The ion chemistry and the source of PM2.5 aerosol in Beijing

Daily PM2.5 aerosol samples were collected at five sites in Beijing for a 3-year period from 2001 to 2003. Concentrations of the water-soluble ions (SO42−, NO3−, Cl−, F−, PO43−, NO2−, CH3COO−, HCOO−, MSA, C2O42−, NH4+, Ca2+, K+, Mg2+, Na+) and 23 elements were measured for a total of 334 samples. A relatively even spatial distribution throughout Beijing and a significant seasonal variation were observed. SO42−, NO3−, Cl−, NH4+, Ca2+, and K+ were the major ions and existed mainly in the form of (NH4)2SO4, NH4NO3, NaCl, KCl, and CaCl2 in aerosol particles. Most ions showed high concentrations in winter and low in summer. Secondary ions, mainly SO42−, NO3−, exhibited high concentrations in both summer and winter due to the secondary transformation accelerated under high humidity and strong solar radiation in summer and the higher concentration of SO2 from coal burning and the lower removal rate in winter. The formations of SO42− and NO3− were determined largely by temperature and NH4+, respectively. Temperature, relative humidity, rainwater frequency, and air mass origin might be the main factors regulating the aerosol distribution. Crustal ions exhibited sporadic but high peaks in spring due to the intrusion of dust from west and northwest of China. The Ca2+/Al ratio was used to indicate the mixing of different dust sources. Factor analysis showed that the secondary formation of coal/biomass burning products, crust, industrial and traffic emissions were the major sources of the fine aerosols in Beijing. Traffic source became more significant with motorization in recent years.