1. Introduction
Industrialization, traffic development, and population concentration in urban areas have caused severe air pollution problems and affected the health and environmental quality of residents who live in cities and nearby areas. Air pollution, atmospheric aerosols, and precipitation acidification are common phenomena that are interrelated [
1]. Air pollutants are accumulated or removed through atmospheric dry and wet deposition. Wet deposition is the primary mechanism for scavenging gas and particle phase pollutants in the atmosphere [
2]. This process is crucial for the nutrient cycle and for scavenging pollutants from the surrounding air. Other than air pollution sources from human activities, the environmental background of a city (such as its location in a basin or near the coast) can affect the distribution of air pollutants and their behavior in the environment. Kulshrestha et al. (2003) [
3] indicated that the chemical properties and characteristics of precipitation differ across regions or sites on the basis of particulates being emitted into the ecological system by humans or natural phenomena. Generally, exploring precipitation sources and transmission is primarily based on the concentration of major dissolved ions in the precipitation, the ion concentration from natural and anthropogenic sources, and the stoichiometry between ions [
4].
The chemical component of precipitation can reveal important information on the level of pollution and help assess acidifying, neutralizing, and eutrophying species in the atmosphere and the deposition and transportation of heavy metal and aerosols [
5]. Urbanization and human activities have a major impact on the chemical component of precipitation. Because the emission sources of these pollutants are complex (they undergo many physical and chemical mechanisms before being deposited as rain), research on the chemical components of precipitation can provide valuable air quality information and assist with differentiating pollution sources [
6]. In the past 30 years, countries across the globe have conducted studies on precipitation chemistry to determine the source of precipitation according to its chemical components, thereby gaining a further insight into the partial and regional distribution of pollutants and the potential impact of scavenging and deposition in the environment [
7].
Acid rain has been among the greatest environmental problems in the last few decades of the 20th century, particularly in the 1980s, when most people believed that acid rain was one of the greatest environmental threats of the time. The danger from acid rain is slow, and only when Europe and North America discovered the precipitation acidification phenomenon did scientists pay attention to this problem [
8]. Acid rain is mainly produced from the emission of a large quantity of sulfur dioxide (SO
2) and reactive nitrogen oxide (NO
x) in human activities, which causes the gradual acidification of the atmospheric and ecological environments. Chen and Chen (2008) [
9] noted that in the 1990s, the economy and population in East Asia grew rapidly. The region became highly urbanized and induced changes in farming patterns. Industrial development and petroleum fuel use increased, and the air pollution problem in the region became more severe. The main species that caused acid rain in the two megacities in China (Shanghai and Shenzhen) were sulfates and nitrates [
6,
10]. The study results revealed that rapid economic development caused the increased emission of acidic substances, which was the main reason for the acidification of precipitation.
Smith (1872) [
11] analyzed the chemical component of precipitation in the industrial city of Manchester and its surrounding areas and discovered that the city’s precipitation contained sulfuric acid (H
2SO
4) or acidic sulfuric acid salts (SO
42−), which made the precipitation acidic. Precipitation in the suburban areas contained ammonium sulfate ((NH
4)
2SO
4) and was slightly acidic. However, precipitation from rural farm villages contained ammonium carbonate ((NH
4)
2CO
3) and was only mildly acidic [
12]. Generally, spatial differences directly affect the chemical properties of wet deposition. Continental wet deposition has a higher concentration of man-made ions than neutral marine rain, particularly when it comes to acidic material in urban and rural atmospheric precipitation. Thus, acid rain has become a global environmental problem [
1]. Changes in the ion concentration in wet deposition are notably related to the source of the air mass, particularly when the air mass source contains other non-sea-salt sources (such as sand dust, biomass burning, and fossil fuel combustion). Some ion concentrations increase because of the special characteristics of the source. According to such characteristics, the main ion concentration can be used to determine the main sources of the aerosol [
4].
Taiwan is located at the junction of the Eurasian continent and the Pacific Ocean, and its climate is affected by the interaction of continental and marine climate patterns. In the winter, continental cold high pressure comes from Siberia as northeast monsoon winds. In the summer, the Pacific marine high pressure comes as southwest monsoon winds. The main rainy season runs from May to June, and the typhoon season runs from July to September. Taiwan’s geographical location is downwind of continental pollutant emissions, which has a significant impact on the wet deposition composition. In northern Taiwan, the historic wet deposition pH is lower than 5.0, particularly during the northeast monsoon wind season. The higher concentration of acid pollutants from the continent renders acid rain more severe during the winter than during the summer. Therefore, acid rain is influenced by climate patterns and exhibits seasonal characteristics. Chen and Chen (2008) [
9] noted that the region’s atmospheric deposition is mainly affected by sea salt, biomass burning, and fossil fuel combustion.
In this study, data were acquired from precipitation samples collected by two wet deposition sampling stations in the southern part of Northeast Asia (Wanli and Banqiao in northern Taiwan) for 12 months. The chemical components of the samples were analyzed, and the results were used to understand the chemical properties of atmospheric pollutants in wet deposition and the reason for their formation. To achieve the research objective, several research methods were adopted for comprehensive analysis, namely pH measurement, ion composition analysis, major ion reference indexes, and correlation coefficient and principle component analysis (PCA). In addition, the wet deposition survey results for the East Asia regions in the Acid Deposition Monitoring Network in East Asia (EANET) (2015) [
13] report were referenced to verify the effects of possible air pollution sources on urban and coastal city environments as well as the differences of effects.
4. Conclusions
This study used the precipitation observation data collected in the Wanli and Banqiao over 12 months and their precipitation chemical properties to determine contribution sources of the atmospheric pollutants. The result against a background of Wanli facilitates a further insight into the impact of local and regional atmospheric pollutant distributions on urban and coastal area environments. The mean pH of wet disposition of this study was far lower than that of 10 East Asian regions (5.31). Wanli had a pH value of 4.63, whereas Banqiao had a pH value of 4.58. Acid rain frequency was 71.7% for Wanli and 85.1% for Banqiao, which signified that problems concerning acidic wet deposition in northern Taiwan remained severe. The total concentration of SO42− and NO3− in the study area is higher than the average of 10 East Asian regions, which may be the main reason. Sulfates and nitrates were the main substances that caused acid rain. These substances came from industrial activity and transportation and were mainly caused by anthropogenic activities.
The analysis results for ion composition and EFs suggested that Cl−, Na+, and Mg2+ are the main substances in Wanli’s precipitation, and they accounted for 69.2% of the total ions. The Na+/Cl− ratio of 0.84 and EFm of 1.02 indicated that marine droplets were the main contributors of atmospheric substances, having an average SSF value of Cl− and Na+ over 85%. For Banqiao’s precipitation, Cl−, Na+, and Mg2+ only accounted for 31.2% of the total ions because the amount of sea-salt ions decreased as the distance from the coast increased. Furthermore, SO42− and NO3− only accounted for 15.8% of the total ions in Wanli’s precipitation but accounted for 32.4% in Banqiao’s precipitation. Because SO42− and NO3− were produced by anthropogenic emissions, they were mainly related to industrial combustion and vehicle emissions. The high percentage of nss-SO42− and NO3− in Banqiao (average over 90%) also showed the impact of rapid industrialization.
The FA, NF, and the acid/alkali balance ratio (NP/AP) revealed contribution of acidic substances and neutralizing compounds to the acidity of precipitation. The results indicated that the lack of neutralizing compounds in the study region’s precipitation caused acidification because this reduced the neutralizing effect on acidic precipitation. For wet deposition in northern Taiwan, the ionizing acidity produced from nss-SO42− and NO3− exceeded the neutralizing reaction of neutralizing cations (such as ammonia ions). Furthermore, sulfuric acid had a greater impact on precipitation acidity than did nitric acid, which showed that the concentration and emission quantity of acid-causing substances and neutralizing compounds in the atmosphere, neutralization capability, were the main factors in the formation of acid rain in the study region.
The EFs, correlation, and PCA indicated that because of Wanli’s geographical proximity to the coast and seasonal climate factors, its main source of pollution came from marine droplets. The emissions also contained other anthropogenic emission sources (such as from transportation tools). Banqiao is urbanized and has a high population density and high traffic volume, which resulted in industrial combustion and vehicle emission being the main emission sources. Banqiao’s atmosphere was affected by local anthropogenic activities, and this effect was compounded by marine sources. This study discovered that precipitation chemistry was related to pollutant sources and transmission. The local and regional distributions of atmospheric pollutants were verified to exert different effects on coastal and urban cities. The precipitation chemical properties exhibited different characteristics in different regions and locations.