Composition and mixing state of water soluble inorganic ions during hazy days in a background region of North China

Abstract

Knowledge of haze particles in background areas of North China is limited, although they have been studied well in urban settings. Atmospheric aerosol particles were collected at a background site in the North China Plain during 16–31 January, 2011. Water soluble inorganic ions of PM2.5 and physicochemical characteristics of individual particles on hazy and clean days were measured by Ion Chromatography (IC) and Transmission Electron Microscopy (TEM), respectively. Average PM2.5 mass concentration was 50.4±29.9 μg m−3 with 62.5±26.8 μg m−3 on hazy days and 19.9±11.5 μg m−3 on clean days. SO4 2−, NO3 −, and NH4 + with a combined mass concentration of 19.0±11.5 μg m−3 accounted for 69.8%–89.4% of the total water soluble inorganic ions. Size distributions of SO4 2− and NH4 + showed one unimodal peak at 0.56–1.8 μm on hazy days, whereas NO3 − appeared as bimodal peaks at 0.56–1.8 and 5.6–10 μm, respectively. Individual particle analyses showed that the dominant aerosols were a mixture of sulfate, nitrate, and carbonaceous species, which together determine their mixing states. 48-h air mass back trajectories on hazy days suggested that air masses crossed the polluted continental areas (such as Jing-jin-ji region and Shandong province) and entrained ground air pollutants 11–19 hours before reaching the background area. During long-range transport particles undergo ageing and tend to be internally mixed mainly due to condensation in the background atmosphere. Our results suggest that hygroscopic and optical properties of these aerosol particles in the background area differ substantially from those in urban areas.