3.1.1 Differences of ions in precipitation between Nam Co station and other sites
The mean concentrations of Ca2+, NO- 3 and SO2- 4 in precipitation at Nam Co Station were the lowest compared to the other sites (Table 1), reflecting the clean atmosphere of this site. Muztagh Ata station is close to large deserts in western China (Mao et al., 2013), so that the concentrations of Ca2+ and Mg2+ ions of precipitation at this station were higher than those at Nam Co Station. Meanwhile, due to the contribution of the human activities around Muztagh Ata station is intensive (Li et al., 2021a), the concentrations of NO- 3 and SO2- 4 in precipitation at Muztagh Ata Station were also higher than those at Nam Co Station. Jiuzhaigou is located on the eastern part of the TP where close to Southwest China. Meanwhile, this site has attracted lots of tourists annually (Gu et al., 2013). Therefore, the concentrations of SO2- 4 of NO- 3in precipitation in Jiuzhaigou were also much higher than those at Nam Co Station. Delhi is one of the most polluted cities in South Asia (Ahmed et al., 2022). Concentrations of all species of ions in precipitation in this city were the highest among all the listed sites. Therefore, ion concentrations of precipitation at Nam Co Station are consistent with those of its low values of aerosol concentrations (Cong et al., 2009; Ming et al., 2010).
3.1.2 Spatial Variation of Precipitation chemistry around the Nam Co Station
Despite low ion concentrations of precipitation at Nam Co Station, significant differences of major ions in precipitation were found at three studied sites around Nam Co Station (Fig. 2). The arrangement orders of cations and anions concentrations in precipitation on the north shore of the Nam Co were the same as that at Nam Co Station. The order of cations and anions were Ca2+>NH + 4 > Na+>K+>Mg2+ and Cl−> NO- 3 > SO2-4, respectively (Fig. 2a, 2c). Ca2+ contributes the most to the total mass of the ions at these two sites, accounting for 43.5% and 62.2%, respectively (Fig. 2a, 2c). Correspondingly, the orders of cations and anions were NH + 4 > Ca2+>Na+>K+>Mg2+ and Cl−>NO− 3>SO2- 4 at the sampling site outside the station, respectively (Fig. 2b).
Because two sets of precipitation samples were collected at Nam Co Station and the site outside the station simultaneously, respectively. Differences in ions of precipitation between these two sites can be investigated. It was shown that despite only 2.2 Km away from each other, significant differences were found between concentrations of Ca2+, Mg2+, SO2- 4, and NO- 3 in precipitation between Nam Co Station and the site outside the station (Fig. 4). Generally, heavy precipitation corresponds to low ion concentrations in each individual precipitation event, which is similar to previous studies and other sites (Fig. S2). It was proved that there is a significant negative correlation between precipitation amount and ion concentration, especially for Ca2+ and NO- 3 (Fig. S2).
Despite the site outside of the station is only around two kilometers away from Nam Co Station, concentrations of several ions in that site were significantly lower than those at Nam Co Station. For example, the mean concentrations of Ca2+ and Mg2+ in precipitation were 31.47 µeq/L and 3.05 µeq/L at Nam Co Station, respectively (Table 1), which were about 2.6 and 1.7 times larger than those outside the station. Normally, Ca2+ and Mg2+ in the atmosphere of the TP are mainly originated from the fine particle of the surface soil (Rao et al., 2016; Wang et al., 2019a), so that Ca2+ is usually used as a representative ion for the continental crust (Adhikari et al., 2019; Wang et al., 2019b). Similarly, it has been proposed that high concentrations of fine atmospheric particles were affected by the low coverage of vegetation near the surface of HTP (Himalayas and Tibetan Plateau) (Liu et al., 2017). In each precipitation event, the concentration of Ca2+ in precipitation at Nam Co Station was normally higher than that outside the station (Fig. 3a). It can be inferred that the effect of crustal dust on precipitation at Nam Co Station is larger than that of its outside because there are more buildings, roads, and less vegetation around Nam Co Station compared with the site outside the station (Fig. S1). The mean concentration of Ca2+ in precipitation on the north shore of the Nam Co was slightly higher than that at Nam Co Station (Table 1). In four precipitation events on the north shore of the Nam Co, the concentration of Ca2+ of the precipitation at the north shore of the Nam Co was the highest among three sampling sites (Fig. 3a). It is because the north shore is far from the mountain, generally less precipitation happens at this site, so that vegetation cover on the north shore was lower than that on the south shore of Nam Co.
Meanwhile, the concentrations of NO- 3 and SO2- 4 in precipitation at Nam Co Station were also significantly higher than those outside the station and on the north shore of the Nam Co (Table 1, Fig. 3b, 3c). Except for mineral dust sources, NO- 3 and SO2- 4 are also representative ions of burning activities such as coal-burning and mobile sources (Sharma and Kulshrestha, 2020; Singh et al., 2014). Large numbers of researchers visit Nam Co Station during the monsoon period. Accordingly, motor vehicles are also used around the station. Therefore, both mineral dust and anthropogenic emissions from very local sources at Nam Co Station are the main reasons for the high NO- 3 and SO2- 4 concentrations in precipitation of Nam Co Station.