Guiyang County is located between 112˚13'26"E - 112˚55'46"E and 25˚27'15"N - 26˚13'30"N, with a total area of 2973 km². Tobacco has been planted since 1953, with tobacco-rice rotation as a main plantation system. Guiyang County has a subtropical humid monsoon climate with an average annual temperature of 17.2˚C, sunshine hours of 1705.4 h, and rainfall of 1385.2 mm [26].

The rainfall sampling device is Ellen ISC-10 automatic continuous sampler for precipitation and dustfall, which is automatically opened during precipitation and automatically closed when the precipitation ends. They were set up in Fangyuan Tobacco Workstation (112˚40'0.03"E, 25˚40'48.65"N, 320.4 m above sea level) and Aoquan Tobacco Workstation (112˚34'35.62"E, 25˚55' 36.21"N, 250.0 m above sea level), about 20 m from the ground on the roof of the buildings to reduce the splash and contamination of surrounding substances.

The rainfall is sampled during the whole year of 2020. 90 and 119 rainfall samples were collected from the two monitoring sites in Fangyuan and Aoquan. The collected rainfall samples were stored in washed 1.5 L PVC bottles and placed in a 4˚C environment saving for backup. pH was measured with a pH meter (PB-21), besides, conductivity (EC) was measured with a conductivity meter (EC215), and total nitrogen, ammonium nitrogen and nitrate nitrogen were measured with a continuous flow analyzer (San++System). The main cations (K⁺, Na⁺, Ca²⁺, Mg²⁺, and Al³⁺) were determined by inductively coupled plasma emission spectrometer (Optima 8000). After the rainfall samples filtered with a 0.22 μm pore size microporous membrane, the concentration of major anions ( SO 4 2 − , Cl⁻) was analyzed and determined by a high-pressure dual-system ion chromatograph (ICS-5000+). The rainfall information at two monitoring sites come from the temperature and humidity recorder (179-TH) installed in the nearby farmland.

The formula for calculating the sedimentation amount of ions is [27]:

F i = P i × C i × A (1)

F m = ∑ i = 1 n F i (2)

F y = ∑ i = 1 n F i (3)

In the above formula, F_i is the sedimentation amount of ions or elements in the sample after each rainfall (kg/hm²/yr), P_i is the daily rainfall (mm), and C_i is the concentration of ions or elements in the sample (mg/L or ug/L), A is the unit conversion factor (A is 0.01 when C_i is mg/L, A is 10⁻⁵ when C_i is ug/L), F_m is the monthly sedimentation amount of ions or elements (kg/hm²/yr), F_y is the annual deposition of ions or elements (kg/hm²/yr).

The formula for calculating the ionic equivalent concentration is [28]:

N = 1000 × C × V / M (4)

C v w m = ∑ i = 1 n ( N i × P i ) / ∑ i = 1 n P i (5)

In the above formula, N is the ion equivalent concentration (ueq/L), C is the ion concentration (mg/L), V is the absolute value of ion valence, M is the ion molar mass (g/mol), and C_vwm is the weighted average concentration of ion rainfall (ueq/L), N_i is the normal concentration of i ions in the sample (ueq/L), P_i is the daily rainfall (mm).

EF_sea and EF_soil are enrichment factors to identify the potential sources of ionic species in rainwater. EF_sea, EF_soil, contribution of sea salt fraction (SSF), non-sea salt fraction (NSSF), terrestrial fraction (TF), crust fraction (CF) and anthropogenic fraction (AF) were calculated using the following equations: [29] [30] [31] [32]

E F s e a = ( X / N a + ) r a i n w a t e r / ( X / N a + ) s e a w a t e r (6)

E F s o i l = ( X / C a 2 + ) r a i n w a t e r / ( X / C a 2 + ) s o i l (7)

% S S F = 100 ( X / N a + ) s e a w a t e r / ( X / N a + ) r a i n w a t e r (8)

% N S S F = % T F = 100 − % S S F (9)

% C F = 100 ( X / C a 2 + ) c r u s t / ( X / C a 2 + ) r a i n w a t e r (10)

% A F = % N S S F − % C F (11)

where X is the concentration of the desired ion, ( X / N a + ) s e a w a t e r is the ratio from seawater composition and ( X / C a 2 + ) c r u s t is the ratio from crustal composition.

Data processing, analysis and graphing were performed using Microsoft Excel 2016 and IBM SPSS Statistics 26.

Table 1 shows the 2020 rainfall data from National Weather Station (No. 57973, 112˚43'29"E, 25˚44'58"N, 329.1 m above sea level) in Guiyang County and the field temperature and humidity recorders (Temperature and Humidity Recorder 179-TH, Beijing Dingxuan Shengshi Technology Co., Ltd.) set in Fangyuan (112˚40'0"E, 25˚40'49"N, 320.4 m above sea level) and Aoquan (112˚34'36"E, 25˚55'36"N, 250.0 m above sea level), it can be seen that 1) the annual rainfall data of Fangyuan and Aoquan monitoring sites are higher than the national station data, The relative errors were 10.5% and 10.2%, respectively. 2) The inter-month high-low sequence and the rising -falling trend of rainfall data at the national station and the two monitoring sites were consistent, and the rainfall was mainly concentrated during the first half of the year (January to June, tobacco season), and it accounted for 67.8%, 72.7% and 77.5% of the annual rainfall at the national station and the two monitoring sites respectively, which was significantly higher than that in the second half of the year (July to December, the rice season).

Figure 1(a) shows the pH variation trend of rainfall in Fangyuan and Aoquan in 2020. The pH of rainfall at two monitoring sites are in the range of 4.92 - 6.17 and 4.93 - 5.69, with an average of 5.62 and 5.27, respectively. The changing trends of pH between the two monitoring sites are slightly different, mainly reflected in the rising and falling trend from January to May. In general, 43.33% of rainfall at Fangyuan site and 73.11% of rainfall at Aoquan site were lower than 5.5 in the characteristic of pH, and acid rain characteristic [33] were obvious. Figure 1(b) shows that there are obvious seasonal changes in the mean value of rainfall EC at Fangyuan and Aoquan monitoring sites. It can be seen that: 1) The EC from January to September were all low, ranging from 6.09 to 56.72 μS/cm and from 11.83 to 30.93 μS/cm, respectively. After that, the electrical conductivity gradually increased and reached the maximum value in November, which were 174.60 and 86.37 μS/cm respectively, and then tended to decrease. 2) The variation range of the rainfall electrical conductivity at two monitoring sites after August (the coefficient of variation is 0.64 and 0.72, respectively) is larger than that before August (the coefficient of variation is 0.54 and 0.32, respectively). The EC of rainfall depends on the concentration of solute-containing salts or other chemical impurities that decompose into electrolytes, besides, the higher the concentration, the higher the conductivity [34]. It can be seen from Figure 1(b) that the high EC values of rainfall generally appear in October~December at both monitoring sites, but the EC of rainfall in January~September are both very low.

The deposition amounts of total nitrogen (TN), NO 3 − -N and NH 4 + -N and their changes in rainfall at Fangyuan and Aoquan monitoring sites are shown in Figure 2 and Figure 3, respectively. The deposition amounts were 22.19 kg/hm²/yr and 20.76 kg/hm²/yr, respectively. The variation trends of TN deposition at the two sites in different months are similar, with a peak in the first half and the second half of the year, respectively. The first peak appeared in March (4.87 kg/hm² and 5.71 kg/hm², respectively), and the second peak appeared in September (3.09 kg/hm² and 3.14 kg/hm², respectively). The annual deposition of NO 3 − -N and NH 4 + -N at Fangyuan monitoring site were 6.80 kg/hm²/yr and 12.54 kg/hm²/yr, accounting for 30.66% and 56.51% of the annual deposition of TN, respectively. At Aoquan monitoring site the annual deposition rates of NO 3 − -N and NH 4 + -N were 6.02 kg/hm²/yr and 8.07 kg/hm²/yr, accounting for 29.00% and 38.86% of the total deposition in TN, respectively. The variation trends of NO 3 − -N and NH 4 + -N deposition in Fangyuan and Aoquan in different months are similar to that of TN, it can be seen that there is a peak in the first half and second half of the year, respectively, and the first peak appeared in March, while the deposition amounts of NO 3 − -N and NH 4 + -N were 1.49 kg/hm², 2.75 kg/hm² and 1.66 kg/hm², 2.01 kg/hm², respectively. Meanwhile, the second peak appeared in September, and the NO 3 − -N and NH 4 + -N deposition amounts were 0.99 kg/hm², 1.83 kg/hm² and 0.86 kg/hm² and 1.46 kg/hm², respectively.

Table 2 shows the annual and monthly equivalent concentrations of main ions at Fangyuan and Aoquan monitoring sites. The sums of the weighted average equivalent concentrations of all ions are 238.88 and 211.21 μeq/L, respectively, and the concentrations of each ion from high to low are SO 4 2 − > NH 4 + > Na + > Ca 2 + > K + > Cl − > Mg 2 + > NO 3 − > Al 3 + and SO 4 2 − > NH 4 + > Ca 2 + > Na + > K + > Cl − > NO 3 − > Mg 2 + > Al 3 + , in which the sum of SO 4 2 − , NH 4 + , Na⁺, Ca²⁺, K⁺ and Cl⁻ concentrations exceeded 90%, which are the dominant ions in rainfall. The weighted average equivalent concentrations of Al³⁺ at the two monitoring sites were lower, which were 3.54 and 4.04 μeq/L, respectively. SO 4 2 − was the main anion at the two monitoring sites, and its weighted average equivalent concentrations were 61.42 and 50.46 μeq/L, accounting for 73.18% and 69.13% of the total anions, respectively. NH 4 + , Na⁺ and Ca²⁺ were the main cations, and their concentrations accounted for 74.39% and 76.99% of the total cations, respectively.

The sums of weighted average equivalent concentrations of main ions in Fangyuan and Aoquan monitoring sites in tobacco and rice seasons were 251 μeq/L, 263.29 μeq/L and 209.46 μeq/L, 456.27 μeq/L, respectively, in which the concentrations were both larger in rice season than taht in tobacco season. And the order were SO 4 2 − > NH 4 + > Na + > Ca 2 + > K + > Cl − > Mg 2 + > NO 3 − > Al 3 + , SO 4 2 − > NH 4 + > Ca 2 + > Na + > Mg 2 + > Cl − > K + > NO 3 − > Al 3 + and SO 4 2 − > NH 4 + > Na + > Ca 2 + > K + > Cl − > NO 3 − > Mg 2 + > Al 3 + , Ca 2 + > SO 4 2 − > NH 4 + > NO 3 − > Cl − > Mg 2 + > Na + > Al 3 + > K + . SO 4 2 − , NH 4 + , Na⁺, and Ca²⁺ accounted for more than 70% of the total average equivalent concentration during tobacco season and rice season in Fangyuan and during tobacco season in Aoquan, while SO 4 2 − , NH 4 + , NO 3 − , Ca²⁺ accounted for more than 85% during rice season in Aoquan.

At Fangyuan monitoring site, the weighted average equivalent concentrations of SO 4 2 − and NH 4 + reached a peak in April (155.84 and 105.11 μeq/L) and December (281.50 and 126.57 μeq/L), respectively. The month-to-month changes of other ions were relatively stable. At Aoquan monitoring site, the weighted average equivalent concentrations of Ca²⁺ and SO 4 2 − reached their peaks in November (466.35 and 323 μeq/L), and the weighted average equivalent concentration of NH 4 + reached a peak in December (148.30 μeq/L), besides, the concentrations changes of other ions are relatively stable from month to month.

There are various reasons for the formation of acid rain in Guiyang County [35] [36], mainly including nitrogen oxides from industrial and automobile exhaust emissions [37], sulfides from coal combustion [38], and from emissions of acid gas from neighboring provinces (Guangdong, Sichuan, Guizhou) [39]. Studies have shown that the areas with severe acid rain in China are mainly distributed in Sichuan Basin and vast area in the south of the Yangtze River [40], and Hunan Province is located in the area with severe acid deposition in China. The energy consumption structure of Hunan Province is dominated by coal [41] [42], so a large amount of SO₂ and NO_x will be generated when coal resources are consumed. These acid gases are the main precursors for the formation of acid rain. Therefore, the formation of acid rain in Hunan Province is related to the energy structure and consumption [43].

In terms of flue-cured tobacco planting, the suitable soil pH for high-quality flue-cured tobacco is 5.5 - 7.0 [44] [45] [46]. There are literatures report that about 2/3 of tobacco fields in Chenzhou were higher in pH (≥7.0) in 2015, which could be attributed to the application of superphosphate fertilizer and the habit of local farmers using fired soil to improve soil quality [47] [48], and may also be related to the continuous supply of Ca²⁺ and Mg²⁺ dissolved from the limestone in the hills and mountains [49]. In 2020, the soil pH values at Fangyuan and Aoquan tobacco fields are in the range of 6.10 - 6.97 (suitable) and 8.16 - 8.40 (higher), respectively. Therefore, acidic rainfall is beneficial to reduce the high soil pH value at Aoquan tobacco fields, as a result, it is conducive to the improvement of flue-cured tobacco quality in Aoquan, but there is a potential adverse effect on Fangyuan tobacco fields.

The high EC values of rainfall generally appeared from October to December at both monitoring sites, due to less rainfall (27.7 mm and 16.3 mm on average) at that time, resulting in more particulate matter and soluble ions in the air [50]. However, the EC values of rainfall were very low from January to September because of its high rainfall (average of 170.9 mm and 174.2 mm, respectively), which effectively reduces the soluble ions and particulate matter in the air. The EC values of the rainfall during the tobacco season (January to July) were low (17.65 - 56.72 μS/cm and 11.83 - 30.93 μS/cm, relatively), indicating that the rainfall was relatively pure with few impurities, which was beneficial to the maintenance of soil and water environment during the tobacco season.

Studies have shown that the annual deposition of TN in farmland in Zhejiang is 18 - 32 kg/hm²/yr [51], besides, the total annual deposition of atmospheric nitrogen in agro-ecological regions in southeastern China can reach 33.93 kg/hm²/yr [52]. In alpine meadow area of Qilian Mountains, the annual deposition of TN, NH 4 + -N and NO 3 − -N is only 3.19, 1.78 and 1.40 kg/hm²/yr, respectively [53]. Also, the annual deposition of TN in Nyingchi City of Tibet is only 2.19 kg/hm²/yr [54]. Comparing with the above data, it can be found that the annual deposition of TN in this study area is higher than that in the western regions with less human disturbance, but equal to or lower than that in the eastern regions with developed agricultural activities. In areas with less anthropogenic interference, there is less exhaust gas from automobiles and factories and less ammonia from agricultural activities, so the TN deposition of rainfall is less. While in the developed eastern areas, ammonia and nitrogen oxides were produced more due to the strong anthropogenic activities such as transportation, industry, and agriculture, so the TN deposition of rainfall is larger. In this study, the ratio of NH 4 + -N to TN at two monitoring sites was higher than that of NO 3 − -N , indicating that the nitrogen deposition at two monitoring sites was more affected by the volatilization of ammonia from agricultural activities [55] [56], especially during the rice season [57] [58], while the effects of nitrate nitrogen generated by factors such as industrial emissions and traffic exhaust [59] are relatively small. Two peaks of nitrogen deposition occurred in March in the first half of the year and in September in the second half of the year, which may be related to the concentrated fertilization for flue-cured tobacco and late rice at that time. The ratios of ammonium nitrogen to nitrate nitrogen deposition at two monitoring sites were 1.84, 1.81, 1.86 and 1.34, 1.46, 1.29 for the whole year, summer and autumn, and winter and spring, respectively. The ratio of NH 4 + -N and NO 3 − -N deposition is higher in summer and autumn (June-November) than that in winter and spring (December-May) as a result of the facts that the frequent agricultural activities and high temperature in summer and autumn can promote ammonia volatilization, while in winter and spring the contribution of agricultural activities to ammonium is reduced, and the nitrate content is slightly increased due to traffic exhaust and fossil fuels [60].

The annual application of pure N in the tobacco season of Chenzhou is generally 157.5 - 195 kg/hm². This study shows that the annual nitrogen depositions in Fangyuan and Aoquan sites are 22.19 and 20.76 kg/hm²/yr, respectively, which is equivalent to 10.7% - 14.1% of the pure N application rate. Flue-cured tobacco is nitrate-liking and ammonium-disliking [61] [62]. During tobacco season, the proportions of NH 4 + -N and NO 3 − -N in annual TN deposition at Fangyuan and Aoquan monitoring sites were 55.84%, 30.34% and 36.32%, 28.56%, respectively. And the proportion of NH 4 + -N in the annual deposition of TN was 84.05% and 27.17% higher than that of NO 3 − -N , which was not conducive to the growth of high-quality flue-cured tobacco in general.

SO 4 2 − is the main anion at two monitoring sites, and its weighted average equivalent concentration is 61.42 and 50.46 μeq/L, accounting for 73.18% and 69.13% of the total anions, respectively. The higher concentration of SO 4 2 − is related to the large amount of fossil fuels combustion and SO₂ emissions from vehicle exhaust [63]. The acidification of rainfall at two monitoring sites was dominated by sulfuric acid rather than nitric acid because the concentration of SO 4 2 − was higher than that of NO 3 − [64]. NH 4 + , Na⁺ and Ca²⁺ are the main cations, and their concentrations account for 74.39% and 76.99% of the total cations, respectively. The higher concentration of NH 4 + is related to local agricultural production activities, because agricultural activities increase the amount of ammonia and particulate ammonia in the atmosphere [65]. The concentration of Ca²⁺ mainly comes from soil and ground dust, etc., as the concentration of Ca²⁺ in rainfall will be increased when the dust, sand dust and construction dust enter the air [66].

Table 3 shows the enrichment factors of main ions components relative to seawater (EF_sea) and soil (EF_soil) at Fangyuan and Aoquan monitoring sites, and Table 4 shows the main ions components contributed source in rainfall at Fangyuan and Aoquan monitoring sites, meanwhile, the analysis method is shown in the literature [67] [68]. The EF_soil of Cl⁻ is much greater than 1 and the EF_sea is less than 1, indicating that Cl⁻ in the rainfall at two monitoring sites mainly comes from marine sources [69], and the proportion of Cl⁻ input from marine sources in both places is greater than 99%. The EF_sea of K⁺ were 34.74 and 35.55, respectively, and the EF_soil were 1.76 and 1.15, respectively, indicating that K⁺ was mainly contributed by terrigenous sources. Most of Mg²⁺ comes from marine sources, and the proportions of marine sources account for 68.02% and 95.35%, respectively. The concentrations of Mg²⁺ and Cl⁻ were significantly and positively correlated at two monitoring sites (p < 0.01, Table 5). The sources of NH 4 + , SO 4 2 − and NO 3 − are mainly contributed by anthropogenic activities, and their anthropogenic contributed source components are 99.35%, 92.43%, 96.88% and 99.25%, 91.90%, 96.88% respectively at two sites. And there were significantly positive correlations among these three ions (p < 0.01, Table 3).The content of SO 4 2 − is the highest, followed by NH 4 + . The content sums of the two ions account for 48.30% and 44.03% of the total main ions at two monitoring sites, and both are mainly affected by human activities, indicating that the soluble ions in rainfall mainly come from human production and life [70].

This study shows that the concentrations of SO 4 2 − and NH 4 + are relatively high in ion compositions of rainfall in Chenzhou tobacco area. The former has both advantages and disadvantages for the growth of high-quality flue-cured tobacco, as it can reduce high pH values of tobacco fields soil in Aoquan which are relatively high at present, but it can also increase the supply of sulfur in tobacco fields while the content of sulfur in the soil of Chenzhou tobacco area is mostly too high for high-quality flue-cured tobacco [71] [72]. High NH 4 + concentration is also disadvantageous for the growth of high-quality flue-cured tobacco, because flue-cured tobacco is nitrate-liking and ammonium-disliking [61] [62].

Note: ** and * indicate significant correlations (two-sided) at the 0.01 and 0.05 levels, respectively.

There are still some shortcomings in this study. 1) Due to the variability of climate, the characteristics of rainfall in different years may be different, and its chemical properties and monthly trends may be different. Generally, several years’ research data is needed to reliably reveal the chemical properties of rainfall in a certain area. However, in this study we only analyzed the research data in 2020, and continuous monitoring is still required. 2) Due to limited conditions, neither information about the types of factories and mines around the two monitoring sites nor relevant data about three-waste-discharges was obtained, which would affect the interpretations of the research results.