Characteristics And Sources Of Carbonaceous Aerosols And Polycyclic Aromatic Hydrocarbons In Total Suspended Particles And PAHs Numerical Simulation In The Tarim Basin

The characteristics and sources of the water-soluble ions,polycyclic aromatic hydrocarbons?PAHs?,and carbonaceous aerosol were investigated and assessed in this thesis based on field measurements of total suspended particulates?TSP?at four sampling sites?Tazhong,Tikanlik,Minfeng and Kashigar?across the Tarim Basin from January to December 2010.?1?Highest annually averaged mass concentration of TSP was measured at Minfeng sampling site,followed by Tazhong,Tikanlik,and Kashigar,respectively.Such spatial distribution of TSPs can be attributed to emission sources,geographic locations,and atmospheric motions over these sampling sites.In general,the p H values of TSP samples in the Tarim Basin showed a mild alkaline characteristic.Five cations in measured TSP samples showed the same ranking at these sites,indicating that Ca²⁺ has the highest level in the atmosphere,followed by Na+,K⁺,NH⁴⁺,and Mg²⁺,respectively.High concentrations of Ca²⁺ and Na+ were most likely attributed to the inorganic salts,such as Na2SO4?Na Cl,and Ca SO4 in the sand dust which could be traced back to local sources in the Tarim Basin.Atmospheric levels of water soluble ions were associated strongly with dust weather.Changes in atmospheric concentrations of K⁺ and NH⁴⁺ might be induced somehow by seasonal agricultural activities.?2?The annual mean concentrations of organic carbon?OC?and elemental carbon?EC?averaged over the four sampling sites in the Tarim Basin were 8.2 ?g/m³ and 2.5 ?g/m³,respectively.OC concentration in the Tarim Basin was lower than that measured in urban areas in eastern China,suggesting that the level of carbon aerosol pollution in the Tarim Basin was relatively low.The carbon component accounted for only 3% of the atmospheric aerosol mass.Similar seasonal variation was also found at the four sites.OC and EC concentrations during the domestic heating season and dust season were higher than other seasons.The OC/EC ratios ranged from 1.58 to 6.57.The annual mean OC/EC ratios at the four sampling sites were greater than 2,manifesting that the dominant fraction of carbonaceous aerosol was OC and has secondary pollutants formation.Our study also showed that the sources and formation of carbonaceous aerosol at urban sites were more complex than that of rural sites.High correlation between water-soluble potassium and OC concentration revealed that the biomass burning was an important source of the carbonaceous aerosol.It was also found that weather conditions exerted little effects on OC and EC concentrations in the Tarim Basin.The correlation coefficients between OC and EC were calculated.OC and EC were associated significantly at the sampling sites,except for Kashigar.This result implies that OC and EC at Minfeng,Tikanlik and Tazhong have the similar sources.Carbonaceous aerosol sources in Kashigar were more complex.Over all,anthropogenic sources exerted important influence on carbonaceous aerosol pollution.In autumn and winter,Kashigar,Minfeng,and Tazhong had higher SOC concentration whereas higher SOC concentrations were ovserved in the wintertime in Tikanlik.Although temperature and solar radiation are relatively higher in summer,the emission of SOC precursors gases VOCs were relatively low,resulting in lower SOC concentrations at the four sites in summer.?3?It was revealed that the annually averaged concentration of 16 polycyclic aromatic hydrocarbons??16PAHs?was highest at the Kashigar site?the largest city in the Tarim Basin?at 390.5 ng/m³ among the four sampling sites,followed by Tikanlik?388.65 ng/m³?,Minfeng?84.54 ng/m³?,and Tazhong?74.07 ng/m³?,respectively.The predominant PAHs in all TSP samples were 4-5 rings PAHs among which Benzo?a?pyrene?Ba P?has the highest concentration at the four sampling sites.2-3 rings PAHs with small molecular weight only accounted for a very small portion of total PAHs.The annual mean ?[Ba P]eq concentrations during the sampling period were 38.40 ng/m³ at Kashigar,34.07 ng/m³ at Tikanlik,10.42 ng/m³ at Minfeng,and 7.22 ng/m³ at Minfeng,respectively.The estimated inhalation cancer risks?ICR?for an adult under 70 years lifetime exposing to PAHs were 3.21×10-3?6.1×10-6-22.5×10-3?at Kashigar,2.85×10-3?2.60×10-5-13.9×10-3?at Tikanlik,8.7×10-4?1.84×10-6-13.5×10-3?at Minfeng and 6.3×10-4?1.47×10-6-3.9×10-3?at Tazhong,respectively.Higher ICRs were found in winter and spring seasons as compared with warm seasons,suggesting higher risks of local residents to PAHs during cold seasons.Overall,the population exposure to PAHs in the Tarim basin has exceeded the maximum acceptable risk level.Using the ratio method we found that PAHs in TSPs in the Tarim Basin were mainly formed by high temperature pyrolysis process.The principal component analysis suggested that the main sources of polycyclic aromatic hydrocarbons in the Tarim Basin include motor vehicle exhaust emissions,coal combustion,fuel combustion,and industrial pollution.Differing from eastern China,diurnal variation in PAHs emission sources in the Tarim Basin is less significant,except for the domestic heating seasons during which there were some fluctuations of PAH emissions,subject to the stability of PAHs emission sources and PAHs contaminants.Backward trajectory analysis revealed that PAHs atmospheric concentrations in the Tarim Basin were influenced by the air flows from Central Asia and Pakistan and northern India,which caused diffusion and migration of PAHs across the Tarim basin.Forward trajectory analysis showed that TSP and PAHs originated from the Tarim Basin could be delivered to downstream regions,such as eastern Xinjiang and northwestern Gansu,as well as eastern China by prevailing westerly winds.We can infer that eastern and southeastern Traim Basin are major sink area of polycyclic aromatic hydrocarbons.?4?The Can METOP model simulations showed higher PHE and Ba P air concentrations in the northwest of the Tarim Basin,consistent with the monitoring results.This implies that the Can METOP model is capable of capturing the primary characteristics of spatial distribution of PAHs atmospheric concentrations.However,the model appeared not to predict correctly the seasonal variation of atmospheric levels of PHE and Ba P.In contrast to the measured concentrations of these two PAH congeners which showed higher levels in winter and spring than the other two seasons,the modeled concentrations illustrated higher PHE concentrations in summer and lowest in winter.This was mainly caused by the lack of seasonal PAH emission inventories which always showed higher emissions in winter and spring because of domestic heating and higher frequency of sand storms.Future improvement of PAHs atmospheric modeling should therefore be gained by developing and establishing seasonal PAH emission inventory.More field monitoring data are also needed to verify modeling results.The numerical experiment results show that,the PHE and Ba P concentrations in the eastern part of the Tarim Basin extered little impact on PHE and Ba P concentrations in southwestern Xinjiang,suggesting that the environmental fate of PAHs in this region was mainly determined by local emission sources.