In the present study, the characteristics of weather conditions and local meteorological variables over the Beijing‐Tianjin‐Hebei (BTH) and the Yangtze River Delta (YRD) regions in the winter are analyzed using the principal component analysis (PCA) method and daily PM_2.5 accumulation rate. Typical synoptic weather patterns over China in the winter can be classified into four types. During the Type 1 synoptic weather pattern, China is under the influence of the Siberian High, and northerly winds prevail; this situation is beneficial to the transport of pollutants from north to south. However, when the Siberian High is weak, southerly winds prevail which may result in the transport of pollutants from south to north. The Type 2 weather pattern refers to a weak high pressure located in the BTH resulting in the accumulation of pollutants. During the Type 3 weather pattern, an intense cold Siberian High moves to the south and affects the northern areas of China. The associated front brings heavy precipitation in the YRD resulting in the wet deposition of pollution. During the Type 4 weather condition, the weak Siberian High is blocked by the northeast cold vortex and moves towards the south, causing the accumulation of pollution in the YRD. The PCA model shows that there are two transport pathways for pollutants to the BTH (YRD) area: one from the YRD (BTH) and Shandong during Types 1 and 2 (Types 1 and 3) situations and the other one from the central provinces during Type 4 (Types 1 and 4).

Long‐range transport of PM_2.5 from the North China Plain (NCP) by cold frontal passage is considered to be an important haze formation mechanism in the Yangtze River Delta (YRD). However, detailed 3D distributions of PM_2.5 and meteorological fields during the front moving through are not fully understood. Herein, in situ observations showed that most haze events in autumn and winter in the YRD were associated with cold frontal passages. A typical cold front episode on November 2–3, 2017, was investigated with unmanned aerial vehicle sounding and model simulations, which both revealed that PM_2.5 in the upper air (∼1.0 km) reached the YRD earlier than that moving at the ground level. When the cold front reached the NCP, air pollutants ahead of the front was lifted from the ground level to the upper air. Subsequently, increased PM_2.5 concentrations and higher wind speed in the upper air resulted in stronger PM_2.5 outflow from the NCP to YRD compared to that at ground level. The maximum PM_2.5 mass contribution from the NCP to YRD was 15.6 μg m⁻³ (accounted for 31.5% of the total PM_2.5 in the YRD), which appeared at ∼1.0 km altitude. Compared to the noncold front period, the PM_2.5 mass transported from the NCP was 10–15 times higher during the cold front episode. Results indicate that the combination of multisite sounding and elaborate simulation can effectively explain the impact of synoptic processes on the transport, distribution, and evolution of air pollution and provide precise pollution control directives.

The strong economic growth in China in recent decades, together with meteorological factors, has resulted in serious air pollution problems, in particular over large industrialized areas with high population density. To reduce the concentrations of pollutants, air pollution control policies have been successfully implemented, resulting in the gradual decrease of air pollution in China during the last decade, as evidenced from both satellite and ground-based measurements. The aims of the Dragon 4 project “Air quality over China” were the determination of trends in the concentrations of aerosols and trace gases, quantification of emissions using a top-down approach and gain a better understanding of the sources, transport and underlying processes contributing to air pollution. This was achieved through (a) satellite observations of trace gases and aerosols to study the temporal and spatial variability of air pollutants; (b) derivation of trace gas emissions from satellite observations to study sources of air pollution and improve air quality modeling; and (c) study effects of haze on air quality. In these studies, the satellite observations are complemented with ground-based observations and modeling.

The COVID-19 pandemic has significantly affected human health and the economy. The implementation of social distancing practices to combat the virus spread, however, has led to a notable improvement in air quality. This study compared the surface air quality monitoring data from the United States Environmental Protection Agency (U.S. EPA)’s AirNow network during the period 20 March–5 May in 2020 to those in 2015–2019 from the Air Quality System (AQS) network over the state of California. The results indicated changes in fine particulate matter (PM_2.5) of −2.04 ± 1.57 μg m⁻³ and ozone of −3.07 ± 2.86 ppb. If the air quality improvements persist over a year, it could potentially lead to 3970–8900 prevented premature deaths annually (note: the estimates of prevented premature deaths have large uncertainties). Public transit demand showed dramatic declines (~80%). The pandemic provides an opportunity to exhibit how substantially human behavior could impact on air quality. To address both the pandemic and climate change issues, better strategies are needed to affect behavior, such as ensuring safer shared mobility, the higher adoption of telecommuting, automation in the freight sector, and cleaner energy transition.

The impact of stratospheric intrusions (SIs) on Central and Eastern China (CEC) with severe ozone pollution presents a range of uncertainties, underscoring the imperative for sustained research endeavors. In this study, we propose a traceability assessment method that can derive high-resolution critical source areas (CSAs) of SIs by utilizing ambient air quality observations, global ozone (re)analysis data, and customized Lagrangian simulations. This approach enables us to facilitate a meticulous and systematic examination of the impact of direct and indirect SIs on tropospheric and near-surface ozone in six important sub-regions within the CEC during the spring and summer of 2019, as well as the unique circulations driving SIs, from a more refined and targeted tracing perspective. The findings reveal that impacts of indirect intrusions are more efficient at monthly scales, with contributions to tropospheric ozone reaching up to twice the magnitude of direct intrusions. The impacts of direct intrusions are more pronounced at daily scales, primarily occurring in May. In terms of contribution to near-surface ozone, the eastern plains frequently witness ozone exceedance events, with the most substantial impact from SIs observed (e.g., contributing 15.8 % and 16.7 % to near-surface ozone in North China and East China, respectively), showcasing a remarkable ability to capture descending lower-stratospheric air. In contrast, the Loess Plateau and Central China, situated in central and western high-altitude regions, receive more intrusive ozone into the troposphere but exhibit minimal contributions to near-surface ozone. The indirect intrusions that generate the above impact come from three to four CSAs located thousands of kilometers away, evenly distributed latitudinally within the westerlies between 40 and 70° N (spaced 70 longitudes apart). These CSAs are intricately linked to the evolution of synoptic-scale Rossby waves or subtropical westerly jets, with western Siberia or Europe and north Africa identified as the most significant CSAs. Conversely, the CSAs for direct intrusions are relatively concentrated, with those influencing the middle troposphere originating from the Tibetan Plateau and those influencing the lower troposphere predominantly located in Mongolia and central Russia, just a few hundred kilometers from the CEC. These sources are associated with typical atmospheric circulations such as the northeast cold vortex and the South Asian High, where the intensity of the intrusion system plays a more crucial role than its frequency of occurrence. This study provides valuable insights for forecasting and mitigating the impact of SIs on ozone pollution in China and contributes to addressing the broader challenges posed by climate change.