Differences in PM 2.5 chemical species and sources since lockdown were reported. • Primary emission reduced while secondary formation enhanced since lockdown. • Emission reduction dominated the improvement of air quality in Wuhan during lockdown.

Compared with the 21-year climatological mean over the same period during 2000–2020, the aerosol optical depth (AOD) and Angstrom exponent (AE) during the COVID-19 lockdown (January 24–February 29, 2020) decreased and increased, respectively, in most regions of Central-Eastern China (CEC). The AOD (AE) values decreased (increased) by 39.2% (29.4%) and 31.0% (45.3%) in Hubei and Wuhan, respectively, because of the rigorous restrictions. These inverse changes reflected the reduction of total aerosols in the air and the contribution of the increase in fine-mode particles during the lockdown. The surface PM 2.5 had a distinct spatial distribution over CEC during the lockdown, with high concentrations in North China and East China. In particular, relatively high PM 2.5 concentrations were notable in the lower flatlands of Hubei Province in Central China, where six PM 2.5 pollution events were identified during the lockdown. Using the observation data and model simulations, we found that 50% of the pollution episodes were associated with the long-range transport of air pollutants from upstream CEC source regions, which then converged in the downstream Hubei receptor region. However, local pollution was dominant for the remaining episodes because of stagnant meteorological conditions. The long-range transport of air pollutants substantially contributed to PM 2.5 pollution in Hubei, reflecting the exceptional importance of meteorology in regional air quality in China.

Herein, we demonstrate that hexagonal boron nitride nanosheets (h-BNNSs) can be used as water dispersible lubricant additive through simple fluoride modification. The fluorination of h-BNNSs (F-BNNSs) is carried out via a facile ball milling of commercial h-BN microflakes in ammonia fluoride solution. The as-obtained F-BNNSs exhibit excellent antifriction and antiwear performance as water dispersible lubricant additive with low concentration of less than 1.0 mg/mL, and their friction coefficients can be lower than 0.08, achieving low friction. This low friction originates because F-BNNSs at a sliding interface roll up to form nanoscrolls induced by the fluoride doping curling effect and subsequent shearing force. These resulting nanoscrolls can slide against the substrate, achieving a rolling/incommensurate contact, thus substantially reducing friction coefficients. The overall lubricant mechanism is further elucidated based on the first-principles simulations. This finding indicates the potential of achieving low, even ultralow, friction of h-BNNSs as water dispersible lubricant additives through composition and structure designing.

Hexagonal boron nitride nanosheets (h-BNNSs), with a crystal lattice structure similar to graphene by over 98%, exhibit good lubrication properties as lubricant additives. However, the poor dispersibility in solvents has limited their wide practical applications as lubricant additives. In the present report, water dispersible Pebax functionalized h-BNNSs (Pebax-BNNSs) have been prepared through a one-step solvent-free mechanical exfoliation process which relies on a simple exfoliation of h-BN layers by shearing force in molten Pebax at 200°C. In this process, Pebax molecules can synchronously react with the dangling bonds formed during the exfoliation process to achieve in situ functionalization of h-BNNSs. The reciprocating friction tests demonstrate that the as-obtained Pebax-BNNSs possess excellent antifriction and antiwear performance as water-based lubricant additive with a low concentration of 0.3 mg/mL under atmospheric condition. The friction coefficients can be <0.01, achieving superlubrication. Further systematical investigations on the wear traces, wear debris, and counter balls propose a "dispersion-compensation-filling repairment" friction mechanism. All these results demonstrate that h-BNNSs can achieve superlubrication as water-based lubricant additives via facile surface modification, making them very promising candidates as lubricant additives in practical applications.