Aerosol samples of PM2.5 and PM10 in a period of intensive haze-fog (HF) events were collected to investigate the chemical characteristics of air pollution in Beijing. The air quality in HF episodes was much worse than that in nonhaze-fog (NHF) days. The concentrations of elements and water-soluble (WS) ions (K+, So4(2-), and NO3-) in HF episodes were more than 10 times higher than those in NHF days. Most of the chemical species in PM2.5 and the secondary species (NH4+, So4(2-), and NO3-) in PM10 showed significant difference between HF from westerly direction (HFW) and southerly direction (HFS). The concentrations of secondary species in HFS were much higher than those in HFW, and other chemical species in HFS were lower than those in HFW. The sources of PM2.5 were more from areas on the regional scale due to its tendency for long-range transport, while PM10 was more limited to the local sources. Aerosol particles were more acidic in HFS and more alkaline in HFW. The secondary species were the major chemical components of the aerosol in HF episodes, and their concentrations increased in the order of NHF < HFW < HFS. High concentrations of the secondary aerosol in HF episodes were likely due to the higher sulfur and nitrogen oxidation rate in aqueous-phase reactions. The serious air pollution in HF episodes was strongly correlated with the meteorological conditions and the emissions of pollutants from anthropogenic sources.

Aerosol samples of PM 2.5 and PM 10 were collected in both summer and winter seasons from 2002 to 2003 synchronously at a traffic site, an industrial site, and a residential site in Beijing, which could basically be the representatives over Beijing. Twenty-three elements and 15 ions together with organic carbon and elemental carbon were analyzed systematically for characterization of Beijing aerosol. PM 2.5 was the major part of the inhalable particles (PM 10 ), as the ratios of PM 2.5 /PM 10 were 0.45-0.48 in summer and 0.52-0.73 in winter. SO 4 2À, NO 3 À , NH 4 + , organic matter, crustal matter, and element carbon were the six dominant species, which totally accounted for 85.8-97.7% of PM 2.5 . Secondary aerosol (SO 4 2À , NO 3 À , and NH 4 + ), road dust or/and long-range transported dust from outside Beijing, industry and motor vehicles emission, coal burning were the major contributors to the air-borne particulate pollution in Beijing. Overall, coal burning and the traffic exhausts, plus the dust from the long-range transport, could be the major sources of the aerosol pollution at Beijing. A relatively even spatial distribution of chemical species in PM 2.5 was found while in PM 10 a significant variation with the highest concentrations at the industrial site in summer and at the residential site in winter was observed. The concentrations of PM 10 , PM 2.5 as well as various chemical species were higher in winter than in summer. The contributions of mineral aerosol from outside Beijing were first estimated with a newly developed element tracer technique, which accounted for 79% and 37% of the total mineral in PM 10 and PM 2.5 in winter, and 19% and 20% in summer, respectively. During the dust storm period from 20 to 22 March, it reached up to 97% in TSP, 79% in PM 10 and 76% in PM 2.5 . This is the technique, firstly, developed for estimating the relative contributions of sources from inside and outside Beijing to the total mineral aerosol and it could provide the basic information in controlling the air-borne particulate pollution at Beijing. r