Preliminary Study Of Sources And Photochemical Age Of Atmospheric Nonmethane Hydrocarbons Based On Stable Carbon Isotopic Composition

Atmospheric nonmethane hydrocarbons (NMHCs) are precursors of ozone and peroxyacetyl nitrates (PAN), and play a key role in atmospheric photochemistry. Some NMHCs are hazardous air pollutants, such as benzene, which will bring undesirable impact on people's health if its concentration runs up to high enough in some microenvironment (e.g. indoor environment). For their importance in atmospheric chemistry and health impacts, the study on NMHCs becomes a hot spot internationally. At present, many studies focus on concentration distribution of NMHCs, and using concentration characteristics to research the sources and photochemical age. But research on sources, transformation and photochemical age of atmospheric NMHCs based on stable carbon isotopic composition are lacked. Due to the photochemical activity of some NMHCs, there would be some uncertainty to use the characteristics of concentration to analysis their sources. And some research revealed that stable carbon isotopic composition of NMHCs could be well used to trace their origins and to study their transformation in photochemical reaction.Sampled by Tenax-TA adsorption tube and determined by Thermal Desorption-Gas Chromatography-Isotope Ratio Mass Chromatography (TD-GC-IRMS), stable carbon isotopic composition of NMHCs in atmosphere of Lanzhou and Taiyuan city and some main pollution sources ( emission by coking processes,air in refuse landfill, lampblack of meal, coal-combustion exhaust of power plant and air in tunnel) were reported in this paper. Based on stable carbon isotopic composition characteristics, preliminary studies of sources and photochemical age of atmospheric NMHCs were conducted. Upon those studies, we concluded:1 Theδ¹³C values of NMHCs from coking processes,air in refuse landfill, lampblack of meal, coal-combustion exhaust of power plant and air in tunnel were -32.1‰^-18.8‰, -33.2‰^-25.7‰, -32.0‰^-24.8‰, -33.8‰^-21.2‰and -28.8‰^-25.5‰, respectively. The averageδ¹³C values of NMHCs in combustion stack gas emission were from -31.6‰to -20.1‰, in coal charging and coke pushing emission were from -32.6‰to -18.81‰, while in ambient air in coking plants the values were -32.3‰^-18.5‰.These values indicated thatδ¹³C values of NMHCs were different from each other in those pollution sources. Therefore it is possible to trace the origins of atmospheric NMHCs based on such discrepancy.2 Theδ¹³C values of atmospheric NMHCs in different functional district were varying, the values ranged from -33.4‰to -23.2‰in commercial district, from -36.0‰to -24.2‰in industrial-transportational area, while in clean area the values were from -30.9‰to -24.2‰. The kinds of compounds detected were different in these functional districts, with the most in industrial-transportational area, while the least in clean area.3 The results of source analysis of atmospheric NMHCs based on stable carbon isotopic composition characteristics showed that tmospheric NMHCs in commercial district were mainly from vehicle exhaust, lampblack of meal and coal-combustion exhaust, besides fossil fuel evaporation, refuse emission and some unknown sources may also have some influence; atmospheric NMHCs in industrial-transportational area mainly rooted in fossil fuel evaporation and coal-combustion exhaust, meanwhile some unknown sources might have a significant contribution; while in clean area, atmospheric NMHCs mainly emitted by plants, lampblack of meal and transportation related sources, other sources such as coal-combustion exhaust and refuse emission may also have some influence.4 The diurnalδ¹³C values of atmospheric NMHCs in winter in Lanzhou city were -30.5‰^-24.3‰, while in summer the values were -31.9‰^-24.2‰. Influenced by coal-exhaust exhaust, the diurnalδ¹³C values of atmospheric NMHCs in winter were larger than that in summer. Based on hydrocarbon isotopic clock and changes of diurnalδ¹³C values of n-hexane and 1-butene in summer atmosphere in Lanzhou city, we calculated their average photochemical ages were 6 d and 25min, respectively.