Molecular Characteristics And Concentration Levels Of Organosulfates In The North And South Atmospheric PM_2.5

In this paper,based on the high resolution Orbitrap mass spectrometry combined with ultrahigh performance liquid chromatography(UPLC-Obitrap MS),a systematic comparison of the pollution characteristics of organosulfates is carried out for methanol extracts of atmospheric PM2.5 from Beijing and Guangzhou in four seasons in 2016 and 2017 which are typical places in north and south.The study explores the regional and seasonal variation of organosulfates from the perspective of composition with structure and concentration level,which can provide data for further exploration of the source and precursors as well as the conversion and generation mechanism of organosulfates in the atmosphere.The metabolomics software MZmine2 was used to untargeted analysis for full scan data of UPLC-Obitrap MS,and the average speices number of screened sulfur-containing compounds from four season samples in Beijing and Guangzhou is as high as 6500 to 8500,and highest values were found in winter and in summer,respectively.There are regional differences in the precursors and conversion mechanisms of sulfur-containing compounds in particulate matter between the two places.The analysis results of the characteristic fragment ions by the tandem mass spectrometry and the sulfur element valence on the surface of fine particles by the X-ray photoelectron spectroscopy(XPS)confirmed that the sulfur-containing organic compounds in PM2.5 mainly existed in the form of organosulfates.The speices number of OS(CHOS)compounds in samples from both places accounted for more than 70%,which exist more isomeric types than NOS(CHNOS)compounds.The organosulfates in Guangzhou PM2.5 are observed with a higher degree of oxidation and a lower degree of isomerization.The speices number of NOS compounds in Beijing samples is much higher than that in Guangzhou samples,indicating that the atmospheric nitrogen oxides pollution is serious in Beijing cities,where more NOx is involved in the formation of NOS compounds.The first 10%of the peaks in both places had the largest number of aliphatic OSs,accounting for more than 50%,and and the proportion of terpenoid OSs is close to 30%.The structure of 265 kinds of organosulfates were confirmed in sample BJ20161128 by characteristic fragment ions,including 179 aliphatic OSs,29 terpenoid OSs,54 aromatic OSs and 3 C1-3 OSs.The high-abundance OS species in both places are aliphatic OSs of which carbon number is even.The high-abundance species in Beijing samples are C8H17O4S1-,C10H19O5S1-,and C18H35O6S1-;The high-abundance species in Guangzhou samples are C12H25O4S1-,C14H29O5S1-and C16H33O6S1-,and C24H47O6S1-has high abundance in both samples.The high-abundance series of aliphatic species which have even carbon dominance,indicated the important influence of anthropogenic sources.There are monoterpenoid nitrogen-containing OSs with high abundance in the samples from both places such as C10H16N1O7S1-,C9H14N1O8S1-and C10H18N1O8S1-,showing the consistent biogenic origin of organosulfates from the north and south.The high-abundance compound C10H16N1O7S1-(m/z 294)which is ubiquitous in the samples has 6 to 7 isomers.PM2.5 in Beijing contains a large amount of highly condensed aromatic OSs with C6-26,O4-10,and DBE of 4-19,whose molecule contains up to 6 aromatic rings(DBE=19),which is the main characteristic that distinguishes it from Guangzhou samples.Only a few aromatic OSs with C22-33 and DBE of 10-14 were identified in Guangzhou that have high molecular weight and high carbon number.The two types are the new organosulfates identified for the first time,supplementing the lack of understanding of aromatic OSs in molecular composition.For the first time,different types of organosulfate standards in structure were used for quantitative/semi-quantitative analysis of various types of OSs.The terpenoid OSs are richest in content of samples in both places,and the content in Guangzhou is generally higher than in Beijing.The concentration of aliphatic and aromatic OSs in Beijing PM2.5 in winter is much higher than that in Guangzhou.The highest average concentration of OSs in Beijing PM2.5 was at 60.59 ng/m3 in winter,and the contribution of OSs to PM2.5 in four seasons were 0.05,0.10,0.10 and 0.13 wt%in the order of spring,summer,autumn and winter,respectively.While the highest average value in Guangzhou occurs in summer,at 55.06 ng/m3,and the contribution of OSs to PM2.5 in four seasons were 0.12,0.19,0.13 and 0.11 wt%in the order of spring,summer,autumn and winter,respectively.Guangzhou may have more serious organosulfate pollution than Beijing.The organic sulfur content of methanol extracts of PM2.5 in Beijing was determined by the trace sulfur and nitrogen analyzer.The highest concentration of organic sulfur is 0.96 μg/m3 in winter;the lowest concentration is 0.23 μg/m3 in spring.The contribution of organic sulfur to PM2.5 ranges from 0.19 to 2.00 wt%.The contribution of the quantified OSs in this paper to organic sulfur is 4.60,3.11,3.39 and 3.33 wt%in the order of spring,summer,autumn and winter,respectively.Although this study makes full use of the data obtained with comprehensively screened,and it also takes into account that electrospray ionization(ESI)source has a large difference in the ionization efficiency of organosulfates with different structures,and standards of different structure types were used to quantify separately,there is still a big gap between the quantified concentration and the actual inventory of organosulfates in the atmosphere.For complex environmental systems such as aerosols,more standards need to be developed and synthesized for quantitative analysis.