Wushaoling Mountain is an important range of the Qilian Mountains, which located in the east of Hexi Corridor and the south of Wuwei City, Northwest China (102° 52′ 00′ ′ E, 37° 12′ 00′ ′ N). Wushaoling Mountain is the convergence zone of the Loess Plateau, Inner Mongolia Plateau and Tibetan Plateau. It has a length of 17 km from east to west and a width of 10 km from south to north. The region is characterized by a temperate semi-arid climate. Annual average temperature is 0° C, with the minimum of -30.6° C and the maximum of 28.1° C. Average annual precipitation is 401.6 mm (Yang et al., 2010, 2012). Annual average wind speed is 4.83 m/s, and the prevailing wind comes from north and southeast.

In this study, we collected a total of 480 precipitation samples from 6 sampling sites in the southern and northern slopes of Wushaoling Mountain, during the period from May 2013 to July 2014 (Table 1). The sampling process was manual operation. Specifically, a tin bucket (diameter of 20 cm) covered with fresh polyethylene film was placed in a height of 1.5 m above the ground at the beginning of each rainfall event. After each rainfall event, the collected rain sample was poured into a polyethylene sample bottle that cleaned with deionized water in advance. The snow samples were collected immediately after the end of the snowfall events. All samples were immediately sealed and kept in cold storage at -18° C. Anti-pollution measures were taken in the sampling process. Then, samples were transported to the laboratory of Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, and immediately stored at low temperature (-15° C). Precipitation samples were melted naturally at room temperature (about 15° C) before they were analyzed for chemical components.

The pH and electrical conductivity (EC) were measured with a PHS-3B pH meter and a DDSJ-308A conductivity meter (Shanghai Precision & Scientific Instrument Co., Ltd., China), respectively. Concentrations of cations (including Ca²⁺, Mg²⁺, Na⁺, K⁺ and NH₄⁺) and anions (including SO₄^2-, NO₃^-, Cl^-, NO₂^- and F^-) were measured by DX-320 and ICS1500 (Diane Limited Company, China), respectively.

Table 1

Table 1

Table 1 Characteristics of sampling sites in the southern and northern slopes of Wushaoling Mountain Wushaoling Mountain Sampling site Latitude Longitude Altitude (m) Samples Northern slope Wushaoling 37° 12′ 00′ ′ N 102° 52′ 00′ ′ E 3045 51 Heisongyi 37° 21′ 29′ ′ N 102° 54′ 45′ ′ E 2326 74 Gulang 37° 27′ 50′ ′ N 102° 53′ 21′ ′ E 2105 83 Southern slope Tianzhu 36° 57′ 54′ ′ N 103° 08′ 17′ ′ E 2425 40 Jinqiangyi 37° 08′ 38′ ′ N 102° 53′ 00′ ′ E 2755 99 Daiqian 37° 15′ 19′ ′ N 102° 34′ 36′ ′ E 3272 133

Table 1 Characteristics of sampling sites in the southern and northern slopes of Wushaoling Mountain

In this study, we used the multivariate analytical technique to obtain the matrix of geochemical data. Specifically, we applied the factor analysis (i.e., principal component analysis (PCA)) to determine the dominant parameters by transforming a large number of correlated parameters into a small number of uncorrelated parameters (Ma et al., 2012; Li et al., 2015). We standardized the data before analysis to minimize the effects of differences in measurement units and to render the data dimensionless (Li et al., 2014). Factor analysis was explored by extracting eigenvectors and eigenvalues of the correlation matrix, and then the less important parameters were discarded (Zhu et al., 2010). Each factor was independent so that the varimax rotation could be used to constrain factors containing the greatest variability (Mahlknecht et al., 2004). In this study, we performed the algorithm of factor analysis with SPSS 22.0 software.

We used the enrichment factor method to calculate the enrichment factor (degree of enrichment) of the interested ion in the objective substance with respect to the reference ion in the reference substance, which can be used to identify the sources of the interested ion (Ren et al., 2012). There are no active volcanoes around the Wushaoling Mountain, so the contribution of volcanic dust can be ignored. Therefore, it is suitable to apply the enrichment factor method in the study area. This method was firstly proposed by Keene et al. (1986). Subsequently, Xiao et al. (1993) summed up the following reference ions of ocean: (1) Na⁺ as a reference ion of ocean when [Cl^-/Na⁺]_{precipitation}≥ 1.165[Cl^-/Na⁺]_ocean and [Mg²⁺/Na⁺]_{precipitation}≥ 0.227[Mg²⁺/Na⁺]_ocean; (2) Cl^-as a reference ion of ocean when [Na⁺/Cl^-]_{precipitation}≥ 0.859[Na⁺/Cl^-]_ocean and [Mg²⁺/Cl^-]_{precipitation}≥ 0.195[Mg²⁺/Cl^-]_ocean; and (3) Mg²⁺ as a reference ion of ocean when [Na⁺/Mg²⁺]_{precipitation}≥ 4.403[Na⁺/Mg²⁺]_ocean and [Cl^-/Mg²⁺]_{precipitation}≥ 5.126[Cl^-/Mg²⁺]_ocean.

In this study, we determined the reference ion of ocean according to the above-mentioned rules, and selected Ca²⁺ as the reference ion of crust according to the study of Taylor (1964). The enrichment factor of the interested ion in precipitation with respect to the reference ion of crust (EF_crust) and the enrichment factor of the interested ion in precipitation with respect to the reference ion of ocean (EF_ocean) were calculated as follows (Okay et al., 2002; Ren et al., 2012):

EF_crust=[X/Ca²⁺]_{precipitation}/[X/Ca²⁺]_crust, (1)

EF_ocean=[X/N]_{precipitation}/[X/N]_ocean, (2)

where X is the concentration of the interested ion in precipitation (mol/L), and N is the concentration of the reference ion in ocean (mol/L). In this study, the value of [X/Ca²⁺]_crust was referred from the crustal data in the study of Taylor (1964), and the value of [X/N]_ocean was referred from the ocean data in the study of Keene et al. (1986). Generally speaking, EF≤ 1 represents that the interested ion in precipitation is diluted, while EF> 1 represents that the interested ion in precipitation is enriched.

The sources of ions in precipitation include anthropogenic activities, terrigenous material, and sea salt aerosol. We therefore calculated the sea salt fraction (SSF), crust faction (CF) and anthropogenic fraction (AF) to identify the sources of ions in precipitation. They were calculated by the following equations (Keene et al., 1986; Li et al., 2015):

SSF(%)=100× [X/N]_ocean/[X/N]_{precipitation}, (3)

CF (%)=100× [X/Ca²⁺]_crust/[X/Ca²⁺]_{precipitation}, (4)

AF(%)=100%-SSF-CF. (5)

During the sampling period (i.e., from May 2013 to July 2014), the pH values of precipitation in the southern and northern slopes of Wushaoling Mountain almost showed the same variation range, with the average value in the northern slope being slightly higher than that in the southern slope. Specifically, pH values of precipitation ranged from 6.81 to 8.68 in the southern slope (average of 7.50) and from 6.66 to 8.54 in the northern slope (average of 7.55). The pH values of precipitation in the northern and southern slopes were almost alkaline. Seasonal variations in pH values of precipitation in the southern and northern slopes of Wushaoling Mountain are shown in Figure 1. It can be seen that the average pH in precipitation was highest in summer both in the southern and northern slopes.

Fig. 1

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	Fig. 1 Seasonal variations in pH values of precipitation in the southern (a) and northern (b) slopes of Wushaoling Mountain. The median of the data is presented by the line inside of the rectangle and “ whiskers” above and below the box display the locations of the maximum and minimum values, respectively. The black square inside of the rectangle represents the outlier.

In the southern slope of Wushaoling Mountain, the EC values of precipitation ranged from 13.15 to 1086.00 μ s/cm, with a mean of 156.07 μ s/cm; while in the northern slope, the values ranged from 17.28 to 971.50 μ s/cm, with a mean of 179.60 μ s/cm (Fig. 2). In the southern slope, the highest EC value was observed on 1 May, 2014 and the lowest value appeared on 15 September, 2013. In the northern slope, the highest EC value was observed on 24 April, 2014 and the lowest value appeared on 11 August, 2013. At seasonal scale, the EC values of precipitation in the southern slope were all lower than those in the northern slope. As shown in Figure 2c, the EC values of precipitation were higher in spring and winter than in summer and autumn. This may be due to the frequent sand dust events in spring and winter.

Fig. 2

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	Fig. 2 Variations of electrical conductivity (EC) in precipitation in the northern and southern slopes of Wushaoling Mountain from May 2013 to July 2014 at daily (a, b) and seasonal (c) scales

Concentrations of ions in precipitation in the northern and southern slopes of Wushaoling Mountain are shown in Table 2. It can be seen that concentrations of Mg²⁺, Ca²⁺, NH₄⁺, F^-, NO₃^-, and SO₄^2- were significant between the northern and southern slopes, while there was no significant difference in concentrations of Na⁺, K⁺, Cl^-, and NO₂^-between the northern and southern slopes.

In the southern slope of Wushaoling Mountain, Ca²⁺and Na⁺ dominated the cations of precipitation (accounting for 83.58% of the total cation composition), while SO₄^2- and NO₃^-dominated the anions (accounting for 83.78% of the total anion composition) (Fig. 3). The chemical type of precipitation in the southern slope was SO₄^2--Ca²⁺-NO₃^--Na⁺, with the four ions accounting for 83.69% of the total ion concentration. In the northern slope of Wushaoling Mountain, also the Ca²⁺and Na⁺ dominated the cations (accounting for 73.18% of the total cation composition), and SO₄^2- and NO₃^-dominated the anions (accounting for 83.82% of the total anion composition). The chemical type of precipitation in the northern slope was SO₄^2--Ca²⁺-NO₃^--Na⁺, with the four ions accounting for 79.77% of the total ion concentration. In addition, the percentage of individual ion concentration in precipitation was in the order of Ca²⁺> SO₄^2-> NO₃^-> Cl^-> Na⁺> K⁺> Mg²⁺> NH₄⁺> F^-> NO₂^- in the southern slope, and in the order of SO₄^2-> Ca²⁺> NO₃^-> Cl^-> Na⁺> Mg²⁺> K⁺> NH₄⁺> NO₂^-> F^-in the northern slope. It can be seen that the order of the percentage of individual ion concentration in precipitation was basically the same in the southern and northern slopes.

Table 2

Table 2

Table 2 Concentrations of ions in precipitation in the northern and southern slopes of Wushaoling Mountain Wushaoling Mountain Na+ K+ Mg2+ Ca2+ NH4+ (μ eq/L) Southern slope 3.50± 0.51a 1.70± 0.18a 1.66± 0.15A 17.78± 1.34A 0.82± 0.06A Northern slope 4.69± 0.78a 1.95± 0.31a 2.74± 0.34B 12.26± 1.06B 1.52± 0.15B Wushaoling Mountain F- Cl- NO2- NO3- SO42- (μ eq/L) Southern slope 0.17± 0.02a 4.74± 0.89a 0.14± 0.03a 10.40± 0.90a 15.69± 1.83a Northern slope 0.11± 0.01b 5.82± 0.95a 0.15± 0.06a 7.69± 0.82b 23.82± 2.83b Note: Mean± SE. Different uppercase letters with a column indicate significant difference of ion concentration in precipitation between the southern and northern slopes at P< 0.01 level; while different lowercase letters with a column indicate significant difference of ion concentration in precipitation between the southern and northern slopes at P< 0.05 level.

Table 2 Concentrations of ions in precipitation in the northern and southern slopes of Wushaoling Mountain

Fig. 3

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	Fig. 3 Percentage of individual ion concentration in precipitation in the northern and southern slopes of Wushaoling Mountain

At seasonal scale, the total ion concentration in precipitation was in the order of spring> winter> autumn> summer in the southern slope of Wushaoling Mountain (Fig. 4a) and in the order of winter> spring> autumn> summer in the northern slope (Fig. 4b). In the southern slope, the concentration of Ca²⁺ in precipitation was higher in spring than in winter, and higher in summer than in autumn. In the northern slope, the concentration of Ca²⁺ in precipitation was also higher in spring than in winter, and higher in summer than in autumn. The concentration of SO₄^2-was highest in winter and spring in the northern slope and southern slope, respectively. The seasonal variation of NO₃^-in precipitation in the northern and southern slopes exhibited the same order, that is, winter> spring> autumn> summer. The concentration of Na⁺ in precipitation showed the same seasonal variation in the northern and southern slopes. The seasonal variation of Cl^-in precipitation was in the order of winter> spring> autumn> summer in the northern slope, and in the order of spring> winter> autumn> summer in the southern slope.

Fig. 4

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	Fig. 4 Seasonal variations of ion concentrations in precipitation in the southern (a) and northern (b) slopes of Wushaoling Mountain

Relationships of ions in precipitation in the southern and northern slopes of Wushaoling Mountain are shown in Tables 3 and 4, respectively. The SO₄^2- in precipitation showed significant correlations with Na⁺, K⁺, Mg²⁺, Ca²⁺, F^-, Cl^- and NO₃^-in precipitation both in the southern and northern slopes. In the southern slope, the NO₃^- in precipitation showed significant correlations with Na⁺, K⁺, Mg²⁺, Ca²⁺, F^-and Cl^-in precipitation; while in the northern slope, the NO₃^- in precipitation was significantly correlated with Na⁺, K⁺, F^-and Cl^- in precipitation. The Cl^- in precipitation was significantly correlated with Na⁺, K⁺, Mg²⁺, Ca²⁺and F^-in precipitation both in the southern and northern slopes of Wushaoling Mountain (Tables 3 and 4). The NH₄⁺in precipitation showed significant correlations with SO₄^2-and NO₃^-in precipitation in the southern slope, indicating that NH₄⁺was the major neutralizing ion for SO₄^2-and NO₃^-. By comparing the correlation coefficients among all ions in precipitation between the southern slope and the northern slope, we found that the correlation coefficients among Na⁺, K⁺, Mg²⁺, Ca²⁺and NO₃^-in the southern slope were higher than those in the northern slope, while the coefficient between F^-and Cl^-in the northern slope was higher than that in the southern slope.

Generally speaking, the ion concentrations of precipitation were higher in the northern slope than in the southern slope of Wushaoling Mountain. This may be due to that the northern slope is near to the sand dust sources, especially near to the deserts. Furthermore, there are many cement factories distributed in the northern slope, leading to the increase in the ion concentrations in precipitation. At seasonal scale, the ion concentrations of precipitation were relatively high in winter and spring both in the southern and northern slopes. This may be related to the low precipitation, high wind speed, and frequent sand dust events in these two seasons. Sand dust events can increase the terrigenous material in the atmosphere (Li et al., 2017b), inevitably resulting in the increase of ion concentrations in precipitation. By contrast, the ion concentrations of precipitation were relatively low in summer and autumn, which may be related to the high precipitation amount. Specifically, the influence of dilution effect from precipitation on the ion concentrations was more obvious in summer and autumn, leading to the low ion concentrations.

Generally speaking, the SO₄^2-was mainly from the local sources. On the one hand, it was from the contribution of weathering effect of the sulphate minerals, which related to the enrichment of CaSO₄ and MgSO₄. On the other hand, it was from the SO₂created by the industrial production and anthropogenic activities. The Ca²⁺ and Mg²⁺ are two important crustal material components, reflecting that the important influences of MgSO₄ and CaSO₄are from the local source particles (Al-khashman, 2005; Eneroth et al., 2007; Li et al., 2014). The NO₃^-was mainly from the pollutant emissions of anthropogenic activities in the southern slope, and from the oil consumption and pollutant emissions of anthropogenic activities in the northern slope. The average ratio of Na⁺ concentration to Cl^-concentration in precipitation was 0.73 in the southern slope and 0.81 in the northern slope (Table 2), being lower than the average ratio of Na⁺ concentration to Cl^-concentration in seawater (0.86) (Al-khashman, 2005; Eneroth et al., 2007). This suggested that the sources of Cl^- in precipitation were complex, and the Cl^- may probably come from the ocean sources, local sources and anthropogenic activities. The ratio of K⁺ concentration to Cl^-concentration in precipitation was 0.36 in the southern slope and 0.34 in the northern slope (Table 2), being higher than the ratio of K⁺ concentration to Cl^-concentration in seawater (0.061). This suggested that the contribution of ocean to K⁺ can be ignored. By contrast, local sources and anthropogenic activities may play an important role in the concentration of K⁺.

Table 3

Table 3

Table 3 Pearson correlation analysis of ions in precipitation in the southern slope of Wushaoling Mountain Na+ K+ Mg2+ Ca2+ NH4+ F- Cl- NO2- NO3- SO42- Na+ 1.000 K+ 0.538* * 1.000 Mg2+ 0.743* * 0.471 1.000 Ca2+ 0.693* * 0.682* * 0.615* * 1.000 NH4+ -0.157 -0.163 -0.169 -0.198 1.000 F- 0.306* * 0.319* * 0.316* * 0.453* * 0.005 1.000 Cl- 0.737* * 0.392* * 0.370* * 0.463* * -0.032 0.339* * 1.000 NO2- 0.081 0.268* * 0.086 0.102 0.016 0.114 0.108 1.000 NO3- 0.483* * 0.579* * 0.356* * 0.576* * -0.210* * 0.452* * 0.411* * 0.038 1.000 SO42- 0.665* * 0.555* * 0.538* * 0.698* * -0.201* * 0.544* * 0.530* * 0.041 0.764* * 1.000 Note: * * means correlation is significant at P< 0.01 level (2-tailed).

Table 3 Pearson correlation analysis of ions in precipitation in the southern slope of Wushaoling Mountain

Table 4

Table 4

Table 4 Pearson correlation analysis of ions in precipitation in the northern slope of Wushaoling Mountain Na+ K+ Mg2+ Ca2+ NH4+ F- Cl- NO2- NO3- SO42- Na+ 1.000 K+ 0.549* * 1.000 Mg2+ 0.579* * 0.256* * 1.000 Ca2+ 0.412* * 0.258* * 0.292* * 1.000 NH4+ -0.173 -0.065 -0.239 -0.119 1.000 F- 0.276* * 0.366* * 0.187 0.292* * 0.152 1.000 Cl- 0.690* * 0.475* * 0.361* * 0.319* * -0.104 0.576* * 1.000 NO2- 0.153 0.228 0.038 0.159 -0.046 0.095 0.101 1.000 NO3- 0.428* * 0.411* * 0.226 0.145 -0.142 0.345* * 0.510* * 0.028 1.000 SO42- 0.630* * 0.452* * 0.343* * 0.324* * -0.021 0.671* * 0.896* * 0.059 0.526* * 1.000 Note: * * means correlation is significant at P< 0.01 level (2-tailed).

Table 4 Pearson correlation analysis of ions in precipitation in the northern slope of Wushaoling Mountain

In the southern slope of Wushaoling Mountain, the first three PC (principal component) factors explained 66.67% of the total variance of precipitation chemistry (Table 5). Specifically, the first PC factor explained 44.24% of the total variance of precipitation chemistry, and this factor had high loading eigenvalues of all ions (with exceptions of NH₄⁺ and NO₂^-). This suggested that ions in precipitation were mainly from the local sources. The second PC factor explained 11.90% of the total variance of precipitation chemistry, and this factor had high loading eigenvalue of NO₂^-. This indicated that industrial emissions, municipal waste incineration and automobile exhaust also contributed to the chemical characteristics of precipitation. The third PC factor explained 10.53% of the total variance of precipitation chemistry, and this factor had high loading eigenvalues of NH₄⁺ and F^-. This reflected the contribution of industrial and agricultural production, and organic decomposition on the chemical characteristics of precipitation.

Table 5

Table 5

Table 5 Eigenvalues and variances of the first three PC (principal component) factors for ions in precipitation in the southern and northern slopes of Wushaoling Mountain Eigenvalue/Variance Southern slope Northern slope PC 1 PC 2 PC 3 PC 1 PC 2 PC 3 Na+ 0.829 0.360 -0.041 0.824 -0.257 -0.020 K+ 0.776 0.120 -0.199 0.640 -0.126 0.351 Mg2+ 0.743 0.127 -0.274 0.608 -0.404 -0.185 Ca2+ 0.869 -0.008 -0.149 0.568 -0.127 0.229 NH4+ -0.256 0.368 0.698 -0.164 0.755 0.237 F- 0.540 -0.248 0.533 0.617 0.562 0.072 Cl- 0.577 0.414 0.278 0.875 0.184 -0.135 NO2- 0.227 0.554 -0.039 0.204 -0.180 0.847 NO3- 0.697 -0.444 0.142 0.586 0.001 -0.255 SO42- 0.774 -0.395 0.209 0.860 0.306 -0.159 Percentage of variance (%) 44.24 11.90 10.53 40.78 13.07 10.98 Cumulative percentage of variance (%) 44.24 56.14 66.67 40.78 53.85 64.83

Table 5 Eigenvalues and variances of the first three PC (principal component) factors for ions in precipitation in the southern and northern slopes of Wushaoling Mountain

In the northern slope of Wushaoling Mountain, the first three PC factors explained 64.83% of the total variance of precipitation chemistry (Table 5). The first PC factor explained 40.78% of the total variance of precipitation chemistry, and this factor had high loading eigenvalues of all ions (with exceptions of NH₄⁺ and NO₂^-). This indicated that ions in precipitation were mainly from local sources. The second PC factor explained 13.07% of the total variance of precipitation chemistry, and this factor had high loading eigenvalue of NH₄⁺ and F^-. This suggested the contribution of industrial and agricultural production, and organic decomposition on the chemical characteristics of precipitation. The third PC factor explained 10.98% of the total variance of precipitation chemistry, and this factor had high loading eigenvalue of NO₂^-. This reflected the contribution of waste gas emissions from industrial production, municipal waste incineration and automobile exhaust.

We used the enrichment factor method to identify the sources of ions in precipitation in the Wushaoling Mountain. It should be noted again that the sources of ions in precipitation include anthropogenic activities, terrigenous material, and sea salt aerosol (Al-khashman, 2005; Li et al., 2014, 2015, 2016; Guo et al., 2015). We calculated the [Cl^-/Na⁺]_{precipitation}=1.35[Cl^-/Na⁺]_ocean and [Mg²⁺/Na⁺]_{precipitation}=0.47[Cl^-/Na⁺]_ocean for the southern slope, and the [Cl^-/Na⁺]_{precipitation}=1.24[Cl^-/Na⁺]_ocean and [Mg²⁺/Na⁺]_{precipitation}=0.58[Cl^-/Na⁺]_ocean in the northern slope, suggesting that Na⁺ was the reference ion of ocean (Xiao et al., 1993).

The enrichment factors of ions in precipitation in the southern and northern slopes of Wushaoling Mountain are shown in Table 6. The EF_crust values of Na⁺ were generally lower than 1 both in the southern and northern slopes, indicating that Na⁺was derived from the crust. The EF_crust values of K⁺ were much lower than 1 while the EF_ocean values of K⁺were much higher than 1, which suggested that K⁺ was mainly derived from the crust. The EF_crust values of Mg²⁺ were lower than 1 while the EF_ocean values of Mg²⁺were higher than 1 in the two slopes, indicating that Mg²⁺ was mainly from the crust. The EF_crust values of Cl^- ranged from 46.83 to 138.05 in the southern slope and from 108.93 to 326.84 in the northern slope; and, only in autumn in the northern slope, the EF_ocean value of Cl^- was higher than 1. This suggested that ocean was the main source for Cl^-. The EF_crust values of NO₃^- were higher than 233.77 and the EF_ocean values of NO₃^- were higher than 717.07 in the southern and northern slopes, indicating that ocean and crust almost exhibited no contribution to NO₃^-, and the main source of NO₃^-was anthropogenic activities. In addition, SO₄^2-was also mainly from anthropogenic activities. The EF_ocean values of Ca²⁺ ranged from 89.66 to 212.01 in the southern slope and from 35.61 to 102.11 in the northern slope, suggesting that Ca²⁺was mainly from the crust. The EF_ocean values of F^-were higher than 1 in the two slopes, which indicated that F^-was mainly from the crust.

In this study, we calculated the source contributions of sea salt fraction, crust fraction and anthropogenic fraction (Equations 3-5, respectively) to further determine the sources of ions in precipitation in the study area (Mahlknecht et al., 2004; Li et al., 2015). The results are shown in Table 7. The NO₃^-in precipitation was mainly from the anthropogenic activities, with contribution of 99.64% in the southern slope of Wushaoling Mountain and 99.69% in the northern slope. Only a small part of SO₄^2-was contributed from the ocean fraction and crust fraction. The contributions of crust fraction and anthropogenic fraction for Mg²⁺and K⁺in precipitation were 76.00% and 93.75% in the southern slope, and 80.35% and 93.43% in the northern slope, respectively. Similarly, the F^-in precipitation in the southern and northern slopes was mainly affected by crust and anthropogenic activities, with contributions of 87.86% and 76.18%, respectively. Different from the other ions, the Cl^-in precipitation mainly came from the sea salt fraction, with the contributions of 98.83% and 99.53% in the southern and northern slopes, respectively.

Table 6

Table 6

Table 6 Enrichment factors of ions in precipitation in the southern and northern slopes of Wushaoling Mountain with respect to the reference ion in the reference substance Southern slope Enrichment factor of ion in precipitation relative to Ca2+in crust Enrichment factor of ion in precipitation relative to Na+ in ocean Na+ K+ Mg2+ Cl- NO3- SO42- K+ Mg2+ Ca2+ F- Cl- NO3- SO42- Spring 0.53 0.19 0.19 138.05 285.29 60.77 8.74 2.84 89.66 3.88 0.71 1342.82 15.53 Summer 0.22 0.18 0.15 46.83 233.77 31.34 19.7 5.25 212.01 10.77 0.57 2601.90 18.94 Autumn 0.32 0.14 0.17 82.30 381.94 53.22 10.04 4.10 147.28 9.97 0.69 2953.28 22.35 Winter 0.25 0.26 0.14 73.55 338.03 58.83 25.50 4.36 192.53 8.32 0.81 3416.75 32.29 Northern slope Enrichment factor of ion in precipitation relative to Ca2+in crust Enrichment factor of ion in precipitation relative to Na+ in ocean Na+ K+ Mg2+ Cl- NO3- SO42- K+ Mg2+ Ca2+ F- Cl- NO3- SO42- Spring 0.91 0.26 0.4 164.63 262.06 110.30 6.88 3.45 52.12 1.99 0.49 717.07 16.39 Summer 0.47 0.24 0.35 108.93 264.58 69.67 12.25 5.91 102.11 4.97 0.63 1418.42 20.28 Autumn 0.51 0.59 0.55 237.19 407.17 169.53 27.85 8.38 93.14 6.17 1.26 1990.94 45.01 Winter 1.33 0.77 0.45 326.84 712.50 239.53 13.82 2.62 35.61 3.66 0.66 1331.98 24.32

Table 6 Enrichment factors of ions in precipitation in the southern and northern slopes of Wushaoling Mountain with respect to the reference ion in the reference substance

Table 7

Table 7

Table 7 Source contributions for different ions in precipitation in the southern and northern slopes of Wushaoling Mountain Source Fraction Na+ K+ Mg2+ Ca2+ F- Cl- NO3- SO42- Southern slope Ocean source SSF (%) 0.00 6.25 24.00 0.00 12.14 98.83 0.32 4.48 Land source CF (%) 100.00 93.75 76.00 100.00 87.86 1.17 0.04 1.95 AF (%) 0.00 0.00 0.00 99.64 93.57 Source Fraction Na+ K+ Mg2+ Ca2+ F- Cl- NO3- SO42- Northern slope Ocean source SSF (%) 0.00 6.57 19.65 0.00 23.82 99.53 0.07 3.77 Land source CF (%) 100.00 93.43 80.35 100.00 76.18 0.47 0.24 0.67 AF (%) 0.00 0.00 0.00 99.69 95.56 Note: SSF, sea salt fraction; CF, crust fraction; AF, anthropogenic fraction.

Table 7 Source contributions for different ions in precipitation in the southern and northern slopes of Wushaoling Mountain

In order to study the indication of precipitation chemistry to sand dust events, we selected three typical sand dust events occurred in the Wushaoling Mountain based on the frequency of sand dust events and the temporal distribution of precipitation. Specifically, we analyzed the changes in chemical characteristics of precipitation after the occurrence of sand dust events. The first sand dust event occurred on 22 May, 2013; the second sand dust event occurred on 23 and 24 April, 2014; and the third sand dust event occurred on 30 April, 2014. It should be noted that for the three selected sand dust events, precipitation occurred after the occurrence of sand dust events.

As shown in Figure 5, Na⁺ in precipitation exhibited a high concentration after the occurrence of sand dust events both in the southern and northern slopes of Wushaoling Mountain. As above-mentioned analysis, Na⁺in precipitation was mainly from the terrigenous material. Sand dust events could increase the terrigenous material in the atmosphere, leading to the increase in the concentration of Na⁺in precipitation. After the occurrence of sand dust events, the concentrations of K⁺, Mg²⁺, SO₄^2-, NO₃^-and Ca²⁺ in precipitation increased in the southern slope and decreased in the northern slope. There may be two possible reasons. On the one hand, these ions were mainly from the terrigenous material and anthropogenic activities. On the other hand, a large number of sand dusts were blown to the southern slope when the sand dust events occurred, thereby increasing the ion concentrations in precipitation in the southern slope and decreasing the ion concentrations in the northern slope. The concentration of Cl^-increased after the occurrence of sand dust events both in the southern and northern slopes, indicating that the sands and dusts contained a high concentration of NaCl.

Fig. 5

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	Fig. 5 Concentrations of ions in precipitation in the southern and northern slopes of Wushaoling Mountain. It should be noted that the first sand dust event occurred on 22 May, 2013 (orange line); the second sand dust event occurred on 23 and 24 April, 2014; and the third sand dust event occurred on 30 April, 2014.