Impact of anthropogenic and natural processes on the chemical compositions of precipitation at a rapidly urbanized city in Northwest China

Abstract

In this study, the chemical composition of precipitation was applied as an indicator to study the influence of urbanization on the atmosphere in Qilian County, a northwest city in China. The results showed that the annual mean pH calculated from the concentration of H+ was 7.35 due to alkaline ion neutralization, and Ca2+ had the largest contribution. Ca2+ and SO42− were the most abundant ions, accounting for 45 and 17%, respectively, of the total ion mass. The correlation coefficient between SO42− and NO3− reaching 0.85 in the urban area indicated the same source. The highest concentration of sulfate in spring was probably related to the strong wind in Northwest China, while the concentration of nitrate was the highest in winter suggesting that most of the nitrate originated from coal combustion, not vehicle emission. The application of factor and cluster analysis indicated that 38.43% of the SO42− and 16.97% of the NO3− in Qilian County were from crustal sources, while 61.5% of the SO42− and 83.03% of the NO3− originated from the human activities. The back trajectory analysis indicated that the pollution emitted from the surrounding areas also made a great contribution to the local atmospheric quality. Thus the communication and cooperation mechanisms with surrounding cities and improvement of energy consumption efficiency should be strengthened in the next few years.