A comparison study on airborne particles during haze days and non-haze days in Beijing

• Water soluble inorganic ions in different size airborne particles during haze and non-haze days in Beijing were analyzed.
• NH4+, K+, NO3− and SO42 − were concentrated in fine particles around 0.56–1.0 μm during haze days, while 0.32–0.56 μm during non-haze days.
• NOx from the vehicle exhaust is playing an important role on fine particle formation.

Airborne particles in Beijing during haze days and non-haze days were collected by an eleven-stage cascade impactor (MOUDI 110, MSP, USA), and the mass concentrations and water soluble inorganic ions of the size segregated airborne particles were quantitatively analyzed. PM10 concentrations during haze days ranged from 250.5 to 519.4 μg m− 3 which were about 3–8 times greater than those (ranged from 67.6 to 94.0 μg m− 3) during non-haze days, and PM1.8 concentrations during haze periods were in the range of 117.6–378.6 μg m− 3 which were 3–14 times higher than those (27.0 to 36.8 μg m− 3) during non-haze days. In comparison with non-haze days, all water soluble inorganic ions investigated in the airborne particles greatly enhanced during haze days. NH4+, NO3− and SO42 − were found to be the dominant water soluble inorganic ions, accounting for 91–95% of the total inorganic ions in PM1.8 during haze days, and 73–81% during non-haze days. The size distributions of SO42 −, NO3−, Cl−, K+ and Na+ exhibited bimodal types, while single mode was found for NH4+, Ca2 + and Mg2 +. Only with exception of Ca2 + and Mg2 +, all ions were concentrated in fine particles around 0.56–1.0 μm of “droplet mode” during haze days, while 0.32–0.56 μm of “condensation mode” during non-haze days. The extremely high mole ratio (> 2) of [NH4+]/[SO42 −] during haze days implied that the main form of ammonium in PM1.8 might be (NH4)2SO4 and NH4NO3. The mass ratio of NO3−/SO42 − was > 1 in PM1.8 during haze days and ~ 1 during non-haze days, indicating that NOx from the vehicle exhaust in Beijing is playing more and more important role on fine particle formation.