Characterization And Sources Of PM_2.5 Water-soluble Ions In Typical Areas In China

Water-soluble ions are the major components of fine particles; therefore the pollution, physical and chemistry characteristics of water-soluble ions are crucial for comprehending influences of fine particles on human health and global climate. Several intensive field campaigns were performed in Beijing and Jinan in the North China Plain and at Mt. Heng in southern China. Water-soluble ions were measured by highly-time resolved instrument to obtain the pollution characteristics and spatio-temporal variations, and the data was analyzed with the aid of many statistical techniques including aerosl inorganic model, principal components analysis and backward trajectory model to discuss the acidity, source and transport pattrern of fine particles. Besides, we discuss the pollution feature and the influence of water-solublel ions on the visibility.To better understand secondary aerosol pollution and potential source regions, highly time-resolved measurement of water-soluble ions in PM2.5was performed from December2007to October2008in Jinan, the capital of Shandong Province. SO42-, NO3-and NH4+were the most abundant ionic species with annual mean concentration of38.33,15.77and21.26μg/m3, respectively, which are among the highest levels reported in the literatures in the world. Well-defined seasonal and diurnal patterns of SO42-, NO3-and NH4+were observed. The fine sulfate and nitrate oxidation ratios (SOR and NOR) were much higher in summer (SOR:0.47; NOR:0.30) than those in other seasons (SOR:0.17-0.30; NOR:0.12-0.14), indicating more extensive formations of SO42-and NO3-in summer. Principal component analysis indicated that the major sources contributing to PM2.5pollution were secondary aerosols, coal/biomass burnings and traffic emissions. The most frequent air masses connected with high concentrations of SO42-, NO3-and NH4+originated from Shandong Province in spring, autumn and winter, while from the Yellow Sea in summer, and then slowly travelled in Shandong Province to Jinan.Highly time-resolved measurements of SO42-, NO3-, and NH4+in PM2.5were simultaneously performed at an urban site and a downwind rural site in Beijing during the2008Olympics to investigate the impacts of control measures and regional transport. The mean concentrations of SO42-, NO3-, and NH4+were18.23,9.47, and9.70μg/m3, respectively, at the rural site. These concentrations were comparable to those of20.74,8.83, and10.85μg/m3at the urban site. Obviously diurnal variations of SO42-, NO3-, and NH4+were observed at both sites and were related to meteorological conditions, primary emissions, and regional transport. PM2.5showed weakly acidic, and aerosol acidity (Hair) was stronger at the rural site than that at the urban site due to higher water concent at the rural site. Principal component analysis indicated that secondary aerosols, photochemical oxidation, traffic emissions and soil were the major factors influencing PM2.5pollution. Air masses from south sector of Beijing had much higher concentrations of SO42-, NO3-and NH4+than other air masses from north and east sectors. The effectiveness of the control measures on SO42-, NO3-, and NH4+was evaluated by comparing the urban site concentrations during three periods:before the full-scale control, after the full-scale control but before the Olympics, and during the Olympics. The high pollution observed after the full-scale control was attributed to the regional transport from the sector south of Beijing. The samples in the air masses from the northwest were selected to minimize the influences of meteorological factors and regional transport, and the results showed a clear reduction of SO42-, NO3-, and NH4+concentrations (approximately35%-69%) after the full-scale control began, suggesting the effectiveness of the control measures in reducing the local secondary inorganic aerosols. A widespread pollution episode was observed during August3-10at the rural site, with regional transport being identified as the main contributor. Secondary transformation evidently occurred during August3-4and contributed more than50%of the rural secondary ion concentrations. During August5-10, the whole region experienced a stable and well-developed plume.Water-soluble ions in PM2.5were 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 of2009, to understand the sources of aerosols in rural central southern China. The mean concentrations of SO42-, NH4+and NO3-in PM2.5were8.02,2.94and1.47μg/m3, indicating a moderate aerosol pollution level at Mt. Heng. Water-soluble ions composed approximately40%of the PM2.5mass on average. PM2.5was weakly acidic with about66%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 of0.56-1μm, 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 of3.2-5.6μm. 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. During the campain a dust storm invaded Mt. Heng and heterogeneous formation of coarse SO42-, NO3-and NH4+in the surface of dust paticles was enhanced. Cloud/fog and rain obviously decreased the concentrations of SO42-, NO3-and NH4+in PM2.5and scavenging ratio increased with the reduce of PM2.5concentrations.Haze study in Jinan and Beijing in the North China Plain (NCP) indicated that the occurence of haze was caused by accumulation and formation of fine particles. SO42-, NO3-and NH4+concentrations in PM2.5on haze days were2-4times those on non-haze day, and increase spending was largest in winter. The higher fine sulfate and nitrate oxidation ratios (SOR and NOR) on haze days reflected that haze days favored to formations of SO4-and NO3-. The size distributions of SO4-, NO3-and NH4+altered from the size-bin of0.32-1μm on on-haze days to0.56-1.8μm on haze days. The influence of fireworks burning on air quality during Spring Festival was also investigated. Display of fireworks emitted varied pollutants (e.g., NO2, SO2and PM), which deduced the visibility and further caused haze. This study compiled the measurement data in North and South China to draw insights into the characteristics and sources of PM2.5, which provide the data for comprehending the PM2.5pollution situation, and theoretical basis for making control strategies of particles pollution. The prestnt study reflected serious water-soluble ions pollution in PM2.5from ground to high altitude in boundary layer, and the pollution was more serious in North China than that in South China. Besides, regional and cross-border transport was important for PM2.5pollution, highlighting the need for regionally coordinated control measures for the secondary pollutants.