Characteristics Of N-Alkanes Pollution In Road Environment In Guanzhong Region In Winter,China

N-alkanes are major organic compounds in atmospheric aerosols,commonly used as markers to investigate the migration and source of particulate matter(PM_2.5).Vehicle emissions are significant anthropogenic sources of n-alkanes,and the vehicle emissions are highly concentrated and distributed in the respiratory belt,which is harmful to human health.In this study,PM_2.5 monitoring sites were selected in the different environments such as urban arterial roads,and road tunnels in the Guanzhong region（GZR）in winter to investigate the concentration,composition,and their sources of n-alkanes.The meteorological elements and traffic volume were observed during the sampling period.Gas chromatography-mass spectrometry was used to quantify the concentrations of n-alkanes in PM_2.5 samples.The levels,carbon number distribution,and daily variation characteristics of n-alkanes measured in different environments were analyzed.The emissions of natural origin and anthropogenic sources contributing to n-alkanes were discussed using a method of principal component analysis（PCA）and multiple linear regression（MLR）.In addition,the emission factors of n-alkanes for gasoline and diesel vehicles were determined based on the principle of mass conservation and multiple linear regression method.The main conclusions are as follows:（1）The concentration of n-alkanes of the PM_2.5 in the Guanzhong region was 222.97±102.82 ng/m³ in winter,and concentration measured in the nighttime was 1.28 times higher than that measured in the daytime.The mass concentration of n-alkanes was mainly contributed by these having carbon number from C16 to C33,with a peak corresponding to C29,accounting for 10.45±2.35%of the total mass level（C16～C38）.The PCA suggested that the n-alkanes was mainly from vehicle emission,plant release and coal burning.（2）The concentration of n-alkanes in the PM_2.5 measured along urban mean roads was258.40±113.08 ng/m³,and the carbon number of n-alkanes was mainly between C16 and C31,having a multimodal distribution patterns.The concentration of n-alkanes depended on the volume of traffic on the road,with obviously diurnal variation,and high concentration occurring on weekdays.Plant emissions contributed 13.42±3.08%to n-alkanes concentration,and n-alkane with C25 was identified as the characteristic species emitted by gasoline vehicles.（3）The concentration of n-alkanes measured inside the Qinling No.1 road tunnel was621.01±396.25 ng/m³ for the left-hand line tunnel with a slope of-2.58%（from Hanzhong to Xi’an,）and 1399.18±1234.85 ng/m³,for the right-hand line tunnel with 2.58%of road slope（from Xi’an to Hanzhong）,respectively.The concentration of n-alkanes in the tunnel had a cumulative effect,with higher concentration farther away from the entrance.The carbon number distribution of n-alkanes in the tunnel follows a normal distribution,with a peak between C19 and C22,which had a characteristic carbon number peak value of diesel vehicle emissions.The concentration of n-alkanes with medium and short chain（C<25）maintained a cumulative effect,while the concentration of n-alkanes having a long carbon chain of C27～C34 had no cumulative effect.（4）Based on the principle of conservation of mass,the average emission factors of n-alkanes from vehicle with two slopes were 468.69±226.24μg/（km·veh）for-2.58%of road slope and 1133.35±478.54μg/（km·veh）for slope of 2.58%,respectively.Using the MLR,the n-alkanes emission factors for diesel and gasoline vehicles had been estimated to be 2317.39μg/（km?veh）and 63.35μg/（km?veh）,respectively.