Comparison of ionic and carbonaceous compositions of PM2.5 in 2009 and 2012 in Shanghai, China

Daily PM2.5 samples were obtained at the site of East China University of Science and Technology (ECUST) in urban Shanghai during 2009-2010 and 2011-2012. The temporal variations of PM2.5 and its chemical compositions including secondary inorganic aerosol (SIA) and carbonaceous components were studied. The concentrations of PM2.5 were (94.04±52.17) μg/m3 and (68.44±41.57) μg/m3 in 2009 and 2012, respectively. The concentrations and contributions of SIA to PM2.5 were significantly higher in 2012 than those in 2009. Sulfate took up above 50% of SIA in 2009 and the corresponding value decreased to 41% in 2012. The increasing trend of NO3-/SO4(2-) mass ratio implied that the contributions of mobile sources were more and more important. The molar ratios of [NH4+]/(2[SO4(2-)]+[NO3-]) were 0.57 and 0.70, lower than 1, which demonstrated that most aerosol samples were ammonium-poor. But the neutralization process of ammonium might affect the formation of sulfate and nitrate, which was indicated by the strong correlation between [NH4+]-[SO4(2-)] and [NH4+]-[NO3-]. The average molar ratio of ammonium to sulfate was 1.74 in 2009, so there was not enough ammonium to neutralize sulfate sufficiently. The higher value of 2.30 in 2012 indicated that sulfate was sufficiently neutralized by ammonium and the predominant production was (NH4)2SO4. Aerosol samples had higher char-EC/soot-EC ratios and lower OC/EC ratios in 2009 than those of samples in 2012. The higher K+/OC values demonstrated that biomass burning made important contributions to carbonaceous components in both 2009 and 2012. All backward trajectories were grouped into four clusters, mainly from the Northwest, the Circum-Bohai-Sea Region (CBSR), the Southwest and the Southeast. The cluster from the Northwest was the most polluted pathway in 2009, while the cluster from the CBSR had more effects in 2012. PSCF model for PM2.5 and carbonaceous components except soot-EC suggested that the Yangtze River Delta Region (YRDR) made significant contributions by short-range transportation in 2009 and 2012. The Northwest, the CBSR and the Pearl River Delta Region (PRDR) were also the potential source areas. However, the source pattern of soot-EC was significantly different from those of other carbonaceous components.