| The chromophores in atmospheric aerosols can effectively absorb solar radiation,which may have a significant effect on the radiation balance of the earth and the photochemical aging of atmospheric aerosols.According to some studies,atmospheric chromophores can be formed or eliminated by chemical reactions such as liquid phase reactions,photochemical synthesis or decomposition.Therefore,it is essential to determine the type and concentration of chromophore in the atmosphere to understand its source and chemical reaction mechanism,and to further evaluate its potential impact on the environment.In this study,the self-built semi-continuous PILS-EEM-TOC system(Particle-into-liquid-sampler combined with EEM spectroscopy and total organic carbon analyzer)was used to demonstrate that the system can provide instantaneous dynamic trend types and concentrations of water-soluble atmospheric chromophores for particle species.By comparing offline collection of atmospheric samples and filtering methods,the results show that the online PILS-EEM-TOC system analysis can provide higher temporal resolution data support,and to the exploration of dynamic atmosphere of the source of the chromophores,generation and elimination processes.The instrument has great potential applications in other directions of atmospheric observation and the existing online monitoring device for broadening the analysis method,can provide three methods of analysis:UV-vis spectra,fluorescent spectra and total organic carbon.The main research contents of this paper are as follows:The PILS-EEM-TOC online observation system was established.PILS coupled three-dimensional excitation-emission matrix and total organic carbon analyzers have been developed and applied in this study.The absorption spectra and EEM spectra of water-soluble aerosol components and the semi-continuous measurement of water-soluble organic carbon were realized.The particulate matter is absorbed by water vapor and condensed into sample droplets,which are collected in the debubbler.The automatic sample injection pump is controlled by programming.After the collected sample is tested by three-dimensional fluorescence spectrum,the total organic carbon is detected.The stability of PILS-EEM-TOC system was verified.The accuracy of the PILS-EEM-TOC system was determined by comparing offline and online sample analyses of WSOC and fluorescence spectra.It was found that the collection efficiency of the online analysis of WSOC was slightly higher than that of the offline collection samples,the average collection efficiency of the online sample was about 121%,and the uncertainty was 124%.The fluorescence spectrum analysis of the online sample was slightly lower than that of the offline sample,and the average collection efficiency of the online sample was about 88%±15%.This result shows that the online sample analysis is similar to the offline sample analysis.The difference may be due to different particulate matter extraction methods.The variation of chromophores under special meteorological conditions was obtained by verifying observations in spring.The primary source and secondary reaction processes of atmospheric chromophores were observed,and the effects of dust storms and rainy days on chromophores type and concentration were studied.Different from the previous online measurement of atmospheric chromophores based on absorption spectra,the developed PILS-EEM-TOC system can provide three-dimensional fluorescence spectra and use parallel factor analysis for analysis,which can reflect the chemical structure information of complex chromophores in more detail than absorption spectra.Therefore,the PILS-EEM-TOC system can be used to dynamically determine the type,source and chemical reaction mechanism of chromophores.The main source of winter haze is primary source,and Maillard-like chromophores are found in ambient samples.The atmospheric observation was carried out during the period of severe pollution in winter,and the possible influence of meteorological conditions on the characteristics of atmospheric chromophores was found after analysis.By analyzing the types of chromophores,we obtained that about 90%of the fluorescent substances in the winter atmospheric chromophores of Xi’an City came from the primary combustion source,and a small part came from the secondary formation,among which the atmospheric chromophores reached the minimum value in the afternoon.The more organic substances in the atmospheric chromophore of Xi’an are mainly humic-likes substances.It is found that one of the sources rises from night to early morning,which may be due to the influence of the height of the boundary layer and the influence of local heating in winter.The control of air pollution in winter cannot be ignored,and the influence of meteorological conditions is also a part that cannot be ignored.It is found that the aging aerosol is the main chromophores in summer,and the oxidation stage is the most important aging pathway.Although the conversion between chromophores in the atmosphere was mentioned in the previous studies,there was no systematic study.This part attempted to explore the influence of photochemistry and atmospheric oxidation process on the dynamic composition of chromophores in the ambient atmosphere under the condition of both strong light intensity and oxidation process in summer.It was found that a large number of secondary organic aerosols were produced in the afternoon,but most of the secondary organic aerosols were non-fluorescent.The highly-oxygenated humic-like substance is the main chromophores of secondary formation,which may be due to the transformation of the less-oxygenated humic-like substance chromophore.The peak of atmospheric photochemical reaction and oxidation reaction occurs in the afternoon,which drives the conversion of atmospheric chromophores,and the main way of conversion is atmospheric oxidation process.At the same time,fluorescence index and humification index were used to measure the aging process of aerosols.It was found that the degree of humification of aging aerosols was higher and the fluorescence index was lower.It is speculated that the aerosol is mainly in the accumulation process of local sources from night to morning,and gradually aging process exists in the noon and afternoon.This finding will contribute to a better understanding of the atmospheric chemistry of chromophore aerosols and provide guidance for the EEM approach to tracer aerosol aging in the atmosphere. |
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