Ozone pollution aggravated by mountain-valley breeze over the western Sichuan Basin, Southwest China.

The impact of thermally driven mountain-valley breezes (MVB) on the atmospheric environment remains poorly understood, especially in ozone (O3)-polluted regions with complex underlying topography. To address this knowledge gap, we focused on the western Sichuan Basin (SCB), situated immediately east of the Tibetan Plateau (TP), which is considered susceptible to MVB coupled with severe O3 pollution in southwest China. We revealed the MVB driving diurnal O3 variations and meteorological mechanisms using surface observations and ERA5 reanalysis data. Local MVB days accounted for up to 47% of cases in the summers of 2015-2022. Driven by the MVB, the near-surface O3 concentrations increased by 8.8%, with 12.7% and 50.0% deterioration in the O3 light and moderate exceedance rates, respectively, on the western SCB edge. The daytime upslope valley breeze with 20% higher wind speed drove the westward transport of rich O3 and precursors from the upwind-polluted inner SCB towards its western edge, and the O3 photochemical production, followed by intensifying solar radiation and air temperature, gave rise to 14.8% of surface O3 concentrations over the western SCB edge. The nighttime downward mountain breeze with a 20% increase in wind speed could transport the rich O3 in the mountainous area to the basin edge, causing O3 levels to increase by 2.8%. In summary, we quantitatively assessed the impacts of MVB on changes in O3 concentrations and air quality along with its meteorological mechanisms, facilitating a comprehensive understanding of meteorological drivers in the atmospheric environment.

Implications of ozone transport on air quality in the Sichuan Basin, China.

Ozone pollution is formed through complex chemical and physical processes closely associated with emissions, photochemical reactions, and meteorological conditions. The objective of this study is to quantify the contributions of meteorological chemical formation, vertical transport, and horizontal transport to air quality during spring and summer in different regions of the Sichuan Basin. The Community Multi-scale Air Quality (CMAQ) with the Integrated Process Rate (IPR) was employed to simulate the months of April and July 2021 in the Sichuan Basin. The results indicate that both the spring and summer chemical formation of ozone in the urban centre show negative values, while the surrounding urban areas contribute positively, with chemical formation ranging from 0 to 10 µg·m-3. The maximum ozone level due to horizontal transport in the urban centre exceeds 20 µg·m-3, whereas horizontal transport in the surrounding urban areas exhibits negative values, with transport contributions concentrated within the range of -5 to 0 µg·m-3. The vertical transport in the central and southern parts of the basin shows positive values, with transport contributions ranging from 0 to 10 µg·m-3, and the urban centre exhibits relatively stronger vertical transport with contributions ranging from 10 to 20 µg·m-3. Although the chemical formation contribution in the urban centre is relatively small due to high nitrogen oxide emissions, vertical and horizontal transport play significant roles and are among the key factors contributing to ozone pollution formation.

Vertical changes of PM2.5 driven by meteorology in the atmospheric boundary layer during a heavy air pollution event in central China.

Regional PM2.5 transport is a crucial factor affecting air quality, and the meteorological mechanism in the atmospheric boundary layer (ABL) has not been fully understood over the receptor region in the regional transport of air pollutants. Based on the intensive vertical measurements of air pollutants and meteorology in the ABL during a transport-induced heavy air pollution event in Xiangyang, an urban site over a receptor region in central China, we investigated the meteorological mechanism in vertical PM2.5 changes in the ABL for heavy air pollution over the receptor region. Driven by northerly winds, regional PM2.5 transport was built from upstream northern China to downstream central China, where the observed ABL structures were unstable throughout the air pollution event. We assessed the ABL structures with meteorological and PM2.5 profiles at growth, maintenance, and dissipation stages, and elucidated the mechanism of regional PM2.5 transport inducing air pollution over the receptor region with the contribution of thermal and mechanical factors. The regional PM2.5 transport was concentrated in the upper ABL over the downwind receptor region with high PM2.5 concentrations at altitudes of 600-800 m, where the transported PM2.5 peaks were downwards mixed by vertical wind shear, forming the vertical PM2.5 transport from the upper ABL to near-surface in the growth stage; the weakened winds and less unstable structures in the ABL favored the sustained pollution with slight vertical PM2.5 changes in the maintenance stage, which was dominated by thermal factors with 87 % contribution; the removal of PM2.5 was triggered by increasing winds from the upper ABL, activating the dissipation of heavy PM2.5 pollution with the mechanical effect accounting for 60 % in the dissipation stage. These findings could improve our understanding of ABL's influence on air pollution over the receptor region with implications for the regional transport of air pollutants in environmental changes.

Terrain effect on atmospheric process in seasonal ozone variation over the Sichuan Basin, Southwest China.

Terrain effect is challenging for understanding atmospheric environment changes under complex topography. This study targets the Sichuan Basin (SCB), a deep basin isolated by plateaus and mountains in Southwest China, by employing WRF-Chem with integrated process rates (IPR) analysis to characterize the terrain-driven seasonal variations of tropospheric ozone (O3) with atmospheric physical and chemical processes. Results show that the basin terrain exerts reversed impacts on regional air quality changes by aggravating summertime and alleviating wintertime near-surface O3 with the relative contributions oscillating seasonally between -40% and 40% in SCB. Similarly, a seasonal shift of vertical O3 structures is dominated by summertime positive and wintertime negative changes in the lower troposphere induced by basin terrain. The key contributions of atmospheric process to near-surface O3 are identified with vertical and horizontal transport, which is dominated by basin terrain with intensifying seasonal and diurnal variations. With the existence of basin, the daytime O3 productions at the near-surface layer are elevated in months of warm seasons (April and July) but inhibited in the cold seasons (October and January), presenting a seasonal transition of primary factor from meteorology to aerosol-radiation forcing on photochemical reactions. Driven by plateau-basin thermodynamic forcing, horizontal O3 transport between the SCB and eastern TP is enhanced by mountain-plains solenoid (MPS), and even nocturnal O3-rich layers contribute to the impacts of vertical exchange on near-surface O3 levels. The terrain effects of deep basin under the interaction of Asian monsoons and westerlies could jointly change atmospheric physical and chemical processes to construct the seasonal and diurnal O3 evolution patterns over the SCB region.

Atmospheric transport drives regional interactions of ozone pollution in China.

In recent years, ozone pollution becomes a serious environmental issue in China. A good understanding of source-receptor relationships of ozone transport from aboard and inside China is beneficial to mitigating ozone pollution there. To date, these issues have not been comprehensively assessed, especially for highly polluted regions in the central and eastern China (CEC), including the North China Plain (NCP), Twain-Hu region (THR), Yangtze River Delta (YRD), Pearl River Delta (PRD), and Sichuan Basin (SCB). Here, based on simulations over 2013-2020 from a well-validated chemical transport model, GEOS-Chem, we show that foreign ozone accounts for a large portion of surface ozone over CEC, ranging from 25.0% in THR to 39.4% in NCP. Focusing on transport of domestic ozone between the five regions in CEC, we find that atmospheric transport can largely modulate regional interactions of ozone pollution in China. At the surface, THR receives the largest amount of ozone from the other four regions (54.2% of domestic ozone in the receptor region, the same in below), followed by PRD (32.3%), SCB (26.7%), YRD (21.1%), and NCP (18.0%). Meanwhile, YRD exports largest amount of ozone to the other regions, ranging from 8.9% in SCB to 28.4% in THR. Although SCB is relatively isolated and thus impacts NCP, YRD, and PRD weakly (< 2.2%), export of SCB ozone to THR reaches 9.3%. The regional ozone transport over CEC, occurring mostly in the lower troposphere, is mainly modulated by the East Asian monsoon circulations, proximity between source and receptor regions, seasonal changes of ozone production, and topography.

Exploring the ozone pollution over the western Sichuan Basin, Southwest China: The impact of diurnal change in mountain-plains solenoid.

The Sichuan Basin (SCB), to the east of the Tibetan Plateau (TP), experiences severe ozone (O3) pollution. Unfavorable atmospheric diffusion conditions are considered the main causes of heavy air pollution over the basin. However, the meteorological impact of thermally driven mountain-plains solenoid (MPS) between the TP and SCB on O3 pollution has not been reported. Here we show the MPS driving the diurnal O3 changes in the atmospheric boundary layer over the SCB based on surface and high-resolution vertical observations, ERA5 reanalysis data, and the WRF-Chem model. The MPS shifts between upslope and easterly flows along the eastern slope of the TP and SCB during the day and downslope westerly flows to the western SCB at night. The daytime MPS flows drive the westward transport of O3-rich air mass in the atmospheric boundary layer from the polluted SCB and accumulate high O3 levels from the western edge of the SCB to the eastern slope of TP, subsequently aggravating O3 pollution in this region. After sunset, the MPS drainage flows carry air containing elevated O3 eastward downslope along the eastern slope of the TP into the nocturnal residual layer, enhancing the O3 concentrations aloft over the western SCB. The high-level O3 in the residual layer is transported downstream by nocturnal prevailing winds and contributes significantly to the next-day surface O3 buildup in the downwind region through daytime vertical mixing (~30 µg m-3 h-1). The present study reveals a transport mechanism driven by the MPS with coupling diurnal changes in the atmospheric boundary layer, which redistributes O3 over the basin and exacerbates O3 pollution along the western edge of the basin. This study has important implications for understanding meteorological drivers on atmospheric environment underlying the complex terrain.

Impact of deep basin terrain on PM2.5 distribution and its seasonality over the Sichuan Basin, Southwest China.

The terrain effect on atmospheric environment is poorly understood in particular for the polluted region with underlying complex topography. Therefore, this study targeted the Sichuan Basin (SCB), a deep basin with severe PM2.5 pollution enclosed by the eastern Tibetan Plateau (TP), Yunnan-Guizhou Plateaus (YGP) and mountains over Southwest China, and we investigated the terrain effect on seasonal PM2.5 distribution and the meteorological mechanism based on the WRF-Chem simulation with stuffing the basin topography. It is characterized that the three-dimensional distribution of topography-induced PM2.5 concentrations over the SCB with the seasonal shift of regional PM2.5 averages from approximately 30 µg m-3 in summer to 90 µg m-3 in winter at surface layer and from summertime 10 µg m-3 to wintertime 30 µg m-3 in the lower free troposphere. Such basin-forced PM2.5 changes presented the vertically monotonical declines concentrated within the lower troposphere below 3.6 km in spring, 2.3 km in summer, 2.6 km in autumn and 4.8 km in winter. Impacts of deep basin aggravated PM2.5 accumulation within the SCB and transport toward the surrounding plateaus contributing approximately 50-90% to PM2.5 levels over the regions of eastern TP and northern YGP. In the SCB, atmospheric thermal structure in the lower troposphere could build a vertical convergence layer between the boundary layer and free troposphere, acting as a lid inhibiting air diffusion, which was regulated by the terrain effects on interactions of westerlies and Asian monsoons, especially the wintertime strong warm lid deteriorating air pollution in the SCB. Furthermore, warm and humid air conditions within the basin prompted sulfur oxidation ratio by +0.02 and nitrogen oxidation ratio by +0.22 effectively producing the secondary PM2.5 in atmospheric environment.

Long-term variations in aerosol optical properties, types, and radiative forcing in the Sichuan Basin, Southwest China.

Long-term variations in aerosol optical properties, types, and radiative forcing over the Sichuan Basin (SCB) and surrounding regions in Southwest China were investigated based on two-decade data (2001-2020) from the Moderate Resolution Imaging Spectroradiometer, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, and the Santa Barbara DISORT Atmospheric Radiative Transfer model. The results showed that the aerosol optical depth (AOD550nm) in the SCB, a major polluted region in Southwest China, experienced an increasing tendency at a rate of +0.052 yr-1 during 2001-2006; thereafter, it decreased speedy up from -0.020 to -0.058 yr-1 over recent years, whereas the interannual variation in Ångström exponent (AE470-660nm) presented a persistently increasing trend during 2001-2020, with a rate of +0.014 yr-1. An improved atmospheric environment but an enhanced fine particle contribution to regional aerosols in the SCB was observed. Over the polluted SCB region, the dominant aerosol types were biomass burning/urban industrial and mixed-type aerosols with the proportions of 80.7%-87.5% in regional aerosols, with a higher frequency of clean aerosols in recent years, reflecting an effect of controlling anthropogenic emission in the SCB owing to governmental regulation. By contrast, few changes were observed in the aerosol types and amounts in the eastern Tibetan Plateau (ETP), where clean continental aerosols dominate with high proportion of 93.7% in the clean atmospheric environment. A significant decline in polluted anthropogenic aerosols was observed below 3 km over the SCB, resulting in the regional aerosol extinction coefficients at 532 nm (EC532nm) were declined by -0.22 km-1 from 2013 to 2020. Notably, the decreases in aerosol radiative forcing within the atmosphere were found in the SCB and the adjacent northern Yunnan-Guizhou Plateau (NYGP) and ETP, with -41.6%, -33.7%, and -13.6%, respectively during 2013-2020. This indicates that such an attenuated aerosol heating rate in the atmosphere, caused by aerosol variation, could alter the atmospheric thermal structure over the SCB and surrounding areas for regional changes of environment and climate in recent years.

Importance of meteorology in air pollution events during the city lockdown for COVID-19 in Hubei Province, Central China.

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 PM2.5 had a distinct spatial distribution over CEC during the lockdown, with high concentrations in North China and East China. In particular, relatively high PM2.5 concentrations were notable in the lower flatlands of Hubei Province in Central China, where six PM2.5 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 PM2.5 pollution in Hubei, reflecting the exceptional importance of meteorology in regional air quality in China.

Nocturnal surface radiation cooling modulated by cloud cover change reinforces PM2.5 accumulation: Observational study of heavy air pollution in the Sichuan Basin, Southwest China.

Surface radiation is crucial to atmospheric boundary layer development and air pollution formation. Several studies have revealed that surface radiation plays a vital role in developing the daytime convective boundary layer that controls the explosive growth of PM2.5 concentration; however, less attention has been paid to the effects of changing nighttime surface radiation on the near-surface temperature inversion layer and PM2.5 accumulation. In this study, we used long-term observations of meteorological and environmental data and atmospheric boundary layer measurements during a severe PM2.5 pollution event to investigate the effect of changes in nocturnal surface radiation on the increase in PM2.5 concentrations. The results showed that surface radiation cooling was enhanced (weakened) by decreased (increased) cloud cover fraction by changing longwave radiation at night; this strengthened (weakened) near-surface temperature inversion intensity and promoted (prevented) the accumulated increase in PM2.5. This observational study using 5-year data further confirmed the cloud radiative effect on the nighttime accumulation of PM2.5 with a significant negative correlation between nighttime averages of surface PM2.5 concentrations and cloud cover fractions. This reveals an important mechanism for the impact of surface radiation cooling modulated by cloud cover change on the nighttime accumulated increase in PM2.5. This finding extends our understanding of air pollutant accumulation at night with potential implications for atmospheric environment change.