| At present,microplastics(MPs,plastic less than 5 mm)has attracted great attention worldwide as a new type of atmospheric particle pollutants that are prevalent in multiple habitats and media,and have the property of long-distance transport.In recent years,MPs have been found to exist in terrestrial indoor and outdoor atmospheric environments,however,research on marine atmospheric MPs is extremely scarce.Atmospheric transport has been recognized as an important route for terrestrial MPs entering the ocean since the early 2019s,yet the link between terrestrial atmospheric MP emissions and ocean impacts remains unclear.Due to the impacts of marine atmospheric MPs on climate change,marine ecological environment and marine atmospheric chemical processes,their characteristics,source-sink and transport in the marine atmosphere are hot issues of global concern,but the relevant researches are currently very limited.This not only limits the understanding of global plastic source-sink dynamics,but also hinders systematic assessment of its potential ecological risks.In this thesis,a series of studies were conducted in the South China Sea(SCS),Western Pacific Ocean(WPO),and Eastern Indian Ocean(EIO),focusing on the characteristics,source-sink,and transport of marine atmospheric MPs:1)In the 2019 National Natural Science Foundation of China(NSFC)study on the EIO voyage,the characteristics and potential sources of atmospheric MPs in the Pearl River Estuary(PRE),SCS,and EIO were investigated through 21 sampling transects from the PRE to the SCS and then to the EIO.It was found that the atmospheric MPs are dominated by MP fiber,with Polyethylene terephthalate(PET,50.0%)as the most abundant component.The results showed that the abundance of MPs over the PRE(4.20±2.50 items/100 m~3)was significantly higher than that over the EIO(0.40±0.60items/100 m~3).However,the abundance of MPs over the SCS(0.80±1.30 items/100m~3)was not significantly different from that over the EIO and the PRE,but both were two to three orders of magnitude lower than that in the terrestrial atmospheric MPs.By comparing the average abundance of MPs in the Intertropical Convergence Zone(ITCZ)and adjacent regions,we hypothesized that the ITCZ may be the receiving center of the south-north transport of atmospheric MPs over ocean.In addition,the backward trajectory model analysis of 21 sampling transects revealed that the atmospheric MPs over the SCS may potentially originated from the Philippines.The atmospheric MPs over the EIO is probably from the Indian subcontinent.2)The abundance of atmospheric MPs over ocean is two to three orders of magnitude lower than that in the terrestrial atmosphere,but the distribution characteristics of atmospheric MPs from coastal cities to offshore waters to the pelagic are limitedly understood.Therefore,in the 2019 NSFC WPO voyage study,we conducted 32 sampling transects in the WPO to not only study the characteristics and potential sources of atmospheric MPs in the WPO,but also to explore the distribution characteristics of atmospheric MPs from coastal cities to offshore waters and then to the pelagic.Similarly,the atmospheric MPs over the WPO are also dominated by MP fibers,and PET is the most common component(67.24%).During the sampling period,the average abundance of atmospheric MPs over the WPO was 0.84±0.70 items/100m~3,which was also two to three orders of magnitude lower than terrestrial atmospheric MPs.Data regression analysis showed that the abundance of atmospheric MPs at sampling stations was power exponential with the mean longitude,and the abundance of atmospheric MPs decreased sharply from Shanghai,China to the East China Sea and then to the WPO.The average abundance of MPs over the WPO(0.43±0.29 items/100m~3,160°E)thousands of kilometers offshore was only 0.30%of that in Shanghai(142.0±142.0 items/100 m~3).The results of the WPO and EIO voyages improve our understanding of the occurrence characteristics and source-sink mechanisms of atmospheric MPs over ocean,and provided reliable baseline data for further model assessment of global terrestrial atmospheric MP transport to the ocean.3)Previous studies mainly focused on the transport of MPs from the land to the ocean,but neglected the transport of atmospheric MPs from the ocean to the land.Based on the research results of atmospheric MPs in the WPO and EIO voyages,we speculate that there is a transport mode of atmospheric MPs from the oceanic atmosphere to the land.During the East Asian Summer Monsoon(EASM)in 2020,an atmospheric MP monitoring study was conducted in the southeast coast of China on the NSFC SCS-PRE voyage.The average abundance of atmospheric MPs in the sampling area during the EASM was 0.39±0.43 items/100 m~3,with high variation between sampling sites.Backward trajectory models and wind fields indicate that the EASM provides a transport pathway for atmospheric MPs from Vietnam,the Philippines,and Malaysia to southeast China.The estimated results showed that EASM transports atmospheric MPs back onto the terrestrial environment at a flux of up to 212.98-213.43kg/EASM/year.Analysis of the data found no significant differences in atmospheric MPs from Vietnam,the Philippines,and Malaysia.This study improves our understanding of the impact of the EASM on atmospheric MP transport and helps to quantify and calibrate the contribution of atmospheric MPs to marine or terrestrial ecosystems.4)Wet-dry deposition plays an important role in the transport of materials to the global ocean,yet there is a lack of researches on the wet deposition of MPs over ocean.Eighteen oceanic wet deposition samples were collected during typhoons Sinlaku and Nuri in 2020 by two NSFC SCS-PRE cruises.This study showed that the abundance of wet deposition of atmospheric MPs over ocean during typhoons was 4.86±2.82items/L,and the flux was 1853.47±2435.37 items/m~2/day,which was higher than other non-typhoon land-based wet deposition studies,indicating that the strong winds and heavy rainfall brought by typhoons aggravate the suspension and deposition of MPs.In addition,the size of MPs in the wet deposition samples was significantly larger than that that of atmospheric MPs over ocean,which indicated that the updraft and turbulence accompanied by heavy rainfall during the typhoon would carry larger MPs into the air before settling into the ocean.This study reveals for the first time the wet deposition characteristics and fluxes of marine atmospheric MPs during typhoons,improving our understanding of the processes and sources of wet deposition of marine atmospheric MPs during extreme weather.In summary,this thesis reveals the prevalence of MPs in the atmosphere in different regions of the ocean through field investigations on a transoceanic scale.Atmospheric MPs over ocean are mainly dominated by MP fibers,with PET as the most abundant component.The backward trajectory model and MP characteristics reveal the potential source of these MPs in the oceanic atmosphere.This study not only shows that the East Asian summer monsoon is the transport pathway of atmospheric MPs from marine environment to land,but also estimates the transport flux of atmospheric MPs in the EASM through the estimation model.Meanwhile,the study reveals the wet deposition characteristics and fluxes of atmospheric MPs over ocean during typhoons.This study provides a new theoretical basis for the land-ocean MP transport,increases the understanding of the source-sink process of MP in the marine environment,facilitates the improvement and validation of terrestrial atmosphere MP to the sea transport model,and provides a new scientific basis for the global plastic cycle and plastic pollution control. |
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