Concentrations and chemical compositions of fine particles (PM2.5) during haze and non-haze days in Beijing

To investigate the chemical properties of PM2.5 and put forward reasonable control measures, daily samples of PM2.5 were collected at an urban site in Beijing from August 4 to September 3 of 2012 using two 2-channel samplers. Chemical analysis was conducted for eight water soluble inorganic ions (WSII, including Na+, NH4+, K+, Mg2 +, Ca2 +, Cl−, NO3−, and SO42 −), organic carbon (OC) and elementary carbon (EC). PM2.5 concentrations ranged from 8.8 to 218.6 μg m− 3, with an average concentration of 80.6 ± 57.3 μg m− 3. WSII, the most dominant PM2.5 constituents contributing 60 ± 18% of its mass, ranged from 3.1 to 172.2 μg m− 3. SO42 −, NO3−, and NH4+ dominated WSII (90 ± 28%) and their concentrations were 1.3–105.7 μg m− 3, 0.5–52.7 μg m− 3 and 0.3–33.5 μg m− 3, respectively. The concentrations of OC and EC were 3.0–28.8 μgC m− 3 and 0.8–7.4 μgC m− 3, constituting 17.6% and 4.9% of PM2.5, respectively. Three serious pollution episodes (haze days) occurred during the campaign. PM2.5 and its chemical species showed substantial increases during haze episodes. The greater enhancement factors for SO42 − (4.5), NO3− (4.0), and NH4+ (4.2) during haze days compared to non-haze days were obtained, suggesting that these secondary inorganic ions play important roles in the formation of haze. The average ratio of NO3−/SO42 − was 0.52. Ion balance calculations showed that PM2.5 samples were acidic during haze periods and close to neutral during non-haze days. Correlation analysis between the major ions was conducted and the results suggested that the main forms of NH4+ might be (NH4)2SO4. In addition, the variations between haze days and non-haze days for OC, EC, and the ratio of OC/EC were discussed.