In 2013, China issued the “Action Plan for the Prevention and Control of Air Pollution” (“Ten Statements of Atmosphere”) and implemented a series of pollution reduction measures from 2013 to 2017. In ...key regions of China, the mass concentrations of particulate matter with aerodynamic equivalent diameters less than 2.5 µm (PM
2.5
) have dropped significantly. However, the contributions of meteorological changes to PM
2.5
reduction are largely uncertain, which has attracted particular concern from the government and the public. Here, we investigated the impact of large-scale and boundary layer (BL) meteorological conditions on aerosol pollution and estimated the contributions of meteorological changes to PM
2.5
reduction based on in-depth analysis and diagnosis of various observed meteorological elements and an integrated pollution-linked meteorological index (PLAM, which is approximately and linearly related to PM mass concentration). In this study, we found that the meteorological conditions worsened in 2014 and 2015 and improved in 2016 and 2017 relative to those in 2013 in key regions in China. In 2017 relative to 2013, only ∼5% (approximately 13% of the total PM
2.5
decline) of the 39.6% reduction in PM
2.5
mass concentrations can be attributed to meteorological changes in the Beijing-Tianjin-Hebei (BTH) region, and only ∼7% (approximately 20% of the total PM
2.5
decline) of the 34.3% reduction can be attributable to meteorological changes in the Yangtze River Delta (YRD) region. Overall, the PM
2.5
reduction due to meteorological improvement is much lower than the observed PM
2.5
reduction in these areas, which indicates that emission reduction during the five-year implementation of the “Ten Statements of Atmosphere” is the dominant factor in the improvement in air quality. The changes in meteorology and climate are conducive to PM
2.5
reduction but do not dominate the substantial improvement in air quality. Similar to the above regions, in the Pearl River Delta (PRD) region, the impact of meteorological changes on the annual averaged PM
2.5
concentration from 2013 to 2017 was relatively weak, and the PM
2.5
reduction was mainly due to emission reductions. During winter 2017 (January, February, and December of this year), the meteorological conditions improved ∼20% in the BTH region (observed total PM
2.5
reduction: 40.2%) and ∼30% in the YRD region (observed total PM
2.5
reduction: 38.2%) relative to those in 2013, showing the meteorological factors played more important role in the decrease of PM
2.5
in winter of these years in the two regions, respectively. The meteorological conditions in winter 2016 were 14% better than those in winter 2017, but the PM
2.5
reduction in winter 2016 was still less than that in winter 2017, reinforcing the significant contributions of the increasing efforts to reduce PM
2.5
emissions in 2017. The substantial progress of strict emission measures was also confirmed by a comparison of several persistent heavy aerosol pollution episodes (HPEs) with similar meteorological conditions. It is found that the decrease of PM
2.5
mass caused by emission reduction increases year by year, especially the decrease of PM
2.5
concentration in 2016 and 2017. In China, HPEs mainly occur in winter, when meteorological conditions are approximately 40–100% worse than in other seasons. This worsening is partly due to the harbor effect of high topography, including downdrafts and the weak wind zone, and partly due to the increasingly stable regional BL structure caused by climate warming. For the formation of HPEs, it occurred under regional stagnant and stable conditions associated with upper-level circulation patterns, including the zonal westerly winds type and high-pressure ridges. After pollution formation, PM
2.5
with mass accumulated to a certain degree can further worsen the BL meteorological conditions. The feedback effect associated with worsening conditions dominates PM
2.5
mass explosive growth. In the context of high air pollutant emissions in China, unfavorable meteorological conditions are the necessary external conditions for the formation and accumulation of HPEs. Therefore, reducing aerosol pollution significantly during the earlier transport stage is critical in reducing persistent HPEs. Currently, even under favorable meteorological conditions, allowing emissions without restriction is also not advisable because aerosol pollution allowed to accumulate to a certain extent will significantly worsen the BL meteorological conditions and close the “meteorological channels” available for pollution dispersion.
Aerosol optical depth (AOD) has become a crucial metric for assessing global climate change. Although global and regional AOD trends have been studied extensively, it remains unclear what factors are ...driving the inter-decadal variations in regional AOD and how to quantify the relative contribution of each dominant factor. This study used a long-term (1980–2016) aerosol dataset from the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) reanalysis, along with two satellite-based AOD datasets (MODIS/Terra and MISR) from 2001 to 2016, to investigate the long-term trends in global and regional aerosol loading. Statistical models based on emission factors and meteorological parameters were developed to identify the main factors driving the inter-decadal changes of regional AOD and to quantify their contribution. Evaluation of the MERRA-2 AOD with the ground-based measurements of AERONET indicated significant spatial agreement on the global scale (r= 0.85, root-mean-square error = 0.12, mean fractional error = 38.7 %, fractional gross error = 9.86 % and index of agreement = 0.94). However, when AOD observations from the China Aerosol Remote Sensing Network (CARSNET) were employed for independent verification, the results showed that MERRA-2 AODs generally underestimated CARSNET AODs in China (relative mean bias = 0.72 and fractional gross error =−34.3 %). In general, MERRA-2 was able to quantitatively reproduce the annual and seasonal AOD trends on both regional and global scales, as observed by MODIS/Terra, although some differences were found when compared to MISR. Over the 37-year period in this study, significant decreasing trends were observed over Europe and the eastern United States. In contrast, eastern China and southern Asia showed AOD increases, but the increasing trend of the former reversed sharply in the most recent decade. The statistical analyses suggested that the meteorological parameters explained a larger proportion of the AOD variability (20.4 %–72.8 %) over almost all regions of interest (ROIs) during 1980–2014 when compared with emission factors (0 %–56 %). Further analysis also showed that SO2 was the dominant emission factor, explaining 12.7 %–32.6 % of the variation in AOD over anthropogenic-aerosol-dominant regions, while black carbon or organic carbon was the leading factor over the biomass-burning-dominant (BBD) regions, contributing 24.0 %–27.7 % of the variation. Additionally, wind speed was found to be the leading meteorological parameter, explaining 11.8 %–30.3 % of the variance over the mineral-dust-dominant regions, while ambient humidity (including soil moisture and relative humidity) was the top meteorological parameter over the BBD regions, accounting for 11.7 %–35.5 % of the variation. The results of this study indicate that the variation in meteorological parameters is a key factor in determining the inter-decadal change in regional AOD.
This paper employs meteorological observation data from surface and high-balloon stations, China Meteorological Administration (CMA) model T639 output data, NCEP reanalysis data, PM2.5 observations ...and modeled HYSPLIT4 trajectory results to study the meteorological causes, including large-scale circulation and planetary boundary layer features, which led to the extended haze episode on January 6–16, 2013 in central-eastern China. It discusses the possible impact of pollutants transported from southern Hebei Province on Beijing. The study's results show that: (1) the re-adjustment of atmospheric circulation from a longitudinal to a latitudinal model provides a valuable interpretation of the large-scale circulation background to the haze episode experienced in the metropolitan regions of Beijing, Tianjin, Hebei and their surrounding regions; (2) the regional atmospheric stratification of the planetary boundary layer is stable and the mixing height is low, suppressing air turbulence in the planetary boundary layer and providing favorable meteorological conditions for the formation of haze; and (3) the southwesterly jet stream with wind speeds of 6–11 m/s at a height of 850–950 hPa and the below-700 m air mass trajectory tracking established using the HYSPLIT4 model interdependently suggest a transport of pollutants from southern Hebei Province to Beijing at 850–950 hPa.
•Large-scale latitudinal atmospheric circulation is beneficial for the haze forming.•Local stable stratification and weak turbulent is favorable for the haze formation.•Pollutants transportation from south Hebei on 850–925 hPa favors Beijing's pollution.
•A virtual ground-based PM2.5 observation network was constructed.•Daily estimations of PM2.5 concentrations at ~1000 sites in China were generated.•The model demonstrated good performance in ...hindcasting historical PM2.5 levels.•This virtual PM2.5 network can be used for reconstructing historical PM2.5 data.
With increasing public concerns on air pollution in China, there is a demand for long-term continuous PM2.5 datasets. However, it was not until the end of 2012 that China established a national PM2.5 observation network. Before that, satellite-retrieved aerosol optical depth (AOD) was frequently used as a primary predictor to estimate surface PM2.5. Nevertheless, satellite-retrieved AOD often encounter incomplete daily coverage due to its sampling frequency and interferences from cloud, which greatly affect the representation of these AOD-based PM2.5. Here, we constructed a virtual ground-based PM2.5 observation network at 1180 meteorological sites across China using the Extreme Gradient Boosting (XGBoost) model with high-density meteorological observations as major predictors. Cross-validation of the XGBoost model showed strong robustness and high accuracy in its estimation of the daily (monthly) PM2.5 across China in 2018, with R2, root-mean-square error (RMSE) and mean absolute error values of 0.79 (0.92), 15.75 μg/m3 (6.75 μg/m3) and 9.89 μg/m3 (4.53 μg/m3), respectively. Meanwhile, we find that surface visibility plays the dominant role in terms of the relative importance of variables in the XGBoost model, accounting for 39.3% of the overall importance.
We then use meteorological and PM2.5 data in the year 2017 to assess the predictive capability of the model. Results showed that the XGBoost model is capable to accurately hindcast historical PM2.5 at monthly (R2 = 0.80, RMSE = 14.75 μg/m3), seasonal (R2 = 0.86, RMSE = 12.28 μg/m3), and annual (R2 = 0.81, RMSE = 10.10 μg/m3) mean levels. In general, the newly constructed virtual PM2.5 observation network based on high-density surface meteorological observations using the Extreme Gradient Boosting model shows great potential in reconstructing historical PM2.5 at ~1000 meteorological sites across China. It will be of benefit to filling gaps in AOD-based PM2.5 data, as well as to other environmental studies including epidemiology.
Although a remarkable reduction in the frequency of sand
and dust storms (SDSs) in the past several decades has been reported over
northern China (NC), two unexpected mega SDSs occurred on 15–20 and ...27–29 March 2021 (abbreviated as the “3.15” and “3.27” SDS
events), which has reawakened widespread concern. This study characterizes
the optical, microphysical, and radiative properties of aerosols and their
meteorological drivers during these two SDS events using the Sun photometer
observations in Beijing and a comprehensive set of multiple satellite
(including MODIS, VIIRS, CALIOP, and Himawari-8) and ground-based
observations (including the CMA visibility network and AD-Net) combined with
atmospheric reanalysis data. Moreover, a long-term (2000–2021) dust optical
depth (DOD) dataset retrieved from MODIS measurements was also utilized to
evaluate the historical ranking of the dust loading in NC during dust
events. During the 3.15 and 3.27 events, the invasion of dust plumes greatly
degraded the visibility over large areas of NC, with extreme low visibility
of 50 and 500 m recorded at most sites on 15 and 28 March, respectively.
Despite the shorter duration of the 3.27 event relative to the 3.15 event,
sun photometer and satellite observations in Beijing recorded a larger peak
AOD (∼2.5) in the former than in the latter (∼2.0), which was mainly attributed to the short-term intrusion of coarse-mode
dust particles with larger effective radii (∼1.9 µm)
and volume concentrations (∼2.0 µm3 µm−2) during the 3.27 event. The shortwave direct aerosol radiative
forcing induced by dust was estimated to be −92.1 and −111.4 W m−2 at the top of the atmosphere, −184.7 and −296.2 W m−2 at
the surface, and +92.6 and +184.8 W m−2 in the atmosphere in
Beijing during the 3.15 and 3.27 events, respectively. CALIOP observations
show that during the 3.15 event the dust plume was lifted to an altitude of
4–8 km, and its range of impact extended from the dust source to the
eastern coast of China. In contrast, the lifting height of the dust plume
during the 3.27 event was lower than that during the 3.15 event, which was also
confirmed by ground-based lidar observations. The MODIS-retrieved DOD data
registered these two massive SDS events as the most intense episode in
the same period in history over the past 2 decades. These two extreme SDS
events were associated with both atmospheric circulation extremes and local
meteorological anomalies that favored enhanced dust emissions in the Gobi
Desert (GD) across southern Mongolia and NC. Meteorological analysis
revealed that both SDS events were triggered by an exceptionally strong
Mongolian cyclone generated at nearly the same location (along the central
and eastern plateau of Inner Mongolia) in conjunction with a surface-level
cold high-pressure system at the rear, albeit with differences
in magnitude and spatial extent of impact. In the GD, the early melting of
spring snow caused by near-surface temperature anomalies over dust source
regions, together with negative soil moisture anomalies induced by decreased
precipitation, formed drier and barer soil surfaces, which allowed for
increased emissions of dust into the atmosphere by strongly enhanced surface
winds generated by the Mongolian cyclone.
The weather conditions affecting aerosol pollution in Beijing and its
vicinity (BIV) in wintertime have worsened in recent years, particularly
after 2010. The relation between interdecadal changes in ...weather conditions
and climate warming is uncertain. Here, we analyze long-term variations of an
integrated pollution-linked meteorological index (which is approximately and
linearly related to aerosol pollution), the extent of changes in vertical
temperature differences in the boundary layer (BL) in BIV, and northerly
surface winds from Lake Baikal during wintertime to evaluate the potential
contribution of climate warming to changes in meteorological conditions
directly related to aerosol pollution in this area; this is accomplished
using NCEP reanalysis data, surface observations, and long-term vertical
balloon sounding observations since 1960. The weather conditions affecting
BIV aerosol pollution are found to have worsened since the 1960s as a whole.
This worsening is more significant after 2010, with PM2.5 reaching
unprecedented high levels in many cities in China, particularly in BIV. The
decadal worsening of meteorological conditions in BIV can partly be
attributed to climate warming, which is defined by more warming in the higher
layers of the boundary layer (BL) than the lower layers. This worsening can
also be influenced by the accumulation of aerosol pollution, to a certain
extent (particularly after 2010), because the increase in aerosol pollution
from the ground leads to surface cooling by aerosol–radiation interactions,
which facilitates temperature inversions, increases moisture accumulations,
and results in the extra deterioration of meteorological conditions. If
analyzed as a linear trend, weather conditions have worsened by
∼ 4 % each year from 2010 to 2017. Given such a deterioration rate,
the worsening of weather conditions may lead to a corresponding amplitude
increase in PM2.5 in BIV during wintertime in the next 5 years (i.e.,
2018 to 2022). More stringent emission reduction measures will need to be
conducted by the government.
The Beijing government implemented a number of clean air action plans to improve air quality in the last 10 years, which contributed to changes in the concentration of fine particles and their ...compositions. However, quantifying the impacts of these interventions is challenging as meteorology masks the real changes in observed concentrations. Here, we applied a machine learning technique to decouple the effect of meteorology and evaluate the changes in the chemistry of nonrefractory PM1 (particulate matter less than 1 μm) in winter 2007, 2016, and 2017 as a result of the clean air actions. The observed mass concentrations of PM1 were 74.6, 90.2, and 36.1 μg m–3 in the three winters, while the deweathered concentrations were 74.2, 78.7, and 46.3 μg m–3, respectively. The deweathered concentrations of PM1, organics, sulfate, ammonium, chloride, SO2, NO2, and CO decreased by −38, −46, −59, −24, −51, −89, −16, and −52% in 2017 in comparison to 2007. On the contrary, the deweathered concentration of nitrates increased by 4%. Our results indicate that the clean air actions implemented in 2017 were highly effective in reducing ambient concentrations of SO2, CO, and PM1 organics, sulfate, ammonium, and chloride, but the control of nitrate and PM1 organics remains a major challenge.
Ti33.3Zr16.7Cu50-xNix (x = 0–16.5, at.%) amorphous filler metals were designed to braze Ti6Al4V alloy and 316 L stainless steel (SS). The effect of Ni addition in filler metals on the wettability, ...joint microstructure evolution and shear strength were investigated. The reaction phase identification and crack initiation mechanism were analyzed in depth. Ni addition weakened the wettability of the filler metal and thickened the brazed seam. The filler metal with 11 at.% Ni was optimized for brazing of Ti6Al4V alloy/316 L SS with the maximum joint shear strength of 318 MPa. FeTi, Fe2Ti, FeCr, and α-Fe phases formed around the transition zone close to 316 L SS substrate, which was the weak part of the brazed joints. The interface between FeTi and Fe2Ti phases was non-coherent with the lattice mismatch of 61.4%, initiating the crack at (β-Ti + FeTi)/Fe2Ti interface. The initiative cracks mainly propagated along the Fe2Ti and FeCr layers with brittle feature. Optimizing the constituents and alloying process for Ti-Cu-based filler metal has huge potential in improving the performance of titanium alloy/steel joint.
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•Ti33.3Zr16.7Cu50-xNix (x = 0 to 16.5%) amorphous filler metals were designed.•The maximum joint shear strength was 318 MPa, higher than reported results.•The reaction phases around the transition zone were confirmed.•The lattice mismatch between FeTi and Fe2Ti phases was 61.4%.
Empirical constraints on orbital gravitational solutions for the Solar System can be derived from the Earth’s geological record of past climates. Lithologically based paleoclimate data from the ...thick, coal-bearing, fluvial-lacustrine sequences of the Junggar Basin of Northwestern China (paleolatitude ∼60°) show that climate variability of the warm and glacier-free high latitudes of the latest Triassic–Early Jurassic (∼198–202 Ma) Pangea was strongly paced by obliquity-dominated (∼40 ky) orbital cyclicity, based on an age model using the 405-ky cycle of eccentricity. In contrast, coeval low-latitude continental climate was much more strongly paced by climatic precession, with virtually no hint of obliquity. Although this previously unknown obliquity dominance at high latitude is not necessarily unexpected in a high CO ₂ world, these data deviate substantially from published orbital solutions in period and amplitude for eccentricity cycles greater than 405 ky, consistent with chaotic diffusion of the Solar System. In contrast, there are indications that the Earth–Mars orbital resonance was in today’s 2-to-1 ratio of eccentricity to inclination. These empirical data underscore the need for temporally comprehensive, highly reliable data, as well as new gravitational solutions fitting those data.
Significance Geological records of paleoclimate provide the only constraints on Solar System orbital solutions extending beyond the ∼50-Ma limit imposed by chaotic diffusion. Examples of such constraints are coupled high and low latitude, Triassic–Jurassic (∼198–202 Ma) sedimentary cyclicity in coal-bearing outcrops from the ∼60° N-paleolatitude Junggar Basin (Western China), and contemporaneous tropical basins. Analysis reveals climate variability dominated by obliquity-scale cyclicity in the Junggar Basin and precession-scale cyclicity in the tropics. Together, these geological records empirically constrain orbital solutions by providing joint g4 − g3 and s4 − s3 secular frequency estimates of the Earth–Mars orbital resonance. These results demonstrate the opportunity for developing a new class of solutions grounded by geological data extending hundreds of millions of years into the geologic past.
This study used the MERRA-2 reanalysis dataset and ground-based and satellite observational data to comprehensively analyze a typical dust storm event in east Asia on 2–7 May 2017 which engulfed most ...of China as well as ocean and Japan, and explore the accuracy and comprehensiveness of the MERRA-2 dataset in the analysis of dust processes. The results of comparison show that the description of the spatiotemporal distribution and evolution of the dust aerosols in the dust event using the MERRA-2 data is consistent with the data of AERONET, National Urban Air Quality Real-time Publishing Platform and Hamawari-8. Gobi Deserts was the most influential source area of this dust event with the highest emissions reaching 1.9 × 106 μg/m3. The vertical motion of the atmosphere can lift dust from the source area above 500 hPa. There were low-pressure troughs at 500 and 850 hPa and the winds behind and in front of the trough led to the high-altitude, long-distance transport of dust. Dust gradually affected the northwest China, north China, northeast China, and even the ocean and Japan on 2–7 May. This study demonstrates that although there is some uncertainty about the source of dust emission in the MERRA-2 model, the data accurately simulated the evolution of the dust event and analyze it comprehensively, while the accuracy of simulating the long-term evolution of dust requires further evaluation.
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