VIETNAM NATIONAL UNIVERSITY, HANOI VNU UNIVERSITY OF SCIENCE FACULTY OF ENVIRONMENTAL SCIENCES Lê Thi Thuy ASSESSMENT OF ACID RAIN PROGRESS IN HANOI CITY DURING THE PERIOD OF 2008-2
LITERATURE REVIEW
Rain water
1.1.1 The concept of rain water
Rain is liquid water droplets condensed from atmospheric water vapor that become heavy enough to fall under gravity, playing a crucial role in the Earth's water cycle It deposits the majority of freshwater on our planet, supporting diverse ecosystems and providing essential water resources for hydroelectric power generation and crop irrigation.
Acid rain, also known as acid deposition, refers to any form of precipitation—such as rain, snow, fog, hail, or dust—that contains acidic components like sulfuric or nitric acid, falling to the ground from the atmosphere According to the European Economic Community (EEC), precipitation with a pH below 5.6 is classified as acid rain, though regulations vary by country; for instance, the USA considers rain with a pH below 5.0 as acid rain, while some Asian countries, including Vietnam, use a threshold of pH < 5.6 Acid deposition occurs in two main forms: wet deposition, which includes acidic rain, snow, and fog, and dry deposition, involving the settling of acidic particles and gases without moisture These acidic substances can deposit quickly onto surfaces or react during atmospheric transport to form larger, more harmful particles When these acids are washed off surfaces during subsequent rains, they flow into water bodies and the soil, posing risks to plants, wildlife, and human health.
Dry deposition of atmospheric particles onto Earth's surface is influenced by the amount of rainfall in a given area In desert regions with minimal rainfall, dry deposition dominates, resulting in a higher ratio of dry to wet deposition Conversely, areas that receive several inches of rain annually experience more wet deposition, reducing the relative contribution of dry deposition Understanding this balance is essential for assessing pollutant deposition patterns across different environments.
Figure 1 This image illustrates the pathway for acid rain in our environment.
Atmospheric concentrations of sulfur dioxide (SO2) and nitrogen oxides (NOx) are the primary chemical precursors that lead to acidic conditions When these compounds react with water, oxygen, carbon dioxide, and sunlight in the atmosphere, they form sulfuric (H2SO4) and nitric acids (HNO3), which are the main agents of acid deposition These gases dissolve easily in water and can be carried long distances by wind, contributing to acid rain, sleet, snow, and fog experienced in various regions.
Human activities are the primary cause of acid rain, as the release of various pollutants into the atmosphere has significantly altered its chemical composition Over the past few decades, emissions from power plants and other industrial sources have introduced large quantities of sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) into the air These pollutants react with water vapor in the atmosphere to form sulfuric and nitric acids, which then fall to the ground as acid rain The increasing levels of these chemicals have led to environmental damage, affecting aquatic ecosystems, soil quality, and plant life.
Burning fossil fuels like coal for electricity generation releases the majority of sulfur dioxide and significant amounts of nitrogen oxides into the atmosphere Additionally, vehicle exhaust from cars, trucks, and buses contributes to air pollution by emitting nitrogen oxides and sulfur dioxide These pollutants are major contributors to the formation of acid rain, which harms ecosystems, water sources, and infrastructure Reducing emissions from energy production and transportation is essential for improving air quality and protecting the environment.
Oveview of dosmestic and worldwide researches
The foundational research on acid rain is documented in Dr Robert Angus Smith's book "Air and Rain," establishing him as the "father" of acid deposition studies His work includes detailed analysis of particulate matter in solid, liquid, and gaseous forms in the atmosphere, highlighting ingredients such as sulfate salts, soda, lime, and iron originating from the sea and ground sources Key pollutants like SO2 and NO2 are identified as primary contributors to acidity in rainwater, alongside organic substances in the air Smith emphasizes the importance of studying the changing composition of rainwater and its substances for medical and environmental evaluations, laying the groundwork for understanding acid rain's impact.
Over the past century, extensive research on acid rain has been conducted globally, yet concerns about its ongoing changes continue to engage generations of scientists and researchers.
The United States was among the first countries to establish an acid deposition monitoring network, with the National Atmospheric Deposition Program (NAD/NTN) founded in 1978 to monitor atmospheric deposition and its environmental impacts This comprehensive program includes three key monitoring station systems: the National Acid Deposition Assessment Program (NAPAP), the Research Monitoring Network (AIRMoN) in collaboration with Canada, and the Mercury Deposition Network (MDN) Data collected through these networks have facilitated numerous studies evaluating the current status and environmental effects of acid rain A 2004 report by the US Environmental Protection Agency (EPA) highlighted that the Northeast United States is particularly affected by acid deposition, underscoring the importance of ongoing monitoring and research.
The highest frequency of rainfall occurs in the Eastern states, where pH values are consistently below 5.0, ranging from 4.3 to 4.7, with New York State experiencing even more acidic conditions with pH levels below 4.3 This region also exhibits elevated nitrate ion (NO₃⁻) concentrations in wet deposition, measuring between 1.5-1.8 mg/L and depositing 14-20 kg/ha annually Additionally, sulfate ions (SO₄²⁻) are prevalent, with wet deposition concentrations of 2-2.5 mg/L and annual deposition of 21-27 kg/ha, indicating significant acidic pollution impacts in this area.
Chinese researcher Li Zong-Jie analyzed the variations of pH and electrical conductivity (EC) in the atmosphere by studying 402 rainwater samples from the Yangtze River source area between 2010 and 2015, revealing a pH range from 4.0 to 8.57 with an average of 6.37, and EC values from 5.2 to 124.4 µS/cm averaging 27.49 µS/cm Seasonal analysis showed that EC was highest in spring (37.62 µS/cm), followed by winter, summer, and autumn, with values of 31.86, 25.75, and 24.04 µS/cm respectively Conversely, pH levels were highest in summer (6.41) and generally higher in spring and winter (6.37 and 6.25) compared to autumn Using the HYSPLIT atmospheric transport model, the study identified industrial pollution and pollutant transport from India and neighboring regions as primary sources of acid rain in the area, additionally highlighting transportation activities as contributing factors.
Agnes et al analyzed the chemical composition of rainwater across 27 European countries from 2000 to 2017, highlighting variations in pH, acidification, neutralization, and wet deposition rates, as well as source contributions Their findings indicate that rainwater chemistry is influenced by atmospheric circulation patterns and regional pollution sources Furthermore, the study reveals that wet deposition rates are affected by climate conditions and local economic activities Sulfate and nitrate are significant contributors to rainwater acidity, primarily originating from industrial and transportation emissions, while ammonium and potassium derive from biomass burning, forest fires, and industrial processes.
Calcium primarily originates from land sources, whereas chloride is believed to come from oceanic inputs Research indicates that key acidifying agents are sulfur and nitrogen compounds released through industrial activities and agriculture, including transportation emissions and farming practices Wet deposition plays a crucial role in removing pollutants from the atmosphere, offering valuable insights into the chemical makeup of rainwater and the long-range transport of pollutants.
Global research on rainwater composition and chemical properties reveals significant findings For example, Liuyi Zhang's 2018 study in Chongqing, China, identified high relative concentrations of ion pairs such as sulfate (SO4^2-) and ammonium (NH4+), highlighting the importance of understanding rainwater chemistry for environmental and public health assessments.
Sulfate (SO4 2-) and calcium (Ca 2+), nitrate (NO3 -) and ammonium (NH4 +), as well as calcium (Ca 2+) combined with nitrate (NO3 -) and ammonium (NH4 +), are key ions responsible for neutralizing acidity in rainwater These ions predominantly originate from anthropogenic activities, such as industrial emissions and vehicular pollution, as well as from dust soil Their presence plays a significant role in mitigating the acidity of rainwater, highlighting the impact of human activities and soil dust on atmospheric chemical composition.
This study investigates the chemical compositions of precipitation at three non-urban sites in Hebei Province, North China, highlighting the influence of terrestrial sources on ionic composition It reveals significant seasonal differences in ion concentrations and pH values, primarily driven by changing air sources and seasonal variations The research emphasizes how the dilution effect on suspended particles affects rainwater chemistry, demonstrating the impact of terrestrial emissions on precipitation quality.
This study examines the chemical composition of rainwater in the northern Eastern Carpathians, Romania, from January 2009 to December 2017, highlighting the influence of long-range transported pollutants By analyzing air mass trajectories, the zonal index, and zonal circulation patterns, the research reveals how atmospheric circulation affects the distribution of chemical elements in precipitation The findings provide insights into the impact of regional and distant pollution sources on rainwater quality, emphasizing the importance of atmospheric dynamics in environmental monitoring and air quality assessment.
“Chemical composition of rainwater and dustfall at Bhubaneswar on the east coast of India” R Das, S.N Das, V.N Misra Chemical composition of
9 rainwater and dustfall was studied in two different stations of Bhubaneswar, located on the east coast of India [25]
Rainwater is essential for people and society, making the analysis of its composition a global concern Understanding the chemical makeup of rainwater is crucial for addressing environmental issues such as acid rain Extensive research on rainwater analysis and acid rain has been conducted worldwide, providing valuable insights that serve as a foundation for studying rainwater components in Vietnam.
Socio-economic development and population growth have driven advancements in industry, transportation, and daily activities, leading to a significant increase in fossil fuel consumption This surge in energy use has contributed to numerous environmental issues, with acid rain being one of the most pressing problems facing our environment today.
Research on rainwater in Vietnam began in the early 1990s, focusing initially on acid rain Raindrops wash hundreds of cubic meters of air, removing airborne dirt and pollutants, which makes rainwater potentially highly toxic Therefore, assessing the chemical components of rainwater is crucial, especially amidst Vietnam's ongoing industrial development According to the Vietnam Environment Status Report, from 1995 to 2005, urban and industrial areas experienced significant air pollution, with acid rain occurring relatively frequently during this period.
The ministerial-level research project titled "Survey and assessment of the pH status of rainwater in the area with measurement data," conducted by the General Department of Meteorology and Hydrology from 1991 to 1993, was Vietnam's first study on acid rain This groundbreaking project focused on monitoring rainwater chemistry at meteorological stations in northern Vietnam, analyzing chemical components in rainwater The research opened new avenues for classifying and quantifying pollution sources that contribute ions to rainwater, advancing understanding of acid precipitation in the region.
Overview of area
Hanoi is situated northwest of the Red River Delta, spanning latitudes from 20°53' to 21°23' North and longitudes from 105°44' to 106°02' East, making it a key geographical location in northern Vietnam The city shares borders with Thai Nguyen and Vinh Phuc provinces to the north, Ha Nam and Hoa Binh to the south, Bac Giang, Bac Ninh, and Hung Yen to the east, and Hoa Binh and Phu Tho to the west Located approximately 120 kilometers from Hai Phong port city, Hanoi's strategic position enhances its significance as Vietnam's political, cultural, and economic center.
Nam Dinh city is strategically positioned 87 km from the city center, serving as one of the three main economic and cultural poles of the Red River Delta region Following the administrative expansion in August 2008, the city now covers a total area of 3,324.92 square kilometers, situated primarily on the right bank of the Red River, with portions extending to the left bank This extensive area enhances Nam Dinh’s significance as a key hub in northern Vietnam's Red River Delta.
Hanoi's topography gradually decreases from north to south and from west to east, with an average elevation ranging between 5 to 20 meters above sea level The northern and western parts of the city are characterized by mountainous and hilly terrain, contributing to Hanoi's diverse landscape This topographical variation influences local climate, urban development, and land use planning across the city.
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Figure 2 Administrative map of Hanoi city [11]
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The Red River is the main river of the city, starting to flow into Hanoi in
Ba Vi district, located outside the city in the Phu Xuyen district area, borders Hung Yen and extends southward to Nam Dinh The Red River flows through Hanoi, covering a length of 163 km—representing approximately one-third of its total length in Vietnam.
Hanoi is bordered by the Da River, which serves as the boundary between Hanoi and Phu Tho Province, and it converges with the Red River north of the city in Ba Vi district The city is traversed by numerous rivers, including the Day River, Duong River, Cau River, and Ca Lo River, contributing to its rich waterways Inside Hanoi, small rivers such as the To Lich River and Kim Nguu River flow through the urban area, along with various wastewater drainage lines that manage the city's sanitation and water management needs.
Hanoi is a unique city renowned for its numerous lakes and remnants of ancient rivers, which contribute to its scenic urban landscape West Lake, covering approximately 500 hectares, is the largest and most significant lake within the city, playing a vital role in Hanoi’s natural beauty Located in the historic city center, Hoan Kiem Lake is a prominent landmark, along with other inner-city lakes such as Truc Bach, Thien Quang, and Thu Le Beyond the city center, Hanoi is home to many large lakes including Kim Lien, Lien Dam, Ngai Son-Dong Mo, Suoi Hai, Meo Gu, Xuan Khanh, Tuy Lai, and Quan Son, enhancing the city's rich aquatic landscape.
1.3.4 The economic growth in Hanoi
Hanoi is a vital economic hub in Northern Vietnam, serving as one of the country's seven key economic regions and a major driver of national development Strategically located in the Northern Delta, Hanoi benefits from favorable natural conditions, excellent transportation links, and abundant human resources, making it a center for politics, culture, education, and economic growth As one of Vietnam's two primary economic engines, Hanoi plays a crucial role in developing the Capital Region, Northern Key Economic Region, and Red River Delta The city’s economic structure has shifted towards modernization, emphasizing the growth of services, trade, industry, and construction sectors while reducing reliance on agriculture Rapid economic growth, with an average rate of 7.2% per year between 2016 and 2018, and substantial investment resources have significantly improved residents' living standards In 2018, Hanoi’s gross regional domestic product (GRDP) reached approximately 920,270 billion VND, highlighting its expanding economic scale.
Per capita income reached 117.2 million VND (approximately 5,134 USD), enabling increased investment in social welfare projects that improve residents' quality of life All commune, ward, and town health stations have now been upgraded to meet national health standards, reflecting significant progress in healthcare infrastructure Additionally, the number of schools meeting national standards and communes achieving new rural standards has exceeded set targets, completing these milestones two years ahead of the planned timeline.
MATERIAL AND REARCH METHODS
Research object
This study examines the progression of acid rain in Hanoi City from 2008 to 2018, focusing on key parameters of wet deposition such as pH, sulfate (SO₄²⁻), nitrate (NO₃⁻), chloride (Cl⁻), ammonium (NH₄⁺), calcium (Ca²⁺), sodium (Na⁺), and magnesium (Mg²⁺) The research analyzes how these indicators have evolved over the decade, providing insights into the severity and changes in acid rain pollution in the region By evaluating these specific components, the study aims to understand the impact of atmospheric pollutants on Hanoi's environmental quality and inform strategies for pollution control and mitigation.
This study analyzes a decade of chemical rainwater data from the Acid Deposition Monitoring Network in East Asia (EANET) at Hanoi station, covering the period from 2008 to 2018 The evaluation utilizes monthly monitoring data, focusing on precipitation and ionic concentrations to assess acid deposition trends Findings highlight variations in ionic composition and acidity levels in rainwater over ten years, providing valuable insights into regional air pollution and acid rain patterns This comprehensive dataset enhances understanding of environmental impacts and informs policy measures for acid deposition mitigation in East Asia.
Scope of study
The spatial range is Hanoi station belongs to the Acid Deposition Monitering Network in East Asia with coordinates 21 0 02’B 105 0 26’Đ The time range of the study is 10 consecutive years from 2008 to 2018.
Database
Secondary data refers to information that has already been collected by sources such as government agencies, company records, or scientific journals, offering a quick and cost-effective way to gather foundational data These data serve as a useful comparison point for primary research and help identify additional information needs However, secondary data can sometimes be outdated, inaccurate, or unsuitable for specific research requirements In this study, rainwater data from 2008 to 2018 were obtained from the Acid Deposition Monitoring Network in East Asia (EANET), providing valuable insights for analysis.
This study analyzes data collected from Hanoi Station, part of the Acid Deposition Monitoring Network in East Asia (EANET), spanning 2008 to 2018 The station provides weekly monitoring data on key parameters including pH, electrical conductivity (EC), and concentrations of SO4²⁻, NO3⁻, Cl⁻, NH4⁺, Ca²⁺, Mg²⁺, Na⁺, and K⁺ Ion concentrations and deposition amounts are systematically calculated and reported following EANET's standardized formulas, ensuring accurate assessment of acid deposition trends over the decade.
Methodology
2.4.1 Collection and synthesis of secondary data method
Collection of relevant domestic and foreign data including rainwater chemistry monitoring in Hanoi, acid rain studies
Effective research on acid rain requires collecting reports and studies from global and Vietnamese sites, enabling identification of suitable approaches for each region and facilitating comparisons with thesis findings The study incorporates both domestic and international data, covering aspects such as natural conditions, socio-economic factors, and the progress of acid rain research Key sources include reputable international journals from publishers like ScienceDirect, Springer Nature, and ResearchGate After gathering relevant references, it is essential to evaluate their applicability, considering regional geographic characteristics and varying research methodologies, to ensure the data is accurately interpreted and appropriately integrated into the study.
2.4.2 Field investigation and survey method
Field investigations before and after research involve interviewing stakeholders and managers to identify sources of acid rain-causing emissions from industrial, agricultural, transportation, and residential activities This process helps assess the progress in reducing acid rain, enabling detailed comparison, interpretation, and clarification of research findings.
2.4.3 Calculation method of acid rain characteristics and treatment data
Raw data on the concentration of ions and rainfall over the months in the 2008-2018 study period are processed and calculated using Excel software
The study utilized Excel software to analyze key rainwater indicators, including average concentration, neutralization factor, correlation coefficient, pH, and rainfall metrics Additionally, XLSTAT software was employed to implement the Seasonal Mann-Kendall model, evaluating trends over time This analysis enabled the researcher to assess the progression of acid rain during the study period, providing valuable insights into environmental changes.
2.4.4 Annual volume - weighted mean precipitation concentrations
Volume weighted concentration (VWM) is calculated by summing the total mass deposited in the sampler divided by the total volume collected during the same period
The volume-weighted mean (VWM) concentrations of major ions of the every precipitation event at the sampling sites were calculated using the formula [36]:
(P1 + P2 + … … + Pn) Where Xi is the concentration of the ions, Pi and n are the rain amount for each rainy event (in mm) and the total number of rainfall events, respectively
The monthly and annual VWM concentrations of ions were calculated according to the data for a given month or for the entire monitoring period
To assess the extent to which crustal species, specifically ammonia and sodium, neutralize precipitation, neutralization factors (NF) were calculated using an established formula An NF value greater than one indicates a higher neutralizing capacity, highlighting the effectiveness of these species in mitigating acidity in precipitation This approach provides valuable insights into the neutralization potential of crustal elements in atmospheric chemistry and environmental monitoring.
Where: [Xi] is the concentration of the alkaline component (Ca 2+ , NH4 + ,
Mg 2+ , Na + , K + ) expressed in àmol /L
The amount of wet deposition (Dw) is calculated using EANET's formula as follows:
Dw: Amount of wet deposition (àmol/m 2 /month)
: Average monthly ion concentrate (àmol/L)
Parameters considered and evaluated in this study include: nss-SO4 2-,
NO3 -, the parameters are thought to be related to emissions SO2 and NOx from socio-economic activities
2.4.7 Marine and crustal enrichment factors
Marine and crustal enrichment factors were calculated according to formulae (1) and (2), respectively, using Na + as reference element for EF seawater and Ca 2+ as reference for EF soil:
EFseawater = [X/Na + ] rainwater /[X/Na + ] seawater (1)
EFcrust = [X/Ca 2+ ] rainwater / [X/Ca 2+ ] crust (2)
In rainwater analysis, X represents the concentration of the element of interest The ratios X/Na+ and X/Ca2+ reflect the element's composition in rainwater, providing insights into its sources Comparing these ratios to those in seawater and crustal materials helps identify the origin of the element, with X/Na+ seawater and X/Ca2+ crust ratios serving as reference points This approach is essential for understanding the environmental and geochemical processes influencing rainwater composition [25].
2.4.8 Non-parametric test method Seasonal Mann-Kendall
The method of using Seasonal Mann-Kendall is a method developed by Mann and Kendall which is applied quite a lot in evaluating trends of seasonal data series [4]
The Seasonal Mann-Kendall non-parametric test is used to assess trends in time-ordered data series, focusing on the relative magnitude of data points rather than their absolute values This approach helps prevent false trends caused by local extreme values, unlike traditional linear trend methods based on least squares Additionally, it does not require the data to follow a specific distribution, making it versatile for various datasets The formulas employed in this method are summarized below [4].
Suppose there is a series of data in chronological order by month (Xi1, Xi2,
…, Xin) with xi representing the data at time i for years from 1,2, n
The Mann-Kendall statistical value at January (Si) is calculated as follows:
Sgn (xij – xik) = 1 if xij – xik > 0
Mann-Kendall statistical values of all months (S') are calculated as follows:
Where m is the number of months in a year, m = 12
√𝑉𝐴𝑅(𝑆 ′ ) 𝑖𝑓 𝑆 ′ < 0 For VAR (S') variance is the variance of S', calculated by:
VAR (Si) is the variance of Si, calculated by the following formula:
18[ni (ni-1)(2ni + 5) - ∑ 𝑔𝑖 𝑝=1 𝑡ip(tip - 1)(2tip + 5)]
Where gi is the number of groups in month i and tip is the number of elements in group p in month i
The Zsk (Z-score) follows a standard normal distribution, N(0,1), enabling easy interpretation of market trends A positive Zsk value indicates an upward trend, while a negative Zsk signifies a downward movement Since Zsk values are drawn from the N(0,1) distribution, traders can efficiently assess whether a chain is trending or consolidating, making Zsk a valuable tool for technical analysis and trend identification.
The Seasonal Mann-Kendall (SKM) test is a non-parametric method used to evaluate seasonal and yearly trends in ion deposition Developed to detect trends in climate-related concentrations of substances and variables, SKM effectively identifies significant changes over time, making it a valuable tool for analyzing seasonal and annual variations in ion deposition patterns.
SMK is typically applied to monthly and seasonal data series, demonstrating robustness by being insensitive to missing or erroneous data The trend slope of SMK is estimated using the Theil-Sen slope method, which is widely regarded as a reliable and monolithic approach for trend analysis Both Sen’s slope and SMK are extensively used in various studies to evaluate wet deposition processes, highlighting their effectiveness in environmental data analysis.
The degree of change in the wet deposition trend is determined by the Sen’s slope and the mean [5]
Kitayam (2012) estimated the annual change (%/year) in ion deposition calculated using the formula:
Change = (Magnitude of the trend slope) / (average deposition) * 100% Yearly changes in ion concentration and precipitation were similarly determined
The chemical components in rainwater are interconnected, with their relationships analyzed through correlation coefficients Specifically, the correlation coefficient (ρxy) is used to evaluate the strength and direction of the relationship between different ions, such as sulfate ions (SO4²⁻) in rainwater Understanding these correlations helps in assessing the interactions between various chemical constituents and their combined impact on rainwater quality.
, NO3 -, Cl - , NH4 +, Ca 2+ , Na + , Mg 2+ and K + ) Correlation coefficient (ρxy) is used to determine the relationship between the two data sets X and Y, (-1≤ ρxy ≤ +1)
The correlation between variables X and Y is positive when the correlation coefficient (ρxy) is greater than zero, indicating a direct relationship Conversely, a negative ρxy signifies an inverse relationship, meaning that as one variable increases, the other tends to decrease When ρxy is close to zero, it indicates no significant correlation between the variables The correlation coefficient is calculated using a specific formula that considers the data points for X and Y, their means, and the overall number of observations Understanding these relationships helps in analyzing the strength and direction of the connection between variables in statistical research.
In which àx, ày: mean value of set X, Y σx, σy: the average deviation of set X, Y is determined by the following formula:
X and Y are data sets for analysis of rainwater ions in Hanoi
2.4.10 Calculate [nss-SO 4 2- ] and [nss- Ca 2+ ]
[nss-SO4 2-] and [nss-Ca 2+ ] are concentration of SO4 2- and Ca 2 non-seasalt, are determined by the following formula:
Calcium (Ca²⁺) and sulfate (SO₄²⁻) ions in the atmosphere originate from both natural and artificial sources Accurate calculation methods are essential to assess the contribution of human-made agents to the presence of these ions, enabling the elimination of natural influences, particularly those from seawater, for more precise atmospheric analysis.
The pAi index is widely used worldwide to identify the primary ionic radicals responsible for rainwater acidification, specifically nss-SO4²⁻ and NO3⁻ ions While pH is commonly employed to assess rainwater acidity, it does not directly indicate the presence of acidifying materials like SO4²⁻ and NO3⁻; instead, pH reflects contributions from ions such as Ca²⁺ and NH4⁺ In this study, both pAi and pH are utilized to comprehensively analyze rainwater chemistry in Hanoi The pAi index is calculated using the formula: pAi = -log10 ([nss-SO4²⁻] + [NO3⁻]) in mol/L, which helps determine the total concentration of these non-sea salt ions in rainwater.
RESULT AND DISCUSSION
Annual volume - weighted mean precipitation concentrations
To ensure the reliability of analytical data and verify ion removal, it is essential to evaluate the balance between anions and cations Data are considered acceptable when the ratio of total anions to total cations in precipitation samples falls within 1± 0.25 In this study, the total anion to cation concentration ratio is 0.8, indicating that all major components in the rainwater sample were accurately measured, thereby confirming the data quality.
Figure 4 Chemical composition percentages of rainwater in Hanoi
The VWM concentrations of the main ionic species followed this order: NH4+ > SO4 2- > NO3- > Ca2+ > Cl- > Na+ > Mg2+ > K+ Sulfate (SO4 2-) was the most abundant anion, with a VWM concentration of 32.9 µmol L⁻¹, accounting for 46.01% of total anions, while nitrate (NO3-) was also significant, with a concentration of 26.9 µmol L⁻¹, representing 37.62% of total anions Among the cations, ammonium (NH4+) and calcium (Ca2+) were the predominant species, indicating their dominant roles in the ionic composition of the studied environment.
SO42- NO3- Cl- NH4+ Na+ K+ Ca2+ Mg2+ H+
The study found that NH4+ and Ca2+ concentrations in precipitation were 54.9 and 23 μmol L−1, respectively, accounting for a significant portion of total cations Meanwhile, the VWM concentrations of Mg2+ and K+ were relatively low, measuring 4.5 and 3.8 μmol L−1, respectively The VWM concentration of H+ ions was the lowest at 2.1 μmol L−1, constituting approximately 2.2% of the total ionic composition in the samples.
pH of rain water
Figure 5 Frequency distribution of the pH values of rainwater in Hanoi during the sampling period.
Natural rainwater is weakly acidic with a pH around 5.6 due to the atmospheric solubility of CO2 in rain droplets Monitoring data from 2008 to 2018 in Hanoi indicates that acid rain (pH < 5.6) occurred approximately 25.75% of the time, with 6.81% of rainwater exhibiting a pH below 5, and this percentage varied significantly over the years, ranging from 8.3% to 75% The pH of collected rainwater ranged from 5.3 to 6.53, with a volume-weighted mean (VWM) pH of 5.8 The distribution of rainwater pH levels over time is visualized in Figure 5.
This study compares the frequency of acid rain occurrences across various station types, including Hanoi's urban station, Ho Chi Minh City’s urban station, Hoa Binh’s rural station, and Yen Bai’s mountainous station The analysis highlights significant differences in acid rain levels, emphasizing how geographic and urbanization factors influence acid rain distribution across these regions.
Figure 6 Frequency distribution of the pH values of rainwater in Ho Chi
Ho Chi Minh Station experiences less acid rain compared to Hanoi Station, with a 6% lower frequency of acid rain (pH < 5.6) during 2014-2015 Both stations are urban environments; however, the data indicates that Ho Chi Minh's air quality in terms of acid rain occurrence is relatively better than Hanoi's over this period This suggests regional differences in air pollution levels, highlighting the need for targeted environmental management in these urban areas.
Figure 7 Frequency distribution of the pH values of rainwater in Hoa Binh
Between 2008 and 2018, the frequency of acid rain was higher in Hoa Binh (39.4%) compared to Hanoi (25.75%), indicating regional variations in acid rain occurrence From 2015 to 2018, Yen Bai (mountainous station) experienced acid rain at a rate 1.9 times greater than Hanoi, with 47.9% versus 25.75%, highlighting the influence of geographic location These findings demonstrate that the occurrence of acid rain depends not only on emission sources but also on atmospheric circulation patterns.
Figure 8 Frequency distribution of the pH values of rainwater in Yen Bai
Seasonal variation of the chemical composition
Table 1 Seasonal variations of the VWM concentrations of the major ions in the rainwater from Hanoi (àmol L -1 )
Ca 2+ Mg 2+ K + Na + Precipitation Dry season 5.15 50.9 88.6 101.7 165 64.1 12.8 12.2 38.1 534 Rain season 6 23.4 25.7 32 54.6 27.8 6.6 7 15.1 1223
The rainfall mainly occurs during the rainy season (from April to
The article discusses how the monsoon climate significantly influences the seasonal variations in water variables, with key ion concentrations exhibiting notable differences between seasons Specifically, the volumetric water mass (VWM) concentrations of primary ions are detailed in Table 1, highlighting distinct seasonal patterns During the dry season, these ion concentrations demonstrate significant shifts compared to the rainy season, emphasizing the impact of seasonal climatic changes on water chemistry.
(DS), all of the concentrations of main ions were higher than in the rainy season
The higher concentration of precipitation ions during the dry season is likely due to the cumulative effect of particulate matter and the reduced dilution effect from limited rainfall Additionally, rainwater in the dry season tends to be more acidified, with a VWM pH value of 5.15 compared to 6 in the rainy season, indicating increased acidity during drier periods.
Figure 9 VWM of major ions in rain season and dry season in Hanoi during 2008-2018 (àmol L -1 )
180 pH NO3- SO42- NH4+ Ca2+
Assessment on wet deposition trend
Analysis of EANET data from Hanoi station reveals seasonal trends in wet deposition, with the lowest H+ ion deposition recorded at 312.986 àmol/m²/month The highest H+ deposition, reaching 3496.912 àmol/m²/month, occurs in August, indicating increased acidity during this period Additionally, nitrate (NO3-) deposition peaks in spring and summer, with a maximum of 17,221.995 mol/m²/month, highlighting seasonal variations in pollutant deposition at Hanoi station.
Deposition of nss-SO4²⁻ and nss-Ca²⁺ varies significantly throughout the year, with maximum values reaching 17,637.445 µmol/m²/month and minimum values down to 138.689 µmol/m²/month Ammonium (NH4⁺) exhibits the highest average sedimentation rate at 7,896.350 µmol/m²/month, with peak deposition amounts up to 38,070 µmol/m²/month, highlighting its prominent role in seasonal sedimentation patterns.
Table 2 The average value of ion deposition, the level of change in deposition, the Sen’s slope, significance level p at Hanoi station during the period from 2000-2018
Variable Mean Kendall's p-value Sen's slope
Table 2 presents the average monthly deposition values of ions, their deposition changes, Sen's slope, and significance levels Ions with p-values greater than 0.05 were deemed statistically insignificant and excluded from further analysis, while those with p < 0.05 were considered significant and used to estimate sedimentation trends These findings help identify ions that significantly influence sedimentation processes, supporting more accurate environmental assessments.
Table 3 presents the annual deposition changes, highlighting that Hanoi experiences the fastest increasing trend in NO₃⁻ deposition at 0.0343% per year, which is statistically significant (p < 0.001) This rise is primarily driven by increased emissions and the contribution of air pollution from neighboring regions, as indicated by emissions data from EDGAR.
[4] from 2000 to 2012 NOx emissions increased from 439.58*10^6- 945.5*10^6
Air pollution in Northern Vietnam has seen a significant increase, with pollutant levels rising by an average of 6.66% annually, reaching 31 kg Duong Hong Son (2013) identified that air pollution from the Northeast region, primarily NO2 originating from neighboring countries, impacts northern Vietnam especially in January, February, June, and August, accounting for 22.31%, 15.66%, 10.78%, and 11.135%, respectively, which influences the NO3 deposition trend in Hanoi The deposition levels of nss-SO4²⁻ and SO4²⁻ were measured at 0.164%, with SO2 emissions in the region increasing markedly from 2000 to 2012, from 324.15 million to 74.555 billion kg, reflecting a 55% rise attributable to northeastern air pollution Additionally, Dao Duy An (2016) reported that the dry deposition rate of SO4²⁻ in northern Vietnam ranges from 12.63% to 28.61%, emphasizing the regional contribution of neighboring countries to sulfur compound deposition.
The deposition of NH4+ ions is increasing at a rate of 0.196% per year, which is statistically significant (p = 0) Ammonia (NH3), a prevalent atmospheric gas, originates from various sources including industry, transportation, soil, and ocean activities In the atmosphere, NH4+ is formed through reactions between NH3 and acids present in rainwater, contributing to environmental and air quality concerns.
The deposits of nss-Ca²⁺ and Ca²⁺ showed a significant average annual increase of 0.205% and 0.206%, respectively (p < 0.0001) Additionally, the deposition of K⁺, Na⁺, and Mg²⁺ also demonstrated upward trends, with annual increase rates of 0.159% (p = 0.002), 0.056%, and 0.026%, respectively These findings indicate a consistent rise in mineral deposit accumulation over time.
Ion H+ concentration is determined through pH measurements and serves as an indicator of acidity levels, with an average deposition value of approximately 312.99 units Over time, the total H+ ion deposition at Hanoi station shows a significant decreasing trend of approximately 0.085% annually Similarly, chloride ions (Cl-) also exhibit a decline in deposition, decreasing by about 0.025% per year, highlighting changes in atmospheric ion deposition patterns in the region.
Jul-98 Jan-04 Jul-09 Dec-14 Jun-20 à m o l/ m 2
Jul-98 Jan-04 Jul-09 Dec-14 Jun-20 à m o l/m 2
Jul-98 Jan-04 Jul-09 Dec-14 Jun-20 à m o l/m 2
Jul-98 Jan-04 Jul-09 Dec-14 Jun-20 à mo l/m 2
Jul-98 Jan-04 Jul-09 Dec-14 Jun-20 à m o l/m 2
Jul-98 Jan-04 Jul-09 Dec-14 Jun-20 à m o l/m 2
Jul-98 Jan-04 Jul-09 Dec-14 Jun-20 à m o l/m 2
Jul-98 Jan-04 Jul-09 Dec-14 Jun-20 à m o l/m 2
Figure 10 The trend of wet deposition of ions over the months in Hanoi between 2000-2018
Jul-98 Jan-04 Jul-09 Dec-14 Jun-20 à m o l/m 2
Jul-98 Jan-04 Jul-09 Dec-14 Jun-20 à m o l/m 2
Jul-98 Apr-01 Jan-04 Oct-06 Jul-09 Apr-12 Dec-14 Sep-17 Jun-20 à m o l/ m 2
Table 3 Level of change in concentration and precipitation by year (%) in Hanoi from 2000-2018
This study investigates the factors influencing ion deposition tendencies by examining ionic concentration and precipitation trends (see Table 3) Notably, the H+ ion concentration shows an annual decrease of 0.155%, which is statistically significant (p = 0.002), indicating important changes in acidity levels affecting deposition processes.
The decreasing trend of H+ ion deposition is primarily due to the reduction in H+ ion concentration in rainwater As the concentration of H+ ions declines, the region experiences a yearly decrease in acid rain This trend indicates improvements in environmental conditions and a reduction in acid rain intensity over time.
NO3- ions significantly increase both their concentration and deposition rates, by approximately 0.346% and 0.343% per year respectively (p < 0.0001) Additionally, ions such as nss-SO4 2-, SO4 2-, Na+, and Mg 2+ showed an upward trend in concentration—at rates of 0.097%, 0.095%, 0.045%, and 0.034% per year—but these changes were not statistically significant (p > 0.05).
Ions NH4 +, Ca 2+ , nss-Ca 2+ and K + tend to increase with values of 0.172%/year (p = 0.001), 0.189%/year (p = 0), 0.192%/year (p = 0) and 0.159
% /year (p = 0.002), respectively In contrast, the Cl- ion concentration tends to decrease by -0.025%/year.
Correlation coefficient
A correlation matrix for the main ions at Hanoi station was calculated using observed EANET data from 2000 to 2018 The results indicate significant relationships between key ions in rainwater, with higher correlation coefficients suggesting that these ions originate from similar rainfall sources and emission processes This analysis helps to understand the interactions between atmospheric ions and their contribution to urban rainfall chemistry.
Global studies have utilized the correlation between main ions in rainwater to assess pollution sources By combining correlation coefficients with meteorological data, researchers can trace the origins of pollution from different areas, providing valuable insights into environmental pollution patterns and their sources.
Table 4 Pearson correlations matrix for the ion concentrations in rainwater of Hanoi
Variables SO 4 2- NO 3 - Cl - NH 4+ Na + K + Ca 2+ Mg 2+
The Pearson correlation analysis of ions in rainwater in Hanoi reveals significant positive correlations between SO4²⁻ and NO3⁻, indicating similar chemical behavior and common precursors like SO2 and NOx Calcium (Ca²⁺) and magnesium (Mg²⁺) are positively correlated, reflecting their shared origin from soil and dust Additionally, strong positive relationships between Ca²⁺, Mg²⁺, K⁺, and SO4²⁻ suggest the formation of mineral acids such as HCl, HNO3, and H2SO4 through atmospheric chemical reactions with carbonate-rich alkaline compounds transported from soil by wind Ammonium (NH4⁺) also shows significant positive correlations with SO4²⁻ and NO3⁻, highlighting its role in atmospheric ion interactions in Hanoi's rainwater.
Ammonia generally occurs in the atmosphere as (NH4)2SO4, NH4HSO4,
Ammonium nitrate (NH4NO3) aerosols form through reactions with sulfuric acid (H2SO4) and nitric acid (HNO3), with high correlations observed between NH4+ and SO4 2- (R = 0.867), as well as NH4+ and NO3- (R = 0.896), indicating the formation of (NH4)2SO4 and NH4NO3 from these reactions on moist soil surfaces These compounds are partly formed due to the reaction of gases with ammonia (NH3) present on the surface of moist soil, especially when relative humidity exceeds 62%, leading to the formation of undissociated NH4NO3 and (NH4)2SO4 The strong correlations suggest that both natural processes and anthropogenic activities contribute to the presence of these aerosols Additionally, SO4 2- shows significant positive correlations with calcium (Ca2+), magnesium (Mg2+), and sea salt ions like Na+ and Cl-, indicating that a portion of sulfate exists as salts associated with soil and sea-originated constituents. -**Sponsor**As a content creator, ensuring your articles are SEO-friendly and coherent is key Need help refining your paragraphs? [Article Generation](https://pollinations.ai/redirect-nexad/noMGxEXW?user_id=983577) can instantly provide you with 2,000-word SEO-optimized articles, saving you time and money Instead of manually rewriting, imagine generating high-quality content effortlessly, like having a dedicated content team without the cost Focus on the core message while this tool handles the SEO optimization!
A significant positive correlation was observed between Na+ and Cl- (R=0.852), indicating their marine origin, while Na+ likely derives from fly ash resulting from coal burning in thermal power plants near Hanoi, which is not close to the sea Additionally, the strong correlation between K+ and Mg2+ suggests that K+ may originate from local agricultural soils, influenced by nitrogenous and potash fertilizers, as well as biomass burning in farmland.
Acid neutralization
Rainwater acidity is primarily influenced by the concentration of acidic ions such as NO3− and SO42−; however, in Hanoi, despite having the highest levels of these ions, the rainwater exhibits a relatively high pH compared to other regions in Vietnam, indicating that acidic ions are not the main factor affecting its acidity Previous research suggests that the presence of basic ions like Ca2+ and NH4+ also plays a significant role in neutralizing acidic components and impacting rainwater pH The ratio of neutralizing potential to acidifying potential (NP/AP), as proposed by Fujita et al (2000), serves as an effective indicator of the rainwater's neutralization capacity and the balance between acidic and basic components.
NP/AP = [nss−Ca 2+ + NH4 +] / [nss−SO4 2− + NO3 -]
The neutralizing potential (NP) is determined by the sum of non-sea salt (nss) calcium and ammonium ions ([nss-Ca²⁺ + NH₄⁺]), while the acidifying potential (AP) is represented by the sum of nss sulfate and nitrate ions ([nss-SO₄²⁻ + NO₃⁻]) Non-sea salt (NSS) concentrations of various ionic species are calculated from measured element concentrations, using sodium as the reference element and assuming that all sodium originates from marine sources.
[18] Non-sea-salt fraction ion concentrations were given by:
NSSX = [X]rainwater − [Na + ] rainwater × ([X] / [Na + ]) seawater,
Which X denotes the measured ions The calculated results of nss-Ca 2+ , nss-SO4 2−, NP, AP and NP/AP of rainwaters in Hanoi and other areas in Vietnam are presented in Table 5 The equivalent ratio of NP/AP in Hanoi, Hoa Binh, Ho Chi Minh, Yen Bai were 1.27, 1.18, 1.23, 1.2, respectively, indicating that alkaline constituents neutralize the acidity [20, 27]
Table 5 The value of neutralization factor (NF) of rainwater from Hanoi and other areas in Vietnam
Yen Bai (2015-2018) nss-Ca 2+ 22.8 14.39 17.52 19.91 nss-SO 4 2- 32.4 20.09 22.2 29.1
NP/AP 1.27 1.18 1.23 1.2 pH 5.8 5.5 5.98 5.23 pAi 4.14 4.42 4.1 4.24 ΔpH 1.66 1.08 1.88 0.99
To determine the degree of neutralization of the acidity, ΔpH is computed as: ΔpH = pH−pAi
The estimated pH, referred to as pAi, corresponds to the measured pH in the absence of neutralization It can be calculated using the formula pAi = − log [nss−SO4 2− + NO3 −], providing an essential measure of soil acidity based on sulfate and nitrate ion concentrations This calculation helps in accurately assessing soil chemical properties for environmental and agricultural applications.
The difference (ΔpH) between the measured pH and the estimated pH (pAi) serves as an effective indicator of acid neutralization capacity in rainwater Calculated values of pAi and ΔpH revealed that rainwater from Hanoi, Hoa Binh, and Ho Chi Minh City have ΔpH values of 1.66, 1.08, and 1.88, respectively, indicating varying neutralization abilities Conversely, Yen Bai's rainwater showed the lowest ΔpH of 0.99, reflecting a weaker neutralizing capacity of its basic cations To assess the neutralization efficiency of these cations, the neutralization factor (NF) was computed using a specific equation, providing further insight into the rainwater's acid buffering potential.
NFXi = [Xi] / ([nss−SO4 2− + NO3 −]
This article discusses the chemical composition of rainwater in Hanoi and other regions of Vietnam, focusing on key ions such as Ca²⁺, NH₄⁺, Mg²⁺, and K⁺, all expressed in μmol L⁻¹ The NF (neutralization factor) values for these ions are summarized in Table 7, providing insights into the acidity neutralization characteristics of rainwater across different areas in Vietnam.
NF values of Ca 2+ , NH4 +, Mg 2+ and K + in the rainwater of the study area are only
The low NF values of Ca²⁺ and NH₄⁺ in rainwater across all Vietnamese cities suggest weak neutralization due to a deficiency of alkaline constituents Analysis of NP/AP, ΔpH, and NF values indicates that limited neutralization capacity is a primary factor contributing to severe acid rain in these areas Despite being rural and mountainous with agricultural economies, Hoa Binh and Yen Bai experience frequent acid rain episodes, highlighting that acid rain occurrence is influenced not only by local emissions but also by atmospheric circulation patterns.
Table 6 The value of neutralization factor (NF) of rainwater from Hanoi and other areas in Vietnam
Area NF(Ca 2+ ) NF (NH 4 + ) NF (Mg 2+ ) NF(K + ) Hanoi
Wet deposition rates of major ions
Figure 11 The average multi annual wet flux depositions (mmol m -2 y -1 ) of major ions at the studied sampling site
Wet deposition rates are primarily influenced by the total annual rainfall, as they are directly correlated with both the amount of precipitation and its chemical composition The chemical content measured in precipitation plays a significant role in determining wet deposition levels, highlighting the strong link between rainfall amount and chemical deposition Therefore, understanding annual rainfall patterns is essential for predicting and managing wet deposition of pollutants.
In terms of cation wet deposition, NH4+ was the highest at 91.3 mmol m⁻² y⁻¹, while K+ recorded the lowest value at 6.8 mmol m⁻² y⁻¹, followed by Mg²+ with 8.1 mmol m⁻² y⁻¹ Among anions, sulfate (SO4²–) showed the largest wet deposition at 56.2 mmol m⁻² y⁻¹, whereas chloride (Cl–) had the smallest value of 20.5 mmol m⁻² y⁻¹.
Enrichment factors
Enrichment factors (EFs) are essential tools for identifying the sources of ions in rainwater, helping to distinguish between marine and terrestrial contributions Sodium (Na) is widely used as the reference element for seawater due to its predominantly marine origin In contrast, elements like aluminum (Al) and calcium (Ca) serve as typical lithophilic reference elements for the continental crust By analyzing EF values using these reference elements, researchers can accurately estimate the relative contributions of marine and terrestrial sources to rainwater composition.
EF values for rainwater compositions were calculated using Ca 2+ as a reference element for crustal source and Na + for marine source [20, 24]
SO42- NO3- Cl- NH4+ Na+ K+ Ca2+ Mg2+ m m ol m -2y -1
An enrichment factor (EF) helps determine whether an element is enriched or diluted relative to a reference source, with EF values significantly higher or lower than 1 indicating enrichment or dilution, respectively An EF close to 1 suggests the element primarily originates from the local source, such as seawater or soil, while EF values greater than 1 imply the influence of additional sources Understanding EF values is essential for identifying pollutant sources and assessing environmental contamination levels.
Table 8 gives the ratios of Cl - , SO4 2+, K + , Ca 2+ and Mg 2+ with respect to
The article discusses the concentration ratios of chloride (Cl^-), sulfate (SO4^2-), nitrate (NO3^-), potassium (K+), sodium (Na+), and magnesium (Mg2+) relative to calcium (Ca2+) in rainwater It highlights the enrichment factors (EF) of these ions, emphasizing their sources and significance The findings indicate that all sulfate (SO4^2-) ratios are analyzed to understand their role in atmospheric composition and potential environmental impacts, with particular attention to crustal and seawater contributions to rainwater chemistry.
Elevated levels of K+ and Mg2+ relative to Na+ in Hanoi's rainwater exceed recommended seawater ratios, indicating significant contributions from anthropogenic and crustal sources Additionally, the ratios of Cl− also suggest the influence of human activities and natural processes impacting the water composition, highlighting the importance of monitoring these ions for environmental health.
The elevated levels of SO₄²⁺ and NO₃⁻ relative to Ca²⁺ in Hanoi's rainwater suggest sources beyond natural crustal inputs Evidence from enrichment factors (EFseawater and EFcrust) supports this, with Cl⁻ showing an EFmarine value of 1.3 and an EFsoil value of 170, indicating significant enrichment from marine sources alongside non-crustal influences Meanwhile, Mg²⁺ and K⁺ typically originate mainly from crustal sources but also show some marine contribution, consistent with findings from other study sites.
The average EF marine value of Ca²⁺ was 66.12, indicating its primarily terrestrial origin Conversely, SO₄²⁻ exhibited an EFmarine value of 28.22 and an EFsoil value of 75.26, reflecting significant enrichment relative to marine and soil sources, with high EF values suggesting that sulfate mainly derives from anthropogenic activities Additionally, NO₃⁻ showed a high EFvalue for soil (EFsoil×5) and was scarcely present from marine sources, implying that human activities are the primary contributors to nitrate pollution.
Table 7 Enrichment factors relative to seawater and soil for rainwater constituents in Hanoi in 2008-2018
K + Na + Mg 2+ SO 4 2- Cl - NO 3 -
Source assessment for different ionic component
The ions in precipitation primarily originated from anthropogenic sources, sea spray, and terrestrial dust resulting from wind erosion, with volcanic and other natural sources contributing negligibly The chemical composition of precipitation reflects the relative contributions of these sources, providing insights into environmental impacts To quantify these contributions, specific equations were used to calculate the ionic components originating from anthropogenic, marine, and crustal sources in rainwater.
%SSF = 100(X/Na + )marine / (X/Na + )rainwater
%CF = 100(X/Ca 2+ ) soil / (X/Ca 2+ )rainwater
Sea spray fraction (SSF), crust fraction (CF), and anthropogenic fraction (AF) represent key sources of ionic species in rainwater According to Table 8, the contributions of these sources vary, with most sulfate (SO4²−), calcium (Ca²+), and potassium (K+) ions primarily originating from non-marine sources This indicates that anthropogenic activities and crustal materials significantly influence the ionic composition of rainwater, highlighting the importance of understanding diverse pollutant sources in environmental studies.
Ca²⁺ and K⁺ predominantly originate from crustal sources, with K⁺ also serving as a chemical marker for biomass burning K⁺ in coarse soil particles reflects terrestrial soil contributions, while fine particles of K⁺ indicate wood combustion emissions The primary source of K⁺ is terrestrial, although distinguishing between soil and wood combustion sources in precipitation remains challenging Cl⁻ mainly derives from marine sources, contributing approximately 89.23%, with minor input from crustal sources.
Anthropogenic sources contribute significantly to the presence of ions in rainwater, accounting for approximately 10.18% of Mg²⁺ and 95.25% of SO₄²⁻, with the majority originating from human activities such as coal combustion and vehicle traffic in Hanoi Mg²⁺ primarily stems from crustal sources, while SO₄²⁻ has a minimal crustal contribution, highlighting the dominant role of anthropogenic emissions It was estimated that about 99.83% of NO₃⁻ in rainwater is derived from human activities, mainly linked to fossil fuel combustion The high emission of SO₂ and NOₓ gases from coal burning and vehicular exhaust results in elevated levels of acidic ions like sulfate (SO₄²⁻) and nitrate (NO₃⁻), contributing to acidification of atmospheric precipitation.
Table 8 Source contributions for different ionic constituents in rainfall measured in Hanoi in the period 2008 -2018
Ions Sea salt fraction (%) Crust fraction (%) Anthropogenic source fraction (%)
Based on the rainwater chemical monitoring data source of the East Asia Acid Deposition Monitoring Network (EANET) at Hanoi station in the period
2008 - 2018, the study has evaluated progress of acid rain in Hanoi city Some main conclusions are drawn as follows:
This study confirms that acid rain (pH < 5.6) continues to occur in Hanoi city, with occurrences ranging from 25% in 2018 to a peak of 75% in 2011, showing irregular fluctuations over the years The primary ionic components found in the rainwater include SO4²⁻, HCO3⁻, Cl⁻, NO3⁻, Ca²⁺, NH4⁺, Na⁺, and K⁺, highlighting the complex chemical composition of acid rain in the region.
During the study period, the concentrations of Mg²⁺, SO₄²⁻, nss-SO₄²⁻, NO₃⁻, NH₄⁺, Ca²⁺, and nss-Ca²⁺ were found to be lower during the rainy season compared to the dry season Additionally, the occurrence and rate of acid rain were significantly higher in the dry season than in the rainy season, indicating seasonal variations in acid deposition levels These findings highlight the impact of seasonal changes on ionic concentrations and acid rain intensity in the study area.
Analysis of ion deposition over time at Hanoi station reveals that H+ and Cl- deposits tend to decrease, with values of -0.085 and -0.025 respectively In contrast, the deposition levels of most other ions show a significant increasing trend (p < 0.005) This indicates that, within the EANET network, ion deposition at Hanoi station is primarily influenced by changes in ion concentrations in rainwater, highlighting the importance of rainwater chemistry in understanding deposition dynamics.
Spearman’s correlation analysis revealed that the chemical characteristics of precipitation are predominantly influenced by sources such as industrial and mining activities, smelting operations, coal combustion, biomass burning, and agricultural activities Additionally, contributions from terrestrial and marine sources are also evident, while sea-salt and non-sea salt contributions, as well as marine and crustal enrichment factors, further highlight the complex interplay of natural and anthropogenic sources shaping precipitation chemistry.
In the study area, the primary ions influencing rainwater pH are nssSO4²-, NO3-, nss-Ca²+, and NH4+ Sulfate ions (nssSO4²-) are the main contributors to acidification, lowering the rainwater pH, whereas calcium ions (Ca²+) play a key role in neutralizing acidity during the rainy season During the dry season, the dynamics of these ions and their impact on rainwater pH may vary, affecting overall water quality.
46 depending on each year, Ca 2 + or NH4 + ions play a key role in neutralizing acidity in rainwater
At Hanoi station, the pH value consistently exceeds the pAi value, indicating that pH is influenced not only by sulfate (SO4 2-) and nitrate (NO3-) ions but also by other ions like calcium (Ca 2+) This relationship highlights the significant impact of various ions on the pH level, demonstrating the complexity of ionic interactions affecting water quality at this location.
Sulphate ions (SO4 2–) exhibited the highest wet deposition rates among all ions, likely due to their atmospheric size As secondary particles formed through gas-to-particle conversion, SO4 2– contributes to ultrafine particle formation, which can further grow via coagulation processes This indicates that sulphate plays a significant role in atmospheric deposition and particle dynamics.
Studies are needed to identify the source of air pollution and the directions of transmission in order to be able to handle it thoroughly
Further studies are needed on the harmful effects of acid rain on the environment and human health
Strengthen air pollution control and pollutant emissions to minimize air pollution and acid rain
Communication and education units enhance people's knowledge about the harmful effects of environmental pollution, to appeal to the entire population to join hands to protect the environment
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