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.
Abstract. The regional transport of air pollutants, controlled by emission
sources and meteorological factors, results in a complex source–receptor
relationship of air pollution change. Wuhan, a metropolis in the Yangtze
River middle basin (YRMB) of central China, experienced heavy air pollution
characterized by hourly PM2.5 concentrations reaching 471.1 µg m−3 in January 2016. To investigate the regional transport of
PM2.5 over central eastern China (CEC) and the meteorological impact on
wintertime air pollution in the YRMB area, observed meteorological and other
relevant environmental data from January 2016 were analyzed. Our analysis
presented noteworthy cases of heavy PM2.5 pollution in the YRMB area
with unique “non-stagnant” meteorological conditions of strong northerly
winds, no temperature inversion, and additional unstable structures in the
atmospheric boundary layer. This unique set of conditions differed from the
stagnant meteorological conditions characterized by near-surface weak winds,
air temperature inversion, and stable structure in the boundary layer that
are typically observed in heavy air pollution over most regions in China.
The regional transport of PM2.5 over CEC aggravated PM2.5 levels,
thus creating heavy air pollution in the YRMB area. This demonstrates a
source–receptor relationship between the originating air pollution regions
in CEC and the receiving YRMB region. Furthermore, a backward trajectory
simulation using a Flexible Particle dispersion (FLEXPART) Weather Research and
Forecasting (WRF) model to integrate the air pollutant
emission inventory over China was used to explore the patterns of regional
transport of PM2.5 governed by the strong northerly winds in the cold
air activity of the East Asian winter monsoon season. It was estimated that
the regional transport of PM2.5 from non-local air pollutant emissions
contributes more than 65 % of the PM2.5 concentrations to the heavy
air pollution in the YRMB region during the study period, revealing the
importance of the regional transport of air pollutants over China as a
causative factor of heavy air pollution over the YRMB area.
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