Formation Mechanisms And Sources Of Nitrate In Marine Atmospheric Particulate Matter Based On Stable Nitrogen And Oxygen Isotopes

The intensification of human activities increases the emission of reactive nitrogen,and the reactive nitrogen produced on land can be transported to the offshore,or even transported to remote oceans over long distances through the atmosphere.However,the deposition of atmospheric nitrogen into the ocean will have an impact on the marine ecosystem.Nitrate（NO₃^-）is one of the main components of active nitrogen in marine atmosphere.Tracing the source and formation mechanism of nitrate in marine atmospheric aerosol is of great significance for limiting nitrogen deposition and understanding the geochemical cycle of marine atmosphere.In this study,atmospheric particulate matter samples were collected from the the cruise among the Yellow Sea and Bohai Sea in the offshore waters of Eastern China and from the cruise from East Asia to the Arctic ocean.The characteristics of chemical components in the aerosols in different sea areas,mainly the target substance nitrate,were discussed.And the oxygen isotope anomaly value of nitrate(?¹⁷O-NO₃^-)was used to analyze the formation mechanism of nitrate aerosols in different sea areas,and nitrogen isotope ratio(δ¹⁵N-NO₃^-)was used to analyze the source of nitrate in marine atmosphere:（1）The uncertainty of end member?¹⁷O of atmospheric ozone will increase the uncertainty of analytical results based on?¹⁷O-NO₃^-for the formation mechanism of nitrate.In this study,the determination method of?¹⁷O of ozone was established based on passive sampling,and the determination of?¹⁷O of nitrate was converted to the value of atmospheric ozone through the absorption of ozone by nitrite.This method has the advantages of portability,low energy consumption.It is relatively feasible and can be realized in remote areas.（2）The atmospheric nitrate concentration(7.2±4.5μg m^-3)in the Bohai Sea and the North Yellow Sea,whose backward air mass trajectories mainly passed over the heavily polluted areas in Eastern China,was found to be similar to that in the polluted land,and nitrate accounted for the highest proportion of water-soluble inorganic components,which is consistent with the phenomenon that nitrate dominates aerosol water-soluble inorganic component in eastern urban agglomeration of China in recent years.However,the concentration of nitrate was significantly lower in the South Yellow Sea(1.7±2.4μg m^-3),whose backward air mass trajectories mainly from the open ocean,and was similar to that in the open ocean,and the proportion of sulfate was higher than nitrate.It can be seen that the atmospheric nitrate in the Bohai Sea and the North Yellow Sea is significantly more affected by land transport than that in the South Yellow Sea.Nitrate aerosol concentration was the lowest(0.2±0.3μg m^-3)in open sea areas from eastern Asia to the Arctic Ocean.Except for the East Asian sea area where sulfate dominated the water-soluble inorganic components of aerosol,chloride and sodium dominated the rest of the sea area,indicating that sea salt aerosols are an important component of atmospheric aerosols in remote open ocean.（3）The atmospheric?¹⁷O-NO₃^-in the Bohai Sea and the North Yellow Sea（34.3±3.0‰）was significantly higher than that in the South Yellow Sea（27.4±4.6‰）,the high values in the Bohai Sea and the North Yellow Sea can reflect the influence of continental pollution on the formation mechanism of marine atmospheric nitrate.The atmospheric nitrate of the Bohai Sea and the North Yellow Sea was mainly generated by the O₃ related pathways（59.0±15.1%）,mainly including NO₃+HC/DMS,while nitrate of the South Yellow Sea was mainly generated by the NO₂+OH（43.9±14.7%）pathway.The?¹⁷O-NO₃^-observed during the cruise from Eastern Asia to the Arctic Ocean ranged from 11.5‰to 36.7‰,and decreased with the increase of latitude.There was a significant negative correlation between?¹⁷O-NO₃^-and day length,and the increase of?¹⁷O-NO₃^-during the return journey was due to the increase of NO₃+HC/DMS pathway caused by the increase of night time.The?¹⁷O-NO₃^-values observed in atmospheric aerosols in the sea ice covered area during permanent sunlight day were lower than those in all current marine and polar studies,which is attributed to the enhancing of chemistry about peroxyradicals and hydroxyl radicals with the presence of sea ice.（4）Theδ¹⁵N-NO₃^-values in the Bohai Sea and the North Yellow Sea and the South Yellow Sea were 5.4±2.3‰and-0.5±7.8‰,respectively.The value of the Bohai Sea and the North Yellow Sea is similar to that of the urban agglomerations of North and Northeast China in the Bohai Rim region,while the value of the South Yellow Sea is significantly lower than that of these polluted cities,which is due to the difference in the relative contribution of nitrate and its precursors from continental transport and from local production between the two sea areas.Contributions of atmospheric nitrate from NO_x of land sources in the Bohai Sea and the North Yellow Sea are significantly higher than those in the South Yellow Sea,while contributions from marine local sources in the South Yellow Sea are higher than those in the North Yellow Sea and the Bohai Sea.The mean value ofδ¹⁵N-NO₃^-from Shanghai to Arctic sea was-4.1±2.5‰,which is lower than that of polluted urban agglomeration and offshore Yellow Sea and Bohai Sea,reflecting that the open ocean is less affected by human beings than that of polluted land and offshore sea.The contribution of land sources of NO_x to nitrate in the oceanic atmosphere outside the Arctic Circle is slightly higher than that inside the Arctic Circle.In the process from near-polluted land to open ocean,the contribution of NO_x from continental sources to atmospheric nitrate decreases compared with that from marine local source.