Contamination Characteristics And Inhalation Exposure Risk Of Atmospheric Nitrated Polycyclic Aromatic Hydrocarbons In Lanzhou Valley,Western China

Polycyclic aromatic hydrocarbons?PAHs?are a class group of important organic pollutants with well-known carcinogenic,mutagenic and teratogenic properties,which are formed during incomplete combustion of fossil fuels rather than intentionally emitted into the atmosphere.Thus they are widely emitted and distributed in atmosphere with high pollution levels.Generally the atmospheric concentration levels of nitrated polycyclic aromatic hydrocarbons?NPAHs?are lower than that of parent PAHs,but the toxicity and mutagenicity of NPAHs were higher than their parent PAHs.NPAHs can originate from both the inadvertent primary emissions of incomplete combustion of fuels?industrial sources,living sources,and vehicle exhausts of mobile source etc?and secondary formation derived from the photochemical reaction of parent PAHs with atmospheric oxidants such as nitrogen oxides etc?OH,NO₃,O₃?under highly oxidizing atmosphere.Therefore,high atmospheric NPAHs levels can pose a potential adverse health effects to local residents.Lanzhou,the capital city of Gansu province,is a typical petrochemical and oil refinery industrialized valley in western China.Xigu district was the first region in China where photochemical smog was found in the middle of 1970s.This photochemical smog event was regarded as a milestone in China's model air pollution research and environmental protection.As an important petrochemical industrialized base in northwest China,Lanzhou suffered from higher emissions of ozone precursors including volatile organic compounds?VOCs?and nitrogen oxides?NOx?as well as semi-volatile organic compounds?SVOCs?such as PAHs.Higher NOx and O₃concentrations of criteria air pollutants would promote the degradation of PAHs and the secondary formation of PAHs derivatives such as NPAHs and OPAHs.Due to the unique closed valley topography,higher altitude and strong inversion layer,Lanzhou valley basin is a natural experimental field to identify the secondary formation sources of NPAHs.However,there were no public reports involving in atmospheric NPAHs emissions and pollution for Lanzhou valley.In the present thesis study,we investigated the concentration levels and contamination characteristics of atmospheric NPAHs in Lanzhou valley using atmospheric passive and active sampling technologies to collect gas phase and particulate matter samples,and to identify its source differences?primary emission or secondary formation?.Furthermore,Lung cancer risks induced by inhalation exposure to NPAHs were assessed based on the BaP equivalent toxicity factor.The results of this thesis are as follows:?1?Contamination characteristics and source identification of atmospheric NPAHs based on PUF-PAS in Lanzhou valley:Air concentrations of?₁₂NPAHs for18 passive sampling sites ranged from 1.6 ng/m³ to 37 ng/m³ in winter and from 2.3ng/m³ to 45 ng/m³ in summer.The mean concentrations of?₁₂NPAHs in the ambient air were 8.6±8.1 ng/m³ in winter and 15±11 ng/m³ in summer.The highest air concentration of?₁₂NPAHs occurred at a heavy traffic site Donggang Bridge?DGB?among 18 sampling sites for both winter and summer season.The lowest air concentration was observed at the background site Guantangou?GTG?.The total concentration of 12 NPAHs were mainly dominated by 1-nitro-naphthalene?1N-NAP?and 2-nitro-naphthalene?2N-NAP?in ambient air,accounting for 32%in winter and45%in summer of?₁₂NPAHs.The ratios of?₁₂NPAHs/?₁₅PAHs were calculated to identify potential sources of NPAHs across the Lanzhou valley basin.The results showed that the ambient air concentration ratios of?₁₂NPAHs/?₁₅PAHs ranged from0.0057 to 0.049 with the mean at 0.021±0.012 in winter and ranged from 0.021 to0.21 with the mean of 0.063±0.046 in summer.The average ratio of?₁₂NPAHs/?₁₅PAHs in summer is about three times higher than that in winter,indicating that the higher temperature in summer promoted the degradation of PAHs and secondary formation of NPAHs in Lanzhou valley.Potential sources of NPAHs were identified in winter and summer based on the correlations among 12 NPAHs,NPAHs and parent PAHs,NPAHs and criteria air pollutants?O₃,NO,NO₂,NO_X?.The results showed NPAHs might be attributed to the primary emissions in winter and the secondary reaction formation in summer.The results showed that higher O₃concentration in industrial district site?Xigu Lanyuan Hotel?in summer might promoted the secondary formation of NPAHs.The higher NO₂ concentration in summer of downtown site?Lanzhou Environmental Monitoring Station?probably played an important role in secondary formation of NPAHs.?2?Observation of dry deposition flux and source identification of atmospheric NPAHs based on PAS-DD in Lanzhou valley:Dry deposition fluxes of?₁₂NPAHs for 18 passive sampling sites ranged from1.4×10² ng/?m²·d?to 6.4×10³ ng/?m²·d?in winter and from 2.3×10² ng/?m²·d?to 5.6×10³ ng/?m²·d?in summer.The mean dry deposition fluxes of?₁₂NPAHs in the ambient air were 1.8×10³±1.9×10³ ng/?m²·d?in winter and 1.4×10³±1.3×10³ ng/?m²·d?in summer.The highest dry deposition flux of?₁₂NPAHs occurred at a heavy traffic site Donggang Bridge?DGB?among 18sampling sites for both winter and summer season.The lowest dry deposition flux of?₁₂NPAHs was observed at the background site Guantangou?GTG?.7-nitro-benzo[a]anthracene?7N-BaA?made the largest contribution to dry deposition fluxes of?₁₂NPAHs,accounting for 28%in winter and 24%in summer.Source of atmospheric dry deposition of NPAHs were discerned based on the dry deposition fluxes ratios of?₁₂NPAHs/?₁₅PAHs.The results showed that dry deposition flux ratios of?₁₂NPAHs/?₁₅PAHs ranged from 0.018 to 0.16 with the mean at 0.067±0.042 in winter,and from 0.017 to 0.28 with the mean of 0.10±0.073 in summer.Higher ratios in summer indicated that the higher ambient temperature in summer promoted the degradation of PAHs and secondary formation of NPAHs in Lanzhou valley,which leading to the higher ratios of atmospheric dry deposition?₁₂NPAHs/?₁₅PAHs in summer.Sources of dry deposition of NPAHs in winter and summer were preliminarily determined based on the correlation among 12 NPAHs and NPAHs and parent PAHs.The results showed that NPAHs might be attributed to the primary emissions in winter and the secondary reaction formation in summer.?3?Gas-particle distribution,particle size distribution,diurnal concentration characteristics and the difference of source contribution identification of atmospheric NPAHs in Lanzhou valley based on active sampling:The concentration of?₁₂NPAHs in gas phase of the three active sampling sites ranged from 0.48 ng/m³ to 17.1 ng/m³with the mean at 3.43±2.01 ng/m³ in winter,and from 0.24 ng/m³ to 9.14 ng/m³ with the mean of 3.69±1.96 ng/m³ in summer.The concentration of?₁₂NPAHs in gas phase was higher in summer than that in winter,indicating that the strong photochemical reaction in gas phase enhanced the secondary formation of NPAHs in Lanzhou valley.The concentrations of?₁₂NPAHs in particulate phase(PM_2.5 and PM₁₀)were higher in winter than that in summer for three active sampling sites,which might be related to the higher primary NPAHs emissions caused by winter heating.The concentrations of?₁₂NPAHs in PM_2.5 in the three active sampling sites ranged from 1.88 ng/m³ to 17.99 ng/m³ with the mean at 17.28±5.89 ng/m³ in winter,and from 1.31 ng/m³ to 4.01 ng/m³ with the mean of 3.56±0.39 ng/m³ in summer.The concentration of?₁₂NPAHs in PM₁₀ in the three active sampling sites ranged from 1.3 ng/m³ to 31.61 ng/m³ with the mean at 11.8±5.63 ng/m³ in winter,and from1.55 ng/m³ to 7.11 ng/m³ with the mean of 3.16±0.86 ng/m³ in summer.The calculated nocturnal/diurnal ratios of?₁₂NPAHs were less than 1 in winter and higher than 1 in summer,which is presumably mainly due to the higher daytime ambient temperature in summer.The atmospheric photochemical reactions were more active during the daytime,which contributed more secondary formation of NPAHs during the daytime.The average concentration of?₁₂NPAHs in the three sampling sites in winter and summer were that the particle phase(PM_2.5+PM₁₀)>gas phase,indicating that NPAHs were more easily enriched in the particle phase.The proportion of?₁₂NPAHs in gas phase was higher in summer than that in winter,indicating that the strong photochemical reaction of gas phase in summer promoted the secondary formation of NPAHs in Lanzhou valley.The concentration of?₁₂NPAHs in winter at three sampling sites was higher in PM_2.5 than that in PM₁₀,indicating that?₁₂NPAHs in winter were mainly concentrated in PM_2.5.Sources of atmospheric NPAHs were further distinguished in Lanzhou valley based on the characteristic compounds of 2+3N-FLA/1N-PYR ratios,?₁₂NPAHs/?₁₆PAHs ratio,OC/EC ratios and SOC/OC ratios in PM_2.5 and PM₁₀.The results confirmed that NPAHs in winter were mainly from primary emissions in Lanzhou valley.The strong temperature in summer strengthened the process of pPAHs transferring to NPAHs by photochemical reaction.The photochemical reactions either in winter or in summer contributed to the total NPAHs.The SOC/OC ratios results in particulate phase exhibited that NPAHs etc contributed large amount of the SOC in Lanzhou valley.SOC in summer might mainly come from the secondary organic derivative formation such as NPAHs etc.?4?Lung cancer risks of atmospheric NPAHs induced by inhalation exposure in Lanzhou valley:Lung cancer risks induced by inhalation exposure to?₅NPAHs were assessed based on the BaP equivalent toxicity factor.The lung cancer risk values of?₅NPAHs in gas phase of background area,urban area and industrial area in winter and summer were lower than the 1.00×10^-5 risk limit in Lanzhou valley,as well as lower than the risk limit for background area in PM_2.5 and PM₁₀ in winter and summer.The lung cancer risk value of?₅NPAHs was also under the 1.00×10^-5 risk limit value.The lung cancer risk value of?₅NPAHs in PM_2.5 and PM₁₀0 were higher than the 1.00×10^-55 risk limit value in winter for urban and industrial districts.This indicated that local residents would therefore be suffered from the potential lung cancer risk due to inhalation exposure to particle-bound NPAHs in winter.The summer risk value of lung cancer in PM_2.5 and PM₁₀ were lower than the risk limit value of 1.00×10^-55 for downtown,indicating that the downtown residents would therefore be at a lower lung cancer risk due to inhalation exposure to particle-bound NPAHs in summer.However,the lung cancer risk value of?₅NPAHs in PM_2.5 of industrial area was higher than the 1.00×10^-5 risk limit value,which further indicated that the strong photochemical reaction in Xigu industrial area in summer promoted secondary formation of NPAHs in fine particles(PM_2.5),which led to the risk of lung cancer exposure to NPAHs in the residents of Xigu industrial district.The lung cancer risk value of?₅NPAHs in PM₁₀ in the summer of industrial area was lower than the1.00×10^-5 risk limit value,indicating that the residents of industrial area would therefore be suffered from the lower lung cancer risk due to inhalation exposure to PM₁₀-bound NPAHs.In summary,the lung cancer risk values of?₅NPAHs in the gas phase and particulate phase of background area,downtown and industrial area in Lanzhou valley were all higher in winter than that in summer.The lung cancer risk values of?₅NPAHs were all at the order of industrial area>downtown>background area.