Pollution Characteristics And Seasonal Variation Of Nitroaromatic Compounds In Atmospheric PM_2.5 In Dezhou And Shanghai

Excessive PM_2.5 is an important cause of smog and endangers human health.Detailed study of its molecular composition helps government departments to specify the exact pollution control policy.Nitroaromatic compounds（NACs）are an important part of nitrogen-containing compounds in PM_2.5 and have been observed in various environmental atmospheres.Studying their regional pollution characteristics and seasonal changes will help characterize the nitrogen-containing compounds in atmospheric PM_2.5 and provide scientific support for pollution reduction and carbon reduction policies.In order to understand the differences in sources and compositions of nitroaromatic compounds in different seasons in central and eastern cities of China,this study selected samples of ambient air PM_2.5 in Shanghai and Dezhou in summer and winter of 2018,including 45 samples of Shanghai atmospheric PM_2.5 and 55 samples of Dezhou atmospheric PM_2.5,and each sample was collected for 24 hours.Ultra-performance liquid chromatography tandem high resolution mass spectrometer（UPLC-Orbitrap MS）was used to analyze PM_2.5samples.The non-target analysis software MZMine was used for data processing.Significant achievements have been made in the following two aspects:Firstly,the molecular characteristics of organic compounds in PM_2.5 were clarified.A total of 1147-4628 organic compounds were detected at the two sites.The basic characteristics of PM_2.5 components in Shanghai and Dezhou showed significant differences in concentration and composition in different seasons,different stations and clean/polluted days.The number and relative abundance of organic matter in winter samples of the two places were higher than that in summer.The amount and relative abundance of organic matter in air PM_2.5samples from Dezhou were higher than that in Shanghai.The amount and relative abundance of organic matter in air PM_2.5 samples from polluted days were higher than that in clean days.CHO,CHON,CHOS and CHONS compounds accounted for 49%,25%,16%and 7%of the total compounds in the PM_2.5 samples of Shanghai clean ozone days,respectively.CHO,CHON,CHOS and CHONS compounds accounted for 31%,28%,16%and 22%of the total compounds in Shanghai’s ozone polluted air PM_2.5 samples,respectively.It can be seen that a large number of CHO compounds may be oxidized to form CHON,CHOS and CHONS compounds through photochemical oxidation of SO₂,O₃,and NO_x,which are abundant in the atmosphere.Secondly,the seasonal changes and sources of nitroaromatic compounds were clarified.This study quantified the concentrations of nine NACs in PM_2.5.The average concentration of NACs in PM_2.5 in Shanghai was 5.50 ng·m^-3 in summer and 21.50 ng·m^-3 in winter during the sampling period.The average concentration of NACs in PM_2.5 in Dezhou was 4.77 ng·m^-3 in summer and 163.06ng·m^-3 in winter.It can be seen that there was a significant difference in seasonal concentration between the two stations.4-nitrophenol was one of the important components of NACs,and 4-nitrophenol accounted for a high proportion of the nine NACs in different seasons at the two sites.The concentration of 5-nitrosalicylic acid was higher in summer,but lower in winter,and the seasonal difference was obvious.Based on the correlation analysis,it can be inferred that the secondary generation of NACs was the main source in Shanghai in summer,and the biomass burning emission in winter was more important than the secondary generation of NACs.