Chemical Composition and Sources of Rainwater Collected at a Semi-Rural Site in Ya’an, Southwestern China

Rain and snow water samples were collected from Sep. 2010 to Jun. 2011 at a semi-rural site in Ya’an, a city located in the rain-belt along the Tibetan Plateau, to characterize the chemical composition and the sources of precipitation. The collected samples were severely acidified with an annual volume-weighted mean (VWM) pH of 4.03 and an annual acid rain frequency of 79%. SO₄^2- and NH₄⁺ were the most abundant ions, followed by Ca²⁺, H⁺, NO₃^-, Cl^-, K⁺, Na⁺, F^- and Mg²⁺. The acidity of samples was predominantly generated by H₂SO₄ and HNO₃, which were neutralized by NH₄⁺ and Ca²⁺ as much as 65%. NH₃ played a major role in neutralizing the acid rain. The average ambient concentration of NH₃ was 174.2 μg/m³ during sampling periods. Different source apportionment methods, including principle component analysis (PCA), enrichment factor (EF), correlation and back-trajectory analysis were used to track the sources of rainwater. The methods suggested that the pollutants in rainwater were from both local and long-distance transport (1:2.2), or they were from anthropogenic actions (86.4%), sea salts (8.1%) and crustal (5.5%) respectively.

• Acid Rain; Chemical Composition; Ammonia; Source Apportionment; Ya’an

1. Introduction

Acid rain has received worldwide attention during the past decades for its notably negative effects on aquatic and terrestrial ecosystems. Specifically, acid rain acidifies surface waters and soils, leads to widespread loss of fish population, forest decay and crop yield decline, and accelerates rust process of wild architectures [1,2]. Acid rain can also bring direct and indirect harm to human health. The main risk components are acidity and heavy metal elements [3].

In the past decades, many studies were conducted on acid rains in south China [4-6]. South China has been regarded as the third largest acid region in the world following Northeast America and Central Europe [4,7]. In these previous studies, major attentions were paid to urban areas because more anthropogenic pollutants, i.e. SO₂ and NO_x, are emitted in the industrialized sites. Although the negative effects of acid rain on rural areas, such as on forests, farmlands and water bodies, are as important as on urban areas, and acid rains in rural areas are usually obvious and caused by a significant influence of long range transport air pollutants [8], the knowledge of acid rains in extensive non-urban areas is still limited.

Ya’an is a famous rainy city located on the rain-belt along the Tibetan Plateau in the southwestern China. In Ya’an, about half of the days in a whole year are rainy. Ya’an is dominated by light rains and the annual average precipitation amount is around 1800 mm. Two reasons make us study the chemical composition and the sources of rainwater at a semi-rural site in Ya’an. By the first reason, Ya’an has abundant rains and it is close to the Tibetan Plateau (see in Figure 1(A)). Due to these unique topographic and climatic conditions, a large amount of pollutants in the long-distance transport air masses in Ya’an are obstructed by high mountains, washed out by rains and deposited in this area. These pollutants, represented by a majority of regional pollutions, can be used to track the origins of air pollutants in the research area. By the second reason, we are in shortage of a study associated with acid rains at rural-urban transit site on the rain-belt along the Tibetan Plateau. As far as we know, the transit regions are usually heavily influenced

by anthropogenic actions, and in the regions environmental pollution has obvious impacts on extensive ecological lands.

Rain is an effective way to remove particulates and dissolved gases in the atmosphere [9]. The chemical compositions and pH values of rainwater are affected by the scavenging of atmospheric pollutants. These pollutants can be of many origins, for example, SO₂ and NO_x emitted from fuel burning and vehicles transforming into and through photochemical reactions and being washed out by rains; NH₃ coming from agricultural sources, such as livestock breeding, fertilizer, soil emission and biomass burning [10], and undergoing gas-toparticle conversion processes to give rise to. Ca²⁺ mainly originates from daytime convection and vehicle/wind-driven roadside dust [11]. Other ions (e.g. Cl⁻, Na⁺, Mg²⁺ and K⁺) are primarily from natural sources such as soils, forest fires, and sea salts [12]. The acidity of rainwater is a result of the balance between acidic ions and alkaline ones upon their neutralization reactions [13]. To better understand the formation mechanism of the severe acidity of rainwater, it’s necessary to gain a deep insight into the chemical compositions of precipitation.

The objectives of the present study are: 1) to analyze chemical compositions and characterize seasonal variation of precipitation at a semi-rural site in Ya’an; 2) to discuss the formation mechanism of the acidity of the rainwater; and 3) to investigate the possible pollution sources as well as their relative contributions to the rains at the research site.

2. Materials and Methods

2.1. Site Description

The sampling site is situated on the roof of a two-story building (29˚58'58" N, 102˚58'44" E) about 7 m tall in Teaching and Scientific Park of Sichuan Agricultural University (TSPSAU). The total area of TSPSAU is 33.3 ha. It comprises a dozen of laboratory buildings, a livestock farm (a small number of animals are experimentally fed in this farm), a small wastewater treatment plant and a large area of experimental fields. The sampling site is kept at least 100 m from possible emission source, and no high obstruction stands nearby.

Figure 1 presents the geographic locations of TSPSAU and Ya’an. As we can see from Figure 1, the campus of Sichuan Agricultural University, which belongs to urban area, is located in the east of TSPSAU. At the same time, the extensive farmland, a typical rural area, is located in the west of TSPSAU. The collection site is a good representative of transit place between urban and rural area.

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