Observation And Simulation Of Atmospheric Polycyclic Aromatic Hydrocarbons In The North China Plain

Polycyclic aromatic hydrocarbons（PAHs）are a group of persistent organic pollutants that seriously endanger human health.Currently,the North China Plain is one of the most polluted regions of atmospheric PAHs in China,and it is urgent to solve this pollution.Developing scientific and effective prevention and control measures is crucial to solving PAHs pollution,and this requires a comprehensive understanding of its characteristics and contributing factors.However,the current understanding of atmospheric PAHs in this region remains incomplete in terms of diurnal variations,weather effects,transport,sources,and risks.Specifically,（1）the dominant factors that affect the diurnal variations of PAHs and its pollution characteristics during fog and haze periods remain uncertain;（2）the differences of atmospheric transport between PAH components are not clear;（3）the latest sources and risks of atmospheric PAHs in the entire North China Plain area are still unknown.These insufficient understanding poses a serious limitation to the establishment of targeted prevention and control measures for atmospheric PAHs in the North China Plain.For this reason,this study conducted simultaneous observations at the summit and foot of Mount Tai(MT_summit and MT_foot)in May and June 2017,and performed a series of numerical simulations using an air quality model that included 16priority control PAHs.The main content and results of the research are presented below:In terms of diurnal variations,this study preliminarily analyzed the diurnal characteristics of atmospheric PAHs in the North China Plain based on observational data.The concentration of PAHs was found to be higher during the day（2.91 ng/m³）than at night（1.88 ng/m³）in MT_summit while the opposite was observed at the MT_foot（6.45 ng/m³ during the day and 11.75ng/m³ at night）,indicating that PAHs have a higher day/night ratio in mountainous than in urban.Subsequently,this study introduced numerical simulations to evaluate the impacts of chemical reactions and atmospheric diffusion on the diurnal variations of PAHs.OH radical was the most important atmospheric oxidant affecting the diurnal variations of PAHs,since the associated PAHs have the highest removal rate and significant diurnal variations,with a daytime rate of76.22%and a nighttime rate of 46.54%across the entire simulation domain.The strong atmospheric diffusion during the day significantly reduced the near-surface concentration of PAHs but increased their concentration in the upper air,which is consistent with the observations.Even though the nighttime emissions of PAHs were lower than those during the daytime,weaker atmospheric diffusion still led to higher near-surface concentrations at night.We also compared the strength of atmospheric diffusion and chemical reaction on the diurnal trends of PAHs.Near the ground,atmospheric diffusion was the most dominant factor in determining their diurnal trends.At high altitudes,their diurnal trends were determined by a combination of atmospheric diffusion and chemical reactions.In terms of weather effects,this study analyzed the mechanism of PAHs pollution during fog and haze in the North China Plain based on observational data.The significant reduction of PAHs concentrations during fog periods was related to the wet deposition of fog droplets,and the decrease in the proportion of low-ring PAHs was attributed to their higher solubility corresponding to stronger wet scavenging.The increase in PAHs concentrations during haze and fog-haze periods was related to unfavorable atmospheric conditions.The change in the proportion of each component during the haze period may be caused by the change of emissions and chemical reactions,while during fog-haze periods,it was related to the wet deposition process determined by solubility.Furthermore,this study focused on the impact of chemical reactions and atmospheric diffusion conditions on PAHs pollution during haze periods through numerical simulations.The reactions between PAHs and atmospheric oxidants were stronger during haze periods than during clear days,with the largest contribution of OH radical.Not only did the consumption of PAHs increase during hazy periods compared to clear days,but the consumption ratio also rose.The unfavorable atmospheric conditions during haze periods resulted in the vertical distribution of PAHs being concentrated close to the ground,which is the dominant factor in increasing near-surface PAHs.In terms of atmospheric transport,this study preliminarily analyzed the characteristics of particle-phase PAHs atmospheric transport in the North China Plain based on observational data.The results showed that local vertical transport was an important source of daytime PAHs at MT_summit,especially for 3-ring components,while regional horizontal transport was an important source of nighttime PAHs,especially for 4-7 ring components.The analysis of typical transport processes revealed that the regional horizontal transport capacity of particle-phase PAHs increased with the number of rings,while the local vertical transport capacity decreased with the number of rings.Subsequently,this study quantified the contribution of each source region to the PAHs at the observation sites by numerical simulation.Local emissions and transport from surrounding provinces were found to be the main sources of PAHs in Mount Tai,with contributions reaching 97.93%and 99.80%at MT_summit and MT_foot,respectively.In addition,this study focused on the mechanisms that cause differences in atmospheric transport of PAH components based on simulated source region results.The results showed that 5-and6-ring PAHs had the strongest transport capacity,followed by 2-and 4-ring components,and the weakest were 3-ring components,which depended on the reactivity.The vertical transport capacity of particle-phase PAHs was enhanced with decreasing vertical temperature,particularly for low-ring components.In terms of sources and risks,this study analyzed the sources of atmospheric particle-phase PAHs in the North China Plain using diagnostic ratios and positive matrix factorization（PMF）based on observational data,as well as calculated the lifetime excess carcinogenic risk（ECR）.The results of the diagnostic ratios indicated that coal and biomass combustion sources contributed most to the atmospheric PAHs at the observation sites.The contribution of coal and biomass combustion sources was further quantified using the PMF model,reaching up to 43.51%and 58.38%at MT_summit and MT_foot,respectively,while the contribution of vehicle emission sources was also significant.The ECR of PAHs were 2.49×10^-5 and 1.06×10^-4 at MT_summit and MT_foot,respectively,and more than 85%were derived from vehicle emissions,coal,and biomass combustion sources.In addition,we quantified the sources and risks of atmospheric PAHs in the entire North China Plain by simulation based on the latest version of the PAHs emission inventory published by Peking University.The results showed that the average concentration of PAHs in the North China Plain was 76.43 ng/m³（68.45 ng/m³ in the gas phase and 7.97 ng/m³ in the particle phase）and the average value of ECR was 1.64×10^-4(3.00×10^-5in the gas phase and 1.34×10^-4 in the particle phase).The emissions associated with anthropogenic activities resulted in the North China Plain being one of the regions with the highest atmospheric PAHs in China.The residential/commercial sector was the main source of atmospheric PAHs in the North China Plain,contributing 45.05%of the total concentration,and was also responsible for a high proportion of the total ECR,accounting for as much as72.24%.The industrial sector was the second largest contributor,accounting for 24.82%of the total PAHs concentration and 19.18%of the total ECR.The above findings could increase the understanding of the characteristics and influencing factors of atmospheric PAHs pollution in the North China Plain,providing scientific basis for the government to achieve precise prevention and control.