| Polycyclic aromatic hydrocarbons(PAHs)and nitro-PAHs are persistent organic pollutants(POPs)widely existing in the atmospheric environment.They are not only harmful to human health,but also important precursors and components of organic aerosols(OAs).Common off-line analytical techniques such as gas chromatography(GC)and high-performance liquid chromatography(HPLC)combined with various sensitive detectors can carry out quantitative and qualitative analysis of PAHs in the atmosphere.These techniques are based on the bulk measurement of aerosols and play a great role in the study of the health effects of PAHs and the source analysis of aerosol.However,due to the complexity of aerosol components,only analyzing the overall distribution characteristics of PAHs is difficult to explore the detailed transformation process of PAHs from a primary emission source to the formation of OAs.The analysis of aerosol mixing state provides a feasible research way to explore the source change and aging process of PAHs in real time.Due to the large molecular weight and low atmospheric content of PAHs,the mixing state of PAHs has not been reported by the existing aerosol time-of-flight mass spectrometer(ATOFMS)and single particle aerosol mass spectrometer(SPAMS).Based on the upgraded high-performance single particle aerosol mass spectrometer(HP-SPAMS),this paper studies and detects the common PAHs and nitro-PAHs standard samples in the ambient atmosphere through laboratory research,analyzes their mass spectrum characteristics,establishes the characteristic ion recognition method,comprehensively evaluates the applicability and reliability of this method combined with a series of verification,and applies it to the analysis of practical atmospheric single particles.1.Establishment and evaluation of analytical methods: In this study,11 kinds of PAHs and nitro-PAHs standard samples were analyzed in the laboratory by HP-SPAMS,and the identification methods of each compound were obtained: "parent ion" and "m/z= 77" characteristic peak determination method.It is found that adjusting different laser energies(0.5,0.7 and 1.0 m J)has little effect on the determination of parent ions.Since PAHs and nitro-PAHs in atmospheric particulate matter are usually mixed with other primary and secondary organics,and these organics will also produce fragment ions with mass charge ratio(m/z)greater than 150 amu,in order to eliminate the interference of these substances on the identification of PAHs and nitro-PAHs,comparative experiments were carried out on the particulate matter emitted from ambient atmosphere,cooking and biomass combustion with and without PAHs and nitro-PAHs standard samples.Compared with similarity algorithm and deep learning algorithm,the reliability of feature ion extraction method is evaluated.In addition,PAHs particles in vehicle exhaust and atmospheric environment are analyzed by this method,and it is found that there are obvious differences in their mixing states.2.Application of analysis method: From September 2 to September 16,2019,HPSPAMS were used to observe in Xianlin Campus of Nanjing University in Nanjing.The identification method established in this paper was applied to the analysis of actual atmospheric single particles,and the particles containing PAHs and nitro-PAHs were discussed based on the identification method.A total of 4,698,666 particles with both positive and negative spectrum information were collected during sampling,of which PAHs and nitro-PAHs particles(9826)accounted for 0.21% of the total collected particles.During the whole observation period,the diurnal variation of PAHs containing particles showed the form of "two peaks and one valley".According to the actual situation around the sampling point,it may be due to vehicle exhaust emission,the increase of PAHs containing particles during the early peak and late peak of traffic flow,while the valley value appeared in the afternoon due to meteorological conditions,reduction of traffic flow and other factors.It was found that the temporal and spatial distribution and diurnal variation of 9-nitroanthracene containing particles were like all kinds of PAHs containing particles,it is speculated that it is likely to come from the same pollution source.3-nitrofluoranthene,2-nitrofluorene and 2,7-dinitrofluorene have very low time correlation with other PAHs-containing particles,and there is no correlation between them.It is speculated that the sources of 3-nitrofluoranthene,2-nitrofluorene and 2,7-dinitrofluorene may be different,and the possible sources are biomass combustion,fossil fuel combustion and PAHs photooxidation reaction.3-nitrofluoranthene and 2-nitrofluorene are significantly affected by regional transmission and may undergo aging and secondary formation during the transmission process.In addition,by comparing with the similarity algorithm and deep learning algorithm,the difference between the total number of particles after merging and the number of particles extracted by the recognition method established in this study is very small,and the overall average coincidence degree of the number of particles after the intersection of the three is high.The average similarity between the similarity algorithm and the deep learning algorithm and the spectrum of this study is 0.85 and 0.83,respectively.The characteristic spectrum of particles containing PAHs and nitro-PAHs extracted by the two algorithms has high similarity with the spectrum extracted by the recognition method of this study.Based on the number of particles,spectral similarity and mass spectrum characteristics,the applicability and reliability of the identification method established in this paper are verified.In conclusion,the analytical method developed in this study is based on the highly sensitive,fast,and high-resolution time-of-flight mass spectrometry technology and establishes an analytical method for the mixing state of single particles containing PAHs and nitro-PAHs in the atmosphere.Single particles containing PAHs and nitro-PAHs can be directly identified by characteristic ion extraction.At the same time,this study also provides a new idea to detect other high molecular weight organics in environmental atmospheric particles,such as aromatic oxygenated compounds and heterocyclic analogues.The study of the mixing state of these organics will help to reveal their formation and evolution process. |
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