Atmospheric Deposition Of PAHs And Its Influencing Factors In Changping District,Beijing

PAHs are a class of persistent organic pollutants that are mainly derived from various combustion processes and are widely present in the atmosphere.Dry and wet deposition is the main removal process of atmospheric PAHs.In order to explore the characteristics and mechanism of atmospheric wet and dry deposition of PAHs in Changping District,atmospheric dry and wet deposition samples were collected from January to November in 2019.An orthogonal test was used to establish a water-soluble PAHs extraction method.A total of 16 priority PAHs were determined by gas chromatography/mass spectrometry（GC/MS）,and the dry and wet deposition fluxes,chemical composition and ring distributions were analyzed.The correlation analysis was used to explore the effects of meteorological factors,air quality and other parameters on the PAHs deposition fluxes.The conclusions were as follows:（1）Through the range analysis of orthogonal test,the experimental scheme was determined to be 250 m L water sample,10 m L dichloromethane was used as the extractant,and the extraction time was 10 min to obtain the highest recovery rate of PAHs.（2）The dry deposition flux of the priority PAHs was averaged to be 2.20μg/m²/d,which was the highest in winter and the lowest in summer.The dry deposition flux presented the trend as traffic zone>educational–residential–commercial mixed zone>agricultural zone.PHE and FLUA were the two most dominant compounds in dry deposition samples.In SJ and GY,2 to 3 ring PAHs accounted for the largest proportion,while the 4 rings was the highest in TYG.The wet deposition flux of the priority PAHs was about 20.11μg/m²/d.The dominant PAHs in wet deposition were PHE,FLUA and PYR.The proportion of the 5～6 ring was higher than dry deposition,indicating that the erosion effect of atmospheric wet deposition on particles（especially fine particles）was higher than that of particles dry deposition process.In dry and wet deposition samples,the deposition flux of water-soluble PAHs was much higher than that of particulate PAHs.（3）The total concentration of the priority PAHs in precipitation was 1276 ng/L.The maximum and minimum concentration appeared in December and July,The maximum and minimum concentration appeared in December and July,respectively,corresponding to precipitation minimum and maximum values.（4）In 2019,about 23.9 kg of PAHs settled to the surface of Chengbei Street of Changping District,of which 12.9 kg of PAHs was in the form of dry depositiont,and about 11.0 kg of PAHs was in the form of wet deposition.The frequency of wet deosition was low,but the deposition flux was high,the concentration of PAHs in precipitation was high,and the removal efficiency of high-ring-number PAHs attached to particles was high,so it was an important process of atmospheric PAHs deposition in Changping District.（5）Water-soluble PAHs dry deposition flux was significantly positively correlated with the concentration of CO and SO₂ in the atmosphere,indicating that gaseous PAHs in atmospheric dry deposition mainly derived from incomplete combustion of fuels,especially coal-burning process.The deposition flux of particulate-associated PAHs had a significant correlation with PM_2.5 and PM_2.5-10 concentrationin in the atmosphere,and the correlation with PM_2.5-10 was better than PM_2.5,indicating that the concentration and particle size of PAHs in atmospheric particles had a significant impact on deposition flux of particulate PAHs.The pollution control of PAHs in the atmosphere should consider the cooperative control with SO₂,CO and particulate matter.Temperature,relative humidity and wind speed were all related to PAHs dry deposition flux.These factors could affect the concentration or deposition velocity of PAHs in the atmosphere,which in turn would affect the dry deposition flux of PAHs.Regression analysis showed that relative humidity and wind speed had the greatest impact on the dry depositiom flux of PAHs,followed by SO₂concentration,temperature,and PM_2.5 concentration,and the least affected CO concentration.（6）The backward trajectory study showed that the air masses from the central and western regions of Inner Mongolia may carry a large amount of PAHs,increasing the atmosphere concentration of PAHs of Changping,and the higher wind speed increased the deposition velocity of PAHs,thus increasing the dry deposition flux of PAHs.（7）Particulate-associated PAHs wte deposition flux had a significant positive correlation with precipitation intensity,proving that heavy rainfall could wash more particulate matter and attached PAHs to the surface,and increase the wet deposition velocity of PAHs in atmosphere particulate.