Characterization and source analysis of water–soluble ions in PM2.5 at a background site in Central China

Abstract

Little updated observation of background aerosols in Central China is available since the clean air actions, despite increasing aerosols pollution in this region. PM2.5 was sampled at Dongting Lake during September 2017 to August 2018 to probe the chemical characteristics of background PM2.5 in Central China. We sampled aerosols at an urban site simultaneously. Annual mean PM2.5 level reached 43.4 μg m−3, and SO42−, NO3−, and NH4+ (sulfate–nitrate–ammonium, simplified as SNA) accounted for 22.8%, 26.8%, and 15.0%, respectively. PM2.5 and most ions levels were highest during winter and lowest during summer. PM2.5 mass was driven by NO3− during cold months (October–March) or SO42− during warm months (September, April–August), suggesting that different emission control measures should be applied in different seasons in future air quality policy of this region. Both SO42− and NO3− were almost completely neutralized by NH4+. Low temperature increased the SO42− concentration and sulfur oxidation ratio. Low temperature also facilitated the partition of NO3− from gas phase into particle phase. Severe particulate pollutions occurred in cold months when the diffusion and wet scavenging of aerosols and volatilization of NH4NO3 were weak, and the NO2 concentration was high. An interesting finding was the highest NO3− concentration and conversion ratios for SNA as well as lowest SO42− concentration in Dongting Lake among major background sites, despite low levels of local gaseous precursors. Industry, secondary SO42−, and NO3− were indentified as main sources of PM2.5 mass based on PMF analysis. Concurrent occurrences of PM2.5 and SNA pollution at Dongting Lake and Wuhan sites as well as back trajectories verified the regional effect. The results will be helpful for investigating water–soluble ions and the major sources as well as geographical origins for PM2.5 at Central China.