The Source, Transport, and Removal of Chemical Elements in Rainwater in China

Abstract

Rainwater is one of the most important parts of water resources and aerosols. The content of chemical elements in rainwater influences air quality significantly. It is extremely important for high-quality sustainable agriculture development and the cultivation of China’s natural landscape to understand and control the sources, transport, and removal of chemical elements in rainwater. Currently, there are some reports on the sources and removal of chemical elements in rainwater; however, these papers do not completely and clearly explain where the chemical elements in the rainwater originate and how they are transported and removed. A review of published literature related to chemical elements in rainwater shows that industrialization and urbanization increase the levels of atmospheric pollutants and trace elements in rainwater, contaminating soil and surface water as well as other natural resources. The Chinese government established a series of sponge cities, rainwater wetlands, rainwater gardens, and biological detention pools to improve the quality of sewage so as to realize the reuse of water resources, the sustainable development of high-quality agriculture, and the cultivation of China’s natural landscape.

1. Introduction

Rainwater is one of the most important parts of water resources and is intimately associated with air quality. It is key in the removal of air pollutants because when rainwater falls, pollutants in the air mix with raindrops. It is also essential for the high-quality and sustainable agriculture development and the cultivation of China’s natural landscape to properly deal with the sources, transportation and removal of chemical elements in rainwater. Although raindrops are small, we cannot ignore the potential harm of rain to life because the process of industrialization and urbanization could release multitudes of detrimental elements into rainwater. Researchers have begun to study the composition of nutrient elements in rain, because the chemical composition of rain provides useful data on the chemical sources and local diffusion pathways of pollutants. This has warned the whole world to pay special attention to the nutrients in rainwater.

Precipitation is the most efficient process for removing different ionic compounds, pollutants, and soluble gases from the atmosphere. The chemical properties of rainwater reflect the quality of air discharged into the atmosphere from natural or man-made sources [1,2,3]. This helps us to assess the relative importance of various sources of chemical elements in rainwater and estimate the risk of future acid rain [4,5,6]. As a global environmental problem, excessive chemical elements in rainwater tremendously influence the integrity of soil, marine ecosystem, wetland ecosystems, forests and human health. Emissions of air pollutants have rapidly increased due to the expanding economic growth, high energy consumption and social activities of humans.

The research of rainwater chemical elements in China has spread all over the country, especially in the Qinghai–Tibet plateau, Yulong Mountain area, Beijing, Chizhou, Shenzhen, Xi’an and Guiyang [7,8,9]. Existing foreign researches in this research field are from Cambodia, western Iran, coastal towns in southwestern Europe, eastern France, Gambia, western India, and Wilmington in North Carolina [10,11,12]. The researchers also studied desert, semi-arid and tropical regions based on local climate diversities. There are many arid and semi-arid areas in the northern desert of China, and the atmosphere there is always filled with soil dust containing high alkaline salt, especially in the spring. Some researchers noted that the components in rainwater showed seasonal characteristics. For instance, northern winter and spring rainwater samples have higher SO₄²⁻ concentrations because coal combustion emits heavy SO₂ [13]. Compared to Beijing, the chemical composition of rainwater in Chizhou has no obvious seasonal change [10].

Because the buffer capacity of aerosols in southern China is insufficient, the annual pH value of precipitation in this area is usually lower and exhibits temporal and spatial diversity. As a consequence, the acidic precipitation has negative effects on the ecosystem by forming complex chemical mixtures before reaching the ground [7,8]. The research on acid deposition mainly focuses on the pH value of precipitation and the distribution of sulfur deposition in the south, with acid rain affecting about 30% of the country [7,11]. The severely affected areas are mainly located in the economically developed areas of southeastern China. The decrease in pH was found to be mainly due to the presence of sulfuric acid, nitric acid, carboxylic acid, carbonated acid, acetic acid, formic acid, and organic acids in the precipitation. Land use and industrial activities are the main driving forces for the chemical changes of the atmosphere and rainwater [14,15,16]. The pollutants in the atmosphere mainly originate from incomplete fuel combustion, fertilizer application, thermal power plants, refineries and so on. In particular, the long-distance monsoon wind transmission and natural, artificial and crust source acidification and neutralization in Shenzhen precipitation make important contributions [11,17]. China ranks third in the amount and frequency of severe acid rain in the world [18,19,20]. Studies of the source and removal of chemical elements in rainwater in various regions are necessary to detect, assess and improve air quality. There are few review papers on related contents in China and abroad, and most of them focus on the variation of rainwater chemical composition in a region and characteristics such as seasonal variation in this region. Qu et al. reviewed the variation of rainwater acidity in 24 cities in China and discussed the observed trends and spatial variability of rainwater acidity and chemical composition in relation to industrialization and environmental changes in China [21]. Majumdar et al. analyzed the chemical composition and sources of rainwater in six homogeneous Indian monsoon regions, showing that it is mainly anthropogenic influences that drive the composition of rainwater chemistry [22]. Meera et al. compared the chemical composition of rainwater around the world, collecting rainwater from rooftop catchments, and concluded that its quality often does not meet drinking water guideline values [23]. No literature has been found that systematically addresses the source–transfer–removal of elements in stormwater. The investigations of chemical elements in rainwater can also help us identify the main sources of anions and cations in rainwater, in order to understand the regional distribution of pollutants and their potential impact on the ecosystem.

2. Method

In this study, by using the digital databases Science Direct and Web of Sciences, a wide range of literature was searched by us for information from peer-reviewed studies published in English from 2002 to 2022 that were identified as focusing on the subfields “Atmosphere”, “Chemistry”, “Soils” and “Engineering”. The search ends on 1 June 2022. The first phase used to identify relevant scientific publications is shown in Figure 1, followed by a content analysis (second phase, see

Table 1. Specific inclusion and exclusion criteria for the phases identification, screening, eligibility, and inclusion of the systematic literature review.

Phase	Inclusion Criteria	Exclusion Criteria	Total N of Excluded Articles
Identification	Keywords: rainwater, chemical composition, transfer, removal	Duplicates	12,849
Screening: Title and keyword screening		No title; no abstract; unrelated topic or measures with no relevance to rainwater; missing data; book chapters; conference papers; editorials	8267
Screening: Abstract screening		Abstract out of scope; no access, not found; no keywords; language (not English or Chinese)	115
Eligibility	Thematic relevance: rainwater, China, chemical composition, organic matter, transfer, collection facility and synonyms	Content out of scope	127

). Pre-defined inclusion and exclusion criteria (as shown in Table 1) were used to present a clear and transparent inclusion process while minimizing bias [24,25]. The review was performed using the combination of the primary terms or keywords: “Rainwater AND chemical composition OR organic matter OR metal element”, “Rainwater AND Chemical Composition AND Transfer” and “Rainwater AND Collection Facility OR Component Removal OR Regulation Reservoir”.

An initial 21,484 publications were retrieved by us, and excluding duplicates, the total number of publications was 8635. The filtering was divided into two parts; the first step was to exclude articles that did not match the topic based on the relevance of the article title [26,27] (see Table 1 for selection and exclusion criteria). The exclusion criteria for articles screened based on article title and keywords were: no title or no abstract, irrelevant topics or measures not related to rainwater, missing data and no research or review papers. The second step for articles screened based on abstracts excluded those with no access, no keywords, out of scope, and in languages other than English or Chinese.

During the qualifying phase, only publications directly or indirectly related to a stormwater chemical constituent or stormwater pollution treatment were included in the keyword and parameter analysis. The content of the 253 selected publications was screened for the keywords “chemical composition”, “transfer”, “removal”, “China” and synonyms to assess the thematic relevance. This process revealed 126 publications and removed articles that were out of the topic scope.

The review also relevantly extended the scope of the article, mainly because the article relies on the implementation of environmental policies, and these measures have a significant impact on the chemical composition of rainwater. In Section 3.1.1 and Section 3.1.2, relevant foreign studies were also selected for comparison, based mainly on the selection of the leading countries, having similar environmental policies, the occurrence of extreme weather in cities (e.g., typhoons, haze, dust storms), industrial similarity, coastal cities, and other characteristics. This helped to analyze the effectiveness of the implementation of environmental policies in the country, as well as trends in the chemical composition of rainwater.

Among the 126 documents finally collected, the literature on chemical composition of rainwater in China involves 17 provinces and cities, and the urban data distribution is shown in Figure 2. Most of the research focuses on first-tier cities such as Beijing, Guangzhou, Shenzhen and Shanghai. There are fewer studies in northern cities than in southern cities, and there are almost no studies in Jilin, Heilongjiang, and Qinghai provinces, but it is enough to give us a good understanding of the sources of the chemical composition of rainwater in China.

3. Results

3.1. The Content of Ions in Rainwater

Based on the search strategy in Section 2 of the methodology, irrelevant literature was removed and representative cities were selected as the basis for discussion. The foreign literature was selected by considering the most recently published influential cities that share the same pollution sources as China. The components and pH values of rural and urban rainwater are significantly different (Table 2). The last updated article in the search literature on the chemical composition of rainwater was by Wang et al. [28] in June 2022 online on the Web. The common components in rainwater include Na⁺, NH⁺, K⁺, Mg²⁺, Ca²⁺, F⁻, Cl⁻, NO₃⁻, SO₄²⁻, SO₄²⁻/NO₃⁻ and so forth.

3.1.1. Regional Change of pH Value in Rainwater

Rainwater pH is a particularly sensitive indicator in atmospheric emissions surveys and is affected by many environmental factors, including anthropogenic activities, geographic and climatic conditions, regional-scale transport, sea fog and ground dust [29,30,31]. The pH of rainwater in dry seasons was found to be higher than that of rainy seasons, which may be due to the neutralization of alkaline soil dust transported from desert and loess soils in northern and northwestern China [32]. For example, rainwater pH in Beijing was lower in the rainy season than that in the dry season [32] and evolved into a much lower level from 1995 to 2012 [9], which is possibly attributed to the urbanization of Beijing city. Compared to the rainwater pH in Beijing and Xi’an, Yan’an had a lower pH [32], which was due to the lower alkaline content of soils in southern China [32]. Among all southern Chinese cities, rainwater in Yan’an [32], Chongqing and Chengdu had the lowest average pH values because of their characteristic climate and high frequency of rainy days with continuous precipitation [10,12]. By contrast, the reduction of pH in Jiuzhaigou was mainly caused by the pollution of sulfur dioxide [33]. In foreign cities, such as Tokyo in Japan, Singapore city, and Newark in the USA, in which rainwater pH is below 4.6 [34], the increased acidity of rainwater was associated with industrial production, automobile exhaust, and agricultural activities [35,36]. Areas such as Pune in India, Ghor Es-Safi in Jordan, Mexico, and Avignon in France exhibited fluctuating rainwater pH values ranging from 4.6 to 7.17 [37], which is mainly due to the high contents of calcium ions in their rainwater.

3.1.2. Regional Change of Major Ions Contents in Rainwater

Based on the comparisons in previous studies in other regions of China listed in Table 2, the annual weighted mean values of the measured species in rainwater were SO₄²⁻ > NH⁺ > Mg²⁺ > NO₃⁻ > Cl⁻ > Ca²⁺ > Na⁺ > K⁺ > F⁻. Among them, SO₄²⁻, NH⁺, Mg²⁺ and NO₃⁻ were the major water-soluble ions in rainwater, accounting for 80.3% of the total ions on average. SO₄²⁻ was the most abundant anion in rainwater samples, with an annual VWM value of 158.62 μeq/L, which accounted for about 27.5% of the total anion mass. SO₄²⁻ is mainly from the large amount of SO₂ produced by fossil fuel combustion, NH⁺ is from animal and human manure and fertilizer application, Mg²⁺ is from soil dust, vehicle exhaust is considered to be the main source of NO₃⁻, and Cl⁻, Na⁺, K⁺ and F⁻ account for very little of the rainwater. There are many different ions, such as sodium ions and ammonium ions, in the rainwater of southern China. Among southern cities, such as Chengdu, Chongqing, Shenzhen, Guangzhou, Kunming and Shanghai, the highest sodium ion content was found in the rainwater of Shanghai (Table 2), and coastal cities are proposed to be the major source of sodium ions [31,38,39]. This is comparable to recent foreign studies, where rainwater sodium and ammonium ion levels in Iran were found to be extremely high. Among them, Kushkabad in Iran had sodium ion content of 440 μeq/L and ammonium ion content of 880 μeq/L, which is related to local biomass burning, agricultural activities, fertilizer use, etc. [10]. The highest levels of ammonium ions in China were in Chongqing and Shanghai, mainly due to gaseous ammonia emissions and incidental industrial production [40,41]. The contents of ammonium ions and potassium ions in Chongqing city were much higher compared to other cities [18]. By contrast, the potassium ion content of rainwater in the northern Beijing region was very high, mainly stemming from the lithosphere and the potassium emission of suspended particulate matter [42].

The highest contents of magnesium ions in rainwater were found in the southern part of Sichuan and in the northern part of Beijing [18,19,43], which are related to soil dust and oceans. The highest calcium ion content was 193.0 μeq/L in Xinjiang [17], followed by 70.7 μeq/L in Jiangxi [44], which was related to the local rock and soil dust. Compared with foreign countries, the contents of magnesium ions and calcium ions were higher in Ematabad, Hamedan, Kushkabad, and Ghor Es-Safi. The content of fluoride ions was generally low in foreign countries [10,14], but higher in Chongqing and Xi’an in China, mainly due to automobile exhaust and industrial pollution [45]. The chloride ion content was consistently high for both China and abroad, especially in Chongqing and Beijing. Biomass burning and ocean were proposed as the main sources of chloride ions in rainwater. It has been hypothesized that changes to the concentration ratio of SO₄²⁻/NO₃⁻ in rainwater over the past 30 years in China influenced the type of acid rain [21]. Nitrate and sulfate are the two main anions in rainwater precipitation. The increase of the SO₄²⁻/NO₃⁻ ratio in northern and southern China indicates that rainwater in these regions is mainly polluted by sulfuric acid [46,47,48]. According to a recent report, acid rains in Hamedan, Kushkabad, Ematabad, Chizhou, Chengdu, Chongqing and Beijing were associated with elevations of SO₄²⁻ [49], in contrast to Mexico, Tokyo, Ghor Es-Safi, Avignon, Jiaozhou, Beijing, and Ya’an. In some areas, acid rains were mainly associated with the increase of NO₃⁻ in rainwater [50]. Atmospheric SO₄²⁻ is mainly derived from natural sources, including volcanic sources, local weathering, atmospheric deposition and sea salt [51], whereas continental, atmospheric and stratospheric factors all contribute to the increase of nitrates, such as biomass burning, NH₃ oxidation, photochemical processes and lightning [52,53]. From 1997 through 2002, acid rain prevailed over approximately 30% of the land area in northern, eastern and southern China [54]. The frequency of acid rain has been decreasing in recent years, and government policies have led to a significant decrease in SO₄²⁻ concentration levels and a subsequent increase in NO₃⁻, which would imply a shift from sulfuric acid to mixed acid rain in China.

Table 2. Change of pH and ionic concentrations (μeq/L) in rainwater.

Site	pH	Ca2+	Mg2+	K+	Na+	NH+	F−	Cl−	SO42−	NO3−	SO42−/NO3−	Time	Reference
Guizhou, China a	5.2	7.0	110.4	4.4	3.0	11.7	0.8	5.2	51.8	24.7	2.1	2019–2020	Zeng et al. 2020 [43]
Jiangxi, China a	5.8	70.7	15.1	33.4	19.9		3.5	38.4	69.4	37.9	1.8	2018–2020	Li et al. 2022 [44]
Beijing, China a	6.7	59.2	76.3	56.0	33.0	141.3		100.1	66.3	59.6	1.1	2017–2018	Xu et al. 2020 [55]
Shanghai, China a	5.5	50.3	130.1	21.1	145.0	342.0		240.0	354.0	138.0	2.6	2016–2018	Luan et al. 2019 [41]
Huzhou, China a	6.2	37.1	57.9	17.3	21.1	194.6		40.0	78.8	41.8	1.9	2016–2018	Yan et al. 2019 [56]
Hangzhou, China a	6.2	29.2	56.8	16.8	16.5	191.1		28.0	58.5	42.3	1.4	2016–2018	Yan et al. 2019 [56]
Zhanjiang, China a		34.4		5.1	14.6	94.2	8.9	99.3		25.3		2015–2019	Zeng et al. 2022 [38]
Jiaozhou, China a	4.8	54.7	107.0	17.2	21.9	64.1	2.6	66.0	93.7	62.9	1.5	2015–2016	Xing et al. 2017 [57]
Ya’an, China a	4.4	16.5	169.6	11.4	8.5	37.9	2.7	20.9	138.4	71.4	1.9	2013–2014	Li et al. 2016 [2]
Nanjing, China a	5.1	15.3	127.5	8.0	20.4	247.9		61.7	135.7	58.1	2.3	2013–2014	Xue et al. 2014 [58]
Beijing, China a	4.9	21.5	346.0	9.2	53.3	273.0	12.0	50.9	357.0	42.6	8.4	2011–2012	Xu et al. 2015 [9]
Xinjiang (Urumqi), China a	6.3	193.0	118.9	45.1	54.4	422.3	10.0	121.5	307.6	39.1	7.9	2010–2019	Zhong et al. 2022 [17]
Ya’an, China a	4.0	24.2	203.7	30.1	13.2	98.4	13.3	37.5	212.3	84.4	2.5	2010–2011	Zhao et al. 2013 [16]
Jiuzhaigou, China a	6.0	38.0	13.4	21.2	41.1	149.8	21.0	37.2	70.5	12.7	5.6	2010–2011	Qiao et al. 2015 [33]
Shenzhen, China a	4.3	36.4	14.7	2.0	11.8	18.1		45.9	59.3	23.7	2.5	2008-2009	Zhou et al. 2019 [11]
Shenzhen, China a	4.9	10.5	24.0	1.1	2.3	21.4	0.5	19.8	38.4	12.3	3.1	2005–2009	Huang et al. 2010 [59]
Guangzhou, China a	4.5	18.0	66.0	9.0	9.0	131.0	12.0	21.0	202.0	52.0	3.9	2005–2006	Huang et al. 2009 [48]
Beijing, China a	6.0		234.0			191.2			248.9	84.1	3.0	2001–2003	Yang et al. 2004 [60]
Chongqing, China b	5.8	20.7	223.8	60.4	40.2	595.6	30.5	96.9	717.8	90.0	8.0	2000–2009	Lu et al. 2013 [18]
Tibet, China a		30.5	15.0	2.5	14.7	70.1		25.6	29.9	6.4	4.7	2021	Wang et al. 2022 [28]
Xian, China a	6.3	28.7	147.4	4.6	22.4	136.6		8.9	145.1	25.3	5.7	2017	Huo et al. 2020 [45]
shenzhen, China a	5.0	23.1	17.4	5.4	6.5	28.2	1.3	27.1	22.4	18.6	1.2	2017	Jian et al. 2019 [61]
Wuxi, China a	5.9	21.3	48.2	8.3	14.9	70.9	1.8	4.6	10.5	4.1	2.6	2016	Wang et al. 2021 [62]
Lijiang, China c	6.1	1.0	20.8	2.0	10.9	50.1	0.6	2.0	23.7	7.0	3.4	2012	Niu et al. 2014 [3]
Hangzhou, China a	4.7	18.9	39.8	7.1	13.4	158.0	8.3	32.2	125.0	35.9	3.5	2011	Wu et al. 2021 [63]
Xi’an, China a	6.6	31.1	229.8	13.8	36.6	425.6	28.7	38.7	489.7	128.8	3.8	2010	Lu et al. 2011 [13]
Chengdu, China a	5.1	1.4	150.5	6.6	16.2	196.6	6.2	8.9	212.8	156.2	1.4	2008	Wang & Han 2011 [29]
Beijing, China a	5.3	8.5	174.0	6.7	38.5	291.0	10.5	67.8	270.0	139.0	1.9	2008	Xu et al. 2012 [19]
Puding, China a	5.4	10.8	33.1	9.1	3.9	155.8	2.8	54.5	152.4	17.0	9.0	2008	Wu et al. 2012 [64]
Beijing, China a	5.1	25.0	185.6	17.7	40.4	607.2	15.7	104.0	315.8	109.0	2.9	2006	Xu & Han 2009 [40]
Shanghai, China a	4.5	50.1	80.7	14.9	29.6	204.0		58.3	200.0	49.8	4.0	2004	Zhang et al. [31]
Tie Shan Ping, China a	4.1	3.0	76.0	8.0	9.0	58.0		11.0	184.0	35.0	5.3	2003	Aas et al. 2007 [65]
Cai Jia Tang, China a	4.3	7.0	112.0	10.0	10.0	60.0		11.0	155.0	60.0	2.6	2003	Aas et al. 2007 [65]
Lei Gong Shan, China a	5.2	7.0	110.4	4.4	3.0	11.7	0.8	5.2	51.8	24.7	2.1	2003	Aas et al. 2007 [65]
Liu Chong Guan, China a	5.8	70.7	15.1	33.4	19.9		3.5	38.4	69.4	37.9	1.8	2003	Aas et al. 2007 [65]
Li Xi He, China a	6.7	59.2	76.3	56.0	33.0	141.3		100.1	66.3	59.6	1.1	2003	Aas et al. 2007 [65]
Chongqing, China a	5.5	50.3	130.1	21.1	145.0	342.0		240.0	354.0	138.0	2.6	2002	Zhou et al. 2003 [49]
Ahvaz, Iran a	6.2	37.1	57.9	17.3	21.1	194.6		40.0	78.8	41.8	1.9	2014–2015	Naimabadi et al. 2018 [66]
India a	6.2	29.2	56.8	16.8	16.5	191.1		28.0	58.5	42.3	1.4	2012–2018	Majumdar et al. 2022 [22]
Oleiros, Spain b		34.4		5.1	14.6	94.2	8.9	99.3		25.3		2011–2012	Moreda-Piñeiro et al. 2014 [66]
Pune, India a	4.8	54.7	107.0	17.2	21.9	64.1	2.6	66.0	93.7	62.9	1.5	2006–2009	Budhavant et al. 2011 [37]
Newark, USA a	4.4	16.5	169.6	11.4	8.5	37.9	2.7	20.9	138.4	71.4	1.9	2006–2007	Song and Gao 2009 [67]
Ghor Es-Safi, Jordan b	5.1	15.3	127.5	8.0	20.4	247.9		61.7	135.7	58.1	2.3	2006–2007	Al-Khashman 2009 [42]
Mexico a	4.9	21.5	346.0	9.2	53.3	273.0	12.0	50.9	357.0	42.6	8.4	2001–2002	Baez et al. 2007 [68]
Hamedan, Iran a	6.3	193.0	118.9	45.1	54.4	422.3	10.0	121.5	307.6	39.1	7.9	2014	Peikam et al. 2021 [10]
Kushkabad, Iran a	4.0	24.2	203.7	30.1	13.2	98.4	13.3	37.5	212.3	84.4	2.5	2014	Peikam et al. 2021 [10]
Nemat Abad, Iran a	6.0	38.0	13.4	21.2	41.1	149.8	21.0	37.2	70.5	12.7	5.6	2014	Peikam et al. 2021 [10]

^a Volume-weighted means. ^b Concentrations of ions and pH were calculated with VWM and arithmetic means, respectively. ^c Arithmetic means.

Table 2. Change of pH and ionic concentrations (μeq/L) in rainwater.

Site	pH	Ca2+	Mg2+	K+	Na+	NH+	F−	Cl−	SO42−	NO3−	SO42−/NO3−	Time	Reference
Guizhou, China a	5.2	7.0	110.4	4.4	3.0	11.7	0.8	5.2	51.8	24.7	2.1	2019–2020	Zeng et al. 2020 [43]
Jiangxi, China a	5.8	70.7	15.1	33.4	19.9		3.5	38.4	69.4	37.9	1.8	2018–2020	Li et al. 2022 [44]
Beijing, China a	6.7	59.2	76.3	56.0	33.0	141.3		100.1	66.3	59.6	1.1	2017–2018	Xu et al. 2020 [55]
Shanghai, China a	5.5	50.3	130.1	21.1	145.0	342.0		240.0	354.0	138.0	2.6	2016–2018	Luan et al. 2019 [41]
Huzhou, China a	6.2	37.1	57.9	17.3	21.1	194.6		40.0	78.8	41.8	1.9	2016–2018	Yan et al. 2019 [56]
Hangzhou, China a	6.2	29.2	56.8	16.8	16.5	191.1		28.0	58.5	42.3	1.4	2016–2018	Yan et al. 2019 [56]
Zhanjiang, China a		34.4		5.1	14.6	94.2	8.9	99.3		25.3		2015–2019	Zeng et al. 2022 [38]
Jiaozhou, China a	4.8	54.7	107.0	17.2	21.9	64.1	2.6	66.0	93.7	62.9	1.5	2015–2016	Xing et al. 2017 [57]
Ya’an, China a	4.4	16.5	169.6	11.4	8.5	37.9	2.7	20.9	138.4	71.4	1.9	2013–2014	Li et al. 2016 [2]
Nanjing, China a	5.1	15.3	127.5	8.0	20.4	247.9		61.7	135.7	58.1	2.3	2013–2014	Xue et al. 2014 [58]
Beijing, China a	4.9	21.5	346.0	9.2	53.3	273.0	12.0	50.9	357.0	42.6	8.4	2011–2012	Xu et al. 2015 [9]
Xinjiang (Urumqi), China a	6.3	193.0	118.9	45.1	54.4	422.3	10.0	121.5	307.6	39.1	7.9	2010–2019	Zhong et al. 2022 [17]
Ya’an, China a	4.0	24.2	203.7	30.1	13.2	98.4	13.3	37.5	212.3	84.4	2.5	2010–2011	Zhao et al. 2013 [16]
Jiuzhaigou, China a	6.0	38.0	13.4	21.2	41.1	149.8	21.0	37.2	70.5	12.7	5.6	2010–2011	Qiao et al. 2015 [33]
Shenzhen, China a	4.3	36.4	14.7	2.0	11.8	18.1		45.9	59.3	23.7	2.5	2008-2009	Zhou et al. 2019 [11]
Shenzhen, China a	4.9	10.5	24.0	1.1	2.3	21.4	0.5	19.8	38.4	12.3	3.1	2005–2009	Huang et al. 2010 [59]
Guangzhou, China a	4.5	18.0	66.0	9.0	9.0	131.0	12.0	21.0	202.0	52.0	3.9	2005–2006	Huang et al. 2009 [48]
Beijing, China a	6.0		234.0			191.2			248.9	84.1	3.0	2001–2003	Yang et al. 2004 [60]
Chongqing, China b	5.8	20.7	223.8	60.4	40.2	595.6	30.5	96.9	717.8	90.0	8.0	2000–2009	Lu et al. 2013 [18]
Tibet, China a		30.5	15.0	2.5	14.7	70.1		25.6	29.9	6.4	4.7	2021	Wang et al. 2022 [28]
Xian, China a	6.3	28.7	147.4	4.6	22.4	136.6		8.9	145.1	25.3	5.7	2017	Huo et al. 2020 [45]
shenzhen, China a	5.0	23.1	17.4	5.4	6.5	28.2	1.3	27.1	22.4	18.6	1.2	2017	Jian et al. 2019 [61]
Wuxi, China a	5.9	21.3	48.2	8.3	14.9	70.9	1.8	4.6	10.5	4.1	2.6	2016	Wang et al. 2021 [62]
Lijiang, China c	6.1	1.0	20.8	2.0	10.9	50.1	0.6	2.0	23.7	7.0	3.4	2012	Niu et al. 2014 [3]
Hangzhou, China a	4.7	18.9	39.8	7.1	13.4	158.0	8.3	32.2	125.0	35.9	3.5	2011	Wu et al. 2021 [63]
Xi’an, China a	6.6	31.1	229.8	13.8	36.6	425.6	28.7	38.7	489.7	128.8	3.8	2010	Lu et al. 2011 [13]
Chengdu, China a	5.1	1.4	150.5	6.6	16.2	196.6	6.2	8.9	212.8	156.2	1.4	2008	Wang & Han 2011 [29]
Beijing, China a	5.3	8.5	174.0	6.7	38.5	291.0	10.5	67.8	270.0	139.0	1.9	2008	Xu et al. 2012 [19]
Puding, China a	5.4	10.8	33.1	9.1	3.9	155.8	2.8	54.5	152.4	17.0	9.0	2008	Wu et al. 2012 [64]
Beijing, China a	5.1	25.0	185.6	17.7	40.4	607.2	15.7	104.0	315.8	109.0	2.9	2006	Xu & Han 2009 [40]
Shanghai, China a	4.5	50.1	80.7	14.9	29.6	204.0		58.3	200.0	49.8	4.0	2004	Zhang et al. [31]
Tie Shan Ping, China a	4.1	3.0	76.0	8.0	9.0	58.0		11.0	184.0	35.0	5.3	2003	Aas et al. 2007 [65]
Cai Jia Tang, China a	4.3	7.0	112.0	10.0	10.0	60.0		11.0	155.0	60.0	2.6	2003	Aas et al. 2007 [65]
Lei Gong Shan, China a	5.2	7.0	110.4	4.4	3.0	11.7	0.8	5.2	51.8	24.7	2.1	2003	Aas et al. 2007 [65]
Liu Chong Guan, China a	5.8	70.7	15.1	33.4	19.9		3.5	38.4	69.4	37.9	1.8	2003	Aas et al. 2007 [65]
Li Xi He, China a	6.7	59.2	76.3	56.0	33.0	141.3		100.1	66.3	59.6	1.1	2003	Aas et al. 2007 [65]
Chongqing, China a	5.5	50.3	130.1	21.1	145.0	342.0		240.0	354.0	138.0	2.6	2002	Zhou et al. 2003 [49]
Ahvaz, Iran a	6.2	37.1	57.9	17.3	21.1	194.6		40.0	78.8	41.8	1.9	2014–2015	Naimabadi et al. 2018 [66]
India a	6.2	29.2	56.8	16.8	16.5	191.1		28.0	58.5	42.3	1.4	2012–2018	Majumdar et al. 2022 [22]
Oleiros, Spain b		34.4		5.1	14.6	94.2	8.9	99.3		25.3		2011–2012	Moreda-Piñeiro et al. 2014 [66]
Pune, India a	4.8	54.7	107.0	17.2	21.9	64.1	2.6	66.0	93.7	62.9	1.5	2006–2009	Budhavant et al. 2011 [37]
Newark, USA a	4.4	16.5	169.6	11.4	8.5	37.9	2.7	20.9	138.4	71.4	1.9	2006–2007	Song and Gao 2009 [67]
Ghor Es-Safi, Jordan b	5.1	15.3	127.5	8.0	20.4	247.9		61.7	135.7	58.1	2.3	2006–2007	Al-Khashman 2009 [42]
Mexico a	4.9	21.5	346.0	9.2	53.3	273.0	12.0	50.9	357.0	42.6	8.4	2001–2002	Baez et al. 2007 [68]
Hamedan, Iran a	6.3	193.0	118.9	45.1	54.4	422.3	10.0	121.5	307.6	39.1	7.9	2014	Peikam et al. 2021 [10]
Kushkabad, Iran a	4.0	24.2	203.7	30.1	13.2	98.4	13.3	37.5	212.3	84.4	2.5	2014	Peikam et al. 2021 [10]
Nemat Abad, Iran a	6.0	38.0	13.4	21.2	41.1	149.8	21.0	37.2	70.5	12.7	5.6	2014	Peikam et al. 2021 [10]

^a Volume-weighted means. ^b Concentrations of ions and pH were calculated with VWM and arithmetic means, respectively. ^c Arithmetic means.

3.2. Sources of Nutrient Elements in Rainwater

3.2.1. Sources of Nutrient Elements in Rainwater

It is important to figure out the sources of chemical elements in rainwater. The ionic composition of rainwater mainly includes Na⁺, Ca²⁺, Mg²⁺, NH₄⁺, K⁺, Cl⁻, SO₄²⁻, NO³⁻, F⁻, etc. (see Figure 3). During haze periods, rainwater will also contain Al³⁺, Ba²⁺ and Fe³⁻ [69]. In this case, the ionic composition of rainwater will be mainly from anthropogenic sources, waves, and weathering-induced ground dust [31,47].

It has been generally found that trace metals, such as Al, Ba, Co, Fe and Ni, are present in rainwater [14], which may be due to the precipitation of these trace metals in soil dust during atmospheric flushing [70]. Fossil fuel combustion, biomass combustion and industrial chlorine factors, as well as crustal factors, are the main sources of trace metals in soil [20,71]. Thus, anthropogenic activities such as fossil fuel burning and smelting are important sources of trace metals in rainwater [70].

Sulfates and nitrates are two major categories of nutrient elements in rainwater precipitation. According to previous reports, NO₃⁻ and SO₄²⁻ are the dominant chemical species during urbanization and industrialization. Coal combustion accounts for approximately 70% of commercial energy production in China [65], resulting in significant SO₂ emissions. This produces a large amount of SO₄²⁻ in the atmosphere [9]. The fumes from biomass combustion are mainly in submicron and cumulative form, and these fumes can be swept into the clouds, increasing the rate of chemical reactions, resulting in the formation of secondary aerosols [72]. Earlier studies in the IGP (Indo-Gangetic Plain) region reported significant increases in NO_X and SO₂ emissions from coal combustion activities [4] and the ratio of NO₃⁻ and SO₄²⁻ in coal-producing cities was <1 [12]. Ghude et al. [73] reported that the IGP region exhibited significant increases in surface NO₂ emissions from thermal power plants and coal-fired industries, which are also sources of SO₂. It is generally found that anthropogenic activities such as the use of ammonium nitrate fertilizers, coal burning, and automobile exhaust are the main sources of NO₃⁻ [14].

Coal combustion is the main source of SO₄²⁻ and NO_x in rainwater in Guiyang, China, which reflects the similarity of their chemical behavior, since atmospheric particles in rain contain the precursors of SO₄²⁻ and NO_x, such as sulfur dioxide and nitrogen oxides [74]. In the atmosphere, nitrate ions released from nitrogen oxide emissions (anthropogenic sources) may react with NaCl particles, eventually releasing NaNO₃ and hydrochloric acid. It has been shown that NO_x is extremely abundant in regions where oceanic air masses intersect [36,75]. This reaction can explain that NO₃⁻ is related to the coarse particles of Na⁺ originating from the ocean. Rao et al. studied the causes for the acidity of rainwater in India, and proposed that the two ions SO₄²⁻ and NO₃⁻ are major contributors, and they may be obtained from the chemical reaction of acidic substances (H₂SO₄, HNO₃ and HCl) with alkaline substances and carbonate substances in the atmosphere. In fact, these substances are enriched in the atmosphere and blown into the air by winds [76]. Chen et al. suggested that NO₃⁻ in rainwater in southern China is directly related to NO_x emissions, and NO_x mainly comes from fuel combustion, including various industrial furnaces and automobiles [77,78]. The combination of NH₄⁺ and SO₄²⁻ may represent acidic pollutants, which are affected by peat combustion and biomass burning, which is possibly related to the removal of aerosols [8].

Biomass burning is the main source of Cl⁻ [16,22,37]. The contribution of Cl⁻ and Na⁺ to the total ion quantity in the rainwater of Sichuan, China is very large, which may be influenced by the subtropical monsoon climate [2]. Tsai et al. [79] reported that the exhaust gas emission of steel raw materials during the sintering process was the cause of chlorine generation. There is a high correlation between Na⁺ and Cl⁻ in rainwater in the Istanbul Basin because these ions are mainly from spray and sea salt [80,81,82]. Cl⁻ is considered to be the predominant presence in rainwater in the Pune region of India, and the transport of sea fog from the marine area to the land contributes to its increase.

K⁺, Ca²⁺ and Mg²⁺ are common components of soil dust in Beijing and the arid and semi-arid regions of northern China [9]. Ca²⁺ is released during the dissolution of calcite in natural soil dust [9] and Mg²⁺ comes from the ocean. Wang and Han [83] demonstrated that Ca²⁺ and Mg²⁺ in southwest China were mainly derived from soil dust, and Zeng et al. [39] supported that the rainwater Ca²⁺ and Mg²⁺ in southwest China mainly originate from calcite dissolution. This indicates that Ca²⁺ and Mg²⁺ in rainwater in most parts of China are mainly controlled by calcite solvation of air dust formed by different carbonate weathering [8]. The concentration of Mg²⁺ is generally affected by soil dust and sea salt particles in the atmosphere [84]. Additionally, K⁺ and Mg²⁺ come from soil enrichment and seawater dilution, and partly from marine sources [8]. The sources of Ca²⁺ and Mg²⁺ on the northwestern coast of Spain may be different, with Ca²⁺ originating from the land and Mg²⁺ originating from the ocean [14]. The sources of ions in rainwater after haze are also different. The contents of Ca²⁺, Mg²⁺ and K⁺ show a strong correlation with haze occurrence, indicating that these ions in rainwater come from same sources, such as the crust (soil) [76]. In the Southeast Asian haze event, Ca²⁺, Mg²⁺ and Na⁺ may also be derived from natural crustal sources [7].

NH₄⁺ is mainly derived from agricultural and livestock manures [8], anaerobic digestion of animal and human feces, and the application of chemical fertilizers [78,85]. Urea in fertilized soils, animal feces, wildfires, biomass burning, peat fires and straw burning are major sources of ammonia [84]. Gaseous ammonia emissions and materials containing NH₄⁺ produce atmospheric ammonia. Studies have shown that NH₃ emissions from human and animal feces can significantly increase the accumulation of ammonium nitrogen in soils in winter. In addition, anthropogenic sources such as agricultural activities and biomass burning are also possible sources [20,71].

K⁺ can be derived from anthropogenic factors, such as fertilizers [7], or from the crust, or from the lithosphere. K⁺ can be used as a tracer for biomass burning [1], waste burning, and vegetation waste decomposition. Large amounts of NH₄⁺ and K⁺ will be emitted from biomass combustion at the beginning of monsoons [35,86]. Weathering of potassium-bearing minerals and potassium feldspar are also possible sources of K+ in rainwater [15]. Due to the special karst landform, rainwater K⁺ in Guiyang mainly comes from the weathering of potassium salt (KCl), potassium feldspar, potassium-containing minerals and biomass burning [15]. Biomass combustion results in the emission of suspended particulate matter K+, such as fly ash and smoke (and NO_x). In Delhi, Ganges Basin, and India, combustion is the main source of K⁺ [87].

Metals, such as Zn, Pb and Ni, are the least abundant elements in rainwater and may come from anthropogenic activities and biomass burning emissions. In Malaysia, a recent study showed that these metals are derived from heavy industry pollution near UKM (National University of Malaysia) campuses, and also from traffic emissions [88]. Behera et al. [69] suggested that Zn concentrations increase during biomass or wood burning, while Ni and Fe originated from industrial emissions [8]. Recent studies pointed to anthropogenic contributions, meteorological conditions and the COVID-19 disease pandemic as influencing the distribution of As, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn elements [89].

3.2.2. Distribution of Pollutants in Rainwater in Different Regions

There are differences in rainwater compositions between northern and southern China, especially in the neutralization capacity of alkaline components. Additionally, the compositions of rainwater vary ranging from large to small cities. The compositional characteristics of precipitation components in megacities (Beijing, Chengdu, Xi’an, Chongqing, etc.) and small cities (Ya’an, Puding, Lijiang, etc.) are obviously different. Moreover, the contents of SO₄²⁻ and NO₃⁻ are the dominant species in urbanization and industrialization [2]. About 97.3% of the acidity in rainwater in Guiyang city is caused by SO₄²⁻ [90], which was further demonstrated by the studies performed in total suspended atmospheric particles in Guiyang [91]. The NH₄⁺ in the Ya’an area ranged between 8.54 and 1797.42 μeq/L, and the content of this ion was abundant, accounting for about 70% of the total cations. Similarly, the most abundant cation in Dalian was also NH₄⁺ [20]. The contents of SO₄²⁻ and NO₃⁻ in rainwater of Lijiang region presented typical anthropogenic characteristics and were particularly higher in August and September when precipitation is heavy. By contrast, during periods of low precipitation (i.e., the dry season from October to May), their contents exhibited a different trend and correlated with pollutants accumulation [3,92].

3.2.3. Seasonal Variation of Elements Content in Rainwater

The compositions of rainwater vary from season to season. Floating particles that stay in the air for longer in spring and winter can lead to an accumulation of ions that ultimately leads to higher ion concentrations in rainwater [77]. In addition, all ion concentrations are slightly higher in spring and autumn than in summer [39]. Seasonal trends are mainly influenced by meteorological factors and source intensity. The general high precipitation in summer and low precipitation in winter, the frequent occurrence of the accompanying winter haze and the appearance of the inverse thermosphere all inhibit the diffusion of elements in the air, which contributes to the accumulation of ions and occurrences of acid rain. In rural areas of China, spring is the time of sowing and the frequent agricultural activities make NH₄⁺ concentrations high [2], and the lower concentrations of K⁺ and NH₄⁺ in summer are due to the decomposition of nitrogen and potassium fertilizers caused by high temperatures [93,94]. In Lushan, Jiangxi, heavy rains and frequent showers in summer reduce the concentration of ions in the air and reduce NO_x and SO₂. In autumn, fertilizers are widely used in agricultural fields, and fertilizer consumption and livestock manure may emit large amounts of NH₃ [44].

The evolution of rainwater compositions is accompanied by the formation of the natural environment and the destruction of vegetation by humans. Long-duration sandstorms with increased wind speed and height in summer induced extreme sandstorms [95,96]. In this case, the pollutant levels in the rain will be extremely high, and the corresponding elements will be abundantly increased.

3.2.4. Effects of Dust Storms on the Composition of Rainwater

As a source of chemical composition, dust storms play a major role in rainwater chemistry [97]. Sand and dust storms are weather phenomena in which strong winds blow up dust and sand on the ground, making the air turbid and reducing horizontal visibility. Dust storms occur frequently in arid, semi-arid and severely desertified areas of land [30]. Northern China is regularly swept by dust storms originating from Mongolia and the desert regions of China, which have become more intense since 1999 [98,99]. Abnormal climate and human overexploitation of water resources can cause dust storms [97,100]. The chemical and physical properties of aerosols during sandstorms are significantly different from those in normal weather. The composition of fine particulate matter in dust storms includes PM2.5 water-soluble ions (the main water-soluble inorganic ions of PM2.5 include inorganic anions such as SO₄²⁻, NO₃⁻ and NH₄⁺ and metal ions such as Fe²⁺, Zn²⁺, Na⁺, K⁺, Ca²⁺ and Mg²⁺), organic pollutants, insoluble components (mainly SiO₂ and Al₂O₃) [30]. The concentrations of the crustal elements Al, Si, Ca, Fe and Ti were evidently enhanced during the sandstorm periods compared to the non-dust storm periods. Dust storms are potential triggers for allergic and non-allergic systemic diseases that can cause pneumonia, and the wind carries contaminated dust to farmlands, oceans, and cities [101,102,103,104].

3.3. Rainwater Nutrient Transport

Chemical elements in rainwater, such as metal pollutants, enter rivers, lakes, and seas in the form of raindrops; see Figure 4. The main pathway for the transformation of chemical elements in rainwater is chemical reactions, including precipitation and dissolution, redox, complexation, colloid formation, adsorption and degradation. Processes of rainwater transport and material cycling persist in the environment for long periods of time, circulating in the lithosphere, atmosphere and biosphere [105,106]. The transfer of ions in rainwater is mainly through mechanical transfer, physicochemical transfer and biological transfer. The mechanical transfer of elements in the rain refers to the movement of spatial positions and the transformation of existing forms in the rain by water or air currents. On the suspended matter, it is migrated and transformed with the flow of water. Elemental mercury can be converted into mercury vapor diffusion that moves with the air. Physicochemical transfer refers to the migration and transformation of elements through a series of physical and chemical actions such as adsorption and desorption, precipitation and dissolution, oxidation and reduction, complexation, chelation and hydrolysis. These processes determine the existing form of rainwater elements, the accumulation state and the degree of potential damage. Biological transfer is the migration of elements in the rain through the processes of the metabolism, growth and death of organisms. It mainly refers to the absorption of certain chemical forms of heavy metals from the sediment by plants through the root system and accumulation in their bodies. Such plants containing heavy metals may cause harm to human health through the food chain if they are eaten by animals or humans. All heavy metals can migrate and accumulate through organisms and exert threats to human health through the interaction of the food chain.

The transfer of elements and their sources go hand in hand, and the sources of various chemical components are transferred to nature in a corresponding way. The transfer of chemical elements is ultimately divided into three pathways; in the first pathway, rainwater-borne elements enter directly into plants and are absorbed and used by them, such as cadmium, copper and lead, 16% of zinc, and a large amount of nitrogen [89]. The chlorides in tap water are very detrimental to most plant growth and rainwater irrigation certainly avoids or reduces this hazard [107]. In the second pathway, in thunderstorms, nitrogen and hydrogen gas synthesize into ammonia, ammonia and carbon dioxide, and rainwater interaction can form ammonium carbon and fall into the soil with the rain; ammonium carbon can help crops generate the required nitrogen element, which belongs to the nitrogen fertilizer [108,109]. However, when the rainwater is excessively acidified, the nutrients potassium, sodium, calcium and magnesium in the soil will be released and leached away with the rainwater. In thunderstorms, as a result of the oxygen and nitrogen in the air under the discharge conditions, the oxides of nitrogen are generated, which then undergo complex chemical changes and finally produce nitrates that are easily absorbed by crops. In the third pathway, due to environmental pollution, rainwater cannot be consumed directly and is treated through water purification plants before being made available for drinking [110]. The elements in the rainwater enter the human body indirectly through drinking [111]. The minerals in the water mainly come from nature, where the rain falls in the mountains and fields, slowly coalesces in the soil, and then passes through the rocks and converges into a river. In this process, the water dissolves the minerals in the soil and ores, so the water is particularly rich in minerals and nature takes on the role of purifying rainwater pollutants.

The transfer of elements in nature follows the principle of the great cycle of elements; however, once the environment is subjected to breakage, the loss of balance in the elemental cycle will impair the ability of ecosystem to restore its function and affect human production and life.

3.4. The Removal Method of Pollutants in Rainwater

3.4.1. Research Progress and Methods of Nutrient Removal in Rain Water

If the content of pollutants in rainwater dramatically increases, it seriously influences the quality of sky and environment. It is necessary to reduce pollutant levels in stormwater by removing harmful chemical elements from stormwater. In recent years, the global climate has been changing, and frequent extreme weather poses a great threat to the urban water environment. Additionally, the shortage of water resources is also an important problem for mankind at present; it causes economic losses and leads to a series of ecological problems. Currently, about 70% of China’s rivers and lakes are subject to different degrees of pollution. Among them, the main pollutants are nitrogen oxides, heavy metals, Chemical Oxygen Demand (COD) and pathogens. If recycling is not considered, these wastes will eventually enter surface water and groundwater.

When rainwater reaches the ground, it also washes polluting elements away from buildings or other facilities, such as material residues of roofs, metals and wastes produced by factories, and chemical fertilizers applied on farmland. Eventually, the chemical elements transported by rainwater flow into river cisterns and infiltrate the underground. Untreated rainwater carries various pollutants, which will lead to the deterioration of the water quality, threaten ecological security, and inhibit the physiological activities of aquatic organisms [112].

Rainwater drainage is the most complicated pollution route in cities. The discharged water cannot be used directly because of the bad quality. The drainage of rainwater increases the flow of stream water, and it also accelerates the erosion of river banks and river bottoms, resulting in changes of river shapes. The suspended solids in rainwater can settle and cause sedimentation, which leads to the silting of rivers [113]. According to a survey by the US National Environmental Protection Agency, about 30% of the pollutants in surface water are caused by non-point source pollution [114]. The research conducted by Che et al. [115] estimated that the pollution load caused by urban runoff in Beijing accounts for more than 12% of the total pollution load, and may even exceed 20% in Shanghai. Storms also cause complex pollution by discharging waste such as cigarette butts, beverage cans and dead leaves into rivers, finally leading to the generation of solid waste in rivers. The deterioration of river water and sediment quality is due to suspended matter, organic matter (hydrocarbons, pesticides, etc.), nutrients (nitrogen and phosphorus) and minerals (heavy metals). A series of effluent elimination treatments should be carried out before they are discharged into water bodies, and most countries are using stormwater constructed wetlands (SCWs) to manage stormwater runoff [116]. According to the scientific literature [117,118,119], various SCWs exist: green roofs, swales, ponds, filters, floating treatment wetlands, retention and infiltration basin valleys, blind ditches, upstream stormwater management and downstream stormwater management.

In related research on SCWs, researchers have focused on the efficiency of SCWs in reducing water discharge [116] and in removing pollutants, including micro-pollutants [120,121,122]. Recently, green roofs are becoming more and more popular in North America (Figure 5). They can control runoff and non-point source pollution in urban areas, and provide multiple advantages [122,123,124], such as aesthetic value, thermal insulation, and noise reduction [125].

Initial rain refers to the runoff formed in different catchment areas and pipelines in the early stage of precipitation [126]. Accordingly, the runoff of 10–15 min in the initial stage of precipitation is generally regarded as the amount of initial precipitation. The initial rainwater pollution is mainly caused by the raindrops rinsing the atmosphere in the early stage of precipitation and scouring urban roads, buildings and wastes [127,128]. The main treatment methods for initial rainwater are constructed wetlands and bioretention ponds [129,130,131]. These two methods have disadvantages, such as large footprints and long processing time. The third method, adsorption, has the advantages of small footprints, high efficiency and low cost. It also has no secondary pollution, and thus is gradually favored by the industry [132].

Usually, artificial ecological facilities are used to intercept and filter the initial rainwater. This can effectively purify the rainwater; additionally, it can play a role in beautifying the city and thus has a certain ornamental value. Commonly used methods include green roofs, permeable pavements, grass-planted trenches, plant buffer zones, rain gardens and ecological green spaces [133,134]. Green roofs can effectively purify rainwater and improve water quality, and have been widely used in the United States, Germany and other countries. In recent years. Using sand coated with nano-silver on roofs to filter rainwater improves microbial removal efficiency [135]. China has begun to pay attention to the use of green roofs. Green roofs can remove most pollutants, but the efficiency of nitrogen removal is not high [136]. Beck et al. [136] added biochar to soil to effectively absorb nitrogen; in this way, the total nitrogen removal rate reached more than 79%.

In the process of rainwater transmission, grass-planted ditches, vegetation buffer zones and other means can be used to intercept and purify the initial rainwater, which can usually be set on the path of the initial rainwater entering the river. Grass-planted ditches can effectively block particulate pollutants in the initial rainwater. A study by Mazer et al. [137] also showed that the removal rate of Suspended Solids (SS) by grass-planted trenches was as high as 60–90%. Vegetation buffer zones are simple to construct and with both purifying and appreciating advantages; thus, they have been widely used [138,139].

Duchemin et al. [140] found that the removal rates of SS and total nitrogen by the preparation of buffer strips can reach 87% and 33%, respectively. Ecological green spaces integrate rainwater collection, treatment and reuse, and therefore are an effective method for initial rainwater control. Wang et al. [141] studied the pollution runoff functions of ecological green spaces that are built with plants, sand and gravel. The research results showed that ecological green spaces have high efficiency in the removal of suspended particles and total phosphorus in rainwater runoff, and the removal rates were over 90% and 47.1%, respectively. By contrast, the removal efficiency of COD was not satisfactory.

3.4.2. Bioretention Systems Treatment

Bioretention systems are composed of trenches or basins filled with porous media. Filtration, transition and drainage layers are the major components of bioretention systems, which can effectively remove nitrogen through chemical reactions, physical processes and biological processes. The main processes are assimilation (e.g., nitrogen absorption), adsorption, mineralization (ammonification), nitrification and denitrification [89,107]. Vegetated bioretention systems remove more nitrogen than non-vegetated bioretention systems [142,143]. Xiong et al. used modified biochar to improve the removal of Cu, Zn, Pb and Cd metal elements from bioretention cells [144]. Plant uptake is one of the main mechanisms by which bioretention systems remove nutrients. Chen et al. [145] studied the absorption efficiency of NH₄⁺-N, NO₃⁻-N, Glycine-N and TSP by three bioretention plants, including Danica, Hemerocallis and Pennisetum. Photocatalytic technology is the currently introduced water treatment technology, which has the characteristics of high efficiency, mild reaction conditions, low secondary pollution and environmental friendliness. The research on rainwater storage tanks in China started relatively late. The first batch of rainwater storage tanks, also called “rainwater gardens”, was built in Suzhou River, Shanghai. Rainwater biological retention ponds appeared one after another in recent years, becoming one of the typical measures of sponge cities. They have the advantages of flexible design, low maintenance cost, wide application range and obvious water purification effects.

3.4.3. Rainwater Storage Tank Treatment

Through surface runoff, heavy metal pollutants in rainwater are enriched, which can accumulate in animals and plants and enter the human body through the transmission of the food chain, posing a great threat to human health and safety. In addition, heavy metals also have the characteristics of a wide pollution range, long pollution duration, and being difficult to control, and thus are one of the most important pollutants in rainwater runoff [146]. Rainwater storage tanks intercept the initial rainwater using ecological, biological, physical and chemical approaches; see Figure 5. Among them, the adjustment storage tank is an effective rainwater storage control method and has been widely used in Japan, Germany and other countries. Germany has built rainwater retention ponds to intercept stormwater [147], Japan has begun to study the use of rainwater storage tanks as early as the 1960s [148], and the United Kingdom has built large sewers and storage tanks to improve the water quality of the Thames River [149]. The Ben-CNT composites can effectively adsorb micro-pollutants and heavy metals [140]. Shenzhen started using a drainage water supply system for water pollution treatment 20 years ago [150]. Using a multistage constructed wetland in Poland, heavy metals and microplastics were removed to varying degrees with the hydraulic loading of the bed [151]. In sum, the construction of rainwater storage tanks plays an important role in the protection of the urban water environment. The use of bacterial carbon and nitrogen metabolisms is capable of removing nitrates from stormwater through a biological process that uses organic nutrients adhered to biofilm supports or through the extraction of molasses from water by autotrophic organisms [152].

4. Discussion

In China, due to the rapid industrial development and the weak awareness of environmental protection, the decline of rainwater pH mainly occurred from the late 1990s to the mid-2000s. In general, acid rain is more severe in northern Chinese cities than in southern cities, and rainwater SO₄²⁻ concentrations are higher in particular northern cities (e.g., Zhengzhou and Xi’an) than in southern cities. This phenomenon can be attributed to the impact of coal combustion, with large coal-fired power plants driving the development of these cities. Following the implementation of the 13th Five-Year Plan (2016–2020), SO₂ emissions in 2020 were 15% lower than in 2015, and rainwater pH in China has increased. The implementation of desulfurization and denitrification technologies in Anyang City and the control of coal combustion in Xi’an have reduced SO₂ by half compared to the period when the policy was not implemented [153,154]. A significant decrease in SO₄²⁻ levels and a concomitant increase in NO₃⁻ would imply a shift in acid rain in China from sulfuric to nitric nature.

Summarizing the ionic composition of rainwater in each region can deepen our understanding of acidic stormwater pollution in different regions and help local government officials develop more effective policies to control acid rain. With high-quality economic growth, a number of environmental control strategies have been implemented. For example, a number of substandard small coal-fired power plants were phased out, and gas desulfurization and dust removal were mandated for new equipment in steel plants. In China, the number of private cars increased tenfold between 2004 and 2020 and vehicle emissions increased significantly. Recently, to effectively control NOx emissions, the use of yellow-label vehicles that do not meet Euro 1 standards was discontinued. The seasons and the nature of urban development should be considered during the execution of the policy. It should be considered whether the development of the city is dominated by industrial development or tourism, which will affect the composition and concentration of ions in rainwater. In tourism cities, once the number of people grows, the cities’ air ion concentration will increase and their high-level critical load should be fully considered to control the number of people entering tourist cities.

The study of stormwater pollution sources such as sea salt, fossil fuel combustion, agricultural production, construction dust, and biomass burning may help develop emission control policies to protect and manage pollution control in the Chinese ecosystem. There are few studies on CO₂ concentration changes during precipitation in China. CO₂ concentrations change when it rains and the main contribution of CO₂ comes from erosion of crustal sources and soil dust in Kuwait Bay. Wind speed, wind direction, temperature and dust storms affect the dissolution of CO₂ in rainwater [155], which will affect the characteristics of rainwater alkalinity.

In places where precipitation is scarce, rainwater harvesting affects human productive life. Clean water shortages are a pressing global water problem, and stormwater utilization has great potential as an effective option to mitigate the water crisis. Some studies have shown that gravity-driven membrane bioreactors and electro-oxidation can disinfect microorganisms (e.g., Escherichia coli) from rooftop stormwater [134]. In Australia, roof rainwater is the main source of drinking water for many rural households. Chlorination is widely used and cost-effective in China, but has not been fully developed and widespread in the Australian case, and hypochlorite can be very effective in retarding bacterial regeneration [156]. The use of photo-assisted electrochemical processes in Iran enables the removal of metallic elements from rainwater, where iron, manganese and lead are removed by oxidation at the photoanode [157]. The photocatalytic method (titanium dioxide) is currently being used in China and Tehran as a photoanode to treat stormwater, a method that significantly removes elements such as iron, manganese and lead [158]. In fact, the electro-oxidation, light-assisted electrochemistry and photocatalysis methods are identical in terms of removal principles, but the media used are different. Different media have different efficiency in removing elements, but practicality and costs need to be considered, and many removal methods are still in the testing and promotion phase in China. Rain gardens are by far the most used method in cities, as they are already embedded in urban buildings when they are built, and are green and ecological. In order to shorten the flow path of rainwater, the focus should be the treatment, disinfection and removal of pollutants from the rainwater source. The rainwater passing through the ground enters the rainwater reservoir and is allocated to different channels of using water according to the rainwater cleanliness level. In order to solve the current outstanding problems of urban flooding, rainwater runoff pollution and water shortage, the Chinese government expects to implement more than 80% of sponge projects in 2030 to create human–water harmonious sponge cities in the whole area.

Urban development affects the ecological balance and disturbs the hydrological cycle, in which precipitation plays an important role. Therefore, many technical solutions have been implemented in China to achieve effective stormwater management. On the other hand, stormwater runoff from urban areas contains a large amount of pollutants and should therefore be treated properly. The quality of the collected water should be categorized into different class attributes. For example, medium water quality can be used for irrigation [159], which may effectively address water scarcity in semi-arid urban areas.

In collecting relevant literature, the authors found that researchers have performed little research on the elemental transfer of rainwater. How do the elements circulate in the atmosphere, biosphere and soil circle? What is the residual state of rainwater elements in plants when they are absorbed directly by plants or flow into the soil to be reabsorbed? These questions have not been solved, and the study of rainwater elemental transfer is still in a vague state. For example, the use of rainwater harvesting ponds in agricultural fields can be an ecological technique for the adaptation of crops to climate [160], but researchers have made direct use of rainwater harvesting ponds, neglecting the analysis of nutrients in rainwater and their transfer in the plant. By using isotopic techniques, scientists can now study the quality and quantity of water resources, and use naturally occurring isotopes in water to determine its origin, age and susceptibility to contamination, and how water moves and interacts above and below ground. According to the current knowledge gaps, several key questions and future research priorities were identified: How are elements in rainwater transferred in plants and in the human body? What is the amount of each element that the human body can receive? The transfer of elemental mercury, for example, can be studied using isotopic labeling methods for environmental mercury contamination and mercury biogeochemistry. Incorporating the results of these studies into improved environmental protection policies and practices would go a long way in promoting ecological security.

People all over the world should join hands to protect the environment, make rainwater clean and sanitary, achieve the United Nations Sustainable Development Goal 6 “clean water and sanitation”, incorporate the views of multiple ecological civilizations, strengthen cooperation, promote win–win cooperation, and work together to build a beautiful earth.

5. Conclusions

Based on this study, it can be concluded that along with urbanization and industrialization construction, the ecological environment has been damaged and the quality of rainwater is not optimistic. In general, in China, the overall acid rain type changed from a sulfuric acid type to a mixed type, and the contribution to NO_X gradually increased with the increase of urbanization, which was mainly influenced by the restriction of motor vehicles and the policy of reducing coal combustion, so the establishment of an automatic acid rain monitoring network should be strengthened to help policy making. The sources of rainwater composition are mainly influenced by man-made and natural influences: the nature of urban development, the geographical location of the city, seasonal changes, extreme weather and airflow trajectories can affect the chemical composition of rainwater, disrupt the hydrological cycle and damage the balance of the ecosystem.

Currently, the establishment of sponge cities can solve the problems of urban flooding, rainwater runoff pollution and water shortage. There are many measures for source treatment and collection, such as green roofs, permeable pavements, planted trenches, planted buffers, bioretention systems and rainwater storage tanks. Solutions such as the new photocatalytic method are being further promoted in order to achieve green economic features. Rainwater that is not treated in time, or is costly to treat again, should be classified according to the quality of the water; medium-quality water can be used for irrigation, which can effectively solve the problem of water shortage in semi-arid urban areas.

Transfer of elements or ions in rainwater occurs between the atmosphere, soil and biosphere, and the exact pathways of transfer need to be further studied. Therefore, scientists should increase the use of isotope techniques to determine the source, age and sensitivity to pollution using naturally occurring isotopes in water to study the quality and quantity of water resources.

Clean water scarcity is a pressing global water issue, and the use of rainwater promises to be an effective option for alleviating the water crisis. Understanding the sources, transport and removal of chemical elements in rainwater, in line with the United Nations Sustainable Development Goal 6 “Clean Water and Sanitation”, can improve rainwater quality and water use, thus contributing to the goal of global environmental governance.