Various forms and deposition fluxes of microplastics identified in the coastal urban atmosphere

Pollution by microplastics has become a major global issue affecting the marine environment. However, there has been little research on microplastics in the atmosphere. Here, we study microplastics in precipitation samples collected in Yantai, a coastal city in Shandong Province, East China. This paper is the first report on the different shapes, deposition fluxes and seasonal variation in microplastics in the coastal urban atmospheric environment. The samples collected were pre-filtered using a set of stainless steel sieves with mesh sizes of 5 and 1 mm. Residues, such as insects and plant debris retained on a stainless steel sieve of mesh size 5 mm were discarded or archived. Residues retained on a stainless steel sieve of mesh size 1 mm were air-dried and placed under a stereomicroscope to select and count putative microplastics with different morphologies and different size fractions of 1−2, 2−3 and 3−5 mm. Samples that had passed through a mesh size of 1 mm were filtered again using a 5-μm pore size nitrocellulose filter for further microscopic observation and the selection and enumeration of smaller microplastics. Some of these samples containing organisms were digested with hydrogen peroxide (30%) to avoid interference during the observation and selection of the microplastics. Putative microplastics were identified using Fourier transform infrared spectroscopy. Four shape types of microplastics, namely fibers, fragments, films and foams were found in the atmospheric samples. The majority were fibers of different colors (white, black, red and transparent) that accounted for about 95% of the microplastics. Fragments accounted for about 4% and films and foams accounted for fewer than 1%. The main polymers were polyethylene terephthalate in the case of most of the fibers, polyvinyl chloride in the case of some fibers and films, polyethylene for the fragments, and polystyrene for the foams. Microplastics with sizes below 0.5 mm accounted for more than 50% throughout the year, followed by the size ranges 0.5−1 mm,1−2 mm and then 2−3 mm. Microplastics with size range of 3−5 mm appeared only in the spring samples at low abundance. In general, the amount of microplastics decreased dramatically with increasing particle size. The 100−300 μm size fraction formed the highest proportion of microplastics below 0.5 mm in size, accounting for about 62%, and those below 50 μm showed the lowest abundance. The annual site deposition flux of atmospheric microplastics attained a maximum of 1.46×105 n/(m2 a), that of the fibers up to 1.38×105 n/(m2 a), and those of the fragments, films and foams up to 6.29×103, 7.65×102 and 2.45×102 n/(m2 a), respectively. Deposition fluxes of different shape types ranged from 0.0 to 6.02×102 n/(m2 d), and the fibers were also the commonest of the four shape types. The deposition flux of microplastics showed some seasonal variation, being higher in the spring, summer and winter and lowest in the autumn. Based on the observation site (1.6 km from the coastline), the length of Yantai urban coastline (100 km) and the annual deposition flux in this study, it was estimated that the total number of microplastics deposited in the urban area was about 2.33×1013 particles, equivalent to about 0.9 to 1.4 tonnes. We suggest that atmospheric microplastics, through precipitation to land and sea, be a key source of microplastics in the coastal and oceanic environments. Moreover, a systematic study is urgently required in the future for a better understanding of pollution and precipitation of atmospheric microplastics and their environmental impact.