Aerosol ionic components at Mt. Heng in central southern China: Abundances, size distribution, and impacts of long-range transport

Water-soluble ions in PM2.5 were continuously measured, along with the measurements of many other species and collection of size-resolved aerosol samples, at the summit of Mt. Heng in the spring of 2009, to understand the sources of aerosols in rural central southern China. The mean concentrations of SO42 −, NH4+ and NO3− in PM2.5 were 8.02, 2.94 and 1.47 μg/m3, indicating a moderate aerosol pollution level at Mt. Heng. Water-soluble ions composed approximately 40% of the PM2.5 mass on average. PM2.5 was weakly acidic with about 66% of the samples being acidic. SO42 −, NO3− and NH4+ exhibited similar diurnal patterns with a broad afternoon maximum. SO42 − and NH4+ were mainly present in the fine aerosols with a peak in the droplet mode of 0.56-1 μm, suggesting the important role of cloud processing in the formation of aerosol sulfate. NO3− was largely distributed in the coarse particles with a predominant peak in the size-bin of 3.2-5.6 μm. Long-distance transport of processed air masses, dust aerosols, and cloud/fog processes were the major factors determining the variations of fine aerosol at Mt. Heng. The results at Mt. Heng were compared with those obtained from our previous study at Mt. Tai in north China. The comparison revealed large differences in the aerosol characteristics and processes between southern and northern China. Backward trajectories indicated extensive transport of anthropogenic pollution from the coastal regions of eastern/northern China and the Pearl River Delta (PRD) to Mt. Heng in spring, highlighting the need for regionally coordinated control measures for the secondary pollutants.

► Water-soluble ions in PM2.5 were first investigated at Mt. Heng in southern China. ► The major factors affecting the variations of fine aerosol were identified. ► SO42 − and NH4+ peaked mainly in the size-bin of 0.56-1 μm, while NO3− in 3.2-5.6 μm. ► Aerosol at Mt. Heng showed different characteristics from those in northern China. ► Long-range transport from coastal regions dominated PM2.5 levels in southern China.