Characterization And Source Apportionment Of Inorganic Water-soluble Ions In PM_2.5 Aerosol Before And During Olympics In Changping

To assess the impact of pollution-control measures on air quality before and during Olympics, more than one month field study was carried out in Changping, a rural site in Beijing. The water-soluble fractions in PM2.5, trace gases (SO2, NO2, O3, and CO), and meteorological factors were on-line measured from 9 July to 25 August in 2008. The chemical compositions, temporal and diurnal variations, source apportionment of water-soluble ionic species in PM2.5 were studied during three different periods in detail. Furthermore, the impacts of meteorological conditions on PM2.5 and water-soluble ions on visibility in haze cases were also investigated. The results were summarized as follows:1. Air quality before and during Olympics in Beijing was better than that of the corresponding period in 2007. Reduction of pollutants concentrations and improvement of air quality were contributed to pollution-control measurement and meteorological conditions before and during Olympics. The pollutants concentrations during Olympics were obviously lower than that before Olympics. Compared with alternative day-driving scheme period, the hourly concentrations of total water-soluble ions, SO2, O3 and CO during Olympics period, were reduced by 55%, 42.9%,30.8% and 41.7%, respectively. The hourly concentrations of SO2, O3 and CO were reduced by 38.5%,24.3% and 32.4% compared with heavily-duty vehicles period, respectively. Except SO2 and NO2, the average hourly concentrations of O3 and CO often exceeded the National Standard of Air Quality. Their excess ratios during three different periods were 4.17%,9.15%,1.31% and 10.98%,29.06%,4.96%, respectively.2. SO42-, NO3- and NH4+ were the dominant water-soluble fractions in PM2.5 in Changping site, accounting for 91.34% of total water-soluble ions concentrations. The hourly concentrations of SO42-, NO3- and NH4+ during Olympics period were significantly lower than those during alternative day-driving scheme period, with 55.4%,61.4% and 56.0% reduction, respectively.3. Most of the water-soluble ions in PM2.5 in both periods in daytime were higher than that in nighttime. Daytime and nighttime concentrations of SO42-, NO3- and NH4+ during Olympics period were 50.0%,33.1%; 56.1%,42.9% and 50.0%,42.1% lower than those during alternative day-driving scheme period. 4. SOR and NOR during Olympics period were 0.68 and 0.41, which were lower than that during alternative day-driving scheme period, indicating the transformation of SO2 and NO2. The average [NO3-]/[SO42-] during Olympics period is 0.44, which is smaller than 0.53 during alternative day-driving scheme period.5. The main sources of water-soluble fractions in PM2.5 are secondary formation(38.69%), combustion(24.14%) and soil(23.45%) according to factor analysis.6. Haze always promoted the accumulation of primary pollutants and formation of secondary pollutants. SO42-, NO3- and BC were the main species that had strong effects on visibility. Meteorological analysis showed that haze was always combined with weak-high pressure system, which would lead to stable weather and prevent pollution dispersion. However, low pressure was the dominating factor during Olympics period, which was affected by the motion of west grooves frequently. Combined with the hard pollution-control measures during Olympics period, the concentrations of pollutants decreased obviously. The back-trajectories analysis suggested that during these two haze cases, air quality in Beijing was mainly affected by the air masses from severe polluted urban areas, including Tianjin, Langfang, Shijiazhuang, Taiyuan, Changzhi, Zhengzhou and Jinan. At the same time, the special terrain condition of northern of Changping site, which was blocked by many hills, would lead to the accumulation of pollutants.