Pollution Characteristics And Transport Of Microplastics In Cloud Water

Microplastics have emerged as a novel class of pollutants.Owing to their diminutive size and light weight,microplastics can be transported over vast distances via air and ocean currents,as well as through precipitation such as rainfall and snowfall.Consequently,these particles adversely impact the quality of regional atmospheric environments and escalate ecological risks.Microplastic pollution has thus become a global issue,with these particles detected in polar regions,oceans,and remote areas.However,the presence of microplastics in clouds remains underexplored,and knowledge of atmospheric microplastic transport routes is limited.To investigate the abundance,transport processes,and sources of microplastics in clouds,a single-stage cloud water collector(ACFC-100)was employed to gather cloud water samples from Mount Tai between June 10 and August 10,2021.Microplastic morphology and composition in cloud water were examined using a stereomicroscope,micro-Fourier transform infrared spectroscopy,Raman spectroscopy,and scanning electron microscope energy spectrum analysis.The influence of regional transport on microplastics in cloud water was assessed using the backward flow trajectory model(HYSPLIT),while the sources of microplastics in clouds were analyzed through concentration-weighted trajectory analysis(CWT).The results revealed that the abundance of microplastics in clouds ranged from 0 to 1.08 MP m-3,with an average value of 0.21 MP m-3.A positive correlation was found between microplastic abundance and cloud droplet numbers,liquid water content,and air temperature,while a negative correlation was observed with air pressure(p<0.01).No significant correlation was detected with other cloud water components,such as dissolved ions.There were differences in the abundance of microplastics in cloud water collected during the day and night..Microplastics in Mount Tai’s cloud water appeared in four shapes:fibers(43%),fragments(32%),films(23%),and microspheres(2%).The size range spanned 8-1542 μm,with an average of 80 μm.Approximately 85%of microplastics were smaller than 300 μm,with the 50-100 μm range showing the highest distribution frequency.Six microplastic colors were identified:blue,black,clear,yellow,orange-brown,and white-gray.Black(36%),blue(24%),and transparent(18%)were the predominant colors.The microplastics in cloud water primarily consisted of polyethylene(PE;25.7%),polyethylene terephthalate(PET;24.3%),polypropylene(PP;21.6%),polyamide(PA;9.5%),and polystyrene(PS;9.5%).Scanning electron microscopy results demonstrated that microplastic surfaces were rough,cracked,and had other particles attached,indicating that the microplastics had undergone mechanical aging and chemical weathering within clouds.Microplastics in cloud water were primarily composed of carbon and oxygen and contained or carried elements such as silicon,mercury,and lead.Experiments on microplastic aging revealed differences in surface element compositions based on various exposure paths.Cloud water adsorption tests confirmed that microplastics had adsorbed small amounts of heavy metals,such as Pb and Hg.Different microplastic components exhibited varying adsorption capacities for heavy metals;PA had the strongest adsorption capacity,followed by PE,with no heavy metals detected in PS and PP.The analysis of microplastic sources in Mount Tai’s cloud water revealed that the shape,size,and composition of microplastics varied across different airflows.Microplastics with the largest proportions in local,southwest,and southeast streams measured above 500μm,below 50 μm,and between 50-250 μm,respectively.The abundance of microplastics was influenced by both airflow height and anthropogenic emissions,with the contribution potential of terrestrial sources surpassing that of marine sources.Regions such as Shandong,Jiangsu,Henan,and Guangxi contributed to the presence of microplastics in clouds,which may originate from everyday plastic products like clothing and packaging bags.Secondary contributions from agriculture,open garbage dumps,waste incineration,and the ocean can also serve as potential sources of microplastics in clouds.This study discovered a significant amount of microplastics in clouds,with their abundance and physicochemical characteristics resembling those previously reported for atmospheric microplastics.This finding demonstrates that microplastics can be transported to clouds above the boundary layer.The presence of microplastics may lead to more complex in-cloud reactions,affecting cloud lifetime and optical properties,which in turn may influence reflectance and global radiation estimates.This research contributes to our understanding of microplastics in the atmospheric environment and provides a data foundation for further investigation into the atmospheric distribution and global circulation of microplastics.