Aging Characteristics Of Individual Aerosol Particles During Long-range Transport In East Asia

Ambient aerosol particles are one of the major reasons for atmospheric pollution,causing a great impact on radiative balance,hydrologic cycle,climate,and human health.Due to the rapid development of the economy and society,the increasing energy consumption led to vast emission of air pollutants,causing severe air pollution issue.East Asia is one of the most populous areas in the world,also is one of the largest and strongest sources of aerosols and gaseous pollutants.During winter and spring monsoons,large amounts of aerosols and gaseous pollutants from the Asian continent can be transported to the North Pacific Ocean,North America,and the Arctic,resulting in a global air pollution event.The mixing structures of aerosols were changed during transport,alter the physicochemical properties of aerosol particles.Current studies on Asian air masses outflow mainly focused on coastal areas,offshore islands,and marginal seas,few investigated the Northwest Pacific Ocean(NWPO),and analysis on marine aerosols using single particle techniques was rare.In this study,aerosol samples were collected at three sites and two cruises in NWPO during 2013 and 2014.Transmission electron microscope(TEM),scanning transmission electron microscope(STEM),atomic force microscope(AFM),and nanoscale secondary ion mass spectrometer(NanoSIMS)were used to obtain the composition,size,morphology,and mixing states of aerosol particles.Based on the information,we investigated the influence of continental air masses transport on marginal seas,Kyushu Island of Japan,and NWPO.Fe acid dissolution hypothesis is that acids formed from anthropogenic and natural emissions can dissolve iron(Fe)in aerosol particles.However,this hypothesis has not been confirmed by field observations.In order to provide indisputable evidence of the Fe dissolution process,we collected aerosol samples during a cruise over the Yellow Sea.The single-particle analysis shows that Fe-rich particles emitted from coal combustion and steel industries were coated with thick layers of sulfate,and the Fe was soluble in the acidic sulfate coating.A series of standard samples were prepared in the laboratory to verify our field results,including coal fly ash and Fe-rich particle samples collected from a coal-fired power plant and a steelworks,sulfuric acid,oxalic acid,and ammonium sulfate.No sulfur and soluble Fe was detected in directly emitted Fe-rich and fly ash particles.The sulfate coating on the fly ash and Fe-rich particles collected over the Yellow Sea was formed via gas-to-particle conversion in the atmosphere,and the soluble Fe in the sulfate coating was from acid dissolution process.This research provides undisputed evidence for acid dissolution,the soluble Fe deposition to the ocean can release bioavailable iron,stimulate the growth of microorganism in the surface of the ocean.Aging of black carbon(BC)has a significant impact on its shape,size,hygroscopicity,and optical properties.In order to investigate the evolution of BC particles as they were transported,aerosol particles were collected at an inland urban site(Jinan,T1)and a coastal urban site(Qingdao,T2)in China,and a coastal rural site(Amakusa,T3)in southwestern Japan.Based on the mixing structures of BC and sulfate,they were classified as bare-like,partially embedded,and fully embedded.Results show that 71%of the BC particles were bare-like at T1,70%were partially embedded at T2,and 83%were fully embedded at T3,suggesting the significant evolution of BC mixing state during transport.Shape factors(SFs)of BC particles were used to indicate their morphological differences from T1 to T3.A circle has a shape factor(SF)of 1.0,lower SF suggests more fractal morphology.Bare-like BC particles at T1 and T2 exhibited a low SF(0.20),indicating their low aging degree.No bare-like BC particle was observed at T3,partially and fully embedded BC particles both had higher SFs(0.69 and 0.73,respectively)at T3 than at T1 and T2.This suggested that secondary aerosols modified the chain-like BC particles to become more compact and eventually fully coated.Significant size growth of BC particles was observed during transport,the particle to BC core diameter ratio(Dp/Dcore)were 1.42,1.78,and 2.49 at T1,T2,and T3,respectively.The increase rate of the coating volume was 50%per hour from T1 to T2 in the dry continental air,much higher than 20%per hour from T2 to T3 in the humid air.Mie theory was used to obtain the optical absorption of individual BC-containing particles at T1,T2,and T3.The results show that the average absorption cross section(?abs)increased from 0.10 at T1,to 0.22 at T2,and to 0.29 at T3.These results suggest that the optical absorption of individual BC particles increased because of their aging during long-range transport.Aerosol particles were collected onboard R/V Dong Fang Hong 2 during the cruise over the East China Sea and the NWPO between 17 Mar.and 22 Apr.2014.In the East China Sea,86.9%of observed particles are anthropogenic particles,the dominated types were S-rich and S-soot particles;sea salt aerosols(SSA)from the natural source were the main type with the percentage of 89.8%in the NWPO.The result suggests that the transport of continental air masses have the greater impact on the East China Sea than the NWPO.Secondary aerosols(i.e.,S-rich and OM coating)accounted for higher percent in the East China Sea than in the NWPO,showing the high aging degree in the East China Sea.SSA particles dominate marine aerosols in the NWPO.Based on the composition and morphology,SSA particles were categorized as fresh SSA,partially aged SSA,and fully aged SSA.For the samples with a high percentage of aged SSA,their air masses were mostly from the northwest(China and Japan)where SO2 and NO2 emission are huge.The aging degree of SSA increased along with the ship sailing westward.The aging of SSA was mainly caused by chlorine depletion due to anthropogenic gaseous pollutants(e.g.,SO2 and NO2).Therefore,in addition to aerosol transport,the transport of Asian gaseous pollutants in the troposphere also has an impact on deep-sea aerosols.This study shows that Asian continental air mass transport has an important influence on the atmosphere of the marginal sea of China,Kyushu island of Japan,and the deep sea of the NWPO.The aging process during air mass transport changes the physical and chemical properties of aerosols.The relative abundance and mixing state of aerosol particles in this study can also provide essential data for further studies and model simulation.