Seasonal and daily occurrence of microplastic pollution in urban road dust

In recent years, microplastic pollution and its potential ecological risks have attracted a lot of attention. The pollution level of microplastics (MPs) in urban road dust has already been demonstrated. However, the occurrence mechanism of MPs in road dust still remains to be understood, especially for the seasonal and daily occurrences. In this study, seasonal and daily pollution characteristics of MPs in the road dust of Ma'anshan city, China were investigated. On average, the abundances of MPs were 7.48 +/- 10.56 n.m(-2) in spring, 5.17 +/- 5.18 n.m(-2) in summer, 4.32 +/- 5.54 n.m(-2) in autumn and 8.58 +/- 7.07 n.m(-2) in winter. There was no significant difference in the abundances of MPs among various seasons (F = 1.448, p = 0.236). As the main polymers, the proportions of PP (Polypropylene), PE (Polyethylene) and PET (Polyethylene Terephthalate) were found to be 71.51%, 22.16% and 1.04% in spring, 48.63%, 16.13% and 24.56% in summer, 59.58%, 23.66% and 7.80% in autumn, and 59.53%, 23.47% and 8.33% in winter, respectively, wherein the abundances of PE and PET varied with seasons (p < 0.05). More specifically, based upon numbers, MPs were dominated by fragments (50.27%) in shape, <1000 mu m (65.02%) in size and black (36.22%) in color. Moreover, the abundances of MPs had a significant correlation with the dust load (p < 0.05). The diversity integrated indices of MPs were 0.42 +/- 0.14 in spring, 0.51 +/- 0.10 in summer, 0.38 +/- 0.17 in autumn, and 0.40 +/- 0.13 in winter, indicating a seasonal difference (F = 3.46, p = 0.02). This suggests that even though the abundance of MPs did not vary with season, their polymers and apparent morphology did change. Particularly, there was no obvious accumulation of MPs with the increase in the antecedent dry days on urban road due to the frequent road-cleaning practice.