Characteristics And Chemical Behaviors Of Atmospheric Non-methane Hydrocarbons In Lanzhou Valley,Western China

Non-methane Hydrocarbons?NMHCs?play an important role in the formation of ground-level Ozone?O₃?and Secondary Organic Aerosol?SOA?.Some of NMHCs,such as benzene and toluene,also have been found to be harmful to human health,ecosystems,and atmospheric environment.Many NMHCs are toxic and some of them,such as 1,3-butadiene and benzene,are carcinogenic.The main anthropogenic sources of NMHCs include vehicular,use of solvent,liquefied petroleum gas?LPG?or natural gas leakage,and industrial sources.Lanzhou,the capital city of Gansu province in the semi-arid region of Northwest China,is a typical industrialized city characterized by large-scale petrochemical and oil refinery industries.The first field evidence of photochemical smog in China was reported in Lanzhou in the late 1970s,which becomes a milestone of China's modern air pollution research.In recent years,due to the unique valley topography,large-scale petrochemical industries,and increasing vehicle numbers,surface ozone has become the primary pollutant in Lanzhou during the summertime.As one of important precursors of photo-chemical smog and ozone,Non-methane hydrocarbons?NMHCs?have attract particular attentions in the scientific communication and government agents.In the present study,we investigated the characteristics and chemical behaviors of atmospheric NMHCs in Lanzhou basin,Northwestern China.The main conclusions:?1?The pollution characteristics and sources of NMHCs:Hourly air concentrations of fifty-three NMHCs were measured at the downtown and suburb of Lanzhou in 2013.The measured data were used to investigate the characteristics of NMHCs pollution.The result shows that annually averaged NMHCs concentration was 38.29 ppbv in downtown Lanzhou.Among 53 NMHCs,alkanes,alkenes,and aromatics accounted for 57%,23%and 20%of the total NMHCs air concentration,respectively.The levels of alkanes,alkenes,and aromatics in downtown Lanzhou were lower by factors of 3-11 than that in west suburb of the city.Principal component analysis?PCA?and multiple linear regressions?MLR?were further applied to identify the dominant emission sources and examine their fractions in total NMHCs.It was found that vehicle emission,solvent usage,and industrial activities were major sources of NMHCs in the city,which contributed to 58.34%,22.19%,and 19.47%of the total monitored NMHCs in downtown Lanzhou,respectively.However,in the west suburb of the city,petrochemical industrial emissions were major contributors to the NMHCs,as characterized by relatively higher ethane?C₂H₄?/ethyne?C₂H₂?and propene?C₃H₆?/ethyne?C₂H₂?ratios.?2?The O₃ and SOA production potential of NMHCs:Propylene-Equivalent?Prop-Equiv?concentrations and the Maximum Incremental Reactivity?MIR?were utilized to estimate the contribution of individual NMHCs compound to ozone formation.The result show that,while the measured alkanes concentrations were the highest at the both sites,their contributions to ozone formation?²0%?were lower than alkenes and aromatics,due to the lower OH reactivity.In downtown,cis-2-butene,propylene,and m/p-xylene contributed mostly to ozone formation potentials,whereas in the petrochemical industrialized west suburb,ethane,propene,and trans-2-Butene played more important role in the summertime ozone formation.The secondary organic aerosols?SOA?obtained by fractional aerosol coefficients?FAC?were 0.54and 4.86 ug/m³ during summertime in downtown and west suburb site,respectively.The aromatic hydrocarbon compounds were expected to be the major components of SOA.?3?The relationship between O₃ and its precursors?NMHCs and NO_x?:To further understand the ozone formation associated with its major precursors,the OZIPR model was applied to assess the relative importance of NMHCs and NO_x to the O₃ formation.Extensive model simulations were carried out during high O₃ episodes at the two monitoring sites.Overall,the modeled concentrations agreed reasonably well with the monitored data.O₃-isopleth plots revealed that the slope of ridgeline?NMHCs/NO_x ratio?is 8:1.Knowing that ozone is formed in the NO_x control regime if NMHCs/NO_x ratio>8 and in the NMHCs control regime if NMHCs/NO_x ratio<8,we estimated the NMHCs/NO_x ratios at the both sampling sites.based to O₃ isopleths,the NMHCs/NO_x ratio at the west suburban petrochemical industrial site was 40,which was almost one order of magnitude higher than that at the downtown site?4.42?in the summertime,indicating that ozone was formed in the NMHCs control area in downtown Lanzhou and in the NO_x control area in the industrial west suburb of the city during the summertime.Therefore,reducing the emission of VOCs would be important measure to control ozone in downtown Lanzhou but NO_x controls may be more effective to reduce ozone formation in the west suburban petrochemical industrial site.?4?The initiation,propagation,and termination process of RO_x radicals:A detailed photochemical box model based on the Master Chemical Mechanism?MCM v3.3?was used to investigate the oxidative capacity and radical chemistry in Lanzhou city.The budgets of RO_x?OH+HO₂+RO₂?radicals were quantified,and initiation,propagation,and termination process in the diurnal variation of ozone chemistry were examined.Results showed that during the high O₃episodes in summer,OH initiation was dominated by the reaction of excited oxygen atoms O?¹D?with water and the photolysis of nitrous acid?HONO?in downtown site.At the west suburban site,the most important production of OH was the reaction of O?¹D?+H₂O,followed by the reaction of O₃ with VOCs and HONO photolysis.At this site,HONO photolysis is less important than that in downtown site during the daytime.The photolysis of HCHO and OVOCs?except for HCHO?were the primary sources contributing to the initiation of HO₂ and RO₂radicals at both sampling sites.Results also revealed that the RO_x termination could be attributed to the reactions of RO_x with NO and NO₂.The self-reactions between radicals also played an essential role at the west suburban site.Overall OH was found to be the predominant oxidant,and NO₃ was a major oxidant in the nighttime chemistry.