Seasonal chemical composition variations of wet deposition in Urumchi, Northwestern China

Urumchi suffered serious air quality problem in recent years. Although the main air pollutants and wet deposition monitoring have been undertaken by Urumchi Environmental Monitoring Center for many years, researches on chemical composition and variations of wet deposition were not there till now. In this paper, wet deposition monitoring data in Urumchi from 2000 and 2005 were selected to perform this evaluation based on data quality and data integrity. The volume-weighted mean (VWM) concentrations of parameters of wet deposition, namely pH, conductivity, SO42−, NO3−, F−, Cl−, NH4+,Ca2+, Mg2+, Na+ and K+ concentrations, were 6.86 and 91.04 μs cm−1, 14.3, 1.63, 0.37, 3.78, 1.22, 4.79, 0.59, 1.05, 0.74 mg L−1, respectively. Acid precipitation appeared only in wintertime in few cases, and its extreme value was 4.96; the maximum emerged during sand storm event in the non-winter season with the value of 9.35. Wet deposition and air pollution characteristics varied with seasons. In order to make a certainty of possible sources of ions in different seasons, principal component analysis was applied, and conclusions were drawn that in wintertime, the predominant contributor to the wet deposition was coal combustion for residential heating; however, in the non-winter season, the situation was more complicated, dust and soil from outside the urban, dust re-suspension, local industries process, motor vehicle emissions all played their roles.