than half of all measured cations. NH4+ and SO42- concentrations were notable for being higher in fog than rain water when compared with other ions. For trace elements, Al, As, Mn and Se were the most abundant elements in fog water; only Al and As were detected in rain water. Seventy-two hour back-trajectory analysis showed that air masses during fog and/or rain events mainly came from the south of Sejila Mountain. Spearman correlation analysis and source contribution calculations indicated that both marine and terrestrial sources contributed to the observed ion concentrations. Considering the higher concentrations of NH4+ and higher ratio of NH4+/NO3- measured in fog than in rain, we suggest that quantification of fog nitrogen deposition and its ecological effect in this area should be given more attention.