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The study of air pollution in a valley is a classic research subject. Compared with flat terrain, the formation and development of haze pollution are more complicated and unique within a deep basin. How a basin or valley plays a role in the horizontal and vertical distribution of air pollutants is poorly understood in highly industrialized deep basins in China due to scarce field observations. We conducted a collaborative experiment of three-dimensional (3D) boundary layer meteorology and pollution at the western Sichuan Basin (SCB) close to the Tibetan Plateau (TP). Generally, the concentrations of PM1 (particulate matter smaller than 1.0 μm), NO, and NO2 largely decline with elevation, while O3 shows a slight increasing trend inside the SCB. Three different types of pollutant profiles and the formation mechanisms are described below. The high PM1 near the surface layer corresponds to the vertical clockwise circulation, i.e., a wind shift with increasing altitude in a clockwise direction. The air pollutants at the central and eastern SCB can be transported to the eastern foothills of the TP by southeasterly winds and then are trapped within the western SCB by a strong surface temperature inversion. The pollutants over the eastern TP also can be dispersed to above the SCB by westerly winds. More aerosol particles are concentrated at about 2.0 km MSL by jointly ascending and descending motion below and above the layer over the valley. The relative uniform PM1 in the vertical direction correlates to the counterclockwise circulation. The trapped pollutants in the western SCB can be transported to the eastern region by westerly winds and then are dispersed to the upper air by unstable stratification.

Tibetan Plateau (TP, hereafter), known as “Third Pole”, is surrounded by the highly polluted regions, such as Indo-Gangetic Plain, Taklimakan and Gobi Deserts and Sichuan Basin. However, the previous in-situ aerosol measurements mainly focused on the southern and northern slopes, while less observations and studies were conducted at the eastern slope of the TP (ESTP). The scientific knowledge on optical properties of aerosols is extremely limited over the ESTP, and in-situ observations at varying altitudes from the heavily polluted regions to the relatively clean Plateau were important for better understanding the light absorption and radiative forcing over the TP. Sichuan Basin (SCB), a highly polluted region due to more rapid economic development, is located in the east side of TP. Therefore, we conducted the first aerosol field experiment at six sites (Chengdu, Sanbacun, Wenchuan, Lixian, Maerkang, Hongyuan) along eastern slope of the TP extending elevation from 500 m to 3500 m. Light-absorbing aerosols are considered to be a key climate driver, and their role may be underestimated in the high-altitude regions. The light absorption of brown carbon (BrC) accounting for that of total carbon increases from 20 % to 50 %, and the mass absorption efficiency of BrC over TP is 2–3 times higher than that inside SCB, especially in winter, which is mainly related to high ratio of secondary to primary organic carbon due to stronger secondary formation and less primary emissions at high altitudes. Contrary to BrC aerosols, EC (elemental carbon) mass absorption efficiency declines with altitude in winter, induced by source difference between the TP and SCB. The more urban sources (motor vehicles, industries, etc.) inside the SCB fail to be transported to the TP due to stable air inside the basin in winter, which also is favorable for aerosol aging to enhance absorption efficiency. The radiative forcing of BrC relative to EC varies from 0.10 to 0.42 as altitude increases with the higher OC / EC ratio over the TP than SCB, and thus the enhanced radiative forcing of BrC relative to EC from polluted SCB to pristine TP is because the concentration of OC decreases more slowly with altitude than does EC. South Asia, a highly particulate matter (PM) pollution region, is an important origin of aerosol particles at the region from western Sichuan Basin to eastern Tibetan Plateau, which is significantly dependent on seasons.

The lack of light–absorbing aerosols vertical distributions data largely limited to revealing the formation mechanism of severe haze pollution in Chinese cities. Based on the synchronous measurements of size–resolved carbonaceous aerosols and meteorological data at near surface level and hilltop (about 620 m above the valley) in Lanzhou of northwest China, this study compared organic and elemental carbon (OC, EC) size distributions at the two altitudes and revealed the key influencing factors in a typical urban valley, China. The winter OC size distributions were typically bimodal with two comparable peaks in the accumulation and coarse modes, while those in summer were unimodal with the highest value in the size bin of 4.7–5.8 μm. The size-resolved OC and EC at near the surface were significantly higher than those at the hilltop. The difference (concentrations and size distributions) of OC and EC between the surface and hilltop in summer was much smaller than that in winter due to stronger vertical mixing and larger summer SOC contributions at the hilltop. The winds paralleling with running urban valley were conducive to dispersing the air pollutants from near the surface to the upper air. The roles of horizontal and vertical dispersions to carbonaceous aerosols were comparable at near the surface, while horizontal dispersion was more important at the hilltop. Furthermore, the vertical dispersion was a main factor controlling size–resolved carbonaceous aerosols under highly polluted conditions in a typical urban valley. This study will provide the basis for regulation of severe haze pollution over complex terrain.

The concentrations, optical and radiative effects of carbonaceous aerosols were essential to studies of the climatic, environmental and health effects. The previous studies less combined numerical simulation with in-situ observations, especially for the aerosol vertical profiles. In this study, we off-line measured vertical profiles of submicron black carbon (BC) aerosols and on-line obtained aerosol optical properties over urban Lanzhou during 26 December 2017 to 11 January 2018. The BC optical properties and radiative effects were evaluated using Optical Properties of Aerosols and Clouds (OPAC) and Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) models. The absorption and scattering coefficients and optical depth of BC aerosols ranged from 9 to 83 M m−1, 3–24 M m−1 and 0.02 to 0.2 respectively, which in average accounted for 50%, 3% and 11% of the optical properties of total aerosols during the study period. BC aerosol radiative forcing (ARF) within ATMOS (top-surface) varying from 16.6 to 108.8 W m−2 accounted for 17.3%–97.4% of total aerosols ARF with an average of 66.6%, and the percentages increased significantly as BC concentrations increased during the period. The mean atmospheric heating rate (AHR) induced by BC aerosols was 1.94 K day−1 ranging from 0.46 to 3.03 K day−1 during the study period. This study contributes to understanding the impacts of light-absorbing aerosols on climate and haze pollution in an urban valley.

To fill the blank on vertical profiles of carbonaceous aerosols during winter, the first in situ vertical measurements of organic and elemental carbon (OC, EC) from ground level to 620 m were conducted by means of high topographic relief in a typical valley city of Lanzhou, northwest China. The key factors (meteorology and sources) affecting the vertical distributions also were revealed by clustering technique and principal component analysis. Mean OC and EC concentrations were (16.2 ± 8.9) μg m−3 and (5.1 ± 2.1) μg m−3 during the study periods near the ground. More uniformly vertical distributions of carbonaceous aerosols during daytime than those during nighttime with OC (EC) decreasing rates of ∼1.45 (∼0.32) μg m−3 100 m−1 during nighttime and ∼0.84 (∼0.09) μg m−3 100 m−1 during daytime. Three distinct types of OC (EC) vertical profiles were determined by clustering technique. The cluster with high concentrations of OC (33.7 ± 5.7 μg m−3) and EC (10.5 ± 7.7 μg m−3) near the ground surface was induced largely by temperature inversion around the ground. The OC (EC) differences between the surface and the hilltop increased by 14.21 (2.45) μg m−3 when inversion layer depth increased by 100 m. Winter carbonaceous aerosols were more significantly affected by atmospheric stratification near the ground, while those were impacted together by vertical and horizontal dispersions at the hilltop. The primary emissions from coal combustion or biomass burning for heating at the hilltop and motor vehicles near the ground also had an important effect on OC (EC) vertical profiles. The emissions from coal burning and gasoline vehicles accounted for 60%–70% of carbonaceous aerosols in urban areas of Lanzhou. Furthermore, the contributions of gasoline vehicles during daytime near the surface (∼20%) and biomass burning at the hilltop (∼15%) were larger due to the impact of human activities.

Regional PM2.5 and ozone pollution and inter-city transportation in the city clusters of the Sichuan Basin (SB) were revealed using k-means clustering and the HYSPLIT backward trajectory model. The PSCF (potential source contribution function) and CWT (concentration-weighted trajectory) methods also were used to identify potential source regions of PM2.5 and ozone in the basin. The regional PM2.5 pollution was obvious with high levels at western and southern SB, while regional pollution characteristics of ozone were less significant than that of PM2.5 inside the basin. Additionally, PM2.5 was often transported directionally from one city to another city, while regional transportation of ozone was less significant than PM2.5 inside the basin, which may be related to the fact that ozone reacted easily with primary emissions during transportation. The Tibetan Plateau (TP) also may be an important source region of high ozone in the city clusters of Sichuan Basin, especially west part of the basin. The results will provide advice to the government to take measures in improving air quality of SB.

To reveal PM1 vertical profiles and key affecting factors in a typical urban valley, daytime and nighttime PM1 (the particles with diameters less than 1 μm) samples were collected with medium volume air samplers during 26 December 2017 to 11 January 2018 at five different altitudes by means of high topographic relief at urban areas of Lanzhou. The synchronous boundary layer temperature and humidity profiles were observed by a microwave radiometer. Daytime PM1 concentrations reduced by about 3.86 μg m−3 when the height above the surface increased by 100 m, which was much lower than that for nighttime (5.68 μg m−3 100 m−1) as particles were easily accumulated near the surface when the air was stable during the nighttime. The three typical PM1 vertical profiles were identified by K-means clustering technique. The most frequent cluster with elevated PM1 concentrations near the surface was closely related to temperature inversion around the ground, while the cluster with relatively uniform PM1 within the boundary layer was mainly induced by unstable atmospheric stratification and thus relatively good vertical dispersion. About 50%–60% of PM1 variations could be attributed to atmospheric stratification near the surface in the valley city, which was much higher than that at the hilltop. The PM1 difference increased by 47.14 (36.91) μg m−3 when inversion layer thickness (intensity) increased by 100 m (1 °C 100 m−1). The newly calculated inversion index considering both inversion layer thickness and intensity explained about 87% of PM1 differences between near the surface and at the hilltop. The vertical dispersion had a more significant effect on PM1 than horizontal dispersion near the surface, while PM1 was more largely affected by horizontal dispersion at the hilltop, which was closely related to the valley terrain.

To explore the possible local climatic and environmental impacts of an onshore wind farm in North China, meteorological observation and satellite remote sensing data around the wind farm were collected, and the soil chemical composition within and outside of the wind farm was determined. Our analysis revealed changes in the spatial patterns of wind, air temperature (Ta), ground surface temperature (Ts), land surface temperature (LST), evapotranspiration (ET), and the normalized difference vegetation index (NDVI) from before to after the construction of the wind farm. The two datasets of meteorological observations showed that wind speed decreased after the construction of the wind farm, with one dataset indicating a significant decrease. Since the construction of the farm, Ta and Ts have shown increasing trends, and ET has shown a significant increasing trend. The nighttime Ta and LST showed opposite trends, with Ta decreasing and LST increasing. The change in LST was the greatest at the wind farm and in its downwind zone, indicating that the operation of the wind farm affects the LST within and beyond the 10 km buffer zone. After the wind farm construction, the NDVI showed a significant positive trend. However, increases in the concentrations of Ti and Co were observed, suggesting that the construction and operation of wind farm may promote vegetation growth and cause soil pollution. The monitoring of wind farms needs to be strengthened to reveal the long-term impacts of wind farms on local climates and ecosystems.

Accurate prediction of the associated hazards resulting from accidental leakage in a large indoor environment is essential for risk analysis and emergency response. Although computational fluid dynamics (CFD) is seen as a promising tool to model complex dispersion scenarios, it is necessary that it be fully validated against reliable experimental data before the predictions can be used with confidence. This paper presents a comprehensive and systematic evaluation of the performance of four commonly used Reynolds Averaged Navier-Stokes (RANS) turbulence models (i.e., standard k-e, Realizable k-e, standard k-ω, and shear-stress transport (SST) k-ω) for predicting heavy gas dispersion in a factory building with various obstacles. The predicted concentrations were compared with tracer (SF6) concentrations measured from a full-scale indoor tracer test, in which 14 sampling sites were distributed around the release source. Several statistical measures were applied to quantify the model performance in and above the breathing zone. The results suggest that the standard k-ω model and the SST k-ω model show the best performance in the breathing zone according to Chang's criteria, producing 78% of predictions (paired in time and space) within a factor of two of the observations. However, none of the RANS models are satisfactory in predicting the spatial-temporal variation of tracer clouds above the breathing zone. Additionally, the SST k-ω model was employed to reproduce the heavy gas dispersion in the building and to analyze the relationship between ventilation rate and concentration in the breathing zone.

The strict Clean Air Action Plan has been in place by central and local government in China since 2013 to alleviate haze pollution. In response to implementation of the Plan, daytime PM

The strict Clean Air Action Plan has been in place by central and local government in China since 2013 to alleviate haze pollution. In response to implementation of the Plan, daytime PM2.5 (particulate matter with aerodynamic diameter less than 2.5 μm) showed significant downward trends from 2015 to 2019, with the largest reduction during spring and winter in the North China Plain. Unlike PM2.5, O3 (ozone) showed a general increasing trend, reaching 29.7 μg m−3 on summer afternoons. Increased O3 and reduced PM2.5 simultaneously occurred in more than half of Chinese cities, increasing to approximately three-fourths in summer. Declining trends in both PM2.5 and O3 occurred in only a few cities, varying from 19.1% of cities in summer to 33.7% in fall. Meteorological variables helped to decrease PM2.5 and O3 in some cities and increase PM2.5 and O3 in others, which is closely related to terrain. High wind speed and 24 h changing pressure favored PM2.5 dispersion and dilution, especially in winter in southern China. However, O3 was mainly affected by 24 h maximum temperature over most cities. Soil temperature was found to be a key factor modulating air pollution. Its impact on PM2.5 concentrations depended largely on soil depth and seasons; spring and fall soil temperature at 80 cm below the surface had largely negative impacts. Compared with PM2.5, O3 was more significantly affected by soil temperature, with the largest impact at 20 cm below the surface and with less seasonal variation.

The relative importance of topography and soil moisture on the initiation of an afternoon deep convection under weak synoptic-scale forcing was investigated using the weather research and forecasting (WRF) model with high resolution (1.33 km). The convection occurred on 29 June 2017, over the Liupan Mountains, west of the Loess Plateau. The timing and location of the convective initiation (CI) simulated by the WRF model compared well with the radar observations. It showed that the warm and humid southerly airflow under 700 hPa was divided into east and west flows due to the blockage of the Liupan Mountains. The warm and humid air on the west side was forced to climb along the slope and enhanced the humidity near the ridge. The accumulation of unstable energy in the middle and north of the ridge led to a strong vertical convergence and triggered the convection. Sensitivity experiments showed that terrain played a dominant role in triggering the convection, while the spatial heterogeneity of soil moisture played an indirect role by affecting the local circulation and the partition of surface energy.

With Tibetan Plateau higher than 4 km to the west, the location of Sichuan Basin is unique all around the world and provides a good platform to study air pollution in the urban agglomerations over the complex terrain. To fill in the blanks on vertical distributions of PM1 (the particles smaller than 1 μm) and carbonaceous aerosols within the basin, by means of high topographic relief, PM1 were off-line sampled during 20 January to 2 February 2018 at eight sites with increasing altitudes from the basin to southeastern margins of the Tibetan Plateau. The regional potential sources for each site were revealed by HYbrid-Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model and concentration-weighted trajectory (CWT) method. The lowest carbonaceous aerosol levels occurred at Lixian, while the highest OC (organic carbon) (EC, elemental carbon) was at Hongyuan (the altitude of 3500 m) (Ande, a rural site) due to more primary emissions. The pollutants inside the basin can be transported the altitudes from 2 km to 3 km by vertical dispersion, but they cannot be dispersed to higher altitudes. The vertical stratification of the pollutants was obvious and easily formed “high-low-high” pattern from Sichuan Basin to southeastern Tibetan Plateau, especially during highly polluted episodes. The regional potential sources significantly varied as the increased altitudes. Regional pollution was significant inside the basin. The sources at the altitudes from 2 km to 3 km originated from southeastern margins of the Plateau and surrounding cities, while those at higher altitudes were transported from southeastern margins of the Plateau. The impact of basic meteorological variables (temperature, wind speed and vapor pressure) on carbonaceous aerosols was opposite between the basin and Plateau sites. This study was essential to understanding formation mechanisms of severe pollution episodes and thus to making control measures for the urban agglomerations inside the mountainous terrain.

PM

Based on existing researches, here we theoretically summarized the characteristics of the atmospheric movement and turbulent transport of energy and substance in the surface layer as well as the ideal and the actual models for the turbulent transport. Then, using the data observed with eddy covariance at the semiarid climate and environment monitoring station (SACOL) in Lanzhou University from May to October during four consecutive years (September 2006–August 2010), we conducted a de-tailed analysis of the turbulent transport in the surface layer, through introducing the relative vertical turbulence intensity to characterize the turbulence strength, $RI_w = \sqrt {\overline {w'^2 } } /(\sqrt {\overline {w'^2 } } + \bar U)$ , and also by adopting the method for controlling data quality at different levels. Our conclusions are: (1) The turbulent transport of energy and substance in the surface layer must obey the law of conservation of energy and the law of conservation of matter, the observed and calculated energy in the surface layer must be balanced, or closed in theory, but the actual observed and calculated energy just approximates the ideal in some degree and is difficult to achieve the energy balance. (2) The energy closure rate depends much on the atmospheric state in the surface layer, and the energy closure rate increases generally with the relative vertical turbulence intensity. (3) By the way of controlling data quality at different levels, it is found that the degree of data quality control can affect the closure rate, but it does not change the fact that the energy closure rate depends on the atmospheric state. (4) The calculation method of surface soil heat flux can affect energy closure rate, but does not change its dependence on the atmospheric state.