Chemical Compositions of Rainfall Water in Nyingchi City, Tibet

Abstract

Understanding precipitation chemistry is highlighted as important worldwide due to its close relationship with air quality and impacts on ecosystems. However, the chemical composition of precipitation is limited in Tibet, where alpine ecosystems are sensitive to global change. Here, rainwater samples were collected in Nyingchi city from January 2021 to December 2021, and a total of 44 samples were obtained. Major ions (NO₃⁻, NH₄⁺, Cl⁻, SO₄²⁻, Na⁺, K⁺, Ca²⁺ and Mg²⁺) were analyzed. Results showed that the predominant ions in the precipitation were Ca²⁺, Na⁺, SO₄²⁻, and Cl⁻. Precipitation was mainly concentrated in summer, accounting for 65.2% of all samples collected during the monitoring period. As a result, ion deposition fluxes were mainly concentrated in summer, accounting for 55%, 53%, 84%, 82%, 61%, 63%, 75.8%, and 37.8% of the annual Ca²⁺, K⁺, Mg²⁺, Na⁺, NH₄⁺, Cl⁻, SO₄²⁻, and NO₃⁻, respectively. Backward trajectory analysis revealed that airmasses were mainly from the southern direction, but the sources varied widely. In addition, Na⁺ and Cl⁻ ions were dominated by the sea source fraction; the ions of Ca²⁺ and K⁺ were dominated by crustal fraction sources. The NH₄⁺ and NO₃⁻ ions were mainly influenced by local pollution. However, SO₄²⁻ was mainly from long distance transports. Our results suggest that ions abundance was varied largely in different direction airmasses in southeast Tibet. Considering that ion deposition fluxes were mainly concentrated in the summer and the airmasses were mainly from the southern direction in this season, the pollutants from the southern direction the environmental effects of those ions should be given more attention in the future.

1. Introduction

Atmospheric precipitation can effectively scavenge pollutants from the atmosphere and is the most effective method of air purification [1,2]. Precipitation chemistry is commonly used to estimate air pollutants at regional scales or the national scale [2,3,4]. Usually, the chemical characteristics of precipitation are affected by both natural and anthropogenic sources [4]. However, in some regions, this situation has changed in recent decades due to human activities. With the intensification of anthropogenic activities, the ionic composition of precipitation has largely depended on anthropogenic sources in human-concentrated areas [1,5,6,7]. Meanwhile, studies on chemical characteristics of precipitation have focused on local and regional areas, especially cities [1,8].

Aerosol sources and the migration and removal processes of chemical substances can affect the chemical concentration of rainwater [9]. The different atmospheric chemical reactions that occur within and below clouds are also important [2]. The chemical composition of rainfall water is influenced by several factors, such as local emissions, pollution sources, meteorological conditions, and process of transportation [4,10,11,12,13]. The chemical composition of atmospheric precipitation reveals the characteristics of regional air pollution to some extent [10,14]. In addition, knowledge of the chemical composition of rainfall not only indirectly reflects the atmospheric quality of specific regions but also provides useful information to help us understand the impact on ecosystems through deposition processes [10]. To date, studies that have focused on precipitation chemistry have been conducted in human-concentrated areas [15,16]. However, the precipitation chemistry in background sites is interesting and useful [17]. The Qinghai–Tibet Plateau is considered to be an area of low human disturbance and low nitrogen deposition, yet it is an area sensitive to global change [18]. To date, the research focused on the chemistry of the Qinghai–Tibet plateau has been relatively limited to areas in China [18,19].

The status of the Tibetan Plateau is unique as it is the highest geographical unit in the world. Thus, it plays an important role in evaluating long-term changes in ecological conditions and environmental status [18]. Human activities on the Qinghai–Tibet Plateau are increasing rapidly. This situation may change the composition of air pollutants and ions in precipitation in the Qinghai–Tibet plateau. Consequently, potential impacts of pollutant and nutrient deposition on local ecosystems are unpredictable. Thus, understanding the precipitation chemistry and quantification of ion deposition fluxes is crucial in the Qinghai–Tibet plateau. To our knowledge, the seasonal variations and potential original sources of precipitation chemistry have not been completely quantified in southeast Tibet. Therefore, the present study sampled precipitations in Nyingchi city, Tibet, with the aim to: (1) quantify the major water-soluble ion deposition fluxes in precipitation and clarify the dominant ions, (2) investigate whether the major ions have the same source in all seasons, and (3) understand if the ion pollution is mainly from local pollution or long-distance transports in those area.

2. Experiments

2.1. Sampling Location

The sampling site was located at Xizang Agriculture and Animal Husbandry College (29.66° N 94.34° E 2990 m a.s.l.), Nyingchi city (Figure 1). Nyingchi city is in southeast Tibet and is mainly influenced by the Indian Ocean and the Pacific warm current. It has a population of approximately 200,000. In addition, Nyingchi city is a tourist city, with no large pollution sources nearby. The potential emission sources were small villages and agricultural fields. The regional climate is that of a cold, temperate, humid, subalpine zone. The monthly mean temperature is in the range of 1–16.2 °C; the annual precipitation amount is in the range of 600–800 mm.

2.2. Precipitation Collection

Bulk deposition samples were collected. Bulk deposition is defined as the amount of wet deposition plus dry particles that sediment during sampling into the collector located in an air environment [2]. As a result, the ion deposition fluxes were slightly higher than the wet deposition fluxes.

Rain samples were collected from January 2021 to December 2021 via continuously open rain gauges (SDM6A, Kailongda Inc., Tianjin, China). All parts are made of stainless steel except the glass water collection bottle. Considering that rainfall events mainly occurred at night, rainfall water samples were collected in the morning of the next day. If rainfall events continued into the morning, we collected samples the next morning. In addition, according to a previous study [18], samples were not collected when the sample’s volume was less than 10 mL. Unfortunately, we did not obtain enough rainfall samples for determination because there were limited rainfall events in January, February, and December. Precipitation was only 6.7mm throughout the whole of winter in Nyingchi city (http://data.cma.cn/data/weatherBk.html, accessed on 30 April 2022). As a result, rainfall samples were analyzed from March 2021 to November 2021. Precipitation samples were collected by hand, and a total of 44 samples were obtained during the monitoring period. After each precipitation event, the samples were thoroughly stirred and immediately transferred to clean polyethylene bottles. The rainwater collection bottle was rinsed with deionized water to eliminate cross-event contamination.

2.3. Chemical Analysis

After each sample collection, rainfall water samples were immediately taken to the laboratory, filtered with 0.45 μm cellulose acetate filter (Tengda Inc., Tianjin, China), and then saved in polyethylene bottles at −18 °C. Water-soluble ions were analyzed by ion chromatography (IC, Dionex Corporation, Idstein, Germany). Three blank samples were measured before samples measurement to eliminate contamination. Anions (Cl⁻, NO₂⁻, NO₃⁻, F⁻, Br⁻, PO₄³⁻, and SO₄²⁻) were analyzed via an ICS-2100 ion chromatograph that consisted of a separation column (Dionex Ionpac AS11) and guard column (Dionex Ionpac AG11). Cations (Na⁺, NH₄⁺, K⁺, Mg²⁺, and Ca²⁺) were analyzed on a DX-600 ion chromatograph that consisted of a separation column (Dionex CS12A) and a guard column (Dionex AG12A), and the detection limit was 0.1 μg/L. The amount of HCO₃⁻ was estimated using the following equation: HCO₃⁻ = 10 ^(pH−5.05) [20].

2.4. Calculations and Statistical Analysis

The wet deposition fluxes of ionic constituents and volume-weighted mean (VWM) concentrations in rainwater were calculated via the following equations:

$$\mathrm{F}={{\displaystyle \sum }}_{i=1}^{n}Ci\times Pi/100$$

C_vwm = 10,000 × V/M × F/Pt

where Ci is the concentration of a particular ion in the ith sample (mg L⁻¹); Pi is the precipitation amount collected within the ith sampling period (mm); and n is the total number of precipitation events. F represents the seasonal and/or annual bulk deposition flux (kg N ha⁻¹); 100 is the unit conversion factor.

C_vwm is the VWM concentration (μeq L⁻¹); M is the molecular weight of ion; V is the valence of ion (absolute value); and Pt is total amount of rainfall per season/year(mm). The unit conversion factor is 10,000.

The Enrichment Factor (EF) is commonly used to estimate the source contributions in rainfall water [18]. In general, Na⁺ can be considered as a typical marine source tracer, whereas Ca²⁺ derives mainly from the continental crust. The EF of an ion concentration in rain relative to the concentration in sea was estimated as follows [20]:

EF_sea = (X/Na⁺)_rain/(X/Na⁺)_sea

EF_soil = (X/Ca²⁺)_rain/(X/Ca²⁺)_soil

EF_sea is the EF of an ion in rainfall relative to in the sea; X is an ion in precipitation; (X/Na⁺)_sea is referenced from Keene et al. [21]; and (X/Ca²⁺)_soil is referenced from Taylor et al. [22].

Sources of ions can be divided into ocean, land, and human activities. We used the following equations to estimate the sources of ionic components from previous studies [18]:

SSF(%) = (X/Na⁺)_sea/(X/Na⁺)_rain × 100

CF(%) = (X/Ca²⁺)_soil/(X/Ca²⁺)_rain × 100

AF(%) = 100% − SSF(%) − CF(%)

SSF represents the sea fraction, CF represents the crustal fraction, and AF represents the anthropogenic fraction; if the SSF (CF) ratio is greater than 1, SSF (CF) is calculated as 1 minus CF (SSF).

The Pearson correlation was calculated based on Σ_anions and Σ_cations. Σ_anions and Σ_cations in each precipitation samples were calculated via the following equation.

$${{\displaystyle \sum }}_{\mathrm{anions}}={{\displaystyle \sum }}_1^{i}Ci/Mi\times Vi\times \mathrm{100,000}$$

$${{\displaystyle \sum }}_{\mathrm{cations}}={{\displaystyle \sum }}_1^{t}Ct/Mt\times Vt\times \mathrm{100,000}$$

where i is the numbers of different anions; t is the numbers of different cations; Ci is the concentrations of ith anions; Mi is the molecular weight of the ith anions; Vi is the valence of anions (absolute value); Ct is the concentrations of tth cations; Mt is the molecular weight of the tth cations; and Vt is the valence of cations (absolute value).

2.5. Backward Trajectory Analysis

Air mass back-trajectory analysis was performed by HYSPLIT 5.2.0 (the Hybrid-Single Particle Integrated Trajectory Model) (http://ready.arl.noaa.gov/HYSPLIT.php; Last accessed on 9 May 2022). The program was provided by the Air Resource Laboratory of the National Oceanic and Atmospheric Administration (NOAA, College Park, MD, USA). Meteorological data were input from the Global Data Assimilation System (ftp://arlftp.arlhq.noaa.gov/pub/archives/gdas1, accessed on 9 May 2022). Then, 72-h backward trajectories were calculated on sampling days, with an arrival height of 1000 m [1]. Cluster analysis was performed using the trajectories based on the total spatial variance (TSV) method [23].

3. Results and Discussion

3.1. Chemical Composition of Precipitation

The VWM concentrations of total cations was 132.8 μeq L⁻¹, whereas the value of the total anions (except for HCO₃⁻) was 63.4 μeq L⁻¹. The estimated VWM of HCO₃⁻ was 66.6 μeq L⁻¹. This result agreed that HCO₃⁻ was the dominant anion species in Tibet [18,19,20]. The Pearson correlation value between the sum of total cations (Na⁺, NH₄⁺, K⁺, Mg²⁺, and Ca²⁺) and sum of total anions (Cl⁻, NO₃⁻, NO₂⁻, NO₃⁻, F⁻, Br⁻, PO₄³⁻, and SO₄²⁻) was 0.812 precipitations, suggesting that the measured ion balance in this experiment was credible.

The VWM ionic concentrations for Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, Cl⁻, NO₂⁻, NO₃⁻, F⁻, Br⁻, PO₄³⁻, and SO₄²⁻ were 30.5, 15.0, 2.5, 14.7, 70.1, 25.6, 0.3, 6.4, 0.29, 0.01, 0.8, and 29.9 μeq L⁻¹, respectively. In addition, the order of concentration abundance of cation concentrations was Ca²⁺ > Na⁺ > NH₄⁺ > Mg²⁺ > K⁺, whereas the order of concentration abundance of anions was SO₄²⁻ > Cl⁻ > NO₃⁻ > PO₄³⁻ > NO₂⁻ > F⁻ > Br⁻. Obviously, Ca²⁺ was the dominant cation, accounting for 52.8% of the total cations; SO₄²⁻ was the dominant anion, accounting for 47.2% of the total cations. Those results agreed with previous studies that focused on southeast Tibet [20]. In addition, both Na⁺ and Cl⁻ concentrations were at a high level (Figure 2). The reason might be that airmasses are mainly influenced by the Indian Ocean and the Pacific warm current (Figure 3). In general, air masses mainly came from the south in different seasons. In addition, the southern trajectory was the only one in the spring, whereas there were three southern trajectory in the summer. In autumn, air masses came mainly from the southeast direction. Moreover, air masses were dominated by long-distance transports in the southern direction in the summer (Figure 3).

3.2. Seasonal Variations in the Ion Concentrations

Due to various pollutant sources in different seasons, seasonal variations of ion concentrations in precipitation are widespread in different regions [5,24,25]. Seasonal variations in cations’ and anions’ concentrations existed within the research area (Figure 4). The VWM cations were 306.4, 132.5, and 64.9 μeq L⁻¹ in spring, summer, and autumn, respectively, whereas the averaged anion concentrations were 126.3, 65.0, and 34.4 μeq L⁻¹ in spring, summer, and autumn, respectively. Both cations’ and anions’ concentrations were the highest in the spring and lowest in autumn. Commonly, the dilution effect of rainwater on precipitation chemistry is present. In this work, both ion concentrations and deposition fluxes had seasonal variations (Figure 3). Interestingly, low concentrations of both cations and anions were observed in autumn, whereas precipitation was concentrated in the summer. One reasonable explanation for this is that the original airmasses’ directions were different between the summer and autumn (Figure 4). India, located in the direction of the source of the air mass, is a human-concentrated area, and ion concentrations in precipitation there are relatively high [26]. During airmass transportation, the air mass may be polluted by sources from India in the southern direction (Figure 4). As a result, high ion concentrations were observed in the summer. In contrast, ion concentrations were relatively low in the Qinghai–Tibet plateau because of its low human activity [18]. As a result, precipitation caused by other airmass directions showed relatively low ion concentrations, and those airmasses contributed low amounts in autumn (Figure 4). In addition, the relationships between precipitation amounts and cations, precipitation amounts and anions, and cations and anions were all calculated (Figure 5). There is no conflict regarding the fact that ion concentrations in rainfall water are negatively correlated with precipitation amount. Considering that bulk deposition was collected in this work, the chemistry of rainfall water may be influenced by the residence time of particles floating in the air [5]. Consequently, ion concentrations were relatively high in some cases (Figure 5).

3.3. Source Assessment

EFsea values of NH₄⁺, K⁺, Mg²⁺, Ca²⁺, Cl⁻, NO₃⁻, and SO₄²⁻ were 55.5, 3.77, 2.17, 52.8, 0.72, 33.9, and 6.93, respectively, which means that these ions, except for Cl⁻, were all enriched by a sea source. EFsoil values of NH₄⁺, K⁺, Mg²⁺, Cl⁻, NO₃⁻, and SO₄²⁻ were 400.2, 0.14, 0.23, 207.5, 592.9, and 164.2, respectively, which means that ions of NH₄⁺, Cl⁻, NO₃⁻, and SO₄²⁻ were enriched by a soil source. In addition, the ions of NH₄⁺, NO₃⁻, and SO₄²⁻ were enriched relative to both marine and soil reference sources, which means that those ions were mainly from anthropogenic sources. Those results agree with a previous study that focused on Tibet [18]. Moreover, the ions of Na⁺ and Cl⁻ dominated SSF sources. The K⁺ and Ca²⁺ ions dominated AF sources. Anthropogenic sources contributed at least 97.9% of the NH₄⁺ and of 96.9% of the NO₃⁻ in precipitation (

Table 1. Source contributions (%) for major ions in precipitation.

	Ca2+	Mg2+	K+	Na+	NH4+	Cl−	NO3−	SO42−
SSF	1.9	46.1	26.5	100	1.8	99.5	2.9	14.4
CF	98.1	53.9	73.5		0.2	0.5	0.2	0.6
AF					97.9		96.9	85.0

). It should be pointed out that the CF source of Mg²⁺ was greater than one. Based on the calculation method, the contributions of CF and SSF sources to Mg²⁺ were comparable.

3.4. Ions Deposition Fluxes and Correlations among Different Components

During our experiment, the precipitation was 782.4 mm. In the same period, the deposition fluxes were 5.48, 2.12, 0.76, 1.38, 11.0, 7.12, 0.12, 3.12, 0.01, 0.21, and 11.2 kg ha⁻¹ for Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, Cl⁻, NO₂⁻, NO₃⁻, F⁻, Br⁻, PO₄³⁻, and SO₄²⁻, respectively. Moreover, the deposition fluxes of NH₄⁺-N, NO₃⁻-N, PO₄³⁻-P, and SO₄²⁻-S were 1.65, 0.713, 0.07, and 3.73 kg ha⁻¹ during our monitoring periods. NH₄⁺-N was the dominant from, and the results agreed with previous studies [27]. In addition, the amounts of precipitation were 75.3, 510.0, and 197.1 mm in spring, summer, and autumn, respectively, and the percentage of precipitation that occurred in the summer was 65.2%.

The deposition fluxes of different ions showed different seasonal variations. Considering that the precipitation was mainly concentrated in the summer, deposition fluxes of different ion species were all highest in this season. In addition, the deposition fluxes were accounting for 55%, 53%, 84%, 82%, 61%, 63%, 75.8%, and 37.8% of the annual Ca²⁺, K⁺, Mg²⁺, Na⁺, NH₄⁺, Cl⁻, SO₄²⁻, and NO₃⁻ deposition fluxes (Figure 6). The deposition flux percentages of Mg²⁺, Na⁺, and SO₄²⁻ were higher than the precipitation percentages in the summer, whereas NO₃⁻, Ca²⁺, and K⁺ were lower than the precipitation percentages in the same period. The reason might be that ions of Mg²⁺, Na⁺, and SO₄²⁻ were mainly from the southern direction, and those ions were abundant in the airmasses from the southern direction (Figure 3). Based on the Pearson correlation analysis, we had been found that a positive correlation was observed between Na⁺ and Mg²⁺, Na⁺, and SO₄²⁻ (

Table 2. The Pearson correlation of different ion deposition fluxes in precipitation.

	Ca2+	K+	Mg2+	Na+	NH4+	Cl−	SO42−	NO3−
Ca2+	1	0.588 **	0.172	0.551 **	0.118	0.570 **	0.513 **	0.464 **
K+		1	0.633 **	0.614 **	0.257	0.561 **	0.434 **	0.172
Mg2+			1	0.414 **	0.032	0.338 *	0.276	−0.036
Na+				1	0.288	0.663 **	0.720 **	−0.069
NH4+					1	0.194	0.221	−0.037
Cl−						1	0.515 **	0.017
SO42−							1	0.003
NO3−								1

The symbol * represents significance at 0.05 level; the symbol ** represents significance at 0.01 level.

). Ca²⁺ and K⁺ were mainly from terrestrial sources (Table 1), and a high positive correlation has also been found between Ca²⁺ and K⁺. Caused by dilution effect of rainwater on precipitation chemistry, the deposition flux percentages of Ca²⁺ and K⁺ were slightly lower than the precipitation percentages in the summer. Compared to other ions, high NH₄⁺ deposition fluxes were observed in the spring. This might be because local agriculture activities can influence atmospheric NH₄⁺ concentrations, and atmospheric NH₄⁺ concentration was at a high level from May to Sep [27]. As a result, the deposition fluxes were high in the spring and summer. Interestingly, the deposition flux percentages of SO₄²⁻ were higher than the precipitation percentages in the summer, and NO₃⁻ was lower than the precipitation percentages in the same period. However, both SO₄²⁻ and NO₃⁻ were significantly positively correlated with Ca²⁺. Acid deposition was mainly caused by the ions of atmospheric SO₄²⁻ and NO₃⁻ [28], and the acid neutralization in the atmosphere was dominated by Ca²⁺ in China [29]. In this work, Ca²⁺ was the dominant cation and neutralizing ion (Figure 2). SO₄²⁻ sourced from long distance transports was neutralized by Ca²⁺, and a high positive correlation was observed between Ca²⁺ and SO₄²⁻. In addition, a high positive correlation between Ca²⁺ and SO₄²⁻ was observed in previous studies [30]. NO₃⁻ sourced from the local environment was neutralized by Ca²⁺, and a high positive correlation was observed between Ca²⁺ and NO₃⁻ (Table 2). In addition, there are often valley winds in autumn afternoons in Nyingchi city. NO₃⁻ may drift into the air, coupling with the ions from terrestrial sources, with the wind and then sediment via dry deposition. In addition, a large contribution of Ca²⁺ and K⁺ to the annual precipitation in comparison with other ions of the autumn period was observed. The ions NO₂⁻, F⁻, Br⁻, and PO₄³⁻ were not measured in some samples, and as a result, we have not calculated their percentages.

3.5. Comparison with Other Research in Other Regions

Compared to the ion concentrations in the precipitation in the other areas, the values were commonly relatively low in Nyingchi city (

Table 3. Comparison of major ion concentrations (in μeq L⁻¹) at sampling sites with other sites.

	Sites	Environment	Years	NH4+	Na+	K+	Ca2+	Mg2+	SO42−	NO3−	Cl−	References
Other Areas	Dahra, Africa	Town	2013–2017	31.3	44.2	12.1	38.5	11.9	19.8	14.5	38.1	[32]
	Iberian Peninsula	Urban	2016–2017	21.5	6.6	3.4	5.9	3.2	13.6	13.2	9.4	[33]
	Goma city	Town	2013	-	170.0	150.0	80.0	60.0	180.0	0	170.0	[34]
		Urban		-	100.0	100.0	160.0	100.0	200.0	50.0	150.0
		Urban		-	130.0	60.0	120.0	160.0	220.0	50.0	130.0
		Urban		-	20	20.0	60.0	20.0	40.0	10.0	30
	Romania	Remote	2009–2017	46.1	27.6	11.2	231.2	25.3	142.7	29.7	38.3	[35]
		Remote		69.7	30.3	20.5	132.1	35.0	65.8	29.8	41.0
		Remote		92.9	41.2	33.8	178.3	44.0	77.0	32.5	49.6
		Remote		65.6	31.8	52.4	140.9	36.2	68.7	30.8	50.5
Other Areas in China	Urumqi	Urban	2010–2019	118.9	193.0	45.1	422.3	54.4	307.6	39.1	121.5	[36]
	Karamay	Urban	2010–2019	88.8	151.9	62.1	389.1	38.3	197.3	58.3	77.2
	Yining	Urban	2010–2019	79.1	131.4	12.6	288.6	39.6	218.2	29.2	30.3
	Wuxi	Urban	2016	48.2	21.3	8.3	70.9	14.9	10.5	4.1	4.6	[37]
	Hangzhou	Urban	2010–2011	39.8	18.9	7.1	158	13.4	125	35.9	32.2	[28]
	Xi an	Urban	2017	147.4	28.7	4.6	136.6	22.4	145.1	25.3	8.9	[15]
	Shanghai	Urban	2016–2018	130.1	503	21.1	342	145	354	138	240	[38]
	Nanjing	Urban	2013–2014	127.5	15.3	8.0	247.9	20.4	135.7	58.1	61.7	[39]
	Huzhou	Urban	2016–2018	57.9	37.1	17.3	194.6	21.1	78.8	41.8	40.0	[40]
	Hangzhou	Urban	2016–2018	56.8	29.2	16.8	191.1	16.5	58.5	42.3	28.0
	Shanghai	Urban	2014–2015	54.3	29.7	10.8	113.0	21.9	92.7	46.1	66.7	[41]
	Shenzhen	Urban	2017	17.4	23.1	5.4	28.2	6.5	22.4	18.6	27.1	[42]
	Shenzhen	Urban	2008–2009	14.7	36.4	2.0	18.1	11.8	59.3	23.7	45.9	[1]
	Chicheng	County	2008	92.4	10.9	17.7	183.0	47.6	162.0	86.5	63.9	[7]
	Huailai	County	2008	57.7	22.2	12.3	391.0	70.9	289.0	169.0	71.4
	Xuanhua	County	2008	117.0	18.9	15.8	210.0	49.3	213.0	117.0	66.2
	Beijing	Remote	2017–2018	76.3	59.2	56.0	141.3	33.0	66.3	59.6	100.1	[6]
	Xi an	Remote	2017	1.1	33.7	13.0	190.3	39.5	71.6	2.6	16.7	[15]
	Guizhou	Remote	2019–2010	110.4	7.0	4.4	11.7	3.0	51.8	24.7	5.2	[43]
Qinghai Tibet Plateau	Shiyanghe	Remote	2013–2014	2.0	5.7	3.8	17.8	3.4	28.7	12.2	8.2	[44]
	Dongkemadi	Remote	2013	9.3	4.1	0.6	5.4	0.3	4.3	4.7	1.0	[16]
	Sejila	Remote	2018	7.1	58.5	13.1	123	28.7	19.4	13.9	40.1	[20]
	Southeast Tibet	Remote	2011–2012	4.9	3.8	1.0	20.8	0.9	2.1	2.2	2.5	[18]
	Nyingchi	City	2021	15.0	30.5	2.5	70.1	14.7	29.9	6.42	25.6	This study

The symbol “-” means no available data.

), which agrees with previous studies [31]. This can be explained by the fact that low human activity and few anthropological ion pollution sources exist in Tibet. Interestingly, Na⁺ concentrations were higher than in most areas in China. The reason is that Nyingchi city is mainly influenced by the Indian Ocean and airmasses mainly from the south (Figure 3). Correspondingly, high Na⁺ deposition fluxes were observed in summer in this work (Figure 6). Similarly, the concentrations of Cl⁻ coupled with Na⁺, mainly from SSF sources, were all at a high level (Figure 2). Due to the airmasses being mainly derived from the Indian Ocean, ion species that were mainly from CF sources were all at a low level (Table 2 and Table 3). In addition, Ca²⁺ was the dominant cation species in Nyingchi city, but the Ca²⁺ concentration in rainfall water was found to be relatively low. Ca²⁺ is a typical crustal ion. However, airmasses came mainly from the southern direction in the summer, when precipitation mainly occurred (Figure 6). This means that the precipitation events were dominated by sea-source airmasses. As a result, low VWM Ca²⁺ concentrations were observed in other research areas. It should be pointed out that, although the concentrations of ion species (NH₄⁺, SO₄²⁻, and NO₃⁻) that are mainly from human activities were all at a relative level, their concentrations were higher than in a previous study that focused on southeast Tibet [18]. Our monitoring sites were located in a city, whereas in previous studies they were located in remote areas. Our results reveal that the deposition fluxes of NH₄⁺, SO₄²⁻, and NO₃⁻ might increase with intensifying human activities in Tibet.

4. Conclusions

Ion concentrations and deposition fluxes were measured in Nyingchi city, Tibet. The results showed that Ca²⁺ and Na⁺ were the dominant cations, and their deposition fluxes were 11.2 and 5.48 kg ha⁻¹ in a year, respectively; SO₄²⁻ and Cl⁻ were the dominant anions, and their deposition fluxes were 11.2 and 7.12 kg ha⁻¹, respectively. Due to precipitation being mainly concentrated in the summer, ions deposition fluxes were all the highest in this season. Considering that atmospheric SO₄²⁻ is an ion that causes acid deposition, and the deposition fluxes were relatively high in Nyingchi city, the environmental effects should be given more attention. In addition, the pollutants from southern direction need be further studied in the future.