Chemical Characteristics of Precipitation in a Typical Urban Site of the Hinterland in Three Gorges Reservoir, China

Liuyi Zhang,Mi Tian,Baoqing Qiao,Jian Cui,Chuan Fu,Fumo Yang,Yimin Huang,Huanbo Wang

doi:10.1155/2018/2914313

Abstract

Major water-soluble ions were analyzed for two-year precipitation samples in Wanzhou, a typical urban site of the hinterland of Chinese Three Gorges Reservoir. The pH values of the precipitation were in the range of 4.0 to 8.3, and the volume-weighted mean (VWM) value was 5.0. The concentration order of anions and cations was as follows: SO42->NO3->Cl->F- and NH4+>Ca2+>Na+>K+>Mg2+, respectively. Good correlations were found between SO42- and NH4+, SO42- and Ca2+, NO3- and NH4+, and NO3- and Ca2+, implying their co-occurrence in the precipitation, most likely as (NH4)2SO4, (NH4)HSO4, NH4NO3, CaSO4, and Ca(NO3)2. The sum of all measured ions was 416.4 μeq L−1, indicating serious air pollution in Wanzhou. NH4+ and Ca2+ were the most important ions neutralizing the acidic compounds in the precipitation; their major sources included agricultural activity and crustal dust. Local anthropogenic activities, for example, coal burning and traffic related sources, contributed most of SO42- and NO3-. The equivalent concentration ratio of SO42-/NO3- was 4.5, indicating that excessive emission of sulfur was the main reason leading to the precipitation acidity in Wanzhou. However, this ratio was lower than the ratio (5.9) in 2000s in Wanzhou, indicating that the contribution of nitric acid to the acidity of precipitation was strengthening.

Highlights

Precipitation is an important means of scavenging airborne pollutants, including in-cloud scavenging and below-cloud scavenging [1]
Monthly mean precipitation amount varied markedly with a peak in the summer and about 70% of the precipitation occurred during the period from May to September. These results were consistent with the long-term average precipitation levels and seasonal variations in Wanzhou [26]
The electrical conductivity (EC) values of single precipitation varied in the range of 3.4 μS cm−1 to 234.0 μS cm−1 with an average of 35.9 μS cm−1, which was larger by a factor of 2.5 than that (14.6 μS cm−1) measured at the global atmospheric background site in Mt

Summary

Introduction

Precipitation is an important means of scavenging airborne pollutants, including in-cloud scavenging (rainout) and below-cloud scavenging (washout) [1]. With the accelerated urbanization and industrialization, excessive pollutants have emitted into the atmosphere in China, for example, nitrogen oxides (NOx), sulfur dioxide (SO2), and particulate matter (PM). These pollutants can dissolve in precipitation and return to surface through wet deposition. It has been observed that precipitation was contaminated worldwide because of the excessive emission of atmospheric pollutants. These pollutants can change the chemical characteristics of precipitation during the scavenging process depending on the solubility [2]. Chemical compositions of precipitation were influenced by the type of pollutant, meteorology, and topographic structure. Na+ and Cl− are abundant along coastal areas [3]; Ca2+ and Mg2+ are abundant in inland areas [4]; Journal of Chemistry

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