Seasonal Characteristics, Sources, and Regional Transport Patterns of Precipitation Components at High-Elevation Mountain in South China

Abstract

To investigate the seasonal characteristics, sources, and regional transport patterns of precipitation components in the high-elevation mountainous regions, field sampling was conducted at Mt. Heng (Hunan, South China) from June 2021 to May 2022. In total, 114 precipitation samples were collected and subjected to chemical analysis, including pH, major inorganic ions, and heavy metals. During the study period, the precipitation at Mt. Heng was generally weakly acidic. The concentrations of metals and acidic anions (NO₃⁻ and SO₄²⁻) were higher in the winter and lower in the summer, whereas the concentration of the primary neutralizing cation, NH₄⁺, peaked during the summer. An association was observed between precipitation pH and metal concentrations, whereby acidic precipitation samples exhibited marginally elevated metal concentrations overall. An additional analysis of winter precipitation chemistry at Mt. Heng revealed an increasing trend of ions from 2015 to 2018, followed by a decrease from 2019 to 2021. This trend coincided with the concentrations of NO₂ and SO₂ in the surrounding cities, reflecting the results of clean air actions. The results of the source analysis revealed five major sources: secondary sources (41.5%), coal combustion (24.7%), a mixed source of biomass burning and aged sea salt (11.6%), dust (10.8%), and industrial emissions (11.4%). Backward trajectory cluster analysis revealed that air masses originating from the northern regions were generally more polluted than those from the southern regions. This study provides fundamental data and scientific support for regional atmospheric pollution control and ecological protection in South China.

1. Introduction

Since the 20th century, anthropogenic activities have significantly increased the levels of airborne contaminants [1]. These pollutants can undergo long-range transport in the atmosphere and be deposited far from their emission sources; this is a critical environmental issue that is currently affecting the healthy development of human society and the normal functioning of natural ecosystems [2,3]. To date, considerable research has been conducted on the dry and wet deposition of atmospheric pollutants, primarily focusing on chemical characteristics, source apportionment, causative analysis, and spatiotemporal distribution [4,5,6,7]. Wet deposition is the primary means by which atmospheric pollutants are removed through rain, snow and fog [8], and rain is the most prevalent type of wet deposition for scavenging atmospheric pollutants [9]. Inorganic ions and trace elements in precipitation have been extensively studied worldwide because of their significant role in acidic wet deposition and their associated toxicity and bioaccumulation potential [10,11].

In China, the chemical composition of rainfall has been thoroughly studied at urban and rural stations [5,12,13]. Nevertheless, chemical data on rainfall in high-elevation mountainous regions remain relatively scarce [6,14]. Typically, high-elevation monitoring stations experience minimal interference from local anthropogenic inputs and are thus considered ideal for assessing pollution induced by regional atmospheric transport [15,16]. Compared with that in low-elevation areas, the precipitation chemistry in high-elevation mountainous regions generally provides a good representation of regional atmospheric pollution conditions [17].

We focus on Mt. Heng, a typical high-elevation area in South China. Given the topography surrounding Mt. Heng, the atmospheric quality at the summit is highly likely to be representative of the regional atmospheric conditions in South China, making it an optimal location for studying the influences of pollution on atmospheric precipitation components in this region [18,19]. According to the 2021 China Ecological and Environmental Bulletin (https://www.mee.gov.cn/hjzl/sthjzk/zghjzkgb/, accessed on 17 September 2025), Mt. Heng is located within the acid rain area of China. However, previous studies have focused primarily on the influence of precipitation acidity on inorganic ions, with few scholars analyzing the effect of acid rain on metals during high-elevation precipitation. Additionally, most existing studies on Mt. Heng precipitation are limited to short-term observations during a single season and predominantly examine either precipitation ions or metals alone [18,20]. Therefore, in this study, year-round measurements of precipitation components, including both inorganic ions and trace elements, are performed at Mt. Heng. The main objectives of this research are to (1) reveal the pollution characteristics and seasonal variations in precipitation components at Mt. Heng; (2) quantitatively identify the contributions of different sources to these precipitation components; and (3) determine the characteristics of regional atmospheric transport influencing the precipitation components at Mt. Heng.

2. Materials and Methods

2.1. Site Description

Mt. Heng is situated in Hengyang city, Hunan. This mountain features a subtropical monsoon climate characterized by intense solar radiation and significant diurnal temperature variations. In summer, Mt. Heng is under the control of low-pressure systems and prevalent tropical air masses, resulting in humid and rainy conditions. In winter, this mountain is dominated by cold high-pressure systems from Siberia and Mongolia, leading to dry and cold weather. The average annual rainfall is approximately 2000 mm.

The precipitation sampling site is located at the Nanyue High Mountain Meteorological Station (Figure 1), which is east of the peak of Mt. Heng. The station is situated at 112°42′ E, 27°18′ N, with an elevation of 1265.9 m. The sampling site lies merely 90 km to the south of Changsha, approximately 400 km south of Wuhan, approximately 500 km north of the Pearl River Delta, and approximately 900 km west of the Yangtze River Delta. These regions represent major industrial zones and population centers in southern China.

2.2. Sample Collection and Chemical Analysis

Precipitation samples were collected at the Nanyue Meteorological Station on Mt. Heng from June 2021 to May 2022 using a bulk sampler, yielding a total of 114 samples. Sampling typically commenced at 08:00 and continued for 24 h. During collection, two layers of polyethylene sampling bags pretreated with high-purity water were placed to line a steel bucket. Upon the cessation of rainfall, the sampling bags were promptly retrieved, and the amount of precipitation was recorded. The collected samples were then transferred into 30 mL polyethylene bottles and immediately frozen at −20 °C for preservation before being transported to the laboratory in Beijing for subsequent analysis.

After thawing, the pH levels of the samples collected during precipitation were measured using a digital pH meter (FE20, METTLER TOLEDO, Greifensee, Switzerland). A portion of each sample collected during precipitation was filtered through a 0.45 µm Sartorius membrane. The filtered sample was then analyzed for water-soluble ions (Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, SO₄²⁻, NO₃⁻, F⁻, and Cl⁻) using an ion chromatography system (ICS-90; Dionex Corporation, Sunnyvale, CA, USA). The average detection limit for all ions was less than 5 μg L⁻¹. Another portion of each precipitation sample was acidified with nitric acid (Sigma–Aldrich, Taufkirchen, Germany) to a pH of less than 2.0 and allowed to stand for 24 h. This prepared aliquot was then analyzed for nine heavy metals (V, Cr, Mn, Fe, Cu, Zn, As, Cd, and Pb) using inductively coupled plasma mass spectrometry (ICP-MS 7500ce, Agilent, Santa Clara, CA, USA). Each sample was analyzed in triplicate. To ensure instrumental stability, internal standard elements (⁷²Ge, ¹⁰³Rh, ¹¹⁵In, and ²⁰⁹Bi) were monitored. An internal standard solution of 1 μg L⁻¹ (Part #5183-4680; Agilent, Santa Clara, CA, USA) was introduced, and the relative standard deviation (RSD) of the internal standards was checked after each data acquisition cycle. Analysis was repeated if any RSD value exceeded 3%.

2.3. Data Analysis and Calculation

2.3.1. Neutralization Factor (NF)

The neutralization factor is an indicator of the potential of alkaline species to neutralize acidic substances in precipitation [21]. The NF can be calculated using the following equation:

$$\mathrm{N}\mathrm{F}\left(\mathrm{X}\right)={\displaystyle \frac{\left[\mathrm{X}\right]}{{[\mathrm{S}\mathrm{O}}_4^{2-}]+{[\mathrm{N}\mathrm{O}}_3^{-}]}}$$

(1)

Here, [X] is the equivalent concentration (μeq L⁻¹) of an alkaline species (Na⁺, NH₄⁺, K⁺, Mg²⁺, or Ca²⁺), while [SO₄²⁻] and [NO₃⁻] are the equivalent concentrations (μeq L⁻¹) of sulfate and nitrate ions, respectively.

2.3.2. Positive Matrix Factorization (PMF) Model

In this study, the source analysis was performed using the PMF 5.0 (https://www.epa.gov) software package recommended by the US EPA (United States Environmental Protection Agency, Washington, DC, USA). The PMF model was first proposed by Paatero and Tapper [22], and it is based on the least-squares method for quantitative source–receptor analysis. The core function of this model was to obtain the source contribution matrix and the source component spectrum matrix to calculate the minimum value of the objective function (Q) [23]. The specific equations were as follows:

$$e_{ij}=x_{ij}-{\sum }_{k=1}^p{g}_{ik}{f}_{kj} (i = 1\dots \dots n; j = 1\dots \dots m; k = 1\dots \dots p)$$

(2)

$$\mathrm{Q}={\sum }_{i=1}^n{\sum }_{j=1}^m{(e_{ij}/s_{ij})}^2$$

(3)

where x_ij is the content of element j in sample i; p is the number of pollution sources; g_ik is the contribution rate of source k to sample i; f_kj is the content of element j in source k; e_ij is the residual matrix; Q is the objective function; n is the number of samples; m is the number of elements; and s_ij is the uncertainty of the element j content in sample i.

2.3.3. Back-Trajectory Cluster Analysis

To investigate the movement trajectories of air masses associated with precipitation at Mt. Heng, we utilized the hybrid single-particle lagrangian integrated trajectory (HYSPLIT) model, developed jointly by the National Oceanic and Atmospheric Administration (NOAA) and the Air Resources Laboratory (ARL). The HYSPLIT model is a comprehensive system capable of simulating the transport, dispersion, and deposition characteristics of various interacting pollutants from local to global scales, evolving from simplified single trajectories based on radiosonde observations. This model has been widely used in atmospheric pollutant transport research because it clearly visualizes air mass and particle movement paths [24,25].

Specifically, 48 h backward trajectories for 114 precipitation events at Mt. Heng from June 2021 to May 2022 were determined using the HYSPLIT4 model integrated within the Meteoinfo software (V3.7.0). The calculations were driven by meteorological data from the Global Data Assimilation System (GDAS), which was obtained from the NOAA website (https://www.nco.ncep.noaa.gov/pmb/products/gfs/, accessed on 12 January 2026). The starting height for each trajectory was set to 1500 m above sea level. For the clustering analysis of these trajectories, the spatial variance clustering algorithm proposed by Dorling, Davies [26] was employed as the primary method. This algorithm grouped air mass trajectories arriving at the receptor site on the basis of spatial similarity by calculating the spatial variation between different trajectory combinations. Ultimately, all the trajectories were classified into six distinct clusters.

2.3.4. Supporting Data

The monthly concentration data for atmospheric pollutants (PM_2.5, PM₁₀, NO₂, SO₂) in Hengyang City covering the period from 2015 to 2021 were obtained from the China National Environmental Monitoring Centre (CNEMC, https://air.cnemc.cn:18007, accessed on 28 September 2025).

3. Results and Discussion

3.1. Seasonal Characteristics of Precipitation Components

3.1.1. pH and Ionic Concentrations of Precipitation

Table 1 presents the statistical parameters of the pH and major ion concentrations in the precipitation samples from Mt. Heng over the entire sampling period, along with comparisons with other mountain sites. As shown, the precipitation at Mt. Heng was generally acidic, with a mean pH of 5.51, which was lower than the reference value of acid rain (pH = 5.6). These findings indicate that Mt. Heng is still affected by regional acid pollution. During the study period, among anions, NO₃⁻ had the highest mean concentration (48.30 μeq L⁻¹), slightly exceeding that of SO₄²⁻ (44.33 μeq L⁻¹). Moreover, the higher concentration of NO₃⁻ relative to SO₄²⁻ suggested that nitrogen oxides (NO_x) from mobile sources (e.g., vehicle exhaust) could have contributed equally to or even more than sulfur dioxide (SO₂) from stationary sources (e.g., coal-fired power plants) to precipitation acidity [27,28]. These findings were consistent with observations from Mt. Lu [6] but differed from those of earlier studies at other high mountain sites, such as Mt. Huang [2] and Mt. Tai [29]. These findings indicated that while SO₂ emissions in China have been effectively controlled in recent years, NO_x pollution has become increasingly prominent. Among the cations, NH₄⁺ was the most abundant, significantly exceeding the concentrations of the other cations. The concentration of NH₄⁺ at Mt. Heng was second only to that at Mt. Tai (129.1 μeq L⁻¹). The concentrations of Ca²⁺, Cl⁻, K⁺, Mg²⁺, Na⁺, and F⁻ at Mt. Heng were generally lower than those at Mt. Tai, Mt. Lu, and Mt. Sejila and were higher than those recorded at Mt. Lulin [30] and Mt. Yulong [31].

Figure 2 and Figure 3 display the seasonal distributions of chemical components and neutralization factors, respectively, in precipitation at Mt. Heng during the study period. Overall, both the precipitation amount and the pH tended to increase in summer and decrease in winter. With the exceptions of NH₄⁺, Ca²⁺ and Mg²⁺, the concentrations of the other ions reached their lowest levels in summer. This phenomenon was primarily attributed to the scavenging and dilution effects caused by the increased rainfall levels during summer [32]. In winter, elevated concentrations of NO₃⁻ and SO₄²⁻ likely reflect not only changes in emissions (e.g., increased fossil-fuel combustion) but also differences in the processes of removal or chemical transformation of components in the atmosphere [33,34]. A comparison of the seasonal concentrations of atmospheric pollutants (SO₂, NO₂, PM_2.5, and PM₁₀) in Hengyang City during the study period revealed that pronounced winter maxima in NO₂, PM₁₀ and PM_2.5, whereas NO₃⁻ and SO₄²⁻ in precipitation also peaked in winter (Figure 4), which is consistent with more stagnant meteorological conditions and enhanced accumulation in winter [34,35,36,37,38]. In contrast, the concentrations of NH₄⁺, Ca²⁺ and Mg²⁺ in precipitation peaked during summer. Furthermore, these three neutralizing cations collectively accounted for more than 50% of the total cation concentration in most summer precipitation events, indicating that their seasonal characteristics could not be attributed to random, isolated rainfall occurrences. Both the concentration and the neutralization factor of NH₄⁺ peaked in summer, primarily because of increased temperatures promoting the volatilization of NH₃ from agricultural activities, such as fertilizer application and livestock breeding [39]. Similarly, the neutralization factors and concentrations of Ca²⁺ and Mg²⁺ peaked in summer, when the air mass was mainly traced to the Yangtze River Delta and are characterized by elevated Ca²⁺ and Mg²⁺ (detailed in Section 3.4).

Table 1. Summary of rainwater pH and ionic concentrations (µeq L⁻¹) at Mt. Heng and comparison with other alpine sites.

Location	Sampling Period	pH	Na+	NH4+	K+	Mg2+	Ca2+	F−	Cl−	NO3−	SO42−	References
Mt. Heng	Average	5.51	13.72	43.58	6.76	6.43	15.97	3.21	15.64	48.30	44.33	This study
	SD	0.50	17.25	62.79	9.82	7.67	12.74	3.50	13.73	79.34	54.56
	Max	6.55	109.1	381.1	76.63	52.07	75.84	15.4	86.35	469.7	314.0
	Min	4.25	1.65	0.32	0.36	0.30	0.19	0.25	6.61	4.14	4.63
Mt. Lu	2018–2020	5.80	19.90	33.60	33.40	15.10	70.70	6.1	38.40	69.40	37.90	[6]
Mt. Tai	2005–2008	4.48	14.30	129.1	10.00	11.40	81.90	6.8	17.90	52.80	179.4	[29]
Mt. Huang	2010–2011	5.03	6.00	25.60	5.50	3.10	51.20	-	63.00	13.00	33.00	[2]
Mt. Lulin	2003–2005	5.12	2.90	12.30	1.40	1.70	3.90	-	7.20	10.80	15.70	[30]
Mt. Yulong	2012	5.89	3.72	13.20	2.46	5.68	37.70	-	1.96	4.00	28.30	[31]
Mt. Sejila	2017–2018	6.20	58.50	7.13	13.10	28.70	123.0	-	40.10	13.90	19.40	[13]
Mt. Jiuzhaigou	2015–2016	5.74	5.74	18.40	34.80	5.60	55.80	-	44.10	9.10	15.90	[40]
Mt. Tian	2003–2004	7.27	28.90	28.80	4.60	26.00	220.3	-	21.00	14.10	75.50	[41]

Table 1. Summary of rainwater pH and ionic concentrations (µeq L⁻¹) at Mt. Heng and comparison with other alpine sites.

Location	Sampling Period	pH	Na+	NH4+	K+	Mg2+	Ca2+	F−	Cl−	NO3−	SO42−	References
Mt. Heng	Average	5.51	13.72	43.58	6.76	6.43	15.97	3.21	15.64	48.30	44.33	This study
	SD	0.50	17.25	62.79	9.82	7.67	12.74	3.50	13.73	79.34	54.56
	Max	6.55	109.1	381.1	76.63	52.07	75.84	15.4	86.35	469.7	314.0
	Min	4.25	1.65	0.32	0.36	0.30	0.19	0.25	6.61	4.14	4.63
Mt. Lu	2018–2020	5.80	19.90	33.60	33.40	15.10	70.70	6.1	38.40	69.40	37.90	[6]
Mt. Tai	2005–2008	4.48	14.30	129.1	10.00	11.40	81.90	6.8	17.90	52.80	179.4	[29]
Mt. Huang	2010–2011	5.03	6.00	25.60	5.50	3.10	51.20	-	63.00	13.00	33.00	[2]
Mt. Lulin	2003–2005	5.12	2.90	12.30	1.40	1.70	3.90	-	7.20	10.80	15.70	[30]
Mt. Yulong	2012	5.89	3.72	13.20	2.46	5.68	37.70	-	1.96	4.00	28.30	[31]
Mt. Sejila	2017–2018	6.20	58.50	7.13	13.10	28.70	123.0	-	40.10	13.90	19.40	[13]
Mt. Jiuzhaigou	2015–2016	5.74	5.74	18.40	34.80	5.60	55.80	-	44.10	9.10	15.90	[40]
Mt. Tian	2003–2004	7.27	28.90	28.80	4.60	26.00	220.3	-	21.00	14.10	75.50	[41]

3.1.2. Heavy Metal Concentrations in Precipitation

Various trace metals were detected in the precipitation samples collected at Mt. Heng (Table 2). Their average concentrations decreased in the following order: Fe > Zn > Mn > Pb > Cu > As > Cr > Cd > V. Compared with those in other high mountain regions globally, most metal concentrations at Mt. Heng were greater than those at foreign mountain sites and sites on the Tibetan Plateau but lower than those at mountain sites in eastern China. This pattern indicated that Mt. Heng was subject to profound influence from regional anthropogenic emissions. The concentrations of these metals in precipitation at Mt. Heng were generally high in winter and low in summer (Figure 5). In winter, increased heating demand intensifies fossil fuel combustion, leading to elevated concentrations of elements associated with combustion processes, such as As, Cd, and Pb [42]. In contrast, increased summer precipitation enhances the scavenging effect of particulate matter from the atmosphere, resulting in the dilution and reduction in metal concentrations [43].

Additionally, we analyzed the influences of pH on metals in the precipitation of Mt. Heng (Figure 6). Metal concentrations were consistently higher in precipitation samples with a pH < 5.6 than in those with a pH > 5.6, suggesting that higher metal concentrations in acidic precipitation at Mt. Heng [44]. Acidic conditions may enhance the dissolution of certain particulate-bound metals, thereby increasing their dissolved fractions in rainwater and resulting in higher measured metal concentrations [45]. A significant inverse relationship was found between rainwater pH and heavy metal concentrations [46], and the concentration of heavy metals in aerosols increased with decreasing pH [47]. In this study, no statistically significant difference was observed in precipitation metal concentrations between the two pH groups, suggesting that pH is not the primary factor governing metal variability. Instead, metal levels in precipitation are more likely modulated by the combined effects of air-mass origin, aerosol particle size, precipitation acidity, and dilution associated with rainfall amount [16,43,46,48]. However, the relatively high concentrations of metals at lower pH (Figure 6) warrant more investigations in future.

Table 2. Summary of rainwater metal concentrations (μg L⁻¹) at Mt. Heng and comparison with other alpine sites.

Location	Sampling Period	V	Cr	Mn	Fe	Cu	Zn	As	Cd	Pb	References
Mt. Heng	Average	0.16	0.45	4.87	36.46	2.23	24.12	1.32	0.22	4.45	This study
	SD	0.19	0.67	6.78	42.52	3.23	25.27	1.61	0.31	6.01
	Max	1.22	5.45	43.39	228.6	15.78	141.3	8.41	2.31	40.13
	Min	0.01	0.04	0.29	2.45	0.15	1.56	0.02	0.0004	0.20
Nam Co Station	2007–2008	0.03	0.4	1.0	16.2	0.8	7.9	–	0.01	0.2	[49]
Mt. Tai	2005–2008	–	0.02	13.8	44.5	3.3	90.6	2.09	0.58	6.48	[16]
Mt. Lu	2011–2012	4.15	6.16	3.15	–	0.18	31.2	7.72	0.24	6.26	[46]
Pyrenees Mountain	2004–2006	–	–	3.4	1	0.4	14.5	0.01	0.02	0.3	[50]
Himalayas	2012–2013	1.83	2.7	29.8	1074	3.7	26.5	0.5	0.1	2.4	[10]
Mansfield Mountain	1998	0.35	–	1.5	–	0.7	4	0.1	0.1	0.5	[51]

Table 2. Summary of rainwater metal concentrations (μg L⁻¹) at Mt. Heng and comparison with other alpine sites.

Location	Sampling Period	V	Cr	Mn	Fe	Cu	Zn	As	Cd	Pb	References
Mt. Heng	Average	0.16	0.45	4.87	36.46	2.23	24.12	1.32	0.22	4.45	This study
	SD	0.19	0.67	6.78	42.52	3.23	25.27	1.61	0.31	6.01
	Max	1.22	5.45	43.39	228.6	15.78	141.3	8.41	2.31	40.13
	Min	0.01	0.04	0.29	2.45	0.15	1.56	0.02	0.0004	0.20
Nam Co Station	2007–2008	0.03	0.4	1.0	16.2	0.8	7.9	–	0.01	0.2	[49]
Mt. Tai	2005–2008	–	0.02	13.8	44.5	3.3	90.6	2.09	0.58	6.48	[16]
Mt. Lu	2011–2012	4.15	6.16	3.15	–	0.18	31.2	7.72	0.24	6.26	[46]
Pyrenees Mountain	2004–2006	–	–	3.4	1	0.4	14.5	0.01	0.02	0.3	[50]
Himalayas	2012–2013	1.83	2.7	29.8	1074	3.7	26.5	0.5	0.1	2.4	[10]
Mansfield Mountain	1998	0.35	–	1.5	–	0.7	4	0.1	0.1	0.5	[51]

3.2. Interannual Trends in Precipitation Components

Previous research on ionic concentrations in precipitation at Mt. Heng was confined to winter periods from 2015 to 2020 [52] and lacked comprehensive seasonal coverage and continuous data for an entire year. This study supplements this gap with newly acquired observational data for all four seasons of 2021–2022, with winter being confirmed as the season with the highest ionic concentrations (Figure 7). By integrating continuous winter precipitation ion data spanning seven years (2015–2021), we systematically examine the interannual evolution characteristics of precipitation ionic concentrations at Mt. Heng during this extended period, thereby addressing previous limitations in temporal continuity and interannual dynamic analysis. The corresponding interannual changes in the winter SO₂ and NO₂ concentrations in Hengyang City, where Mt. Heng is located, are also shown in Figure 7.

The total ionic concentration in precipitation at Mt. Heng tended to increase from 2015 to 2018, began to decrease in 2019, and reached its lowest value in 2021. From 2015 to 2018, the winter concentrations of SO₄²⁻, NO₃⁻, and NH₄⁺ in precipitation at Mt. Heng increased from 79.0, 57.6, and 112.2 µeq L⁻¹ to 114.2, 105.2, and 172.6 µeq L⁻¹, respectively. As important precursors, the atmospheric concentrations of NH₃, SO₂ and NO₂ directly influence the formation of NH₄⁺, SO₄²⁻, and NO₃⁻, NH₄⁺ [27,28]. As shown in Figure 8, both the SO₂ and NO₂ concentrations in Hengyang peaked in 2018, which coincided with the highest recorded SO₄²⁻ and NO₃⁻ concentrations in the precipitation at Mt. Heng. These findings indicate that regional anthropogenic emissions—such as coal combustion, industrial processes, vehicle exhaust, and agricultural activities—were intense from 2015 to 2018, which led to the substantial formation of secondary ions through atmospheric oxidation and neutralization reactions, subsequently influencing high-elevation precipitation chemistry.

After 2018, the SO₄²⁻, NO₃⁻, and NH₄⁺ concentrations in precipitation at Mt. Heng decreased significantly, paralleling the declines in SO₂ and NO_x levels in Hengyang. This trend is closely associated with the intensified implementation of China’s three-year action plan to fight air pollution launched in 2018. Specific measures, including ultralow emission upgrades for coal-fired boilers, industrial desulfurization and denitrification, stricter vehicle emission standards, and control of agricultural ammonia emissions, effectively reduced the emissions of precursors responsible for secondary ions, such as SO₄²⁻, NO₃⁻, and NH₄⁺. Furthermore, the COVID-19 lockdown measures during the observation period reduced anthropogenic activities and corresponding atmospheric pollutant emissions, which contributed to decreases in precipitation ion concentrations [53].

3.3. Analysis of the Sources of Precipitation Components

To identify the major sources of inorganic components in precipitation events at Mt. Heng, we employed an integrated approach combining correlation analysis with a positive matrix factorization (PMF) model. For the PMF analysis, F⁻ was excluded because its signal-to-noise (S/N) ratio was less than 0.5; thus, it was classified as a weak species. Through simulation, comparison, residual matrix analysis, factor interpretability assessment, and validation of the fitting results, the optimal solution was determined to be a five-factor model. The results of the correlation analysis for inorganic components during the observation period are presented in Figure 9. The source profiles and contribution percentages of the factors are shown in Figure 10.

Factor 1 was characterized by the highest loadings of K⁺ (54.94%), Cl⁻ (49.41%), and Na⁺ (38.52%). Cl⁻, Na⁺, and K⁺ were significantly correlated (0.62 < r < 0.72). K⁺ is generally regarded as a tracer for biomass burning [54]. Additionally, Cl⁻ and Na⁺ are widely used as tracers for marine air masses [55]. Therefore, Factor 1 was identified as a mixed source of biomass burning and aged sea salt. Factor 2 was represented by high loadings of Ca²⁺ (73.1%) and Mg²⁺ (49.56%). These crustal elements, which were strongly and significantly correlated (r = 0.71), likely originated primarily from surface rock weathering and construction dust [6]. Consequently, this factor was attributed to a dust source. Factor 3 had high loadings for certain elements, such as Cu (49.48%), As (53.01%), Cd (47.78%), and Pb (42.73%). These elements represent coal combustion. Cu can be emitted from the combustion of coal and fuel oil [56]. As, Cd and Pb are potentially toxic trace elements in coal [57,58]. Furthermore, the correlation coefficients between Cu and both Cd and Pb exceeded 0.6, indicating significant correlations. The exceptionally high correlation between As and Cd (r = 0.88), coupled with the significant correlation between As and SO₄²⁻ (r = 0.62), further supported their likely common origin from coal combustion processes.

Factor 4 exhibited high loadings for V (58.92%), Cr (61.08%), Mn (56.17%), Fe (76.42%), and Zn (74.18%). Correlation analysis revealed generally weak correlations between metals such as Fe and Mn and the crustal elements Ca²⁺ and Mg²⁺ (e.g., Ca²⁺ vs. Fe: r = 0.15). In contrast, strong intercorrelations were found among Fe, Mn, V, and Cr (e.g., Fe vs. V: r = 0.82; Mn vs. V: r = 0.88), suggesting that Fe, Mn, V, and Cr were unlikely to be derived from soil dust. Cr and V are typically emitted during metal smelting, such as in alloy steel production, where they are used to increase toughness and corrosion resistance [59]. In addition to crustal sources, Mn, Zn, and Fe can originate from metal smelting and iron/steel manufacturing processes [20]. Given that Hunan is renowned as the “Hometown of Nonferrous Metals” in China with a developed and concentrated presence of metal smelting and iron/steel industries [60], this factor was attributed to industrial emissions. Factor 5 was characterized by high loadings of NH₄⁺ (69.64%), SO₄²⁻ (60.24%), and NO₃⁻ (49.03%). These three ions exhibited extremely strong positive correlations with each other (r > 0.87). SO₄²⁻ and NO₃⁻ originate primarily from fossil fuel combustion, such as coal-fired power plants and vehicle emissions [27,61]. NH₄⁺ is generally derived initially from agricultural emissions, such as nitrogen fertilizer use and livestock farming [39]. These ions are secondary aerosols formed through atmospheric oxidation and neutralization reactions of gaseous precursors (SO₂, NO_x and NH₃) emitted from anthropogenic activities, representing typical secondary pollutants. Therefore, this factor could serve as a secondary source, which is the most significant contributor to the inorganic components in precipitation at Mt. Heng.

In summary, five principal sources were identified for the precipitation components at Mt. Heng: a mixed source of biomass burning and aged sea salt (11.6%), dust (10.8%), coal combustion (24.7%), industrial emissions (11.4%), and secondary sources (41.5%).

3.4. Regional Transport Patterns of Precipitation Components

To investigate the effects of different transport pathways on the precipitation components at Mt. Heng during the observation period, we analyzed the transport routes of six clustered air mass trajectories (Figure 11) and the contributions of different components and sources within each cluster (Figure 12).

Cluster 1 originated from the south of Henan province and passed through Wuhan and Changsha before reaching Mt. Heng. This cluster presented the highest concentrations of As, Cd, Pb, and SO₄²⁻ and the second highest concentrations of NH₄⁺ and NO₃⁻. This pattern was driven primarily by fossil fuel combustion from coal-fired power plants and vehicles and agricultural emissions along the transport path across the North China Plain, leading to high contributions from secondary sources (53.6%) and coal combustion sources (16.9%). Cluster 2 primarily originated from the Pearl River Delta region and passed through Guangxi before arriving at Mt. Heng. The inorganic component concentrations in precipitation from this cluster were generally moderate. The concentrations of NO₃⁻ and SO₄²⁻ were second only to those in clusters 1 and 3, whereas the Zn concentration was second only to that in cluster 6. Consequently, this air mass was significantly influenced by secondary sources (33%) and industrial emission sources (25.6%). Cluster 3 mainly originated from the Yangtze River Delta region and passed through Anhui and Jiangxi Provinces. This cluster had the highest concentrations of Na⁺, Ca²⁺, NH₄⁺, Cl⁻, and NO₃⁻ among all six clusters. This characteristic was attributed to several factors: the coastal location of the Yangtze River Delta led to marine aerosol influence, enhancing the mixed source contribution (20.2%); urbanization processes generated construction and surface dust, contributing significantly to Ca²⁺ and Mg²⁺ levels (21.6%); and intensive vehicle activity and agricultural emissions strengthened the roles of secondary sources (27.4%). Additionally, industrial activities (18.5%) collectively contributed moderate concentrations of metal elements. Thus, cluster 3 resulted from the combined effects of secondary sources, dust, mixed sources, and industrial emissions.

With the exception of K⁺, the concentrations of precipitation components in cluster 4 were generally low, reflecting the weak anthropogenic emission intensity in its source regions (Laos, Vietnam). The high K⁺ concentration likely arose because of the developed tropical agriculture and less stringent controls on biomass burning (e.g., crop residue burning) in Southeast Asia, allowing K⁺ from biomass burning (38.4%) to be transported over long distances to Mt. Heng. Cluster 5 primarily originated from Xinjiang and Mongolia in northwestern China. Only the concentrations of dust-related elements such as Ca²⁺ and Mg²⁺ were relatively high in this cluster. This phenomenon occurred because of the arid climate, exposed land surfaces, and frequent dust storms in northwestern China, which allowed the air masses to carry significant amounts of Mg²⁺ and Ca²⁺ during transport, increasing their concentrations [42]. Therefore, the dust source (42.8%) had the greatest influence on cluster 5. Cluster 6 originated from local areas surrounding Mt. Heng. This cluster showed high concentrations of V, Cr, Cu, Zn, and Fe, and local nonferrous metal smelting and iron/steel manufacturing industries contributed strongly to industrial emission sources (44.8%).

In summary, the chemical characteristics of precipitation at Mt. Heng clearly differed on the basis of transport pathways. Compared with those from the south, air masses arriving from the north generally carried higher concentrations of pollution components. Furthermore, the contributions of different sources varied significantly across the clusters, reflecting the distinct regional atmospheric pollution characteristics associated with each transport path.

4. Conclusions

In this study, the seasonal variations, sources, and regional transport characteristics of precipitation components at Mt. Heng in South China were investigated. The main findings can be summarized as follows:

The average precipitation pH value at Mt. Heng is 5.51. The concentrations of the primary neutralizing cations, NH₄⁺ and Ca²⁺, peaked in the summer, whereas those of the acidic anions NO₃⁻ and SO₄²⁻ peaked in the winter. The metal concentrations in precipitation were higher in the winter and lower in the summer, and the concentration of metallic elements is relatively high in acidic precipitation. Additional analysis of winter precipitation chemistry at Mt. Heng revealed an increasing trend of ions from 2015 to 2018, followed by a decrease from 2019 to 2022. Combined correlation analysis and PMF source apportionment revealed secondary sources (41.5%) as the dominant contributors to inorganic components of precipitation on Mt. Heng, followed by coal combustion sources (24.7%), a mixed source of biomass burning and aged sea salt (11.6%), industrial emissions (11.4%), and dust (10.8%). Pronounced regional transport characteristics were observed. Compared with those from Southeast Asia (K⁺) and the Pearl River Delta (NO₃⁻, SO₄²⁻ and Zn), air masses originating from the northwestern China (Mg²⁺ and Ca²⁺), the local regions (As, Cd, Fe and SO₄²⁻), and the Yangtze River Delta (Na⁺, Ca²⁺, NH₄⁺, Cl⁻, and NO₃⁻) carried higher concentrations of precipitation components. Moreover, the contributions of different sources varied significantly across trajectory clusters, reflecting distinct regional atmospheric pollution features.