As rapid industrialization has substantially increased environmental pollution and public concern, much attention has been given to the evaluation of the pollution level which caused by atmospheric deposition and acid rain [1,2].

Rainwater, although purifies pollutants in the air, it is becoming a sources of pollution, for example, from wildfire, geological activity, dust storm [3] and human and industrial activities [1,4] to the environment. The effect of acid rain depends upon the direction and speed of wind, therefore, the widespread damage to the neighboring countries is both directly and indirectly massive [5]. Crop, soil and surface water are easily exposed to chemical wet (rain, snow, fog, etc.) and dry precipitates (gas, fine particles, etc.). The chemical and physical damages by rainwater to forest and agricultural products [6], soils and microorganisms [7], streams and lakes [8], buildingv [9,10] and humans [11] were monitored by previous studies.

In the past, only few studies concerning chemical investigation of rainwater and assessment of its emission sources (natural vs. anthropogenic) to reduce the consequences of pollution on ecosystems were investigated [12-15]. They defined the emission sources, dust from sea salt and soil dust (i.e., Ca²⁺, Mg²⁺, /) as natural sources and intensive coal burning, gases and particles emitted from industries and vehicles (i.e.,) as anthropogenic sources. Their findings of rainwater chemistry correlated with spatial-temporal and meteorological conditions indicated that the impact of pollution transport from distant emission sources may play an important role at the study site, as local pollution alone cannot account for the high levels of ion concentrations in the analyzed rainwater [12].

South Korea is located in East Asia, on the southern half of Korean Peninsula jutting out from the far east of the Asian land mass. There were no noticeable damages by atmospheric acid rain until now. In addition, the previous studies simply characterized the rainwater (and acid rain) properties [16-20]. Different from Western Europe, South Korea has the unique characteristics including the considerable change of monthly precipitation and the neutralization of acid rain by alkali components in the air. However, public awareness of environmental pollution and geological circumstance of South Korea that locates on the east side of China where sulfurous acid gas emission was originated by rapid industrialization has increased and demanded the continuous monitoring on physiochemical properties of yellow dust and acid rain.

In this study, rainwater samples were collected from April to October (farming period) and November to March (non-farming period), 2009-2011 in Suwon region, Gyeonggi province, South Korea. The rainwater chemistry including the contents of ionic species and their temporal variation was used for factor analysis, which was then used to presume the natural and anthropogenic sources depending upon the farming and non-farming periods.

The average monthly rainfalls during farming and non-farming periods were 1050 mm and 286 mm, respectively. Seventy-eight percentage of the total rainfall was concentrated during farming period and the highest amount of rainfall was observed in August in 2010 (Figure 2).

The chemical properties of rainwater are essential to determine the causative agent of acidity and neutralization. Table 1 lists the component ratio of each ion. The cations observed during farming and non-farming periods were > Na⁺ > Ca²⁺ > K⁺ > H⁺ > Mg²⁺ and Na⁺ > > Ca²⁺ > H⁺ = Mg²⁺ > K⁺, respectively. The anions during farming and non-farming periods were > >Cl⁻ and > > Cl⁻, respecttively. During both farming and non-farming periods, Na⁺ and NH₄⁺ were occupied by 60% among other cations, and and were occupied by more than 80% among other anions. This is consistent to previous studies [17,19,20]. The high values of and reflect a more important influence from local pollution sources such car and plant and may be a good indicator of common agricultural practices. The elements of natural sources (e.g., Ca²⁺, Mg²⁺, /) have lower concentrations during farming period, which was caused by more frequent rain events. Arsene et al. [12] also mentioned that the amount of rainfall can influence the levels of the main ions in rainwater, which indicated that the smaller level of ion is caused by the higher amount of rainfall due to dilution effect.

Figure 3 shows the monthly changes of SS-SO42- (Sea-Salt sulfate) and NSS- (Non-Sea salt sulfate) in rainwater. The concentration of NSS- discharged from an anthropogenic source was calculated using NSS- = [] − [Na⁺] ´ 0.121 and the sea-salt sulfate was excluded in the calculation. In case of Na⁺, there was no anthropogenic source except for sea salt; therefore, it was assumed that the component of sea water is consistent with the one of sea salt particles [23,24]. The coefficient of 0.121 was used as an equivalence ratio of [Na⁺]/[] in sea water. During farming and nonfarming periods, the total amounts of sulfate dissolved in rainwater were 113.5 and 177.0 ueq·L⁻¹, respectively. The sulfate content in December was particularly highest. The content of NSS- in sulfate during farming and non-farming periods were 93.7 and 87.1%, respectively [17,19,20].

The observed pH values (pH_obs) were measured to assess the neutralization of rainwater acidity and Figure 4 shows the comparison of pH values measured from the

Table 1. Cationic and anionic composition ratios of rainwater collected in Suwon during farming (April to October in 2010 and 2011) and non-farming (November to March in 2009 and 2010) periods.

study site (pH_obs) with the theoretical ones (pH_the) from the reference [25]. The pH_the value was calculated using: pH_the = −log([H⁺] + [NH₄⁺] + [Ca²⁺]). The pH measurements biased on the right side of 1:1 line as shown in Figure 4 indicates that the neutralization of rainwater was considerably progressed during farming and nonfarming periods.

Figure 5 shows the monthly variation of NH₄-N and NO₃-N. The annual volume-weighted average concentration of inorganic N in rainwater was high during the non-farming period (295.3 ueq·L⁻¹) and the ones during farming period were higher from April to June and lowest in September. This indicates that cations well represented a seasonal variability. It was known that NH₄ was associated with the most intense traditional agricultural activities, particularly during farming season. However, this study found that the content of NH₄ was also high during non-farming season.

The average monthly ion contents during farming and non-farming periods are listed in Table 2. Overall inorganic N contents were much higher during farming period than non-farming period due to high volume of rainfall during farming period. Precipitation was mostly concentrated over Suwon region in August (310.5 mm) and the N loading into soil through rainfall was 5.03 kg·ha⁻¹. In spite of excessive amount of N in rainwater, its availability for crop growth was not high because it was directly discharged into neighboring lakes and streams. In case of sulfate ion (), the highest amount was monitored in soil during August (15.9 kg·ha⁻¹). From April to October, the major observed ion contents for crop growth, , Ca²⁺, K⁺ and Mg²⁺, were 49.4, 5.5, 18.1 and 1.4 kg·ha⁻¹, respectively. Based on previous reference, it was presumed that sulfuric acid from sulfur dioxide (SO₂), Na⁺/Mg²⁺ and Ca²⁺ were from the combustion of fossil fuel and coal, sea salt, and soil/building

Table 2. Monthly change of inorganic ion contents in Suwon during farming and non-farming periods.

Table 3. Source identification of ionic species in rainwater by factor analysis.

materials [25].

The assessment of emission source for chemical ions in rainwater was determined based on the PCA and the results are shown in Table 3. The factor loadings showed that the data synthesized into three factors with Eigin values, cut-off at greater than unity and explained about 80% of the total variance.

The fact that many activities by human and heavy traffic existed in Suwon region was reflected in the selection of factors for statistical analysis. During farming period, three factors were chosen as the following: 1) K⁺ and Cl⁻ from natural sources (sea and soil); 2), NH₄⁺ and Na⁺ from natural sources (sea and soil) and anthropogenic sources (agricultural activity, vehicular emission + fossil fuels Combustion), and 3) Ca²⁺, Mg²⁺ and from natural source (sea and soil) and anthropogenic source (vehicular emission). In case of non-farming period, three factors were chosen as the following: 1) NH₄⁺, and from anthropogenic sources (human and animal waste, vehicular emission + fossil fuels combustion), 2) Na⁺, Cl⁻, Mg²⁺ and K⁺, and 3) Ca²⁺ from natural source (sea and soil).

This study was carried out with the support of “Research Program for Agricultural Science & Technology Development (Project No. PJ008507)”, National Academy of Agricultural Science, Rural Development Administration, Republic of Korea.