Study On Variation Of Trace Volatile Organic Compounds In The Atmosphere Of Beijing

Atmospheric volatile organic compounds (VOCs) are not only one of precursors of photochemical ozone and secondary aerosol formation but also VOCs do harm to human beings directly. An automated accumulation sampling system and a method of two-step Cryo-Concentrated System combined with Gas Chromatography/Mass Spectrometry (CCS-GC/MS) are introduced to measure volatile organic compounds(VOCs) in Beijing from 1999 to 2005. The injection volume is enlarged from 1～5cm3 to 500～1000 cm³ and then the lowest limit volume concentrations of the measurement is expanded from 10^-6 to 10^-12 using CCS-GC/MS instead of the normal method of GC/MS. The method is evaluated by a set of special experiments and the results are demonstrated. When injection volume of air sample reaches 1 000cm3, the lowest limit is 7×10^-12～40×10^-12 . CCS-GC/MS may identify all the objective chemical species in the atmosphere samples, with average 2.5s bias of retention time for 39 GC peaks. Within 0～400×10^-9(v/v), the concentration of all the objective compounds can be calculated by standard curve individually ( average r2 above 0.99 ) and accurately. The average recollect rate is 100.8%±5.6%, and the bias of precision is 2%～14%.The standard matter factor transfer method is used to calibrate the results of VOCs from 1999 to 2004 and the database of 67 VOCs (30 alkanes, 12 alkenes , 6 aromatics and 19 halocarbons ) from 1999 to 2005(sampling time : 14:00 per Thursday, spot: flat roof of 32meters of 325 meters meteorologic tower) in Beijing is established . The database can provide a mass of sequential data to study the trends and variations , model parameter setting , validation and source apportionment .The concentrations of weekly samples are measured simultaneously from 2000 to 2005. Alkanes ( 66.8±21.8ppbC ) represent the highest average concentration of the total VOCs , followed by aromatics ( 59.2±22.2ppbC ) , alkenes (14.1±3.1ppbC ) and halocarbons ( 13.6±5.8ppbC ) . The most abundant species of Beijing's atmosphere is benzene ( 4.0±0.7ppbV ) , followed by...