| Particular matter (PM) is one of the major pollutants in China. Radiation balance, visibility and human health can be strongly influenced by high concentrations of PM. Production rates of relevant heterogeneous reactions can be dramatically enhanced also by significant levels of PM. Due to anthropogenic local emission and regional transport effect, haze events have been constantly observed in the Yangtze River Delta (YRD), and severely disturbed citizen’s ordinary living and working conditions.In this study, long-term field observations of gaseous pollutants (O3, SO2, NO, NO2 and NOy), aerosol scattering/absorption and mass concentration of PM2.5 were performed from December 1,2013 to November 30,2014, at a regional background site (SORPES, Station for Observing Regional Processes of the Earth System) in Nanjing. Measurement data were analyzed to find out the characteristics of PM2.5 and visibility. Two intensive field campaigns were conducted during the study period, the formation mechanism of haze pollution in summertime and wintertime were discussed.Results of long-term measurement showed that the mean levels of O3, SO2, NO, NO2 and NOy were 27.9 ppbv,6.1 ppbv,6.2 ppbv,29.7 ppbv and 37.3 ppbv, respectively. The observed mass concentrations of PM2.5 (36%) frequently exceeded the National Air Quality Standard Ⅱ throughout the campaign. Annual mean mass concentration of PM2.5 was 72.5 μg/m3 and exhibited a "W" shape inter-annual variation. According to the result of data analysis, vigorous photochemical reaction and high concentration of O3 were the major causes of the summertime PM2.5 peak; while wintertime peak was heavily affected by meteorological factors, such as low temperature and precipitation amount, and increasing levels of precursors, such as SO2 and NOx. The strong scattering effect caused by high concentration of PM2.5 also made Nanjing a suitable place to encounter haze events, and most events were observed in June and December. When haze occurred, concentrations of PM2.5 and gaseous pollutants were 25-50% higher than ordinary days.Two intensive campaigns were done in December 2013 and June 2014, several haze events were observed and their formation mechanisms were discussed and analyzed. According to synoptic pattern analysis, the high temperature and low air pressure during summertime may provide a proper environment for convective diffusion of the air masses, which means that more pollutants may come to Nanjing from the heavily polluted YRD city clusters; and the near-surface low air pressure condition during wintertime could lead to an accumulation of pollutants, while air masses containing high-level pollutants were mainly coming from the northern part of China. Besides the regional transport of the pollutants, haze events caused by the secondary formation in the local area were also observed. In the two summertime cases, SO42- was the primary pollutant (26.1% and 23.1%) of the PM2.5· NO3- of 17.9% and 21.6% were also found. The results suggested that the SO42- and NO3- were mainly formed through the photochemical reaction and gas-phase oxidation, the NO3-were lower because of the thermal decomposition of NH4NO3. In the wintertime cases, NO3- was the most significant pollutant (26.1% and 23.1%) of PM2.5, while SO42- was about 17.9% and 21.6%, respectively. The daytime homogeneous reactions contributed significantly to the formation of sulfate and nitrate, while the nocturnal oxidation and catalysis by the metallic ion at night may contribute to the formation of sulfate. |
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